Completion of designing and manufacturing of the coil support structure of W7-X

In February 2000, the project called coil support structure for the Wendelstein 7-X fusion machine was started. Since October 2009 the full production of this big (80 tons) and complex component is now completed and delivered at IPP Greifswald. The W7-X coil system consists of 20 planar and 50 non-planar coils. They are supported by a pentagonal 10 m diameter, 2.5 m high called coil support structure (CSS). The CSS is divided into five modules and each module consists of two equal half modules around the radial axis. Currently, the five modules were successfully assembled with the coils meeting the tight manufacturing tolerances. Designing, structural calculation, raw material procurement, welding & soldering technologies, milling, drilling, accurate machining, helium cooling pipe forming, laser metrology, ultra sonic cleaning and vacuum test are some of the key points used all along this successful manufacturing process. The lessons learned in the large scale production of this difficult kind of support structure will be presented as relevant experience for the realization of similar systems for future fusion devices, such as ITER.     

FE analyses and tests in support of Wendelstein 7-X trim coil development

The paper presents an overview of the finite element (FE) analyses on the trim coils (TCs) and tests performed at IPP, and focuses on some specific aspects of the TC assembly. Global FE models (GM) of two types of TCs predicted the coil deformations and reaction forces on the supports. Dead weight, temperature rise during operation up to 80 °C, electromagnetic loads of 19 field configurations, and outer vessel (OV) deformations have been taken into account. In addition, local models were created for the critical winding pack (WP) regions as well as for each support clamp, and were analyzed with boundary conditions determined by the GM. The detailed WP sub-models were developed to verify the insulation strength against local stresses. The simulations were supported by various mechanical tests under static and cyclic loads. Aim of these experiments was to determine the time dependent behavior of the silicone rubber pads which are used as interfaces between TC and supports, and to check and optimize the assembly tools as well as procedures for the TCs and their supports.     

Structural analysis of W7-X: Overview

The Wendelstein 7-X (W7-X) modular stellarator is in the assembly phase at the Max-Planck-Institut für 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.     

Design and analysis of the W7-X divertor scraper element

Thehigh heat-flux divertor of the Wendelstein 7-X large stellarator experiment consists of 10 divertor units which are designed to carry a steady-state heat flux of 10 MW/m². However, the edge elements of this divertor are limited to only 5 MW/m², and may be overloaded in certain plasma scenarios. It is proposed to reduce this heat by placing an additional “scraper element” in each of the ten divertor locations. It will be constructed using carbon fiber composite (CFC) monoblock technology. The design of the monoblocks and the path of the cooling tubes must be optimized in order to survive the significant steady-state heat loads, provide adequate coverage for the existing divertor, be located within sub-millimeter accuracy, and take into account the boundaries to other in vessel components, all at a minimum cost. Computational fluid dynamics modeling has been performed to examine the thermal transfer through the monoblock swirl tube channels for the design of the monoblock orientation. An iterative physics modeling and computer aided design process is being performed to optimize the placement of the scraper element within the severe spatial restrictions.     

Processing of the quench detection signals in W7-X

The Wendelstein 7-X (W7-X) project uses superconductive coils for generation of the magnetic field to keep the plasma. One of the important safety systems is the protection against quench events. The quench detection system of W7-X protects the superconducting coils, the superconducting bus bar sections and the high temperature superconductor of the current leads against the damage because of a quench and against the high stress by a fast discharge of the magnet system.Therefore, the present design of the quench detection system (QDS) uses a two-stage safety concept for discharging the magnetic system. This paper describes the present design of the system assembly from the quench detection unit (QDU) for the detection of the quench to the quench detection interface (QDI) to implement the two-stage safety concept.     

Evaluation of the mechanical behavior of W7-X central support connections by means of semi-automated FE analysis

The stellarator Wendelstein 7-X is under construction at the Max-Planck-Institut für Plasmaphysik in Greifswald. Its superconducting coil system is fixed by a massive structure. During machine operation the coils exert high forces and moments against each other and the central support structure (CSS). Therefore, the detailed analysis of the coil to CSS connections, the so-called central support elements (CSE), is a critical item. The major details of the design have been frozen; nevertheless, there is still need for detailed analysis of the CSEs due to assembly issues, and later on for exploring operational limits of the machine. These analyses have to be performed quickly, reliably, and shall provide results in a standardized form to enable timely responses to the assembly team. Special numerical tools – finite element (FE) parametric models of CSEs – have been developed for the purpose of such analyses. In the models, the geometry, material properties, contact conditions, loads as well as results presentation are defined in a parametric way. The use of the developed models for the definition of the final weld parameters, bolt preloads, assessment of acceptable tolerances, and optimal positions of the CSE-wedges before welding is also discussed.     

