Transient-MHD-Induced Transitions In Metastable Tokamak Plasmas*

When small resonant magnetic perturbations (RMPs) are present in flowing plasmas, they can produce metastable states. Namely, tokamak plasmas can be in benign high-slip, flow-screened states or in low-slip, mode-penetrated states that produce magnetic islands which can be problematic and growing. It has been demonstrated recently in slab model calculations with NIMROD [1] that MHD-type perturbations of large enough magnitude and duration can induce transitions into low-slip states. In tokamaks, while static RMPs are typically produced by field errors and applied fields, sawteeth and ELMs cause nonlinear resonant field responses. MHD-transient-induced transitions in tokamaks include RMP ELM suppression and excitation of neoclassical tearing modes (NTMs). We are developing reduced fluid-based models of these processes in tokamak plasmas that include near-axisymmetric geometry, low collisionality effects, and poloidal and toroidal flow (radial electric field) responses to MHD transients. Applications to RMP ELM suppression and NTMs will be discussed.

[1] M.T. Beidler, J.D. Callen, C.C. Hegna, and C.R. Sovinec, “Mode Penetration Induced By Transient Magnetic Perturbations,” report UW-CPTC 18-3, June 25, 2018, available via cptc.wisc.edu.