Thermo-stress analysis of actively cooled diagnostic windows for quasi-continuous operation of the W7-X stellarator

Finite element method calculations have been performed in order to develop and demonstrate the design of actively cooled windows for the W7-X stellarator which are adequate for diagnostics in the UV, VIS and IR spectral regions at quasi-stationary thermal loads of 50 kW m−2, mainly originating from short-wavelength radiation. The results have shown that sapphire which is suitable for the spectral range from 150 nm to 4.5 μm is the only material for large window diameters (about 13 cm) which could cope with the expected maximum power loads for W7-X of 50 kW m−2 for more than 20 min. Other materials like fused silica, MgF2, ZnSe and ZnS can only be used for smaller diameter windows (<5 cm) or lower thermal loads. CaF2 and BaF2 are unacceptable as W7-X window materials because of strong in-plane distortions during long pulses. The investigations show in which operating regimes the different materials are suitable with regard to pulse duration and power load for windows of different diameters and thicknesses.