Problems of laser-plasma thruster development for space applications are analyzed. Results of laboratory research
concerning the choice of solid-state laser, operating mode and target material are considered. Characteristics of micro-thruster
with diode pumping are discussed.
The results of the experimental study of UV and IR lasers pumped by electron beams are presented. The accelerators with radially convergent electron beam for pumping gas mixtures under pressure up to 3 atm were used. Laser radiation energies of up to 2 kJ, 200 J, 100 J, 50 J have been obtained at the wavelengths of 308 nm, ~2.8 μm, 250 nm and 1.73 μm, 2.03 μm, correspondingly.
The experimental results of the studies on process of radiation interaction of the pulsed CO2 - laser with polytetrafluoroethylene (PTFE) are presented. A dependence of PTFE transmission factor at λ = 10.6 μm versus temperature in the range -90 - 300°C has been obtained.
Low energy electron beam excitation of dense helium and neon was used for light production in the extreme ultraviolet (XUV) wavelength range. The entire system was completely filled with the working gas avoiding the use of vacuum equipment for light production, propagation and detection. Emission spectra from He and Ne are dominated by the second continua with peak intensities at 80 and 83 nm, respectively. The hydrogen Lyman-α line was observed as the dominant impurity line. This XUV light source was used for transmission measurements of LiF near its absorption edge.
Properties of plasma produced in volume nanosecond high-pressure discharge and its formation conditions under elevated pressure in air, nitrogen, krypton, argon, neon and gas mixtures Ar-N2, Ar-Xe, CO2-N2-He in the gap with the cathode having small curvature radius have been investigated. Time-amplitude characteristics and radiation spectra of plasma in different gases in the range of 230-600 nm were defined. Lasing in the gas mixture Ar-Xe at active length of 1.5 cm has been achieved by excitation through volume nanosecond high-pressure discharge. Comparison of spectral radiation in nitrogen, krypton, argon, and neon under pumping of mentioned gases by volume discharge in non-uniform electric field, by nanosecond e-beam, and by volume pulsed discharge in the uniform electric field with high initial voltage was made. Lasing has also been obtained in the gas mixture CO2-N2-He under pumping by e-beam initiated discharge. The electron beam was formed in a gas diode filled the same gas mixture.
Investigation of the modes of thin metal and ceramic films (0,3-1 μm) removal from the glass and stainless steel substrate by IR and UV laser radiation has been performed. It is presented that the efficient film destruction and removal occurs due to laser induced thermal strength under laser fluxes of ~0.5 J/cm2 not sufficient for phase transformation.
Formation of microscopic size colored dots on titanium foil surface by pulsed IR-laser irradiation was studied. It was found that the dot on titanium surface changes its color from yellow to blue as the exposure is increased. According to our numerical calculation, the colored dot formation is accompanying with titanium surface melting and partial evaporation. At pulse-periodical irradiation mode it can result in foil break-through.
An e-beam pumped Xe-laser is experimentally investigated with additions of molecular gases. For a pump pulse duration from hundreds of nanosecond to 1 μs, the highest laser energies are reached without molecular additions at a comparatively low e-beam current density (~ 10 kW cm-3 atm-1). However, in setups with specific pump powers above 40 kW cm-3 atm-1 and a working mixture pressure limited by strength of the laser chamber, molecular additions result in an increase in the radiation energy and efficiency. In wide-aperture facilities with high pump powers, molecular additions improve the distribution of the radiation power density over the laser-beam cross section.
The interaction of pulsed periodical CO2-(λ~10.6 µm), and Xe-(λ~1.73 μm) lasers radiation with surfaces of polymers has been studied. Correlation between parameters of surface erosion (area of crater and amount of evaporated material versus laser focus position and number of pulses) was investigated using teflon (polytetrafluoroethylene-PTFE) and vinypros (plasticized semi-transparent polyvinylchloride) sample. Monitoring of erosion track on surfaces was performed through optical microscopy. It has been shown that at pulsed periodical CO2-radiation interaction with teflon and vinypros the sputtering of polymers with formation of submicron-size particles occurs. Dependencies of particle sizes, form and sputtering velocity on laser pulse duration and target temperature have been obtained.
