Observation of quasi-periodic frequency sweeping in electron cyclotron emission of nonequilibrium mirror-confined plasma

Abstract

-Chirping frequency patterns have been observed in the electron cyclotron emission from strongly nonequilibrium plasma confined in a table-top mirror magnetic trap. Such patterns are typical for the formation of nonlinear phase space structures in a proximity of the wave-particle resonances of a kinetically unstable plasma, also known as the "holes and clumps" mechanism. Our data provides the first experimental evidence for acting of this mechanism in the electron cyclotron frequency domain.Introduction. -Resonant interaction of electromagnetic waves and charged particles plays an important role in the dynamics of magnetoactive plasma confined in space and laboratory magnetic traps. One of the most intriguing manifestations of such interaction is emission of broadband radiation with regular variations of dynamical spectra, e.g. quasi-periodic bursts with a frequency sweeping, resulting from the development of kinetic plasma instabilities. Such events are common features of experimental plasmas. Kinetic instabilities are caused by the presence of positive gradients in the velocity distribution of resonant particles, whose formation is universal for both space and laboratory plasma. In space magnetic traps, the sources of free energy are formed due to different acceleration mechanisms of particles, such as betatron acceleration, plasma-wave turbulence and magnetic reconnection. Under laboratory conditions, the energetic particles with an anisotropic velocity distribution can be formed due to the features of plasma heating, when the energy of the external source is embedded in a specific region of the phase space, such as provided by resonant cyclotron heating or neutral beam injection in magnetic fusion experiment. Spatial gradients can also result in instabilities of waves for which the diffusion in real space is coupled to the diffusion in velocity space due to conservation of invariants of particle motion in inhomogeneous systems energy; this type of instability is exploited in the so-called alpha channeling proposals [1,2]. As a rule, plasma confined in laboratory traps consists of at least two components, one

Interpretation of quasi-periodic frequency sweeping in electron cyclotron emission of nonequilibrium mirror-confined plasma sustained by high-power microwaves

“…Power/frequency (dB/Hz) Figure 3. Dynamic spectra of enhanced microwave emission during the decay stage of argon plasma: experimental data [25] (top) and simulation with BOT code.…”

“…In the experiment, we see that instabilities develop near the discrete frequencies corresponding to low eigenmodes of an empty (with no plasma) cylindrical vacuum chamber [25]. Besides, only modes with the transverse electric field near the axis are pronounced [36].…”

“…This model, commonly referred to as the Berk-Breizman theory, has been used to study the Alfven wave turbulence driven by high-energy ions in toroidal magnetic traps [12,[15][16][17][18][19][20][21][22][23]; as well as some other MHD processes, such as fishbone [12] and ion-acoustic [24] instabilities, have been considered. This paper was motivated by the first laboratory observations of potentially the same mechanism acting in a much higher electron cyclotron (EC) frequency domain reported in [25]. We study the electron cyclotron kinetic instabilities of plasma with fast electrons sustained by high-power microwaves (with maximum intensity about 10 kW/cm 2 at 37.5 GHz) and confined in the table-top mirror magnetic trap.…”

“…Resonant heating results in the plasma with at least Figure 1. Dynamic spectrum of enhanced microwave emission during the decay stage of nitrogen plasma after ECR breakdown in the SMIS-37 open trap facility [25]. The ECR heating is operating during 0-1 ms (not shown in the plot).…”

Observation of multiple chirping events in electron cyclotron emission of non-equilibrium mirror-confined plasma

“…Chirping frequency patterns have been observed in the electron cyclotron emission from a strongly nonequilibrium mirror-confined plasma created by powerful microwave radiation of gyrotron (37.5 GHz, 80 kW) under electron cyclotron resonance (ECR) conditions [1]. An example of multiple chirping events in the spectrum of the electron cyclotron plasma emission is shown in fig.…”