Tearing mode analysis in tokamaks, revisited

A new Δ' shooting code has been developed to investigate tokamak plasma tearing mode stability in a cylinder and large aspect ratio (ε≤0.25) toroidal geometries, neglecting toroidal mode coupling. A different computational algorithm is used (shooting out from the singular surface instead of into it) to resolve the strong singularities at the mode rational surface, particularly in the presence of the finite pressure term. Numerical results compare favorably with Furth et al. [H. P. Furth et al., Phys. Fluids 16, 1054 (1973)] results. The effects of finite pressure, which are shown to decrease Δ', are discussed. It is shown that the distortion of the flux surfaces by the Shafranov shift, which modifies the geometry metric elements, stabilizes the tearing mode significantly, even in a low-β regime before the toroidal magnetic curvature effects come into play.