B. Mendelevitch

Final design of W7-X divertor plasma facing components – tests and thermo-mechanical analysis of baffle prototypes

Abstract The plasma facing components (PFCs) of the W7-X are designed in detail. The current design of the target plates, baffle plates and wall protection is presented which takes into account the requirements of the plasma heating, diagnostic systems and mounting. Prototypes of baffle elements were tested with heat loading to investigate the long term behaviour. The experimental results are compared with finite element calculations of the temperature and stress distributions in the elements. Based on these activities, the fabrication of the W7-X divertor PFCs and the graphite covered wall protection for W7-X can be initiated.

Water-cooled target modules for steady-state operation of the W7-X divertor

The stellarator WENDELSTEIN 7-X (W7-X) includes water-cooled plasma facing components (PFCs) to allow steady-state operation and to provide an efficient particle and power exhaust up to 10 MW for a maximum pulse duration of 30 min. Ten divertor units are arranged along the helical edge of the fivefold periodic plasma column. The three-dimensional shape and positioning of the target surfaces are optimized to address physics issues for a wide range of experimental parameters, which influence the topology of the boundary. The three-dimensional target surfaces are reproduced by a series of consecutive plane target elements as a set of parallel water-cooled elements positioned onto the frameworks of target modules. The design and arrangement of target modules and elements are described.

Divertor concept for the W7-X stellarator and mode of operation

A favourable property of the stellarator concept is the potential of stationary operation within a magnetic configuration maintained by a superconducting coil system. For proof of principle the stellarator Wendelstein 7-X is presently under construction at Greifswald, Germany, and the start of operation is planned for 2007. The magnetic configuration of the confinement is a non-axisymetric three-dimensional configuration with a helix-like magnetic axis and five identical magnetic field periods. As a first-step divertor design, an open divertor structure has been chosen, which benefits from the inherent divertor property of the magnetic configuration. The system will allow an effective particle and energy exhaust for a wide range of plasma and magnetic parameters. Experimental tools, e.g. localized heating, various heating schemas, gas feed and pellet injection, impurity doping and variation of the pumping speed together with appropriate diagnostics are provided. The purpose is to investigate different modes of operation for the divertor system and to evaluate an extended database for further improvement of the divertor. The main heating method will be 140 GHz ECR as a cw heat source of 10 MW. Additional heating schemes are ICRF and NBI.

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.

Concepts and prototype elements of the low-Z wall protection for the stellarator W7-X

Abstract Stationary operation, envisaged at WENDELSTEIN 7-X, demands for an active cooled wall protection system. To minimise the radiation losses of the plasma during long pulse operation the inner surface of the vacuum vessel has to be covered completely with materials having low atomic numbers. Unlike from the target plates (receiving a high heat flux of up to 10 MW/m 2 ) that consist of Carbon Fibre Composite (CFC) brazed on a cooling structure, a more simple solution for the Plasma Facing Components (PFC) on the basis of CFC tiles clamped on a cooling structure or larger stainless steel (SS) -panels covered with B 4 C will be developed for the less loaded baffle areas of the divertor and the 120 m 2 wall protection. The choice of the components to be installed in the various areas for wall protection will be determined mainly by the conditions of the physics of the plasma, additional loads by plasma heating (mainly neutral beam injection, NBI and electron cyclotron resonance heating, ECRH), demands by the diagnostics and the geometrical restrictions at different locations in the vacuum vessel.

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.

Optimisation of target plates for the W7-X divertor at stationary operation

The open divertor structure of the Wendelstein 7-X stellarator allows an effective particle and energy exhaust for a wide range of plasma and magnetic parameters. Ten 3D-shaped surfaces are located along the helical edge of the five-fold rotational symmetry magnetic configuration of the machine. The plasma outflow concentrates onto the target plates, so that the plasma is decoupled from the wall. The target plates are designed to withstand a maximum stationary heat flux of 10 MW/m2. The present optimised design of the target plates, which takes into account the latest results of the physics, the requirements of the heatings and diagnostics systems, the manufacturing costs and the mounting operations in order to build a technical reliable system is presented.

Modification of the target design of the W7-X divertor for steady-state operation

Actively-cooled divertor and wall protection systems are planned to be installed from start of operation in 2010 in the stellarator WENDELSTEIN 7-X (W7-X). The W7-X plasma facing components are designed to remove up to 10 MW stationary power. To reduce the actively-cooled target areas, the local angle of incidence of the field lines to the target areas has been increased by 1/spl deg/. This modification makes it possible the keeping of the full range of plasma and magnetic parameters to be studied experimentally. Two different loaded zones are defined: the higher loaded zone of about 20 m/sup 2/ designed to withstand a maximum stationary heat flux up to 10-12 MW/m/sup 2/, and the lower loaded zone of about 6 m/sup 2/ designed to withstand a maximum stationary heat flux up to 1 MW/m/sup 2/. The selected technology for the higher loaded zone is based on CFC flat tiles bonded on water cooled CuCrZr heat sink. For the lower loaded zone, graphite tiles are clamped on a CuCrZr heat sink which is brazed on a stainless steel water cooling tube. The design is now sufficiently detailed to start the fabrication.