Investigation of Multi-Mode MHD Behavior in Shaped HBT-EP Discharges

We report on investigations into the effect on multimode MHD of a newly installed poloidal field (PF) coil. The coil will allow the circular, limited HBT-EP to follow the main thrust of research towards a fusion reactor, which has been directed toward plasmas that are shaped and diverted. The coil shapes the high field side of the plasma up to and including imposing a PF null, while retaining compatibility with existing diagnostics and control systems. Multimode dynamics have been detected in naturally-rotating kink modes and during the response to 3D resonant magnetic perturbations.\footnote{Levesque, \textit{et al.}, \textit{Nucl Fusion} \textbf{53}, 073037 (2013).} Shaping changes both the resonant helical characteristics of MHD instabilities and the plasma response to external excitation and active control. Calculations using the TokaMac and DCON codes\footnote{Maurer, et al., \textit{Plasma Phys Contr F} \textbf{53}, 074016 (2011).} have predicted that the coupling between the two least stable MHD kink modes would be reduced when edge $q^*$ is near resonance. The RMP response, mode structure, and multi-mode content of diverted plasmas are compared to limited ones. Preliminary results show shaping changes the relative strengths of the $n=1$ and $n=2$ modes.