B. Missal

Manufacture of cryostat components for Wendelstein 7-X

Abstract Wendelstein 7-X (W7-X) is a follow-up of the successful stellarator W7-AS and is presently being built at the Greifswald branch institute of IPP. One of the main parts of the stellarator is the cryostat, which provides the thermal protection of the coil system. Construction of the cryostat of W7-X is based on the experience gained during design, construction and test of the demo-cryostat [Fusion Eng. Des. 56–57 (2001) 861]. The design of the main components of the cryostat, in particular the plasma vessel, the outer vessel, the ports and the support structure is finished, and manufacture by European industry has started.

Project design progress on plasma and outer vessel exhaust gas system based on LOCA safety analysis of W7-X stellarator

Wendelstein 7-X (W7-X) is a fully optimized low-shear stellarator and shall demonstrate the reactor potential of this fusion plant. It is presently under construction at the Greifswald branch institute of IPP. The cryostat of W7-X stellarator has a torus shape which consists of a plasma vessel as an inner vessel, the outer vessel and the ports, which connect the plasma vessel (PV) with the outer vessel (OV). In operating mode several postulated hazard events which could occur on W7-X stellarator were identified. A set of actions were initiated, in order to mitigate or minimize the possible damaging consequences especially for both main W7-X vessels (OV and PV). The document presents the requirements of the exhaust gas safety systems currently under design for both vessels with overpressure protections working in failure mode.

Overview of main-mechanical-components and critical manufacturing aspects of the Wendelstein 7-X Cryostat

Wendelstein 7-X (W7-X) will demonstrate the possibility of a stellarator for a future fusion power plant. This stellarator fusion experiment is at present in the assembly phase at the Max-Planck-Institut fur Plasmaphysik (IPP). The main advance of the static plasma is caused by the three dimensional shape of the coils. But inside the Cryostat this extravagant geometry of the coils efforts also a three dimensional contour of the main mechanical components. One of the ambitious challenges is how to build up such complex machine. The manufacturing of these complex devices have been demanded the newest manufacturing methods. At 2014 Wendelstein 7-X will be the worlds largest superconducting helical advanced stellarator. The toroidal plasma vessel geometry follows exactly the three dimensional shape of the plasma. It contains the plasma with a great diameter of 11m and an average plasma diameter of 1.1 m. To control the plasma geometry it is necessary that all the 20 planar and 50 non planar coils are not only extreme narrow positioned to the Plasma Vessel but also within a tolerance of 1.5 mm to each other. To meet this requirement and to withstand the high magnetic forces a complex coil support structure was created. The Central Support Ring have to bear the coils but the different inter coil supports canalize the forces by very stiff connections on one side and sliding areas on the other side. The coils and the support structure are enclosed within the Outer Vessel with its domes and openings. The Outer Vessel, the Plasma Vessel and the ports generate the boundaries for the Cryostat. The vacuum inside provides thermal insulation of the magnet system which is cooled down to 4 K. The 254 ports secure the access to the Plasma Vessel with all the supply lines and the diagnostics. Due to the different thermal movements the Plasma Vessel, Outer Vessel and the Central Support Ring have to be supported separately. The Central Support Ring is held by 10 cryo legs. The Plasma Vessel supporting system is divided into two separate systems, allowing horizontal and vertical adjustments to centre the Plasma Vessel during thermal expansion. Beside an overview about the main components in the cryostat like the plasma vessel, the outer vessel, the ports and the different support systems this paper describes the most demanding manufacturing methods. The author delineates some disparate and special problems during the manufacturing of the components at the companies in the different European countries.

Lessons learned from designing and manufacturing of the coil support structure of W7-X

Wendelstein 7-X (W7-X) is a fully optimised low-shear stellarator and shall demonstrate the reactor potential of this fusion plant. It is presently under construction at the Greifswald branch institute of IPP. The W7-X coil system consists of 20 planar and 50 non planar coils. They are supported by a pentagonal 10 m diameter, 2.5 m high called coil support structure (CSS). The CSS is divided into 5 modules. The full central ring structure (80 tons) had been completed and delivered at IPP Greifswald since January 2010. Currently, the four first modules were successfully assembled with the coils meeting the tight manufacturing tolerances. Since 2004, the manufacturing of CSS has represented a technical challenge at industrial level and the need for proven techniques and manufacturing processes in accordance to the highest quality standards. The production of these components has required a management of monitoring for quality and tests especially for the raw material and the machining. For instance on complicated geometries of CSS, the cast austenitic steel was the preferred material for complex parts. Casting austenic steel is rather economic to manufacture and has good mechanical properties at low temperature and a good weldability. Designing, structural calculation, raw material procurement, welding & soldering technologies, milling, drilling, accurate machining, helium cooling pipe forming, laser metrology, ultra sonic cleaning and vacuum test are some of the key points used all along this successful manufacturing process. The main project management and detailed technical challenges will be presented. The lessons learned in the large scale production of this complex and major component such as W7-X coil support structure will be presented as relevant experience for the realisation of similar big components for future fusion devices, such as ITER.