Low-frequency oscillations in the central cell of the TMX tandem mirror experiment

Abstract

Two modes of low-frequency oscillations have been observed in the central cell of the tandem mirror experiment (TMX). A mode at about 7 kHz has m=1 and probably drives radial transport at large radii. The mode identification is uncertain. A mode at about 13 kHz has m=0. The two end plugs oscillate 180° out of phase with each other and in phase with the amplitude (envelope) of the ion cyclotron frequency oscillations in each plug. This mode is identified as a sound wave; the frequency is apparently locked to the E×B rotation frequency, probably through an associated m=1 component. Neither mode severely limits confinement in the central cell, and both may be controllable. The lower-frequency mode is sensitive to the density profile and to the fueling and is not always present. The higher-frequency mode may be less important (or absent) in devices in which the plugs are stable at the plug’s ion cyclotron frequencies.