Protection system for the superconducting coils in WENDELSTEIN 7-X

doi:10.1016/S0920-3796(03)00266-7

Fusion Engineering and Design

Volumes 66–68, September 2003, Pages 1041-1044

https://doi.org/10.1016/S0920-3796(03)00266-7 Get rights and content

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.

Introduction

Magnetic inclusion of the plasma is achieved through superconducting coils. These allow loss-free operation and thus are an important condition for continuous operation. The magnetic field of WENDELSTEIN 7-X is produced by five groups of ten non-planar coils each. Two further groups of ten planar coils each serve to achieve different configurations of the magnetic field. The five non-planar coils and the two planar coils have almost identical electric parameters with typical inductances of about 1.1 and 0.4 H, respectively.

The power supply for the superconducting coils is provided by seven identical and independent high-current rectifiers, so that each coil group can be supplied separately with a precise direct current of up to 20 kA.

Section snippets

Protection system for W7-X

The greatest failure for the superconducting coils is the so-called quench, when part of the superconductor suddenly becomes normal-conducting. In the superconductor used for W7-X, the normal-conducting zone spreads at a speed of 90–100 m/s. Unless the power supply is switched off immediately, the sudden loss of superconductivity and the now effective ohmic heating of the conductor would quickly heat the coil to about 350 °C, thereby destroying it.

In order to avoid this, the power supply modules

Protection system requirements

Coil energy is transformed into heat through an external resistor. The resistor material selected is pure nickel (Ni 99.6), which combines a high thermal coefficient with a low resistivity and a high melting temperature.

The characteristic curves which directly specify the requirements for the switching devices were optimized by means of computer simulations through variation of the resistance values and the active masses of the nickel resistors.

In order to keep the hot-spot temperature in the

Components of the protection system

The circuit shown in Fig. 1 was selected [2].

Characteristic for this circuit is the redundant construction with standard components, so that reliable data are available for the failure rate.

During normal operation, the switches have the following initial positions:

BPT and BPS open.
SD1 and SD2 closed.
CB1 and CB2 closed.
ES in working condition.

The current flows from the rectifier via the load break switches through the superconducting coils.

In case of a quench signal, a switching sequence is

Determining magnetic field compatibility

Due to the immediate proximity to the experiment of only 6 m, the components must operate reliably within magnetic stray field of up to 51 mT. The specified magnetic stray fields are composed of the stray fields of the stellarator and components in immediate vicinity of the protection-system components and the self fields of the current bus bars feeding the superconducting coils.

For this reason a magnetic field test facility was built up at the Greifswald branch institute (test volume max. H:

Summary

The superconducting coils of the W7-X experiment require a fast, extremely reliable and constantly available protection system. This is achieved through redundant construction of the components and permanent monitoring of the discharge resistor.

All relevant components were tested both with respect to important technical parameters and their usability under the effect of magnetic stray fields. Great care was taken to create testing conditions that correspond to subsequent operating conditions by

References (2)

R. Heller, W. Maurer, A. Ulbricht, F. Wüchner, G. Zahn, I. Schoenewolf, Abschlußbericht zum Test der Wendelstein 7-X...
Th. Rummel, O. Gaupp, G. Lochner, J. Sapper, Quench Protection of the Superconducting Magnet System of Wendelstein 7-X,...

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