Surface charging of RF windows caused by RF operation

Alumina rf windows are used in the output portion of the klystrons in order to isolate vacuum from atmosphere and pass rf power. The window breakdown is one of the difficulties to develop klystrons. The breakdown phenomena are considered that the stored electrons caused by the multipactoring (secondary electron multiplication in the rf field) and /or by the electron implantation into the alumina ceramics are discharged at the threshold rf-power. In this study, surface potentials are measured in air for the some kinds of alumina ceramics after rf operations. High power rf tests are performed using an S-band (2856 MHz) resonant ring with the maximum transmission power of 220 MW (2μs, 50 pps). It revealed that the measured surface potentials are around some hundred volts and that the potential distributions are different in each alumina sample. (author)