Plasma Response in Single Null and Double Null Plasmas*

The 3D plasma response to applied non-axisymmetric magnetic fields decreases significantly as the plasma shape transitions from single null to double null divertor configuration, indicating a role of the separatrix and scrape-off-layer in determining the internal plasma response. This may provide a reason as to why full RMP ELM suppression in double-null divertor configuration plasmas has been elusive—coupling to the high-field side (HFS) plasma response has been previously correlated with ELM suppression. In single null plasmas, the measured plasma response via magnetics shows both a HFS and low-field side (LFS) component consistent in size with previous results. Additionally, internal kink-like perturbations in the steep pedestal gradient region measured by Soft X-Ray (SXR) imaging are observed in the X-point region. Transitioning from a single null to balanced double null configuration reduces or damps the HFS magnetics response by 60% and similarly damps the perturbations measured by SXR imaging on the inner side or HFS of the X-point. Linearized time-independent extended MHD modeling similarly shows a reduction in HFS response in double null configurations.