Plasma Response in Single Null and Double Null Plasmas and Implications for ELM Suppression

The 3D plasma response to applied non-axisymmetric magnetic fields decreases as the plasma shape transitions from single null to double null divertor configuration. Measurements from the DIII-D tokamak show the response measured on the high-field side (HFS) drops while remaining relatively constant on the low-field side (LFS) in both the external magnetics and internal SXR. The drop in the HFS response is shown over a range of $n_{e,ped}$, $\beta_{n}$, and $q_{95}$ for both $n=2$ and $n=3$ perturbations indicating a robust effect. Linearized 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. This may provide a reason as to why RMP ELM suppression in double-null configurations has been elusive—coupling to the HFS plasma response has been previously correlated with ELM suppression.