Drift Instability in Collisionless Alkali Metal Plasmas

Abstract

Measurements of low-frequency (ω≪Ωi) oscillations in collisionless, low-β, alkali metal Q-device plasmas are reported. These oscillations are conclusively identified as density-gradient-driven drift waves by the agreement between experiment and the predictions of the linear theory with regard to frequency, wave-number, and stabilization by ion Landau damping. The linear theory of collisionless drift waves is modified to take into account effects of cylindrical geometry. Also considered are radial gradients in the zero-order diamagnetic drift and plasma rotation frequencies. The enhancement of resonant particle effects by the finite length of the plasma column is computed. The occurrence of discrete radial eigenfunctions is found to depend on the radial variation of the equilibrium plasma potential. The observed dependence of amplitude on r agrees with that predicted by the theory. The experimental results demonstrate the importance of centrifugal and Coriolis force effects in determining the oscillation frequency. Finally, the transition between collisionless and collision dominated drift waves is explored, and the relationship between the observed amplitudes and the predictions of several nonlinear theories is examined.