We present data related to the repetitive pulse performance of Reltron HPM tubes. While the tubes using thermionic cathodes are usually limited only by the power supply and/or cooling system, the repetition rate for the high-peak-power tubes (which use explosive emission cathodes) is limited by the rise in the vacuum pressure. In particular, if the tube pressure significantly exceeds 10^-4 torr, the tube fails to operate properly, and the voltage pulses are significantly degraded. We have developed a simple model to describe the phenomenology. We have also begun to explore alternate cathode materials which evolve less gas than velvet. A CsI-coated graphite-fiber cathode appears most promising at present.

A model to account for the shortening of the relativistic magnetron microwave pulse length based on radial cathode plasma motion is put forth and corroborated with measurements. If the dense, conducting, cathode plasma (and not the cathode itself) sets the diode electrostatic and microwave boundary conditions, its motion changes the magnetron resonance condition. Time varying microwave power envelopes and frequencies are both expected and observed. Based on the time scales, the main plasma ion constituent is inferred to be hydrogen and its source is likely adsorbed water. Results of an experiment to defeat the pulse- shortening mechanism are presented and compared to the results from similar experiments conducted by other researchers.

The problem of pulse shortening was raised at this conference two years ago, and in the intervening time, efforts to address the problem have been initiated. There is already some significant progress at our laboratory and at some others. This paper reviews the progress and describes the directions of research and industrial efforts underway at this time. The subject is an exciting one in view of the potential applications that would benefit from the extension of pulse length in Gigawatt class microwave tubes. The paper discusses some of the implications of success in this regard.

The Hard-Tube MILO (Magnetically Insulated transmission Line Oscillator) is a gigawatt-class L-band high power microwave tube driven by a 500 kV, 60 kA electron beam. It is nearly identical to the MILO reported by Calico et al., with the principle difference being that the Hard-Tube MILO has been constructed using conventional-tube brazing techniques for the rf joints, while the earlier version of MILO used finger-stock connections for the rf joints. This paper reports on recent experimental improvements to the tube that have allowed us to generate 1.5 - 2.0 GW pulses of 175 ns duration; a 2.5 times improvement in the pulse width over the original tube. In addition, we report on experiments to identify the breakdown mechanism in the vacuum radome for the Vlasov antenna used to radiate the microwave pulse. Finally, details of an optimized version of the Hard-Tube MILO that should allow us to generate over 3.0 GW are presented.

Recent theoretical studies have shown that presence of plasma in the high power microwave sources, when properly introduced, may enhance the efficiency of the source. These findings have been confirmed by numerical simulations for some high power microwave sources. The enhancement has been partly attributed to the neutralizing effect of the positively charged plasma on lowering the disruptive space charge effects of the electron beam used to generate the rf. This attribute allows the se of higher current beams for more power. Another favorable effect of the presence of plasma might be a modification of the dispersion relation leading to higher group velocities for the travelling rf waves. This aspect of the plasma presence might help in designing more efficient rf extraction mechanisms. Mode selection may also be favorably affected by the presence of plasma in the source. Here we present the results of numerical simulations carried out to study the effect of plasma on a gyrotron device. Both neutral as well as positively charged plasmas have been explored. The gyrotron is run in TE₀₁ mode and generates a 13 GHz steady oscillation without any competing modes. The effect of the plasma at several different densities has been explored. The presence of plasma at the densities explored do not show any significant enhancement of the gyrotron operation. Other plasma densities are being studied. The PIC codes used in simulations were the MRC 21/2 dimensional code MAGIC and the Phillips Laboratory 3-D code ICEPIC.

Diagnostic experiments and operation of multi-megawatt gyrotrons utilizing rectangular cross-section resonant cavities are currently under investigation. The goal of the experiments is to achieve gyrotron generation of high power- long pulse microwaves. A rectangular cross-section resonant cavity has the advantage that it is a linearly polarized source of microwaves. The output microwaves have a high degree of polarization control by changing the magnetic field in the interaction region. Diagnostics include cold tests of the microwave cavity, heterodyne mixer measurements of operating frequencies, and beam alpha (Vperp/Vparallel)/spatial distribution measurements using glass witness plates. Operation at microwave powers greater than 10 MW and high polarization power ratios (vertical/horizontal) greater than 100 have been observed in experiments. The electron beam was produced by MELBA (Michigan electron long beam accelerator) with the following parameters: -0.8 MV, 1 - 10 kA diode current, 0.5 - 1.0 microsecond pulse length.

