Toroidal Mode Number Dependence of RMP ELM Suppression in DIII-D

A comprehensive comparison of RMP ELM suppression using n=1,2,3 fields in DIII-D is presented, highlighting the differences in the radial structure of perturbations, pump-out, and core mode-locking thresholds between the toroidal mode numbers. DIII-D has demonstrated RMP ELM suppression using n=2,3 fields, however robust suppression with n=1 fields has remained elusive. From recent dedicated RMP experiments using n=1 fields, a ~100 ms phase of suppression was observed at q₉₅=3.9 using DIII-D’s two rows of internal six-coil arrays (I-coils), but was not sustained. A further window of suppression was observed at q₉₅=5.1, also using only the I-coils. A combined in- and ex-vessel coil configuration, optimized via 3D ideal MHD modeling to enhance edge-to-core resonant flux, produced stronger density pump-out but no suppression. These results illustrate the difficulty in attaining robust suppression with n=1 fields using DIII-D’s coil set. This analysis shows that while n=1 fields may be more efficient at large distances, the cost in performance, risk of locking, and coil design are serious challenges that make higher n more attractive.