Development of MHD simulation capability for stellarators

Experiments have shown that stellarators and heliotrons are subject to soft limits and loss of equilibrium due to changes in magnetic topology as plasma-beta is increased. We expect that non-ideal, time-dependent simulations, such as those in Refs. [1-2], will contribute to the understanding of this behavior. An efficient numerical representation is important for achieving resolution at realistic values of physical parameters, so we are developing a variant of the NIMROD code that features 3D geometry with the logical-to-physical coordinate mapping using a 2D spectral-element/1D Fourier representation. The logical periodic coordinate is a generalized toroidal angle. The only restrictions are that the mapping is continuous and periodic. Improved convergence is demonstrated through anisotropic heat-transport computations. We also compare the numerical ideal-MHD spectra and divergence-constraint properties of different formulations for the magnetic advance, using magnetic field or vector potential with either H1 or edge elements. [1] K. Ichiguchi, et al., PPCF 55 014009 (2013); [2] T. A. Bechtel, C. C. Hegna, and C. R. Sovinec, this meeting.