Updates on Numerical Implementation and Testing of NIMSTELL*

NIMSTELL [1] solves visco-resistive MHD equations in 3D geometry. Cylindrical coordinates (R, Z, φ) of the finite element mesh and the perturbed and steady state fields are represented as Fourier series in the generalized toroidal coordinate. Vector potential is expressed using an H(curl) basis of arbitrary polynomial degree to ensure a divergence-free magnetic field. Parallel block-diagonal preconditioning has been developed for accelerating the large sparse linear solves required for each time-advance of velocity, temperature, and vector potential. The matrix blocks may span over multiple Fourier components and may have an overlap of several components. This strategy is effective in cases where the Fourier components of dependent fields are coupled due to 3D geometry. An interface between NIMSTELL and DESC [2] has been developed to obtain flux-aligned meshes and the corresponding equilibrium fields. Linear tearing is calculated in helically shaped toroidal configurations with elliptical cross-sections of eccentricities up to 0.8, whose equilibria are obtained using the NIMSTELL-DESC interface. Progress on calculations of linear interchange in a straight stellarator is also reported.

References: [1] C. R. Sovinec and B. S. Cornille, BAPS 66(13), PP11.00092 (2021); [2] D. W. Dudt and E. Kolemen, PoP 27, 102513 (2020).