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

氧化铝陶瓷的机械处理不仅改变了表面粗糙度，而且改变了表面形貌。对于光滑表面，介质材料的SEE系数依赖于入射角，而对于粗糙表面，SEE系数与入射角度无关。此外，机械处理后氧化铝的SEE系数降低，尤其是氧化铝纯度在95%或更低时。除了蓝宝石外，SEE系数随着温度的升高而减小。这种下降是由于高温下较大的声子和电子散射引起的。氧化铝陶瓷的SEE系数随纯度的增加而增大。在650℃下的测量表明，所有氧化铝样品的SEE系数都随着温度的升高而减小。在室温下，氧化铝陶瓷的表面电荷随着纯度的提高有减小的趋势。表面电荷随温度的升高而减小。单晶氧化铝(蓝宝石)即使在高温下也表现出比其他样品更高的表面电荷。结果表明，SEE系数随纯度的增加而增大。高纯氧化铝陶瓷的SEE系数越高，击穿阈值越低。这表明SEE系数似乎并不直接影响击穿强度。与氧化铝陶瓷相比，蓝宝石具有更高的表面电荷和更低的击穿强度。从充电堆积的角度来看，表面电荷过高是有害的，会导致表面放电。因此，表面电荷是决定电绝缘子击穿强度的关键因素之一。因此，表面电荷低、无F心缺陷的材料有望成为高性能绝缘体的候选材料。此外，根据高功率传输试验的结果，讨论了表面电荷与样品击穿强度的关系。

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.