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

Abstract

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.

Tuning of the rotational transform in Wendelstein 7-X

“…Such models were provided through a metrology campaign on W7-X [9]. These 'as-built' coil models are then deformed using electromagnetic (EM) simulation tools (ANSYS) [17]. In this model a 2.5 T field was assumed, and the coils slightly stiffened for improved fit to measurements (displacement and strain gauges [18]).…”

Wendelstein 7-X Magnets: Experiences Gained During the First Years of Operation

“…So far, some changes in the pre-tension of the bolts, which fixes the coils to the central support ring, have been observed, but far away from critical values. 9 There are no signs of too-high ohmic resistances of the joints, neither in the joints that connect two bus bar sections with each other nor in the joints that connect the bus bars with the cold end of the current leads.…”

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

“…-Close monitoring is strongly required; safety margins included in the design are reduced by unexpected deviations with no clear explanation [4].…”

“…Its 70 superconducting coils (NbTi CCIC, AlMgSi (6063) jacket) arranged in the fivefold symmetric manner are extraordinary not only due to complex 3D shapes of 50 non-planar coils, but also due to the large set of structural and thermal sensors on the coils and their support structures to monitor the behavior of the non-linear support system (see Fig. 1 and [4]). All components of the magnet support system have been operated up to 70% of design loads to fulfill physics program with ∼ 2.5 T of magnetic induction on the plasma axis.…”

“…All components of the magnet support system have been operated up to 70% of design loads to fulfill physics program with ∼ 2.5 T of magnetic induction on the plasma axis. The search for optimized parameters for physics programs in steady-state operation phase OP2 (from2021) resulted in new magnetic configurations, which have to be carefully checked before their use [4], [5]. Successful compensation of main magnetic field errors [6] have been done by five normal conducting trim coils (TCs) installed on the cryostat and accessible for checking and maintenance.…”