The next step in the Wendelstein stellarator line is the large superconducting device Wendelstein 7-X, currently under construction in Greifswald, Germany. Steady-state operation is an intrinsic feature of stellarators, and one key element of the Wendelstein 7-X mission is to demonstrate steady-state operation under plasma conditions relevant for a fusion power plant. Steady-state operation of a fusion device, on the one hand, requires the implementation of special technologies, giving rise to technical challenges during the design, fabrication and assembly of such a device. On the other hand, also the physics development of steady-state operation at high plasma performance poses a challenge and careful preparation. The electron cyclotron resonance heating system, diagnostics, experiment control and data acquisition are prepared for plasma operation lasting 30 min. This requires many new technological approaches for plasma heating and diagnostics as well as new concepts for experiment control and data acquisition.

Abstract-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 3.5x3.3 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 2.8x2.2 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.

Wendelstein 7-X (W7-X) is a fully optimised low-shear stellarator and shall demonstrate the reactor potential of a HELIAS-type fusion device. It is presently under construction at the Max-Planck-Institute for Plasmaphysics (IPP), Greifswald site. The superconducting magnet system enables continuous operation, limited only by the plasma exhaust cooling water system whose capacity is designed for 30 minutes full power operation. Companies spread all over Europe participate in the manufacture of this large and complex machine. Whereas most of the component production progresses well, the coil delivery has been hampered over the years by a variety of problems. Another cause of concern was the mechanical structure which -with the progressing of in-depth analyses -turned out to be increasingly complex.As of now the situation has widely stabilized, and component manufacture and assembly is making reasonable progress.

Abstract-The magnet system of the Wendelstein 7-X stellarator (W7-X) consists of 50 non-planar and 20 planar coils. In particular, the winding packs of the non-planar 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.

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