DIELECTRIC-PROPERTIES DURING ELECTRON-IRRADIATION OF ALTERNATIVE MATERIALS FOR GYROTRON WINDOWS

Recent work on high power gyrotron windows has focused interest on som e homopolar insulators as alternatives to sapphire due to their combin ed low dielectric loss and high thermal conductivity. The two main can didates at this moment, CVD diamond and high resistivity silicon, have been studied. As an indicator of their radiation behaviour, loss tang ent and permittivity at about 15 GHz have been measured under 1.8 MeV electron irradiation at RT. In the case of silicon the previously obse rved radiation-induced decrease of loss tangent has been confirmed rea ching a lower saturation level of 3.5 x 10(-5) at higher doses, and fa lling with increasing frequency. An even more important observation is that the sensitivity to ionizing radiation dropped by 4 orders of mag nitude due to the radiation dose. First results for diamond are also p romising, only a small degradation at relatively short times being see n with no further changes up to the maximum dose used.