M. Smirnow

Wendelstein 7-X high heat flux components

The actively water-cooled In-Vessel Components (IVCs) of the stellarator Wendelstein 7-X consist of the divertor, the first wall protection components, the port liners, each designed for different loading conditions, and the associated pipework, the control coils, the cryo-pump system, the Glow discharge electrodes, and a set of diagnostics. The divertor, designed for high heat fluxes, is a set of 10 target and baffle units arranged along the plasma surface. The design and production of these high heat flux (HHF) components is a challenging task. The divertor target elements, which are based on flat CFC (carbon-carbon fibre composite) tiles bonded via active metal casting onto CuCrZr cooling structures required intensive development and testing to reach a reliable performance; removing, under stationary conditions, 10 MW/m2. Industrially manufactured high quality target elements have been delivered and assessed, and the process of incorporating them into assembly units, so-called modules, has begun. The time scale for the completion of the HHF divertor has been held for the last four years and the final delivery of the HHF divertor is still planned in 2017. In parallel to the realization of the divertor the remaining IVCs have been defined, developed, designed and fabricated and the installation of many of these components has begun. Some of these components can also be expected, for a short period of time, to receive high heat loads approaching those of the divertor. These components will be described, in detail, from conception to realization.

Status of High Heat Flux Components at W7-X

The actively water-cooled in-vessel components (IVCs) of the stellarator Wendelstein 7-X consist of the divertor, the first wall protection components, the port liners, each designed for different loading conditions, and the associated pipework, the control coils, the cryo-pump system, the Glow discharge electrodes, and a set of diagnostics. The divertor, designed for high heat fluxes (HHFs), is a set of 10 target and baffle units arranged along the plasma surface. The design and production of these HHF components is a challenging task. The divertor target elements, which are based on flat carbon-carbon fiber composite tiles bonded via active metal casting onto CuCrZr cooling structures required intensive development and testing to reach a reliable performance; removing, under stationary conditions, 10 MW/m2. Industrially manufactured high quality target elements have been delivered and assessed, and the process of incorporating them into assembly units, so-called modules, has begun. The time scale for the completion of the HHF divertor has been held for the last four years and the final delivery of the HHF divertor is still planned in 2017. In parallel to the realization of the divertor, most of the remaining IVCs have been defined, developed, designed, and fabricated and the installation of many of these components has begun. Some of these components can also be expected, for a short period of time, to receive high heat loads approaching those of the divertor. These components will be described, in detail, from conception to realization.

Actively Water-cooled Plasma Facing Components of the Wendelstein 7-X Stellarator

Abstract The actively water-cooled plasma facing components (PFCs) of the Wendelstein 7-X stellarator consisting of the first wall protection and the divertor systems have a total surface area of about 265m2. The complex 3D geometry of the plasma and plasma vessel with 244 vessel ports dedicated to diagnostics, heating systems and water-cooling pipe-work together with the need to minimize the space taken and the significant heat loads expected on the components presents significant design and manufacturing challenges. The actively water-cooled divertor, made of 100 target modules, has an area of 19 m2. Each target module is formed from target elements made of CFC flat tiles bonded with the bi-layer technology to CuCrZr heat sinks. In total 16,000 tiles are bonded to the 890 target elements. A full-scale target module prototype has been manufactured to validate the design, the selected technological solutions and the inspection methods to be used in the serial module fabrication. About 30% of the target elements have been delivered and the production of the remaining elements should be completed by 2014. The fabrication of the components of the first wall protection, 320 stainless steel panels and 170 heat shields, is almost completed.

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.

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.