High Power Microwave Gas Breakdown Experiments at 110 GHz

We report the results of measurements of breakdown in gases induced by megawatt-level W-band radiation, covering the pressure range 5 - 760 Torr. A microwave beam, generated by a 1.5 MW, 110 GHz gyrotron with a 3-microsecond pulse duration, is focused by a lens into a pressurized chamber at a peak intensity of 5 MW/cm$^2$. Breakdown takes place in the volume of gas, without any field-enhancing objects to initiate the discharge. Data is taken in air and argon. Breakdown threshold data is seen to follow a Paschen-type curve of E vs. p, similar to DC breakdown, having a minimum threshold level at a pressure of about 40 Torr in air and 80 Torr in argon. The spatial structure of the breakdown plasma changes from a diffuse, uniform discharge at low pressure to a periodic array of quarter-wavelength-spaced filaments at high pressure. The transition in observed structure coincides with the measured transition between the low-pressure, diffusion-controlled and the high-pressure, collisional breakdown regimes.