Macroscopic Behavior of a Supersonic Plasma Flow in a Divergent Magnetic Mirror

Macroscopic behavior of a high-beta and supersonic plasma flow produced by a magneto-plasma-dynamic arcjet (MPDA) was investigated in the HITOP device. A macroscopic instability was observed as a bulk rotation of the plasma plume eccentrically around the center axis of the vacuum vessel. We investigated whether it was caused by current-driven or pressure-driven instability. A plasma current circulating in the plasma plume was controlled by changing discharge current, magnetic field configuration and mass flow rate and by inserting a copper-mesh grid. It was found that the plasma plume was macroscopically stable when the current extending into the plasma plume was low and the safety factor was more than unity. This indicates that the observed macroscopic instability was mainly caused by a current-driven one.