Low-Pressure Helicon-Plasma Discharge Initiation via Magnetic Field Ramping

Abstract

Discharge initiation at low pressures and flow rates is investigated in the Madison helicon experiment flowing helicon source. At low pressures (below 14-sccm flow rate), a threshold magnetic field exists for discharge initiation which depends on RF power and gas flow rate. Above the threshold magnetic field, RF discharges start only after a significant delay (approximately seconds) and sometimes will not start at all. This threshold magnetic field is interpreted using electron multipactor arguments. A technique is described for initiating discharges at low flow rates and pressures (lambdaen,iz > Lsystem) and high magnetic fields (above the threshold value). Without a static magnetic field present, the RF power is turned on, and a lower density (< 1011 cm-3) unmagnetized discharge occurs. The magnetic field is then applied, and the discharge transitions to the higher density (up to 1013 cm-3) regime. Using this method, magnetized discharges can be started at flow rates as low as 1 seem (1.8 times 10-4 torr at z = -91 cm, 1.7 times 10-5 torr at z = 105 cm) at 500 W in a 1.04-kG magnetic field. This technique can be used to initiate low-pressure helicon discharges for basic plasma science experiments and other applications.