Experimental Investigation of Filamentary Arrays in a Breakdown Plasma Generated by a 1.5 MW, 110 GHz Gyrotron

Summary form only given. We present detailed experimental results of surface flashover (or breakdown) at a dielectric window transmitting a focused Gaussian beam of high-power millimeter waves. The experiments have been conducted using a 110 GHz gyrotron with maximum output power of 1.5 MW and pulse length of 3 microseconds. Results so far have been at atmospheric pressure. Diagnostics include optical emission measurements using photodiodes and a camera and microwave transmission measurements. We find a highly periodic, near-hexagonal structure in the time-integrated images of breakdown plasma columns. These plasma filament arrays are an unexpected outcome of these studies. Present research concentrates on a study of the evolution of the structured plasma by using photodiodes directed at different regions of the plasma. These results along with more accurate threshold power measurements for different dielectric window materials such as polycarbonate, high-density polyethylene, and Teflon, will be presented.