Dielectric and mechanical properties of neutron irradiated KU1 and KS-4V glass

The special ‘quartz glass’ grades KU1 and KS-4V, which are considered for International thermonuclear experimental reactor (ITER) plasma diagnostics as radiation-hard optical materials, were characterised for their mechanical and dielectric properties after neutron irradiation to 1021 and 1022 n/m2 (E>0.1 MeV). As a reference to these grades which differ in their OH content (KU1: 800–900 ppm, KS-4V<0.2 ppm), another commercial silica grade (Infrasil 301) with low OH content (<10 ppm) was studied. Dielectric measurements performed at 90–100 GHz proved that permittivity increase occurred after irradiation to 1022 n/m2, which goes along with a density increase. Dielectric loss is much higher (tan δ=10×10−4) in KU1 than in the grades with low OH content (tan δ=3×10−4). The mean strength levels, which are measured for disks of 17 mm diameter applying the ball-on-ring test, depend sensitively on the surface quality. Whereas for Infrasil 301 with optical surface quality, mean values up to 380 MPa are obtained, disks with typical surface roughness after machining (Ra≈0.5 μm) show mean values of 150 (±30) MPa irrespective of the grade. The results for all silica grades irradiated to 1022 n/m2 fall systematically to the high end.