Experimental investigation of air breakdown utilizing a 1.5-MW, 110 GHz gyrotron

Summary form only given. We present experimental results from air breakdown utilizing a 1.5 MW, 110 GHz 3 μs pulse length gyrotron beam in atmospheric pressure air. The beam is focused to a peak intensity of 5 MW/cm² and the plasma formed is a two-dimensional array of filaments oriented along the electric field lines with spacing one quarter of the microwave wavelength (~0.68 mm) that propagate back toward the microwave source. The effect of beam polarization on air breakdown structures is examined with a slow and fast gating camera. The periodic filament arrays that are repeatedly observed with the linearly polarized beam disappear when the gyrotron beam is given a circular polarization. This discovery fits with the explanation that array development arises from the result of diffraction of the beam on plasma filaments, and filament formation therefore requires the beam to have linear polarization. A fast gating, high-resolution spectrometer and a broadband spectrometer are used to study breakdown plasma temperature and electron density. Furthermore, diodes are used to measure power reflection and transmission through the plasma. Breakdown field/intensity threshold, power transmission/reflection, and plasma temperature and density measurements are all important in predicting the transmission of high-power millimeter-waves through atmospheric air at various altitudes.