Design principles for handmade electrical insulation of superconducting joints in W7-X

The superconducting magnet system of the Wendelstein 7-X (W7-X) experiment consists of 50 non-planar and 20 planar coils, 121 bus bars and 14 current leads. The connection between bus bars, coils and current leads will be provided by 198 joints. The joints have to be insulated manually during the assembly of the machine in constraint positions and a tight environment. In general the insulation is based on glass tapes impregnated with epoxy resin and special G10 insulating pieces embedded in the glass tape insulation. In critical areas Kapton^®-foils are embedded in the insulation. All types of insulation were qualified at mock-ups in a 1:1 model of the expected environment in W7-X. The qualification programme comprises thermal cycling between room temperature and 77 K and high voltage tests under air, under vacuum and under reduced pressure (Paschen test). The paper describes the main principles used for different types of handmade Paschen-tight insulations in W7-X and the visual and electrical tests during and after assembly.     

Designing and manufacturing of the coil support structure of W7-X

Wendelstein 7-X (W7-X) is a fully optimized low-shear stellarator and shall demonstrate the reactor potential of this fusion plant. It is presently under construction at the Greifswald Branch Institute of IPP. The superconducting magnet system will allow continuous operation, limited only by the plasma exhaust system whose capacity is designed for 30 min full power operation. The Wendelstein 7-X (W7-X) coils and structures are part of the largest superconducting fusion device being constructed at present. They represent a technical challenge at industrial level and the need for proven techniques and manufacturing processes in accordance to the highest quality standards. The production of these components requires a management of monitoring for quality and tests. The coil system consists of 20 planar and 50 non-planar coils. They are supported by a pentagonal 10 m diameter, 2.5 m high coil support structure (CSS). The CSS is divided into five modules. Each module consists of two equal half modules. The manufacturing status of the CSS and the main project management and technical challenges will be presented. 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.     

Configuration space control for Wendelstein 7-X

The Wendelstein 7-X stellarator (W7-X) is a superconducting fusion experiment, presently under construction at the Greifswald branch of the Max-Planck-Institut für Plasmaphysik. W7-X is a device with high geometrical complexity due to the close packing of the components in the cryostat and their complex 3D shape e.g. of the superconducting coils. The tasks of configuration space control are to ensure that all these components do not collide with each other under a set of defined configurations, i.e. at the time of assembly, at 4 K or for various coil currents. To fulfill these tasks sophisticated tools and procedures were developed and implemented within the realm of a newly founded division that focuses on design, configuration control and configuration management.     

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.     

Influence of construction errors on Wendelstein 7-X magnetic configurations

Wendelstein 7-X, currently under construction at the Max-Planck-Institut für Plasmaphysik in Greifswald, Germany, is a modular advanced stellarator, combining the modular coil concept with optimised properties of the plasma. The magnet system of the machine consists of 50 non-planar and 20 planar superconducting coils which are arranged in five identical modules, forming a toroidal five-fold symmetric system. The majority of operational magnetic configurations will have rotational transform ι/2π = 1 at the boundary. Such configurations are very sensitive to symmetry breaking perturbations, which are the consequence of imprecisely manufactured coils or assembly errors. To date, all 70 coils have been fabricated, and the first two half-modules of the machine have been assembled. The comparative analysis of manufactured winding packs and estimates of the corresponding level of magnetic field perturbation are presented. The dependency of the error fields on the coil assembly sequence is considered, as well as the impact of the first assembly errors. The influence of different construction uncertainties is discussed, and measures to minimise the magnetic field perturbation are suggested.     

Finite Element Analyses and Instrumentation Layout for Single Coil Testing of TF Coils in HT-7U

The HT-7U tokamak is a magnetically-confined full superconducting fusion device, consisting of superconducting toroidal field (TF) coils and superconducting poloidal field (PF) coils. These coils are wound with cable-in-conductor (CICC) which is based on UNK NbTi wires made in Russian '. A single D-shaped toroidal field magnet coil will be tested for large and expensive magnets systems before assembling them in the toroidal configuration. This paper describes the layout of the instrumentation for a superconducting test facility based on the results of a finite element modeling of the single coil of toroidal magnetic field (TF) coils in HT-7U tokamak device. At the same time, the design of coil support structure in the test facility is particularly discussed in some detail.     

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.     

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.     

How should we test the ITER TF coils?

