Theoretical Estimation of Electromagnetic Loss From the Movement of Superconducting Coil in the W7-X Stellarator

We investigated the dissipation, occurring when a superconducting coil experiences a transient in the magnetic field generated by other coil. Such mechanism could be relevant during energizing the main magnetic field in the W7-X machine, when the mutual forces between superconducting coils could lead to their deformations or movements. Significant paths for current loops have been identified, and the formulas allowing the estimation of dissipated heat derived. All contributions to the heat generation due to eddy currents, coupling and hysteresis losses could be included into the calculations. Finite element simulations have been used to determine the typical transient of magnetic field that could be expected inside the W7-X coil due to mechanical movement. We found that one can foresee magnetic field changes of about 20 mT occurring during 40 milliseconds. Applying the formulas derived for transient loss for the case of such field change, the maximum dissipation connected with one event was estimated to be less than 0.16$ mJ/cm^3$, which is tolerable for the magnet design of W7-X.