Overview of main-mechanical-components and critical manufacturing aspects of the Wendelstein 7-X cryostat

Wendelstein 7-X (W7-X) will be the world's largest superconducting helical advanced stellarator. This stellarator concept is deemed to be a desirable alternative for a future power plant like DEMO. The main advance of the static plasma is caused by the three dimensional shape of some of the main mechanical component inside the cryostat. The geometry of the plasma vessel is formed around the three dimensional shape of the plasma. The coils and their support structure are enclosed within the outer vessel. The space between the outer, the plasma vessel and the ports is called cryostat because the vacuum inside provides thermal insulation of the magnet system which is cooled down to 4 K. Due to the different thermal movements of both vessels and the support structure have to be supported separately. 10 cryo legs will bear the coil support structure. The plasma vessel supporting system is divided into two separate systems, allowing horizontal and vertical adjustments. This paper aims to give an overview of the main mechanical components of the cryostat. The authors delineate some disparate and special problems during the manufacturing of the components at the companies in Europe. It describes the current manufacturing and assembly.     

Thermo-mechanical tests on W7-X current lead flanges

Fourteen pieces of high temperature superconducting current leads (CL) arranged in seven pairs, will be installed on the outer vessel of Wendelstein 7-X (W7-X) stellarator. In order to support the CL, it is provided with two glass fiber reinforce plastic (GFRP) flanges, namely, the lower cryostat flange (CF) remaining at room temperature and upper radial flange (RF) at about 5 K. Both the flanges i.e. CF & RF experience high mechanical loads with respect to the CL, due to the evacuation of W7-X cryostat, cool-down of cold mass including the CL, electro-magnetic forces due to current & plasma operations and self weight of CL. In order to check the integrity of these flanges for such mechanical loads, thermo-mechanical tests were carried out on these flanges at room temperatures and at liquid nitrogen (LN2) temperatures. The details of test set-up, results and modeling are described in the paper.     

Thermal and structural analysis of the W7-X magnet heat radiation shield

The thermal insulation of the W7-X – cryostat consists of multi-layer insulation (MLI) and an actively cooled thermal shield. The shield is cooled by He gas flowing in pipes, which are flexibly attached via copper braids. The paper presents the basic mechanical and thermal layout of the complex plasma vessel shield, which is made of a glass fibre compound with three embedded copper nets.Main mechanical loads on the shield are electromagnetic forces resulting from rapid shut down of the magnet system, and the self weight. Design and calculations were performed iteratively to determine the number as well as orientation of electrical insulation gaps within the copper nets, and the number and positions of the supports.It is shown that the maximum displacements of the panels, the maximum forces on the supports, and the shield temperatures fulfil the requirements.     

Design and manufacturing status of trim coils for the Wendelstein 7-X stellarator experiment

The stellarator fusion experiment Wendelstein 7-X (W7-X) is currently under construction at the Max-Planck-Institut für Plasmaphysik in Greifswald, Germany. The main magnetic field will be provided by a superconducting magnet system which generates a fivefold toroidal periodic magnetic field. However, unavoidable tolerances can result in small deviations of the magnetic field which disturb the toroidal periodicity. In order to have a tool to influence these field errors five additional normal conducting trim coils were designed to allow fine tuning of the main magnetic field during plasma operation. In the frame of an international cooperation the trim coils will be contributed by the US partners. Princeton Plasma Physics Laboratory has accomplished several tasks to develop the final design ready for manufacturing e.g. detailed manufacturing design for the winding and for the coil connection area. The design work was accompanied by a detailed analysis of resulting forces and moments to prove the design. The manufacturing of the coils is running at Everson Tesla Inc; the first two coils were received at IPP.     

Design and assembly technology for the thermal insulation of the W7-X cryostat

The Max-Planck-Institut für Plasmaphysik in Greifswald is building up the stellarator fusion experiment Wendelstein 7-X (W7-X). To operate the superconducting magnet system the vacuum and the cold structures are protected by a thermal insulated cryostat. The plasma vessel forms the inner cryostat wall, the outer wall is realised by a thermal insulated outer vessel. In addition 254 thermal insulated ports are fed through the cryogenic vacuum to allow the access to the plasma vessel for heating systems, supply lines or plasma diagnostics.The thermal insulation is being manufactured and assembled by MAN Diesel & Turbo SE (Germany). It consists of a multi-layer insulation (MLI) made of aluminized Kapton with a silk like fibreglass spacer and a thermal shield covering the inner cryostat surfaces. The shield on the plasma vessel is made of fibreglass reinforced epoxy resin with integrated copper meshes. The outer vessel insulation is made of brass panels with an average size of 3.3 × 2.0 m². Cooling loops made of stainless steel are connected via copper strips to the brass panels. Especially the complex 3 D shape of the plasma vessel, the restricted space inside the cryostat and the consideration of the operational component movements influenced the design work heavily. The manufacturing and the assembly has to fulfil stringent geometrical tolerances e.g. for the outer vessel panels +3/?2 mm.     

