Evolution and Stabilization of Flutes in a Magnetic Mirror

Abstract

This paper deals with flutes as observed in a hot-ion, low-β plasma. This plasma was established in an adiabatic, simple mirror trap by Lorentz or gas-collisional ionization of an energetic H0 beam. The studies emphasize: (1) the detailed evolution of the flute mode and comparisons with linear theory and (2) the stabilizing effects of increased electrical connection of the plasma to conducting end walls (line tying). The evolution is traced as a function of density from a stable drift wave at low density to threshold for a growing, unstable wave which quickly exhibited nonlinear frequency behavior. Evolution of the stable drift wave is much as predicted by the linear theories. Instability threshold seems to occur at a somewhat higher density than that calculated even from theories treating bounded plasmas. The line-tying experiments used movable end walls as a control on the electrical connection. Threshold was raised with end walls close to the hot plasma, in a fashion that is in qualitative agreement with a recent calculation. Two possible explanations of the nonlinear frequency behavior of the unstable wave are proposed.