Thomas Rummel

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.

Test Results of the High Temperature Superconductor Prototype Current Leads for Wendelstein 7-X

The Karlsruhe Institute of Technology (which is a merger of former Forschungszentrum Karlsruhe and Karlsruhe University) is responsible for the design, construction and testing of the high temperature superconductor (HTS) current leads for the stellarator Wendelstein 7-X (W7-X) which is presently under construction at the Greifswald branch of the Max-Planck-Institute for Plasma Physics. The current leads are of the binary type, the HTS part covering the temperature range between 4.5 K and 60 K while the heat exchanger covers the range between 60 K and room temperature being cooled by 50 K He. In total 2 prototypes and 14 series current leads are required with a nominal current of 14 kA and a maximum current of 18.2 kA. The paper describes the design and first test results of the prototype HTS current leads.

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

Power supplies for the WENDELSTEIN 7-X stellarator

Abstract The magnetic confinement of the plasma in WENDELSTEIN 7-X (W7-X) will be determined by 50 superconducting non-planar coils and 20 superconducting planar coils. The coils are grouped in five periodic modules with ten coils each connected in series. In order to be able to vary the magnetic configuration and hence increase the experimental flexibility the five groups of non-planar coils and two groups of planar coils will be powered individually resulting in seven independent power supplies. To fine tune the magnetic configuration, shift the plasma axis and to modify the plasma edge an additional set of ten normal conducting control coils will be mounted. Other aims are to correct small field errors and to distribute the heating power on the target plates over a wider range. In order to correct field errors a direct current is necessary, whereas an alternating current is essential for the second aim. The paper describes the design of the two types of power supplies for W7-X as well as the results of the first tests of components.

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.

Test Arrangement for the W7-X HTS-Current Lead Prototype Testing

The Karlsruhe Institute of Technology (KIT) is responsible for the design, construction and testing of the high temperature superconductor (HTS) current leads for the stellarator Wendelstein 7-X (W7-X) which is presently under construction at the Greifswald branch of the Max-Planck-Institute for Plasma Physics. Because the W7-X current leads are mounted with the warm end at the bottom a special test cryostat has been built and is attached to the main cryostat of the TOSKA facility of KIT. Two prototypes of these current leads are tested in this cryostat under W7-X relevant conditions. The test conditions with respect to cryogenic, control, current supply, data acquisition and quench detection of the test setup for the prototype test at TOSKA is described. The performance of the MC plug used to power the HTS current leads is described in detail.

Quench protection for the superconducting magnet system of 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), Germany. The magnetic confinement of the plasma is achieved by 70 coils. In case of a rapid shutdown (e.g., caused by a quench) a very reliable protection system has to reduce the current in all coils within seconds. Different concepts applying linear and nonlinear resistors, which operate at room temperature or at 80 K, were studied. To switch the current electronic and mechanical breakers were considered. The paper presents the concepts studied and describes in detail the selected solution.

The Wendelstein 7-X Trim Coil System

The magnet system of the fusion experimental device Wendelstein 7-X (W7-X) contains superconducting as well as normal conducting coils. Seventy superconducting coils are forming the steady state main field to confine the plasma. Inside of the plasma vessel, ten control coils, made of copper, will be placed to modify the strike points of the plasma at the divertor. In addition, a set of five normal conducting, water cooled trim coils will increase the experimental flexibility by providing a means to balance the divertor heat loads among the five field periods. The coils will be placed at the outer surface of the cryostat of W7-X. There are four coils (type A) with equal shape; the fifth coil (type B) has a slightly different shape due to space restrictions. The coils have dimensions of 3.5 × 3.3 m with 48 turns and will be operated with currents of up to 1.8 kA (type A). The other coil (type B) has a smaller size of 2.8 × 2.2 m, compensated by a higher number of turns and a higher operation current of 1.95 kA. Five independent power supplies are being fabricated to operate the coils with a maximum of flexibility. The concept is based on four-quadrant power supplies using Insulated-Gate-Bipolar-Transistors. The trim coil package consisting of the five coils plus the five power supplies is being designed and built in collaboration between IPP, Germany and PPPL, USA, partly funded by the Department of Energy.

The current leads of the Wendelstein 7-X superconducting magnet system

The stellarator fusion experiment Wendelstein 7-X (W7-X) is presently under assembly at the Greifswald branch of the Max Planck Institute for Plasma Physics (IPP), Germany. The W7-X superconducting magnet system basically consists of 50 non planar and 20 planar coils including the interconnecting bus bars, and the support structure. The seven electrical circuits with ten coils each in series, are supplied by 14 current leads (CL) operating between the cryogenic and ambient temperature environments. A special feature of these feed troughs is their upside-down orientation to save space in the vicinity of the machine. Basic electrical CL requirements are maximal steady state currents of 18.2 kA and voltage strengths of 13 kV to ground. A W7-X current lead consists of a copper conductor which also acts as a heat exchanger at the warm end side, in its continuation of a high temperature superconductor part, and at the cold end side of a copper bar with integrated Nb3Sn rods. All components are fully contained within a CL vacuum chamber which is separated from the main W7-X cryostat vacuum. The high voltage electrical insulation is built up of a glass tape epoxy resin system. Mechanical support of the current leads is achieved by a warm and cold glass fiber reinforced plastic flange. There are three He cooling circuits: one for the bus bar and contact cooling, one for the cold end of the CL (both at about 5 K), and one for the CLs heat exchanger (entrance temperature about 50 K). After intensive tests of two prototypes the series production has been established and completed. The current leads were tested at room temperature, and with electrical currents up to the maximum current at cryogenic temperatures.

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