Observation of the Discharge Structure in 303 GHz Millimeter-Wave Air Breakdown

In this study, we present observations of the discharge structure in air breakdown using a high power millimeter wave beam at 303 GHz. The discharge is ignited by focusing the incident beam using a parabolic mirror. At the focal point, the discharge is under the overcritical condition. Plasma filaments extend along the electric filed vector of the incident beam. The pitch size of each filaments is one-quarter of the wavelength. After the ignition, the condition turns to be subcritical and the filaments begin to separate into granular plasmas. Filaments then extend in parallel to the incident beam in the E-k plane while in the H-k plane, a diffusive structure was formed. Dynamic evolutions of the complicated discharge structure were observed in both electric and magnetic field planes of the incident beam in high resolution and a clear difference between the structures in E-k field and H-k field planes was observed for the first time.