Strong nonlinearity of 3D equilibrium responses in DIII-D RMP plasma

A strong nonlinearity in the 3D equilibrium response to an RMP field is found in the DIII-D plasma. In n=1 RMP field phase-flip experiments, differences in the n=1 plasma response are observed in each phase, although the RMP field is periodically symmetric. To understand this plasma response nonlinearity, a 3D equilibrium including the error field, which breaks the field periodicity, is studied. The island width at φ_RMP=185 is larger than the width at φ_RMP=5. Because, the error field amplifies an m/n = 1/1 island, the island redistributes the toroidal current density. Thus, the redistributed toroidal current density amplifies the further nonlinear evolution of the m/n = 1/1 island. This is the reason for the strong nonlinearity that appears after the phase flip of the RMP field.