Compensation of Wendelstein 7-X construction errors by optimisation of module positions

Wendelstein 7-X, currently under construction at the Max-Planck-Institut fur Plasmaphysik in Greifswald, Germany, is a modular advanced stellarator, combining the modular coil concept with optimised properties of the plasma. Wendelstein 7-X magnetic configurations are rather sensitive to magnetic field perturbations caused by manufacturing deviations of the winding pack shapes from their designed values and by positioning errors during the machine assembly. In order to minimize the impact of these errors it was decided to optimize the position of each of the five machine magnet modules individually, based on up-to-date measurement data concerning the coil alignments available at the moment of calculation. This paper presents a choice of a corresponding quality function for the magnetic field evaluation. Results of the optimisation are shown for the sequential assembly of the first three machine modules. The influence of the step-by-step sag of the machine base on the magnetic configuration is also considered.