First Nonlinear Stellarator Applications of NIMSTELL

The NIMSTELL simulation code is being developed to study macroscale dynamics in stellarator configurations. As a variant of the widely used NIMROD code [JCOMP 195, 355 2004], NIMSTELL uses many of the same numerical methods but with generalization for 3D configurations. The time advance is semi-implicit, and the spatial representation uses a plane of nodal spectral elements, combined with 1D finite Fourier series for the generalized toroidal angle. Unlike NIMROD, NIMSTELL's geometric information is also expanded in Fourier series over the toroidal angle. Our first nonlinear stellarator applications focus on magnetic tearing. One case is the W7-A tearing case of Ref. [Nikulsin, et al., PoP 29, 063901, 2022], and NIMSTELL computations agree with the fluctuation energy and island width computed by JOREK. Another case has the quasi-axisymmetric (QA) geometry of [Landreman & Paul, PRL 128, 035001, 2022] with an added current profile to make the (2,1) mode resonant and tearing-unstable. As with the W7-A case, the QA instability saturates in a large magnetic island. In addition to the physical results, we briefly describe important computational performance aspects. This work is supported by U.S. DOE grants DE-SC0024548 and DE-FG02-99ER54546.