A. W. Degeling*, R. W. Boswell and T. E. Sheridan

PRL, RSPhysSE, Australian National University, ACT 0200
alexander.degeling@anu.edu.au

A helicon wave mode (W - mode) that exhibits a peak downstream density of greater than 1012 cm-3 in argon (with bright blue ArII emission along the axis) has been observed. The experimental conditions are : argon gas pressure of 3 millitorr, dc magnetic field of 100G and rf power input of 2 to 3 kW at 13.56 MHz into a source region of 18 cm radius and 50 cm length using a double half-turn antenna. B-dot probe measurements indicate that the wave phase velocity while the blue mode operates is about 2.5x106 m/s, which has been shown previously , to be the optimum velocity for resonant wave heating of plasma electrons to increase the ionisation rate.

A very interesting property of the high density mode is that it is unstable on timescales of a few milliseconds. We believe this is due to a neutral pressure decrease in the source caused by ion pumping while the blue mode is operating. Once the W - mode is quenched the ion pumping stops and the pressure increases in the source, allowing the W - mode to return in a few milliseconds. A simple model reveals that the ionisation rate from the W-mode must be peaked at a particular density in order to cause the observed instability. This provides furthert evidence that resonant wave heating of plasma electrons is a significant contributor to the ionisation rate in W - mode. Further our model predicts that the W - mode should become stable at sufficiently high input power. This has also been verified experimentally.