Abstract:The W 7-X Stellarator (R = 5.5 m, a = 0.55 m, B<3.0 T), which is presently being built at IPP-Greifswald, aims at demonstrating the inherent steady state capability of stellarators at reactor relevant plasma parameters. A 10 MW ECRH plant with cwcapability is under construction to meet the scientific objectives. The physics background of the different heating-and current drive scenarios is presented. The

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Recently designed optimized stellarator experiments have suffered from very tight construction tolerances, but some level of deviation of the coil system is unavoidable during fabrication of the coils and assembly of the coil system. In this paper, we present a new approach that incorporates reduced sensitivity to construction tolerances of the coil system into the optimization sequence. The approach was tested within the framework of the existing coil optimization scheme for Wendelstein 7-X. The results are compared with those of a coil set obtained by the original optimization. The result is a more optimal coil system with less stringent tolerances, such that small deviations cause reduced deterioration of the properties important for fusion performance.

In this paper, vacuum flux surface mapping is used to measure the 1/1 error field in Wendelstein 7-X. Two methods for discriminating the 1/1 perturbation from other co-resonant modes are applied: measurements of the helical axis shift in a magnetic configuration with the rotational transform barely above one on the axis; and measurements of the edge island structure with cancellation of intrinsic 5/5 islands by divertor control coils. The normalized radial 1/1 amplitude in Boozer coordinates is found to be about 0.8 • 10 −4 in the first case and about 0.5 • 10 −4 in the second one, with the discrepancy being attributable to different currents in the planar coils. The 1/1 error field can be reliably compensated by using error field trim coils with currents about 100 A. This compensation also helps to identify the 2/2 error field of about 0.6 • 10 −4. During plasma operation in divertor configurations, it is confirmed that the present 1/1 field leads to asymmetries in the power distribution and that the 1/1 compensation improves this symmetry.

Plasma core fuelling is a key issue for the development of steady-state scenarios in large magnetically-confined fusion devices, in particular for helical-type machines. At present, cryogenic pellet injection is the most promising technique for efficient fuelling. Here, pellet ablation and fuelling efficiency experiments, using a compact pellet injector, are carried out in Electron Cyclotron Resonance and Neutral Beam Injection heated plasmas of the stellarator TJ-II. Ablation profiles are reconstructed from light emissions collected by silicon photodiodes and a fast-frame camera system, under the assumptions that such emissions are loosely related to the ablation rate and that pellet radial acceleration is negligible. In addition, pellet particle deposition and fuelling efficiency are determined using density profiles provided by a Thomson Scattering system. Furthermore, experimental results are compared with ablation and deposition profiles provided by the HPI2 pellet code, which is adapted here for the stellarators Wendelstein 7-X (W7-X) and TJ-II. Finally, the HPI2 code is used to simulate ablation and deposition profiles for pellets of different sizes and velocities injected into relevant W7-X plasma scenarios, while estimating the plasmoid drift and the fuelling efficiency of injections made from two W7-X ports.