Engineering Challenges of W7-X: Improvement of Numerical Modeling and Mechanical Monitoring After Commissioning and First Phase of Operation

Abstract

The largest modular stellarator Wendelstein 7-X (W7-X) has successfully passed commissioning and first phase of operation in Greifswald, Germany. The limiter configurations of plasma with 2.5 T of magnetic induction on the plasma axis produce already considerable loads (MN) in the W7-X systems. The sophisticated W7-X superconducting magnet system with its non-linear support system is instrumented with an extensive set of mechanical and temperature sensors. Measurement results showed that magnet system behavior is in good correspondence with original predictions from numerical models. However, several areas require modeling improvements and/or proper adjustment of parameters to reflect "as-built" situation. Moreover, high temperature dependence of strain gauge signal accuracy in the range below 10 K requires its compensation in order to avoid fault alarms during monitoring. The work is considered as benchmarking of numerical models and as a preparation for upcoming more demanding phases with longer plasma pulses to guarantee safe and reliable W7-X operation with different divertor configurations. Both results of W7-X measurements and implemented improvements as well as lessons learned so far are also given.

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

“…First priority was given to instrumentation of Inconel bolts of CSE connections (see fragment in Fig. 4) under bending and tension, critical region with expected yielding on each half module of the CSS and in the corners of coil blocks for bolted lateral support bridges [9]. Other SGs are located in areas with moderate stress, mainly intended to support the FE model benchmarking.…”

“…Part of the MI sensors are distributed symmetrically over the five W7-X modules and the others asymmetrically in order to check the MS symmetry and to cover more locations, respectively. Strong temperature dependence of the SG resistance jeopardizes the measurements at the temperature region below 10 K; therefore, special compensation procedure is to be implemented [9].…”

“…5) Ramping up with simultaneous monitoring and structural assessment is developed and to be tested. 6) Temperature compensation with Kalman filter prediction (see [9] for details) is to be fully tested.…”

“…The "workhorse" of the engineering team is a MS global 72° FE model (v.6) prepared in ANSYS. The main improvements are presented in [9] for v.6.01, while recent activities have been devoted to the introduction of model modifications in order to deliver more accurate signal predictions (v.6.02). The driving factor was the plan to double the current levels in planar coils during the next operation phase (see Table I).…”

Engineering Challenges in W7-X: Lessons Learned and Status for the Second Operation Phase

“…To verify the structural integrity of the cryostat and magnet system during commissioning and operation several accompanying measurements were prepared [21,22]. The most relevant load cases for structural monitoring of the cryostat were the evacuation and baking of the vessels.…”