AVS 45th International Symposium
    Plasma Science and Technology Division Tuesday Sessions
       Session PS1-TuM

Paper PS1-TuM9
Very High Density Helicon Mode Operation in WOMBAT

Tuesday, November 3, 1998, 11:00 am, Room 314/315

Session: Pulsed Plasmas
Presenter: R.W. Boswell, Australian National University
Authors: R.W. Boswell, Australian National University
A. Degeling, Australian National University
Correspondent: Click to Email
A Helicon Wave mode which exhibits a peak downstream density of greater than 10@super 12@ cm@super -3@ in argon (so called the "blue mode" because of bright ArII emission along the axis) has been oberved. The experimental conditions are : argon gas pressure of 3 mtorr, dc magnetic field of 100g and rf power input of 2 to 3 kW at 13.56MHz into a source region of 18 cm radius and 50 cm length using a double half - turn antenna. The nominal plasma density is about 10@super 11@cm@super -3@ when tuned for the normal Helicon wave mode under these conditions, and the blue mode appears sporadically for intervals of about 2 ms every few seconds. By pulsing the rf power for a duration of a few milliseconds it was found that the blue mode could be tuned to and operated more consistently, however the duration of the mode never exceeded a few milliseconds until the power level was increased above 4kW (where the blue mode operated continuously). B-dot probe measurements indicate that the wave phase velocity while the blue mode operates is about 3x10@super 6@ m/s, which has been shown previously to be the optimum velocity for resonant wave heating of plasma electrons to increase the ionisation rate.The instability of the mode on time scales of a few milliseconds may be due to a neutral pressure decrease in the source caused by ion pumping while the blue mode is operating. Once the blue mode is quenched the ion pumping stops and the pressure increases in the source, allowing the blue mode to return in a few milliseconds.