Manufacturing and assembly status of main components of the Wendelstein 7-X cryostat

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Abstract

The stellarator fusion experiment Wendelstein 7-X (W7-X) is at present in assembly at the Max-Planck Institut für Plasmaphysik (IPP).

The toroidal plasma with a ring diameter of 11 m and an average plasma diameter of 1.1 m is contained within the plasma vessel. Its form is dictated by the shape of the plasma. The form of the plasma is controlled by the coil system configuration. To control the plasma form it is necessary that all the 20 planar and 50 non-planar coils should be positioned within a tolerance of 1.5 mm. To meet this requirement a complex coil support structure was created, consisting of the central support ring and the different inter coil supports. The coils and the support structure are enclosed within the outer vessel with its domes and openings. The space between the outer and the plasma vessel is called cryostat because the vacuum inside provides thermal insulation of the magnet system, and the entire magnetic system is then be cooled down to 4 K. Due to the different thermal movements the plasma 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.

This paper aims to give an overview of the main components in the cryostat like the plasma vessel, the outer vessel, the ports and the different support systems. It describes the current manufacturing and assembly status and the associated problems of these components, using pictures and text. This paper does not describe the general assembly situation or time schedules of the Wendelstein 7-X.

Introduction

Wendelstein 7-X (W7-X) is the world's largest superconducting helical advanced stellarator (Fig. 1). It is presently under construction at the Max-Planck Institut für Plasmaphysik (IPP) in Greifswald, Germany. W7-X is provided with an internal vacuum vessel, the ‘plasma vessel’ (PV), whose complex toroidal shape follows the designed contour of the stellarator plasma. The PV is the first barrier for the ultra-high vacuum (≤10−8 mbar) of the plasma chamber. The superconducting magnet system, the coil support structure, the liquid helium cooling pipes and the thermal shield are enclosed between the PV and a second external toroidal vacuum vessel, the ‘outer vessel’ (OV). An intermediate vacuum (≤10−6 mbar) is established between the vessels.

Section snippets

Plasma vessel

The plasma vessel consists of 5 similar modules, each extending for 72°, which are joined together along radial–poloidal and radial–toroidal planes. The inner surface is protected against the plasma heat by the actively cooled panels of the in-vessel components. Its outer wall is covered with the thermal shield consisting of an actively cooled glass fibre and copper shell with multi-layer super insulation which minimises the thermal radiation from the PV to the coils [1].

The maximum outer

Ports

The 254 ports provide the access to the plasma vessel. All ports are equipped with bellows which vary between 100 mm circular to 1170 mm × 570 mm rectangular sizes. The bellows allow relative movements between the outer and the plasma vessel. During various load-cases the axial and lateral stiffness of all bellows will create a resulting spring-force which acts directly on the vessel supports. The ports can be divided in two main groups. The so-called diagnostic ports contain the different systems

Outer vessel

The outer vessel and the plasma vessel form the boundaries for the cryostat. The outer vessel is designed as a torus with an outer diameter of approximately 16 m. The internal diameter of the cross-section is 4.4 m. It is made of the same material as the plasma vessel. The nominal wall thickness of the shell is 25 mm. The outer vessel is also made of 5 modules. Unlike the PV each OV module is divided into an upper and a lower half-module shell. The outer vessel rests on 15 supports, which are

Support structure of the magnetic system

The cryostat encloses different support systems for the magnetic system. The so-called Central Support Structure (CSS) consists of several types of structural components. The main one is the Central Support Ring (CSR) to which the superconducting coils are connected through Central Support Elements (CSE).

The CSS is subjected to various loads during the life of the W7-X experiment. The main loads are generated during coil de-energisation and subsequent magnetic forces. Additional loads can occur

Conclusion

The assembly of the first two modules is well in progress. As described some of the named main parts in the cryostat were not only delivered but also assembled. They could show that the assembly requirements were achieved. In general it can be resumed that all parts which are named in this paper are on the recent internal time schedule. From this point of view there are no doubts to complete the assembly up to 2014 (Fig. 2, Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8).

Because of the

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Cited by (2)

  • Structural analysis of W7-X: From design to assembly and operation

    2011, Fusion Engineering and Design
    Citation Excerpt :

    The planar support elements (PSE) connect the two types of PLC (A, B) to the NPC. One PSE per coil (PSE-A1, PSE-B1) is a fixed bolted connection, while other PSEs follow the NSE design [4]. The cryostat system consists of the plasma vessel (PV), outer vessel (OV), the ports and the machine base.

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