A. Panin

Overview of Progress on the EU DEMO Reactor Magnet System Design

The DEMO reactor is expected to be the first application of fusion for electricity generation in the near future. To this aim, conceptual design activities are progressing in Europe (EU) under the lead of the EUROfusion Consortium in order to drive on the development of the major tokamak systems. In 2014, the activities carried out by the magnet system project team were focused on the toroidal field (TF) magnet system design and demonstrated major achievements in terms of concept proposals and of consolidated evaluations against design criteria. Several magnet system R&D activities were conducted in parallel, together with broad investigations on high temperature superconductor (HTS) technologies. In this paper, we present the outcomes of the work conducted in two areas in the 2014 magnet work program: 1) the EU inductive reactor (called DEMO1) 2014 configuration (power plant operating under inductive regime) was the basis of conceptual design activities, including further optimizations; and 2) the HTS R&D activities building upon the consolidated knowledge acquired over the past years.

Structural Analysis of a Prototype Fast Shutter for ITER cCXRS Diagnostic

Optical lifetime of the first mirror is a critical issue for the ITER upper-port-plug core-charge-exchange-spectroscopy diagnostic. A fast shutter is engaged to protect the mirror from depositions between measurements. The prototype shutter will be examined in a test vacuum vessel that is now under development in Forschungszentrum Jülich, Germany. Being located near the plasma, the shutter operates under severe thermal and electromagnetic (EM) loads. The multifield analyses conducted for the shutter are presented in this paper. Since the fast shutter can operate within 0.7 s, its static structural analysis should be accompanied by dynamic studies. This paper gives details about the numerical strategy used for a multifield ANSYS modeling of a complex structure. The shutter structural performance under the service, thermal, and EM loadings is in line with the requirements. A solution for a problem of high local thermostresses revealed by the analysis is proposed. Problems connected with other possible port-plug-shutter layouts are discussed.

Design of Narrow Support Elements for Non Planar Coils of Wendelstein 7-X

The stellarator Wendelstein 7-X is presently under construction and assembly in Greifswald, Germany. One of the main structural elements which have to take the electromagnetic forces of the superconducting coil system are the narrow support elements (NSE). They are placed between the non planar coils and have to take very high compressive forces while relative sliding and tilting of the coils must be allowed. The design has been optimised with regard to a proper load distribution among all support elements taking also into account manufacturing and assembly tolerances. The paper describes the design, analysis and tests which have been carried out for the NSEs

Approaches to numerical modeling in development process of complex structures for fusion devices

Development of components for fusion device normally comprises numerous simulations to validate their design compatibility with the specified loads according to applicable codes and standards. A strategy of multifield finite element analyses is determined by an iterative nature of design process, and a level of a structure complexity dictates analysis approaches and modeling technique. Nowadays powerful software exploiting the strength of modern computers allows simulating structures in a very detailed way, even for some types of coupled field analyses. As some drawback of this tendency, many efficient numerical tools, used in the past, are not frequently engaged. Sometimes huge numerical resources are spent to solve trivial problems. Direct CAD-analysis meshing of structures in all their complexity may mix effects of different importance and may lead to overlooking some key structure features. Computational problems with convergence in nonlinear analyses often make modeling less flexible to design changes. This paper gives examples of engineering simplified modeling made with clear understanding of an analysis goal and a role of simulations in design process, like for the W7-X stellarator busbar system. Engineering methods to perform multifield analysis including electromagnetic transients for some ITER diagnostics are also presented. An engineering material model to calculate steel superconducting structures working in inelastic range is discussed.

Approaches to Analyze Structural Issues of the European DEMO Toroidal Field Coil System at an Early Design Stage

Because of its ambitious goals, the DEMO project faces many technical challenges. The mechanical performance of the superconducting toroidal field coil (TFC) system is mostly determined by the high electromagnetic forces. A structural evaluation of the TFC of the EU DEMO 2014/2015 baseline configurations is presented. The emphasis is focused on the parametric study with the use of simplified numerical modeling. Physical interpretation of the numerical results makes the system mechanical behavior transparent and directly leads to the design recommendations. An express approach is proposed to reconstruct the detailed conductor stress state from the homogenized winding. This allows for sorting out the WP designs with respect to their robustness against acceptance criteria while giving a way for their improvement. An analysis workflow for the TFC system stress analysis is proposed. The main features of a semianalytical procedure allowing for the coil predimensioning prior to the 3-D numerical simulations are highlighted. It features the structural optimization of the layered winding, thus defining the minimum space required for the coil winding.

