F. Füllenbach

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.

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.

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.

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.

Turn-key supply for the power supplies of the control coils of wendelstein 7-X experiment

Abstract In the Max-Planck Institute for Plasma Physics, IPP, in Greifswald, a new stellerator is being built for fusion research experiments, the so called Wendelstein 7-X Experiment. JEMA Company has designed, manufacture and supply the turnkey system which energises the ten control coils in the stellerator. The system comprises ten independent Switch-mode power supplies, and the central distribution stations for power, cooling and control facilities. Each power supply must provide a controlled current compounded of direct current and 0–20 Hz bandwidth ac current in a range of almost 3 kA at low voltage, 30 V, in four quadrants. Positive and negative voltage and sinusoidal current with low harmonic distortion is required by load. The selected design is based on Switch-mode-power supply technology (SMPS) providing the system with a very high regulation response (very low current and voltage ripple) and excellent input power factor, keeping the grid total harmonic distortion (THD) under minimum values. The sources can be operated locally as independent systems or they can be co-ordinated from a main control keeping current synchronisation requirements. The system in its entirety requires a high efficiency due to thermal considerations and reduced space. The system presented here introduces a decisive solution for the future power supplies to superconducting or regular coils, where the regulation, precision and ripple are the basic requirements for the design. During year 2000 the design stage was covered and in 2001 a first prototype for the power supplies was manufactured and successfully tested at factory. The entire system has been installed over this year after the series fabrication and it is currently working at IPP facilities, waiting for the acceptance tests scheduled before end of the year. This paper aims to explain the different stages the company went through in order to get validation of the power supply system and make it running at its last location.

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.

Design and Prototype of the High Voltage In-Service-Tests on the Superconducting Magnet System of Wendelstein 7-X

The stresses induced during the energizing and de-energizing process of the Wendelstein 7-X super conducting magnet system could cause cracks in the embedding of the winding packages and consequently a ground fault that would not be detected during normal operation as the operating voltage of the magnet is system is too low. With a ground fault present during a fast discharge, the middle point grounding needed to half the voltage could be rendered useless and one coil group could be stressed with the full voltage of up to 4 kV which would 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. With the In-Service-Test, a voltage of 2.5 kV can be applied during the operation of the magnets especially after significant changes in the coil current. The measured leak current can then be used to determine any changes in the quality of the insulation. In a first step, the feasibility of such a system has been evaluated with a simulation model of one coil group with Simplorer. The next step was to find a design which can be implemented in the given magnet power supply system and to build and test a prototype for final confirmation of the concept.