The interaction of Xe- (λ~1.73 µm) and XeCl- (0.308 μm) lasers radiation with surfaces of metal and TiN-ceramic coatings on glass and steel substrates has been studied. Correlation between parameters of surface erosion (area of crater and amount of evaporated material versus laser focus position and number of pulses) was investigated. Monitoring of laser induced erosion on smooth polished surfaces was performed using optical microscopy. The correlation has been revealed between characteristic zones of thin coatings damaged by irradiation and energy distribution over laser beam cross section allowing evaluation of defects and adhesion of coatings.
Presently, from the world data available on interaction of laser radiation with metal and dielectric surfaces and development of experimental diagnostic techniques by itself it is possible to raise a reverse question, namely, restoration of laser radiation energy spatial distribution through surface imprint. With this in mind, we have made the detailed morphology of imprint of a pulse HF-laser interaction with carbon steel surface through atomic force microscope. Interaction of the radiation of a relatively short pulse of ArXe-laser ((tau) ½equals400 ns, (lambda) equals1,73 micrometers , Q ~ 20 J/cm+2)) with carbon steel surface of 40X (4140 (USA)) type besides the surface layer hardening leaded to decrease of steel hardness at below original value on depth of about 10 - 12 micrometers . This effect can be related to occurrence of deformation and thermal residual stresses in the subsurface layer. At multiple-pulse interaction (~500 pulses) of UV-laser focused radiation ((lambda) equals0,222 or 0,308 micrometers , (tau) ½equals 12 or 20 ns) with the steel 40X (4140 (USA)) surface a distinct interference imprint of laser beam is formed after diffraction at outer diaphragm being determined by its form.
The complex of experimental installations for studying of laser radiation interaction with surface of metals has been established. At titanium surface irradiating depending on the accumulated laser radiation energy, the surface color might be changed from bright yellow to red and deep-blue colors. The presented results testify to the possibility to use the change of titanium surface color at heating by laser irradiation in the open air to obtain dot raster images. Presently, form the world data available on interaction of laser radiation with metal and dielectric surfaces and development of experimental diagnostics techniques by itself it is allowed to raise a reverse question, i.e., restoration of laser radiation energy spatial distribution through surface imprint. Using imprints, it is also possible to make an express diagnostics of multi-layer surface coatings. With this in mind, we have made the detailed morphology of imprint of the pulsed HF-laser interaction with carbon steel surface through atomic force microscope.
Experimental study of Xe and Kr radiation spectrum temporal dynamics under pumping by nanosecond e-beam have been carried out for the purpose to determine the dependence of UV radiation efficiency of molecular-ion continua (MIC) on pressure. The results obtained allow to draw a conclusion, that with increase of gas pressure it does happen the change of the main radiating molecules from double-ionized to single ionized ones, and the efficiency of MIC radiation in inert gases decreases. To obtain the laser oscillation at MIC of inert gases is more probable at low pressures, when absorption in active medium is small and MIC radiation efficiency is maximal. It is necessary to use a selective narrow-band resonator in this case. From the point of view of efficient use of e-beam energy the optimal excitation geometry is the longitudinal relatively to optical axis.
The results of an experimental study on the temporal evolution of molecular-ion continua of rare gases using electron- and ion beam pumping are presented. The temporal evolution of radiation bands, which make up the molecular-ion continuum (MIC) has been measured. A hypothesis for a relaxation cascade of excitation energy transfer from doubly charged ions (Rg++n) to singly charged ions (Rg+n) has been analyzed. Results obtained using electron-beam pumping and a comparison with data obtained by other pumping methods allows us to draw the conclusion that for MIC in rare gases, the local maxima's wavelengths do practically not depend on pumping power and pumping method. The ratio of intensities in different maxima during plasma relaxation, however, and the behavior of each maximum with respect to pressure variations do depend on specific excitation power.
