Physics of auroral phenomena : proceedings of the 38th annual seminar, Apatity, 2-6 march, 2015 / [ed. board: A. G. Yahnin, N. V. Semenova]. - Апатиты : Издательство Кольского научного центра РАН, 2015. - 189 с. : ил., табл.

LABORATORY STUDIES OF KINETIC INSTABILITIES UNDER DOUBLE PLASMA RESONANCE CONDITION IN A MIRROR-CONFINED NONEQUILIBRIUM PLASMA M.E. Viktorov, S.V. Golubev, D.A. Mansfeld, V.V. Zaitsev Institute o f Applied Physics Russian Academy o f Sciences, Nizhny Novgorod, Russia e-mail: mikhail.viktorov@appl.sci-nnov.ru Abstract. We study the stability of a dense nonequilibrium plasma of the electron cyclotron resonance (ECR) discharge in an open magnetic trap immediately after the end of heating. The observed instability is accompanied by pulse-periodic generation ofhigh-power electromagnetic radiation at a frequency that is close to the frequency of the upper hybrid resonance and the double gyrofrequency of electrons and by synchronous precipitations of fast electrons from the trap. It is shown that the observed instability is connected with excitation of plasma waves under the conditions of double plasma resonance in the decaying plasma of the ECR discharge. 1. Introduction Studies of kinetic instabilities in a nonequilibrium plasma, which is produced in an open magnetic trap by high- power electromagnetic microwave radiation under the conditions of electron cyclotron resonance, are of fundamental interest, in particular, since they allow one to model physical processes in the solar corona and the magnetospheres of the Earth and other planets. For example, plasma instabilities in the magnetic traps on the Sun are sources of high-power wideband radio emission (the so-called Type-IV radio bursts), which is interpreted as emission of plasma waves by fast electrons at the frequency of the upper hybrid resonance, followed by transformation of these waves into electromagnetic ones, e.g., as a result of scattering by thermal ions [1]. In the case of double plasma resonance, when the frequency of the upper hybrid resonance coincides with one of the electron gyrofrequency harmonics, the growth rates of instability of plasma waves increase sharply [2, 3], which leads to appearance of intense narrow bands near the harmonics of the electron gyrofrequency in the radio emission spectrum (the so-called zebra structure). It should be noted that manifestations of the double plasma resonance effect in astrophysical plasmas are not rare. Suffice it to say that the zebra structure is observed not only in the radio emission of solar bursts, but also in the Jovian decameter radio emission [4], in the radio emission of the Earth’s magnetosphere [5], and even in the radio emission of pulsars [ 6 ]. Therefore, confirmation of the double plasma resonance effect in a laboratory plasma experiment seems to be a fairly important problem. Under laboratory conditions, the cyclotron resonance heating of electrons allows one to create a nonequilibrium two-component plasma with a dense, relatively cold (background) fraction with isotropic velocity distribution of particles and a less dense, high-energy fraction of nonequilibrium electrons with anisotropic distribution function (and the greater energy of the motion transverse to the magnetic field than the energy of the longitudinal motion). In the nonequilibrium plasma of the ECR discharge, kinetic instabilities can develop as a result of interaction of energetic electrons with electromagnetic waves [7-13]. Specifically, in a dense plasma, when the condition coce < cope ( coce and (ope are the gyrofrequency and plasma frequency of electrons, respectively) is fulfilled, instability o f plasma waves can occur at frequencies near the frequency of the upper hybrid resonance, +согре . These instabilities of a dense nonequilibrium plasma are studied immediately after the end of ECR heating in this paper. The observed instability is accompanied by generation of high-power electromagnetic radiation at a frequency close to the upper hybrid frequency and double gyrofrequency of the electrons. We discuss the possible connection between the above-specified intense radiation and the manifestation of double plasma resonance in the decaying plasma of the ECR discharge. 2. Description o f the experiment The plasma of the ECR discharge maintained in a magnetic mirror trap was studied in the experimental setup, whose layout is shown in Fig. 1. Generation and heating of the plasma under conditions of electron cyclotron resonance are maintained by gyrotron radiation with an operating frequency of 37.5 GHz, a power of 80 kW, and a pulse duration of 1 ms. The gyrotron radiation is injected to the discharge chamber center along its axis through a Teflon window brought out of the magnetic trap and a matching system, which is located in the mirror of the magnetic trap and used also as a plasma collector. Resonance heating of the plasma is performed at the fundamental harmonic of the gyrofrequency in an open axisymmetric magnetic trap. The zone of ECR absorption is located between the mirror and the central cross section of the trap and corresponds to the magnetic-field inductance equal to 1.34 T. The axisymmetric discharge chamber 7 cm in diameter is located in the magnetic trap with a length of 20 cm and a *P hysics o f Auroral P henom ena", Proc. XXXVIII A nnual Seminar, Apatity, pp. 64-70, 2 0 1 5 © Kola Science Centre, Russian Academy of Science, 2015 Polar Geophysical Institute 64