Imaging of Atmospheric Air Breakdown Caused by a High-Power 110-GHz Pulsed Gaussian Beam

We present the images of regular filamentary plasma arrays produced upon the breakdown of air at atmospheric pressure at the focal region of a high-power 110-GHz pulsed Gaussian beam. The source of the millimeter wave beam is a gyrotron that can generate up to 1.5-MW output power with 3-mus pulselength. This unique plasma structure exists only at high pressures. With decreasing pressure, the structure changes into layers of curved plasma sheets and into more familiar diffuse plasma. A main cause of the formation of the regular array structure appears to be the reflection from filaments. The successive generation of conductive filaments modifies the incident field pattern and creates local hot spots upstream of the existing filaments with regular spacing of roughly a quarter wavelength.