Improved poloidal convergence of the MARS code for MHD stability analysis

Most codes for the analysis of magnetohydrodynamics (MHD) stability experience difficulties when the equilibrium is highly shaped or has an X-point. In particular, codes using Fourier expansion in the poloidal direction may need a large number of Fourier components to give reasonably accurate values of the growth rate. In the present work, it is shown that the poloidal convergence can be improved by a judicious choice of normalization factors for the perturbation quantities and multipliers for the equations. Using this method, a new version of the MARS code has been produced with significantly improved poloidal convergence.