High Field RF Breakdown of Pressurized SF₆

Pure N₂ and SF₆, as well as their mixtures, are evaluated for high electric field breakdown tested at pressures ranging from 750 torr to 1650 torr at 2.85 GHz. Previous research concerning the breakdown characteristics of pressurized SF₆ and SF₆ mixtures at S-Band frequencies is limited, likely due to the high electric fields required to breakdown pressurized SF₆. A stepped impedance transformer is used in conjunction with a traveling wave resonator to obtain the high electric field amplitudes necessary to break down the gases. Starting with the output from a 3.5 MW coaxial magnetron the electric field amplitude in the test piece at the center of the stepped impedance transformer yielded a maximum of about 150 kV/cm RMS field. Using Pure SF₆ as a baseline, the measured breakdown field shows a mostly linear dependence upon pressure in a range from 750 torr to 1350 torr, while some leveling out tendency is observed at pressures greater than 1350 torr. Since pure N2 exhibits a much lower breakdown threshold, 60%, compared to pure SF₆, mixing the two gases also results in a lower effective breakdown threshold; however, the reduction in the electric field breakdown threshold is not strictly proportional. For example, a 60/40 SF₆ to N2 mixture resulted in a 90% breakdown field while a 20/80 mixture still yielded about 80% in the high-pressure regime.