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Numerical survey of predicted peeling response in edge localised mode mitigated and suppressed phases on ASDEX upgrade

Using the MARS-F linear MHD code (Liu et al 2000 Phys. Plasmas 57, 3681), a numerical survey of the plasma response to applied resonant magnetic perturbations in ASDEX Upgrade edge localised mode (ELM) control experiments is conducted, to clarify the role of triangularity and the peeling response in the suppression mechanism. The peeling response is found to decrease with increasing triangularity, due to an increase in the coil-plasma gap reducing the effective vacuum field. Therefore the prior hypothesis that the requirement of high triangularity for suppression access is due to the requirement of a sufficiently large peeling response (Nazikian et al 2016 (In XXVI IAEA Int. Conf. on Fusion Energy, Kyoto, Japan) 1–8) is suspected to be incorrect. A secondary hypothesis is proposed, that in high triangularity the drive of the resonant response by the peeling response may be boosted by enhanced poloidal harmonic coupling, which could explain the requirement of high triangularity for suppression access. It is shown that in fact the poloidal harmonic coupling between the resonant and off-resonant components decreases with triangularity, and therefore this hypothesis is also rejected. Finally an alternative hypothesis is discussed, that high triangularity is required to access suppression because the associated enhanced pedestal stability allows the edge deformation to be large enough to control the density, without the reduction in stability due to boundary deformation destabilising ELMs. A rigorous test of this hypothesis requires models to be developed to compute the stability of experimental 3D equilibria.