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

Abstract

Abstract-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 minutes with an input 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 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. R&D 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 about 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 R&D 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.

Summary of the production of the divertor target elements of Wendelstein 7-X

“…The quality of the delivered target elements was systematically assessed as follows: visual inspection and in particular pictures and archiving of the interface between tiles and heat sink on both sides (~32400 pictures) to detect possible cracks [11]; dynamic pressure test and the measured values have to lie within ±15% of the mean measured value per type; He-leak testing in a vacuum oven with a leak rate ≤ 5.10 -7 Pal/s at room temperature and 3.2 MPa internal He pressure, and a leak rate ≤ 5.10 -6 Pal/s at 160°C and 2.5 MPa internal He pressure (1 cycle). In case of repair, the number of cycles is increased to 3.…”

“…The explanation of this leak is based on logical assumption which is a combination between possible impurities in stainless steel raw material and cleaning issues during the electron beam welding. The selected solution was the local galvanic copper-coating, 0.3 mm thick and 20 mm long, of all target element connectors of phase 2 [11].…”

Development activities of the high heat flux scraper element

“…The approach chosen for the design of the scraper finger uses, as far as possible, existing validated technology. The target elements of the W7-X divertor use flat tile technology and can withstand steady state heat fluxes of 10 MW/m² [9]. Physics simulations have shown that localized heat fluxes as high as 20 MW/m² will load the scraper element surface.…”

Water-cooling system of the W7-X plasma facing components

“…The full long pulse operation of the machine is the operation phase 2 (OP2). In OP2 the TDU is replaced by an actively water cooled divertor made of target elements [5]. The water cooling system of the plasma facing components has been originally conceived for OP2.…”