Secondary electron emission from alumina RF windows

The development of alumina RF windows for use at the output section of high-power RF sources is an important issue for accelerators. The breakdown of RF windows is caused by 1. the multipactor effect (electron multiplication on the surface) and/or 2. the discharge of accumulated charges (due to the multipactor effect). Measurements of secondary electron emission (SEE) and surface charging were carried out to determine the durability of various alumina ceramics in this application. Since excessive surface heating takes place during surface discharge, SEE yields at high temperature were also measured. The results indicate that SEE yields become lower at high temperature, and that higher purity alumina ceramics show higher SEE yields. High-power tests using a resonant ring were conducted in order to observe surface charging and electron accumulation. Higher purity alumina ceramics showed superior performance in the high-power tests, probably due to lower surface charging.