Field Direction Dependent Particle Heating in a Helicon Source

Experiments have demonstrated that ion phenomena, such as the lower hybrid resonance, play an important role in helicon source operation. Damping of the slow branch of the bounded whistler wave at the edge of a helicon source (i.e., the Trivelpiece-Gould mode) has been correlated with the creation of energetic electrons, heating of ions at the plasma edge, and anisotropic ion heating. Here we present ion velocity distribution function measurements, and electron density and temperature measurements, on both sides of a m = ± 1 helical antenna in a helicon source as a function of the antenna frequency and magnetic field strength. These measurements were obtained for two different ambient magnetic field directions. For both field directions, significant perpendicular ion heating at frequencies near the lower hybrid resonance was observed. Three crucial differences in the two background field directions were found. (1) Hotter ions were observed for one of the field directions, (2) edge heating of the ions was observed for only one of the field directions, and (3) dramatic differences in the downstream electron density were found. The preferred direction for optimal coupling of RF energy into the particles is when the magnetic field is such that the helical antenna couples to a m = +1 wave that is launched into the region of interest (upstream or downstream of the antenna). The resonant behavior at the lower hybrid frequency suggests that the enhanced energy coupling occurs specifically for m = +1 Trivelpiece-Gould waves. At low plasma densities (as in the edge of a helicon source), the lower-hybrid resonance is dominated by ion motion and therefore ion energization is expected. At higher plasma densities, it is coupling to the electrons that dominates and right-hand polarization (m = +1) is optimal for coupling to the electrons. We include measurements of the wave polarization at the plasma edge for both magnetic field directions.