The generation of Ultra-Wide Band Pulses nanoseconds is a challenging problem that involves generating pulses with 100 pico-second rise times and voltage of 500 kV with pulse widths of the order of less than one to a few nanoseconds. A critical step involves switching high voltages with precision. The use of both gas and oil for the switching insulating medium have been accomplished with varying results. The Phillips Laboratory is pursuing both media in the gas switched Hindenburg series of pulsers and in the study of oil switches that promise good performance in compact packages. This paper reports on progress in gas switching associated with the new H-5 pulser and with the use of earlier Hindenburg pulsers to investigate the UWB properties of oil switches. We compare the design strategies and techniques of oil and gas switching in the context of pulsers of interest.

The Air Force Phillips Laboratory, in collaboration with the Army Research Laboratory, is developing lateral geometry, high-power photoconductive semiconductor switches (PCSS) for use in phased-array, ultra-wideband sources. The current switch utilizes an opposed contact geometry with a 0.25 cm gap spacing and is an extension of previous work on 1.0 cm PCSS devices. This work presents the development and demonstration of the 0.25 cm PCSS under both ideal laboratory conditions and potential source conditions. The laboratory configuration consists of two high-bandwidth transmission lines connected with a PCSS. The potential source configuration consists of a vector-inversion pulse generator (Blumlein) commuted with a PCSS. The 0.25 cm PCSS is shown to operate at 20 kV charge voltage, 65 ps rms switching jitter, less than 450 ps risetime and greater than 1 kHz pulse repetition rate when triggered using a compact, high-power laser diode.

An ultra-wideband, nonperturbing, electric-field sensor is being developed that uses the linear electro-optic effect and is packaged in a suitcase-sized optical configuration. The methodology has been demonstrated on the optical bench with ZnTe and used to measure an applied electric field. The immediate goal is to demonstrate the sensor up to 5 GHz and apply it to unknown fields in a configuration that uses DAST crystals, which are significantly more sensitive than ZnTe. This sensor eventually will be applied to the measurement of electromagnetic pulses, preserving the amplitude, polarization, and phase content of the detected signal. Preliminary measurements reported here verify the crystal sensitivity and response linearity. Variations in optical configurations are compared on the basis of sensitivity.

A Mach-Zehnder interferometer designed as an electric field transducer operates without metal electrodes by incorporating a novel substrate configuration. The lithium niobate device uses reverse poling of one of the interferometer arms, which provides opposing optical phase changes in the two interferometer arms when placed in an electric field. The fabricated devices exhibit a measured minimum detectable field of 0.22 V/m(root)Hz and frequency response greater than 6 GHz. Theoretical calculations show that fields in excess of 330 kV/m can be detected before appreciable distortion occurs.

The process of virtual cathode formation in a gap is critically examined via particle simulations. It is found that the limiting current obtained from the electrostatic approximation is valid only in the deeply non-relativistic regime. When the injection energy reaches 30 keV, the transients in the injected current may produce an inductive voltage that can significantly lower the limiting current from the classical, electrostatic value. The self magnetic field is unimportant, however. At mildly relativistic energy (250 keV, the self magnetic field becomes very important in the determination of the limiting current. The inclusion of a strong external magnetic field along the mean flow reveals two quasi-steady states, whose presence is unique to the high voltage regime. Implications of this study on simulation codes are explored.

The formulation is given for the recoil force on the linear dynamic polarization current of a relativistic test particle participating in collective bremsstrahlung in a nonequilibrium beam-plasma system. Possible contributions arise from terms involving interaction of the total field of the test particle with its own induced linear dynamic polarization.

In this paper, a physical description of the mechanism of microwave generation in Super-Reltron is presented, and the analytical formulas of the beam bunching in the modulation cavity and the prebunched beam emitting in the radiation cavity are derived.

Cylindrical cavity with large apertures on its sidewall for coupling with rectangular waveguides is often used in frequency or phase-locking vircators. In this paper the above structure is studied in detail. The fields in the cavity is properly expanded by derived mode functions. By making reasonable approximation of the boundary conditions, the process of derivation and the formation of the characteristic equation is greatly simplified. The characteristic equation and model coefficients are obtained. The reliability of the derived characteristic equation is tested. Numerical results of characteristic frequency and quality factor changing with dimensions of coupling holes are given.

This paper describes progress in a cooperative research program of national scale that is focused on crucial research issues in the development of high energy microwave sources. These have many applications in the DOD and industry. The program includes participation by and cooperation among University, Industrial and Government research laboratories.

