The Role of Gas Pressure and Magnetic Fields on RF Breakdown*

RF breakdown/arcing is a major power-limiting factor in antenna systems used for RF heating and current drive in fusion experiments. The factors that contribute to breakdown include gas pressure, gas type, magnetic field, material coatings, and local plasma density. The effects of these factors on RF breakdown are being studied in a resonant 1/4-wavelength section of vacuum transmission line terminated with an open circuit electrode structure. The electrode structure is designed to determine the role of the RF electric field strength and magnetic field orientation on the breakdown process. The formation of arcs at the electrode or plasma in the transmission line limits the voltage that can be sustained in the system. Initial results show that the maximum RF electric field that can be sustained without breakdown starts to degrade as the gas pressure approaches 1 mTorr and can change with gas type. The addition of an external magnetic field causes the formation of a plasma at pressures below 1 mTorr. Details of the experimental results and future plans will be presented.