Resolving the Rotation Threshold for RMP-ELM Suppression

Varying the neutral beam injection (NBI) mix reveals pedestal-top toroidal rotation thresholds for RMP-ELM suppression. The threshold rotation increases when the pedestal width is reduced. Observations are interpreted by considering zero-crossings in the flow as expected for the RMP penetration mechanism. A critical radius for the ExB rotation (wExB) zero-crossing is found at the exit from ELM suppression. No such constraint is observed for the electron perpendicular flow (w⟂e), challenging linear two-fluid models. The increased threshold in the narrow pedestal is consistent with a more outward critical radius for the wExB zero-crossing. The observed thresholds are not found at constant injected NBI torque, due to variations in the torque deposited locally at the edge. Understanding the rotation threshold is of great importance in predicting the ability of RMPs to suppress ELMs in future tokamaks such as ITER, where the rotation is expected to be small and consequently the zero-crossings far from the pedestal.