F. Schauer

The W7-X project: scientific basis and technical realization

The Wendelstein 7-X Stellarator (W7-X) is the next step device in the stellarator line of IPP Garching. A new branch of IPP is being built at Greifswald, Germany, to house W7-X. The design of W7-X is based on physics principles, which are discussed in the light of experimental results from the W7-AS stellarator. The experiment aims at demonstrating the inherent steady state capability of stellarators at reactor relevant plasma parameters and is therefore equipped with a modular superconducting twisted coil system. The 3D magnetic configuration of W7-X asks for a special divertor solution for steady state heat removal and decoupling of the vessel wall from the plasma. The status of the design and construction of W7-X including heating systems, divertor and diagnostics is presented.

Construction of Wendelstein 7-X - Engineering a steady state stellarator

The next step in the Wendelstein stellarator line is the large superconducting device Wendelstein 7-X, presently under construction in Greifswald. 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 at reactor relevant plasma conditions, as required for an economic fusion reactor. Such steady-state operation requires development of special technogies to be discussed in this paper.

Design and construction of WENDELSTEIN 7-X

Abstract Following the approval by EURATOM and the German government WENDELSTEIN 7-X (W7-X) is presently the largest fusion project under construction. W7-X is a helical advanced stellarator (HELIAS) which produces the confining magnet field only by magnet coils that enables steady-state operation. W7-X aims to demonstrate that the HELIAS configuration has the potential for a future power reactor. The successful application of new technologies for manufacturing prototypes and the positive results gained from tests allowed to design the machine in detail and to order major components. The geometry of the non-planar magnet coils has a considerable impact on the design of the machine in particular on the shape of the plasma vessel, positioning of the plasma-facing components and the size and orientation of the ports. The requirement for steady-state operation has consequences for many subsystems of W7-X. The magnet coils need to be superconducting and cooled to liquid helium temperature. Gyrotrons shall continuously provide 10 MW of ECR heating power. The divertor must be cooled to withstand heat loads of up to 10 MW/m 2 . The schedule of W7-X is determined by the delivery dates of the non-planar coils, the plasma vessel and the outer vessel. Start of commissioning and scientific operation is scheduled for spring 2006.

Specific Features of Wendelstein 7-X Structural Analyses

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, that is, 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 toward 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. This paper focuses on the specific features in the development, evolution, and realization of analysis strategies, implemented numerical approaches. Further specific subjects are standards and codes, 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 numerical 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.

Tracking of the magnet system geometry during Wendelstein 7-X construction to achieve the designed magnetic field

Wendelstein 7-X, currently under commissioning at the Max-Planck-Institut fur Plasmaphysik in Greifswald, Germany, is a modular advanced stellarator, combining the modular coil concept with optimized properties of the plasma. Most of the envisaged magnetic configurations of the machine are rather sensitive to symmetry breaking perturbations which are the consequence of unavoidable manufacturing and assembly tolerances. This overview describes the successive tracking of the Wendelstein 7-X magnet system geometry starting from the manufacturing of the winding packs up to the modelling of the influence of operation loads. The deviations found were used to calculate the resulting error fields and to compare them with the compensation capacity of the trim coils.

Coupled FEM-DBEM method to assess crack growth in magnet system of Wendelstein 7-X

The fivefold symmetric modular stellarator Wendelstein 7-X (W7-X) is currently under construction in Greifswald, Germany. The superconducting coils of the magnet system are bolted onto a central support ring and interconnected with five so-called lateral support elements (LSEs) per half module. After welding of the LSE hollow boxes to the coil cases, cracks were found in the vicinity of the welds that could potentially limit the allowed number N of electromagnetic (EM) load cycles of the machine. In response to the appearance of first cracks during assembly, the Stress Intensity Factors (SIFs) were calculated and corresponding crack growth rates of theoretical semi-circular cracks of measured sizes in potentially critical position and orientation were predicted using Paris’ law, whose parameters were calibrated in fatigue tests at cryogenic temperature. In this paper the Dual Boundary Element Method (DBEM) is applied in a coupled FEM-DBEM approach to analyze the propagation of multiple cracks with different shapes. For this purpose, the crack path is assessed with the Minimum Strain Energy density criterion and SIFs are calculated by the Jintegral approach. The Finite Element Method (FEM) is adopted to model, using the commercial codes Ansys or Abaqus;, the overall component whereas the submodel analysis, in the volume surrounding the cracked area, is performed by FEM (“FEM-FEM approach”) or alternatively by DBEM (“FEM-DBEM approach”). The “FEM-FEM approach” considers a FEM submodel, that is extracted from the FEM global model; the latter provide the boundary conditions for the submodel. Such approach is affected by some restrictions in the crack propagation phase, whereas, with the “FEM-DBEM approach”, the crack propagation simulation is straightforward. In this case the submodel is created in a DBEM environment with boundary conditions provided by the global FEM analysis; then the crack is introduced and a crack propagation analysis has been performed to evaluate the effects of the crack shape and of the presence of nearby cracks on the allowed number of EM load cycles.

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.

Assembly and test of the W7-X demo-cryostat

An overview is given on the status of the demo-cryostat project for the WENDELSTEIN 7-X stellarator. Construction and assembly of the prototype are finished, and the test period is near completion. The intention of this project was to get experience with design and construction of W7-X-components, as well as with assembly of this complex system. The goal is now practically achieved, and it could be demonstrated that the W7-X cryostat can be built with reasonable effort. Many of the solutions found can be adopted directly for W7-X, or are starting points for further improvements. A short description is given of the cryostat, its assembly, and of the most important tests which were performed so far.

Cryotechnology for Wendelstein 7-X

Abstract For the underlying task to cool the Wendelstein 7-X (W7-X) superconducting coil conductors and the divertor cryo-pumps, an extended cryo-system is required. Besides the magnet coils and cryo-pumps proper, the coil housings and supports, the thermal shields of the cryostat and cryo-pumps, and the current leads (CL) have to be cooled. This is achieved by a refrigeration plant including coolant transfer lines and distribution boxes, and a widespread pipework within the cryostat. All these components are finally defined, and purchase actions are under way.

Wendelstein 7-X Mechanical Instrumentation System for Commissioning and Operation

The modular stellarator Wendelstein 7-X in Greifswald, Germany, is currently in the state of commissioning. The sophisticated superconducting magnet system with 50 non-planar and 10 planar coils will be operated at 4K and has to sustain high electromagnetic loads. The likewise quite complex cryostat comprises the outer vessel, plasma vessel, and 254 ports of different types connecting the plasma and outer vessels. The magnet and cryostat systems are instrumented with more than 800 strain gauges, distance, and contact sensors. Implementation and expected results of this extended mechanical instrumentation is the scope of this paper.