Summary of Research and Development Activities for the Production of the Divertor Target Elements of Wendelstein 7-X

The realization of the high heat flux divertor of the stellarator Wendelstein 7-X requires the production of 890 target elements. Since the divertor has to operate for 30 min with an input continuous wave (cw) plasma power of 10 MW, the target elements need to be actively water cooled. A target element is made of a CuCrZr copper alloy heat sink armored with carbon fiber reinforced carbon (CFC) NB31 tiles. It is designed to remove a stationary heat flux of 10 MW/m² on its main area and 5 MW/m² at the end adjacent to the pumping gap. Research and development activities proved to be essential for the preparation and the support of the on-going serial fabrication. The most critical issue is the stable production of the bonding between the CFC tiles and the heat sink as the divertor surface consists of ~ 16 000 tiles. The fast unexpected failure of one tile may reduce the whole divertor performance and, consequently, restrict the stationary machine operation. Two main research and development phases allow the design improvements of the target elements compatible with industrial and economic constrains of the series production as well as the development of quality controls such as a visual inspection system able to easily detect and fast record cracks at the interface between CFC tiles and heat sink.