Predicting nonresonant pressure-driven MHD modes in equilibria with low magnetic shear

Wright, A. M.; Ferraro, N. M.; Hudson, S. R.; Dewar, R. L.; Hole, M. J.

doi:10.1063/5.0032489

Predicting nonresonant pressure-driven MHD modes in equilibria with low magnetic shear

Journal Article · Thu Jan 21 00:00:00 EST 2021 · Physics of Plasmas

DOI:https://doi.org/10.1063/5.0032489· OSTI ID:1779754

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Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States); Australian National Univ., Canberra, ACT (Australia)
Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)
Australian National Univ., Canberra, ACT (Australia)

Nonresonant internal modes can be difficult to anticipate as there is no resonant surface in the plasma. However, equilibria that are unstable to multiple nonresonant magnetohydrodynamic (MHD) modes may be more prone to global loss of confinement since these instabilities generate spatially extended linear displacements, potentially enhancing magnetic field line chaos via nonlinear interactions. Here, we successfully predict the unstable nonresonant pressure-driven modes for equilibria with zero shear in the plasma core, irrational q on axis, and a central pressure gradient, which is consistent with pre-crash profiles in sawtoothing tokamak plasmas in the large-aspect-ratio limit. A criterion for identifying nonresonant modes most likely to be unstable is developed from the convergents of the continued fraction representation of q₀. A higher-order analysis of the standard Energy Principle reveals the conditions under which these modes are expected to dominate. Linear growth rate spectra, as a function of toroidal mode number (up to n = 30), calculated using the initial-value extended-MHD code, M3D-C1, recover the characteristic dependence observed for ideal infernal modes. Nonresonant modes have also been invoked in some ideal sawtooth crash models. Finally, this work provides a mechanism to predict the mode numbers of infernal modes and, potentially, the width of some post-sawtooth-crash profiles.

View Accepted Manuscript (DOE)

Research Organization:: Princeton Plasma Physics Laboratory (PPPL), Princeton, NJ (United States)

Sponsoring Organization:: Australian Research Council; Simons Foundation; USDOE

Grant/Contract Number:: AC02-76CH03073

OSTI ID:: 1779754

Alternate ID(s):: OSTI ID: 1761251

Journal Information:: Physics of Plasmas, Journal Name: Physics of Plasmas Journal Issue: 1 Vol. 28; ISSN 1070-664X

Publisher:: American Institute of Physics (AIP)Copyright Statement

Country of Publication:: United States

Language:: English

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