1.5 MW, 110 GHz gyrotron breakdown in air

We present experimental measurements of breakdown in atmospheric pressure air created by a 3 μs pulse length, 1.5 MW, 110 GHz gyrotron producing a quasioptical beam. The linearly polarized focused beam creates a plasma that exhibits a filamentary array of streamers. We use a fast gating camera to measure the time evolution of the plasma filaments and their free space expansion velocity. Microwave measurements show that the scattering of incident microwaves by the plasma through an angular distribution agrees with what is predicted by diffraction of the microwave beam after being absorbed by the plasma. We use a fast gating, high-resolution spectrometer and a broadband spectrometer to measure breakdown plasma temperatures and compare with previous microwave breakdown experiments.