Compatibility of RMP ELM Suppression with Double-Null Configurations

Modeling of DIII-D plasmas spanning shapes from single to double-null (DN) reveals new insights into the nature of resonant magnetic perturbation (RMP) conditions necessary for edge-localized mode (ELM) suppression. The suppression of ELMs with RMPs has proven difficult in DN configurations, where no device has thus far shown any hints of suppression. Modeling using the GPEC code finds a reduced high-field side response closer to dRsep=0, shown by a lower perturbed field and resulting synthetic diagnostic measurements. This is consistent with what has been observed in experiments on DIII-D [Shafer IAEA 2020]. While common metrics for suppression do not illustrate a clear distinction between SN and DN cases, the pedestal top resonant field does show a ~15% decrease at dRsep=0 in modeling. Effective island widths inferred from this 3D ideal MHD model are also assessed, where maximum widths in lab coordinates indicate a threshold of ~15x the ion gyroradius for ELM suppression. These effective island widths have a distinct dip near the DN shape and are compared to critical island widths from drift kinetic simulations. These results indicate that ELM suppression may be possible in DN with sufficiently large RMP coil amplitude, however shaping and profile effects need to be further investigated.