Experiment planning using high-level component models at W7-X

The superconducting stellarator Wendelstein 7-X (W7-X) is a fusion device, which is capable of steady state operation. Furthermore W7-X is a very complex technical system. To cope with these requirements a modular and strongly hierarchical component-based control and data acquisition system has been designed.The behavior of W7-X is characterized by thousands of technical parameters of the participating components. The intended sequential change of those parameters during an experiment is defined in an experiment program. Planning such an experiment program is a crucial and complex task. To reduce the complexity an abstract, more physics-oriented high-level layer has been introduced earlier. The so-called high-level (physics) parameters are used to encapsulate technical details.This contribution will focus on the extension of this layer to a high-level component model. It completely describes the behavior of a component for a certain period of time. It allows not only defining simple value ranges but also complex dependencies between physics parameters. This can be: dependencies within components, dependencies between components or temporal dependencies.Component models can now be analyzed to generate various views of an experiment. A first implementation of such an analyze process is already finished. A graphical preview of a planned discharge can be generated from a chronological sequence of component models. This allows physicists to survey complex planned experiment programs at a glance.     

Resource checking and event handling within the W7-X segment control framework

ITER, Wendelstein 7-X, LHD, and TORE SUPRA are experimental facilities designed to lead the way to steady state fusion devices. These experiments require strategies to sustain a discharge in case of unforeseen events, e.g. heat overloads of plasma facing components or the failure of a plasma heating source. A recovery strategy is needed to get the discharge back for physics exploitation. For this purpose the W7-X segment control framework provides means for automated event detection along with options to formulate and initiate a recovery strategy. Besides handling of failures and degradation there are events that represent a desired plasma physical effect. An example for this kind of event is a transition to from Low to High-Confinement mode. These events indicate that a certain plasma state is reached and scientific examination can be altered thus enabling event-driven multiple experiments per discharge. Examples of both kinds of events will be presented and compared to other approaches in the community.     

Asymmetry of W7-X magnet system introduced by torus assembly

The magnet system of the stellerator Wendelstein 7-X (W7-X) consists of 5 modules of 14 superconducting coils with complex 3D shape each. After manufacturing the coils and assembly of the modules on temporary stands, the position of each module on the machine base was successfully optimized to minimize the electromagnetic (EM) field asymmetry. This asymmetry originates from inevitable geometric deviations of the coils from the target shape due to manufacturing and assembly tolerances.However, new deviations were introduced after module optimization due to bolting the modules of the magnet system together to a torus, removing temporary supports and further loading of the machine base with weight of additional components.In this paper, the geometrical deviations along the centre line of the coil currents are assessed through detailed step-by-step non-linear finite element (FE) simulation of the assembly procedure of the complete torus. The model is evaluated against measured displacements and reaction forces monitored during consequent assembly steps. The results are being used to quantify the obtained field asymmetry and countermeasures to minimize it.     

Pre-series and testing route for the serial fabrication of W7-X target elements

The fabrication of the actively cooled high-heat flux divertor of the WENDELSTEIN 7-X stellarator (W7-X) requires the delivery of 890 target elements, which are designed to withstand a stationary heat flux of 10 MW/m². The organization of the manufacturing and testing route for the serial fabrication is the result of the pre-series activities. Flat CFC Sepcarb^® NB31 tiles are bonded to CuCrZr copper alloy cooling structure in consecutive steps. A copper layer is active metal cast to CFC tiles, and then an OF-copper layer is added by hot isostatic pressing to produce bi-layer tiles. These tiles are bonded by electron beam welding onto the cooling structure, which was manufactured independently. The introduction of the bi-layer technology proved to be a significant improvement of the bond reliability under thermal cycling loading. This result is also the consequence of the improved bond inspections throughout the manufacturing route performed in the ARGUS pulsed thermography facility of PLANSEE. The repairing process by electron beam welding of the bonding was also qualified. The extended pre-series activities related to the qualification of fabrication processes with the relevant non-destructive examinations aim to minimize the risks for the serial manufacturing and to guarantee the steady-state operation of the W7-X divertor.     

