Microstructural effects of temperature and neutron irradiation on optical windows

Optical windows, installed on the vacuum vessel in ITER, require glass-metal joints, which must be leak tight to preserve vacuum and to confine the tritium. The joints will be subjected to temperature variations up to 250 deg. C and intense neutron fluxes. To better understand the impact of such environment on the joints' performance we analyzed the effect of thermal cycling and neutron radiation on the microstructure of the glass-metal interface of a fused silica viewport using SEM. We observed that thermal cycling up to 150 deg. C created cracks in the joint near the glass element. Apparently, the big difference in thermal expansion coefficient between the glass and the metal makes it difficult to withstand the thermal cycling test without damage. Subsequent neutron irradiation up to 1018 n/cm2 modified the cracks structure only slightly, but the glass-metal interface became more uniform and small pits were observed with SEM.