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This PDF file contains the front matter associated with SPIE Proceedings Volume 1576, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.
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Using the "MAGIC" code, computer simulation is performed to design a cathode structure which provides a radial distribution of electron beam density matching the field distribution of the TM05 mode. Thus, very narrow band TM05 mode microwave radiation can be generated in an open end waveguide. The simulation results show that the vircator can indeed be operated at the selected frequency 10.4 GHz with a very narrow bandwidth(∆ω/ω<1%).
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The magnicon is a new scanning beam (deflection-modulated) microwave amplifier related to the gyrocon. We discuss the design of a high perveance, magnetic-field-immersed frequency-doubling magnicon amplifier at 11.4 GHz. This magnicon will operate with a field-emission diode, but is intended to be a prototype for future thermionic devices with potential application to advanced linear accelerators and to space power beaming.
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The recirculating hub charge causes nonlinear feedback between the input and the output in Crossed-Field Amplifiers. By varying some control parameters, such as the secondary emission coefficient or the recirculating charge fraction, the operation exhibits transitions from steady state, to limit cycles and finally to unpredictable (chaotic) behavior. The amplifier dynamics is described by a truncated system of fluid equations following guiding center trajectories1. Numerical integration of these equations yields very good agreement with experimental results, while reducing the computation time by three orders of magnitude compared to particle simulations. A pass-to-pass map is employed to investigate whether the chaotic transitions in CFAs follow the 'universal' rout to chaos for nonlinear-dissipative systems.
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It is known that the working regime of the magnetron is defined by thwo generalized parameters, namely H and J. H=H/Hcr - is the parameter of the supercritical state (where H - is the strength of working magnetic field, Hcr is the strength of the critical magnetic field), J=Je/Je* - is the parameter characterizing the proximity to the regime of the emission limitation by the space charge field...
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The interest evinced to high stability sources of radiation is explained by their broad application both in a physical investigation and a technical supplements. Herewith, the peculiar attention is spared to the elaboration and research of millimeter wave range electron-vacuum devices of the middle and larger power.
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Recently, two-dimensional (2D) plasmons in thin semiconduc. tor electron layers have been widely studied /1/. 2D plasmons are well-defined excitations at the frequencies ω in the far-infrared range where the condition uτ≫1 can be satisfied for the realistic relaxation time τ∼10-12s.
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Electron synchrotrons can serve as infrared sources with special characteristics which are particularly well matched to certain types of experiments. The electrons emit from a small area into a narrow solid angle, resulting in a source several orders of magnitude brighter (and at very long wavelengths, greater overall power) than a blackbody source. The NSLS infrared beamline at Brookhaven National Laboratory realizes these advantages, and the source has been used to explore, among other things, the properties of high-Tc copper oxide superconductors.
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Very high magnetic fields are required to determine accurately the resonance position, width and shape of broad magneto-optical absorptions. A strip-line was developed for use in pulsed magnetic fields. This was used for sensitive measurements in ferromagnetic 3d metals. Paramagnetic resonances were measured in Gd-doped high Tc superconductors; an additional resonance was observed at low temperatures which was sensitive to the oxygen content.
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The photoresponse of granular Bi2Sr2CaCu2O8 and of epitaxial YBa2Cu3O7-δ thin films has been investigated at wavelengths between 10 μm and 944 μm. In addition to a bolometric signal we observe for the granular Bi-based films a nonbolometric response at temperatures T <80 K. The dependence of the signal on intensity, applied magnetic field and wavelength is consistent with an ac josephson effect occurring at the grain boundaries.
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In the past four years there have been many reports of detection of far- infrared radiation in High Temperature Supercondutors1,2. We report here some additional observations of this phenomena, in particular we have measured a threshold of bias-current below which no response is observed to radiation in the range 85-150 cm-1.
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We present experimental results of far-infrared reflectivity investigations of RBa2Cu3O7-δ (R=Er, Tm, Dy) and YBa2Cu4O8 superconducting materials in the temperature range 10-300 K. In order to determine the phonon parameters such as frequency, half-width and oscillator strength, we performed a Kramers-Kronig transform of the reflectivity spectra and than fitted the bands in the conductivity spectra to lorentzian lineshapes.
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Two YBCO oriented epitaxial films on SrTiO3 substrates prepared by laser ablation technique were used for mid-infrared optical Investigations. Both ac-ori rented (i.e. a→ (or b→) and c→ lattice vectors lie in the substrate plane) and a→b→-oriented films were deposited and annealed at the same technological cycle and had thickness about 4000A. Special attention was paid to the determination of quality and orientation. X-rays difraction showed only presence of 00n peaks for (ab)-orientation and 110, 220 peaks for (ac)-film. Lattice constants from X-rays measurements were used to calculate the oxigen contents of films by empyrical formula 7−δ=76.4−5.95⋅c→ [A], which gives δ≈0.08+0.12.
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Ellipsometry is based on exploiting the polarization transformation that occurs as a beam of polarized light is reflected from or transmitted through an interface or film. With this method optical constants can be determined very fast, very precisely and without further assumptions (no Kramers-Kronig analysis). We have developed a rotating analyzer ellipsometer for the far-infrared wavelength range from 30 to 1200 μm. Applications are in the characterization of semiconductors and also, together with a cryogenic reflection unit, of high-Tc superconductors.
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The investigation of thin film features on high reflectivity materials by optical methods is complicated due to small useful signal. Surface electromagnetic waves (SEW) is considered to be very perspective method for analysis of such materials. For SrTiO3 SEW exist in FIR region (ν=88 - 807cm-1), except narrow gaps, and its propagation distance is about a few millimeters [1,2]. So, it can be easy measured. In the present paper we report the measurements of the optical features of YBa2Cu3O7-x films on SrTiO3(100) substrate by FIR SEW amplitude and phase spectroscopy at temperatures 80 - 300K.
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Far-infrared and submillimeter-wave astronomy are relatively new branches of astronomical research. This is due to the poor transmission of the terrestrial atmosphere in this spectral range. A restricted number of narrow spectral windows permit the observation of astronomical objects directly from the ground. Reliable observations have therefore to be performed from satellites [1], stratospheric-balloon gondolas [2,3] or aircraft [4].
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Far infrared emission spectroscopy is a powerful technique for the measurement of a number of molecules that are important in stratospheric (i.e., ozone layer) chemistry. The technique is particularly applicable to polar molecules with large, hydrogenic rotational constants [Chance et al., 1985]. These molecules include diatomics (HCl, HF, HBr, and OH), and nearly symmetric prolate molecules with large perpendicular components of their dipole moments (HOCl, H2O2, and HO2). They have strong rotational spectra in the far infrared that persist to sufficiently high energy (80 cm-1 and above) that their spectra do not suffer great interference from the rotational lines of H2O and O3 which dominate the far infrared atmospheric spectrum. Thus, this type of molecule can be measured with great sensitivity from about 80 to 200 cm-1.
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Ozone is the most important minor constituent of the upper atmosphere. It plays a very important role in the chemistry and photochemistry of the stratosphere. For this reason, a large number of studies have been performed on this molecule. In particular, pure rotational and vibrational-rotational spectra have been recorded with several techniques (1).
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Lineshape parameters of ozone rotational transitions in the submillimeter wave region are required for an accurate determination of mixing ratio of this molecule in stratosphere. In order to obtain very accurate concentration profiles, it is very important to have a. good knowledge not, only of lineshape parameters, but also of their temperature dependence.
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Since 1978, the Microwave Sounding Unit (MSU) onboard the NOAA american satellites provides the meteorological community with an enhanced temperature profiling capability. In addition to the 60GHz oxygen band observations, the future generation of these polar orbiting satellites will also include a humidity sounder up to 190GHz : AMSU-B.
To help understanding the radiative transfer in the atmosphere at these high millimeter wave frequencies, the UK Meteorological Office and the Laboratoire de Météorologie Dynamique (France) have jointly developed a microwave receiver at 89 and 157 GHz, frequencies corresponding to the AMSU-B window channels. The sounder has been flown in two international experiments in 1990. In this paper, we will focus on the gaseous absorption problem.
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The melting layer is assumed to be composed of spherical melting snow particles. Size distribution and average dielectric constant are used to characterize the melting snow particles. Both the size distribution and the dielectric constant are found to be connected with the physical and meteorological parameters. The cross sections, the albedo, and the effective reflectivity factor are computed in a frequency range of 1-100 GHz by using the Mie scattering for five size distributions and rain rates below 12.5 mm/h. Numerical results show that the difference between the melting layer attenuation and the attenuation of an equal path length in rain with the drop size distribution that results from the melting is at maximum around 25 GHz. It has also been seen that the peak of the effective reflectivity factor is disappeared in the melting layer when the frequency is higher than 70 GHz.
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A measurement system for the observation of the influences of the local climate on the transmission behaviour at millimeter waves has been developed and tested. Because of the required high resolution for path attenuation changes the coherent measurement principle has been selected, thereby simultaneosly avoiding a biologically relevant power density anywhere in the open transmission path. Both transmitter and receiver antennae have been placed at the same end of the bidirectional transmission path to facilitate the coherent measurement, whereas the other end consists of a metallic reflector.
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There are many atmospheric trace gas species involved in the three catalytic cycles that contribute to stratospheric ozone depletion. The OH molecule is central to one of this cycles yet its concentration as a function of altitude is very difficult to measure in the stratosphere. The stronger rotational transitions lie at frequencies greater than 1500 GHz, significantly above the realm, of traditional submillimeter heterodyne instruments. Both Fabry-Perot and Fourier transform spectrometers have been used to observe OH in the atmosphere. Due to their inherent lower resolution these instruments can obtain atmospheric profiles only in a limb viewing geometry, thus they must be lofted above the stratosphere via balloons. This significantly constrains their seasonal and geographical survey capabilities. In this paper we Investigate the feasibility of utilizing a far infrared laser heterodyne instrument, operating aboard an aircraft, for the detection of OH and other relevant trace gas species in the stratosphere.
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For some years the main aim of investigations of gyrotrons is the creation of sources ECRH of fusion plasmas at frequenceies over 100 GHZ with outputs of 1 MW - level in pulses with duration of several seconds (up to CW). Nowadays the fabloration of gyrotrons with average output from 10 to 100 kW at frequencies ∼30 GHz and higher has been begun for technological applications. Nevertheless the basic investigation of cyrotrons are still bound with problems of increase in output power. Pulse duration and frequency.
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A recent multimode simulation[1] of a high power gyrotron[2] has suggested that mode competition with backward waves is responsible for the lower than expected efficiency at high current. The present effort is an attempt to compare the predicted and measured features of parasitic modes in a particular cavity.
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We present a simulation study of mode competition in a highly overmoded gyrotron cavity, modeled after the approximate design parameters of a 280 GHz, 1 MW TE+42,7-mode gyrotron at the Massachusetts Institute of Technology (MIT). This study addresses 1) the problem of achieving gyrotron operation in the design mode, and at a value of magnetic detuning sufficient to achieve high efficiency operation, and 2) the mode purity of the final state.
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On the basis of a generalized theory we compare the cold-cavity and the self-consistent methods of calculation of mode competition in gyrotrons. This comparison enables us to determine conditions, when the cold cavity approximation is justfied. A specific example is presented.
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The output power from a gyrotron is usually extracted through a window which is not perfectly transmitting. Thus, a fraction of the power is reflected back into the interaction region. This reflection can affect the operation of a gyrotron in several ways1. First, the reflection modifies the Q factor of the cavity and changes the operating characteristics such as start current and efficiency. Second, due to the so-called long line effect the spectrum of stationary modes in the cavity becomes dense with a spacing equal to the reciprocal of the round trip travel time from the cavity to the window and back. A third consequence is the effect of the variation of reflection coefficient with frequency on mode competition. We will examine these effects with the aid of simple models and time dependent multimode numerical simulation. One result of our studies is that even for small reflection coefficients if the delay time is sufficiently long the time dependence of the radiation field in a single transverse mode can become quite complicated.
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Gyrotron design is almost exclusively based on a time-domain analysis of the tube performance. Such a procedure is cumbersome mainly with respect of taking also device/circuit interactions into account. We present a network-theoretical approach to the gyrotron oscillator analysis which circumvents such difficulties.
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In a preceding paper (Jensen, Schtinemann: "Network-Theoretical Approach to the Gyrotron Oscillator", 16th Conf. IR & MMW, Lausanne 1991) we have developed a network-theoretical approach to gyrotron oscillator analysis. As in normal microwave oscillator theory, it formulates an oscillation equation in frequency-domain in which both the device and the load admittance are generalized now to a nonlinear (and mainly frequency- insensitive) and to a linear admittance matrix, respectively. The latter describes the frequency-dependent passive circuit.
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There is more interest in electrostatic electron cyclotron resonance maser (EECRM) ,which is similar to Orbitron, because of its attractive advantages to gyrotron[1-4], e. g. , it may be voltage tunable, it may operate at high cyclotron harmonic and nonrelativistic case, it requires no magnetostatic field. By now, most articles for EECRM and Orbitron dealed with an ideal model, i. e. , the space-charge effects were neglected. Whereas, one can see from the results given below that the space-charge effects have a important role in EECRM, and may change parts of EECRM characteristics.
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Bending a short bunched electron beam from the linac by the magnetic field, we have observed coherent synchrotron radiation, which is a new source of far-infrared and millimeter waves with high brightness. Spectra of coherent Cherenkov and rf wake field radiation have also been observed. This paper is a review of our experiments by S- and L-band electron linacs at Tohoku University and Kyoto University.
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The idea behind this particular Gunn oscillator-frequency multiplier combination which is a follow up on an earlier project (1), was that in combination with SIS receivers which have ℒO requirements in the 100 nW range,efficiency and maximum output power of a source can be traded for convenience and economy. Instead of cascading carefully optimized varactor doubler and triple stages, this frequency source only rectifies the radiation from the Gunn oscillator and filters for the desired harmonic output frequency.
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Measurements of the dielectric and optical properties of materials (complex refractive indices, dielectric constants, magneto and electro-optic coefficients etc.) over wide ranges of frequency above 300 GHz are made by Fourier transform spectrometry and dispersive Fourier transform spectrometry. These techniques utilise mercury arc lamps as broadband noise sources with noise temperatures of about 2000 K. For frequencies below 300 GHz the spectrometric methods are still applicable but the power available from mercury arc lamps is very low (< 2000 K) and below about 100 GHz spectrometric measurements require long integration times to reach acceptable signal-to-noise ratios.
Waveguide mounted noise diodes are used for power calibration of microwave systems at frequencies below 60 GHz. They have fairly high noise temperatures but are limited in frequency coverage to a single waveguide band.
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The low-noise high efficiency Q-band MESFET oscillators are developed. Their noise, power and temperature characteristics are investigated. The possibility of the 2th and 3th harmonic power generation are demonstrated.
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We have modified the frequency of a propagating R.F. signal using a pulsed plasma discharge. In our experimental work, we have utilized two frequency shifting mechanisms. The first is to use the plasma as an nonphysical plunger to upshift the frequency of the signal.
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A structure of active layer for relieble efficient gunn diodes is suggested. Such layer was grown by MBE and good gunn diodes were achieved.
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One of the means of lowering of noise of sources of electromagnetic oscillation is the use in generator high Q-quality osciallation system. This way especially actual in elaboration solid-state generators of extremely high-frequency band (E,F,G, band), where the using of the method of outer synchronisation and other methods stabilisation is rather problematically. The open resonator (OR) with the echelette is one of the versions of the dispersion open oscillating system. The diffraction reflection grating echelette has brightly pronounced angle dispersion and this helps to realize the selection of spectrum as an transversal as an longitudinal modes and this is of a great importance. The last property does these gratings very attractive from the point of view their use in the open oscillation system of the solid-state and electron-vacuum tube devices.
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The difficulties connected with the development of MM-wave generators of middle and large power in with the shortening of the wavelength. The main cause of this is the decreasing of the geometrical sizes of the elements of slow wave system (SWS) of TWT and, accordingly, the decreasing of transversal cross-section of its floating-drift channel and the increasing of heat loads. The improvement of heat-physics characteristics of TWT is achieved by the using slow wave system with the extensive surface of type of coupled cavity circuit or modification of ribbed structures, working at spatial harmonics. The increasing of the geometrical sizes of the elements of slow wave circuit entails the increasing of the operating voltage, RF-loss, the decreasing of electron flow perveance and electron efficiency. The increasing of the beam perveance and the efficiency in TWT is achieved by the using multi-beam electron flows /1/. At this the increasing of output power, the decreasing of operating voltage and the broadening of operating bandwidth take place. The possibility of the realization of TWT with staircase, slow wave structure to obtain middle and large output power in UHF-region was shown in /1,2/.
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Creation and investigation of Gann-diode low-noise comb spectrum oscillator (CSO) are discussed. Considered oscillator can build the base for coherent frequency synthesizers forming oscillations directly in millimeter band.
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After four years of world wide research on high Tc, superconductors of the cuprate type a variety of different material families is known, with Tc values up to 125 K. Progress in material preparation and in the development of experimental and theoretical methods has lead to a basic understanding of various material properties, however, the nature of the pairing mechanism is yet unknown. In this paper some basic properties will be discussed. A second point will be concentrated on important optical properties from microwave to u.v. frequencies. Condensation of charge carriers can be accompanied with anomalous phonon behavior; at Tc anomalous changes of oscillator strengths of infrared active phonons and anomalous shifts of resonance frequencies of some phonons are observed. Condensation of charge carriers leads, furthermore, to a behavior of the (anisotropic) optical properties that may be described by superconducting energy gaps. For excitations along the copper oxide planes the optical properties can be described as arising from a superconducting energy gap 2∆(0)/kTc≈7 . However, there remains at low temperatures intra-gap absorption that extends from microwave to far-infrared frequencies. In a third part of the paper, microwave and far-infrared applications of high Tc superconductors will be discussed, namely microwave antennas, Fabry-Perot filters and
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We report on the application of high-Tc superconductors as reflectors for far-infrared Fabry-Perot resonators. At low temperatures and frequencies smaller than the superconducting gap frequency the optical properties of YBa2Cu3O7-δ are mainly determined by inductive currents. Therefore YBa2Cu3O7-δ thin films on MgO, which is transparent in the far-infrared, are suitable for Fabry- Perot resonators with high peak-transmissivity. We report on a resonator with first order resonance at a frequency of 80 cm-1, a quality factor of 55 and a peak transmissivity of 0.16.
