Influence of up-down asymmetry in plasma shape on the plasma response to RMPs and it's implication on ELM control

The 3D plasma response provides a candidate explanation for the inability to access RMP-ELM suppression at DN plasma shapes in tokamaks, namely that the resonant coupling is reduced in such shapes. Recent RMP-ELM control experiments in KSTAR found that ELMs can be suppressed at LSN shape, while no suppression is obtained in DN shapes when dR_sep<0.5 cm. To understand the influence of up-down asymmetry in plasma shape on RMP-ELM control, the 3D plasma response is investigated in both experiment and modeling by systematically morphing the plasma shape, whilst keeping other equilibrium quantities largely unchanged. It reveals that the edge resonant component of plasma response is strongly suppressed at DN shapes. This is validated through targeted comparison with experiments at KSTAR and EAST. KSTAR simulations show that edge resonance decreases as plasma shape approach DN, while EAST magnetic measurements are used to validate simulations. However, the linear MHD model of plasma response still has limitations in modeling ELM suppression access, as it cannot explain the inability of ELM suppression at USN shapes in KSTAR, nor predict the critical dR_sep. These findings indicate that the plasma shape should be taken into consideration to maximize access to RMP-ELM control in future devices.