SURFACE FLASHOVER ON ALUMINA RF WINDOWS FOR HIGH-POWER USE

Breakdown phenomena on alumina rf windows for S-band high-power use ha ve been investigated using a resonant ring. Occasional flashovers were observed on TiN-coated surfaces, though the multipactor effect is sup pressed by the coating. The flashovers show luminescence with a tree-l ike pattern, resulting in a limitation of the transmittable power thro ugh the window. This flashover threshold depends on the dielectric mat erials; a sapphire with F-center defects shows a lower threshold. For further investigations of the flashover mechanism under an rf field, i n situ measurements of the charges built up on the surface both before and after flashover and high-speed photography observations of the fl ashover avalanches have been made. The results indicate an electron ac cumulation; 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.