Confinement of Plasma Heated by Ion Cyclotron Resonance in the C Stellarator

Abstract

Microwave interferometric measurements of electron density, measurements of the particle flux to the divertor and of electron and ion cooling rates, have been used to obtain confinement times τc of deuterium plasma (Te up to 80 eV, Ti up to 200 eV, and mean density 0.5 to 5 × 1012 cm−3) in the C stellarator. These times are consistent with a Bohm-type pumpout law, τc = 10−8 2πr02 B/Te sec, where B is in G, r0 (plasma radius) in cm, and Te in eV, and are found to be insensitive to shear, to Ti, and to longitudinal current density with associated drift velocity in the range 0.2 to 7 times the plasma acoustic velocity. The enhancement of plasma temperature produced by ion cyclotron heating is accompanied by enhanced fluctuations in the frequency range of 10 kc/sec to 1 Mc/sec. The particle loss calculated from the fluctuations seems consistent with the measured value of τc, although the measured τc may be determined by other effects, such as the scrape-off layer effect. No improvement in respect of confinement or of fluctuations was detected in a minimum-average-B geometry of 1% well depth. The plasma confinement may have been limited by the lack of detailed toroidal equilibrium. Theoretically, the plasma is unstable both to the universal and to the resistive and kinetic ballooning modes within the observed frequency range and with growth rates within the plasma duration.