51 / 2021-06-19 05:22:50

Limits of RMP ELM Suppression in Double Null Plasmas

RMP, ELM Suppression, double null

New DIII-D results provide a candidate explanation for why achieving ELM suppression by resonant magnetic fields (RMPs) remains elusive in double null (DN) diverted configurations: the lack of ELM suppression in DN correlates with a damped high-field side response of field-aligned structures that could be indicative of a missing resonant tearing needed to stop inward growth of pedestal. The lack of ELM suppression is found despite favorable conditions for RMP suppression in lower single null (LSN)—low Ω_E×B aligned with a resonant surface at the pedestal top under the model that this results in resonant tearing inhibiting the inward growth of the pedestal otherwise leading to an ELM [1]. The 3D plasma response to applied RMPs measured on the high-field side (HFS) drops in plasma shapes transitioning from LSN to DN and recovers in upper single null (USN). The plasma response on the low field side (LFS) remains relatively constant during the shape transition. The reduced HFS response is found broadly across a range of |dRsep|≲1 cm indicating it is not restricted to exactly balanced DN or specific pedestal conditions. Linear time-independent extended MHD modeling similarly shows a reduction in HFS response in double null configurations. Conceptually, the additional null adds radial shear to externally driven field-aligned modes on the LFS and therefore may inhibit the coupling to the HFS. These results are consistent with using HFS response as a proxy for local tearing drive responsible for ELM suppression by stopping inward growth of pedestal, consistent with previous results showing correlation of HFS response and tearing drive needed for ELM suppression [2].

This work was supported in part by the US Department of Energy under DE-AC05-00OR22725, DE-FC02-04ER54698, and DE-AC02-09CH11466.



[1] Snyder P.B. et al 2012 Phys. Plasmas 19 056115; Nazikian R. et al 2015 Phys. Rev. Lett. 114 105002; Paz-Soldan C. et al 2019 Nucl. Fusion 59 056012.

[2] Paz-Soldan C. et al 2015 Phys. Rev. Lett. 114 10500