Temperature dependence of the surface charging on the high power rf windows and minimization of surface charging for the surface discharge suppression

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Mechanical treatments of alumina ceramics changed the surface roughness as well as the surface topographies. The SEE coefficients of dielectric materials depend on the incident angle for smooth surface, but not for rough one. Moreover, the SEE coefficient of alumina decreased after mechanical treatments, particularly for alumina purity of 95% or less. Decreasing in the SEE coefficients with temperature was confirmed, except for sapphire. This decrease is caused by the larger phonon and electron scattering at high temperature. The SEE coefficients of alumina ceramics increased with the purity. Measurements at 650℃ showed that the SEE coefficients decreased with temperature for all alumina samples. At room temperature, the surface charging of alumina ceramics has a tendency to become smaller for high purity. The surface charging decreases with temperature. Single-crystal alumina (sapphire) shows higher surface charging than other samples even though at high temperature. It is revealed that the SEE coefficient increases with purity. The higher SEE coefficients of higher purity alumina ceramics do not result in a lower breakdown threshold. This indicates that the SEE coefficients seem not to affect the breakdown strength directly. Comparing to alumina ceramics, sapphire has higher surface charging and lower breakdown strength. The high surface charging is harmful from the viewpoint of pile-up of charging which can lead to the surface discharge. Therefore, the surface charging is one of crucial factors that determine the breakdown strength of electrical insulator. Accordingly, materials having low surface charging without F-center defects are promising candidates as high-performance insulators. Furthermore, relationship between surface charging and the breakdown strength of the samples is discussed based on the results of the high-power of transmission tests.