Zebra-like spectral patterns in electron cyclotron emission of nonequilibrium mirror-confined laboratory plasma

We investigate the nonequilibrium mirror-confined plasma created and sustained by high-power microwave radiation of a gyrotron under the electron cyclotron resonance condition (ECR discharge). Resonant plasma heating results in the formation of at least two electron components, one of which, more dense and cold, determines the dispersion properties of the high-frequency waves, and the second, a small group of energetic electrons with a highly anisotropic velocity distribution, is responsible for the excitation of unstable waves. Dynamic spectra and the intensity of stimulated electromagnetic emission are studied with high temporal resolution. Interpretation of observed data is based on the cyclotron maser paradigm. In this context, a laboratory modeling of non-stationary wave-particle interaction processes have much in common with similar processes occurring in the magnetosphere of the Earth, planets, and in solar coronal loops.

During the developed discharge phase, we registered microwave emission in a direction along the ambient magnetic field at frequencies about a half of electron cyclotron frequency. Every radiation pulse is strongly correlated with precipitations of energetic electrons. At a large density of the background plasma during the stationary ECR discharge stage cyclotron instabilities of the extraordinary waves are suppressed, because their dispersive properties are strongly modified by the background plasma. Emission of dense plasma at frequencies below electron cyclotron frequency is most naturally related to the whistler mode instability.

The distinctive feature of this type of instability is the presence of the selected frequencies (more than ten) in the spectrum, which are arranged equidistantly relatively to each other. These frequencies of spectral components are slightly changing in time while the distance between them remains constant. In the present work, we study features of the observed whistler waves with such a frequency modulation and discuss the origin of this modulation.

The work is supported by the Council on grants of the President of the Russian Federation (grant MK-2593.2019.2).