Air-breakdown limits for microwave-pulse propagation. Part 1. Experiments

Breakdown power levels for short pulses of 2.856-GHz microwaves in a test section of WR-284 waveguide filled with air at several pressures in a range extending from 0.1 Torr to several hundred Torr were measured. The traveling wave input was a pulse with a risetime of approximately 50 nanoseconds, a length of 100 ns (fwhm), and an amplitude of approximately 15 MW. Input power transmitted power was monitored with directional couplers and fast diodes: reflected power was also monitored on some measurements. Breakdown power for a given air pressure is defined as the power level (generally on the leading edge of the pulse) at which extreme attenuation of the transmitted pulse occurs. Results are presented because they are the only single pulse microwave data that exist for pulse lengths below 100 ns. The data are compared with predictions of simple cascade theory, existing microwave breakdown data, video pulse breakdown data, and recent LLNL fluid model predictions.