Neutron irradiation studies on window materials for EC wave systems

Fast neutron irradiation experiments are reported to study tolerable fluence levels of the primary window of electron cyclotron (EC) wave systems near the torus. Model discs of CVD diamond grades (30/40 mm diameter) were irradiated to fluences between 1020 and 1022 n/m2 (E>0.1 MeV). For a torus window demonstrator, a large area diamond disc of (106 mm diameter) was irradiated to 0.9.×1021 n/m2. Mechanical strength tests were performed with specimens (11 mm diameter) cut from medium size diamond discs (62 mm diameter). In contrast to dielectric and mechanical properties, the most sensitive material parameter is thermal conductivity for which at 10−4 dpa a significant reduction by about a factor of 2 has to be accounted for. Comparative dielectric measurements for Quartz discs and special silica glass grades (Infrasil, KU1) showed that loss in quartz stayed at the tan δ-level of 10−4 at least up to 1022 n/m2; loss in the unirradiated KU1 glass reached the 10−3 level.