April 4-6

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Author: Carl Sovinec
Requested Type: Poster
Submitted: 2022-03-04 14:59:16

Co-authors: Sanket Patil

Contact Info:
University of Wisconsin-Madison
1500 Engineering Drive
Madison, Wisconsin   53706-1

Abstract Text:
The NIMSTELL code is being developed to solve fluid-based models of macroscale stability in stellarators and torsatrons. Previous application of the NIMROD code to these configurations was limited by the axisymmetric constraint of the numerical grid, leading to weak numerical convergence with respect to the toroidal representation [1-3]. NIMSTELL is a significant refactoring of NIMROD with a 3D grid achieved through Fourier expansion of geometric coefficients over a generalized toroidal angle. To satisfy the divergence constraint precisely, NIMSTELL also replaces the H1 element/Fourier magnetic-field expansion with an H(curl)/Fourier expansion of vector potential. Linear computations use either simple analytical equilibria or externally generated equilibria. The latter is made possible by a new interface to the recently developed 3D equilibrium code, DESC [4]. The interface calculates the necessary transforms of the geometry and equilibrium fields. DESC’s inverse coordinate representation makes it convenient to create a flux-aligned grid, which is an important advance with NIMSTELL. Linear verification computations prove convergence on magnetic tearing and resonant interchange on straight and twisted cylindrical meshes. Eigenmodes of the latter have coupled axial Fourier harmonics, as is the case for toroidal harmonics in stellarator computations, exemplified by tearing modes in shaped toroidal equilibria imported from DESC. Findings and developments for the solution of NIMSTELL’s algebraic systems are also presented. [1] M. G. Schlutt, et al. NF 52, 103023 (2012); [2] N. A. Roberds, et al., PoP 23, 092513 (2016); [3] T. A. Bechtel, Stellarator Beta Limits with Extended MHD Modeling Using NIMROD, PhD Dissertation, UW-Madison, 2021; [4] D. W. Dudt and E. Kolemen, PoP 27, 102513 (2020).
*Work supported by US DOE grant DE-SC0018642.