Surface flashover on alumina RF windows for high-power use

Breakdown phenomena on alumina RF windows for S-band high-power use were investigated using a resonant ring. Occasional flashovers were observed on TiN-coated surfaces, though the multipactor effect is suppressed by the coating. The flashovers show luminescence with a treelike pattern, resulting in a limitation of the transmittable power through the window. This flashover threshold depends on the dielectric materials; a sapphire with F-center defects shows a lower threshold. For further investigations of the flashover mechanism under a RF field, in situ measurements of the charges built up on the surface both before and after flashover and high-speed photography observations of the flashover avalanches were made. The results indicate an electron communication; its relaxation leaves positive charges on the surface. The relaxation time is found to be <40 ns. The velocity of the avalanche growth under the RF field is estimated to be >10⁶ m/s