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The concentration of the electromagnetic field inside WG dielectric resonators [1] has been emphatized as a cause of sensitivity enhancement in the techniques of ESR spectroscopy [2]. This work is an experimental study dedicated to the detection of optical and infrared radiation utilizing as a thermal detector a small polycristalline sample of high-Tc superconducting material placed inside a WG resonator and using the techniques of microwave measurements [3]. The experimental set-up, as depicted in Fig 1, consisted essentially in a simple transmission-cavity spectrometer operated in the frequency range from 18 to 26.5 GHz (K-band). The resonator was a 20 mm alumina disk, 3 mm thick, excited in the WGH modes of resonance with dielectric rod waveguides, and supported by a liquid nitrogen cold finger in high vacuum conditions. The transmitted power was detected by a crystal receiver. A few tens of μg of YB2C3O7-δ powder [4], was pressed at the bottom of a .9 mm hole, in a region of high electromagnetic field intensity. The hole was drilled in radial direction into the curved surface of the disk and was about 1.5 mm deep. A microwave sweep oscillator of 10 dBm of output power was fixed at the resonant frequency of the loaded resonator, the temperature being sligthly lower than the critical temperature of the superconducting material. The transmitted power level was recorded while a red-laser beam was directed inside the hole. The recorded signals, as reported in Fig 2, give indications about the response time, and the sensitivity attainable. The upper trace represents the transmitted signal variation obtained illuminating the resonator hole with a non focalized 1 mw CW red laser beam. The rise time of the signal was of τ=.5 s , and it was found to be proportional, within limits, to the sample mass. The voltage signal was proportional to the incident microwave power. The lower trace represents the transmission curves of the resonator as a function of time: with "laser on" (the first two peaks), and with "laser off" (the three peaks to the rigth) . These curves were obtained by sweeping repeatedly the microwave source +/−10 MHz around the frequency of 24.4 MHz, i.e. the resonant frequency of the WG disk resonator with laser "off'. The resonant frequency shift under "laser on" conditions was of 4 MHz and the relative Q-factor variation was of about 17%. Both the resonant frequency and the Q-factor diminished as the sample temperature raised over Tc. The observed signals are attributable to the phase transition of the superconducting sample under the heating effect of light. Chopped laser signals up to 10 Hz could be recorded in a straigth video- detection scheme, with a signal to noise ratio of 10. The loaded Q-factor was always of the order of 103. The quasi black-body absorption characteristic of the powder charged hole, acting much as a light-trap, makes the device capable of detecting infrared radiations in a wide range of wavelengths. The maximum voltage "responsivity" (at 0 hertz) was of about 200 V/W , with a voltage noise less than of .1 mV/(Hz)1/2. Experiments with higher Q-factor dielectric resonators and epitaxially deposited thin superconducting films for characterization purposes at microwave frequencies are in project.
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The work introduced below has extended previous reports of far infrared (FIR: 10 cm-1 < ω < 100 cm-1) responses from granular Y-Ba-Cu-O [1] and Tl-Ba-Sr-Cu-O [2] films to the Bi2Sr2CaCu2O8 system for the first time. More importantly, the photosignal was found to depend on √Power over a two orders of magnitude range of incident powers. The detection mechanism is therefore believed to arise from the interaction of current biased intergranular Josephson junctions with radiation induced screening currents. A non bolometric response is also observed at low temperatures at a mid infrared frequency of 903 cm-1.
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We report on the use of high temperature superconductor material for fabricating of a resonator of a far-infrared solid state laser. In our experiment the active medium consisted of a p-germanium crystal at 4.2 K in crossed electric and magnetic fields. We used a SrTiO3 plate as highly reflecting mirror and a YBa2Cu3O7-δ thin film on MgO plate as output coupling mirror. The spectra of the laser with the superconducting mirror was measured. We demonstrate also highly reproducible operation of the laser.
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Since the discovery of high-Tc superconducting cuprates, the interest in envisioning various radiation detecting mechanisms has been considerably revived. If the implementation of Josephson micro-junctions is greatly inhibited by difficulties to setup multilayer processes, simpler devices - that involve a single superconducting film deposition step - are technologically more accessible.
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We have fabricated HTSC bolometers with a responsivity that is essentially wavelength independent in the range from λ ∼ 0.6 μm to 450 μm. They consist of a thin film of YBaCuO on a SrTiO3 substrate and were patterned by photolithography in the form of a micro-bridge (20 x 20 μm2). A maximum responsivity of S = 0.1 V/W and an NEP = 2.10-7 W/√Hz have been obtained when the element was operated near the transition temperature of 90°K. The frequency response varies like S ≈ 1/√f which indicates a strong thermal coupling of the micro-bolometer to the substrate. Higher responsivities (S ≈ 800 V/W and an NEP of 2.10-11 W/√HZ) have been obtained using a second type of detector element in the form of a meander covering an area of 1 mm2. For these elements a frequency response S ≈ 1/f and a cut-off frequency of 0.1Hz have been found.
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A microwave phase shifter has been made by using Bi-Sr-Ca-Cu-O bulk superconductor. In this paper we describe the fabrication of microwave phase shifter and present the expermental results on the relation between phase and frequency, also phase and bias current.
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The generation of ultrashort far- infrared laser pulses is of interest for applications in solid state physics, plasma physics and also for the understanding of fundamental processes occuring on a short-time scale in far-infrared gas lasers. However, far-infrared lasers are generally low-pressure narrow-band systems which implies that in this wavelength range short pulses cannot be produced by conventional means. We therefore investigate the generation of ultrashort far-infrared pulses with laser gases which emit by coherent processes such as superradiance or Raman emission.1 In the process of superradiant emission2 the atoms are coupled together by their common radiation field, and thus decay cooperatively. For a number N of emitting atoms or molecules the pulse intensity is therefore proportional to N2 and the pulse duration to 1/N. Since N can be large, this process offers the possibility of producing intense short pulses. In the case of Raman emission3 the pulse can be shorter than the inverse linewidth of the transition if the pump pulse is broad band. Hence, the pulse duration is determined by the larger of either the pump-pulse bandwidth or the laser linewidth.
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We report operation of a passively mode-locked tunable high-pressure CO2 laser and its use for generation of far-infrared laser pulses. Mode-locking of the CO2 laser was performed with p-doped Germanium as saturable absorber. We obtained tunable radiation consisting of trains of short pulses of about 1 ns duration. By optical pumping of CH3F gas superradiant emission resulted in generation of subnanosecond far-infrared laser pulses at a frequency of about 35 cm-1.
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Using a continuously tunable high pressure CO2 laser as pump source, we obtain laser action at 58 lines, 48 detected for the first time, from D2O gas. 53 of these lines, covering the frequency region from 26 cm-1 to 236 cm-1, were found to occur by stimulated Raman scattering with a maximum bandwidth of the Raman gain regions of 17 GHz. We made use of these broad amplification profiles for generation of subnanosecond far infrared pulses.
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In this paper, an overview is presented on the frequency tunable methylfluoride Raman laser. By use of a continuously tunable high pressure CO2 laser as pump source, tunable far-infrared radiation can be obtained from all the stable isotopes of methylfluoride which have symmetric top structure. Using these gases, 12CH3F, 13CH3F, 12CD3F and 13CD3F, as FIR media about half of the frequency region from 8 cm-1 to 73 cm-1 can be covered with tunable intense pulsed radiation. We studied quantum effiencies and bandwidths of the tuning intervals for the different gases and also for different pump schemes.
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The para-H2 Raman laser has been developed intensively as a high intensity tunable laser source in the wave-length range of 13-18μm. The successive conversion to the second Stokes wave can expand wavelength up to 46μm. However, the increase of wave-length is accompanied by the substantial decrease of Raman gain. Practically, the wavelength which is obtained by the second Stakes conversion re- mains below 30μm even with the use of a liquid-nitrogen-cooled Raman cell1). In this report, we generate a high intensity far-infrared pulse from a para-H2 Raman laser being operated at room temperature as the second Stokes wave. The longest wavelength extended to 42.5μm. Powers of the pump pulse and the first Stokes pulse were fully depleted and the converted energy was estimated to be more than 100 mJ with a pulsewidth of about 50 ns.
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Utilization of active media of different isotopic composition enables to expand significantly the spectral range of optically pumped FIR Raman lasers. In this work CD3F FIR emission was investigated using grating tunable TEA-CO2 laser. Experimental investigation of possibility to obtain superradiant emission on different FIR lines under relatively low pump intensities (up to 5 MW) was the main objective of this work. We used multi pass cell with total optical length of about 15 m in order to diminish the emission thresholds. The experimental conditions were analogous to /1/, where the output characteristics of 12CH3F, 13CH3F mixture FIR Raman laser were investigated. The pressure of the active media was not optimized.
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Theoretical studies of the efficiency of cw optically pumped far infrared lasera (OPFIRL) tend to neglect transversal variations of the pump-intensity (see e.g. passes through the low absorbing cavity. Realistic pump field distributions appear to be different due to the common practice of focussing the pump beam through a small coupling hole, thus introducing large intensities of the "primary beam" that will always dominate the overall intensity profile /2/. The specific consequences of radially varying pump fields will be discussed in two steps.
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Very little research on optically pumped FIR-ringlasers has been reported in the past, despite the fact that such lasers have proven to be more stable FIR-monochromatic sources than the usual standing wave resonator types (Heppner, Weiss 1978).
The ringlaser offers much better isolation between the pump and the FIR-laser due to the elimination of interaction between the two. This is similar to pumped dye ringlasers in the visible part of the EM-spectrum.
Unlike most ringlasers in the visible range unidirectional emission is a general feature of FIR-ringlasers. Hence an optical diode is not required to suppress simultaneous bidirectional operation.
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A theoretical model was set up to calculate numerically the output power of far infrared lasers with an active medium where vibrational-translational relaxation predominates, e.g. formic acid HCOOH, in dependence of the pump beam. The calculation is based on an actual waveguide ring laser system similar to that described in [1] with a pump power of 15W.
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Recent results will be presented on high frequency (100-300 GHz) megawatt gyrotron experiments being conducted at MIT. Two gyrotron oscillators are being investigated: one operating at 148 GHz in the TE16,2,1 mode, and the other at 280 GHz in the TE42,7,1 mode. These gyrotrons operate with 3μsec pulses, but can be scaled to cw operation. The highest power at 148 GHz has been achieved with a two-section cavity. Powers up to 1.2 MW with an efficiency of 32% were measured at 48 A. The 280 GHz experiment will begin shortly, and will use a single cavity with a diameter of 23 wavelengths. This will be a severe test of the gyrotron's ability to excite a single mode and suppress competing modes. Measurements of the electron beam spatial and velocity characteristics for the 140 GHz gun will also be presented.
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A 120 GHz, 1MW, TE15,2,1 whispering gallery mode gyrotron was designed and constructed. As this tube is a demountable type, it is possible to change the important parts of the gyrotron such as a magnetron injection gun, a cavity and a window. In the first experiment, an output power of 610 kW was obtained at 120 GHz with an efficiency of 30.6 %. The maximum efficiency of 32.2 % was achieved at an output power of 506 kW.
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Varian is carrying out the development of a high-power, CW gyrotron at a frequency of 110 GHz. The design of a tube capable of generating an output power of 1MW CW has recently been completed. The tube will employ a TE22,2,1-mode cavity and utilize an output coupling concept where the collection of the spent electron beam is separated from the outgoing microwave power.
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The user-friendly programming environment DAPHNE has been developed for the optimization of Gyrotrons. It is embedded into the ASTRID system which is an integrated program development system including the module MiniM to define the geometry of the device, the CASE module to describe the boundary conditions, a MESH module to specify an autoadaptive grid, a solver for elliptic partial differential equations, as well as a powerful graphics system. These ASTRID modules are connected together by means of the data management system MEMCOM. External programs (magnetic field solver, particle pusher, wave/beam simulator, heat deposition) can be added by interfacing them to MEMCOM. The different program modules are executed by means of a specially concepted command language. Domain decomposition techniques are used to specify the geometry, to build locally structured meshes and to represent graphically the results.
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The top energy parameters of gyrotrons depend on the quality of helical electron beams ( HEB) which are formed mainly by magnetron injection guns (MIG). The limitations of the HEB current are related to the instability which has various specific manifestation (increase of noise,spontaneous growth of the emitting current,breakdowns), but always cause a sharp worsening of the HEB quality.
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Industrial gyrotrons are being developed for more than fifteen years in the Soviet Union. Gyrotrons, that are developed and manufactured by industrial firms, completely meet requirements inside the Soviet Union. Gyrotrons are used mainly for scientific researches and specifically for fusion experiments. These tubes are used in Kurchatov Institute of Atomic Energy (Moscow), in its branches in Troitsk, Leningrad, Kharkov , Novosibirsk.
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AC space charge self-fields in gyroiron beams are studied via a set of particle equations. The study addresses the linear stability of beams in tapered magnetic fields as well as the nonlinear-multi frequency saturation of space charge instabilities.
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In experimental investigations of gyrotrons an important discrepancy between the theoretically predicted efficiency and measured values has been observed. [1] Since the energy conversion efficiency depends critically on the electron velocity, it is important to be able to measure this latter one. A non-intrusive measurement is only possible with optical methods.
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A 110 GHz superconductor insulator superconductor (SIS) tunnel junction receiver has been developed and used in regular astronomical observations on the 4m radio telescope at the Department of Astrophysics, Nagoya University. The SIS junction consists of a sandwich structure of Nb/AlOx/Nb, and is cooled to 4.2K with a closed cycle He-gas refrigerator. The receiver exhibits a best double side band noise temperature of 17±2 K at 110GHz. The receiver has a good response over this input frequency range of 98-115 GHz.
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Today superconducting tunnel junctions are the best mixer elements in the 0.7 to 3 mm wavelength range. This open structure mixer, built on the basis of such a junction approaches the best waveguide systems (1). Its particular merrits are low cost, easy assembly, and convenient tuning by one single element. The mixer element is a series array of 3 PbBi junctions deposited on a fused-quartz substrate. The junctions are positioned in the middle of a planar dipole structure. In order to isolate the bias circuit and the IF amplifier band stop filters were deposited on both ends of the dipole. The usual perodic high-low impedance structure was used for these filters. A substrate holder presses the substrate with moderate force towards an elliptical immersion lens made from crystal quartz. This concept was pioneered by (2). Special features of our design are (i) the junction composition, providing a gap voltage of 10mV for the array, (ii) the use of a resonant dipole feed with choked bias connections, (iii) the imaging by one single aspheric lens of crystal quartz. The lens-junction assembly is mounted in a copper block which also carries the IF connector. A 3 mm screw in the substrate holder provides a movable backplane behind the dipole feed. On both sides of the mixer block superconducting coils with 6000 turns each provide the necessary magnetic field. The results below were measured at 100 mA corresponding to 110 Gauβ at the junction. At this flux density the Josephson effect is at its first minimum. Mixer, coils, and a low noise preamplifier are mounted in a ℓHe bath dewar which has a quiescent hold time of 50 hours. An uncoated 1.05 mm crystal quartz window on the 77 K radiation shield rejects 10 μm radiation and has its resonant transmission at 340 GHz. A 0.25 mm Mylar window, resonant at 350 GHz, seals the vacuum. The local oscillator, (4) is combined with the signal by a 35 μm Mylar foil transmitting nominally 96% of the signal and 4% of the ℒO power. Apart from the immersion lens in the mixer block there is no further lens in the signal path. The ℒO source is focused onto the mixer by a plastic lens between the beam combiner and the ℒO output. The receiver noise was measured for a number of frequencies (fig 1) using the Y-factor method where an absorber at 290 K and a wet absorber at 77 K stored in ℓN2 were brought into the signal path. The best value of 214 K at 342 GHz, measured with a bath temperature of 1.5K, coincides with the resonance of the IR-filter. The pumped and unpumped i/v curve together with the calibration curves for hot and cold load are given in fig.2. Receiver noise as a function of IF frequency (fig 3) was measured in 50 MHz bandwidth and coincides with the noise curve of the preamplifier. The angular diagram (fig 4) was measured using a chopped cold load with a 2 degree aperture moved in 1 degree steps. It meets the design goal of a clean, symmetrical f/10 beam and would match the Cassegrain beam of e.g. the 30 m IRAM telescope on Pico Veleta without additional optics.
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For many years TRAM has used PbBi/in-oxide/Pb SIS mixers in the heterodyne receivers for radioastronomy at the telescope installations of Pico Veleta and Plateau de Bure. The frequency coverage of these receivers are (75-115)GHz, (130-180)GHz, and (210-270)GHz. The waveguide mixers, which use Nb/Al-oxide/Nb junctions are subjected to progressive updating. This paper concentrates on the work at (210-270)GHz and (320-370)GHz.
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RF coupling to Superconductor-Isolator-Superconductor (SIS) junctions suffers from their large geometric capacitance (45 to 60 fF/μm2 for Nb-Al2O3-Nb junctions). Junction areas smaller than 1 μm2 would be necessary for submillimeter wavelengths but pose considerable difficulties in fabrication. We show mixer performance calculations for moderately sized junctions using an integrated open circuited microstrip stub to tune out the junction capacitance.
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Present state-of-the-art Schottky and SIS mixer receiver noise temperatures (TRX) range from roughly 300K at 0.3 THz to 3000K at 1 THz, while TRTRX for InSb hot electron mixers varies from 300K to 500K in the same frequency range. It is natural to ascribe the much more gradual frequency dependence of TRX for the hot electron mixer to its bulk nature, which minimizes parasitic reactances. A hot electron mixer requires an electron gas with high mobility which can be heated by the LO power, and a change in the mobility or carrier concentration as a result of this heating. Smith et al. (1987) pointed out that the two-dimensional electron gas in the channel of FIFET devices at liquid helium temperatures constitutes a potentially excellent nonlinear medium of the required type, which would also attain a much wider IF bandwidth than the InSb mixer (GHz versus a few MHz).
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We report on the work being undertaken at Bath University in collaboration with British Aerospace on subharmonic mixers using planar GaAs diodes. This project has been underway for several years but recent results at 160 GHz using air-bridged diodes are very promising. We shall present these results together with our most recent work on the modelling of subharmonic mixers.
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The design and preliminary results of an experimental fundamental 300 GHz mixer are reported. The mixer has a modular construction consisting of three major parts (see Fig. 1). They are the waveguide connector with claw flange, the diode mount unit with IF-connector (see Fig. 2) and the backshort unit with micrometer drive. This separable building-up enables on the one hand a simple replacing of the diode mount in case of damage by another mount held on hand. And of course the procedure of contacting itself is facilitated by observing the whisker through the waveguide under a microscope. And on the other hand separated experimental studies of capabilities of the single units independent of another are made possible. So for example different types of backshorts can be tested. Furtheron one limitation of broadband mixer design, i.e. the coaxial mounting structure /2/, is experimentally investigated. Therefore a special IF-coupling is realized at the diode mount. The waveguide for 300 GHz has the dimensions 0.864 x 0.432 mm2 and is not height reduced because of a practical fabrication limit of the whisker length of 0.35 mm. The honeycomb Schottky diode is glued on a turned bush fixed on the center conductor of the glass bead of a K-connector (see Fig. 3). So a one step lowpass filter, which provides a short circuit for 300 GHz, and a good mechanical stability are achieved. Furtheron the extremely short way for the IF provides a very small dependence of the IF-impedance versus frequency. The IF-impedance at a bias current of 1 mA shows (see Fig. 4) in the bandwidth 0.04 - 10 GHz a mismatch loss of less than 0.09 dB and even for the bandwidth 0.04 - 15 GHz less than 0.7 dB related to 50 Ohm. The IF-impedance is investigated with no LO-power applied. Nevertheless the results are transferable. For low level of LO-power (less than 1 mW) there is no significant deviation of IF-impedance. And even for high LO-power the IF-reflection coefficient shows a similar variation of phase versus frequency but at a higher amount of reflection. And the small dependence of phase variation is decisive for a broad-band power and noise matching. In connection with a backshort (the same as used in /3/) in form of a slotted hollow cylinder (made of 0.05 mm thick CuBe sheet) the mixer shows as video detector in the frequency range 290-300 GHz a responsitivity of 280 ± 50 V/W at a bias current of 3 microamperes and a chopping frequency of 1 kHz. Results in mixer operation will be reported in the future.
