Th. Rummel

Status of WENDELSTEIN 7-X construction

WENDELSTEIN 7-X (W7-X) shall confirm the favourable plasma properties and the high density and beta limits of the helical advanced stellarator and demonstrate steady-state operation. The magnetic configuration of W7-X is characterized by a set of 50 non-planar and 20 planar superconducting coils. The magnet system will be manufactured and assembled to a precision of a few millimetres and maintain its symmetry during cool-down to cryogenic temperatures. Power supplies allows one to adjust the magnetic field with a precision of a few milli Tesla and safely dump the magnet energy in case of a quench. The plasma vessel gives maximum space for the plasma and is kept symmetrically with respect to the plasma by dedicated means. Steady-state heating is achieved by 10 MW ECRH. Energy and particles are controlled by a continuously working divertor. All plasma-facing surfaces are covered by CFC, graphite or B4C, respectively. The paper reviews the status of the construction and describes details of the design.

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.

Protection system for the superconducting coils in WENDELSTEIN 7-X

Abstract Apart from scientific aims, an important technical goal of WENDELSTEIN 7-X is to demonstrate stationary operation. This is a fundamental fusion-reactor requirement and a characteristic advantage of stellarators over tokamak fusion reactors. Magnetic confinement is achieved by means of 70 coils with a diameter of up to 5 m and a magnetic induction of up to 3 T on the axis. To allow continuous operation, the coil windings are made of superconducting material. Because of the great amount of stored magnetic field energy of up to 1 GJ, the protection system is an essential part of the power supply system. This text describes the protection system for the superconducting coils as well as key preliminary experiments which demonstrate the suitability of selected components of the protection system for the WENDELSTEIN 7-X experiment.

Wendelstein 7-X Commissioning of Superconducting Magnet System

The Wendelstein 7-X stellarator (W7-X), one of the largest stellarator fusion experiments, is presently being taken into operation at the Max Planck Institute for Plasma Physics in Greifswald. The main objective of the experiment is to prove the reactor relevance of the optimized stellarator concept. The W7-X experiment has a superconducting magnet system with 50 nonplanar and 20 planar coils grouped in five equal modules, which are electrically connected in 7 circuits with 10 coils of each type. The connections between the coils are made by superconducting the bus bars using the same NbTi Cable-in-Conduit Conductor as used for the superconducting coils. Particularly developed high-temperature superconducting current leads feed the current into the cryostat by bridging the temperature gradient from room temperature down to the 4-K level. Seven power supplies provide individual currents in the seven circuits. The quench detection system permanently checks the superconducting system regarding the occurrence of a quench. In case of a quench, the magnet safety system has to be activated, and a set of switches lead the current into the dump resistors. The commissioning of the magnet system was successfully performed until July 2015 with tests of the complete magnet system functionality needed for plasma operation at a magnetic field of 2.5 T.

Production and Tests of the Discharge Resistors for Wendelstein 7-X

One of the goals of Wendelstein 7-X is to demonstrate steady state operation which is an inherent property of stellarators and essential for a future power plant. Magnetic confinement in Wendelstein 7-X will be archived by 50 non-planar and 20 planar coils with diameters of up to 5 m and masses of typically 6 t. To achieve steady state operation, all coils are superconducting. The large amount of stored energy in the coils of up to 900 MJ requires a fast and reliable protection system. In case of a quench or the need for rapid shut-down the current in the coils has to be commutated to external discharge resistors and ramped down within 10 s. In order to limit the voltage for the breakers, a resistor made of pure Nickel with low starting resistance and high thermal coefficient of the specific electric resistance was chosen. The paper describes briefly the results of simulations prior to the production as well as results of simulation during production, based on material tests during production. After the production all resistors were tested under real W7-X conditions regarding amount of stored energy, heat impact and current rise time. It was confirmed that the resistor are usable for the operation in W7-X

Design, Tests, and Repair Procedures for the Electrical Insulation of the Superconducting W7-X Magnets

The Wendelstein 7-X (W7-X) stellarator fusion experiment is presently under construction at the Max-Planck-Institut fu¿r Plasmaphysik in Greifswald Germany. The W7-X machine has a superconducting magnet system with 50 non-planar and 20 planar coils grouped in five equal modules. The operation of the W7-X machine requires a reliable insulation system of the superconducting magnets. Starting in the design phase the components have to consider the insulation system, weak points of the insulation must be identified and countermeasures must be taken, e.g. for the crossings of conductors inside the windings, the shape of handmade insulated areas and the quench detection cable design. The manufacturing has to be based on qualified procedures and materials, e.g. the vacuum pressure impregnation of the glass epoxy insulation as well as the hand-made glass epoxy wrappings must be thoroughly qualified and tested during the whole manufacturing process. A set of accompanying tests, especially high voltage tests, during the manufacturing and assembly process helps to assure the quality of the insulation.

