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 o fkinetic instabilities under double plasma resonance condition in a mirror-confined nonequilibrium plasma occur when the processes of wave excitation and stimulated wave scattering compete. Stimulated scattering leads to the exit of the growing packet of plasma waves from the resonance angle interval to the attenuation regime, which results in a breakdown of the instability. In the simplest case, this regime is described by the Lotka-Volterra system of equations [18]: — = T w -& v w ’, = £ w w * - v w \ (6) dt t dt Here, w and w are the amplitudes of plasma turbulence in the intervals of the angles corresponding to the instability and attenuation, Г is the instability growth rate, £ is the coefficient of stimulated scattering of plasma waves, and v is the damping rate of plasma waves in the nonresonance angle region. Equations ( 6 ) describe periodical solutions which correspond to closed trajectories in the phase plane (w, w*) around a critical point with the coordinates w 0 = v/£ and w* = Г/£ (the center-type equilibrium). If the modulation in the plasma wave energy density is weak (|w -w 0 | « w 0 ,|w * -w j |« w 0 ), then the oscillation period is determined by the formula T = - ^ = - (7) VTv In the case of a deep modulation (jw- w0| « w0, |w* - W q | » the period of oscillations is (*) v {N CTC J У where Dc is the Debye radius. Based on Eq. ( 8 ) for the observed oscillations period T = 200 ns, one can estimate the damping rate of plasma waves in the nonresonance angle region: v « 7.5 x 10 7 s”1. This decrement estimate exceeds the effective frequency of electron-ion collissions vw« 6 x 10 6 s _1 at the plasma density Nc » 2 x 10 12 cm ' 3 and the background plasma electron temperature TQ~ 5 eV, which correspond to the moment of generation of intense high- frequency radiation. On the other hand, the damping rate of plasma waves in the nonresonance plasma angle region turns to be approximately equal to the instability growth rates, which can be connected with attenuation of plasma waves at the same fast electrons that generate plasma waves in the resonance angle interval. In this case, periodic ejections of fast electrons from the trap, which coincide in time with the increase in the intensity of plasma waves, are connected with generation of plasma waves on the normal Doppler effect, which results in fast electrons’ losing part of the energy of their motion across the magnetic field. As a result, generation of plasma waves leads to electrons entering the loss-cone and precipitating further out of the magnetic trap. Another reason for the modulation in the observed radiation can be fast magnetosonic plasma oscillations in the trap. The period of such oscillations [19], Тт5 ~ 2.6 , Rl * 90 ns (9) ■P. at Rj_ = 3.5 cm, the Alfven velocity vA = 10 cm/s, and the ion-acoustic velocity vs ~ 6x10 cm/s, which is approximately twice as low as the observed period. This fact can be connected with that Eq. (9) is obtained for an infinite plasma cylinder with no allowance for the actual geometry of the trap. In the case of fast magnetosonic oscillations, periodic precipitations of fast electrons are explained by periodic changes in the mirror ratio of the trap. In our case, the characteristic attenuation time of fast magnetosonic oscillations is determined by viscosity (see review work [ 2 0 ]) and amounts to about 1 0 0 fts, i.e., the order of magnitude of the high-frequency radiation intensity. This means that one cannot exclude the modulation of the plasma radiation by fast magnetosonic oscillations. 5. Conclusions Thus, we can assume that the high-power quasiperiodic bursts of electromagnetic radiation, which are observed in the experiment after the plasma heating stage, are connected with the phenomenon of double plasma resonance. In contrast with space plasma, where double plasma resonance is observed in spatially inhomogeneous plasma at several frequencies (zebra structure), the double plasma resonance occurs in the laboratory plasma at a certain time moment in the process of plasma decay, when the decreasing frequency of the upper hybrid resonance coincides with the harmonic of the electron gyrofrequency. In this case, the growth rate of plasma waves at the frequency of the upper hybrid resonance increases by approximately an order of magnitude as compared with the case of no resonance. As a result, the intensity of radio emission from the trap increases significantly. The duration of high- frequency radiation in this experiment is as short as 30 fis only. During this time, the harmonic of the electron gyrofrequency changes by approximately 0.5 GHz, i.e., by the observed width of the radiation band, and the conditions of double plasma resonance stop being fulfilled. In this case, the pulse regime of generation of plasma waves and synchronous pulsing precipitations of fast electrons from the trap can be connected with either the 69