The magnetoacoustic cyclotron instability of an extended shell distribution of energetic ions

Abstract

The magnetoacoustic cyclotron instability is a mechanism by which waves on the perpendicular fast Alfvén-ion Bernstein branch can be excited through cyclotron resonance with an energetic ion population. It is a candidate emission mechanism for the superthermal ion cyclotron radiation, apparently associated with the products of fusion reactions, that has been observed from tokamak plasmas. In the present paper, an extended shell model is adopted for the energetic ion distribution function, fα(v)∼nα exp[−(v−v0)2/vT2]. An analytical formulation of the dispersion relation is obtained, whose numerical solution yields quantitative information on the role of vT in stabilizing wave growth at ion cyclotron harmonics. The results show that, for typical plasma parameters of interest, the degree of instability is significantly depressed, relative to its level for vT=0, once vT/v0≂0.1. Gaps appear in typical multiple cyclotron harmonic excitation patterns for 0.1≤vT/v0≤0.2, and most harmonics are stable for vT/v0≥0.25. Thus the energetic ion shell-driven magnetoacoustic cyclotron instability typically occurs only when the shell is relatively narrow in velocity space.