Design Requirements for the Plasma Edge Control System in Wendelstein 7-X

Ten normal conducting control coils will be installed inside the plasma vessel of the WENDELSTEIN 7-X (W7-X) stellarator to modify the magnetic configuration at the plasma edge. The aim is to correct small error fields and to distribute the plasma power on the target plates over a wider area (sweeping). If this two functions are to be combined the power supplies have to deliver a direct current with a superimposed alternating current, the first to correct the error fields and the second for the sweeping. The paper describes possible solutions to combine these two main requirements. The main parameters of the power supply will be presented and some points to be taken into account in the design of the control coils are discussed. The synchronisation of the 10 power supplies is performed by a master control system. Requirements for these control system will be pointed out and the final scheme of the control system will be discussed in detail.

W7-X DEMO coil cryogenic tests in the CEA/Saclay test facility

In the frame of the W7-X stellarator project, CEA cooperates with Max-Planck-Institut fur Plasmaphysik to perform the acceptance tests of all 70 superconducting coils of the W7-X magnet system. The test facility is now complete and its performance is being checked using the W7-X prototype coil. The main objective of the tests of the series coils is to demonstrate their proper function and to determine their margin of operation. Since many coils will be tested and compared to each other, it is important to measure the margin of operation in a reproducible way with sufficient accuracy. Either increasing the current, increasing the temperature or mixed operation can induce quenches. The test results on the prototype coil are analyzed with respect to the temperature and current margin and compared to the expected values calculated from superconducting strand data. The paper summarizes the results of these tests and the methods of evaluation.

Feasibility Study of a High Voltage In-Service-Tests on the Superconducting Magnet System of Wendelstein 7-X

During the energizing and de-energizing process of the Wendelstein 7-X magnet system, the forces induced in the superconducting coils could lead to stresses in the insulation of the coils, the interconnecting bus system, the current leads and the mechanical support structure. Such stress could cause a ground fault that would not be detected during normal operation. In normal condition the voltage across a coil group is halved by a middle point grounding system. If a ground fault is present during a fast discharge, the coil group could be stressed with the full voltage of up to 4 kV which could endanger the high voltage integrity of the whole superconducting magnet system. To prevent such undesirable occasions, the insulation of the superconducting magnet system shall be monitored by a high voltage In-Service-Test. That means that a test voltage shall be applied during the operation of the magnets. Therefore, it is planned to temporarily lift the middle point of the grounding system by a high voltage source of 2.5 kV and to measure the leak current. To evaluate the feasibility of such a system, a simulation model of one coil group has been created in Simplorer. Considering detailed parameters of the coil group, the model shall support the evaluation of how the high voltage influences the superconducting magnet system and shall determine the leak currents with and without certain damages in the insulation. The paper describes the Simplorer model with its different simulation scenarios and summarizes the results and findings.

W7-X Progress

Abstract The WENDELSTEIN 7-X stellarator (W7-X) is the next step device in the stellarator line of IPP and is presently under construction at the Greifswald branch institute. The experiment aims at demonstrating the steady state capability of a stellarator machine at reactor relevant parameters. An important feature of W7-X is the high geometrical accuracy of the magnetic configuration which implies tight tolerances in the construction and assembly phases. The magnetic system consists of 50 non planar and 20 planar superconducting coils. Critical components are the coil support elements connecting the coil to the central mechanical structure and the inter-coil elements connecting the coils one to the other. Efficient thermal insulation of the superconducting coils is achieved by high vacuum and multi-layer insulation. The plasma vessel is composed of 10 half-modules welded together during the assembly phase. A 10 MW ECRH system with CW-capability operation at 140 GHz is required to meet the scientific objective of W7-X. The paper will report the recent progress on W7-X with particular emphasis on the components where high technology solutions have been applied.