Study of Microinstabilities in Anisotropic Plasmoid of Thermonuclear Ions

Abstract

The following article presents the results of investigation of microinstabilities in the anisotropic synthesized hot ion plasmoid (SHIP). Plasmoid is located in a small mirror section that is installed at one side of the GDT facility in Budker Institute of Nuclear Physics, Novosibirsk, which is an axially symmetric magnetic mirror device of gas dynamic trap type. The magnetic field on axis is in the range of 2.5 Tesla and the mirror ratio is ~ 2. The additional mirror section is filled with background plasma streaming from the central cell of GDT. To create the population of hot ions with strong anisotropy two focused neutral beams with energy of 21–23 keV are injected perpendicularly to the direction of magnetic field. Ionisation of the beams generates the high-energetic ion component with the density of about 5x1013 сm–3 and mean energy about 13 keV. The distribution function of fast ions is thus strongly anisotropic in the phase space with the ratio E⊥ / E& ~ 50. To define the type and the parameters of the developing microinstability a set of high-frequency electrostatic and magnetic probes was used. The microinstability observed in the additional section of GDT is the Alfven ion cyclotron instability (AIC), because of small azimuthal wave numbers, magnetic field vector rotating in the direction of ion gyration and oscillation frequency below the actual ion cyclotron frequency. AIC instability threshold was registered at the following plasma parameters: fast ion density n > 3 · 1013 см–3, ratio of ion pressure to magnetic field pressure β ≈ 0.02, anisotropy A ≈ 35, ai / Rp ≈ 0.23, where ai is the ion gyroradius and Rp is the plasmoid radius.