The next step in the Wendelstein stellarator line is the large superconducting device Wendelstein 7-X, currently under construction in Greifswald, Germany. 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 under plasma conditions relevant for a fusion power plant. Steady-state operation of a fusion device, on the one hand, requires the implementation of special technologies, giving rise to technical challenges during the design, fabrication and assembly of such a device. On the other hand, also the physics development of steady-state operation at high plasma performance poses a challenge and careful preparation. The electron cyclotron resonance heating system, diagnostics, experiment control and data acquisition are prepared for plasma operation lasting 30 min. This requires many new technological approaches for plasma heating and diagnostics as well as new concepts for experiment control and data acquisition.

The stellarator W7-X will be equipped with two Neutral-Beam-Injector (NBI) boxes for balanced injection. Each NBI box has 2 tangential and 2 radial source positions. For the experimental start-up phase each NBI box will be only equipped with 2 ion sources. For the selection of the initial 2 NBI source positions per box three physical aspects were examined (transmission and duct power deposition, shine through and heating efficiency) Using hydrogen injection the heating power to the plasma under typically planned conditions should be 1.3MW for the tangential sources and 1.1MW for the radial sources (Deuterium: 2MW for the tangential sources, 1.8MW for the radial sources). The tangential source positions all have similar heating efficiencies. One of them suffers from the lowest duct transmission (highest power-load to the duct). The same source hits a component with a low power-load capability. The W7-X inner wall design will be modified in order to enhance the maximum power-load capability of that component. For the radial source positions there is no clear physics advantage of one position over the other. Taking all aspects into consideration the decision was made to use one tangential source and one radial source per box during the experimental start-up phase.

The operation of W7-X stellarator for pulse length up to 30 minutes with 10 MW input power requires a full set of actively water-cooled plasma facing components. From the lower thermally loaded area of the wall protection system designed for an averaged load of 100 kW/m² to the higher loaded area of the divertor up to 10 MW/m², various design and technological solutions have been developed meeting the high load requirements and coping with the restricted available space and the particular 3D-shaped geometry of the plasma vessel. 80 ports are dedicated alone to the water-cooling of plasma facing components and a complex networking of kilometers of pipework will be installed in the plasma vessel to connect all components to the cooling system. An advanced technology was developed in collaboration with industry for the target elements of the high heat flux (HHF) divertor, the so-called "bi-layer" technology for the bonding of flat tiles made from CFC NB31 onto the CuCrZr cooling structure. The design, R&D and the adopted technological solutions of plasma facing components are presented. At present, except the HHF divertor, most of plasma facing components has been already manufactured.

Abstract-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/m 2 .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.

The stellarator experiment Wendelstein 7-X (W7-X) is designed for stationary plasma operation (30 minutes). Plasma facing components (PFCs) such as the divertor targets, baffles, heat shields and wall panels are being installed in the plasma vessel (PV) in order to protect it and other invessel components. The different PFCs will be exposed to different magnitude of heat loads in the range of 100 kW/m2 to 10 MW/m2 during plasma operation. An important issue concerning the design of these PFCs is the thermo-mechanical analysis to verify their suitability for the specified operation phases. A series of finite element (FE) simulations has been performed to achieve this goal. Previous studies focused on the test divertor unit (TDU) and high heat flux (HHF) target elements. The paper presents detailed FE thermo-mechanical analyses of a prototype HHF target module, baffles, heat shields and wall panels, as well as benchmarking against tests. Xuebing PengMr.Max Planck Institute for Plasma Physics, EURATOM Association,Greifswald, Germany 14 September 2012 Dear Dr. Ing.Olaf, Please accept the attached manuscript of the paper, named Thermo-mechanical analysis of Wendelstein 7-X plasma facing components, for SOFT-27 special issue. The paper is about the thermo-mechanical analysis of Wendelstein 7-X plasma facing components, including the high heat flux divertor targets, baffles, heat shields and wall panels, in order to verify their suitability for steady state operation.I greatly appreciate you taking time to read this letter and the paper, and am look forward to hearing from you. Best regardsSincerely yours, Xuebing Peng. Cover Letter Research Highlights Thermo-mechanical analysis of HHF divertor module TM-H09 shows that it can withstand heat loads of 10 MW/m 2 in steady state when the supports was optimized accordingly.  FE calculations indicate that the tiles of baffles and heat shields can withstand stationary heat loads of 250 kW/m 2 , but that the pulse length of plasma operation must be limited depending on the number of full load (500 kW/m 2 ) cycles.  Gap requirements for wall panels during assembly were defined based on several FE calculations. The stellarator experiment Wendelstein 7-X (W7-X) is designed for stationary plasma operation (30 minutes). Plasma facing components (PFCs) such as the divertor targets, baffles, heat shields and wall panels are being installed in the plasma vessel (PV) in order to protect it and other in-vessel components. The different PFCs will be exposed to different magnitude of heat loads in the range of 100 kW/m 2 to 10 MW/m 2 during plasma operation. An important issue concerning the design of these PFCs is the thermo-mechanical analysis to verify their suitability for the specified operation phases. A series of finite element (FE) simulations has been performed to achieve this goal. Previous studies focused on the test divertor unit (TDU) and high heat flux (HHF) target elements. The paper presents detailed FE thermo-mechanical analyses of a prototype HHF target modu...