Intense On-Axis Plasma Production and Associated Relaxation Oscillation in a Large-Volume Helicon Source

Abstract

A helicon wave mode with a peak downstream density of greater than 1018 m-3 in argon that exhibits bright ArII emission along the axis has been characterized. The experimental conditions are: Ar gas pressure of 1-5 mTorr, external magnetic field of 70-150 G and radio frequency (rf) power input between 2 and 4 kW a 13.56 MHz using a double half-turn antenna into a source of 9 cm inner radius and 50 cm length that opens into a diffusion chamber 45 cm radius and 200 cm length. Radial profiles of the density in the source and downstream show that plasma production is strongly concentrated on axis, B-dot probe measurements indicate that the wave phase velocity in this discharge mode is between 2 and 2.5 × 106 m/s, which has been shown previously to be the optimum velocity for resonant wave heating of electrons to increase the ionization rate. An interesting property of the high-density mode is that it is unstable on timescales of a few milliseconds and that a relaxation oscillation occurs between the high- and low-density modes. It is believed that this is driven by the depletion of neutrals in the source region due to ionization and momentum exchange with ions leaving the source. Show more