Plasma response to external fields with finite current gradient

The displacement of the plasma edge in response to an external magnetic perturbation is calculated in slab geometry. Experiments in DIII-D have shown that varying the magnetic field imposed by the external coils can cause an edge displacement of approximately 2 cm, roughly 6{\%} the plasma half-width. A previously derived model [A. Cole and R. Fitzpatrick, Phys. Plasmas 13, 032 503 (2006)] for the plasma response to an external field perturbation is extended to include effects essential to explaining this data, where equations describing the drift-MHD stability are extended to apply to an equilibrium in slab geometry with a non-zero current gradient flanked by vacuum regions. This system of equations is solved for the modified ExB velocity in viscous force balance. The maximum displacement of the plasma boundary calculated is comparable to experimental results, but only occurs near the ideal MHD limit. This result indicates that the inclusion of a non-zero current gradient and a vacuum region in the slab equilibrium specification is perhaps the simplest method to calculate realistic plasma displacements due to external fields.