R. Stadler

Construction of Wendelstein 7-X - Engineering a steady state stellarator

The next step in the Wendelstein stellarator line is the large superconducting device Wendelstein 7-X, presently under construction in Greifswald. Steady-state operation is an intrinsic feature of stellarators, and one key element of the Wendelstein 7-X mission is to demonstrate steady-state operation at reactor relevant plasma conditions, as required for an economic fusion reactor. Such steady-state operation requires development of special technogies to be discussed in this paper.

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.

A proposed scraper element to protect the end of the W7-X divertor target elements

The three main areas of the W7-X divertor target are: the horizontal target, the vertical target and the high iota tail, with a pumping gap between the horizontal and vertical targets. For each of the standard operational scenarios the target has been designed so that, based on vacuum field calculations, the plasma strike points lie away from the ends of the target. Each target consists of CFC armoured water cooled poloidally running target elements capable of operating reliably in these central regions at up to 10MW/m2 in steady state operation, as necessary for the standard configurations. Due to the U-turn shape of the cooling channel at the pumping gap end of the elements the cooling is not optimal and a reduced performance is expected. However, recent studies of experiment scenarios, also now taking into account the influence of bootstrap currents, showed that the strike point drifts over the ends of the target elements adjacent to the pumping gap on the L/R time scale; potentially leading to damage of the elements in these scenarios. Various operational methods have been studied to satisfactorily avoid this situation without success. The paper describes design changes being made to improve the ability of the end of the elements to accept reasonable power levels, however it is not expected that the elements in these regions will meet the full power requirements. Hence, this paper also describes the design of a protection element; the purpose of this so-called “scraper element” is to intercept the power to the targets before the full power reaches the sensitive end of the elements. This is done without reducing significantly the pumping at the pumping gap and addresses issues such as how to control local impurity flows. The programme of work concerning design and technology qualification still needed to realise this scraper element is described.

Design and Test of Wendelstein 7-X Water-Cooled Divertor Scraper

Heat load calculations have indicated the possible overloading of the ends of the water-cooled divertor facing the pumping gap beyond their technological limit. The intention of the scraper is the interception of some of the plasma fluxes both upstream and downstream before they reach the divertor surface. The scraper is divided into six modules of four plasma facing components (PFCs); each module has four PFCs hydraulically connected in series by two water boxes (inlet and outlet). A full-scale prototype of one module has been manufactured. Development activities have been carried out to connect the water boxes to the cooling pipes of the PFCs by tungsten inert gas internal orbital welding. This prototype was successfully tested in the GLADIS facility with 17 MW/m2 for 500 cycles. The results of these activities have confirmed the possible technological basis for a fabrication of the water-cooled scraper.