The international thermonuclear experimental reactor (ITER) toroidal field (TF) magnet system consists of 18 superconducting coils using a 68 kA Nb₃Sn conductor. In order to guarantee the performances of these coils prior to their installation, the test of at least one prototype coil at liquid helium temperature and full current is required. The test of all coils in the two-coil test configuration, with successive charging of each coil to nominal current is recommended. This requires a large test facility.     

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.     

Present status and one upgrading method with a cold compressor of the EAST sub-cooling helium cryogenic system

Since 2006,the superconducting toroidal field (TF) coils of the Experimental Advanced Superconducting Tokomak (EAST) have been successfully cooled by supercritical helium at a temperature of 4.5 K and a pressure of 4 bara in eleven experiments.To obtain higher operating currents and magnetic fields it is necessary to lower the operating temperature of the TF coils.The EAST sub-cooling helium cryogenic system,with a warm oil ring pump (ORP),was tested twice in cool-down experiments,which made the TF coils operate at 3.8 K.However,the long term operational stability of the sub-cooling system cannot be guaranteed because of the ORP's poor mechanical and control performance.In this paper,the present status of the EAST subcooling helium cryogenic system is described,and then several cooling methods below 4.2 K and their merits are presented and analyzed.Finally,an upgrading method with a cold compressor for an EAST sub-cooling helium cryogenic system is proposed.The new process flow and thermodynamic calculation of the sub-cooling helium system,and the main parameters of the cold compressor,are also presented in detail.This work will provide a reference for the future upgrading of the sub-cooling helium system for higher operation parameters of the EAST device.     

AC operation of large titanium sublimation pumps in a magnetic field: Results of the test stand for the W7-X neutral beam injectors

A neutral beam injection (NBI) system is being built for the Stellarator experiment Wendelstein 7-X (W7-X) currently under construction at IPP Greifswald. The NBI system consists of two injectors which are essentially a replica of the system present in the Tokamak experiment ASDEX-Upgrade at IPP Garching. A vacuum system with high pumping speed and large capacity is required to ensure proper vacuum conditions in the neutral beam line. For this purpose, large titanium sublimation pumps (TSP) are installed inside the NBI boxes, consisting of 4 m long hanging wires containing Ti and the surrounding condensation walls. The wires are DC ohmically heated up with 142 A to Ti sublimation temperature. A TSP system has been operated since many years in the AUG-NBI system, sublimating Ti in the pauses between the plasma discharges, when no magnetic field is present. However, at W7-X the superconducting coils generate a magnetic field permanently during experimental campaigns, whose stray B field with a maximum of 30 mT, affects the TSPs. Operated with DC, the wires would be deflected against the surrounding panels due to the Lorentz force. A simple possible solution is heating with AC, which reduces the wire deflection amplitude, inducing a risky wire oscillation. The feasibility of the AC operation in an equivalently strong B field such as the stray B field around W7-X has been demonstrated in a test stand for different AC waveforms and frequencies. Several test campaigns have shown no qualitative difference in the pumping properties between AC and DC operation of the TSP and no critical dynamic behaviour of the wires.     

Manufacturing and assembly of the plasma- and outer vessel of the cryostat for Wendelstein 7-X

Wendelstein 7-X is an advanced helical stellarator, which is presently under construction at the Greifswald branch of IPP. A set of 70 superconducting coils arranged in five modules provides a twisted shaped magnetic cage for the plasma and allows steady state operation. Operation of the magnet system at cryogenic temperatures requires a cryostat which provides thermal protection and gives access to the plasma. The main components of the cryostat are the plasma vessel, the outer vessel, the ports, and the thermal insulation. The German company, MAN Diesel & Turbo SE Deggendorf (former MAN DWE GmbH Deggendorf), is responsible for the manufacture and assembly of the plasma vessel, the outer vessel and the thermal insulation. This paper describes the manufacturing and assembly technology of the plasma and outer vessel of the cryostat for Wendelstein 7-X.     

Manufacturing and assembly status of main components of the Wendelstein 7-X cryostat