Conventional CW millimeter amplifiers (coupled-cavity TWTs or gyro-klystrons) are limited in power by the maximum current hat can be accommodated in a single beam. Cathode current density, beam optics, and the magnetic field necessary to confine the beam, combine to limit beam current and add cost and bulk to the device. If the microwave source is designed as a pulsed klystron operating at a high voltage, larger lateral as well as axial dimensions can be employed. Beam optics become easier and permanent magnet periodic focusing is possible. A higher efficiency also results, because of the low perveance. A number of klystrons can then be fabricated on single substrate, using a deep- etch lithography technique. They can be water-cooled individually, and operated in parallel. Several such modules can be stacked to form a klystron `brick,' requiring a relatively low voltage for the peak and average power produced. The `brick' can be provided with a single output, or with individual, spatially-combined radiators. The design of a 4 X 10 X 1.5-inch module producing 500 kW peak, 500 W average at 91 GHz, and operating at 120 kV, 10 A, will be described.

A study of the efficiency of a coaxial virtual cathode oscillator is presented. The coaxial geometry has many physical parameters that can be changed to alter performance. The parameters of interest include the placement of a ring cut in the anode base and the polarity of the system. The ring creates a decelerating field for the electrons and tends to keep them in the right-phased region of the virtual cathode. The ring is varied in width and in position from the center line with the results normalized to the no ring geometry. The results for a positively and negatively pulsed system are also given. Comparisons of frequency, efficiency, and particle dynamics of the positively and negatively pulsed systems are given. MAGIC, a 2-1/2 dimensional particle-in-cell code, and SOS, a 3 dimensional particle-in-cell code, are used to simulate the different geometries.

A simple circuit model is used to investigate the transit time oscillator driven by a high current diode. A novel condition for the onset of oscillation is derived in terms of the diode impedance. The initial growth is calculated, and the saturation level is numerically computed using the 1D model. The 1D results are in excellent agreement with a full scale 2D particle-in-cell simulation. The success of the much simpler 1D model allows a close examination of the roles played by the convection current and by the displacement current, as well as the modification in the transit time due to the intense space charge within the gap.

The cyclotron resonance maser (CRM) array was proposed recently by our group as a compact high-power microwave source operating at low-voltages. In this paper, we present a CRM array experiment with two electron beams, in a 2D array periodic-waveguide. Spectral measurements are displayed for fast- and slow-wave interactions. This two- beam experiment leads to the construction of a multi-beam CRM array at Tel Aviv University.

One of the ways to increase the generated pulses power of the microwave emission in virtual cathode systems is to make use of several devices simultaneously. To solve this problem it is necessary to investigate the virtual cathode system operation in the regime of the amplification of the electromagnetic wave from an external signal. Results of the investigation of the virtual cathode operation in presence of the external high-frequency signal are given in this work. The phase locking conditions of the high-power virtual cathode triode for the external signal have been determined. The phase locking band dependence on system and electron beam parameters have been established.

Plasma motion in TWT-like microwave devices with slow wave structures filled by plasma (hybrid structures) is considered. The reason of plasma motion is high frequency pressure force caused by microwave excited by electron beam. Redistribution of a plasma density changes microwave excitation conditions and the self-consistent dynamics of plasma and excited microwaves appears in this way. This connection between plasma and microwave amplitude distributions can be a reason of power limitation. If the beam current is large enough, nonstationary regime of beam- wave interaction arises. Microwave generation in this case has the form of short pulses sequence. Conditions of nonstationary regimes of microwave radiation and its connection with hybrid structures properties were obtained. Influence of self-consistent plasma motion on TWT-device spectral and energy characteristics is investigated.

New mechanisms of plasma bunch acceleration and short wave amplification are proposed. This mechanisms are based on the possibility of effective interaction of charged particle with a field of an intensive transverse electromagnetic wave. This interaction exists despite of both the absence of a resonant interaction of the particle and the wave and the absence of high frequency pressure forces. The possibility of the interaction results from the rigorous solution of the problem of movement of a charged particle in electromagnetic field with an arbitrary strength. It is shown, that this interaction can be used both for effective acceleration of plasma bunches and for electromagnetic wave acceleration. The strength of longitudinal electric field, that accelerates plasma ions can be more than the intensity of the transverse field of accelerating electromagnetic wave.

The solution of the problem of radiation of a oscillator that moves with an arbitrary velocity through the medium with a periodical heterogeneity (crystal) is presented in the report. There is general formula that describes the power of radiation of the oscillator. Both longitudinal and transverse fields of the charge have been taken into the consideration. So, these formula described the radiation both relativistic and non-relativistic oscillators. It was found that the non-relativistic oscillators can effectively radiate high numbers of harmonics. The importance of this result is determined by the fact that it is easy to form the ensemble of non-relativistic oscillators. Moreover, the density of this ensemble may be done much high than the density of relativistic and restricted by a solid density only. The presence of such the radiation opens the new approach to the problem of the short wavelength radiation excitation (non-relativistic one).

