Electron Cyclotron Heating Limiting Access to RMP ELM Suppression

DIII-D experiments explored varying the ratio of on-axis electron cyclotron heating (ECH) to neutral beam injection (NBI) heating and a loss of edge localized mode (ELM) suppression is reliably observed at an ECH/NBI ratio close to 1. Resonant magnetic perturbations (RMPs) are considered the main tool to suppress ELMs for ITER, but experiments using ECH – the dominant heating source for ITER – thus far suggest this is challenging for RMP ELM suppression. So far, an access condition for RMP ELM suppression related to applied external heating or resulting temperatures is not being documented. This poses a serious concern for RMP ELM suppression at ITER. In DIII-D experiments studying this, torque input is still sufficiently high to keep the plasma rotation above the observed thresholds during ECH. Changes in plasma beta and hence the plasma response to the RMP field do not explain the loss of ELM suppression. Neither can the change in peeling-ballooning stability as a consequence of the altered pedestal. Comparing the pedestal to NBI heated plasmas suggests the ion/electron temperature or heat flux ratios are impacting RMP control through turbulence in the pedestal. Replacing a fraction of the on-axis ECH with near pedestal top electron cyclotron current drive preserves ELM suppression.