The superconducting magnet system of Wendelstein 7-X (W7-X) consists of five identical field periods (modules). Magnetic field errors arise if the modules are not exactly identical. Even small deviations in the coil shapes of the same type or misalignments of coils or modules break the periodicity of the system and cause error field components.

Simulation of the magnetic field perturbations that are expected has been done by the analysis of existing winding packages and statistical extrapolations of inaccuracies expected during assembly steps. A numerical experiment has shown that assembly errors should contribute significantly more than manufacturing errors of individual coils.

Compensation of the magnetic field perturbation can be done with the help of the coil adjustment during the assembly or by the individual adjustment of all five modules. Further compensation of field errors is possible with additional coils. The existing control coils in W7-X can be used for error field compensation; however, their efficacy is limited. Therefore, solutions employing normal-conducting trim coils outside the cryostat vessel are also considered here.