Strategies of Numerical Modeling in Component Development for Fusion Devices

The development of components for fusion devices includes numerous simulations to validate their design compatibility with specified loads according to applicable codes and standards. The strategy of multifield finite element analyses is determined by the iterative nature of the design process. The level of structural complexity dictates the analytical approaches and modeling techniques. Even though nowadays powerful software exploiting the performance of modern computers allows structures to be simulated in a very detailed way, direct meshing of CAD models with all their complexity is often not beneficial for nonlinear and multifield analyses. Reasonable modeling simplifications, submodeling, and integration with analytical modeling and testing can optimize the analysis process. This paper gives examples of simplified modeling for engineering purposes with a clear understanding of the analysis goal and the role of simulations in the design process, such as for the W7-X stellarator busbar system. Engineering methods to perform multifield analyses including electromagnetic transients for ITER diagnostic port plug components are also presented. An engineering material model is discussed for calculating the superconducting coil steel structures operating in the inelastic range.

Selected Design Solutions for the Integration of the CXRS Diagnostic in to ITER Upper Port Plug No. 3

Diagnostic plug for the ITER core charge exchange recombination spectroscopy (core CXRS) is located in the upper port 3. It transfers the light emitted by interaction of plasma ions with the diagnostic neutral beam (DNB). Conceptual design study of the core CXRS port plug has indicated several challenging technical problems: (1) likely too short lifetime of the first mirror, (2) quite contradictory requirements to the first mirror holder, (3) harsh environmental conditions for the “shutter”, that is a movable element protecting the first mirror, (4) a task to combine a sufficient structural integrity and nuclear shielding capability of the plug with a wide enough optical path, (5) excessive electromagnetic loads caused by the halo current and applied at the plug as a whole. This paper describes possible design solutions for the listed technical problems.

Sensitivity Study of Mechanical Behavior of Busbar System Designed for Wendelstein 7-X Stellarator

The Wendelstein 7-X stellarator is now under construction in Greifswald, Germany. The superconducting busbar system for the stellarator coils is designed and manufactured at the Forschungszentrum Jiilich, Germany. The electromagnetic forces on the long busbars, considerable relative tilting of the coils carrying the busbar supports, lack of available space required for the robust design of these supports, collisions with other systems during operation as well as the cooling down process had to be taken into account during design phase. Having a complex layout and complying with different, often contradictory requirements, the busbar system turned out to be sensitive to the manufacture and assembly tolerances. The modular structure of the magnet system implied the modular sequence of the busbar system production. The mechanical behavior of the busbar system has been numerically simulated for the module 5. The developed global model reflects main important features of the complex system in a relatively simple way. It proved to be useful tool for frequent iterations during design period and for numerous tolerance studies. The paper describes the main features of the numerical model and some results of the sensitivity study of the busbar system.

Studies of protection and recovery techniques of diagnostic mirrors for ITER

In optical diagnostic systems of ITER, mirrors will be used to guide the light from plasma towards detectors and cameras. The mirrors will be subjected to erosion due to fast particles and to deposition of impurities from the plasma which will affect adversely the mirror reflectivity and therefore must be suppressed or mitigated at the maximum possible extent. Predictive modeling envisages the successful suppression of deposition in the diagnostic ducts with fins trapping the impurities on their way towards mirrors located in the end of these ducts. To validate modeling predictions, cylindrical and cone-shaped diagnostic ducts were exposed in TEXTOR for 3960 s of plasma operation. After exposure, no drastic suppression of deposition was observed in the cylindrical ducts with fins. At the same time, no detectable deposition was found on the mirrors located at the end of cone-shaped ducts outlining the advantages of the cone geometry. Analyses of exposure provide evidence that the contamination of exposed mirrors was due to wall conditioning discharges and not due to working plasma exposure. Cleaning by plasma sputtering was performed on molybdenum mirrors pre-coated with a 100 nm thick aluminum film. Aluminum was used as a proxy of beryllium. During exposure in electron cyclotron resonance-generated helium plasma, the entire coating was sputtered within nine hours, leaving no trace of aluminum and leading to the full recovery of the specular reflectivity without detrimental effects on the mirror surface.

The Superconducting Bus-Bar System for Wendelstein 7-X - A Challenge to Engineering

Forschungszentrum Julich is contributing to the Wendelstein 7-X project among others with the superconducting bus-bar system. The scope is the design, qualification, manufacturing, and assembly of the superconducting bus bars and its appropriate supports. An overall concept of the bus-bar manufacturing was developed with the goal to optimize the process, to simplify the system assembly, and to provide easy transportation. A suitable insulation set up fulfilling all specifications was developed. For the qualification of the insulation and the fabrication process different samples have been fabricated and tested. The bus-bar support system has to take considerable loads due to cool down shrinkage, electromagnetic forces, and displacements imposed by the magnet system. General considerations led to the choice of a system of standardized components which allows assembly of specific adjustable supports for each position. The complexity of the system geometry and its loading required an iterative approach of support design and stress analysis. Finally, an assembly sequence for the bus-bar system was found matching the technological requirements of the bus itself with all geometric and other constraints of the assembly facility.