Experimental studies on magnetic field and pressure dependence of high power microwaves at a dielectric surface
The number of anticipated military applications of high power microwaves has greatiy increased in the past few years. Several limitations in the operation of high power microwave devices have been identified including breakdown at dielectric interfaces. These limitations prevent modem devices from being used to their full capabilities. Current research has focused on different materials, geometries, and coatings of dielectric windows. To truly understand how to maximize a dielectric window's capabilities, the basic physics of breakdown at a dielectric interface must be understood.
In the work reported here, studies are made using a simple interface geometry for testing at power density levels up to 10 MW/cm^. An alumina sample with a planar geometry is placed in a rectangular waveguide such that the sample surface is parallel to the microwave electric field and perpendicular to the microwave propagation. A 4 MW, S-band magnetron is coupled to a traveling wave resonant ring to produce power levels for breakdown up to 100 MW. High speed (nanosecond) and high spatial resolution diagnostics are used to study the breakdown phenomena at the sample surface.
The studies identified evidence of secondary electron emission avalanche occurring on the sample surface initiating breakdown as well as evidence of the microwave magnetic field contributing significantly to the avalanche effect for high power breakdown. This was corroborated by a significant difference in breakdovm levels on the upstream interface side of the sample compared to breakdown on the downstream side. Further studies compared gas breakdown to gas/interface breakdown and identified pressure regions in which breakdown is avalanche dominated or collision dominated.