Sliding weight supports for W7-X magnet system: structural aspects

Abstract

The Wendelstein 7-X (W7-X) stellarator is presently under commissioning at the Max-Planck-Institut für Plasmaphysik in Greifswald. The coil system consisting of 70 superconducting coils of seven different types is supported by a massive central support structure (CSS), and thermally protected by the cryostat. The magnet system weight is borne by supports which are bolted to the cold CSS. These ten so-called cryo-legs penetrate through the cryostat wall to the warm machine base. The design of the cryo-legs incorporates glass-reinforced plastic tubes to guarantee relatively small thermal conductivity. In order to ensure free thermal shrinkage of the magnet system and to reduce stresses in the cryo-legs, sliding and rotating bearings are used as interfaces to the machine base. Tie-rods between the machine base and the warm ends of the cryo-legs prevent toroidal rotation of the magnet system, as well as any other horizontal shifts due to asymmetric loads. The assembly of the magnet system introduced some vertical imperfections in the cryo-leg positions causing considerable additional internal stresses which were not considered during the design stage. In addition, originally not planned trim coils induce unsymmetrical cyclic loads. Therefore, the previously used method to analyse one magnet system module with periodical boundary conditions is not applicable. Consequently, a model of the complete magnet system, including all five modules, was created and analysed. Fatigue analyses of the cryo-legs under the new cyclic loads, applied on top of the approximately 100 t static weight, have been performed in order to evaluate the lifetime. The paper presents the progress in structural analyses of the W7-X magnet system under the as-built conditions, loads due to the trim coil operation, and results of the weight support fatigue analysis.

Cyclic Behavior of Wendelstein 7-X Magnet System During First Two Phases of Operation

“…1). With such design, only toroidal tie-rods fix the MS position without restrictions for the shrinkage during cooldown to 4 K [12].…”

Engineering Challenges of Wendelstein 7-X Mechanical Monitoring During Second Phase of Operation

“…A detailed description of the different aspects has been published already during the design, development, and commissioning of the system. 4,[7][8][9][10][11][12][13] Following assembly and commissioning, five steps are usually required for the full analysis of MS behavior. Four of them are pretty straightforward calculations…”

“…1. central support bolt preload degradation 7,10,12 2. glass-reinforced plastic tubes of cryolegs 8 3. cracks in lateral support elements 9 4. bolted lateral support at module interface with some sliding development 12 5. measured displacements exceed FE predictions in some configurations. 12 The relevant components require permanent monitoring during MS operation.…”

Mechanical Monitoring Issues in Preparation to Next Step of Wendelstein 7-X Operation

“…1 and 3). Apart from displacement measurements of cold components of the MS, also the 3-D-movements of all ten MS weight supports (cryolegs [8]) need to be monitored. A special 3-D displacement measurement unit of pyramid type has been developed and installed at the (warm) bottom of the cryoleg [5].…”