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

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4 Author(s)
Matt Wiebold ; Dept. of Electr. & Comput. Eng., Univ. of Wisconsin-Madison, Madison, WI, USA ; He Ren ; C. Mark Denning ; John E. Scharer

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.

Published in:

IEEE Transactions on Plasma Science  (Volume:37 ,  Issue: 11 )