A study of low frequency instability and transport in a helicon plasma

Equilibrium behavior of the helicon plasma discharge source turned up a puzzling result. The electron density has been observed to monotonically increase and then saturate or even decrease as the static magnetic field strength is increased. This does not agree with customary equilibrium diffusion characteristics. Lower hybrid resonance heating has been suggested as a possible candidate. Efforts to calculate the power balance between helicon wave power deposition and an assumed loss mechanism have been employed as well. Both fall short in explaining the observed effect. The possibility of an instability was investigated as a mechanism responsible for enhanced radial loss. The wave-like nature of detected fluctuations was investigated and loss rates were measured parallel and perpendicular to the magnetic field direction. As the magnetic field strength was increased for constant input power, a low frequency electrostatic instability was discovered. This instability has characteristics of the resistive drift, Kelvin-Helmholtz, and Simon-Hoh instabilities. Measured radial loss rates dominate the equilibrium as the instability initially grows. The onset of the dominant radial loss rate corresponds to the onset of the instability. At higher magnetic field strengths, axial loss from ions that have gained significant subsonic speeds then increases.