Pre-series and testing route for the serial fabrication of W7-X target elements

The fabrication of the actively cooled high-heat flux divertor of the WENDELSTEIN 7-X stellarator (W7-X) requires the delivery of 890 target elements, which are designed to withstand a stationary heat flux of 10 MW/m2. The organization of the manufacturing and testing route for the serial fabrication is the result of the pre-series activities. Flat CFC Sepcarb® NB31 tiles are bonded to CuCrZr copper alloy cooling structure in consecutive steps. A copper layer is active metal cast to CFC tiles, and then an OF-copper layer is added by hot isostatic pressing to produce bi-layer tiles. These tiles are bonded by electron beam welding onto the cooling structure, which was manufactured independently. The introduction of the bi-layer technology proved to be a significant improvement of the bond reliability under thermal cycling loading. This result is also the consequence of the improved bond inspections throughout the manufacturing route performed in the ARGUS pulsed thermography facility of PLANSEE. The repairing process by electron beam welding of the bonding was also qualified. The extended pre-series activities related to the qualification of fabrication processes with the relevant non-destructive examinations aim to minimize the risks for the serial manufacturing and to guarantee the steady-state operation of the W7-X divertor.