Filamentary arrays in breakdown plasmas generated by a 1.5 MW, 110 GHz Gyrotron

We present the observation, for the first time, of large, regular arrays of plasma filaments during surface and volume breakdown in air at atmospheric pressure using a focused, linearly polarized Gaussian output beam of a pulsed gyrotron. Both volume breakdown in free space and surface breakdown at/near a dielectric window have been studied. A gyrotron with an internal mode converter that emits 110 GHz microwave pulses with up to 1.5 MW peak power and pulse length of 3 microseconds has been used as the source of the microwave beam. Diagnostics include optical emission measurements using photodiodes and a camera, and microwave transmission measurement. The formation of the observed regular filamentary array can be explained by field enhancement approximately a quarter wavelength upstream of each existing filament due to diffraction from the filament.