Identification of multi-modal plasma responses to applied magnetic perturbations using the plasma reluctance

Authors:  Nikolas C. Logan, Carlos Paz-Soldan, Jong-Kyu Park and Raffi Nazikian

Abstract:  Using the plasma reluctance, the Ideal Perturbed Equilibrium Code (IPEC) is able to efficiently identify the structure of multi-modal magnetic plasma response measurements and the corresponding impact on plasma performance in the DIII-D tokamak. Recent experiments demonstrated that multiple kink modes of comparable amplitudes can be driven by applied nonaxisymmetric fields with toroidal mode number n = 2. This multi-modal response is in good agreement with ideal magnetohydrodynamic (MHD) models, but detailed decompositions presented here show the mode structures are not fully described by either the least stable modes or the resonant plasma response. This work identifies the measured response fields as the first eigenmodes of the plasma reluctance, enabling clear diagnosis of the plasma modes and their impact on performance from external sensors. The reluctance shows, for example, how very stable modes compose a significant portion of the multi-modal plasma response field and that these stable modes drive significant resonant current. This work is an overview of the first experimental applications using the reluctance to interpret the measured response and relate it to multifaceted physics, aimed towards providing the foundation of understanding needed to optimize nonaxisymmetric fields for independent control of stability and transport.
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