Analysis and correction of intrinsic non-axisymmetric magnetic fields in high-β DIII-D plasmas

Rapid plasma toroidal rotation, sufficient for stabilization of the n I resistive wall mode, can be sustained by improving the axisymmetry of the toroidal magnetic field geometry of DIII-D. The required symmetrization is determined experimentally both by optimizing currents in external n = 1 correction coils with respect to the plasma rotation, and by use of the n = I magnetic feedback to detect and minimize the plasma response to non-axisymmetric fields as beta increases. Both methods point to an intrinsic similar to7 G (0.03% of the toroidal field), m/n = 2/1 resonant helical field at the q = 2 surface as the cause of the plasma rotation slowdown above the no-wall beta limit. The drag exerted by this field on the plasma rotation is consistent with the behaviour of 'slipping' in a simple induction motor model.