Basing on energetic processes studying in the near-field radiator zone, a new concept of antenna synthesizing for ultrawideband electromagnetic pulse radiation has been suggested. The results of experimental investigations of the antennae developed with using of this concept for high-power applications are presented. The antennae have small dimensions, high electrical strength, cardioid pattern with linear polarization of the pulse radiated and they are ideally adapted to be used as a steering antenna array element. A high-voltage nanosecond bipolar pulse generator design to excite antennae is described.

The nonlinear susceptibilities in the millimeter range are defined in this paper. In this range, except the field of lattice phonons, the main source of the nonlinearity is the interaction of radiation with the gas of free carriers. The external electric field of an incident wave E causes the variation of the carriers distribution, so their mean energy, i.e. the carriers temperature, changes: T_e equals T_e(E). This leads to the change of the relaxation time (tau) ((tau) equals (tau) (T_e)), and, as a consequence, to the variation of the dielectric constant E. Thus, the dielectric constant depends on the amplitude of electric field of the wave and the nonlinear susceptibilities may be calculated by dielectric constant expansion in terms of the field degrees. The nonlinear susceptibilities are calculated accounting for the main processes of the carriers pulse relaxation for GaAs and InSb. The spectral and the temperature dependencies of the susceptibilities are obtained.

The experimental and theoretical results of coaxial diode and the theoretical results of coaxial vircator are presented in this paper. The cathode is a cold, field- emitting graphite ring and needle-shaped copper applied to a grounded cylinder. The anode is a semi-transparent cylinder located inside of, and concentric to the cathode cylinder. The anode cylinder is pulsed positive. The coaxial vircator generates microwave by injecting a radial electron beam into cylinder such that the space-charge limited current is exceeded. A virtual cathode forms and oscillates in radial position and amplitude, generating microwaves which are extracted by an attached waveguide with a circular cross- section. Analytic and PIC simulations were used to study coaxial diode and vircator, with aid of the two dimensional PIC code, KARAT. The comparisons between the theoretical and the experimental results for a coaxial diode are presented.

The results of the breakdown of microwave pulses in air have been described in this paper. The experimental study has been done in a waveguide, with the frequency of 9.37 GHz, and the peak power up to 200 kW, pulse width from 0.3 microsecond(s) to 2.0 microsecond(s) , single shot and repetition rate up to 970 Hz, and the processes of the breakdown of repetitive pulse were also recorded for ten pulses. The analytic methods for threshold power density are presented also. The comparisons between the theoretical and the experimental results for the breakdown threshold are presented, and in good agreement.

A single-pulse high power microwave detector was described in this paper, which was developed based on hot carriers effect in P-type semiconductors for measuring the power of high power microwave. The detector consists of P-type silicon as sensor unit and standard wave guide and is characterized by its capability of undertaking much higher power of microwave (about 6 orders higher than ordinary crystal detectors), fast response (response time is less than 1.7 ns), good linearity and large amplitude of output pulse, and can offer 3.0 GHz to 20 GHz performance with various waveguide. Also included in this paper are the calibration procedure of the detectors and results. The detector can be proved to be an efficient technical approach to solve the problems in measurements of high power microwaves.

The forming of accelerating voltage and current pulses with the given parameters is one of the most significant problems for creating the high-efficiency generators founded on a virtual cathode triode. The experiments have shown, that the nonlinear elements of an electrical circuit influence on the growing slope, amplitude and pulse length. One of nonlinear elements is the radiating triode. The results of theoretical investigations of the emission influence on the virtual cathode impedance and the forming of voltage and current pulses in the circuit with a inductive energy accumulator and exploding wires have been presented in this work. A model has been elaborated, including the influence of the electromagnetic radiation emission on the impedance of virtual cathode triode. The numerical simulation of the transient processes in the electrical circuit has been carried out. The dependences of the voltage and current pulses parameters, the emission efficiency, beam geometry and resonance chamber have been received.

In this report we present results of synthesis and experimental investigations of multigenerator coherent microwave systems based on symmetrical and nonsymmetrical schemes.

