The main objective of the Wendelstein 7-X (W7-X) stellarator is to demonstrate the integrated reactor potential of the optimized stellarator line. An important element of this mission is the achievement of high heating-power and high confinement in steady-state operation. Such an integrated plasma operation has not yet been demonstrated and represents the major scientific goal of W7-X. The way towards this goal is staged. In the first phase, called OP 1.1, December 2015-March 2016, a limiter configuration was used. In this paper, the preparation of the first operation phase as well as lessons learned during the first commissioning and the operation phase are discussed, while the physics results from OP 1.1 are reported elsewhere (Wolf et al 2017 Nucl. Fusion 57 102020).

In the Wendelstein 7-X stellarator with its twisted magnetic geometry the investigation of plasma wall interaction processes in 3D plasma configurations is an important research subject. For the upcoming Operation Phase i.e. OP1.2, three different types of material probes have been installed within the plasma vessel for the erosion/deposition investigations in selected areas with largely different expected heat load levels, namely, ≤ 10 MW/m 2 at the Test Divertor Units (TDU), ≤ 500 kW/m 2 at the baffles, heat shields and toroidal closures and ≤100 kW/m 2 at the stainless steel wall panels. These include 18 exchangeable target elements at TDU, about 30000 screw heads at graphite tiles and 44 wafer probes on wall panels, coated with marker layers. The layer thicknesses, surface morphologies and the impurity contents were pre-characterized by different techniques and subjected to various qualification tests. The positions of these probes were fixed based on the strike line locations on the divertor predicted by field line diffusion and EMC3/EIRENE modeling calculations for the OP1.2 plasma configurations and availability of locations on panels in direct view of the plasma. After the first half of the operation phase i.e. OP1.2a the probes will be removed to determine the erosion/deposition pattern by post-mortem analysis and replaced by a new set for the second half of the operation phase, OP1.2b.

WENDELSTEIN 7-X, a superconducting fusion experiment, is presently under construction at the Greifswald branch of the Max-Planck-Institut für Plasmaphysik (IPP). The magnetic confinement of the plasma is achieved by 70 coils. The accuracy of the magnet system plays an important role in order to achieve the technical and scientific goals.The accuracy of the coil system is determined by the accuracy of the shape of the winding packs, the accuracy of the final machining of the casings and the accuracy of the assembly of the stellarator.The contribution gives an overview about the accuracy requirements for the coils and presents a strategy to control the geometrical shape of the winding packs as well as of the casings. The achieved accuracy of the winding packs is in full agreement to the requirements. It can be postulated that the complicated shape of the nonplanar coils does not lead to an insufficient accuracy.

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