The effect of plasma shape and neutral beam mix on the rotation threshold for RMP-ELM suppression

Abstract

Varying the neutral beam injection (NBI) mix reveals a clear pedestal-top rotation threshold for edge localized mode (ELM) suppression by resonant magnetic perturbations. Guided by expectations for the RMP penetration mechanism, the rotation threshold is found to correspond to a critical radius for the ExB rotation zero-crossing. No such critical radius is observed for the electron perpendicular rotation zero-crossing. Varying the amount and ratio of power in different NBI source geometries (termed the NBI mix) also reveals that the rotation threshold can be crossed at widely varying total injected NBI torques. Computing the local torque density at the edge, the rotation threshold is found to be crossed when the local edge NBI torque is negative in nearly all discharges. Reducing the upper triangularity (Δ<sub>up</sub>) from the ITER-similar value of 0.3 to 0.1 significantly reduces the pedestal height and width. This in turn: 1) Increases the rotation threshold and yields a more outward critical ExB rotation zero-crossing location. 2) Decreases the density threshold, consistent with a comparable collisionality range at lower pedestal temperatures. 3) Increases the input torque requirement, due to observed lower confinement and smaller intrinsic torque. These findings represent an important step along the road to predicting ELM suppression access conditions in future tokamaks such as ITER, where the toroidal rotation is expected to be low and consequently the rotation zero-crossing far from the pedestal-top.