ELM suppression by resonant magnetic perturbations at DIII-D

Edge localized mode (ELM) suppression by resonant magnetic perturbations (RMPs) is being studied at DIII-D with ITER-similar plasma shapes and edge collisionalities. Suppression at low collisionality correlates with overlap of vacuum magnetic field islands in a quantified edge layer at moderate q. Suppression is successful with n = 3 perturbations from one or two rows of internal coils, but it fails for n < 3 and for poloidally-tall and distant coil arrays. These observations are interpreted in terms of differences of spatial helical harmonic spectra the different magnetic perturbations: excessive nonresonant harmonics may brake plasma rotation before the RMP is large enough to suppress ELMs.