Development activities for the target elements of the Wendelstein 7-X divertor

The 19.6 m² high heat flux loaded area of the divertor of the stellarator Wendelstein 7-X (W7-X), which follows the twisted plasma shape, consists of 890 individual actively cooled target elements. A target element is made of a water-cooled CuCrZr copper alloy heat sink armored with CFC NB31 tiles and is specified to withstand a stationary heat flux of 10 MW/m² on its main area and 5 MW/m² at the end adjacent to the pumping gap. Development activities have been undertaken to prepare and support the on-going serial fabrication of the target elements, focusing on the reproducibility of the element performance and the associated quality assurance system. Two main pre-series phases with full-scale prototypes were necessary to validate the selected design and the manufacturing route compatible with industrial and economic constraints. The control of the bonding of the CFC tiles to the heat sink is the most critical issue since the divertor surface consists of about 16,000 tiles and the fast unexpected failure of one tile may restrict the machine operation. A visual inspection system has been specially developed, which allows the easy detection and the recording of cracks at the interface between CFC tiles and heat sink Development activities were essential for the successful on-going target element fabrication.