Measurement of the electromagnetic torque on a magnetic island could be an attractive method for error field identification in the early phase of ITER operation. Previous DIII-D experiments (Strait 2014 Nucl. Fusion54 073004; Shiraki et al 2015 Plasma Phys. Control. Fusion57 025016) have demonstrated the principle of this approach using a stationary or slowly rotating island, while recent developments in magnetic data analysis (Sweeney and Strait 2019 Phys. Plasmas26 012509) allow the field of a rapidly rotating island to be readily distinguished from that of the wall currents induced by its rotation. In a recent experiment, a 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 time-dependent 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 agree well with those from more conventional methods.
This paper describes a method to identify error fields in fusion devices by measuring the electromagnetic torque on a rotating magnetic island. The authors used a rotating n=1 magnetic perturbation to force a saturated island to rotate, allowing them to sample all toroidal phases and infer the phase and amplitude of the error field from the time-dependent torque balance on the island.