On the accuracy of port assembly at Wendelstein 7-X

Wendelstein 7-X uses 254 ports for diagnostic and supply purposes. Actually 176 ports are final adjusted and welded. The major number of ports meets the general position tolerances of typically 4, …, 8 mm after assembly without countermeasures. 3D metrology turned out to be an essential factor to achieve required adjustment accuracy as well as to control welding process. The measurement accuracy of typically 0.3, …, 0.6 mm proved to be appropriated for all adjustment and control processes inside the experimental hall. A consequent application of 3D metrology can substitutes trail assembly steps and saves process time. Even reduced tolerances of special ports (AEV and AEK-V2) are achieved using appropriated assembly, welding and metrology procedures.     

Thermo-mechanical analysis of the Wendelstein 7-X divertor

The stellarator Wendelstein 7-X (W7-X) has a divertor consisting of 10 units installed inside the plasma vessel (PV). It was decided not to install the long pulse high-heat flux (HHF) divertor targets at the first two years stage of W7-X operation and to start with an adiabatically cooled test divertor unit (TDU) and shorter plasma pulses operation. This allows to accumulate operation experience with much simpler components, and as a result to adjust accurately the actively cooled HHF divertor which replaces the TDU for the stationary operation. Finite element (FE) analyses have been performed for better understanding of thermo-mechanical problems of divertor targets, and to guide the design of the TDU and HHF divertors. This paper presents the detailed results of the temperature response, the deformation and thermal stress of the divertor components.     

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.     

Hydraulic analysis of the Wendelstein 7-X cooling loops

Actively water cooled in vessel components (IVC) are required for the long pulse operation of the stellarator Wendelstein 7-X (W7-X). In total, the cooling pipes have a length of about 4.5 km, supplying the coolant via 304 cooling circuits for the IVC. Within each cooling loop, the IVC are organized mostly in parallel. A homogeneous flow through all branches or at least the minimum specified flow in all of the branches of a circuit is crucial for the IVC to withstand the loading conditions. A detailed hydraulic simulation model of the W7-X cooling loops was built with the commercial code Flowmaster, which is a 1-D computational fluid dynamics software. In order to handle the huge amount of pipe-work data that had to be modelled, a pre- and post-processing macro was developed to transfer the 3D Catia V5 CAD model to the 1-D piping model. Within this model, the hydraulic characteristics of different types of first wall components were simulated, and compared with their pressure drop measurements. As a result of this work, the need for optimization of some cooling loops has been identified and feasible modified solutions were selected.     

Thermal analysis of the Mirnov coils of Wendelstein 7-X

Mirnov coils are used to measure fluctuations of the magnetic field which are in particular generated by magnetohydrodynamic (MHD) modes. The underlying plasma currents have a multipolar structure in a poloidal cross-section. Therefore the amplitude of the magnetic fluctuations decays quickly with increasing distance from the plasma edge. It is hence important to place the Mirnov coils as close to the plasma edge as possible where they are exposed to high thermal loads. Two types of Mirnov coils are proposed to be used in Wendelstein 7-X (W7-X). Type 1 (44 Mirnov coils) should be mounted on the plasma side of wall protection panels with a graphite cap to shield them from direct plasma exposure. Type 2 (137 Mirnov coils) will be located behind the tiles of the heat shields. An important issue concerning the design of these Mirnov coils is to verify their suitability for steady state operation from the thermal point of view. Both steady state and transient finite element thermal analyses were performed for the Mirnov coils under different conditions and with different designs. The paper presents detailed thermal analyses of the Mirnov coils.     

Challenges in the realization of the In-Vessel-Components of Wendelstein 7-X

The In-Vessel Components (IVC) for the Wendelstein 7-X stellarator at the Institute for Plasma-Physics (IPP), to be installed for the initial phase of operation, are nearing completion and a significant fraction of the components was delivered in 2011 and 2012. Due to the considerable amount of different components including many variants, the timely realization required a comprehensive management approach, not only covering the demanding technology and system requirements, but also coordination, planning and control issues. A variety of tools were set up to address the technical, financial and timescale challenges. The implementation of this comprehensive management approach is illustrated by the production of the water-cooling system of the IVC. Careful design and manufacture of these components is needed to fulfil the cooling function under high vacuum conditions within very restricted available space. The evolution of the complexity of these components together with changes of boundary conditions had to be managed, integrated into the overall project planning and adequately resourced.     

