Laboratory modelling of plasma waves excitation and transformation to electromagnetic emission in a mirror-confined plasma

Description

Abstract D2.3-0025-21 (oral), id.979.

The kinetic instabilities of plasma confined in an open magnetic trap are relevant to understanding various types of radio emission in space plasma, for example, in the magnetospheres of the Earth and the planets, the Sun, and certain types of stars. The high efficiency of the kinetic wave generation mechanism is due to the low group velocity of plasma waves (in comparison with electromagnetic waves), which ensures they enjoy an extended interaction time with nonequilibrium particles resulting in a high integral gain. Emission from the plasma is observed due to various mechanisms for the transformation of plasma waves into electromagnetic waves, for example, as a result of scattering by thermal ions. In view of the universality of the physical mechanisms of radiation generation, essential aspects of natural systems can be reproduced in laboratory magnetic traps under controlled and reproducible conditions. The technique reported here exploits a plasma generated by irradiating a mirror confined plasma using mm-waves from a gyrotron under electron-cyclotron resonance conditions. In such a discharge, a two-component plasma is created with a dense cold (background) fraction with an isotropic particle velocity distribution and a less dense high-energy fraction of nonequilibrium electrons with an anisotropic distribution function. The spectral characteristics of the electromagnetic radiation of a dense nonequilibrium plasma in the range from 2 to 47 GHz were studied, which corresponds to high harmonics of the electron gyrofrequency (up to the 5th) and includes the upper hybrid frequency range. It has been experimentally shown that the plasma emission spectrum is determined by the arrangement of electron cyclotron harmonics. The significant dependence of the emission frequency on the magnetic field confirms the hypothesis that the detected radiation is a result of the generation of electron Bernstein modes in the plasma at harmonics of the electron gyrofrequency, which are then transformed into electromagnetic modes and leave the source region. Based on the data obtained, it can be concluded, for the first time, that the operation of two different mechanisms of transformation of plasma waves into electromagnetic ones has been experimentally demonstrated: scattering of plasma waves by thermal ions without changing the wave frequency and scattering of two identical plasma waves with a frequency doubling. A numerical calculation of the increments of electrostatic plasma modes at harmonics of the electron gyrofrequency in a dense plasma was performed, as well as the spatiotemporal structure of the dynamic spectra of dense plasma radiation in an open magnetic trap using codes based on the Vlasov-Maxwell equations for the most realistic set of distribution functions in velocity space. A comparison of experimental and theoretical data will lead to an understanding of the mechanisms of electromagnetic radiation generation in magnetic traps and the features of the radio emission spectra observed in natural conditions.

Period	28 Jan 2021 → 4 Feb 2021
Event title	COSPAR Scientific Assembly 2020
Event type	Conference
Conference number	43rd
Location	Sydney, AustraliaShow on map
Degree of Recognition	International