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.     

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.     

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.     

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.     

Status of Wendelstein 7-X construction

Wendelstein 7-X (W7-X) represents the continuation of fusion experiments of the stellarator type at the Max-Planck Institute for Plasma Physics (IPP). The aim of W7-X is to demonstrate the suitability for a fusion reactor of this alternative type of magnetically confined plasma experiment. W7-X is being built at Greifswald in the northeast of Germany. The size of device (725 tons, height of 5 m, diameter 16 m) and the superconductive magnet system distinguish W7-X from earlier stellarators at IPP. The paper provides a summary of the status of the main components, the mastering of the technical challenges during component acceptance testing and during machine assembly. Latest results of the assembly work are especially highlighted. The scope of the construction of W7-X was modified and additional acceleration measures were implemented to mitigate risks and delays. Some aspects of these changes are explained in this paper.     

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.     

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.     

Analysis of the accident with the coolant discharge into the plasma vessel of the W7-X fusion experimental facility

Fusion is the energy production technology, which could potentially solve problems with growing energy demand of population in the future. Starting 2007, Lithuanian Energy Institute (LEI) is a member of European Fusion Development Agreement (EFDA) organization. LEI is cooperating with Max Planck Institute for Plasma Physics (IPP, Germany) in the frames of EFDA project by performing safety analysis of fusion device W7-X. Wendelstein 7-X (W7-X) is an experimental stellarator facility currently being built in Greifswald, Germany, which shall demonstrate that in the future energy could be produced in such type of fusion reactors. In this paper the safety analysis of 40 mm inner diameter coolant pipe rupture in cooling circuit and discharge of steam–water mixture through the leak into plasma vessel during the W7-X no-plasma “baking” operation mode is presented. For the analysis the model of W7-X cooling system (pumps, valves, pipes, hydro-accumulators, and heat exchangers) and plasma vessel was developed by employing system thermal-hydraulic state-of-the-art RELAP5 Mod3.3 code. This paper demonstrated that the developed RELAP5 model enables to analyze the processes in divertor cooling system and plasma vessel. The results of analysis demonstrated that the proposed burst disc, connecting the plasma vessel with venting system, opens and pressure inside plasma vessel does not exceed the limiting 1.1 × 10⁵ Pa absolute pressure. Thus, the plasma vessel remains intact after loss-of-coolant accident during no-plasma operation of Wendelstein 7-X experimental nuclear fusion facility.     

Manufacturing of the Wendelstein 7-X divertor and wall protection

The in-vessel components of Wendelstein 7-X (W7-X) with a total surface of 265 m² comprise the divertor and the wall protection. The high heat flux (HHF) and lower heat flux (LHF) target, the baffle, the end plates closing the divertor chamber, a cryo vacuum pump (CVP) and a control coil form one divertor unit. Steel panels and the graphite heat shield protect the wall, including the ports. The HHF target elements, the steel panels and the control coils are manufactured by industry. The remaining components will be manufactured by the Max-Planck-Institute für Plasmaphysik (IPP) at its Garching workshops. For all components the final acceptance tests will be performed by IPP. This paper summarizes the main aspects for manufacturing, the preceding development and qualification tests as well as the final acceptance tests for the in-vessel components.     

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.     

Gate valve and shutter control system of the fusion experiment Wendelstein 7-X

The plasma vessel of the fusion experiment Wendelstein 7-X (W7-X) is a plasma vessel covering a plasma volume of about 30 m³. The vacuum conditions for plasma experiments inside the plasma vessel are supposed to be in a range of 1 × 10⁻⁸ mbar (ultra high vacuum conditions) after evacuation and conditioning. The 254 ports of the plasma vessel allow an external access to the inner space of the plasma vessel. Ports for heating and diagnostic systems are equipped with gate valves or with shutters. The vacuum gate valves are used as a controllable mechanical and a vacuum disconnection point between diagnostics and heating systems on the port side and the inner plasma vessel on the other side. The shutters are responsible for an optical and thermal protection for port windows or installed equipments inside the ports. After an overview of the main requirements for the control of the huge number of gate valves and shutters for the operational phases 1 and 2 of W7-X the design and realization of a centralized control system for controlling and observing all shutters and the majority of gate valves of the machine Wendelstein 7-X will be introduced and discussed.     

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.     

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.     

Piezo-valve controller for the gas inlet system of the fusion experiment Wendelstein 7-X

The gas inlet system of the fusion experiment Wendelstein 7-X (W7-X) comprises eleven gas inlets around the torus for controlled provision with working gases in the torus. This fast gas inlet system is designed for different operating modes of W7-X, from short discharges with only a few seconds durations to steady state plasma operation with operation time of 30 min. Piezo valves of type FGIS (FGIS: Fast Gas Injection System from General Atomics) are used as actuators for the W7-X gas inlet system.The design of an intelligent control unit for the FGIS Piezo valves are introduced and discussed. The integration of the valve controller units into the W7-X control component “W7-X gas inlet” and their planned application in an experiment run is described.     

The procurement and testing of the stainless steel in-vessel panels of the Wendelstein 7-X Stellarator

320 In-vessel water cooled stainless steel panels, poloidal closure plates and pumping gap panels, covering an area of approximately 100 m², are used in Wendelstein7-X to protect the plasma vessel. The panels are manufactured at Deggendorf, Germany by MAN Diesel & Turbo SE. The panels consist of a laser welded sandwich of stainless steel plates together with a labyrinth of cooling channels and have a complicated geometry to fit the plasma vessel of Wendelstein 7-X. The hydraulic and mechanical stability requirements whilst maintaining the tight tolerances for the shape of the components are very demanding. The panels are designed to operate at up to an average heat load of 100 kW/m² and a maximum heat load of 200 kW/m² with a water velocity of approximately 2 m s^?1. High heat flux testing of an un-cooled panel at a time averaged load of 200 kW/m² for 10 s were successfully performed to support the start up phase of Wendelstein 7-X operation. Extensive testing both during manufacture and after delivery to IPP-Garching demonstrates the suitability of the delivered panels for their purpose.     

