Abstract
Alumina rf windows are used at the output portion of the klystrons (2856 MHz, 50 MW in maximum, 4 μs, 50 pps) in KEKB linac. Breakdown of the windows is one of the most serious problems as klystron failures. The breakdown is caused by 1) multipactor, and 2) surface discharges. The multipactor is the resonant multiplications of the secondary electrons in the rf fields and the surface discharges are probably due to the release of the electrons accumulated on the surface. In both cases, the alumina surfaces are considered to be charged. In this report, the surface potentials are evaluated by two ways. One way is to measure the surface potentials with the window structure. The measured potentials are the potentials by which incident electrons are affected. Another way is to measure the potentials with the reverse side connected to the earth. The measured potentials correspond to the surface charge density. The surface potentials at multipactor region (3 MW) and surface discharge region (220 MW) are measured. The higher surface potentials at higher transmission power are obtained. The accumulated charges are considered to be determined by the field-emitted current mainly emitted at the edges of the ceramics. The threshold of the rf breakdown is considered to be improved by thicker TiN coating around the periphery of the ceramics, which reduces the primary electrons.
