K. Risse

Transition From Construction to Operation Phase of the Wendelstein 7-X Stellarator

Assembly of the superconducting stellarator Wendelstein 7-X is well advanced, and commissioning of the device is being prepared. A first draft of the commissioning tasks has been developed and will be discussed in this paper.

Progress in the Design, Manufacture and Testing of the W7-X Superconducting Magnets

The W7-X machine is a low-shear stellarator of the Wendelstein line being assembled in Greifswald, Germany. The manufacture of its superconducting magnets is in a well advanced phase. After the acceptance tests in the factory, the magnets are tested in cold conditions in a dedicated facility at CEA Saclay. This paper gives an overview of the status of the manufacture of the W7-X coils, including the production of the superconductor, the windings and casings for the magnets, the final assembly and in-factory test procedures. Several design changes and re-work have been implemented in the course of the manufacture due to more detailed engineering analyses or weaknesses and quality problems found in some components. The status of the cold tests of the coils will also be presented

Fabrication of the superconducting coils for Wendelstein 7-X

The Max Planck Institute of Plasmaphysics is building up the stellarator fusion experiment Wendelstein 7-X (W7-X) at the branch institute in Greifswald. W7-X continues the line of stellarator experiments at IPP. To allow for steady state operation W7-X has a superconducting coil system with 50 non-planar and 20 planar coils. The coil system is grouped in five equal modules, each consisting of two mirror symmetric half modules. The half modules are assembled from five different non-planar coils, two planar coils and a sector of the coil support structure. All cryogenic parts are enclosed in a cryostat to protect them from ambient temperature. The magnet system was ordered from the European industry. The production of superconductor, winding packs and encasings are under way. The main focus of this contribution aims on the fabrication state of the coil system.

The Trim Coils for the Wendelstein 7-X Magnet System

The magnet system of the fusion experiment Wendelstein 7-X (W7-X) consists of superconducting as well as normal conducting coils. 50 non planar superconducting coils are forming the main field, 20 planar superconducting coils allow varying the shape of the plasma. Inside of the plasma vessel 10 normal conducting control coils will be placed to modify the strike points of the plasma at the divertor. In addition a set of five normal conducting trim coils has been designed to allow the correction of error fields and to increase the experimental flexibility. The coils will be placed at the outer surface of the outer vessel of W7-X. Four out of five coils have identical size and shape. They have dimensions of 3.5 × 3.3 meters with 48 turns and will be operated with currents of up to 1.8 kA. The other coil type has a smaller size of 2.8 × 2.2 meters, but a higher number of turns and a higher operating current of 1.95 kA. Both types of trim coils will be made of square copper hollow profile with an integrated cooling channel. Five independent power supplies will be used to energize the coils. The present concept is based on four-quadrant power supplies. The control system will allow the local control as well as the remote control of the five power supplies from an external control room.

Design Changes and Impact on the Production of the Non-Planar Coils for the W-7X Experiment

The Max-Planck-Institut fur Plasmaphysik is building the stellarator fusion experiment WENDELSTEIN 7-X (W7-X) at the branch institute in Greifswald, Germany. The 50 non-planar superconducting coils are currently manufactured by industry and use a specially developed NbTi cable-in-conduit conductor. The manufacture of the coils includes the conductor, winding of the conductor, vacuum pressure impregnation as well the embedding of the winding pack into a cast steel casing, the final machining and the installation of the helium cooling system on the casing. Design changes became necessary during production in the connection areas of the casings due to more detailed design and structural analysis. In addition, the insulation and the welds in the termination area need to be improved. New quench detection cables were qualified. LINAC inspections allowed to check the presence of casting defects in the thick casings. Additionally high voltage tests in vacuum under Paschen-minimum conditions turned out to be a very sensitive inspection method to check the integrity of the electrical insulation

Comparative Analysis of Impulse and Impedance Tests to Detect Short Circuits Within the W7-X Magnets

