Hot Electron Plasma Equilibrium and Stability in the Constance B Mirror Experiment

An experimental study of the equilibrium and macroscopic stability property of an electron cyclotron resonance heating (ECRH) generated plasma in a minimum-B mirror is presented. The Constance B mirror is a single cell quadrupole magnetic mirror in which high beta (3 K 0.3) hot electron plasmas (T, ~ 400 keV) are created with up to 4 kW of ECRH power. The plasma equilibrium profile is hollow and resembles the baseball seam geometry of the magnet which provides the confining magnetic field. This configuration coincides with the drift orbit of deeply trapped particles. The on-axis hollowness of the hot electron density profile is 50 t 10%, and the pressure profile is at least as hollow as, if not more than, the hot electron density profile. The hollow plasma equilibrium is macroscopically stable and generated in all the experimental conditions in which the machine has been operated. The hollowness of the plasma pressure profile is not limited by the marginal stability condition. Small macroscopic plasma fluctuations in the range of the hot electron curvature drift frequency sometimes occur but. their growth rate is small (Wi/W, < 10-2) and saturate at very low level (B/B < 10-). Particle drift reversal is predicted to occur for the model pressure profile which best fits the experimental data under the typical operating conditions. No strong instability is observed when the plasma is near the drift reversal parameter regime, despite a theoretical prediction of instability under such conditions. The experiment shows that the cold electron population has no stabilizing effect to the hot electrons, which disagrees with current hot electron stability theories and results of previous maximum-B experiments. A theoretical analysis using MHD theory shows that the compressibility can stabilize a plasma with a hollowness of 20-30 percent in the Constance B mirror well. Although the Constance plasma is not typically in a regime in which MHD theory is valid, the stabilizing effect of compress-ibility suggests that the geometric effects which relate to the field line curvature play a significant role. The observed plasma stability cannot be fully explained by the theories based on the effects such as line-tying, finite electron or ion Larmor radius, and compressibility.