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Pressure Dependence of Plasma Structure in Microwave Gas Breakdown at 110 GHz

Recent studies of 110 GHz microwave discharges in air at atmospheric pressure have demonstrated formation of a large array of quarter-wavelength-spaced plasma filaments. Here we present measurements showing that as pressure is decreased from atmosphere to a few torr, the discharge transitions from a well-defined array to a smeared-out array and finally to a diffuse plasma. Despite the distinct nature of breakdown phenomena at high microwave frequencies, the pressure dependence of the breakdown threshold field is seen to follow a Paschen-type curve. Data for air and argon at 110 GHz are compared with previous low-frequency data.