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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