The data on optimization of output parameters (output energy and laser efficiency) and distribution of laser radiation energy on the output window cross-section of the wide- aperture Xe-laser pumped by a radially-convergent e-beam with pulse duration of approximately 0,5 microsecond(s) at the FWHM were obtained. It is shown experimentally, that the requirement of homogeneity of output laser radiation across the aperture limits the energy deposition value from below, and the requirement of maximal laser efficiency restricts the energy deposition value from above. In the experiment the maximal laser efficiency as 1,5 - 2% was obtained at the specific energy deposition of 12 - 14 mJ./cm3atm. The calculations show that with cleaning of the operating mixture from molecular gases impurities the laser efficiency and energy output can be increased in 1.5 - 2 times. The negative influence of impurity gases at optimum range of pumping power is decreasing of electron temperature and as a consequence decreasing of the upper laser level pumping.
The results of investigation into the use of UV-laser produced plasma for high-current commutation are summarized. It is shown that exciplex lasers can be used as efficient triggers for megavolt gas switches. If the laser spark length is comparable with the switch gap, the triggering delay time and jitter are independent on the arc position in the gap. In other cases, the shortest delay time was observed under focusing on the electrode surface. The plasma formed by exciplex lasers on the metal targets has been studied. The plasma formation thresholds and plasma expansion velocities have been measured.
The fluorescence of dense rare gas targets has been produced and analyzed using X-ray flash, e-beam and ion beam as excitation techniques. For the first time to our knowledge an extended database of argon and krypton molecular UV emissions, i.e. the Molecular Ion Continua, has been obtained with a very good agreement between spectra whatever the pumping technics is used. The estimation of potential surface for both singly and doubly charged argon trimers and the consideration of previous ab initio calculation for the argon dimer enables us to confirm some former hypothesis but also to suggest new pathways likely to be at the origin of the population of the upper states radiating in the Molecular Ion Continua.
The results of experimental study of Ne emission in the wavelength range of 200 - 600 nm under excitation by electric discharge and short-duration e-beam are presented. It was found that temporary behavior of the emission intensity distinguished in different spectral regions. The nature of the radiation in the ranges of 120 - 300 nm and of 300 - 500 nm was suggested due to transitions of molecular ions and those from Rydberg states Ne2R, respectively.
Time correlated optical emission spectra of argon, krypton and xenon in the wavelength range from 110 to 450 nm, and 0 to 6000 ns time interval, recorded at the Munich Tandem accelerator using heavy ion beam excitation with 2 ns beam pulses, were measured in order to clarify the origin of the so called third rare gas excimer continua. Experiments were performed at xenon and krypton pressures between 50 and 250 mbar, and argon pressures between 230 and 1500 mbar. All spectra clearly show different distinct peaks, emerging at different time delays after excitation. These spectral maxima are interpreted as arising from excimer emissions by separate radiating species, formed by gas kinetic processes. While the spectral shape of the components, forming the third continuum radiation in the heavy rare gases krypton and xenon turned out to be of complicate structure, in the case of argon all wavelength spectra could be reproduced by fitting a limited number of Gaussian functions with fixed center wavelengths and fixed widths to the data. Hereby, six distinct maxima, appearing at four different times after the excitation pulse, could be identified.
The results of experiments on measurement of broadband UV radiation efficiency in Xe and Kr pumped by nanosecond e- beam are presented. The values of efficiency of the 3-rd continuum in Kr and Xe, calculated on the basis of experimental data, do not exceed 0.25% in gas pressure range 0.25 - 2.5 atm. It indicates that the main part of energy of a band is radiated by single-ionized molecular ions Rgn+* (where n equals 2.3). The influence of ions of a type Rgn++ is more pronounced at lower pressure p < 0.5 atm, where the efficiency of broadband radiation in Kr increases.
