Observation of Plasma Instability with Rotational Effects in a Mirror Machine

Abstract

Two types of hot plasmas were produced and confined between magnetic mirrors by plasma injection followed by slow magnetic compression in the Table Top III device: (1) a ``hot‐electron plasma,'' in which the plasma was highly compressed and the electron temperature was 10 to 25 keV, and (2) a ``hot‐ion plasma'' in which the compression was less but the ions were hotter than the electrons. In both cases, regimes could be found in which gross instability occurred. The plasma column was observed to leave the axis and spiral, as a whole, outward to the chamber walls with a radial velocity of about 5 × 10⁵ cm/sec. On the basis of the observations and a comparison with theory this behavior is interpreted as a simple azimuthal mode number M = 1 interchange instability accompanied by a rotation arising from the fact that the plasma has a net charge. No multiple flutes (M > 1) have been observed during this motion, although occasionally simultaneous existence of two plasma columns has been detected. The lower critical density for the onset of the instability was approximately that density for which the Debye distance was equal to the plasma diameter. Stabilized regimes were also obtained by operation with high injected densities or at higher base pressures and the plasma remained confined for many milliseconds. There was also an intermediate regime in which instability could be deliberately ``triggered'' by a finite electrostatic perturbation in a plasma that had been quiescent for milliseconds. Some aspects of the observed stabilization, particularly the nonobservance of multiple flutes, occurring under conditions in which simple hydromagnetic theory predicts instability for all values of M > 0, seem to be explainable in terms of a ``finite‐orbit'' theory, although other effects seem also to be present.