Plasma Response to Perturbed Nonaxisymmetric Fields in Double Null Configurations

The plasma response to external nonaxisymmetric perturbations is a key ingredient in the observed ELM suppression from external fields. Dedicated experiments in DIII-D have confirmed a prior observation that the magnetic response on the inboard high field side in balanced double null (DN) plasmas is greatly reduced compared to the signal in similar single null (SN) discharges, as well as to the outboard signals. This is a key part of the puzzle in explaining ELM suppression: full suppression has not been observed in DN in any experiments despite the large density pumpout that typically occurs in those conditions. Analysis of sequences of equilibria with scans in the magnetic balance and edge safety factor q₉₅ indicates that there is a relatively narrow window in magnetic balance over which the inboard signal is suppressed by a factor of five. In this range, the density pumpout is enhanced and the ELMs are mitigated but not suppressed. Comparisons of various code predictions for the response with external magnetic data and internal SXR data confirm the observed inboard magnetic field suppression. The results are discussed in the context of the key competing models for ELM suppression and mitigation.