Showing papers by "Joris Fellinger published in 2013"

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

Abstract: 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.

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

Abstract: 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.

Influence of assembly and operation asymmetries on Wendelstein 7-X magnetic field perturbations

Abstract: Wendelstein 7-X is a modular advanced stellarator realizing a 5-period Helias configuration. An important part of the planned operational plasma scenarios is characterised by a rotational transform ι/2π =1 at the plasma boundary. Such configurations are very sensitive to symmetry breaking perturbations, resonant with the value of the rotational transform at the boundary and violating the toroidal periodicity of the magnetic field. The most critical consequences of magnetic field perturbations are modifications of the island topology, which can result in uneven loads on the divertor targets and affect the plasma performance. In order to keep the magnetic configuration of the machine as designed and to be prepared for further compensation methods, the level of possible magnetic field perturbations needs to be quantified. The initial sources of symmetry breaking perturbations are coil manufacturing and positio

Numerical modelling in the construction of Wendelstein 7-X

Abstract: 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.

FEM-DBEM coupled procedure for assessment of cracks in lateral supports of the magnet system of Wendelstein 7-X

Abstract: . The superconducting coils of the magnet system of Wendelstein 7-X (W7-X) 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. The Miner's rule was adopted to allow for different load combinations. The predefined semi-circular initial crack shape and crack growth limit, were set in such a way to avoid multiple crack coalescence even if such restrictions could have a significant effect on N. These results have been published in a first paper, whereas, in the current paper, this work has been extended with analyses of propagation of cracks with different shapes and affected by nearby cracks. For this purpose, the Dual Boundary Element Method (DBEM) was applied in a coupled FEM-DBEM approach: the crack path is assessed with the Minimum Strain Energy density criterion and the SIFs are calculated by the J-integral approach. The Finite element method (FEM) was adopted to model the overall component whereas the DBEM was adopted for the submodel analysis in the volume surrounding the cracked area. With this approach, the effects of the crack shape and the presence of nearby cracks on the allowed number of EM load cycles of the machine were demonstrated.