Fabrication and installation of equilibrium field coils for the JT-60SA

Recently, fabrication of the first superconducting coil in JT-60SA tokamak (EF4 coil) was finished. EF4 coil has ten double pancake (DP) coils. All DP coils were stacked up to form the winding pack. In order to check the manufacturing error of DP coils, their circularities (in-plane ellipticities) were evaluated for all DP coils. Positions of conductors for each DP coil were measured before curing process. Error bars of the current centers, which were used for the index for DP coil's circularity, ranged between 1.1 and 2.5 mm. During stacking the DP coils, the positions of these coils were optimized in order to cancel the error of circulation of the winding pack. As the result, the manufacturing error of the radial current center was achieved 0.6 mm for the winding pack. This value was an order of magnitude smaller than the required manufacturing error of EF4 coil.     

Analysis of dark current images of a CMOS camera during gamma irradiation

We report on the behaviour of the dark current images of the Event Detection Intelligent Camera (EDICAM) when placed into an irradiation field of gamma rays. EDICAM is an intelligent fast framing CMOS camera operating in the visible spectral range, which is designed for the video diagnostic system of the Wendelstein 7-X (W7-X) stellarator. Monte Carlo calculations were carried out in order to estimate the expected gamma spectrum and dose for an entire year of operation in W7-X. EDICAM was irradiated in a pure gamma field in the Training Reactor of BME with a dose of approximately 23.5 Gy in 1.16 h. During the irradiation, numerous frame series were taken with the camera with exposure times 20 μs, 50 μs, 100 μs, 1 ms, 10 ms, 100 ms. EDICAM withstood the irradiation, but suffered some dynamic range degradation. The behaviour of the dark current images during irradiation is described in detail. We found that the average brightness of dark current images depends on the total ionising dose that the camera is exposed to and the dose rate as well as on the applied exposure times.     

The implementation of the Wendelstein 7-X control a data acquisition concepts at VUV/XUV overview spectrometers HEXOS

HEXOS (high efficiency extreme ultraviolet overview spectrometer) is an optimized set of four efficient VUV/XUV spectrometers. It is suitable for a complete coverage of the wavelength range of interest with sufficient spectral resolution. The spectrometers cover the entire wavelength range of 2.5–160 nm with high performance (up to 9999 spectra at spectra rate of 1000 s^?1). To operate according to the Wendelstein 7-X (W7-X) control and data acquisition guidelines all necessary concepts for safety, autonomous and subordinated operation, and segment program controlled experiment operation will be implemented at HEXOS. The design of the HEXOS control and data acquisition system and the implementation of the main W7-X control and data acquisition concepts are described. An outlook on the test phase at the TEXTOR (Tokamak Experiment for Technology Oriented Research) device and the commissioning phase at W7-X is given.     

Towards assembly completion and preparation of experimental campaigns of Wendelstein 7-X in the perspective of a path to a stellarator fusion power plant

The superconducting stellarator device Wendelstein 7-X, currently under construction, is the key device for the proof of stellarator optimization principles. To establish the optimized stellarator as a serious candidate for a fusion reactor, reactor-relevant dimensionless plasma parameters must be achieved in fully integrated steady-state scenarios. After more than 10 years of construction time, the completion of the device is now approaching rapidly (mid-2014). We discuss the most important lessons learned during the device assembly and first experiences with coming major work packages. Those are (a) assembly of about 2500 large, water-cooled, 3d-shaped in-vessel component elements; (b) assembly of in total 14 superconducting current leads, one pair for each coil type; and (c) assembly of the device periphery including diagnostics and heating systems. In the second part we report on the present status of planning for the first operation phase (5–10 s discharge duration at 8 MW heating power), the completion and hardening of the device for full power steady-state operation, and the second operation phase (up to 30 min discharge duration at 10 MW heating power). It is the ultimate goal of operation phase one to develop credible and robust discharge scenarios for the high-power steady-state operation phase two. Beyond the improved equilibrium, confinement, and stability properties owing to stellarator optimization, this requires density control, impurity control, edge iota control as well as high density microwave heating. Of paramount importance is the operation of the island divertor, which is realized in the first operation phase as an inertially cooled conventional graphite target divertor. It will be replaced later on by the steady-state capable island divertor with its water-cooled carbon fiber reinforced carbon target elements.     

Performance and statistical quality assessment of CFC tile bonding on the pre-series elements of the Wendelstein 7-X divertor

Extensive high heat flux (HHF) testing of pre-series IV targets was performed to establish the industrial process for the ongoing production of the actively water-cooled target elements which will be needed for the installation of the Wendelstein 7-X (W7-X) divertor. Finally, 890 components covered with about 18,000 CFC tiles will be installed.The examinations of the elements with 10 MW/m² cycling up to 10,000 pulses, 16 MW/m² cycling and screening tests up to 32 MW/m², confirm the robustness of the design and in particular of the applied CFC bonding technology. The results of the IR examination of the initial tests have been assessed statistically. The paper presents a detailed statistical analysis based on the Six-Sigma method of the surface temperature increase of the CFC tiles tested for 100 cycles at 10 MW/m². Assuming that the series elements will behave in a similar fashion to the pre-series elements this statistical assessment can also be performed for the series elements.