The magnet system of the Wendelstein 7-X stellarator (W7-X) consists of 50 nonplanar and 20 planar coils. In particular, the winding packs of the nonplanar coils are densely packed and compressed in the area of inter-layer joints and coil terminations, which brings some risk of inter-turn and inter-layer short circuits. The paper describes methods deducted from impulse and ac tests performed on the coils, which enable demonstrating the absence of short circuits and weak points of low inter-layer insulation resistance. In the impulse test, frequency and damping are compared for coils with and without short circuits and coils with an external low resistance bypass. With ac voltage, a drastically reduced impedance clearly indicates a short circuit. Therefore both tests represent excellent QA (quality assurance) tools for checking the uniformity of the electromagnetic performance of a serial production of magnet coils

Accuracy of the construction of the superconducting coils for WENDELSTEIN 7-X

WENDELSTEIN 7-X, a superconducting fusion experiment, is presently under construction at the Greifswald branch of the Max-Planck-Institut fur Plasmaphysik (IPP). The magnetic confinement of the plasma is achieved by 70 coils. The accuracy of the magnet system plays an important role in order to achieve the technical and scientific goals. The accuracy of the coil system is determined by the accuracy of the shape of the winding packs, the accuracy of the final machining of the casings and the accuracy of the assembly of the stellarator. The contribution gives an overview about the accuracy requirements for the coils and presents a strategy to control the geometrical shape of the winding packs as well as of the casings. The achieved accuracy of the winding packs is in full agreement to the requirements. It can be postulated that the complicated shape of the nonplanar coils does not lead to an insufficient accuracy.

Wendelstein 7-X trim coils — Component safety aspects and commissioning strategy

The stellarator fusion experiment Wendelstein 7-X (W7-X) is currently under construction at the Max-Planck-Institut für Plasmaphysik in Greifswald, Germany. Five normal conducting trim coils have been designed to allow for fine tuning of the main magnetic field during plasma operation. To limit the mechanical stresses in the coil, the proper functioning of the coil cooling system must be carefully monitored. Two independent systems will monitor the coil temperature. In addition, flow monitors in the outlet hydraulic line of each coil will determine if the required cooling water flow is present. The trim coil system will be provided as a part of a collaboration program between the Princeton Plasma Physics Laboratory, Oak Ridge National Laboratory, and the Wendelstein 7-X project, and is funded by the U.S. Department of Energy.

Challenges for the Wendelstein 7-X Magnet Systems During the Next Operation Phase

During the first operation phase OP1.1 of Wendelstein 7-X, the magnet systems were not operated up to its maximum capabilities. During the next operation phase OP1.2, a big step in the direction to a full current operation will be taken. The superconducting magnet system consists of the two different coil types: the nonplanar coils (NPCs) and the planar coils (PLCs). With respect to OP1.1, the NPC current in OP1.2 will be increased slightly, but will be doubled in the PLC. Also a reversal of the current direction in the PLC will be required. Tests during and after OP1.1 showed that it might be advantageous to reduce the electrical stress during fast discharges. Therefore, the magnet protection system was optimized. In order to avoid the risk of a quench, the magnet system is being operated with a certain temperature margin with respect to the critical temperature of the superconductor. The safety operation system will be updated to secure automatic observation and reaction. The five trim coils are normal conducting coils mounted at the outer surface of the cryostat. They were operated during OP1.1 up to 2/3 of the maximum current. Therefore, full current operation needs to be tested for the first time. For OP1.2, also measures were studied and installed to minimize the cross-link between the trim coils and the superconducting main field coils.

Purpose and design 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, not avoidable tolerances can result in small deviations of the magnetic field disturbing the toroidal periodicity. In order to have a tool to influence this field errors additional five normal conducting trim coils were designed to allow a fine tuning of the main magnetic field during plasma operation. The detailed design of the coils and the manufacturing will run within a collaboration program between the Princeton Plasma Physics Laboratory, Oak Ridge National Laboratory, Los Alamos National Laboratory and the Wendelstein 7-X project funded by the U.S. Department of Energy.