XXXVIII international conference on plasma physics and CF, February 14 – 18, 2011, Zvenigorod.
AlfvéN ION-CYCloTRon instability in a mirror trap with highly anisotropic hot ion Component
Chernoshtanov I.S., Tsidulko Yu.A.
Budker Institute of Nuclear Physics, SB RAS, 630090, Novosibirsk, Russia
The Alfvén ion-cyclotron (AIC) instability threshold and unstable mode structure in a mirror trap containing strongly anisotropic () hot ion component and a background cold plasma are studied in the present work (here is the anisotropy factor, is the magnetic well longitudinal scale length and is the hot ion Larmor radius). These plasma features are specific for the compact cell of Gas-dynamic trap (SHIP-GDT), where AIC instability was observed experimentally [1].
Traditionally, the criterion (where ) is used [2,3] for the instability threshold estimation. The criterion is based on WKB calculations made in [4] for bi-Maxwellian plasmas with moderate anisotropy and with no cold background.
The unstable perturbation wavelength in bi-Maxwellian plasmas is limited by the inequality (see [5]). (This is condition of an inverse population for resonant ions.) From the other hand, the wavelength should to be less than hot plasma longitudinal scale length . It yields the simple condition , indicating that in the strongly anisotropic plasma case the instability threshold is higher than the traditional estimation.
In the present work, an integral equation for unstable modes is derived. Numerical solving of the equation allows to calculate the threshold value of as well as dependences of frequency and growth rate on for a given , and plasma sizes. In the limit and large plasma radius the equation has a simple analytical solution, which yields the asymptotic stability threshold .
The cold plasma addition reduces the minimal wavelength limitation. From the other hand, it raises a wave energy portion leaving the hot plasma region. Numerical calculations show that threshold value of non-monotonically depends on the ratio of the cold and hot plasma densities when , and the threshold monotonically increases when .
References
[1]. A.V. Anikeev, P.A. Bagryansky, M.S. Korzhavina, V.V. Prikhodko. In proc. of XXXV International (Zvenigorod) conf. on plasma physics and CTS, Zvenigorod, 2009, p. 14.
[2]. Casper T.A., Smith G.R., Phys. Rev. Let., 1982, 48, p.1015-1018.
[3]. Post R.F., Nucl. Fusion, 1987, 27, p. 1692.
[4]. D. C. Watson, Phys. Fluids, 1980, 23, p. 2485.
[5]. Davidson R.C., Ogden J.M., Phys. Fluids, 1975, 18, p.1045.
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