Benchmarking extended MHD modeling of resonant magnetic perturbations against measurements of islands and screening on DIII-D

Experiments where resonant magnetic perturbations (RMPs) are applied to L-mode plasmas are used to test the fundamental plasma response physics on island formation in resistive MHD models. Fine torque scans reveal that large RMP-induced $n$=1 islands open at multiple mode-rational surfaces ($m$=2,3,4) at low rotation, but are screened elsewhere. Time-independent linear resistive MHD simulations with the M3D-C1 code predict a narrow region centered at at $\omega_{e\bot}$ $<$ 0 where resonant fields are found to be weakly screened, though strongly screened elsewhere. Experimentally, the island formation window is wider and centered at $\omega_{e\bot}$ $<$ 0. Nonlinear resistive MHD simulations in the absence of rotation are performed with the SIESTA code. A series of meta-stable equilibria are modeled with an increasing helical 2/1 perturbation. These meta-stable equilibria demonstrate the transition from even-parity screening currents at the rational surface at low perturbation levels to odd-pairty Pfirsch-Schl\"uter currents when a large island is present.