The stellarator fusion experiment Wendelstein 7-X (W7-X) is at present in assembly at the Max-Planck Institut für Plasmaphysik (IPP).The toroidal plasma with a ring diameter of 11 m and an average plasma diameter of 1.1 m is contained within the plasma vessel. Its form is dictated by the shape of the plasma. The form of the plasma is controlled by the coil system configuration. To control the plasma form it is necessary that all the 20 planar and 50 non-planar coils should be positioned within a tolerance of 1.5 mm. To meet this requirement a complex coil support structure was created, consisting of the central support ring and the different inter coil supports. The coils and the support structure are enclosed within the outer vessel with its domes and openings. The space between the outer and the plasma vessel is called cryostat because the vacuum inside provides thermal insulation of the magnet system, and the entire magnetic system is then be cooled down to 4 K. Due to the different thermal movements the plasma vessel and the central support ring have to be supported separately. The central support ring is held by 10 cryo legs. The plasma vessel supporting system is divided into two separate systems, allowing horizontal and vertical adjustments to centre the plasma vessel during thermal expansion.This paper aims to give an overview of the main components in the cryostat like the plasma vessel, the outer vessel, the ports and the different support systems. It describes the current manufacturing and assembly status and the associated problems of these components, using pictures and text. This paper does not describe the general assembly situation or time schedules of the Wendelstein 7-X.     

Configuration Management for Wendelstein 7-X

A complex system like the large superconducting Wendelstein 7-X stellarator necessitates a dedicated organizational structure which assures permanent consistency between the requirements of its system specification and the performance attributes of all its components throughout its life time. This includes well-defined processes and centrally coordinated information structures. For this purposes the department Configuration Management (CM) has recently been established at W7-X. The detailed tasks of CM for W7-X are oriented along common CM standards and comprise configuration identification, change management, configuration status accounting and configuration verification. While the assembly of W7-X is proceeding some components are still under procurement or even under design. Thus design changes and non-conformances may have a direct impact on the assembly process. Highest priority has therefore been assigned to efficient control of change and non-conformance processes which might delay the assembly schedule.     

The in-vessel components of the experiment WENDELSTEIN 7-X

The in-vessel components of the WENDELSTEIN 7-X stellarator consist of the divertor components and the wall protection with its internal cooling supply. The main components of the open divertor are the vertical and horizontal target plates which form the pumping gap, the cryo-vacuum pumps and the control coils. The divertor volume is closed by graphite shielded baffle modules and with divertor closures. All these components are designed to be actively water-cooled. For the first commissioning phase planned in 2014, an inertial-cooled test divertor will be installed instead of the actively water-cooled high heat flux divertor. The wall protection consists of graphite-protected heat shields in the higher loaded areas and stainless steel panels in the lower loaded regions. The wall protection cooling circuits are connected through 80 supply-ports via so-called “plug-ins”. It is envisaged to protect the diagnostic ports by panel-type port-liners. Special graphite-shielded port liners are used on the diagnostic injector and the neutral beam injector ports. The in-vessel components are mainly manufactured and tested at the Max-Planck-Institute für Plasmaphysik in its Garching workshop. Panels, high heat flux target elements and control coils are delivered by industrial partners. Manufacturing of the KiP (“Komponenten im Plasmagefäß”) is in plan. Delivery of the components will be in time.     

Winding machines for the manufacturing of superconductive coils of the main European fusion research machines

The successfull construction of large magnets passes through the development and application of non-conventional manufacturing processes.A difficult and delicate step in the manufacturing of superconducting coils is the conductor winding technique. It is often a challenging and technologically advanced process, developed according to the requirements of each project.An important aspect during the winding is to avoid any deformation of the cable cross section leading to a damage of the strands and to maintain the design features of the cable.A second aspect is to assure the suitable repeatability and a production rate for an industrial process.The winding line is a system of different machines linked and tuned together properly designed for each project.An adapted software assures the overall process control.TPA realized for ANSALDO Superconduttori the winding lines for many projects: TFMC (NET-TEAM), CMS (INFN-CERN), WENDELSTEIN W7-X (Max Planck Institute, IPP), etc.The experience acquired in this field by ANSALDO Superconduttori and by TPA (as manufacturing tools and equipments supplier) has been acknowledged by CERN with “The CMS Gold Award” of the Year 2004. The paper describes the main features of the winding lines, the main problems, the technical solutions used for the above mentioned projects and the new ideas for the forthcoming ones.     

High Temperature Baking Test and Study on Axial Insulation Breaks

Axial insulation breaks are needed in ITER superconducting magnet system, which are used for separation of high voltage area from grounded cooling pipes system. To determine the maximum safe temperature that the insulating break can withstand without damage, such as preventing damage by overheating during welding of the insulating breaks to the helium cooling pipes for the superconducting magnets and high temperature baking to remove moisture, the glass transition temperature was tested by using the dynamic mechanical analysis with the standard sample made from epoxy resin. Furthermore, the high temperature baking test of axial insulation break was performed, further helium tight test and high voltage tests indicate the baked insulation break is in good condition.     

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.     

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.