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In millimeter wave space communication applications, where packaged devices are usually involved for enviromental and EMS protection reasons, distributed and parasitic effects become important for the design and performance evaluation of frequency translating mixers. Hence, the accuracy of the ususally adapted ideal exponential resistive diode-based mixer model is considered to be not adequate for the prediction of the spiurious-response suppression related to such millimeter wave frequency converters. In this paper the effects of parasitic and distributed diode capacitances in a single-tone spurious products evaluation for space qualified mixers, are investigated for frequencies up to 120 GHz.
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A new and efficient analysis approach for the microwave and millimeter-wave MESFET mixers is developed. The approach incorporates the harmonic balance large signal analysis, implemented using novel, robust and fast numerical algorithms, into an extended small signal analysis originally developed for diode mixers. Simulated performances of a GaAs MESFET mixer circuit using the developed analysis and the commerically available software LIBRA agree very well.
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Significant electric potentials have been observed along thin metallic films deposited on rippled or randomly rough dielectric substrates, under pulsed laser irradiation [1,2]. Experimental work investigating this effect with an eye toward developing pulsed laser detectors will be discussed.
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The energy of infared and millimeter radiation is well suited to elementary excitations in solids and provide in this way an excellent tool to investigate the corresponding physical mechanism by resonant and nonresonant spectroscopy. Due to the large number of competing transitions caused by different physical processes the experimentalist envisages quite often the problem to identify correctly the resonance structure in question. Magnons, quasi-free carrier transitions, phonons and impurity excitations superimpose within the spectrum of experimental data. This means that there is need for a filter to discriminate the transition in question from the other perturbing effects. In many cases magnetic fields provide this filter function, if the energy levels involved can be tuned by those fields.
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Magneto-optical absorption was measured in Hg1-xMnxTe (x=0.13) and Hg1-y-xCdyMnxTe (x=0.12; y=0.03) using far infrared lasers and pulsed magnetic fields up to 35 tesla. The paramagnetic resonance was found to behave a differently at high and at low magnetic fields. Results are related to the spin-flop field for the antiferromagnetically coupled spin pair.
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HgSe:Cr belongs to the class of dilute magnetic semiconductors (DMS) [1]. These materials are (in most cases) II-VI compounds, where transition metal magnetic ions substitute randomly the nonmagnetic cation of the host lattice, thus introducing new physical properties into the system. In the material we investigated (HgSe based), mercury is replaced by magnetic chromium ions, which forms a very new and yet unexplored material in the family of narrow gap DMS. The samples were grown on the University of Warsaw.
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HgSe:Fe is a semimagnetic semiconductor exhibiting exceptionally high mobility of the charge carriers due to the special properties of a Fermi-level pinned system [1]. For these cases the carrier concentration is constant n ≈ 5 ⋅ 1019cm-3 independent of the Fe-doping.
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We report the first data on the influence of resonant, selective photo excitation in the visible range on the FIR-cyclotron resonance of InGaAs/InP quantum wells. The experimental setup consists of an optically pumped FIR-laser magneto-spectrometer in connection with a 15 Tesla superconducting magnet and and the output of a monochromator guided directly onto the sample surface. In this way the excitation energy could be tuned and matched to the interband and excitonic transitions in both the quantum wells and the substrate. Both, intensity change and shift of the magnetic resonance position in the observed FIR-transmission were recorded for different, selected excitation energies for different FIR-radiation wavelenghts. The observed structures are interpreted in the context of the cyclotron resonance of electrons and holes, as well of intersubband transitions. Investigation of the time-resolved response function of the signal provides additional information on the physical mechanism involved. The presented experimental results demonstrate the efficiency of this method for the investigation of systems lacking initial carrier population.
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We report the first observation and extensive study of Spin Defect Induced Optical Excitation (SDIOE) from the singlet ground state (GS) to the lowest triplet excited state (LES) of the S=1 linear chain Heisenberg antiferromagnet (LCHA) Ni(C2H8N2)2NO2(ClO4) (NENP).
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Magneto-absorption in InSb has been measured by Dresselhaus et al[1] for the first time. Since then the magneto-optical properties of this material have been thoroughly studied at low temperatures in the past forty years[2-5], but only at low temperatures. Studies above room temperature are limited to transport measurements[6,7] which give only indirect information about the electronical properties. Using far-infrared radiation (FIR), the condition wr>1 (where r is the scattering time and w the FIR frequency) is fulfilled at 600K due to the high mobility in InSb. It is therefore interesting to study the optical properties of semiconductors under these conditions.
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Investigation of Impurity conduction band which formed as a result of interaction of two electron (H--like) impurity states presents a great interest both for physics of irregular systems and for physics of semiconductors. Detailed investigation of the process of the formation of H--band were carried out in [1,2]. However up to now there are not numerous experimental works on H--band because it is rather difficult to separate the conduction on H--band from the other types of conduction.
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Recent theoretical and experimental achievements on far-infrared (FIR) laser oscillation in p-Ge are reported. The experimentally observed cyclotron-resonance and inter-valence band laser oscillations are explained by theories. The Luttinger Hamiltonian describing the valence bands in the presence of crossed electric and magnetic fields are solved and the distribution functions of holes are derived to evaluate (negative) absorption coefficients in the FIR-range. Experimental results studying the influence of uniaxial stress on the oscillations are also presented and compared with the theory.
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In recent years it has become possible to achieve high power stimulated far infrared (FIR) radiation due to transitions between the light and heavy hole subbands of p-Ge crystals subjected to crossed electric (E) and magnetic (B) fields /1,2/. The population inversion arising between the light and heavy hole bands under such a crossed field situation has been qualitatively explained by a strong interaction of the heavy holes with optical phonons and an accumulation of light holes in momentum space /3/. According to this semiclassical picture, where the quantizing effect of the applied magnetic field is neglected, the expected stimulated emission is broadband and the obtained spectra should be independ of the values of the electric and magnetic field for a constant ratio E/B. In this work we present for the first time results of a detailed, systematic investigation of the dependence of the emission spectra of stimulated light-heavy hole emission on the absolute values of the electric and magnetic fields for a constant ratio E/B.
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An Inversion between Landau levels (LL's) of the light hole (lh) subband of germanium is achieved when the crystal is subjected to crossed electric and magnetic fields /1,2/. This leads to stimulated far infrared (FIR) emission at the cyclotron resonance (CR) frequency. The emission spectrum consists of a single line linearly tunable with magnetic field /3/. Lasing was found to depend critically on the orientation of the electric and magnetic field with respect to the crystallographic axes. Characteristically the laser emission is observed in two field regimes - which we call here low field (1) and high field (2) - depending on conditions of sample doping and external stress. In general, low concentration samples (NA − ND = 8 × 1012 cm-3) oscillate in region (1) giving frequencies between 30 cm-1 and 50 cm-1, and high concentration samples (NA − ND = 6 × 1013 cm-3) oscillate in region (2) with frequencies between 65 cm-1 and 85 cm-1. The latter samples howerver can be made to oscillate also in range (1) by applying uniaxial stress /4/.
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The hot holes population inversion (PI) and FIR amplification mechanism in germanium in crossed electric and magnetic fields at low temperature were proposed in 1979 [1]. The hot holes lasers were constructed for the first time in 1982 at LSTU [2]. The stimulated emission due to light holes (LH) - to heavy holes (HH) transitions (Fig.1) was observed later by Andronov with coworkers and Komiyama with coworkers [3]. The stimulated intersubband emission provided with nonselective resonator covers the spectral ranges λ = 80...120 and 150..210 μm.
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High frequency modulation of semiconductor laser output radiation is one of the most important problems of optoelectronics. The method of laser output radiation modulation by pumping current is widely used at present. However, it is followed by falling modulation efficiency at modulating signal frequencies exceeding several gigaHertz. A new method of effective modulation heterolasers radiation was proposed in [1,2] consisting of charge carrier heating in an active laser area by the external microwave electric field. The present paper is dealing with the discussion of new results of theoretical and experimental study in the field of heterolaser radiation microwave modulation and formation of picosecond optic pulses through the heating charge carriers in the active area of injection laser on the double heterostructure n+AlxGa1−xAs−GaAs−p+AlxGa1−xAs with classic and single quantum well.
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Temporal variation of mode structure of an optically-pumped 385μm-D2O laser is investigated numerically, based on semiclassical theory. Initially, multi-longitudinal FIR modes around the Raman transition are allowed to grow under a pulsed, single-mode pump field corresponding to the 9R(22) transition of CO2 molecule, to simulate our experimental conditions.
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One of the ways of the enhancement of optically pumped FIR laser's efficiency is the utilization of appropriately polarized pump and FIR fields. It was shown in [1], that the optimum of field polarizations depends on the the types of the transitions involved: Q, R or P type. General principles of gain calculation for different field polarizations are given there. The results are calculated through direct summing of spectral components. Note that the quantization axis z lay in the plain of field polarization vectors. In [2] the quantization axis z was selected to be perpendicular to both pump and FIR electric field vectors, parallel to their wave vectors. The problem of polarization dependence of laser efficiency was investigated both experimentally and theoretically, also by direct summing of spectral components.
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Discrete lines with frequencies corresponding to the optical transitions from excited to ground states of acceptor impurity have been discovered in the radiation spectra of FIR pulsed hot hole p-Ge laser [1]. The main reason for lasing is light to heavy hole transitions gain. The population inversion arises due to the light holes accumulation in crossed electric and magnetic fields at helium temperatures. The experimental results show the additional amplification on the transitions between excited and ground states of acceptor impurity.
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The tunability of ASE laser with transverse feedback was studied theoretically and experimentally. We found theoretically that a miniature OFFIRL with transverse feedback would give FIR laser with very broad frequency characteristics as shown in Fig.l.
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Experimental results of the first operation of a quasi-optical gyrotron on the second harmonic are presented. With a large (mirror separation d = 329mm) unoptimized Fabry-Pérot resonator (ohmic losses = diffractive losses), a nearly monomode emission (ƒRF≃197GHz) at a power level of 8kW has been measured for a beam current of 3A (Vb = 78kV). Numerical simulations show that operation at the second harmonic is highly sensitive to velocity spread.
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The design of a quasioptical gyrotron resonator with large mirror separation is presented. The large mirror separation reduces the peak ohmic heating density on the resonator mirrors to 400 W/cm2, well within the limits for cw operation. The longitudinal mode density in the resonator is also reduced to 0.2% by increasing the mirror separation, with approximately 20 modes within the interaction bandwidth of the resonator. This design will allow the rf radiation to propagate out of the vacuum system without any reflections, allowing relatively simple diagnosis and analysis of the output power. The large mirror separation also leaves room for a beam splitter to be inserted into the interior of the resonator to reflect a small amount of power out a side port for diagnosis. This will allow direct measurement of the resonator modes including any trapped harmonic modes.
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An 85 GHz quasioptical gyroklystron has been designed and fabricated. The experiment will investigate longitudinal mode selection, efficiency enhancement, and phaselocking in an open resonator. This paper presents results of cold tests of the prebunching resonator.
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At the University of Maryland we are developing high power gyroklystron amplifiers with properties suitable for driving future electron-positron colliders[1-2]. Our initial experiments have involved a sequence of six different two-cavity configurations operating near 9.87 GHz in the TE011 mode. The energy is derived from a rotating beam in 425 kV, 80-200 A, 1 μs pulses. With velocity ratios near one, the studies have culminated in peak powers approaching 24 MW with efficiencies and large-signal gains of 33% and 34 dB, respectively. In the following paragraphs we describe the final two-cavity experiment and summarize our initial investigation into advanced configurations.
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We present experimental results of a 10 GHz TE01 mode three-cavity gyroklystron. The beam is produced by a pulse line modulator and magnetron injection gun, which can operate to 433 kV and 225 A with 1 μs flat-top and at a rep rate of 3 Hz. Microwave power is measured by a mode-selective directional coupler and flowing methanol calorimeter. Mode purity is determined by a large anechoic chamber. Initial testing of the first three-cavity circuit has produced a peak power of 23 MW with efficiency of 27% and pulse energy of 36 J. We have a maximum gain of 39 dB at a peak power of 21 MW.
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Effects of magnetic field profiling and tuning on the performance of Ka-band gyrotron traveling wave amplifiers are examined experimentally and verified by theoretical calculations. Regimes of stable and unstable operations are systematically investigated and sources of spontaneous oscillations are identified. It is shown that, through magnetic field profiling and tuning, the gyro-TWT can be optimized for maximum gain, bandwidth, or efficiency. In particular, a power level of 50 kW has been achieved with magnetic field profiling. The performances of severed and unsevered gyro-TWT's are also compared. The addition of a sever is shown to result in very significant improvement in gain and stability. Critical functions of a sever in a gyro-TWT will be also discussed.
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A wideband low voltage millimeter wave gyrotron traveling wave amplifier (Gyro-TWT) is under investigation at NRL. The device incorporates precise axial tapering of both the magnetic field and the interaction circuit for broadband low voltage operation. The amplifier is a single-stage reflection-type device operating in the TE10 rectangular waveguide mode at the fundamental cyclotron frequency with a 33 kV, 1.6 A beam. By employing an electron beam with α of near 0.6, stable operation is observed at full beam current. Small signal gain in excess of 20 dB is observed over a wide (33%) instantaneous frequency range (27-38 GHz). Using a high power narrowband driver at 35 GHz, saturated efficiencies of about 10% (Pout>5 kW) have been obtained, in good agreement with theory. Attempts to increase the efficiency have been hampered by the onset of in-band oscillations. An experimental study investigating the dependence of the start oscillation current threshold on the relevant experimental parameters (α, magnetic field, reflection coefficient) has been performed.
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For many applications it would be desirable to operate a gyroklystron amplifier at the second harmonic, as this would reduce the applied magnetic field. In previous work1 harmonic amplification has been examined for cold, weakly relativistic beams; i.e. nk⊥rL ≪ 1 where n is the harmonic number, k⊥ the perpendicular wavenumber, and ]rL the Larmor radius. We will extend this analysis to include moderately relativistic and relativistic beams with a spread in velocity. The distinction between moderately relativistic and relativistic is that in the former we assume that the beam is weakly relativistic but nk⊥rL, is not necessarily small, and in the latter we use the full set of equations, including guiding center drift and changes in axial momentum. A comparison is made among the three models.
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An extremely high power, second harmonic TE21 gyro-TWT utilizing a MIG is described along with a moderately high power, fourth harmonic gyromagnetron-TWT employing a cusp gun. Self-consistent simulation results of a 500 kW, 20% efficient, 7% bandwidth, 100 kV, 25 A, second harmonic, two-stage gyro-TWT and a 40 kW, 15% efficient, 6% bandwidth, 50 kV, 5 A, π mode, fourth harmonic gyro-TWT will be presented.
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For gyro-devices operating at high cyclotron harmonics one of the main problems is to prevent the self-excitation of parasitic modes resonant with the fundamental cyclotron frequency. In previous papers the important class of possible instabilities in such tubes has not been considered yet, namely, the parametric instabilities.
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This paper reports on the design, fabrication, and testing of a series of GaAs Schottky barrier photodiodes with extrapolated RC time constants in excess of 100 GHz.
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Using frequency multiplier as local oscillator is not only more economical but also can keep the advantages possessed by microwave equipment, such as the frequency stabiliity, in the millimeter-wave system. A novel approach that provides third hamonic output with low conversion loss and broad banwidth is described in this paper.
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The design and testing of a broadband harmonic mixer at 300 GHz, pumped via a flexible dielectric fiber, are reported. Harmonic mixing proves to be an useful technique (/1/,/2/) in the high frequency millimeter wave region, where the sensitivity of diode detectors is degraded and a second fundamental oscillator for heterodyne mixing is rarely available. Furtheron the handling of an harmonic mixer is more flexible, because multiplying and diplexing are combined in one component and because the LO feed takes place at a lower frequency, at which flexible supply is available.
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As major applications for millimeter and submillimeter electromagnetic waves have developed there has arisen a need for instruments which will allow measurements to be made of the power carried by free-space propagating beams and of the energy carried in short pulses. Such instruments should be calibrated absolutely and re-calibration should be a straightforward matter. This paper will report on the power and energy meters we have developed for the 60 - 900 GHz range.
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A pyroelectric detector has been built and characterised for operation within W-band. The various characteristics reported include detectivity, responsivity, NEP, polarisation sensitivity, dynamic range, linearity, and reflectivity (vswr). The pyroelectric detector performs well at these wavelengths and as the detector is inexpensive to build, and is robust, it ought to find many an opportunity for use within millimetre-wave laboratories.
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Simulations of frequency multipliers require good knowledge of the nonlinearities of the devices. In this paper we report the first experimental measurements of the nonlinearities of a single barrier varactor (SBV) diode over extended voltage range. A similar SBV structure is also analyzed theoretically. Theoretical characteristics are in reasonable agreement with the measured curves.
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Small area, low capacitance Schottky barrier diodes are currently available from only two suppliers in the world, and very considerable technological effort is required to obtain stable, low-noise operation when junction capacitances of 1 fF or less are required. Such diodes have been reported in the literature [1], and yield low noise and low conversion loss [2]. As a result of the increasing need for such diodes at frequencies above 500 GHz, and the possible need for tailoring specifications to particular space or radioastronomical requirements, a consortium of four German technical universities and a group of potential users combined to initiate fabrication and measurement of such diodes. This paper reports the first results.
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Apart from the great success of SIS-junctions as mixers in heterodyne receivers there are several other reasons to intensify investigations of SIS-junctions as direct detectors. These devices can be produced as single junctions and arrays for use. in waveguide systems or in planar antennas by standard lithographic techniques. Impedance matching structures and filter elements can be incorporated easily. Recent developments1,2 in junction technology show strong improvements of junction performance. Areas of 0.5μm2 and leakage currents as low as 50nA have been realized. Since the final sensitivity (NEP) of SIS-junctions as direct detectors is limited ultimately by the shot noise of the bias current (NEP∼√IL), a low leakage current IL, is highly important for these devices. Using these improved SIS-junctions, NEP's down to 10-17W/√Hz seem to be possible with He4-cooled devices. A crude estimate for the time constant of a SIS-direct detector yields the inverse of the gap frequency h/∆ or the sumgap frequency h/2∆, which gives very small values of 1 − 10psec.
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The microwave radiometer has a series of advantages it has been applied widely in many fields [1] [2], such as romete sensing and guidance. In addition, the structure of the total power radiometer is the most simple and its sensitivity is the highest, so it is the optimum choice for airborne imaging system. Because of the influence of the gain fluctuation and other interference, the total power radiometer works unsteadily and the practical sensitivity become lower. In order to overcome these defects, periodic calibration technique is developed.
