Surface Charging of RF Windows

Klystrons used in an accelerator have alumina rf windows installed in their output portion in order to isolate vacuum from atmosphere and pass rf power. Window breakdown probably related to the surface charging is one of the difficulties to develop klystrons. The surface potentials of as-received alumina samples are measured. Secondary electron emission coefficients (δ) on the charged area (more than + 40V) are less than 0.2, which results from secondary-electron trapping on the charged surface. High power tests using a resonant ring are examined. Positive charging is considered to be due to multipactoring (a resonant multiplication (in rf fields) of secondary electrons) and surface flashover. After the high power tests, electrons are irradiated in order to modify the surface charging. The effects of the irradiation are also studied by measurements of surface potentials.