T. Bräuer

Tracking of the magnet system geometry during Wendelstein 7-X construction to achieve the designed magnetic field

Wendelstein 7-X, currently under commissioning at the Max-Planck-Institut fur Plasmaphysik in Greifswald, Germany, is a modular advanced stellarator, combining the modular coil concept with optimized properties of the plasma. Most of the envisaged magnetic configurations of the machine are rather sensitive to symmetry breaking perturbations which are the consequence of unavoidable manufacturing and assembly tolerances. This overview describes the successive tracking of the Wendelstein 7-X magnet system geometry starting from the manufacturing of the winding packs up to the modelling of the influence of operation loads. The deviations found were used to calculate the resulting error fields and to compare them with the compensation capacity of the trim coils.

Progress, Challenges, and Lessons Learned in the Construction of Wendelstein 7-X

This paper reports on experiences gained during the construction of the large superconducting optimized stellarator device Wendelstein 7-X (W7-X). The goal of the project is to demonstrate the fusion reactor potential of optimized stellarators and to operate for the first time fusion-relevant plasmas under full steady-state conditions. In addition, the validity of the engineering concept is to be proven under operating conditions. The actual construction of the device is now running since more than ten years and will require about three further years of assembly and integration. The current technical status of W7-X is reviewed, the major construction challenges are highlighted, and conclusions are drawn from past experiences that may also be useful for other large-scale science projects.

Three-dimensional photogrammetric measurement of magnetic field lines in the WEGA stellarator

The magnetic confinement of plasmas in fusion experiments can significantly degrade due to perturbations of the magnetic field. A precise analysis of the magnetic field in a stellarator-type experiment utilizes electrons as test particles following the magnetic field line. The usual fluorescent detector for this electron beam limits the provided information to two-dimensional cut views at certain toroidal positions. However, the technique described in this article allows measuring the three-dimensional structure of the magnetic field by means of close-range photogrammetry. After testing and optimizing the main diagnostic components, measurements of the magnetic field lines were accomplished with a spatial resolution of 5 mm. The results agree with numeric calculations, qualifying this technique as an additional tool to investigate magnetic field configurations in a stellarator. For a possible future application, ways are indicated on how to reduce experimental error sources.

Prospects of X-ray imaging spectrometers for impurity transport: Recent results from the stellarator Wendelstein 7-X (invited)

This paper reports on the design and the performance of the recently upgraded X-ray imaging spectrometer systems, X-ray imaging crystal spectrometer and high resolution X-ray imaging spectrometer, installed at the optimized stellarator Wendelstein 7-X. High resolution spectra of highly ionized, He-like Si, Ar, Ti, and Fe as well as H-like Ar have been observed. A cross comparison of ion and electron temperature profiles derived from a spectral fit and tomographic inversion of Ar and Fe spectra shows a reasonable match with both the spectrometers. The also measured impurity density profiles of Ar and Fe have peaked densities at radial positions that are in qualitative agreement with the expectations from the He-like impurity fractional abundances, given the measured temperature profiles. Repeated measurements of impurity decay times have been demonstrated with an accuracy of 1 ms via injection of non-recycling Ti, Fe, and Mo impurities using a laser blow-off system.

Progress and challenges in the construction of wendelstein 7-X

This paper reports on experiences gained during the construction of the large, superconducting, optimized stellarator device Wendelstein 7-X. The goal of the project is to demonstrate the fusion reactor potential of optimized stellarators and to operate for the first time fusion-relevant plasmas under full steady-state conditions. In addition the validity of the engineering concept is to be proven under operating conditions. The actual construction of the device is now running since more than ten years and will require about three further years of assembly and integration. The current technical status of Wendelstein 7-X is reviewed, the major construction challenges are highlighted, and conclusions are drawn from past experiences that may also be useful for other large-scale science projects.