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M.T.Faber, W.K.Gwarek [1] and S.A.Maas [2] introduced to use standard rectangular metal waveguide fabricating microwave mixer. We attempt progressively using nonradiative dielectric waveguide ( NRD-guide ) to replace metal wave-guide. Thus a kind of Ka-band NRD-guide integrated receiver front-end is presented in this paper. This integrated receiver front-end has many advantages, for example, structure simple, noise figure tow, and so on. It will find many potential applications in radar, communication and navigation systems.
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The transport of charge carriers in semiconductors is fundamental to electronics — usually however one measures average properties only, e.g. the mobility due to collisions averaged over all carrier energies. Several works have shown that radiation in the far infrared can selectively interact with certain subensembles of the carrier distribution, and thus make possible to perform state-selective measurements of e.g. the scattering time (ns to fs time scale). By using step-tunable pulsed gas lasers, with power up to 1 MW, a nonthermal population can be induced resulting in saturated transmission, saturated photo-conductance and nonlinearities in other effects such as in the photon drag effect. A great advantage over common optical pumping in the near infrared or visible spectrum is that the far-infrared radiation does not produce any extra electron-hole pairs.
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We have recently [1] described modifications to an NPL far infrared cube interferometer in which the radiation is brought to a focus in each arm to enable amplitude and phase reflection measurements on small (area > 3x3mm2) solid samples to be made by dispersive Fourier transform spectroscopy. We now describe the use of this instrument to study the complex far infrared reflectivity of a series of semiconductor structures consisting of three epitaxial layers (CdTe/CdxHg1-xTe/CdTe) deposited on GaAs substrates. The epilayers were deposited by plasma enhanced MOCVD [2] at temperatures as low as 180°C, with thicknesses in the range 1-2 μm and Cd concentrations in the range x=0.24-0.61.
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We report a FIR photoelectrical study of shallow acceptors in seraimagnetic p-Hg1-x-yCdxMnyTe (0,02≤X≤0,22 .0,04≤Y≤ 0,12) alloys. Acceptor concentration in the samples deduced from Hall coefficient, was in the lange (2-3,5) x 1014cm-3.
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A selection of far infrared techniques has been used to investigate guided waves and surface and interface polaritons propagating in a series of CdTe/CdxHg1-xTe/CdTe heterostructures deposited on GaAs substrates by plasma enhanced MOCVD. The thicknesses of the epilayers were in the range 2-4 μm and the mixed crystal (CMT) layers had Cd concentrations in the range x = 0.24-0. 6 1 .
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A novel metal-SiO2-InP MISFET (metal-insulator semiconductor field effect transistor) structure, incorporating a modulation doped channel and the self-aligned gate feature of Si MOSFETs, is proposed. Analytical results on current-voltage and transconductance characteristics are presented. Significant enhancement of performance over conventional MISFETs, employing SiO2 as an insulator, is reported.
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CsFeCl3 is an interesting singlet ground state magnet which has no long range magnetic ordering at zero magnetic field.1) Recent calculation by Suzuki based on the dynamical correlated-effective-field approximation shows that most of magnetic properties of singlet ground state system can be described by effective spin Hamiltonian (S=1), which cosiders only the ground state and the excited doublet.2) In this paper we will study these low lying energy levels in CsFeCl3 by submillimeter wave ESR.
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Tetramerisation of the TCNQ-stacks in TEA(TCNQ)2 at low temperatures, and the formation of electron pairs that follows from it, results in an energy diagram consisting of a singlet (S=0) and a triplet (S=1). Susceptibility measurements show this effect as a deviation from the normal Curie-law behaviour caused by TCNQ-impurities.
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We have studied the influence of Far Infra-Red (FIR) radiation on the current of Double Barrier Resonant Tunneling Devices (DBRTD), consisting of consecutive layers of GaAs/AlGaAs. The current-voltage- (I(V)-) characteristic of these devices is strongly nonlinear and shows for device 1 (5nm well) two regions of negative differential resistance (NDR) and more than twenty for device 2 and 3 (60nm, 120nm well), since for these latter devices the energy levels are closer spaced due to the wider quantum wells [1].
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Spectral analysis of pZnSb-nCdTe heterostructures (HS) obtained by a laser-pulse epitaxy method has shown that such HS exhibit photo-sensitivity beyond the absorption edge at T=78 K for a narrowband semiconductor. Up to a wavelength λ=7μm [1]. This photo-sensitivity value is only 6 times smaller than its maximum at a fundamental absorption wavelength λ=4.3μm. Due to HS thickness, this effect cannot be explained by the usual tunneling
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Tunable coherent far-infrared (FIR) spectroscopy is a new domain. The first experiments appeared some ten years ago.
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Recently we have published a listing of 3377 assigned IR absorption lines of CH3OH between 960 and 1100 cm-1, i.e. in the region overlapping with the CO2 laser emissions1, and a work where we have reviewed all the available literature on CH3OH far infrared (FIR) laser emissions2. In this latter work we have considered 495 laser lines emitted by the normal isotopic species of CH3OH, disregarding, for the moment, several additional laser lines excited by tunable CO2 TEA lasers. 205 FIR laser lines of CH3OH have been assigned up to now.
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In the framework of our systematic investigation project of the infrared and far infrared spectrum of CH3OH by high resolution Fourier transform spectroscopy, we have currently assigned more than 35000 absorption lines. Two listings of 12700 assigned lines below 200 cm-1 have already been published1,2, and a listing of almost 8700 lines between 200 and 350 cm-1 is in press.
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The Radiofrequency excitation of gas lasers is very attractive because it allows to operate at larger pressures with moderate transverse voltages. In this way it is possible to deposit a high energy density in short waveguides with relatively large section, to obtain an easy and large mode tuning. Efficient high pressures and high energy density discharges are possible only in a pulsed regime, because of the insufficient gas cooling. The pulsed emissions are affected by frequency chirp which acts as a mean linewidth enlargement. In spite of this effect, a mean 20-40 MHz linewidth can be always obtained. We have built two RF lasers with 506 and 920 MHz mode tuning respectively. Pulsed emission with 300-1000 PPS, 150-600 W peak power, and 0.3-3 W mean power can be obtained. By using the standard He reach laser mixtures at 1-1.5 bar we obtain the best performance, but normally we use an enexpensive He free CO2∶N2=1∶2 at 0.25-0.4 bar obtaining 65 full mode tuned lines in the longer device and 40 the the shorter one. By using both the laser we have performed a high resolution (10MHz) spectroscopy of gases which are interesting in the FIR emission, looking to a further use of our lasers in FIR laser pumping experiments
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We have been developing the TEA CO2 laser/D2O laser system and the detection system for the ion Thomson scattering experiment on the Heliotron E device in Kyoto University. Up to now a considerable effort has been concentrated on the improvement of the laser output characteristics such as (i) output energy, (ii) pulse duration and (iii) spectral purity. Recently, we have succeeded in obtaining high power, single mode TEA CO2 laser beams with good reproducibility, by developing a relatively compact TEA CO2 laser system. It consists of a main laser oscillator (Lumonics 601) and a triple-pass amplifier (Lumonics 620). The main laser (601) is injection-locked by the output pulse from a hybrid laser consisting of a cw CO2 laser section and a TEA CO2 laser section (Lumonics K921S). The cavity length of the main laser (601) is feedback controlled by making use of the cw CO2 laser signal, so that the single mode output is obtained with high probability. The output pulse duration of the main laser is also maximized by properly choosing the injection time after the initiation of the main discharge. The typical pulsewidth of the main laser in the case of the pulsed injection locking, is about 0.6 μs and the output energy is about 4 J at the line of 9R22. Then the laser beam is amplified by a triple pass amplifier up to 100 J . The burn patterns of the amplifier output beams show roughly uniform intensity distribution at the input window of the D2O laser. The 4 m long, unstable D2O laser consists of a set of concave and convex mirrors and a wire grid. We measured the output power of the D2O laser with a Scientech power meter and the maximum power of 200 mJ was obtained at a pressure of about 5 Torr. The measurements of the D2O laser emission linewidth were already carried out by the MIT 1) and Lausanne groups 2). However, the detailed spectral structures of the D2O laser were not obtained yet, because the frequency resolutions were limited to the bandwidth of filter used in their detection systems, say, 80 MHz, which are broader than the longitudinal mode spacings.
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Methyl alcohol and its isotopes are the most common and efficient medium for far infrared laser generation. The usual pump source is the regular 12CO2 laser. By using other CO2 isotope lasers it is possible to excite other absorption transitions which can produce new FIR laser lines and also help in understanding fine details of the spectroscopy of methanol molecule. In this work we have pumped 13CD3OH with 13CO2 laser lines for the first time and have also reinvestigated the 12CD3OH molecule. Six new FIR laser emissions were observed. Based on previously known molecular parameters, and Fourier-Transform data we propose tentative assignments for three of the observed FIR lines in 12CD3OH.
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Recently some results of a systematic investigation about the vibrational C-O stretching band of 13CD3OH have been presented [1,2]. In this work, we have studied the part of the I.F.T. spectrum that refers to the C-O stretching Q branch of the molecule, which presents absorption transitions in good coincidence with CO2 10R(22), 10R(24), 10R(26) and 10R(28) pump lines.
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The high-resolution Fourier transform infrared (FTIR) spectrum of CH318OH has been investigated in order to probe the vibrational band structure with an ultimate view to identifying transitions for reported optically-pumped far-infrared (FIR) laser lines. Spectra were recorded from 1050 to 1620 cm-1 and 1925 to 2050 cm-1 at 0.004 cm-1 resolution on a DA3.002 Bomem Fourier transform spectrometer. Several vibrational bands occur in these regions. For the in-plane CH3-rocking mode, series in a combination band from the ground state to torsionally excited levels of the CH3-rock have been observed and assigned. Extensive transition series have also been identified for the asymmetric CH3-deformation band, the OH-bending band, and the C-O stretch vco=2−0 overtone band. Analysis is in progress to determine molecular constants in the excited states.
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The various isotopic species of methyl alcohol have proved in recent years to be fertile sources of optically-pumped far-infrared (FIR) laser lines. The relatively exotic 13CD3OH form, in particular, turned out to produce the second most efficient FIR line known,1 creating considerable spectroscopic interest. We have examined spectra of this molecule previously ,2-4 and determined some of the molecular constants for the ground, C-O stretch and CD3-rock vibrational states, as well as possible assignments for a few IR-pump/FIR-laser transition systems. Spectroscopic and FIR laser studies have also been carried out by Pereira and co-workers,5,6 and many additional FIR laser lines have been found. Because several of the known FIR lines are pumped by CO2 laser lines of the 9R branch lying above the C-O stretch band, we have extended our spectroscopic investigations into the higher IR regions to identify new vibrational bands which might account for the FIR laser emission. Further motivation came from the apparent large increase in torsional barrier height found for the CD3-rock state, which prompted our curiosity about the barrier heights for other vibrational modes. In this paper, we discuss our progress to date in the study of new IR spectral regions of 13CD3OH, and the determination of molecular constants and FIR laser assignments.
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A major goal of the KfK gyrotron development program is the design and construction of a high power, long pulse 140 GHz gyrotron for electron cyclotron wave applications in fusion research. At the W7AS stellarator in Garching a TE0,3 gyrotron with a pulse length of at least 100 ms, mode purity better than 97% and an output power of 100 kW was constructed, tested [1], and successfully used for plasma heating of neutral beam sustained, high density plasmas [2]. A pulse length of 500 ms with rectangular modulation at 200 Hz, duty factor of 50% and peak power 100 kW was attained (see Fig. 1).
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Future plasma heating experiments using electron cyclotron waves call for gyrotrons capable of delivering output powers at the 0.5 - 1.0 MW level at frequencies around 140 GHz. To reduce the ohmic losses (in the cavity wall) at such high output powers to manageable values and to reduce the mode competition problem, a highly overmoded resonator is required to operate the tube in an asymmetric mode. As a planned step towards a 1 MW tube, following the TE10,4 experiments [1], a gyrotron working in a TE22,n mode (with n=4,5,6) with an internal quasi-optical mode converter will be considered (see Fig. 1). Results of Russian experiments indicate that these modes are promising candidates for 1 MW cw operation [2, 3].
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On the basis of the kinetic theory general relations for the starting current and frequency shift in a gyrotron are derived in the case of eccentricity of the electron beam in a cavity. The influence of a disalignment of the beam and cavity on the gyrotron operation is discussed and illustrated.
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A mode converter for whispering gallery mode gyrotrons has been designed and experimentally demonstrated to have 95% calorimetric efficiency. Experiments were performed on a megawatt power level, 3μs pulsed gyrotron operating in the TE16,2,1 whispering gallery mode at 146 GHz. The gyrotron cavity employs a nonlinear uptaper to minimize radial mode conversion. About 97% of the output power is in the TE16,2 mode. A doubly curved reflector, designed using ray optics and vector diffraction theory, was recently built to focus the full radiation pattern. Experiments show that the converter focuses at least 92% of the incident radiation into a Gaussian-like TE16,2 focal spot, about 1.5 cm in width. Small fractions of other spatial modes were found to form focal spots in the far field. Analysis of the power in the other focal spots allows for a good quantitative measurement of the gyrotron output mode content, potentially on a single shot basis.
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The linewidth of a single 140 GHz gyrotron pulse has been measured using a time domain sampling technique. A spectral resolution of 60 kHz could be achieved, whereas the pulse length of the gyrotron did not exceed 600 μs. The paper describes the measurement set-up and preliminary results are presented.
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A double conversion receiver with center frequency of 140 GHz and 18 GHz bandwidth is presented. A spectral resolution of up to 100 MHz is achieved by use of a contiguous filterbank. Thus, real time spectral analysis of 100 MHz resolution is possible. It is planned to use the device to monitor mode competition of the 500 kW 140 GHz TE10,4 gyrotron, which is currently under test at KfK (Kernforschungszentrum Karlsruhe).
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A high power TE31 gyro-BWO utilizing a MIG is described which will yield continuous tuning over a full band. The interaction tube has been severed to destroy all modes without a threefold azimuthal symmetry.
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This paper summarizes the recent progress of engineering efforts in the area of active circuit techniques for quasi-optical applications. Typical grid type structures are described. In addition, a number of schemes for locking the output for a coherent power combining are reviewed. Some methods for characterizing these components are reviewed.
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In recent years, much work has been done on quasi-optical components that interact directly with waves propagating in free-space without need for a separate receiving antenna, [1-5]. By embedding different devices in a periodic planar array, processing such as filtering, amplification [5], phase shifting [2], and mixing [4] may be performed on an incident wave. The advantages of such an approach are many. At millimeter-wave frequencies, the dimensions of waveguides become quite small and the losses associated with the skin effect become severe. Furthermore, solid-state components have relatively low power-handling capabilities. Planar grids of solid-state devices overcome these deficiencies by distributing the energy over an array of devices. In addition, the losses associated with metal waveguide walls are eliminated. Tuning can be accomplished using mirrors and dielectric slabs. The structure of the grids is compatible with modern IC fabrication techniques which lends the approach conveniently to scaling down to shorter wavelengths. A complete system using quasi-optical grids can be realized by simply cascading the grids one-after-another to form a receiver or a transmitter. The basic components that make up a heterodyne receiver are a local oscillator, amplifier, and mixer. Typically, a signal is received first by an antenna and fed to a waveguide or transmission-line circuit. The signal is then amplified and down converted to an IF frequency. In this paper we present designs and results for a grid oscillator, mixer, and amplifier that could be used to build a quasi-optical millimeter-wave heterodyne receiver.
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Microwave quasi-optical integrated circuits has a potential for low cost manufacturing, simple design, small size and high efficiency. Several works for the quasi-optical components were reported[1]. Due to its planar configuration, monolithic technology can also be applied. In order to meet these requirements, in particular simple design and small size, we proposed a layered structure with a slot formed in the ground plane of the microstrip circuit[2][3]. The slot is electromagnetically coupled through a microstrip to slotline transition to realize the layered structure. Thus, an antenna and a circuit are fabricated on both sides of a dielectric substrate.
This paper describes the design and the experimental results of the 24 GHz negative resistance oscillator circuit for layered quasi-optical transmitter components. The technology for the 10 GHz negative resistance oscillator was scaled for use at a higher frequency with a HEMT (HJFET) as the oscillator source and a 1-λ, slot antenna as the radiator. The design concept for layered quasi-optical transmitter compoments was confirmed by fabrication and measurement at 24 GHz.
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A varactor-tuned Gunn oscillator was constructed in an image-line circuit ingrated with a grating antenna. The combined circuit exhibited a 2 percent electronic tuning range corresponding to a frequency steerable range of 4 degrees. The circuit can be produced at low cost in the millimeter-wave frequency band.
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A Gunn device has been integrated with a coplanar waveguide resonator and a planar, endfire notch antenna. The circuit provides a power output of more than 15 mW at 37 GHz. The integrated active antenna offers advantages of small size, light weight, low cost, good reproducibility and excellent performance.
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Recently, quasi-optical power combining arrays using FETs have been demonstrated [1,2], where the active elements are distributed on a plane between partially reflecting dielectric slabs. Spatial injection locking is achieved by illuminating the array with a plane wave; locking bandwidths are typically low.
In this paper, we demonstrate a new type of power combining array using two-port FET oscillators as the active elements [3]. The oscillators are arranged in a square planar array, with the output of each connected to a patch radiator. The key difference between this approach and the work cited above is that the oscillators are injection locked to a single external source by means of a Wilkinson-type 16 way power divider, rather than by parasitic coupling between the oscillating elements.
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The paper presents a complex designed for recording and treatment of characteristics of electromagnetic fields of the millimeter wavelength range radiated and scattered by objects of a different type. Among them are antenna systems, diffractional objects both of a simple and complex form, radiotransparent materials.
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In the successful approach to millimeter-wave and particularly submillimeter wave circuity, it has become apparent the techniques commonly used in the microwave region are inapplicable. It has recently been demonstrated that laser-excited highly conductive bulk semiconductor (silicon or gallium arsenide) can be used for high-speed optoelectronic controlled devices getting signals up to the submillimeter-wave region [1]. The advantages of optical control include short response time, high modulation rates, inherent high dc and reverse-signal isolation, compatibility with optical fibers, immunity to electromagnetic interference and low cost. With the rapid development of solid-state millimeter devices and electrooptics, it became apparent that this devices and systems can be controlled by optical illumination.
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Cyclotron emission sources of the far infrared radiation have unique features to be continuously (magnetic field) tuned and narrow band ones [1],[2]. We show practical realization and examples of application of the transmission spectrometer based on the bulk GaAs and InSb cyclotron resonance sources.
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Using the principles of quasi-optics, a high resolution ESR spectrometer operating at 250GHz has been constructed. It has been used to study the temperature dependent linewidths of PDT, a nitroxide spin probe, over a range of temperatures for which motional narrowing theory is valid. The narrowest lines that we have studied to date are 89 microtesla (890 mG) peak to peak in a Larmor field of 8.9 tesla and demonstrate the resolution of the spectrometer. We have studied 0.5mM PDT in toluene-d8, an isotropic solvent, and 2.0mM PDT in phase V, a thermotropic liquid crystal, in order to compare our results with careful work at X-band (9.5 GHz).
