Further Development of the W7-X Magnet System Finite Element Global Model in Preparation for Enhanced Operation Phase

Abstract

The Wendelstein 7-X (W7-X) stellarator equipped with a large cryogenic magnet system (MS) has been enhanced for the long pulse operation at the Max-Planck-Institute for Plasma Physics in Greifswald, Germany. In parallel with the process, the MS global finite element model (GM) has been developed further to consider two interrelated issues: i) the effect of winding pack (WP) embedding (EB) process and ii) planar coil case pins and bolts plastification. The procedure of implementing the EB effect in ANSYS is based on the death/birth feature of particular elements, special fixation algorithm and careful check of the unavoidable artificial stress level. The approach to get a refined prediction of plastic strain/ stress levels for multiple pins and bolts uses the detailed local pin/bolt models to check the GM stiffness representation and to deliver the target values using an interpolation procedure. If the main focus during first two experimental campaigns, for roughly 13 months, was on the static structural strength of the MS, the issue of cyclic behavior is addressed in the updated GM post-processing procedure. The post-processing routine predicts how many electromagnetic cycles and cool down-warm up cycles with particular loading patterns could be safely withstood with required margins by the system components. This paper is focusing on the structural cyclic behavior modelling of the W7-X magnet system components. Several related issues are addressed, such as: 1) Specific features of winding pack embedding modelling, 2) Reasonably simplified modeling of multiple pins and bolts in the GM, 3) Preliminary assessment of coil case pin failure. In addition, lessons learned so far regarding the gradual modifications of the GM are also summarized briefly.