Multimode Plasma Response in the EAST Tokamak.*

A multimodal plasma response to n $=$ 2 RMPs was found in EAST tokamak low $\beta_{N} $ plasmas. The signature of the multimodal response is the difference in the radial ($B_{r} )$ and poloidal ($B_{p} )$ magnetic field dependencies on the applied phasing (poloidal structure). A difference in the 3D coil phasing that maximizes these two responses is observed only in response to n$=$2 fields, while the n$=$1 $B_{r} $ and $B_{p} $ have the same phasing dependence. Neither the maximum $B_{r} $ nor $B_{p} $ agrees with the best phasing for ELM mitigation in experiments. GPEC modeling accurately reproduces the experimental measurements only when multiple eigenmodes of the plasma response are included. The measured plasma response is not dominated by resonant current drive from the external field, with non-resonant contributions playing a large role. These results on EAST demonstrate a new type of multimode response based on the variation of the polarization of the plasma response to the 3D field. The results clearly demonstrate the danger of associating any one sensor array with desired physics consequence of 3D fields and the need for 3D modeling to predict optimal 3D field configurations in multi-modal plasmas.