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Dependence of threshold capacity of breakdown on pressure of gas in various operating modes of the generator of microwave-radiation with jumps of a phase


V.I. Karas, I.V. Karas, A.G. Zagorodny, V.I. Zasenko, I.F. Potapenko, A.N. Starostin

New types of beam–plasma generators of intense stochastic microwave radiation were developed and put into operation at the National Science Center Kharkov Institute of Physics and Technology (Ukraine). In the present paper, we have discussed the results of theoretical and experimental studies and numerical simulations of the normal and oblique incidence of linearly polarized electromagnetic waves on an interface between a vacuum and an overcritical plasma. The main results of our investigations are as follows: (i) for the parameter values under consideration, the transmission coefficient for microwaves with a stochastically jumping phase is found to be one order of magnitude greater than that for a broadband wave with the same spectral density; (ii) the electrons are shown to be heated most efficiently by obliquely incident waves with a stochastically jumping phase and, in addition, the electron distribution function has a high-energy tail; and (iii) necessary conditions for gas breakdown and for the maintenance of a microwave discharge in stochastic fields in a light source have been determined. The anomalously large transmission coefficient for microwaves, the anomalous character of the breakdown conditions, the anomalous behavior of microwave gas discharges, and the anomalous nature of collisionless electron heating have been attributed to stochastic jumps in the phase of microwave radiation. In theoretical and experimental researches proved to be true, that in difference from a regular signal earlier, for MWRSJP minimal breakdown power poorly depends on pressure of working gas at its reduction from optimum value. For carrying out of the further numerical comparisons with experimental researches on breakdown of a mixture of gases by microwave radiation with a stoсhastic jumping phase the theoretical researches of dispersive properties of the created coaxial waveguide and a collisionless electron heating by MWRSJP and a gas ionization are carried out. Modeling of particle motion in a field of random waves has shown that stochastic jumps of their phases enhance diffusion in coordinate and velocity space. However at finite time interval diffusion does not rise monotonously when a phase jumping frequency increase, but attains a maximum at certain frequency. The results of theoretical and experimental studies for a gas breakdown and for the initiation of a microwave discharge in a low pressure gas by pulsed microwave radiation with a stochastically jumping phase and the peculiarities of a such wave propagation across plasmas that created by it have been presented. It is shown that a breakdown power minimum for microwaves with a stochastically jumping phase depends weakly on a working gas pressure at its increase from optimal (50 Pa for Ar at radiation frequency about 450 MHz) pressure that is caused the anomalous nature of collisionless electron heating, a weak diffusion of electrons and a diminution of losses connected with the elastic and nonelastic collisions between electrons and neutrals. This situation allows to extend the discharge existence region in a direction of lower pressures. Results of experimental researches of breakdown in rarefied gas by pulse microwave-radiation with stochastic jumps of a phase in various operating modes of the beam-plasma generator are discussed. It is shown, that for stochastic microwave-radiation with jumps of a phase a range of pressure of working gas in which its breakdown is carried out, as against regular microwave-radiation of the same spectral range, practically not changing for high pressures, it is very strong (almost on the order) extends aside low pressure that is caused effective collisionless by heating electrons, the weakened diffusion and decrease in losses on elastic and not elastic impacts. It allows to expand area of existence of the category and to lower losses of energy on its maintenance.

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