Intense on-axis plasma production and associated relaxation oscillations in a large volume helicon source

A helicon wave mode with a peak downstream density of greater than 10(18) m (-3) in argon that exhibits bright ArII emission along the axis has been ch aracterized. The experimental conditions are: Ar gas pressure of 1-5 mTorr, external magnetic field of 70-150 G and radio frequency (rf) power input b etween 2 and 4 kW a 13.56 MHz using a double half-turn antenna into a sourc e 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 sour ce and downstream show that plasma production is strongly concentrated on a xis. B-dot probe measurements indicate that the wave phase velocity in this discharge mode is between 2 and 2.5x10(6) m/s, which has been shown previo usly to be the optimum velocity for resonant wave heating of electrons to i ncrease the ionization rate. An interesting property of the high-density mo de is that it is unstable on timescales of a few milliseconds and that a re laxation oscillation occurs between the high- and low-density modes. It is believed that this is driven by the depletion of neutrals in the source reg ion due to ionization and momentum exchange with ions leaving the source. ( C) 1999 American Institute of Physics. [S1070-664X(99)00809-5].