The effect of the plasma position control system on the three-dimensional distortion of the plasma boundary when magnetic perturbations are applied in MAST

When resonant magnetic perturbations are applied in MAST, the plasma edge boundary experiences a three-dimensional (3D) distortion, which can be a few percent of the minor radius in amplitude, in good agreement with ideal 3D equilibrium modelling. This displacement occurs in plasmas both with radial position feedback control applied, and without feedback. When position feedback control is employed, an applied non-axisymmetric field can lead to an exacerbated edge displacement due to an ancillary axisymmetric position correction, with the direction of the correction dependent upon the phase of the applied field with respect to the toroidal position of the sensors used in the controller. This suggests that future machines reliant upon resonant magnetic perturbations for controlling ELMs should consider using a plasma control system capable of applying a position correction which accounts for the non-axisymmetry of applied magnetic perturbations.