A. Carls

Structural Analysis at the Transition From W7-X Construction to Operation

The Wendelstein 7-X modular advanced stellarator is in the commissioning phase at the Max Planck Institute for Plasma Physics in Greifswald, Germany. The focus of the numerical analysis has been shifted from support of the machine design and assembly toward preparation of the commissioning steps, assessment of possible field disturbances under operational loads, and exploration of operational limits. This paper emphasizes on the development, evolution, and realization of new analysis strategies, as well as on implemented numerical approaches for electromagnetic, thermal, and structural analyses. Remarkable results of the first comparisons with measurements from the extended mechanical instrumentation system are presented in detail and cover evacuation/flooding of the cryostat and plasma vessel, the first cooldown of the magnet system (MS), and both the superconducting coil groups and integral MS commissioning. Finally, some lessons learned during the transition phase are highlighted, which might be relevant for other large fusion machines.

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.

Numerical modelling at the transition from W7-X construction to operation

The Wendelstein 7-X modular advanced stellarator is in the commissioning phase at the Max Planck Institute for Plasma Physics in Greifswald, Germany. The focus of the numerical analysis has been shifted from support of the machine design and assembly towards preparation of commissioning steps, assessment of possible field disturbances under operational loads, and exploration of operational limits. The paper emphasizes on the development, evolution and realization of new analysis strategies, as well as on implemented numerical approaches for electromagnetic, thermal and structural analyses. Remarkable results of first comparisons with measurements from the extended mechanical instrumentation system obtained during evacuation and flooding of the cryostat, the first cool-down of the magnet system and also during first part of the superconducting coil groups commissioning are presented in detail. Finally, some lessons learned during the transition phase are highlighted which might be relevant for other large fusion machines.

Numerical modelling in the construction of Wendelstein 7-X

The Wendelstein 7-X modular stellarator is in the final assembly phase at the Max Planck Institute for Plasma Physics in Greifswald, Germany. The design and assembly of the “basic machine”, i.e. without in-vessel components, diagnostics and periphery, is completed. Structural parameters such as bolt preload, initial gap widths for contacts between structure elements, final magnet module positions, etc., were specified on the basis of detail numerical modeling and are now implemented. The focus of the numerical analysis has been shifted towards fast consideration of nonconformities and changes in assembly procedures, to preparation of commissioning, assessment of possible field disturbances, and exploration of operational limits. In parallel the analyses of in-vessel components, diagnostics and periphery are being continued. The paper focuses on the development, evolution and realization of analysis strategies, implemented numerical approaches and most remarkable results, and on a few specific issues like parameterization and complex finite element model structuring. Further subjects are reasonable safety margins in relation to expected tolerances and uncertainties, and the confirmation of analysis results by tests as well as their benchmarking with alternative models in different codes. Finally, some lessons learned so far which might be relevant for other large fusion machines are highlighted, and a brief outlook on future work is given.

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

The largest modular stellarator Wendelstein 7-X (W7-X) has successfully passed the first phase of operation in Greifswald, Germany. The sophisticated W7-X superconducting magnet system (MS) with its nonlinear support system has been carefully monitored using an extensive set of mechanical and temperature sensors. This paper focuses on detailed consideration of cyclic MS behavior during operation with limiter configurations of plasma. Measurement results are carefully compared with predictions from updated numerical models and critical issues are highlighted. As a result, the structural monitoring tool is extended to follow enhanced requirements and expectations. This paper is a preparation for upcoming more demanding phases with longer plasma pulses to guarantee safe and reliable W7-X operation with different divertor and scraper element configurations. The procedure to establish required sensor configurations, and to analyze and release new plasma regimes being compatible with W7-X component design values is also described.

Commissioning Results of the Superconducting Magnet System of Wendelstein 7-X

The superconducting fusion device Wendelstein 7-X went into operation in December 2015. One of the most important steps was the commissioning of the superconducting magnet system. It consists of 70 superconducting coils, 14 high temperature superconductor (HTS) current leads, and more than 100 superconducting bus bars. The system is divided into seven electrical circuits with ten coils, associated bus bars, and two current leads each. The commissioning of the magnet system was performed in two major steps: In the first step, the seven coil circuits were separately energized and operated at several current levels in a range between 2 and 12.8 kA. In the second step, all seven circuits were operated together. The adjustment of the quench detectors, the evaluation of the thermal and mechanical behavior, and the test of the magnet safety system were further work packages. Fast discharges were initiated to check the proper behavior of the magnet safety system. Steady-state operation of up to 8 h was performed to adjust the helium mass flows. The commissioning was successfully completed, the main goal of the magnet system is fully reached, which is also confirmed by magnetic flux measurements and the results of the first plasma operation.