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We present a setup for far infrared multireflection measurements on highly reflecting materials in magnetic fields up to 30 Tesla, following a suggestion of Prof. Genzel [1]. The method concerns the determination of the reflectance (R8) of a metallic surface which is integrated in an optically stable Fabry-Perot resonator. In resonance, the reflectivity of the resonator shows a sharp minimum (R10) and the absorption in the sample is amplified by the multiple reflections.
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Differential magnetooptical Fabry - Perot - Interference structures are an excellent tool for the determination of buried refractive index profiles which exist within microstructures used by modern semiconductor technology. The coherent superposition of the phases and amplitudes of circularly polarized IR - waves respond sensitively but differently to the variation of physical - and profile - parameters. The method is working non - destructive and contactless and its accuracy is comparable with other destructive methods used to determine profile functions such as SIMS, RBS, spreading resistance ect.. Especially the accurate analysis of doping profiles which are more than 10 μm below the surface is possible.
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A Mach-Zender interferometer has been constructed at the National Synchrotron Light Source at Brookhaven National Laboratory. The interferometer consists of two fixed sets of mirrors positioned at both ends of the interferometer. In the middle, a movable part contains two sets of mirrors facing on each side the fixed mirror sets. The interferometer is of the differential type and the beams are folded on each side of the center part for compactness of the instrument. The beamsplitters may be of mylar or metal mesh.
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Nonlinear dynamical properties of carrier transport in semiconductors like self-sustained oscillations and chaotic fluctuation are of strong current interest. In order to understand dynamical phenomena in high-purity semiconductors on a microscopic basis, detailed knowledge of the recombination kinetics of charge carriers is needed. A powerful method to study the kinetics of carriers bound to shallow impurities represents saturation spectroscopy using high-power FIR. lasers [1] [2]. Saturation of photoionisation yields the product στ where σ is the photoionisation cross section and τ the recombination time. To evaluate τ the cross section σ must be known. As many high-purity materials are only available in form of thin epitaxial layers, σ cannot be measured with sufficient accuracy by simple transmission spectroscopy because the absorption is usually to small. A much more sensitive method is given by the photoacoustic effect.
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Contacting II-VI-semiconductors has always been a problem and this problem still isn't solved up to now. Therefore we investigate CdxHg1-xTe epitaxial layers (0 ≤ x ≤ 0.22) by means of a contactless microwave bridge technic in a frequency range between 8 GHz and 12 GHz. These layers are grown on Cd0.96Zn0.04Te substrates by molecular beam epitaxy (MBE). The sample with a layer thickness between 1 μm and 5μm is inserted into a closed rectangular waveguide in such a way that we face the problem of microwave transmission in a partially filled waveguide (see Fig. 1).
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Ellipsometry is a well known technique of measurement of the optical constants of different substances. To determine the optical constants of small samples in FIR someone should focus irradiation on the sample surface by means of lens or mirror to avoid diffraction effects and to use all energy of the beam. So the problem of converged beam ellipsometry arises. This decision is valid in approximation of parabolic equation or Fresnel integral.
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Investigation of behaviour of a nonlinear Fabry-Perot interferometer with a paramagnetic filling in the conditions of inhomogeneous broadening of the electron paramagnetic resonance line is of interest for different implementations in the spectroscopy, quasioptics and dynamic nuclear polarisation.
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In an optically pumped far-infrared (FIR) laser the molecular gas is excited from its ground vibrational state to a higher vibrational state by an external laser source (usually a CO2 laser). Population inversion is obtained between two rotational levels within the excited vibrational state. The laser cycle is closed by the relaxation of molecules from the excited vibrational state to the ground state .
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For most applications of Far Infrared (FIR) lasers, sufficient output power on as many lines as possible is as important as good beam quality. In order to provide optimum coupling to the EH11 intracavity mode and thus to achieve optimum power and a Gaussian beam profile, hybrid metal mesh couplers are prefered over most other coupling techniques, since no aperture diffraction occurs. Application of our mesh-couplers to FIR lasers yields output powers of tens of milliwatts on many lines, even when pumped with moderate powers in the 10-60 Watt range. Gaussian beam profiles with low divergence angles were achieved in all cases.
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Utilization of metallic grids stacks for F.I.R filters needs a good knowledge of optical properties of each grid. We study here an experimental method in order to determine the wave's change of phase by reflection on a wire grid. We use a Perot-Fabry interferometer constituted by these grids.
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Experiments of damage to optical components induced by intense infrared radiation are reported.
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In every kind of laser it is desirable to have an output element that couples uniformly over the whole cross section. In addition one often wishes to have a variable coupling degree to optimize transmission for highest output power during operation.
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The combination of a wide frequency tuning range with the high output power in a waveguide CO2 laser is necessary for effective pumping of FIR lasers. This problem deals with the realization of severe selection of both a vibration-rotational transition and a single resonator mode in waveguide lasers with a diffraction grating. The main intention of our work was the detailed theoretical and experimental investigation of the frequency-selective properties of laser resonator using a square-bore waveguide, a diffraction grating and a distant plane mirror (near Case 1 of waveguide resonator). Its results are the further development of the investigation of the near Case 1 of a waveguide resonator selective properties [1-3].
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Strong resonant self-action of radiation, saturation of Stokes signal amplification and pump field absorption, and dynamic Stark effect result in a complex dependence of a refractive index and amplification/absorption coefficients on intensities and frequency offsets of the pumping and generated radiation in the active medium. Numerical simulation of propagation of diffracting FIR and pumping waves enables to investigate the influence of nonlinear beam interaction on the effectiveness of generation and to optimize energetic and spatial characteristics of the output FIR radiation.
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An exessive study of the effect of medium on the frequency of IR-Absorption bands is presented. Due to the special importance of the (X-H) bonds, this study was restricted to the (N-H) and the (0-H) bonds of disphenylamine and (∝-raphthol) respectively.
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Theoretical calculation on the output power density of optically pumped far-infrared laser was usually based on three-level system model for simplicity. Considering on the far-infrared self-absorption effect of the operating gas medium, the ready-made calculation should be renewed. The calculating method of self-absorption was proposed.
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Shapiro steps by NbN thin-film Josephson junctions (JJ) have been recently observed at above 1THz1. Frequency mixing around 1THz may be hopefully achieved with these types of JJ's.
However, it is necessary to obtain FIR coherent sources with appropriate performance for these experiments. We should also count large power loss in quasi-optical components usually used in the submillimeter region: this demands a rather high power source (20∼30mW, > 500 ∼ 600GHz). Candidate sources for this purpose are optically pumped and discharge pumped FIR gas lasers. We have developed a high-power optically pumped FIR laser modifying an Apollo 125 model and examined its characteristics. The laser will be used as a calibration source for FIR spectroscopy and for testing submillimeter detectors and mixers, too.
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Peniotron development activities at Research Institute of Electrical Communication, Tohoku University in Sendai are directed along three directions, auto-resonant peniotron oscillator (ARPO) with a high efficiency, the backward wave peniotron oscillator (BWPO) with a wide band frequency tunability and higher cyclotron harmonic peniotron oscillator for a high frequency generation. This paper will review the recent work performed in these three fields.
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There is a current need for high power, high efficiency and high frequency amplifier operating at low magnetic field and low beam voltage. Amplifier based on Harmonic peniotron interaction in slotted waveguide structure appears to satisfy these requirements. However, mode competition becomes a problem at high frequency. In particular we study the competition between higher harmonic gyrotron and peniotron modes. The theory and code developed earlier based on slow-time scale formulation are extended to include multiple harmonic interaction. Numerical results for a planned 35 GHz peniotron experiment will be presented.
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The thin beam model of an axis encircling relativistic electron beam in a uniform, constant, external magnetic field in a cylindrical wave guide is used to develop a unified theory of the various interactions of such a beam with the electromagnetic field. The model includes the gyrotron interaction (longitudinal displacement of the beam) and the peniotron interaction (transverse displacement of the beam). Space charge effects are included. The resulting self-fields influence the boundary conditions of the perturbed electromagnetic field across the electron beam. For a beam with only azimuthal equilibrium flow and for propagation perpendicular to the equilibrium external magnetic field the well known gyrotron instabilities are recovered. In addition, it is shown that the transverse beam modes (slow peniotron) are unstable for resonant interaction with the TE wave guide mode. A general dispersion relation for propagation at any angle is derived for a beam with arbitrary energy and arbitrary pitch angle. This dispersion relation includes gyrotron, CARM, peniotron and HARP interactions and the coupling between TE and TM wave guide modes. For weak space charge, analytic expressions for the growth rates of the Doppler shifted cyclotron resonance interaction of TE and TM modes with the gyrotron and slow peniotron modes are obtained.
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A particle simulation approach is used to determine optimum design parameters.
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Electrons from a cyclotron autoresonant accelerator can be injected into an axisymmetric static B field on helical orbits having spatial harmonics which allow wavenumber matching to guided fastwaves at the temporal harmonics of the accelerator frequency. As a result, power transfer from beam to wave can occur as a first-order process with quadratic spatial growth. Examples at 94, 140 and 280 GHz are given which require magnetic fields no stronger than 10 kG.
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The autoresonant peniotrons with βph=1 (1Tmo-waves) working at high harmonics in the homogenous magnetostatic field are perspective sources of MM-waves of the middle and the large power.
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Regimes of microwave electromagnetic oscillations amplification and generation occurring when a cylindrical hollow helical electron beam interacts with a transverse - nonhomogeneus electromagnetic TE wave in a waveguide Ca peniotron-like CRM) are considered theoretically. The analysis has been made by an averaging method. The transverse distribution of the TE wave in a waveguide is approximated us multipole structures series. The helical electron beam effectively interacts with one of the multipole structures when the cyclotron resonance condition w±hvz−pwc≈0 is realized (w and wc - working and cyclotron frequencies correspondly, h - a wave number, vz - a longitudinal speed, p=1,2,3,... - a resonance number).
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The actual task of the electronics is the development of the electron-vacuum sources of MM and SUBMM waves, the operating principle of which is based on a new and more effective mechanisms of interaction of the beam with the RF-field. The peniotron mechanism, based on the nonsynchronism of the rotation of magnetoguided beam electrons and transversally nonuniform electrmnagnetic field, which has been predicted for the first time in USSR /1/, is the perspective one. Autoresonant reqime of the peniotron gyrodevices, which combines in itself the merits of MCAR and peniotron namely the high efficiency of electron-wave interaction (theoretically right up to 100%) at the high harmonics of cyclotron frequency, the large Doppler frequency gain, the possibility of the prolonged braking of charged particles in the homogeneous magnetostatic field because of the autoresonance effect, was discovered and has been examined in recent years at the laboratories of USA, USSR, Japan /2-4/.
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Optical techniques such as electro-optic sampling and photoconductive sampling are currently used to characterize microwave and millimeterwave devices. These techniques are extremely important to the microwave scientific community because they offer several advantages over conventional network analyzer measurements. Namely, they have broader band capability (over 100 GHz) and permit on-wafer, non-invasive measurements. Jitter-free time synchronization between the microwave signal and the laser pulse train is essential to these sampling techniques. Several approaches to realize synchronization have been reported. We developed one possible approach whereby a narrow electrical pulse (with broadband content) is generated on-wafer, using a fast photoconductive switch [1]. Measurements performed in the time domain are then converted to S-parameter measurements by Fourier transformation.
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The set-up and capabilities of a VNA Wiltron 360 extension to the frequency range 70.5 - 82.5 GHz are reported. The swept signal (1.5 - 13.5 GHz) emitted by the test-set is up-converted (see Fig. 1) in a broadband mixer by a fixed local oscillator (BWO at 69 GHz). A 70 GHz cut-off filter (suppression of the carrier better than 80 dB causes an amplitude error less than 0.001 dB and a phase error less than 0.006 degrees /1/) behind the mixer allows only the upper sideband to pass, whose amplitude and phase information is known because the baseband serves as a reference (automatically in the test-set). After passing the device-under-test in transmission the signal in the frequency range 70.5 - 82.5 GHz is down converted to the baseband in a second mixer, which is pumped by the same local oscillator via a flexible phase stable (see Fig. 2) dielectric fiber with corresponding transitions to rectangular waveguide (see Fig. 3). So geometrical lengths of devices up to 25 cm can be compensated and different geometrical positions of the ports, too. This is a significant improvement related to rigid rectangular waveguide for compensation /2/ and also related to a proposed quasi- optical line stretcher /3/. After amplifying this signal is fed in the test-set of the VNA. Because the up and down conversion is done by the same source, no problems (e.g. source locking, measurement of high Q-factors) occur in spite of a FM deviation of nearly 1 MHz of the local oscillator. A noise floor of -80 dB relative to a thru calibration (at minimum bandwidth and 512 points averaging) is achieved, although not optimized single ended harmonic mixers, which require directional couplers to separate and combine the signals, respectively, are used in the fundamental up and down mixing operation. A second problem caused by using single ended mixers in reflection is the cross-talk via the dielectric fiber. So three isolators (isolation better than 75 dB) are provided to suppress the cross-talk. Furthermore the amplitude and phase reproducibility is better than 0.1 dB and two degrees, respectively (see Fig. 4).
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The current systems for making phase and amplitude measurements proposed by the large manufactures in the millimeter range of frequencies are restrictive, in that they only reaches 110GHz, and they is very expensive. Using the H.P. 8510 20Ghz vector analyzer we have managed to make an extension which at present reaches frequencies upto 170GHz with 40 dB dynamic range.
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The use of a new 8-1000 GHz Vector Network Analyzer (1) gives complete information (amplitude and phase) on the detected microwave, in a very wide frequency band, covering the millimeter domain and a large part of the submillimeter.
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A quasi-optical isolator was designed to avoid stability problems of heterodyne detection receivers arising from time variable standing waves in radio telescopes. These effects cause serious distortions of the spectrum because they are proportional to the field, while the detected power is dependent on the field square [1]. In addition to the investigation of the isolator performance the origin of the standing-wave effects in heterodyne-detection receivers was examined measuring their suppression.
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Quasi-optical bandpass and bandstop notch filters have been designed employing the synthesis techniques that are generally employed in waveguide filter design. The performance of a bandstop filter consisting of a number of coupled Fabry-Perot resonators will be reported.
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Metallic plates of finite thickness perforated periodically with circular holes can be used as diplexers, called "dichroic", as part of quasioptical multiplexers in the millimetric range. The purpose of this work is to investigate their filtering properties, to provide a method to design dichroic filters in an efficient way and to draw conclusions about their field of application. Computed results are compared to measurements and the design of a diplexer for a breadboard of the atmospheric radiometer METEOSAT II is presented.
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A three-probe microstrip measurement scheme has been successfully used for input impedance and S-parameter measurements. This paper reports several alternative configurations for these measurements.
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The estimation of the uncertainty of the measuring by six-port reflectometer is of great importance in a technical and practical point of view, however, only a few studies of the error analysis have published, and most of them were constrained to special structures and/or calibration methods. In this paper a general analysis of the uncertainty and a general method of simulation is discussed, and then a measure - "effective radius" of "vague circle" - is given to estimate the uncertainty.
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The polarization structure research of millimeter waves is the urgent problem of plasma physics, radio communication, microwave geophysics, radioastronomy and it is a complicated problem in the radiophysics. The total polarization analysis of radio waves (as a rule stochastic and partially polarized) suggests a simultaneous or a quasisimultaneous measurement of four factors of coherency matrix or four Stokes parameters. In the millimeter waves region this type of analisys is performed extremely rarely and is characterized by low accuracy. A new differential method of a total polarization analysis presented by the author is mean of the space structure investigation of the arbitrary as well as slightly polarized waves. The method is based upon a quasisimultaneous wave decomposition in three orthogonal bases (two linear at 45° with respect to each other and one circular), switching of decomposition conditions, measurement of power difference of the orthogonally polarized components and on the construction of the Stokes parameters in the differential form [1]. The method suggested turned out especially effective in the millimeter wave region.
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An electric-field cross-correlation Fourier transform spectrometer worked in the region of millimeter waves is described. It uses two controllable phase variation pseudorandom binary sequences (PRBS) to modulate individually two microwave fields devived from the same source, so two wide bandwidth microwave signals with variable phane delay are obtained. The Fourier transform of the cross-correlation function of these microwave signals is the cross-power spectrum. The principle and fundamental structure of the spectrometer are presented and experimental results are given in 8mm-wave range.
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Experimental investigations of nonlinear characteristics of the transfer coefficient (G) and bistability phenomena in a quasioptical resonator with the ruby layer saturated under the electron spin resonance conditions (ESR) are presented. Experiments have been carried out on the special radiophysical complex BURAN /1/ at the electromagnetic wave frequency ν = 75⋅109H3 and in the temperature range T = 0.9 K + 4 K and with the static magnetic field H<6 T. A two-mirror half-symmetric open resonator has the mirror aperture about 2a=8λ, the radius of a spherical mirror R = 15λ , the quality factor Q ≈8⋅103 and operates on the spectral mode TEM0011. A ruby layer (thickness h = λ/4) is placed near the plane mirror.
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Recently the application of the equivalence principle to the analysis of metal- and dielectric-loaded cavities and waveguides has been suggested in [1]-[5]. In comparison to other methods, the original problem is reduced with respect to its dimensions. This drastically reduces the size of the characteristic matrix.
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The Quality factor (Q) of a Waveguide is the measure of its efficiency of power transmission. The Quality factor is inversely proportional to the total loss (conductor loss + dielectric loss) produced in the guide.
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The finite-difference method is used in the analysis of the propagation characteristics of an infinite array of rectangular dielectric waveguides. The formulation developed is general and may be applied to the solution of other problems, including those with anisotropic dielectrics and with a continuous variation of the index of refraction profile in the waveguide cross section.
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The NRD-guide filter is investigated by a method which combines the building block approach of multimode network theory with the rigorous mode matching procedure. A comparison between the results calculated by the present approach and measured data is given and very good agreement has been found; the effectiveness and practicality of the present method are thus verified.
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Since the groove nonradiative dielectric (GNRD) guide was proposed in 1987,[1] a series of papers for investigating theoretically and experimentally the GNRD structure have been published, and then be reviewed.[2] In which, the advantages of the GNRD guide in both the assembling and the dispersion characteristics than that of conventional NRD guide were described. The grooves in the coupled GNRD guides are particularly valuable for precisely fitting two dielectric strips with a specific distance. Some samples of the GNRD coupler based on coupling between two identical dielectric strips had been fabricated and tested. Unfortunately, their bandwidths were generally narrow like that of the NRD couplers.
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In this paper, the vector field formulas of TEMpi-modes in a concave spherical quasi-opitcal cavity are deduced by the complex-source-point theory. The resonant frequencies of TEMpi-modes are also obtained.
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The mode-matching method is used to analyse the dispersion characteristics of single and asymmetrical double groove guides, and the unified formulae of Nth-order approximation dispersion equations are presented. A general program for calculating the dispersion characteristics of the groove guides has been completed. Some comparisons between the results obtained by present method and those in the literature are made, and very good agreement has been seen. The effectiveness and the reliability of the formulae and the program are thus justified. The theoretical coupling characteristics of an asymmetrical double groove guide coupler are given to demonstrate how flat coupling can be obtained with this coupler.
