Error Field Identification through Torque Balance on a Saturated Island

Measurement of the electromagnetic torque on a magnetic island could be an attractive method for error field identification in the early (PFPO-1) phase of ITER operation. Previous DIII-D experiments [1,2] have demonstrated the principle of this approach using a stationary island, while recent developments in magnetic data analysis [3] allow the field of a rotating island to be distinguished from that of the wall currents induced by its rotation. In a recent experiment, a slowly rotating n=1 magnetic perturbation forced a saturated magnetic island to rotate, thus sampling all toroidal phases periodically in a single discharge. The phase and amplitude of the error field are inferred from analysis of the torque balance on the island, including torques from the error field, the applied magnetic perturbation, and the wall currents induced by rotation of the applied perturbation and the island. Results will be compared with those from more conventional methods. --- Refs.: [1] E.J. Strait, NF 2014; [2] D. Shiraki, NF 2014; [3] R.M. Sweeney, PoP 2019