Reduced Breakdown Delay via Memory and Penning Effects in High Power Microwave Dielectric Window Discharges

Development of high power microwave (HPM) distributed discharge limiters relies critically on minimizing the delay time between HPM incidence and diffuse plasma creation. Breakdown is achieved by illuminating a gas cell with a train of $\sim $25kW, $\sim $2 kV/cm, 800ns-long pulses at 41 HZ repetition rate. Using mixtures of neon with small concentrations of argon or xenon at pressures between 5-350 torr, we have observed breakdown in $<$100ns for particular choices of gas composition and pressure. Breakdown times predicted by published theoretical models\footnote{Y.Y. Lau, J.P. Verboncoeur, H.C. Kim, ``Scaling laws for dielectric window breakdown in vacuum and collisional regimes,'' Appl. Phys. Letters, Vol. 89, 261501-1.} are approximately 3-5 times longer than our experimental observations. Careful study of experimental trends suggest surface charge accumulation on the gas cell's polycarbonate window and Penning-like effects in mixtures of noble gases may explain the observation of breakdown times shorter than the theoretical models predict.