Slow wave ion heating and parametric instabilities in the HELIX helicon source

The primary focus of the experiments described here is to determine the mechanism responsible for intrinsic ion heating in helicon sources. Two possible mechanisms have been identified: ion Landau damping of the slow wave and parametrically driven instabilities. Consistent with ion Landau damping of the slow wave, the perpendicular ion temperatures 35 cm downstream of the RF antenna are largest when the RF frequency matches the local lower hybrid frequency; the condition at which the slow wave has a maximum in perpendicular wave number (perpendicular with respect to the applied magnetic field) due to a lower hybrid resonance. The ion temperatures also peak at the edge of the plasma where theory predicts the slow wave should have the largest amplitude and perpendicular wave number. Consistent with ion heating due to parametrically driven instabilities, parametrically driven low frequency waves are observed for the same conditions at which the ion temperatures 5 cm downstream of the RF antenna are largest. The measured characteristics of the low frequency wave suggest that the wave is an electrostatic ion acoustic wave. The electrostatic and electromagnetic features of the parametrically driven waves as a function of magnetic field and RF frequency are also presented and discussed.