SURFACE FLASHOVER ON ALUMINA RF WINDOWS FOR HIGH-POWER USE

Breakdown phenomena on alumina rf windows for S-band high-power use have been 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 tree-like 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 an 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 have been made. The results indicate an electron accumulation; 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 > 10(6) m/s.