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Many kinds of IR and millimeter wave components, e.g., distributed feedback lasers, filters, etc., are based on layered periodic structures. The latter are known to support electromagnetic waves guided by separate layers (waveguide modes) or interfaces (surface modes), as well as by the spatial period d of the superlattice (Bragg-type modes). Besides of these, there exist special bulk modes which can be described as plane electromagnetic waves in an effective homogeneous anisotropic medium [1]. Theoretically, the dispersion branches of these excitations are subject to change in the presence of dc magnetic field resulting in the tunability of electromagnetic wave power transmission [2]. However, there are little experimental data on the transmission line shape, particularly at the bulk mode resonance minimum which have been found to be very deep. Present experiments have been carried out by using high dynamic range technique revealing an unexpected additional line.
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The resolving power of classical optical microscopy is restricted to about one half of the wavelength because of diffraction. In the far infrared this limits the resolution to a rather macroscopic scale of about 100 μm. On the other hand it is well known that a truely microscopic confinement of radiation much below the diffraction limit can be obtained by using a transmitting aperture smaller than the wavelength. In this case the illuminating field consists primarily of evanescent waves bound to the aperture. The object to be investigated is placed within this field at a small distance less than the diameter of the aperture. Examples of λ/20 resolution have been given both at visible and at microwave frequencies.1,2 Massey reported on a near field microscope with a resolution of λ/4 for the far infrared.3
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Thin film niobium nitride Josephson tunnel junctions have been used as harmonic mixers in experiments to measure the frequency of various submillimetre wavelength laser lines.
The Josephson junctions were used as harmonic mixers with an 18-40 GHz microwave synthesiser providing the local oscillator signal. Harmonics up to the 140th of ∼18 GHz, generated within the junction, mixed with optically pumped laser lines up to 2.5 THz to produce output signals around 20 MHz.
Results of frequency measurements and data on the performance of the mixing system, including signal to noise ratios and frequency response, will be presented.
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A quasi-optical flux-flow submillimeter oscillator is described. The electromagnetic wave was radiated out of an printed antenna into the free space passing through a hyperhemispherical lens and then collected onto a detector. The oscillation was confirmed over the frequency range of 145 to 480 GHz. The output power of ∼1 μW was obtained at maximum for the tested devices.
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Measurements of small absorption coefficients of weakly absorbing or trace gas in the atmosphere has been one of the most important applications of photoacoustic devices. Normally, the major factor limiting the sensitivity of these systems is the spurious signal produced by the heating of the cell windows. We present a simple photoacoustic configuration that eliminate the spurious window and external background signals and enhance the main (real) signal of infrared gaseous photoacoustic systems, optimizing the signal to noise ratio.
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A broadband reflectometer based on high speed IMPATT oscillators has been developed. The reflectometer is capable of sweeping a bandwidth of 26.5-110 GHz in a time of 1.25mS. The operating frequency range is divided into five subbands. Each subband has an associated polarizer and custom broadband antenna. The description of one subband is detailed in Figure 1.
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Electron temperature profiles are routinely measured on FTU tokamak from second harmonic Electron Cyclotron Emission in the extraordinary polarization. Spatial resolution of the diagnostic is 2 cm; time resolution achieved for the whole ECE spectrum by a fast scanning Fourier Transform Spectrometer is 5 ms, while about 1 μs is obtained at twelve radial locations simultaneously by means of a twelve channel grating polichromator.
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A 2-D multichannel interferometer designed for tomographically inferring contours of electron density in the WT-3 tokamak plasmas (R≃0.65 m, a≃0.2 m, Bτ≤1.75 T) with high time resolution is described. A frequency shift of wlF=1.45 MHz for a local beam splitted from a 337 μm HCN laser beam is obtained by use of a super rotating grating /1/, which allows the time resolution of the density measurements as fast as τ≃10 μsec.
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The problem of measuring the distribution function of plasma electron component concentration along the coordinate axes and further determination of its absolute value in case of average densities and sizes can be solved by means of a resonator method using the barrel-shaped open resonators (BOR) excited on the higher-type oscillations in the millimeter wavelength range.
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In this presentation we introduce a new concept for an infrared photoconductor and demonstrate that such a detector can, theoretically, exhibit unit quantum efficiency at selected frequencies. The idea is based on establishing a relatively high finesse absorption-cavity internal to the detector element and tuning the front surface reflectivity and the dopant concentration of the detector to achieve improved performance. A theoretical analysis demonstrates this concept and provides the relevant design parameters. This approach offers many other advantages over conventional photoconductors as well as impurity-band-conduction approach. Among those are enhanced photoconductive gain, improved noise performance, and better immunity against ionizing radiation.
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The Microwave Tokamak eXperiment at LLNL was designed to explore the physics of microwave interactions in plasmas using high power millimeter wave sources. We have used 140 GHz power from two sources, a gyrotron capable of ∼ 1MW pulsed, 400 kW CW, and a free electron laser with 100-400 MW pulses of ∼ 10 ns for experiments, with results that we will describe here. Our interests center on non-linear absorption phenomena (FEL) and on heating, transport, and MHD control (FEL and gyrotron) in tokamak plasmas. In the first phase of the FEL experiments we were theoretically in the linear absorption regime, and our plasma transmission experiments gave results consistent with linear theory (with refraction included).
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A gyrotron based around an 11T superconducting magnet is currently being operated at Strathclyde University. As with our previously developed gyrotrons, it makes use of a two electrode configuration with a field immersed, field emission cold cathode1.The initial results from this system, in the lower frequency range up to 110GHz, will be presented.
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The linear and the nonlinear analyses have been performed for the Electron-Cyclotron Maser interaction, between arbitrary rf field modes (TM, TE, HE or EH) and an axial velocity electron beam. This interaction [1-2] is capable of giving wave growth, with a conversion efficiency as high as 100% in principle. The underlying idea is based on the observation that for a wave field with transverse components (E1,B1), if the axial electron velocity initially satisfies v > E1 / B1 which requires that q = E1 / cB1 < 1, then transverse electron momentum is produced coparallel to the wave electric field, with the resulting current doing work on the wave, while the resonance is maintained with an appropriate value of the uniform axial magnetostatic field B0.
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The concept of electrostatic electron cyclotron resonance maser (EECRM) was proposed in 1984[1]. Its mechanism is that: injecting an relativistic or nonrelativistic cyclotron electron beam into the coaxial wave-guide system (see Fig. 1) , an radial electrostatic field between the inner and outer conductors is used to restrict the electron cyclotron motion , while electrons interact synchronously with coaxial high-frequency fields (TE, TM or TEM modes) , resulting in the coherent radiation. So it is necessary to give a detailed analysis to the electron motion in the EECRM system.
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We describe a redesign of the MIT long-pulse CARM oscillator experiment including the incorportation of an improved Bragg resonator design. The first MIT long-pulse CARM oscillator experiment, which was carried out last year, resulted in the observation of CARM operation at ∼ 30 GHz. However, the excited modes were the result of window reflections rather than Bragg resonator modes, and the device efficiency was low. Two substantial improvements are being incorporated into the CARM oscillator redesign. First, a new 700 kV electron gun, built by Thomson Tubes Electronique, will substantially improve the beam quality. Simulations predicting a substantially enhanced beam quality will be presented. Second, a new improved Bragg resonator design will be used. In this improved design, both scattering matrix and coupled-mode theory codes will be used to optimize the mode purity and cavity Q for the desired operating mode (TE11). The use of Hamming and Kaiser-Muller k-space filters allow the design of Bragg resonators with substantially improved mode properties. The improved design will be described, and simulations of the CARM oscillator operation will be presented.
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Cyclotron autoresonance maser (CARM) amplifiers are under investigation as a possible source of high-power (>100 MW), high-frequency (>10 GHz) microwaves for powering the next generation of linear colliders. A design for a high-power, short pulse, 17.136 GHz CARM amplifier, utilizing a 500 kV linear induction accelerator, is presented.
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A slow-time-scale, single-particle theory of the CARM oscillator with a Fabry-Perot resonator is presented for the case of mirror separation much greater than the Rayleigh length. Analytical results are obtained for the small-signal gain, efficiency and threshold current. The optimum nonlinear efficiency is presented in terms of F—μ plots.
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Quasioptical Cavity have many applications and potentials at millimetre and submillimetre wavelength. An investigation in quasioptical gyrotrons (QOG) has made substantial progress[1-5].
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A general formula of the energy transfer rate of the transverse-electric-mode gyrotron, CARM, and nonwiggler FEL oscillators is rigorously derived within the Vlasov framework. The present result can be used to simultaneously deal with the influences of the finite cavity length, spreads, higher harmonics, and nonresonance. It is examined to be in agreement with the Madey theorem of these devices for a cold electron beam.
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For the numerical solution of cyclotron resonance masers theory equations, describing the annular rotating electron beam - TE electromagnetic field interaction processes, so called Wave Method is proposed to be applied.
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In this work by the computer simulation the transient processes and the character of steady-state oscillation regimes were investigated in a gyro-BWO with an uniform waveguide and a non-profieled magnetic field. The self-consistent model is based on the assumption of the slow time and space scale variation of the e.m. wave amplitude.
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Dual polarisation systems, imaging systems and complementary systems operating at cm/mm/IR/EO wavelengths have been singled out for future research, after a brief description of the roles of current mm-wave systems. The underlying aims have been an enhancement of the performance of present systems as well as the extension of their capabilities during inclement weather. The scene is now set for widespread, high volume, commercial applications of mm-wave systems. These systems derived considerable benefits from R&D undertaken for military systems and the MIMIC program. Communication systems are also destined to attract significant funds during this decade.
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Several major millimeter-wave system developments are proceeding in the United States. The systems include satellite communications, missile terminal guidance, airborne synthetic aperture radar, and airborne radiometric imaging equipment. These are supported by significant technology thrusts, including the MIMIC (Microwave and Millimeter Wave Monolithic Integrated Circuits) program, and a major effort to measure target and background radar returns at several wavelengths.
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This paper describes a compact, high speed 95 GHz scanned antenna designed for collecting radar data from an airborne platform. This antenna consists of a corrugated horn feeding an f∶1 parabolic reflector through a hole in the scanning mirror, as shown in Figure 1. Radiation exiting the horn is thus collimated by the parabola and is incident on the flat scanning mirror, from whence it is steered to the desired angle. The collimated output is incident on the scanner in such a way that the angle is doubled, so that greater angular coverage can be obtained for a given mirror angular velocity. The scanner operates in only one axis; coverage of the other dimension is given by the forward motion of the aircraft.
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The 3246th Test Wing at Eglin AFB FL and the Georgia Tech Research Institute (GTRI) at Atlanta GA are jointly developing a multispectral Airbone Instrumentation System (AIS) to be used in the collection of terrain (clutter) backscatter signatures as well as target-in-clutter signatures. The system will contain both a 95 GHz instrumentation radar system and a two color infrared (IR) camera system. The system is being developed to address the RDT&E data requirements for a variety of USAF and US Army weapon systems to include data required for modeling and analysis, hardware-in-the-loop simulators, and coincidental sensor/seeker under test baseline performance evaluation. The data quality and specific system parameters are such that the "scene" data collected can be merged or synthetically blended with the recent target signature collected under a number of joint services target data base collection programs. By establishing a "comparable" data base of high quality scene data, the sensor/seeker community will have a high quality, highly flexible data base for concept development and evaluation. Conicidental 95 GHz and 3 5 micron and 8-12 micron signatures of the scene will be collected, providing data for multimode sensor/seeker development. The characteristics of the 95 GHz radar include coherent, wide bandwidth (8 inch range resolution), polarimetric operation. The IR cameras available provide a variety of resolutions.
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The high transmit powers provided by free electron laser (FEL) sources in combination with the narrow antenna beamwidths achievable at millimeter wave (MMW) frequencies offer potential for use in a number of radar applications. Potential applications of high power millimeter wave sources include satellite imaging, low angle radar tracking, radar astronomy, and a number of other possible applications such as atmospheric research, space debris detection, and space vehicle tracking.
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This paper describes a 35 GHz and 95 GHz instrumentation radar system and the associated data acquisition/processing system developed by the Georgia Tech Research Institute (GTRI). The radar are polarimetric coherent and wide bandwidth providing high range resolution. Polarimetric signatures on the order of 4-6 inches. High cross range imaging equal to or better that the range resolution is generated using inverse synthetic aperture radar processing (ISAR). In addition to systems development data calibration procedures have been developed which provide the needed quality of data to support the development and evaluation of more complex processing concepts (e.g. image processing, polarimetric processing). The key parameters of the radar units are listed in table 1.
The paper will describe the design of the radar units as well as the overall function of the data acquisition and processing software. Radar performance data will be presented which will demonstrate the effectiveness of the various calibration techniques. Data examples will include RCS polar plots, high range resolution profiles, and ISAR plots with and without correction of system errors.
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Ranges operation of previously investigated LADAR Cooperative target acquisition and tracking systems have been enhanced from 10 to 30 km using dual mode LADAR/RADAR systems. All weather ranges above 400 km require L band RADAR which employ very high power Tx and a bulky antenna.
The use of active transponders on cooperative targets is investigated. Equations for range computations of active transponders in LADAR/RADAR systems are developed and several results are discussed.
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In recent years, millimeter wave (MMW) radar stationary target detection algorithms have employed methods which exploit the geometric nature of targets and clutter in order to eliminate high amplitude clutter returns. Traditional adaptive amplitude thresholding techniques (e.g., constant false alarm rate, CFAR, processing) could not yield the desired high probabilities of detection because thresholds had to be set high enough to eliminate the high amplitude clutter. Man-made targets and high amplitude clutter tend to have different geometrical characteristics. In order to improve detection results while keeping false alarms to a minimum, these differences must be exploited.
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The main functions of RF windows in fusion reactors are: to act as a first barrier for tritium and radioactive dust, and to provide vacuum containment. They must have low RF losses. The lifetime must be satisfactory (>5 years); nevertheless maintenance by remote handling must be foreseen. Valves or shutters are needed to improve safety. The tokamak windows will be placed in a screened area where they will receive a maximum dose of about 1016-17 n/cm2 over their life, to avoid degradation of their dielectric and mechanical properties. The coolant must be an inert fluid to avoid damages to other materials of the machine in case of leakage. In ITER [1], the most demanding working conditions are those of the windows of the Electron Cyclotron (EC) system which should support 1 MW, CW in the 120 - 140 GHz range (probably at fixed frequency) in a gaussian or HE11 mode. In the Lower Hybrid (LH) system the windows will have to transmit 1 MW, CW at about 5 GHz.
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The theory of waveguide windows has been well known for a long time. Nevertheless, in some recent works, several formulas have been used which lead to different results, especially for window absorption. Since the problem of heat generation (e.g. in high-power windows) often has to be solved by people not specialized in rf- or optical engineering, it seems to be useful to summarize the necessary formulas and to discuss their application. This paper presents a set of consistent equations for reflection, transmission, and absorption of a single waveguide mode, passing through a plane transverse window. Special cases, such as a resonant, an antiresonant, and a sheet window, are discussed. The scattering matrix formulation allows an easy calculation, even of more complex multilayer window structures (e.g. double disk windows or coated disks).
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The design of millimeter (mm)-wave windows for high power gyrotrons must provide a minimization of the reflected power in order to assure an efficient operation of the tube. This aspect is usually accounted for in single disc or double disc structures by choosing the geometry in such a way that the reflected beams arising from the multiple reflections can interfere destructively. As the electrical length of the disc is determined by its permittivity, a good knowledge of this material parameter is indispensable for this concept. Due to the dielectric power absorption, a temperature profile will develop and thus the temperature function of the permittivity is required for modelling the window performance.
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Broadband complex refractive index and complex dielectric permittivity spectra are measured at several cryogenic temperatures for crystalline sapphire over the frequency range 100 - 450 GHz. The dispersive Fourier transform spectroscopy was employed together with a special interferometer for low temperature operation. At 180 GHz the absorption coefficient value only reduced to about one half for both 76 K and 5.8 K compared to its value at 300 K. At 280 GHz the absorption value at 76 K remains to about one half of its value at 300 K but drops to about 20 percent at 5.8 K. The earlier reported temperature squared dependence (decrease) in the absorption coefficient value is totally absent in our data at frequencies below 300 GHz. The average value of the real part of the dielectric permittivity decreases with decreasing temperature.
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High resistivity silicon with d.c. conductivity levels (σdc) of 10-4 (Ωcm)-1 at 300 K has been identified as a low dielectric loss material at mm-wavelengths [1] and was also proposed as an alternative window material for high power gyrotrons because the dielectric loss tangent (tanδ) falls well below that of sapphire, the presently used material, in the 100 GHz range [2]. A major increase, however, is expected for the power levels that can be transmitted through cryogenically cooled sapphire windows because with decreasing temperature T, a strong reduction of tanδ is observed by different authors [3,4] apart from other beneficial trends in operation relevant thermophysical parameters. For Si, tanδ at 77 K was found to be at similar level or even above that at 300 K [5]. A more detailed data set is presented for which the features of extrinsic conduction in semiconductors yield a adequate description.
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Radiation-induced increase of the dielectric loss tangent ( tanδ ) is a major problem for the realization of mm-wave windows in Electron Cyclotron wave systems in next generation fusion devices. In oxyde materials with dominant ionic bonding character, such as Al2O3 or MgAl2O4, increased loss levels are observed after neutron irradiation which can be related to displacement damage. Few experiments have been published so far on the transient effects effective only under irradiation, but at least for purely ionizing radiation, such as X-rays, none of such in-beam effects have been observed. A different situation can be expected for alternative window materials with a homopolar lattice, where contributions from free charge carriers to the dielectric loss dominate those from the ionic lattice polarization.
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Dielectric properties of fused silica, MgO, AlN and BN were measured using an open resonator at frequencies around 100 GHz. The resonator is of semiconfocal type and the frequency variation method is used. By making the parameter ∆=1 for every sample and determining resonator length precisely, the standard deviation of measurements less than 0.1% in permittivity and about 10% loss tangent was achieved.
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A mechanism limiting the lifetime of high-power millimeter wave windows is the degradation of the ceramic which is exposed to gas. We first observed this phenomena on the air side (atmospheric pressure) of a plane circular waveguide window (Al2O3 ceramic, ∅ = 70 mm) of a TE0,3 mode gyrotron, which was operated with a series of 100 ms pulses at a power of about 100 kW [1]. This degradation occured at the center of the window in an almost circular symmetry. Figure 1 shows the damage pattern at the time of first observation. It consisted of several thin concentric circles (width ≈ 0.3 mm, diameter ≈ 0.5, 2.5, 4.6, 6.7, 8.8 mm) which where not completely closed. From their radial location and from the window thickness one may conclude a TE0ln (or TM0,n) mode with n ≈ 19 being involved in the damage process. It is also observed that the circles occur only near the inner maximum of the TE0,3 mode power density. With further operation of the gyrotron the damage grew, the fine structure smeared out, and the resulting pattern approximately corresponded to the inner maximum of the TE0,3 mode power density (Figure 2).
