Asymmetry of W7-X magnet system introduced by torus assembly

• Torus assembly introduces magnet system asymmetry up to 2 mm.
• Asymmetry due to torus assembly is of same magnitude as caused by uncertainties of magnet system properties (material, contacts, geometry and initial preload).
• Asymmetry is input for EM field error calculation.

The magnet system of the stellerator Wendelstein 7-X (W7-X) consists of 5 modules of 14 superconducting coils with complex 3D shape each. After manufacturing the coils and assembly of the modules on temporary stands, the position of each module on the machine base was successfully optimized to minimize the electromagnetic (EM) field asymmetry. This asymmetry originates from inevitable geometric deviations of the coils from the target shape due to manufacturing and assembly tolerances.However, new deviations were introduced after module optimization due to bolting the modules of the magnet system together to a torus, removing temporary supports and further loading of the machine base with weight of additional components.In this paper, the geometrical deviations along the centre line of the coil currents are assessed through detailed step-by-step non-linear finite element (FE) simulation of the assembly procedure of the complete torus. The model is evaluated against measured displacements and reaction forces monitored during consequent assembly steps. The results are being used to quantify the obtained field asymmetry and countermeasures to minimize it.