Plasma heating experiments at frequencies near the lower hybrid frequency have been carried out at modest powers (0.05-4 kW). The antenna structure operating at 68 MHz was comprised of two plates driven out of phase on the exterior of the plasma. High electron heating efficiency (> 40%) in both helium and argon plasmas was observed with only a weak density dependence. At low densities (ne ⪅ 1 × 1011 cm−3), the heating was uniform across the plasma while at higher densities the heating was preferentially on the exterior portion of the plasma. The heating of the exterior of the plasma was found not to correspond to absorption at the lower hybrid resonance layer. The electron heating efficiency was found to be a weak function of the RF-power when the incident RF-power was varied from 1 to 15 times the experimentally observed threshold power for parametric instabilities. Ion temperatures were determined by measuring the Doppler broadening of an argon ion line using a Fabry-Perot interferometer. Low-efficiency main-body ion heating (1–3%) was observed.
Parametric instabilities and plasma heating in an inhomogeneous plasma