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A three-level, generation-recombination model has been developed to explain the unusual response law and ultraphotosensitivity of the Avalanche Breakdown far infrared detector1,2). The detector consists of Sb-doped n-germanium operated at a bias level above the point of impact ionization breakdown. Under these conditions, the signal response current, is, has been shown to depend on far infrared signal power, P, as is = K(P)1/2, where K is a proportionality constant, over more than six orders of magnitude in P. Furthermore, an apparent photomultiplication factor of the order 106 - 107 has been observed. Signals as low as 10-14 watts in a 10-7 sec pulse have been seen, corresponding to the arrival at the detector of only 4-5 photons per pulse. Earlier attempts to provide a model of this detector3), including a two- level scheme by Penin4), have not proven satisfactory.
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A heterodyne receiver has been developed for operation in the frequency range 625-650 GHz. The goal of the ongoing project is to provide an effective tool to study upper atmospheric trace gases. The receiver is designed to operate on the research aircraft FALCON at flight altitudes of 10-12 km. Receiver performance and tuning capability were tested during a first flight campaign in April '91, with measurements of stratospheric ozone (647.8 GHz) and HCl (625.9 GHz).
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Heterodyne spectroscopy can achieve resolving powers of more than 106 in the far-infrared spectral region. However, in the range 30-300μm (10-1THz), there are problems with both detectors and local oscillator (LO) sources. At longer wavelengths GaAs Schottky barrier diodes have been employed with great success, using optically-pumped far-infrared lasers (OPFIRLS), carcinotrons or harmonic generation from millimetre wave sources as LOs. Superconductor-Insulator-Superconductor (SIS) mixers, which require much less LO power than Schottky diodes, are excellent devices at wavelengths longer than 600μm. A great advantage of all rectifying detectors is the very large intermediate frequency (IF) bandwidth available. Typically this might be ±20GHz, allowing spectroscopy over a useful wavelength range (eg ∆λ = 12μm at 300μm) even with fixed frequency LOs.
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We report on the performance of antenna-coupled submicron thin-film Ni-NiO-Ni diodes used as detectors for 10 μm CO2-laser radiation. The fabrication process of the diodes and the antenna design have been described elsewhere1,2.
We investigated the laser-induced signal by varying the angle between the antenna axis and the linearly polarized electric field vector of the incident laser radiation for different temperatures between T=77 K and T=335 K.
As reported previously1,2 the recorded voltage consists of a polarisation dependent and a polarisation independent part. The period between the two maxima in the curve is 180 degree. The difference between a voltage peak and a voltage valley is found to be half of the maximum total signal. Experiments performed in the past1 with MOM diodes of different contact sizes (1 μm2 and 0.12 μm2) demostrated that the polarisation dependence is strongly related to the size of the contact area and was therefore attributed to antenna coupling.
For the experiments described in this abstract the MOM diodes were heated to 335 K and cooled to T=77 K. The part of the response of the diode not affected by the polarisation decreases up to 55% whilst the polarisation dependent part exhibits no change with temperature or slightly increases for some diodes. These experiments confirm definitely that the polarisation dependent part of the signal is due to antenna coupling whereas the polarisation independent part is mainly caused by thermal effects.
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The concept of far infrared photocathodes involving work functions at interfaces between lightly doped and heavily doped silicon has been previously discussed and device structures were shown to have response out to 60 microns.[1] Si, Ge and InGaAs p-i-n structures with different interfacial workfunctions giving rise to longwavelength thresholds up to 200 microns are reported here. Preliminary estimates of responsivity and detectivity for these non-optimized samples are encouraging. Work function estimates based on the different cutoff wavelengths show that this approach can be used to tailor the cutoff wavelength by changing the impurity concentration near the metal-insulator transition.
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GaAs Schottky barrier mixer diodes have been successfully used in heterodyne receivers throughout the millimeter and submillimeter wavelength ranges [1,2]. These receivers combine excellent spectral resolution (∆ν/ν=10-6) and exceptional sensitivity, and have given scientists an important tool for use in such fields as radio astronomy [3], atmospheric studies [4], chemical spectroscopy [5] and plasma diagnostics [6]. Although superconducting devices (SIS junctions) have recently achieved better performance at millimeter wavelengths [7], it is not certain when, or if, SIS technology will be extended to terahertz frequencies. Since there are a great many scientific programs which rely on submillimeter wavelength heterodyne technology, including studies of ozone depletion [8] and space based astronomy [9], it is critical that the Schottky technology be pushed to its fundamental limits.
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High purity GaAs exhibits excited state far-infrared photoconductivity in the temperature range from 2K to 4.2K. The response is characterized by an exceptionally sharp peak which is magnetically tunable over a broad range. This dominant peak, at 35.8 cm-1 (279 μm), belongs to the 1s-2p transition of the residual shallow level impurities, and its response is over an order of magnitude above the continuum. The preliminary results of the measurements of responsivity, dark current, and NEP of this device are reported.
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Fabrication processes of Pt/GaAs Schottky diodes have been studied for obtaining submillimeter wave detector/mixers with high performance. We have been studying and fabricating dot-matrix Pt/GaAs Schottky diodes which have advantages of high speed responsivity and room temperature operation. Noise characteristic of the diode is one of the important measure for the submillimeter wave detector/mixers.
The noise characteristics are influenced by defects localized near Pt/GaAs interface, which defects are introduced in processes of wet etching, chemical surface treatments, reactive ion etching and deposition of SiO2 insulating layer. We have measured the diode noise characteristics, as the diode temperature was changed. Effects of chemical processes, composition of GaAs surface, damages by SiO2 sputtering on the diode noise have been measured.
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The wide-spread use of Schottky microjunctions to perform efficient and fast detection, low level mixing, sideband or harmonic generation in the terahertz range has given rise to a sustained interest for an adequate optimization of these devices.
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Electron transport in submioron GaAs Shottky diode has been simulated under the barrier capacitance modulation conditions by high frequency (f>100 GHz) voltage pumping signal.
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Free-Electron Lasers (FELs) [1] are coherent sources of high-power electromagnetic radiation. In the microwave part of the spectrum, high efficiencies can be achieved at power levels in the multi-megawatt range. FELs can be operated as amplifiers or oscillators, and so have applications in a variety of areas ranging from electron cyclotron resonance heating (ECRH) to RF acceleration [2]. In this paper, results of the de- sign and operation of a 27 GHz Bragg resonator Free-Electron Maser (FEM) oscillator and a 35 GHz high-gain FEM amplifier are presented and discussed. The experiment is driven by a 700 kV, 900 Ω HV modulator, and operates with a pulse length of 1μs at power levels in the 1-3 MW range.
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Free electron laser amplifiers are investigated as sources of high-average-power (1 MW) millimeter to submillimeter wave radiation (200 GHz - 600 GHz) for application to electron cyclotron resonance heating of magnetically confined fusion plasmas. As a stepping-stone to higher frequencies and cw operation a pulsed amplifier (τpulse ≃ 80 ns) at 98 GHz is being developed. Status is reported on this experiment which investigates linear gain amplification with use of a sheet electron beam (transverse cross section = 0.1 cm × 2.0 cm, Vbeam = 440 keV, Ibeam ≃ 10 A) and short-period wiggler (lw = 0.96 cm) and with expected output of 140 W. Predictions of gain and efficiency from a 1-D universal formulation are presented. Beam propagation results, with wiggler focusing as a means of sheet beam confinement in both transverse dimensions, through the 54 cm (56 period) pulsed electromagnet wiggler are discussed. Peak wiggler fields of 5.1 kG on-axis have been achieved.
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This paper reports the first operation of a Sapphire-based Cerenkov Free Electron Laser (CFEL). The CFEL is a tunable source of coherent millimeter and sub-millimeter radiation. The phase velocity of light in Sapphire is about one tenth of that in free space so that Cerenkov radiation is obtained at modest electron beam energies; our electron gun supplies 10's of A/cm2 in the 20 to 100 keV range. The Cerenkov emission is confined by the resonator and can build to watts of power. The resonator geometry is critical both in terms of a passive cavity Q and because of its role in the electron/field interaction. Frequency, oscillation threshold current, and power have been measured.
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The study of non-linear time resolved spectroscopy requires the generation of fast high power optical pulses. Recently, short FIR pulses of considerable power were reported using cavity-dumped FIR lasers [1,2]. However, minimal pulse widths (5-50 ns) are then determined by the length of the laser resonator.
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Generation of microwave radiation from a long pulse rotating electron beam in a circular wiggler magnetic field (Circular Free Electron Laser) has been studied. The beam energy is 300-400 kV typically and the beam current is 200 A. The radiation frequency is measured to be 12.7 GHz with the grating spectrometer. Measured value agrees with the theoretical prediction. The interaction between the beam mode and the TM waveguide mode is discussed based upon the dispersion relation.
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A 1 MW, cw, Free Electron Maser is being designed at the FOM-Insitute. The specifications are optimized for ECR heating on the next step tokamak NET/ITER. In order to track the evolving plasma, the EC power source has to be tunable over 10 % of the central frequency of 200 GHz.
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Coherent Cherenkov radiation in the far-infrared region have been observed for the first time using an electron bunch consisting of 2x1011 electrons from a linear accelerator. Angular distributions of the radiation measured in air showed the Cherenkov cone pattern. The shift in the radiation peak is presented as a diffraction phenomenon, depending upon the wavelength.
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Modified orbitron (M.O.) millimeterwave oscillators were constructed and the experiments began in order to improve the efficiency, pulse duration, and jitter time beyond the levels achievable with conventional orbitrons (C.O.s). The necessity of modifications was justified through characterization experiments of C.O.s. Computer simulation of a M.O. showed unambiguous development of the negative mass instability on an orbiting beamlet in a non-volatile logarithmic potential-well. Although significantly long pulses are also possible, extremely narrow pulses useful for a high resolution or impulse radar were successfully produced because of no jitters in a M.O. before the wave emission.
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Free-electron-laser (FEL) offers the promise of a continuously tunable source of extremly high-power coherent radiation from millimeter waves to visible wavelengths. And a great deal of attention has been focused on the electromagnetically pumped FEL(EM-FEL) in recent years due to the potential of short-wavelength operation with electron beams of relatively modest energy. In the case of a magnetostatic wiggler, the wavelength of the output radiation scales as λ≈λw/2Υ2 where λw denotes the wiggler period and Υ is the relativistic factor for the beam. In contrast, the wavelength of the output radiation for an electromagnetic-wave wiggler scales as λ≈λw/4Υ2. As a result , for fixed wiggler periods and electron beam energies, the electromagnetic-wave wiggler will produce shorter output wavelength. Although various proposals for developing short-period magnetostatic wigglers and the associated electron beam technology are being studied, there are still many difficulties to overcome in order to get shorter or/and visible output wavelengths of FEL. On the other hand, The EM-FEL can produce any given laser frequency with electron beam of modest energy using a variety of wavelengths' electromagnetic-wave wigglers in principle.
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Scharlemann and co-workers1 studied optical guiding in a short-wavelength free-electron laser (FEL) by using a weakly guiding optical fiber to model the bunched electron beam, and Bhattacharjee and co-workers2 presented a theoretical approach to the problem of optical guiding in a Raman FEL, in which the radiation wave is expanded as a complete set of vacuum waveguide modes. Generally speaking, different vacuum waveguide modes possess different wave number. In this sense, they are non-coherent. However, the dispersion relations of vacuum waveguide modes can be modified by introducing an electron beam so that some of them might have the same wave number and become coherent modes under proper conditions. Based on this, we have developed a multimode coherent coupling theory to investigate optical guiding in a Raman FEL amplifier. Under the assumption of a small-radius electron beam passing through an ideal linear wiggler magnetic field, the coherent coupling multimode dispersion equation in a rectangular waveguide is obtained. A special attention is paid to coherent coupling of TE10 mode with TE30 mode. Solving numerically the dispersion equation shows that the coherent coupling of TE10 mode with TE30 mode can result in optical guiding if proper operating parameters are chosen. However, the optical guiding is of strong frequency-dispersion.
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The Fresnel zone plate antenna has seen further development in the recent past. Analytical results and measured data have been obtained at various frequencies to describe the efficiency, bandwidth, far-field pattern, and focal-region and off-axis behavior. Both transmission (i.e., lens-like) and reflective configurations have been used [1,2]. Horn antennas have typically been used as feeds, but recent measurements at 220 GHz have utilized a dipole (or an array of dipoles) for a feed [3,4]. Off-axis feed methods have recently been developed for reflective zone plates (of elliptical cross-section). This arrangement can produce a better far-field pattern (lower sidelobes), because the feed blockage is reduced.
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The requirements for wide-band multifunction phased arrays are outlined; although canonical phased arrays are ideally suited for narrow band operation and have graceful degradation of performance when some of their elements fail, other array configurations are shown to be more suitable for phased arrays required to operate over wide bands. Novel controlled redundancy arrays are shown to meet all the requirements of wide-band multifunction phased arrays.
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When antenna arrays are used for broadband operation or when they are designed to be thinned, difficulties with grating lobes can arise. This paper presents a variation of a synthesis technique due to Cottony [1] whereby the use of multiplicative processing overcomes these difficulties in the principal azimuthal plane.
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The Deep Space Network has a need for a frequency selective surface (FSS) to simultaneously receive X- and Ka-bands on a beam waveguide antenna where a ultra-low-noise maser is used at Ka-band (Fig. 1). The plate is required to pass circular polarized wave at (1) the Ka-band downlink (31.8–32.3 GHz) with a low insertion loss (possibly as low as 0.04 dB), (2) the Mars Observer spacecraft Ka-band Beacon Link Experiment (KABLE) frequency (33.6–33.8 GHz) with an insertion loss of 0.2 dB, and (3) a high-power Ka-band uplink (34.2–34.7 GHz), while at same time reflecting the X-band downlink (8.4–8.5 GHz). A thin FSS with apertures or patches is not mechanically suitable for these requirements, therefore a thick metallic plate with rectangular apertures is considered. Mechanical constraints also require an oblique angle of incidence, while bandwidth considerations dictate the use of a skew grid (Fig. 2).
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Annular synthesis antenna systems, first proposed as imaging systems, are shown to meet most of the requirements of ideal power combiners operating at mm-wavelengths.
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Integrated horn antennas have been proved to be very well suited for millimeter-wave applications. They consist of a strip-dipole antenna suspended on a thin dielectric membrane inside a pyramidal cavity etched in silicon [1]. The antennas are free from surface-wave losses and fully monolithic. However their large flare-angle of 70°, which is inherent in the anisotropic etching of < 100 > silicon, limits their gain below 13dB and their 10dB beamwidth above 90°. In this paper a quasi-monolithic dual-mode horn is introduced which increases the gain to 20dB with a high circular symmetry and a high 97% Gaussian coupling (fig. 1). As shown in figure 1, a machined section has been superimposed on a standard integrated horn antenna to increase the radiating aperture to 3.56λ. The junction of the integrated section and the machined section acts as a mode converter which mainly converts some of the incident TE10 mode to the TE12, TM12 modes. The modes are properly phased on the radiating aperture by selecting the flare-angle and the length of the machined section. This results in highly symmetric, low cross-polarization far-field patterns.
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While being well established at classical microwave frequencies, array antennas are on their way to conquer the millimeterwave region. One unconventional attempt shall be demonstrated in this paper.
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Quasioptical techniques are necessary for submillimeter mixers. Several feed antennas have been proposed to replace a conventional waveguide feed. Among them an equiangular spiral antenna is shown to have advantages of axially symmetric radiation pattern and of extremely broad frequency characteristics [1]. It was successfully coupled with a submillimeter SIS mixer [2]. It is important to find how its radiation pattern is dependent on geometrical shape of the spiral, on the position of a metal reflector behind the spiral, and on operating frequencies.
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The limitation of the slotline antennas for submillimeter wavelengths mainly comes from the requirement on the substrate thickness. For a dielectric constant of eleven the required thickness is ≈ 10 μm at 350 GHz. One way to overcome this problem is to make the slotline antenna on thick substrate and place one more thick substrate from the top, i.e. to make a sandwich slotline antenna. Experimental results on a sandwich slotline antenna at 350 GHz will be presented together with scaled model measurements (at 30 GHz). The effect of an air-gap and a lens will be discussed.
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This paper presents a unique design concept for a dual-frequency,electronically steerable, MM-wave phased array antenna using Monolithic Microwave Integrated Circuit (MMIC) technology. Such an antenna design will be found most attractive for satellite communication and missile seeker applications, where size, weight and power consumption are the most stringent requirements. The block diagram of a dual-frequency (20 GHz/30 GHz), electronically steerable, phased array antenna using MMIC technology is shown in Figure 1. Specific details on the critical RF components are also shown in Figure 1. Because of the location of the radiating elements on the hemispherical surface, the antenna is inherently capable of providing electronic scanning over 360 deg. in AZ-plane and over plus/minus 60 deg. in EL-plane. This electronically steerable phased array antenna will generate multiple beams in the AZ-plane with consistent bandwidth and pattern symmetry. This design exploits true integration of printed circuit antenna technology, GaAs active device technology, MM-wave microstrip technology and printed circuit time delay phase shifting element technology. This design employs microstrip periodic structure transmission line sections as time delay phasing elements, which requires no input power. The printed circuit microstrip line sections will be used as feed networks. Two distinct dual- frequency, antenna element fabrication techniques, namely, stacked-patch element (Figure 2a) and collocated, interleaved-patch element (Figure 2b), were investigated in terms of cost, size, performance and complexity. Tradeoff studies performed seem to indicate that stacked-patch element approach will offer minimum size and weight. The physical dimensions shown in Figure 1 and 2 are computed for 20 GHz and 30 GHz operations. All the electronics, control circuits and RF MMIC circuits can be located within the hemispherical structure. The input power, overall system size and weight will depend on the antenna gain requirement, inter-element spacing, number of elements and phase shifter types. It is important to mention that delay line phasing networks using printed circuit periodic structures will provide optimum flexibility in phasing of the antenna elements with minimum loss. The proposed antenna architecture for a dual-frequency operation using a common aperture and MMIC technology seems to offer an antenna system with minimum cost, size, weight and power consumption.
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In this paper, a rhombic slotline millimeter wave integrated antenna is proposed. It is analyzed by using the principle of duality and experimentally studied at Ka-band.
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The dispersive Fourier transform spectroscopic technique was improved to extend the frequency range at the long millimeter wavelength range for study of real and imaginary parts of complex dielectric permittivity and complex magnetic permeability magnetic materials. The use of a 110, 000 Gauss superconducting magnet alligns all magnetic dipoles with the precessional frequency centering around 330 Gllz. Complex dielectric permittivity and magnetic permeability were then evaluated for Ba- and Sr- ferrites over the frequency region 30 - 300 GHz.
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It has been shown (1) that the reflection of an absorbing medium covered with a convenient transparent film gives interference fringes irregularly spaced because of the specific phase shift at the reflection on the absorbing material. Two equations allow the determination of n and k (n = n - jk is the complex refractive index corresponding to a monochromatic wave E = Eoejwt), if we know the fringes position and the corresponding reflection power at the minima and maxima.
