P. van Eeten

Design and test of the support elements of the W7-X superconducting magnets

The Wendelstein 7-X stellarator is presently under construction at the Max-Planck-Institute for Plasma Physics in Greifswald with the goal to verify that a stellarator magnetic confinement concept is a viable option for a fusion power plant. The superconducting magnet system has to fulfill demanding requirements regarding magnetic field, loads, manufacturing and assembly. The magnet support system consists of several types of structural components. The main one is the central support structure (CSS) to which the superconducting coils are connected through Central Support Elements (CSE). These are bolted interfaces that allow for flange opening to reduce loads on the components. The non-planar coils (NPC) are toroidially interconnected via lateral support elements (LSE) and narrow support elements (NSE). NSE are contact supports consisting of Al bronze pads that allow for sliding under large compressive loads between the coils. The planar coils (PC) are connected to the NPC through planar support elements (PSE). At the module and half-module separation planes Contact Elements (CTE) connect the neighbouring NPC. An integrated programme of design, FE analysis, experiments and assembly trials has been undertaken. The NSE experimental program provided confidence that the pads can cope with the requirements regarding loads and cycles. Weld trials provided procedures for installing the LSE whilst keeping shrinkage and distortion within tight limits. Tests have been carried out to provide insight on the functioning of the CSE, in particular of the bolts and high performance Superboltreg-nuts during pre-load. This paper gives an overview of the integrated program on the W7-X support elements.

Main Results and Critical Issues of W7-X Structural Analysis

The goal of the Wendelstein 7-X (W7-X) stellarator project is to demonstrate that this type of machine is a viable option for a fusion power-plant. At present the W7-X experiment is in the assembly phase at the Max-Planck-Institut for plasma physics in Greifswald, Germany. The reliable prediction of the structural behavior of the W7-X machine is only possible by employing complex finite element (FE) analyses with a hierarchical set of FE models. A special strategy has been developed for the structural analysis which is under implementation now. This paper gives an overview of the analysis strategy, the applied structural criteria and critical issues, and focuses on the most remarkable results. The main attention is paid to the components that have been changed or optimized recently.

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.