The next step in the Wendelstein stellarator line is the large superconducting device Wendelstein 7-X, currently under construction in Greifswald, Germany. Steady-state operation is an intrinsic feature of stellarators, and one key element of the Wendelstein 7-X mission is to demonstrate steady-state operation under plasma conditions relevant for a fusion power plant. Steady-state operation of a fusion device, on the one hand, requires the implementation of special technologies, giving rise to technical challenges during the design, fabrication and assembly of such a device. On the other hand, also the physics development of steady-state operation at high plasma performance poses a challenge and careful preparation. The electron cyclotron resonance heating system, diagnostics, experiment control and data acquisition are prepared for plasma operation lasting 30 min. This requires many new technological approaches for plasma heating and diagnostics as well as new concepts for experiment control and data acquisition.

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Technical challenges in the construction of the steady-state stellarator Wendelstein 7-X

Towards assembly completion and preparation of experimental campaigns of Wendelstein 7-X in the perspective of a path to a stellarator fusion power plant

Wendelstein 7-X (W7-X) represents the continuation of fusion experiments of the stellarator type at the Max-Planck Institute for Plasma Physics (IPP). The aim of W7-X is to demonstrate the suitability for a fusion reactor of this alternative type of magnetically confined plasma experiment. W7-X is being built at Greifswald in the northeast of Germany. The size of device (725 tons, height of 5 m, diameter 16 m) and the superconductive magnet system distinguish W7-X from earlier stellarators at IPP. The paper provides a summary of the status of the main components, the mastering of the technical challenges during component acceptance testing and during machine assembly. Latest results of the assembly work are especially highlighted. The scope of the construction of W7-X was modified and additional acceleration measures were implemented to mitigate risks and delays. Some aspects of these changes are explained in this paper.

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Status of Wendelstein 7-X construction

Deep learning for plasma tomography using the bolometer system at JET

Assembly of the superconducting stellarator Wendelstein 7-X is well advanced, and commissioning of the device is being prepared. A first draft of the commissioning tasks has been developed and will be discussed in this paper.

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Transition From Construction to Operation Phase of the Wendelstein 7-X Stellarator

The Wendelstein 7-X (W7-X) modular stellarator is in the assembly phase at the Max-Planck-Institut fur Plasmaphysik (IPP) in Greifswald, Germany. The goal of the project is to demonstrate that this type of machine is a viable option for a fusion power-plant. The “pentagonal” magnet system of the machine encompasses 50 non-planar and 20 planar superconducting coils with sophisticated support structure. Structural reliability of components as well as resulting deformations and displacements during various modes of operation have to be considered not only for the magnet system but also throughout the whole cryostat whose main components are the plasma vessel, outer vessel, ports, and thermal insulation. A reliable prediction of the W7-X structural behaviour is only possible by employing complex finite element (FE) simulations with a hierarchical set of FE models. A special strategy has been developed and implemented for the task. The design is basically completed, main parameters are defined, and most of the W7-X components are manufactured. Therefore, the focus in the analysis is being shifted to the creation of parametric FE models which allow performing fast analyses of possible non-conformities, changes in the assembly procedure, and future exploration of operational limits. This paper gives an overview of the implemented analysis strategy, the applied safety margins, and focuses on the most remarkable results.

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Structural analysis of W7-X: Overview

Structural analysis of W7-X: From design to assembly and operation

The Wendelstein 7-X (W7-X) stellarator project goal is to demonstrate that the stellarator is a viable option for a fusion power plant. W7-X is in an advanced state of construction and has entered the assembly phase at the Max-Planck-Institute fur Plasmaphysik (IPP) in Greifswald, Germany. The W7-X “pentagonal” basic magnet system is highly sensitive to parameter variations; the cryostat comprises two vessels, which are interconnected elastically by 299 ports. The strategy of the structural analysis for this complex mechanical system is being developed and implemented with the ultimate goal to create a tree of numerical models which reliably predict the stellarator structural behaviour. This paper gives an overview of the strategy, addresses the critical issues and focuses on the most interesting results of the analyses.

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Structural analysis of W7-X: Main results and critical issues

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.

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

Wendelstein 7-X (W7-X), a modular advanced stellarator, is presently under construction at the Max-Planck-Institut fur Plasmaphysik in Greifswald. Its magnet system (MS) consists of 50 non-planar and 20 planar superconducting coils which are arranged in five identical modules. The cryogenic cooling pipe system (CPS) for the MS is one of the remaining major components under manufacture. Since it is the easiest to adapt, its design and structural analysis had to be postponed until the design of the main MS components like coils, structure, and bus-bar system, influencing the layout and mechanical behaviour of the piping, had been finalized. The complex three-dimensional routing of the cooling pipes is governed by avoiding clashes with the structure during cool-down and operation. Most of the pipe supports allow sliding and limited rotation in order to reduce stress levels, forces and moments in the pipes and supports. This given fact, in combination with high deformations of the supporting magnet system, makes the CPS behaviour highly non-linear. Furthermore, several connected components such as valves and coil terminals cannot take any significant loads. As a consequence, the transmission of forces and movements through these interfaces has to be limited by kinematic separation, using flexible hoses and compensators. The non-linear finite element (FE) model created for the first MS module to be assembled includes 159 pipe sections with a total length of 470 m. The system encompasses approximately 80 flexible hoses and compensators, as well as 136 supports with clamps interconnecting up to 6 neighbouring lines. This paper presents the strategy of the CPS analysis, the FE model, as well as the various boundary conditions and limitations due to design requirements. The results of this most challenging parametric study are discussed, including variations of hose and compensator flexibilities as well as of the support gap sizes.

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D. Zacharias

Papers

Structural analysis of W7-X: Overview

Structural analysis of W7-X: From design to assembly and operation

Structural analysis of W7-X: Main results and critical issues

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

Structural analysis of the W7-X cryogenic pipe system