Surface breakdown phenomena in alumina rf windows

The multipactor and flashover phenomena of alumina rf windows used in a high-power klystron have been investigated. Multipactoring due to the high yield of secondary electron emission takes place during rf operation. A spectrum analysis of luminescence due to multipactoring shows that multipactor electron bombardment causes F centers in alumina, thus leading to surface melting. From the results of a high-power examination of rf windows with several kinds of alumina ceramic, it was found that alumina with a crystallized grain boundary and without any voids between the boundaries, thus having a low loss tangent value, is not liable for F centers, even under multipactoring. Flashover in a tree-like pattern of alumina luminescence occasionally takes place on a TiN-coated surface where multipactoring is suppressed. A sapphire window, whose surface was polished and having pre-existing F centers, shows a lower flashover threshold. The annealing effect of polished window surfaces was also investigated