Initial Results of NIMSTELL, the Stellarator Variant of NIMROD

NIMSTELL is a substantial refactoring of the NIMROD code to make nonlinear stellarator MHD computations efficient. The changes include 1) expanding geometric information in finite Fourier series over a generalized toroidal angle and 2) using vector potential from the H(curl) space. Previous stellarator applications of the standard NIMROD code [for example, 1-3] suffered slow convergence of the Fourier expansion with uniform meshing over the geometric toroidal angle and were limited to configurations where the coils and plasma could be separated by a toroidally symmetric surface. The new developments for geometry are overcoming these challenges, and magnetic field is inherently divergence-free with the use of vector potential in H(curl). This presentation focuses on initial nonlinear MHD applications of NIMSTELL, including heating and MHD topological evolution in straight and toroidal configurations. The efficacy of the spectral-element/Fourier representation with approximately flux-aligned 3D meshing is demonstrated. [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.