An original and physically real scheme (circuit) is considered in present report. This scheme consists of two ensembles of generators coupled with each other and with common load according to a nonsymmetrical scheme. This report has shown that the multi-pole circuit satisfies some resonance relation, for which favorable and unfavorable interactions between generators take place simultaneously. Some mathematical aspects of the steady-state problem are briefly discussed. We have obtained eight roots of characteristic equation. Analysis of this roots exposed unique properties of the system and role of nonsymmetry. It allowed to propose the practical algorithms of realized of the steady-state operation.

The results of experimental investigations of the system of two strongly coupled relativistic magnetrons with power supply from two independing linear induction accelerators which are continuing at the Institute of Nuclear Physics are reported in the present paper. Two different types of connecting lines (symmetrical and non-symmetrical) between magnetrons were used. The results of the first experiments were presented at the SPIE conference in 1995. Authors investigated symmetrical system of two strongly coupled relativistic magnetrons with power addition in the common load. Net output power of such a system obtained experimentally was 1.9 times higher than that of a single magnetron. In the system of two strongly coupled relativistic magnetrons with power subtraction in the common load the output, power was 25% against the power of a single magnetron. In the present report we discuss the results of experimental investigations of symmetrical and non- symmetrical systems of two relativistic magnetrons with the significant detuning of intrinsic frequencies, and results of power addition and subtraction of magnetron signals in the common load and free space.

This paper is dedicated to the investigation of electromagnetic fields composition of an electron beam propagating through different electrodynamics superdimensional structures--the drift tube, the corrugate waveguide, the waveguide partially filled by a dielectric. Excitation equations for radiation and wake fields are written in a matrix form. The directed orthogonalization algorithm with semiinversion may be used to solving that matrix equations. The example of a finite-dimensional electron ring-bunch which moving through the superdimensional drift tube and the waveguide filled by a dielectric is discussed. The energy stored in different parts of the electromagnetic field is calculated. Pictures of lines of force, longitudinal and radial distributions of electromagnetic fields components are presented.

High-power Cherenkov type microwave device based on the unmatched dielectrical liner (maser) is studied by the methods of the effective linear theory. All theoretical investigations of amplification and generation of microwaves are made by the model of the well-known experimental device. Electrodynamical properties of using a slow-wave structure are calculated in the absence and in the presence of an electron beam inside it. It is shown that the existence of mismatches at the ends of the structure converts it into a passing resonator. A device based on the latter has corresponding resonance properties which determine the discrete generation spectrum and its starting conditions. As a result generation band of the device consists of a series of separate generation lines. The calculated bandwidth is compared with its experimental value. The process of generation band tunability is studied too. In conclusion some features of the considered device are discussed in detail.

A two-sectional slow-wave structure drift-tube influence on the operation regime of a three-centimeter-wavelength range multiwave Cerenkov generator has been investigated both experimentally and theoretically. Stable and powerful generation regimes have been found to correspond to the drift tube lengths at which the starting conditions calculated from a linear theory are realized only for one longitudinal electron-electromagnetic resonance. An important condition of stable generation regimes with a narrow radiation spectrum is an optimal correlation between a magnetic field and a gap between the electronbeam and the slow-wave structure. It has been shown theoretically by means of a nonlinear code that synchronization of the nearest longitudinal resonances of TM₀₁-mode by electron beam results in both increase and decrease of generation efficiency compared to the interaction of the beam and field of one longitudinal resonance.

The operating principles and basic design equations of the relativistic two-stream amplifier (RTSA) are presented. A dispersion relation is obtained for two relativistic electron beams interacting with a slow wave structure for RF power extraction. A conceptual design is presented for the development of a high-power X-band RTSA.

We use the cold-fluid plasma equations to consider the nonlinear effects of a strong, relativistic RF electric field (with a frequency, w, and a wavevector, k) which is propagating on the background electron density profile in a relativistic crossed-field, electron vacuum device. Earlier, we had shown that in the nonrelativistic case, when k and w are such that a wave-particle resonance, w equals v_dk, can occur at the edge of a Brillouin sheath, then the Brillouin sheath becomes strongly unstable to a Rayleigh instability, with the instability being driven by the strong negative density gradient at the edge of the Brillouin sheath. As a consequence of this instability, the average DC density profile becomes strongly modified and is driven away from the classical Brillouin flow by the RF field, and is driven toward stationary solutions of a nonlinear diffusion equation. From this nonlinear diffusion equation, one can predict the DC current flow through a device and also can predict the shape of the stationary DC electron density profile. Also we have demonstrated that such stationary solutions do exist and can be calculated. Further, we showed that when one combined these stationary solutions with the RF field solutions, then the total solution would generate the standard spoke structure, long seen in numerical simulations. Here, we shall extend these calculations into the relativistic regime and discuss their form.