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 improvement of the target elements of Wendelstein 7-X divertor

The actively cooled high-heat flux divertor of the Wendelstein 7-X stellarator consists of individual target elements made of a water-cooled CuCrZr copper alloy heat sink armored with CFC tiles. The so-called “bi-layer” technology developed in collaboration with the company Plansee for the bonding of the tiles onto the heat sink has reliably demonstrated the removal of the specified heat load of 10 MW/m² in the central area of the divertor. However, due to geometrical constraints, the loading performance at the ends of the elements is reduced compared to the central part. Design modifications compatible with industrial processes have been made to improve the cooling capabilities at this location. These changes have been validated during test campaigns of full-scale prototypes carried out in the neutral beam test facility GLADIS. The tested solution can remove reliably the stationary heat load of 5 MW/m² and 2 MW/m² on the top and on the side of the element, respectively. The results of the testing allowed the release of the design and fabrication processes for the next manufacturing phase of the target elements.     

Studies for the design of the Wendelstein 7-X Thomson Scattering polychromators

The aim of the Thomson scattering system is the measurement of electron temperature and density profiles with high time and spatial resolution. The whole Thomson scattering optical system is optimized to minimize losses of scattered light with wavelengths from 700 nm up to 1064 nm. A five-channel polychromator serves as spectral analyzer of the scattered light. The light is carried to the polychromator input via fiber bundles of 3 mm diameter. High performance dielectric interference filters will be used for spectral analyzing. Their transmission curves will be chosen according to the range of electron temperature that is supposed during W7-X operation (from 10 eV up to 10 keV). The design of the polychromator must be optimized for a high throughput and size compactness. The article describes three possible polychromator setups. The main difference between these designs is the usage of relay and field lenses. The optical properties of the designs will be derived both by measurements and by simulations. Limitations considering high Numerical Aperture (N.A.) and fiber bundle size are discussed.     

Quality assurance on the welding work during the assembly of Wendelstein 7-X

At the Max-Planck-Institut für Plasmaphysik (IPP) in Greifswald (Germany) the stellarator experiment W7-X is presently being assembled. During this assembly many different weld connections are made, which are very important for the proper functionality of the experiment. This concerns mainly the structural integrity and the leak tightness. The quality requirements for the weld seams are high (mainly class B according to DIN EN ISO 5817), because the complex machine must operate reliably for more than 15 years and the possibility of any repair or change of important components is very small. To guarantee a high quality of the welds they are submitted to different tests. The applied test methods are depending on the function of the weld, the wall thickness, the seam geometry, and the material. The main test methods are visual testing (VT), penetrant testing (PT), radiographic testing (RT), ultrasonic testing (UT), leak testing (LT), permeability testing and macros. The paper will describe the application of these test methods and show their need by examples of typical weld imperfections.     

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.     

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.     

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.     

Assessment of cracks in lateral supports of the magnet system of Wendelstein 7-X

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 factor (SIF) of theoretical cracks of various sizes in potentially critical position and orientation were predicted in a fast approach. For each crack size, N was based on the SIF, derived from beam theory, and on Paris’ law parameters determined in fatigue crack growth rate (FCGR) tests, thus leading to tolerable maximal crack sizes and distances between cracks. It was proved that the actual crack dimensions remained below these values or turned out to be only superficial. Afterwards, (extended) finite element method (XFEM and FEM) and boundary element method (BEM) models were developed to project the SIF of most critical tolerated cracks, considering new FCGR tests and the local stress state in more detail. N appeared highly sensitive to the assumptions which were therefore critically reviewed.Finally, the limit for load combinations of different amplitudes was determined using Miner's rule. As a result it was shown that the predefined number of W7-X operation cycles is not jeopardized by any of the detected cracks.     

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.     

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,manufacture and testing of the glow discharge electrodes for Wendelstein 7-X

The electrodes for the Wendelstein 7-X glow discharge system have been designed, tested and manufactured. The compact design relies on a cooled housing, integrated into the first wall cooling system, and a calotte-shaped graphite anode. The new mounting concept avoids the need of active cooling of the anode due to an improved thermal conduction. Comprehensive tests of a prototype electrode had been carried out in laboratory and in the ASDEX Upgrade Tokamak during two operation campaigns. The electrode showed excellent and reliable long-time discharge behavior and fulfilled all the requirements regarding temperature limits and maintainability resulting from the steady-state operation of W7-X.     

Development of a thermo-hydraulic bypass leakage test method for the Wendelstein 7-X target element cooling structure

A facility for testing the cooling structure to ensure the quality of the target element heat sink is under construction at IPP Garching. This test bed has been built up to do a proof of concept study with a hydraulic mock up, built from the same material as the target elements, but can be opened for artificial gap manufacture.A bypass in the cooling structure was the main reason for the overheating of CFC tiles, which resulted in a defect by cracking of the CFC–AMC-Cu interlayer in pre-series 3. The cooling structure is built from two half-shells, in both a half pipe is milled [1]. To correlate the thermal response function to the artificial bypass gaps the time constant of the cool down was used. The results of the measurements are presented and compared with the calculated results.     

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.