Multimodal plasma response to 3D magnetic perturbations and its impact on the divertor heat fluxes

Observation of changes in the three-dimensional (3D) structure of the heat and particle fluxes at the divertor plates of the DIII-D tokamak, correlated with changes of the poloidal spectrum of applied 3D fields, are explained by the multimodal nature of the plasma response, and this provides new tools for the control of the divertor heat fluxes in ITER and beyond. Detailed comparisons between measured and predicted edge poloidal structure of the plasma response to applied magnetic perturbations in DIII-D highlight the capability of resistive MHD codes to correctly capture the multimodal nature of the plasma response. Singular value decomposition analysis of the magnetic measurements of the plasma response to stationary applied perturbations, combined with resistive MHD simulations performed with the MARS-F code, shows the presence of two dominant modes: one radially localized at the edge that extends to the whole poloidal circumference and one poloidally localized at the low field side midplane that extends radially to the core. Radial profiles of the infrared and D?? emission show a larger 3D structure at the divertor plates when the former mode is dominant and no correlation with the amplitude of the latter. Resistive MHD simulations done with the M3D-C1 code coupled with the field line tracing code TRIP3D confirm the link between magnetic footprints and the edge resonant mode. This mechanism can also explain the observed inability of the error field correction based on the mitigation of the core mode to reduce the presence of 3D magnetic footprints at the divertor in DIII-D L-mode plasmas. Furthermore, this suggests it will be challenging to decouple the RMP-ELM control problem, for which the edge-resonant coupling is a prerequisite, from the divertor heat flux one.