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

The Wendelstein 7-X (W7-X) modular stellarator is in the assembly phase at the Max-Planck-Institut für Plasmaphysik in Greifswald, Germany. The design of the “basic machine”, i.e. without in-vessel components, diagnostics and periphery, is largely completed, structural parameters such as bolt preload, initial conditions for contact elements, etc. are defined, and most of the components are manufactured and partly assembled. Therefore, the focus of structural analysis was shifted towards fast analyses of non-conformities, changes in the assembly procedure, and exploration of operational limits. Assembly-related work is expected to continue until commissioning of the machine, however, with decreasing intensity. In parallel the analysis requirements for in-vessel components, diagnostics and periphery will increase.This paper focuses on the most remarkable results, on special problems which had to be solved, on strategic issues like parameterization, complex finite element model structuring and benchmarking with alternative models in different codes, on assumptions of reasonable safety margins and expected tolerances, and on confirmation of analysis results by tests. Finally it highlights some lessons learned so far, which might be relevant also for other large fusion machines, and gives an outlook on future work.     

Design analysis and manufacturing of the cooling lines of the in vessel components of WENDELSTEIN 7-X

All in vessel components (IVCs) of W7-X are actively cooled. Inside the plasma vessel about 4 km of pipes will be installed, supplying water to the IVC. 226 cooling circuits with 78 variants are necessary. The cooling circuits enter the cryostat and the plasma vessel through ad hoc flanged penetrations called “plug-ins”, which provide for the vacuum boundary between the plasma chamber and the torus hall atmosphere. The plug-ins are installed inside the W7-X ports. Some of the plug-ins are also used for the diagnostic cables. In total eighty plug-ins will be produced and installed. The inlet/outlet cooling lines are connected to the plug-ins using a welded hydraulic connector. The layout of the cooling lines is rather complex in consideration of the limited space and the routing between many component parts. Additionally the differential thermal expansion of the lines with respect to the supporting structures during the different operation scenarios had to be compensated by ad hoc supports and adjustments in the flexibility of the lines.     

Simulations of W7-X magnet system fault scenarios involving short circuits

The stellarator Wendelstein 7-X (W7-X) which is presently under construction at the Max-Planck-Institut für Plasmaphysik in Greifswald [(C. Beidler, G. Grieger, F. Herrnegger, E. Harmeyer, J. Kißlinger, W. Lotz, H. Maassberg, P. Merkel, J. Nührenberg, F. Rau, J. Sapper, F. Sardei, R. Scardovelli, A. Schlüter and H. Wobig., Physics and engineering design for Wendelstein 7-X, Fusion Technol., 17 (1990)148–168)], uses 70 superconducting coils, arranged in 7 groups to create the magnetic confinement for the plasma. A wide variety of tests and investigations are performed in order to ensure the later safe operation of the device. Much attention is also paid to the proper insulation of all the parts. These measures are costly and time consuming but are necessary in order to avoid the severe consequences of faults–—especially short circuits–—during operation. If a short circuit would happen during an emergency switch-off, the discharge of any shorted inductance would be delayed, and the coupled magnetic flux of the discharging system would induce additional currents into this shorted part. The currents and forces developing in such a case depend not only on the short circuit resistance and the critical current of the superconductor, but also on the shorted inductance itself, its magnetic coupling to other inductances, and its position within the system. The paper describes the influences of these factors and presents simulation results for different fault scenarios involving short circuits across a coil group, a single coil, different double layers, and a single turn. Maximum currents result from a shorted outer double layer, maximum forces from a shorted coil group, depending on its position in the magnet system.     

W7-X neutral-beam-injection: Selection of the NBI source positions for experiment start-up

The stellarator W7-X will be equipped with two Neutral-Beam-Injector (NBI) boxes for balanced injection. Each NBI box has 2 tangential and 2 radial source positions. For the experimental start-up phase each NBI box will be only equipped with 2 ion sources. For the selection of the initial 2 NBI source positions per box three physical aspects were examined (transmission and duct power deposition, shine through and heating efficiency).Using hydrogen injection the heating power to the plasma under typically planned conditions should be 1.3 MW for the tangential sources and 1.1 MW for the radial sources (deuterium: 2 MW for the tangential sources, 1.8 MW for the radial sources). The tangential source positions all have similar heating efficiencies. One of them suffers from the lowest duct transmission (highest power-load to the duct). The same source hits a component with a low power-load capability. The W7-X inner wall design will be modified in order to enhance the maximum power-load capability of that component. For the radial source positions there is no clear physics advantage of one position over the other. Taking all aspects into consideration the decision was made to use one tangential source and one radial source per box during the experimental start-up phase.