Accuracy of the Construction of the Superconducting Coils for WENDELSTEIN 7-X

Abstract

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.

Optimized finite-build stellarator coils using automatic differentiation

“…For coils of approximately the same dimensions as the W7-X coils (18cm by 18cm), we have the result that the mean shift ∆r ≈ 2.5mm, while the maximum shift is about 11mm. The coil tolerances in the W7-X experimental design are ±2mm [34]. Because the mean coil shift is of the same magnitude as the coil tolerances, and the maximum shift much Figure 9: W7-X: (a) ∆r, the mean shift in the coil position between finite-build and filamentary coils, is plotted in blue as a function of coil size δ.…”

Identification of important error fields in stellarators using the Hessian matrix method

“…The residual error is exponentially reduced with increasing N F .Since N F = 10 has the minimum error, we will use N F = 10 for future calculations. Then the total number of degrees of freedom, as listed in equation (12), is 1024 = 16 × (6 × 10 + 3 + 1).…”

“…The primary criteria were the resonant magnetic perturbations, B 11 , B 22 , B 33 & B 44 since W7-X has a m/n = 5/5 island chain outside the last closed flux surface (LCFS). Numerical investigations [12,13] showed that the resonant magnetic field perturbations were the most sensitive to rotations of coils and modules, after the effects of manufacturing errors, shifts and rotations of individual coils and modules were compared. With numerous efforts and advanced manufacturing techniques, W7-X superconducting coil system was built and assembled with impressively high accuracy.…”

Technical challenges in the construction of the steady-state stellarator Wendelstein 7-X

“…In order to be able to achieve the high precision of coil manufacturing, a soft aluminium-conduit has been chosen for the superconductor, which allows for easier bending to the required accuracy [17]. Later, the finished winding pack was heat treated to harden the aluminium and to achieve the required strength of the conductor [6].…”

“…Symmetry-breaking perturbations can arise either from non-symmetric deviations in the coil shape during the coil manufacturing or from position displacements and coil deformations during the assembly process. In order to be able to achieve the high precision of coil manufacturing, a soft aluminum conduit has been chosen for the superconductor, which allows for easier bending to the required accuracy [17]. Later, the finished winding pack was heat treated to harden the aluminum and to achieve the required strength of the conductor [6].…”