DIELECTRIC MEASUREMENTS ON SAPPHIRE FOR ELECTRON-CYCLOTRON WAVE SYSTEMS

Three different grades of sapphire have been investigated in the unirradiated state at 25-300 K and 145 GHz. The initial T-2 dependence of dielectric loss was found to become steeper below 150 K, reaching exponents around 4 at 70 K. In the HEMEX grade, the loss starts to level off to constant values below about 100 K. A specimen subjected to the RIED effect under electron irradiation showed a similar behaviour but reaching higher constant loss levels. Fast neutron irradiations were performed in the fluence ranges of 10(20)-10(24) n/m(2) (E > 0.1 MeV). No significant property changes in the dielectric properties at 70-300 K were observed in HEMEX grades irradiated up to 10(21) n/m(2). The of the present data base for the assessment of tolerable fluence levels for a cryogenically-cooled torus window in electron cyclotron wave systems is discussed.