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

The largest modular stellarator, the Wendelstein 7-X (W7-X), has completed its second phase of operation, OP1.2, in Greifswald, Germany. The inertially cooled divertor installed between mid-2016 and mid-2017 has allowed a wider range of plasma configurations in comparison with the first operation phase, OP1. The sophisticated W7-X superconducting magnet system is further loaded up to 70% of its maximum design loads for all main components. The extensive set of mechanical sensors clearly shows a highly nonlinear system response, which is in rather good correspondence with the predictions from the available advanced numerical models. However, there are also significant deviations observed in several areas. Therefore, modeling improvements and/or parameter variation analyses are necessary to clarify the issues in preparation for the upcoming, more demanding phase OP2 (2021+) with the actively cooled divertor and longer plasma pulses to guarantee safe and reliable W7-X operation. The updated strategy to release multiple new plasma configurations being compatible with W7-X component design values is described briefly. In this approach, the numerical model linearization in the vicinity of an accurately analyzed point is a key method to accelerate the process and to highlight areas for vacuum field parameters not allowed for plasma operation due to structural criticality. A brief overview of the W7-X measurement results, the observed deviations with numerical models, and the implemented improvements, as well as the lessons learned so far, are presented.