High-frequency conductivity of photoionized plasma
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High-Frequency Conductivity of Photoionized Plasma M. V. Anakhova and S. A. Uryupina, b a National
b
Research Nuclear University “MEPhI,” Kashirskoe sh. 31, Moscow, 115409 Russia Lebedev Physical Institute, Russian Academy of Sciences, Leninskii pr. 53, Moscow, 119991 Russia e-mail: [email protected] Received November 25, 2015
Abstract―The tensor of the high-frequency conductivity of a plasma created via tunnel ionization of atoms in the field of linearly or circularly polarized radiation is derived. It is shown that the real part of the conductivity tensor is highly anisotropic. In the case of a toroidal velocity distribution of photoelectrons, the possibility of amplification of a weak high-frequency field polarized at a sufficiently large angle to the anisotropy axis of the initial nonequilibrium distribution is revealed. DOI: 10.1134/S1063780X16080018
1. INTRODUCTION The velocity distribution of photoelectrons in a plasma created via ionization of atoms by laser radiation differs substantially from Maxwellian (see, e.g., [1–7]). As a result, the properties of the photoionized plasma differ from those of a weakly nonequilibrium plasma [8–13]. One such property is the high-frequency plasma conductivity, which characterizes the response of photoelectrons to the electromagnetic field at a frequency that is much higher than the effective electron collision frequency. When considering the response of the photoionized plasma to such a high-frequency field, it is natural to disregard variations in the initial highly nonequilibrium photoelectron distribution caused by their mutual collisions and collisions with ions. If the velocity distribution of photoelectrons is anisotropic, then another reason for its variation is aperiodic electromagnetic instability [14– 17]. This instability can also be ignored if the field frequency is much higher than the instability growth rate, which, in turn, is less than the electron plasma frequency by a factor equal to the ratio between the maximum velocity of photoelectrons and the speed of light. The latter ratio is usually quite small. Under these restrictions on the field frequency, we derive a general expression for the tensor of the highfrequency conductivity on the basis of the Fokker−Planck electron−ion collision integral. The initial distribution of photoelectrons is assumed to be known. The general expression is analyzed for two types of initial distributions: bi-Maxwellian and anisotropic toroidal. These two distributions are used to approximate the distributions of photoelectrons produced via tunnel ionization of atoms in the field of linearly or circularly polarized radiation, respectively. It is shown that, in the case of a bi-Maxwellian distribu-
tion, the real part of the conductivity tensor is highly anisotropic. The real part of the conductivity is minimal along directions with the maximum effective electron temperature. For a toroidal photoelectron distribution, the real part of the conductivity tensor is also highly anisotropic. It is found that a
