10 - Electrostatic Ion Cyclotron Waves and Ion Heating in a Magnetic Field

Summary

INTRODUCTION.

It is well known that the electrostatic ion cyclotron (EIC) waves may be destabilized by drifting electrons (current) through the stationary ions or by ion beams along magnetic field (Drummond and Rosenbluth, 1962; Kindel and Kennel, 1971; Perkins, 1976; Okuda et al., 1981). EIC waves are particularly important in an isothermal plasma (T_e ≈ T₁) where the ion acoustic waves may be stable due to Landau damping. The threshold drift speed is a fraction of electron thermal speed so that EIC waves may easily be excited under a variety of conditions.

Laboratory experiments as well as space craft measurements have been reported recently on the observation of EIC waves in tokamaks (TFR Group, 1978), linear devices (Yamada and Hendel, 1978), and space plasmas (Kintner et al., 1979). Large amplitude density fluctuations, heating of ions across magnetic field and anomalous cross-field particle diffusion have been observed in these measurements.

There are a number of theoretical considerations on the linear as well as nonlinear behavior of the current—driven EIC instabilities (Drummond and Rosenbluth, 1962; Kindel and Kennel, 1971; Okuda and Ashour-Abdalla, 1982). Linear theory predicts that the unstable modes satisfy

where p is the ion thermal gyroradius,. is the ion gyrofrequency, and n is an integer representing a cyclotron harmonic.

When the electron drift speed is above the threshold, EIC waves grow to large amplitude until limited by nonlinear effects. In the absence of electron source, plateau formation on the electron distribution due to quasilinear diffusion gives rise to the nonlinear saturation of the EIC waves resulting in a modest ion heating (Drummond and Rosenbluth, 1962). There are situations, however, in which a flux of fresh electrons constantly replenish the distribution function of electrons so that complete stabilization due to plateau formation cannot take place. Plasma heating experiments by injection of electron beams (Yamada and Hendel, 1978), ion beams (Eubank et al., 1979) and field-aligned auroral currents where ionosphere acts as a reservoir of fresh electrons are such examples. For these cases, the duration of beams is much longer than the characteristic time scale of the EIC waves so that a presence of beam source play an important role on the nonlinear behavior of the EIC waves, ion heating and plasma transport.