The results of the experimental study of UV and IR lasers pumped by various methods are presented. The accelerators with radially converging or planar e-beams pumping gas mixtures at pressures up to 3 atm and self-sustained discharges were used. The highest laser radiation energies in the UV up to 2 kJ have been obtained at (lambda) equals 308 nm. Output of 100 J at (lambda) equals 1.73 micrometer in Ar-Xe mixture and 50 J at (lambda) equals 2.03 micrometer in He-Ar-Xe mixture was obtained from the e-beam laser with active volume of 600 l. Output energies of 110 J at (lambda) equals 308 nm and 90 J at (lambda) approximately 250 nm, respectively, were achieved in compact high-power e-beam laser with chamber of 20 cm in diameter and 30 l active volume. This e-beam geometry was shown to be very promising for excitation of non-chain HF- laser media. HF-laser efficiency with respect to deposited energy as high as approximately 10% and energy up to 200 J at (lambda) approximately 2.8 micrometer were demonstrated. Amplification of XeCl-laser beam from master oscillators and amplifiers under conditions of strong amplified spontaneous emission is considered. Investigations of CO2 laser excited by e-beam controlled discharge and e-beam ignited discharge were performed. The highest laser output in the IR of 3 kJ have been obtained at (lambda) equals 10.6 micrometer.
KEYWORDS: Electron beams, Xenon, Energy efficiency, Laser energy, Diodes, Infrared lasers, Digital signal processing, Signal processing, Pulse generators, Chemical species
An experimental investigation was made of the parameters of lasing as a result of atomic transitions in xenon observed when Ar-Xe and He-Ar-Xe mixtures were excited by an electron beam in a laser chamber with an active volume of approximately 600 liters. Electron beam from six double acceleration diodes were injected axially symmetric into this chamber. This ensured a more efficient utilization of the pump energy and a uniform distribution of the laser radiation energy over a transverse exit aperture in the maximum specific input energies. An output radiation energy of the order of 100 J was generated with an efficiency of about 2% at lambda equals 1.73 micrometer and of the order of 50 J with an efficiency of about 1% at lambda equals 2.03 micrometer. An analysis was made of the experimental results and of criteria for the optimization of the pump parameters in order to improve the output characteristics of a wide-aperture laser operating on the basis of atomic transitions in xenon.
Results of the experimental study of powerful lasers pumped by a radially convergent electron beam and powerful excilamps are presented. Laser radiation energies of 110, 90, 100, and 50 have been obtained at (lambda) equals 308, 249, 1730 and 2030 nm, respectively. Excilamp average power of 130 W have been obtained at (lambda) approximately 222 and 208 nm, the efficiency was up to 14%.
A great number o recent papers were devoted to the investigation or a Penning plasma laser under the conditions or electron-beam and nuclear puinpin,g and to the experimental and theoretical investigations or plasmochemical and radiation processes in the active medium (see , tor example , [1 1 ) . But the question about maximal erriciency o the laser is opened yet and this is the main purpose or the paper.
E. Abdullin, Alexander Bugaev, S. Gorbachev, Anatoly Efremov, Boris Koval'chuk, Nikolai Koval, Sergey Loginov, Mikhail Lomaev, Victor Skakun, Victor Tarasenko, V. Tolkachev, Andrei Fedenev, E. Fomin, P. Shchanin
The results of an experimental study of the laser action on XeCl*, KrF*, and N2 molecules and by the atomic transitions of xenon and neon are presented. Laser radiation energies of 110, 90, 3, 100, 50, and 0.5 J have been obtained at (lambda) equals 308, 249, 358, 1730, 2030, and 585 nm, respectively. The experiments were carried out on two accelerators producing radially convergent electron beams, that pumped laser chambers 20 mm in diameter with a beam current of FWHM duration 0.3, 30, and 100 microsecond(s) .
An experimental apparatus for studying laser action in metal vapors and gaseous media pumped by e-beams from compact accelerators at 10 - 750 degree(s)C is described. The results obtained from He-Cd, He-Zn, He-NF3, He-Ne-Ar, and Ne-H2 mixtures are reported. The requirements to a high-efficiency metal vapor laser medium pumped by e-beam are examined.
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