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

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 (lambda_en,iz > L_system) 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.