Magneto-Acoustic Instability in Disk CCMHD Generator

Abstract

Magneto-acoustic Instability of nonequilibrium plasma in a disk MHD generator was investigated experimentally and analytically. At a low load resistance, a striated discharge structure was observed and it was found that the resonance line spectrum of cesium and recombination radiation fluctuated simultaneously under the conditions of some seed fractions. Furthermore, it was found that the fluctuation of recombination radiation became smaller when a high enthalpy extraction was obtained. On the other hand, at a high load resistance, a large fluctuation of static pressure with frequency of 2[kHz] which was thought of a magneto-acoustic wave was observed and it seemed that the pressure wave propagated upstream against the flow. Optical measurements suggested that the fluctuation of electron density did not depend on the fluctuation of electron temperature in the case of existence of the pressure wave. In addition, growth rates of magneto-acoustic wave were calculated by a liner perturbation theory, and the growth rate was found to get smaller with increase of seed fraction because the Hall parameter becomes lower at higher seed fraction. This analytical prediction agreed well with the experimental result. The analysis also indicated that the wave was amplified by a spatial gradient of the collision force between electrons and heavy particles which was caused by an increase in local collision frequency.