A. Dudek

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.

Stability Test of a Superconducting W7-X Coil With Respect to Mechanical Disturbances

The superconducting magnet system of the Wendelstein 7-X (W7-X) stellarator experiment consists of 50 non-planar and 20 planar coils which are supported by the central support structure and inter-coil structure elements. This highly loaded support system is prone to mechanical disturbances like stick-slip effects. On the other hand, the coils are built up from cable-in-conduit-conductors (CICC) whose strands are highly compressed by Lorentz forces during operation. Residual elastic energy release within a cable can be triggered by shock waves and corresponding frictional heat generation may occur. The released energy might come into the order of the conductor stability limit and possibly cause a quench. An experiment was performed to simulate the impact of such mechanical disturbances on W7-X coils with stability margins corresponding to different operation conditions. A non-planar coil installed within the magnet test facility was energized and then hit by a pendulum via a stainless steel transfer rod. The test has shown that mechanical disturbances expected in W7-X are not able to induce a quench in any of the foreseen W7-X operation scenarios.

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.

Preload Experiments on Wendelstein 7X Attachment System of the Superconducting Coils

The attachment system of the Wendelstein 7X (W7-X) superconducting coils joins the coil casing support blocks to similar blocks on the main coil support structure using a high strength stud connection. The joint has to withstand the deadweight and the large electromagnetic loads from the coils; it is designed to comply with the large coil deformations during operation and works at 4 K at the allowable load limits. Preloading tests have been performed to determine the behavior of the connection at room and cryogenic temperatures and to define the preloading to be applied during assembly. A full scale mock-up of a single stud connection has been manufactured. During the experiment execution the attachments system had to be improved in order to achieve a high preload, having a limited space for the fitting elements. The paper describes the tests and summarizes the results obtained

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.

Building block support structure for HELIAS Stellarator reactors

An engineering design study is being performed on an enhanced version of a “Helical Advanced Stellarator” (HELIAS) reactor which is based on a fourfold up-scaled W7-X. The coil sizes, their maximal conductor field of 12.3 T, and their operation conditions are similar to those of the ITER TF coils which means that many of the ITER magnet technologies can be applied. The mechanical structure of the first design iteration step has been sub-divided into manageable building block panels for easier manufacture and assembly. The screwed panels between the coils are dismountable which opens up the possibility to separate the torus for maintenance as an alternative to the usually considered exchange of divertor and blanket components through the ports.