Computation of linearly unstable resistive MHD modes in tokamak and stellarator plasmas using NIMSTELL

Calculating the nonlinear MHD behavior of stellarator plasmas on resistive timescales is important for understanding and predicting soft beta limits and loss of equilibrium. The code NIMSTELL [1] has been developed with these goals in mind. It uses a finite-element grid in the poloidal plane, and Fourier series in a generalized toroidal angle for all perturbations and steady-state fields, including the physical coordinates (R, Z, φ). An interface between NIMSTELL and DESC [2], a 3D ideal MHD equilibrium code, has been developed for obtaining flux-aligned finite-element grids and the corresponding equilibrium fields. As a first step toward nonlinear calculations, linearized visco-resistive MHD equations have been solved using NIMSTELL. Benchmarking against NIMROD has been completed for tearing modes in a circular tokamak. Here, we investigate tearing in a toroidal plasma with a helically twisting elliptical cross section of eccentricities ranging from 0.4 to 0.8. First results for the linear MHD stability of a set of QHS configurations [3] are also presented.

References:

[1] C. Sovinec and B. Cornille, BAPS 66(13), PP11.00092 (2021).

[2] D. W. Dudt and E. Kolemen, PoP 27, 102513 (2020).

[3] A. Bader, et al., JPP 86, 90580506 (2020).