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We present a study of the localized vibrational mode of H- ions substituting F- ions in the calcium fluoride lattice. Using a tunable high pressure carbon dioxide laser we determined the linewidth and the saturation intensity of the H- local mode infra-red absorption line for temperatures between 10 K and 110 K. Our results indicate that energy relaxation of an excited H- vibrational mode occurs mainly by decay into three phonons with a lifetime for the localized mode of about 17 ps at low temperatures (T ≤ 10 K), which results in a Lorentzian line of width 0.33 cm-1. An analysis of our results shows that line broadening is caused mainly by energy relaxation at temperatures up to 60 K and by phonon Raman scattering at higher temperatures.
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We have studied the magnetic excitations of Fe2SiO4 and Co2SiO4 by means of Fourierspectroscopy in the far infrared and in an external magnetic field. For the analysis of our experimental data we have introduced a spin wave model and have determined the values of the parameters, exchange interactions and anisotropy constants.
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We have measured the transmission spectra of CaCoSi2O6 with a mm-wave spectrometer for different orientations of an applied external magnetic field relative to the crystallographic axes. The results are discussed on the basis of group theoretical considerations and by means of a spin-wave calculation.
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Hydrogen Deuteride is a molecule with a small permanent electric dipole moment (about 0.8 x 10-3 Debye) and a corresponding weak vibro-rotational spectrum. Wick (1935) predicted the existence of the HD dipole moment, entirely due to non-adiabatic effects, coupling the electronic and nuclear motion. This molecule is of special interest for astrophysical studies and the smallness of its dipole moment has also stimulated fundamental studies on collision induced dipole moments.
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Potassium dihydrogen phosphate (KDP) undergoes a paraelectric (PE) to ferroelectric (FE) phase transition at 123 K. The primitive cell containing two formula units changes from the tetragonal symmetry (I42d) in the PE phase, to the orthorhombic symmetry (Fdd2) in the FE phase. The optical fourfold c axis of the PE phase, becomes the ferroelectric axis below 123 K.
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In contrast to normal metals, the fundamental dielectric dispersion in semi-conductors and superionic conductors takes place at rather low frequencies, 1011-1012 Hz. The main changes in the spectra of superconductors during the superconducting phase transition also occur in the same frequency interval . On the base of systematic experimental study of submillimeter spectra of a variety of materials the general features of temperature behaviour of conductivity and dielectric permittivity of nonconventional conductors are analysed. The phenomenologycal model for describing the evolution of conductivity and dielectric permittivity spectra during the transition from free motion of charge carriers to their localization is developed. The high sensitivity of the submillimeter dielectric measurements for determination of the charge carriers characteristics in semiconductors and ionic conductors and for testing the quality of conventional and high-temperature superconductors is demonstrated.
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Magnetic structure of the yttrium orthoferrite YFeO3 (space group Pbmn – D162h can be treated as consisting of the two sublattices antiferromagnetically ordered (TN = 640 K) along the a-axis and slightly canted towards the c-axis. Investigating low-frequency AFMR mode (300 GHz at zero magnetic field) at temperatures 4.2 – 70 K and magnetic field up to 14 T we observed five additional well resolved and rather intensive impurity modes [1]. Since no magnetic impurities were measured in the sample by x-ray-fluorescent analysis with the accuracy 10-5, these "impurity" modes were assigned to the electron transitions between the levels of a 6S5/2 multiplet of the Fe+3 ions which occupy the 4c-positions of the Y+3 ions instead of their regular 4b-positions. Because of these positions symmetry the 6S5/2 multiplet is split into six non-equidistant levels both by an exchange interaction with the host matrix and by a crystal field. The impurity ions are magnetized by the effective field approximatly 9 T along the c-axis opposite to the weak ferromagnetic moment. Magnetic field applied alond the antiferromagnetic axis (a-axis) induces spin reorientation (SR) from the a- to the c-axis with partial softening of the AFMR mode [2] that results in crossing of the AFMR and impurity modes during SR at H < Htr (Htr - field of the SR completion, see Fig.). "Impurity" spins coupled with the host matrix also rotate from the c- to the a-axis, so that at the fields H > Htr they are directed strictly opposite the applied field. Thus, after SR complition the relative arrangement of the sublattices magnetisation, impurity spins and magnetic field is quite the same as in the case H ∥ c-axis [1] and in addition, we can investigate the crossing of the ascending AFMR mode with the five descanding "impurity" modes.
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To avoid edge diffraction effects of quasi-optical aperture antennas serving as launchers for helically propagating modes [1] - which can in geometrical optics be approximated as rays forming a circular caustic - the aperture distribution has to be tailored such that the output rays are Gaussian-like distributed along the caustic. This can be achieved by slight deformations of the launcher feed waveguide.
A system of differential equations to optimize the shape of the deformations is derived. These formulas also can be applied for the design of any system containing two reflectors. We present numerical results for practical designs of launchers for rotating higher order volume modes.
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A three-dimensional synthesis technique is applied to the design a quasi-optical antenna system which transforms the output of a gyrotron operating in a whispering-gallery mode (WGM) into a Gaussian free-space beam. The two shaped reflector surfaces of the antenna system are synthesized numerically using a procedure based upon geometrical optics principles. An outline of the synthesis technique together with numerical results for a focused Gaussian output are presented.
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In order to get maximum power transfer from a waveguide mode into the gaussian freespace mode it is necessary to transform a rectangular part of a cylindrical caustic into the caustic of a gaussian beam. The design of a quasi optical antenna is reduced to a caustic transformation problem which can be solved by at least two mirrors. The gaussian beamwidth and the direction of the beam can be choosen as a free parameter.
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Bragg reflectors consisting of periodic cylindrically symmetric corrugations can produce an adjustable reflection up to nearly unity in definite modes through the principles of constructive interference. Window tapering of the corrugation leads to a high mode purity and a ripple free frequency dependence. Bragg reflectors as mode converters have been optimized using the coupled mode theory and a scattering matrix code.
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The use of a diffraction grating arranged in the -1 Littrow mount as an output coupler for a quasi-optical Fabry-Pérot resonator at microwave frequencies (100 GHz) was suggested in 1990 [1]. A planar grating with curvilinear grooves (in order to match the Littrow condition everywhere on the surface for a given Gaussian beam) gives a power coupling efficiency of 85%, which is limited by the depolarization induced by the curvature of the grooves. An ellipsoidal grating with linear, equidistant grooves minimized the depolarization but gave approximately the same global efficiency because of distortion. We report low power tests on an improvement of the second scheme, based on curved grooves on an ellipsoidal surface, which gives a global efficiency of 94%.
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We examine the distortion and cross polarization of a Gaussian beam diffracted from a planar grating with curvilinear grooves or from an elliptical grating with curved grooves placed in the Littrow mount. The electromagnetic field diffracted from the gratings in the 0th order and propagating to an HE11 waveguide has been numerically computed. The output patterns, distortion, and cross polarizatign have been obtained for the gratings; the results agree well with the cold test results. The comparison between the gratings shows that the elliptical grating is more effective than the planar grating.
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Vlasov type high power microwave launchers are studied experimentally and modeled using two dimensional finite element analysis with absorbing boundary conditions. In this paper we consider a combination of two launchers, as well as the effects of choices of material geometry in the near field. Testing is presently underway of several launchers combined into an array.
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For ECH in the large helical device, we are carrying out R&D of millimeter transnission line of the mirror relay system and corrugated waveguide system. For many higher modes to propagate in the waveguide transmission, identification of mode and measurement of mode-conversion coefficients are important for the design of system. As the method of mode-identification, multihole k-spectrometer and measurement of radiation pattern have been used. Though the former can be applied to the waveguide with the smooth wall, it seems to be difficult to measure the propagation mode directly in the corrugated waveguide and is necessary to calibrate sensitivity of mode.
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The main peculiarities of a Free Electron Laser (FEL) [1] are directly related to the physical process with which the coherent radiation is produced. In all free electron devices, the active medium is no more represented by an atomic or molecular system, but by an electron beam (e-beam) which is "pumped" by a structure (a magnetic undulator in the FEL case) which provides synchronism between the e-beam oscillations and the phase velocity of the electromagnetic (e.m.) wave generated in the same structure. The output frequency is therefore not related to fixed quantum level transitions, but to the e-beam energy and to the undulator geometry, and therefore can be quite easily changed making the FEL a tunable source. Also the saturated output peak and average power can in principle be very high and not limited by the dissipation capability of an active medium like in conventional lasers.
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The focusing process and ability of a bifilar helical wiggler, which is often used in Raman Frer Electron Lasers, is analysized in detail.
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The Orbitron MASER is a negative mass unstable device, which can be used to produce millimeter and submillimeter RF radiation. In its basic design, the Orbitron is a coaxial structure with a high positive potential between the outer cathode and inner anode wire. Electrons are supplied to this system by either an internal glow discharge or by an oxide coated hot electron emitter. These electrons, due to an initial angular velocity, go into orbit around the positive central wire and emit a frequency that is inversely proportional to the radius of their orbits. Emission, from the pulsed form of this device, has been observed at a frequency of 1 THz.
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A nonlinear self-consistent frequency domain analysis of the orotron is presented. The problem is separated into a linear and a nonlinear part. In the frequency domain, the cavity and load are described by a linear admittance, whereas the behaviour of the electron beam under the influence of the e.m. field is described by a nonlinear admittance, which depends on both frequency and amplitude of the e.m. field. An operating point is found when the load admittance equals the beam admittance. The shape of the device line, i.e. the locus of the beam admittance in the complex plane, gives useful informations about the optimum system parameters. To check the analysis, the results have been compared with time domain calculations.
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The grating-coupled oscillator (GCO) as configured at Dartmouth has produced coherent, tunable radiation over the wavelength range of 1 cm to 400 microns. This device utilizes a periodic metal grating in conjunction with a mildly relativistic electron beam (15- 130 keV) to produce Smith-Purcell radiation. Confocal mirrors placed at each end of the grating structure form a resonant cavity that allows for the growth of the radiation field to occur, thus enhancing the radiative process. In the past, a TM interaction model has been used to determine operational aspects of the device such as dispersion, electronic tuning, and start oscillation current. Experimental results have been compared in detail to theoretical predictions and excellent agreement has been found. Correlating the TM formalism with the dipole radiation viewpoint of Smith and Purcell has resulted in a deeper understanding of how these devices operate. The Smith-Purcell model suggests that the use of new resonator designs may result in the extension of the wavelength of the device to the 10 micron region.
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The FELIX project (Free Electron Laser for Infrared eXperiments) involves the construction and operation of a rapidly tunable FEL users facility. The spectral region between 8 and 80 μm is addressed in the first stage of the project.
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Advances made recently in the development of electron beam sources to drive free-electron lasers (FELs), have opened the door to novel generators of radiation for the far infrared. The quality of all such electron beam-based sources ultimately depends upon the characteristics of the electron beam. A key element of this project is the RF electron gun, originally designed to serve as the injector on the highly successful Mark III FEL. This electron gun (See Figure 1) has properties well suited to the development of a variety of unique short pulse, mode-locked devices which can operate from the microwave to the far-infrared. The gun produces macropulses of 1 MeV electrons, which is an ideal energy for the spectral region of interest to us. These macropulses have a duration of five microseconds and are further subdivided into a train of micropulses 1-2 picoseconds in length.
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In resonance SHF-generators with prolonged interaction, the efficiency of the beams interaction with the electro-magnetic field is first of all determined by the intensity of the field En acting on the electors. There are different ways to define this field while studying the subject theoretically. In the one-wave orotron theory [1], the concept of synchronic field is introduced to definite En; it is suppose that En = Eo∙æ, where Eo is the field of a high quality open resonator (OR), and æ the transformation coefficient. In paper [2] æ is defined for an infinite periodic structure (PS) and has a maximum value of 2/Π. The field distribution form is taken into account in the discrete orotron theory [3] where the synchronic field is not introduced.
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As a kind of high power millimeter and submillimeter source, gyrotron and FEL have both been studied thoroughly both experimentally and theoretically. The incorporation of these two, the Cyclotron-Undulator Hybrid FEL (CUHF), however, though seeming promising in its less restrictive requirement on longitudinal beam velocity spread in experimental operation, has not yet been researched sufficiently theoretically, even its mechanism has not yet been convincingly explained.
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Cylindrical microstrip lines, or non-planar microstrip lines, are used in the development of several microwave integrated circuits to be mounted on curved or cylindrical conducting surfaces [1]-[3].
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The finite-difference method is used for the analysis of coupled rectangular waveguides. The formulation, initially used for the analysis of single rectangular waveguides, was extended to include coupled structures. Practical examples are examined and some of the results are compared with those obtained using the mode matching technique.
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The study of microstrip patch antennas on anisotropic and anisotropic dielectric substrates has been made with increasing interest. This is basically due to their low size and light weight.
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This paper presents a study of the propagation characteristics of coupled microstrip lines on ferrimagnetic substrates. The analysis is based on Galerkin's method applied in the Fourier transform domain. Numerical results are found as a function of various structural parameters and of biasing magnetic field.
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A thin layer with its dielectric constant two-dimensionally (2D) periodically distributed can bring many interesting characteristics to a planar guiding structure. In applications, the thickness of this thin layer is very small comparing with the wavelength, and the feature of the whole structure instead of details inside the thin layer is most interested. Therefore, the layer can be considered as a infinitesimal thin film for a simpler and efficient analysis.
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An analysis of losses in the metal-dielectric waveguide formed by the wedge-shaped metallic conductor with the external angle XT and circular dielectric rod located on the wedge is presented. Cross-sectional view of the waveguide is depicted in Fig.1.
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The periodic dielectric structures on which the surface waves propagating at an angle to the gratings have recently been finding increasing applications in optical and millimeter wave integrated circuits. Strong interest in researching this kind of problem appears in the literature. For the oblique incidence case the electromagnetic boundary value problem becomes a three dimensional vector one that supports only hybrid modes and requires the coupling between TE and TM modes. There are two types of methods used to analyze the dispersion characteristics of the dielectric periodic structures. One is coupled-mode approach which expands the fields in the periodic structure in terms of eigen-modes in unperturbed wavegiude and offers relatively simpler analysis procedure with an assumption that the periodic layer acts as only a small perturbation in a planar multilayered medium. This approximation yields good results only if the periodic change is sufficiently small, so that its use may produce erroneous results in many practical cases; the other is Floguet-Bloch expansion method which uses an expansion of fields in terms of Floque space harmonics and the propgation of EM waves along periodic dielectric structures is formulated as a rigorous boundary value problem. The rigorous treatment gives very accurate results. However, it is rather complex and requires time consuming and elaborate high precision computer program to yield accurate quantitative results, it is not economic from viewpoint of engineering.
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In this paper, the external Circuit impedance looking outside from two active devices are first calculated by the technique combining Galerkin method with collocation method. The devices are abreast mounted in a reatangular waveguide cavity. The calculated results are very useful for designing the power combiner.
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There exists a wide class of industrial and economical problems, which solution is related to the multizone scene analysis, that drastically increases the efficacy of identification of the latter, in particular, under intricate observation conditions. The examples of such problems are the search of focal points of leakage in main gas and other product pipelines, proximate analysis of the condition and the type of roadway covering for transport means' control automation and safety, search of people during rescue operations in the mountains, on the sea, etc.
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Absorption measurements of powders of lysozyme-water and crystals of L-alanine have been performed in the 10 to 60 cm-1 spectral region at the National Synchrotron Light Source at Brookhaven National Laboratory.
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The atmospheric windows between the few lines of water vapor in the millimeter wave range have received much attention in the past years. Significant departures from theoritical impact and kinetic laws have been brought to light by experimental observations, which are mostly field measurements. Only few laboratory experiments are available, which require a high sensitivity because of the low absorption coefficients to be measured: about 10-8 cm-1 (4 x 10-3 dB/km) per torr of pure water vapor, and 5 x 10-7 cm-1 (0.2 dB/km) per torr of water vapor in an atmospheric mixture with nitrogen.
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The pressure-broadening of the 3(1,3) - 2(2,0) and 4(1,3) -3(2,1) transitions of water at 183 and 380 GHz respectively, were studied over a wide temperature range. These included broadening by 02, N2, He, and H2 with the temperature dependence between 80K and 600K. Low temperature measurements were taken in a collisionally cooled cell with variable temperature injectors. Results for the temperature dependence of the pressure broadening parameters are compared with theoretical calculations and with experimental values of other researchers.
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A millimetric source is obtained by difference frequency mixing in a nonlinear crystal It allows with a two-beam spectroscopie mounting, investigation of spectral absorption near 2cm-1. The complete mounting is described. The measurement method is detailed.
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Infrared spectroscopy was applied for the investigation of the combined effects of γ-irradiation and heat on poly (vinyl alcohol)(PVA). The samples were exposed in an atmosphere of air at room temperature to doses ranging from 0.5 to 15 Mrads. The obtained results revealed that exposure to γ-radiation produces no considerable changes in the IR spectra of PVA apart from very slight changes in the intensities of the absorption bands. The crystallinity of the irradiated samples was determined by following the induced changes in the absorbances of the crystalline bands.
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It was shown /1,2/ that laser FIR emission has an influence on biological objects, in particular it has the effect on cells growth velocity /2/. Investigation of FIR emission - biological object interaction mechanism needs to carry out experiments on biological macromolecules.
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Of great interest for the researches is the influence of electromagnetic emission on biological objects. This paper describes the effect of far-infrared radiation on the process of growth of A.laidlawii cells as a function of culture age.
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The results of investigations of materials used today as polarized nuclear targets (PNT) such as deuterium ammonia ND3 and stable complexes CrV soluted in 1.2-propilenglycol are presented.
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In recent years, several papers have been published dealing with enclosed or surface-waveguides of elliptical cross section. Examples have included hollow metal waveguide, enclosed waveguide filled with one or more dielectrics, coaxial transmission lines, dielectric surface waveguides, and waveguides employing anisotropic media. Some of these types have been investigated experimentally, and several have been used in practical microwave-frequency applications, such as in feed lines for antenna installations.
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The Istituto di Fisica del Plasma is preparing an Electron Cyclotron Resonance Heating experiment on the plasmas of the FTU tokamak in Frascati (Rome). The total power involved is 1.6 MW at 140 GHz, the transmission lines consisting of oversized corrugated waveguides, with quasi optical mode converters and launching systems.
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The use of dielectric resonators at millimeter wave frequencies has been limited by their almost exclusive use in the fundamental TEOld resonant mode. This leads to the resonator having very small dimensions and a diminishing Q-factor. The use of whispering-gallery mode resonances within a cylindrical dielectric resonator, offers both oversized dimensions and higher values of Q-factor.
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The paper presents the results of theoretical analysis and experimental study of the processes of spatial filtration of the millimeter wave range electromagnetic fields using the properties of the resonant fields and discrete representation of spatial signals.
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As is known, some phenomena in semiconductors, e.g. Faraday (1) and field displacement (2) effects, can be used for the electromagnetic radiation control in the millimeter wavelength range. In waveguide devices based on these effects a highly conductive semiconductor element is placed in the part where the wave electric field is maximum. It causes a sharp mismatch of impedance in the waveguide and great reflection losses.
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The new technique of time-domain analysis of discontinuities in planar transmission line is presented in this paper. The analysis model of system, the concept of transfer function and the technique of FFT are employed.
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