Impact of DC electric fields on HPM induced surface flashover

Summary form only given. The introduction of a high voltage DC electrode into an experimental setup used for observing high power microwave surface flashover has shown to significantly vary the total delay time for this type of breakdown. The experiment utilizes an S-band magnetron operating at 2.85 GHz to produce a 4 MW, 3 mus pulse. A plasma switch mounted in a WR-284 waveguide reduces the 10-90% rise time of the pulse to ~50 ns, and it reflects the pulse towards a dielectric window to induce surface flashover. A wire electrode charged to plusmn20 kV is inserted into the center of the dielectric window and oriented perpendicularly to the major electric field component of the TE₁₀ microwave mode. The DC field from the electrode influences charge carriers in the flashover region, forcing potential breakdown initiating charged particles away from or towards the surface, dependent on polarity and particle charge sign. Initial tests were conducted in pure N₂ at a pressure of 125 torr. The low probability of negative ions (stable negative N₂ ions do not exist) appearing in the volume simplifies the interpretation of the experimental results by allowing for the existence of primarily electrons and positively charged ions. A significant increase (~50%) in the average total delay time for the case of a positively charged electrode has been observed. An increase in the average statistical delay was also observed as well as a decrease in the presumed formative delay for both voltage polarities. The apparent electron production rate in N₂ was estimated to be 2 e/mus and 8 e/mus under HPM pulse application in the case of positive and negative DC voltages, respectively. Results of further tests conducted in Argon and Krypton-85 in Argon balance are presented along with a statistical analysis of measured delay times.