Nonlinear behavior of multiple-helicity resistive interchange modes near marginally stable states

Abstract

Nonlinear behavior of resistive interchange modes near marginally stable states is theoretically studied under the multiple-helicity condition. Reduced fluid equations in the sheared slab configuration are used in order to treat a local transport problem. With the use of the invariance property of local reduced fluid model equations under a transformation between the modes with different rational surfaces, weakly nonlinear theories for single-helicity modes by Hamaguchi [Phys. Fluids B 1, 1416 (1989)] and Nakajima [Phys. Fluids B 2, 1170 (1990)] are extended to the multiple-helicity case and applied to the resistive interchange modes. Nonlinear amplitude equations of the multiple-helicity modes are derived, from which the convective transport in the saturated state is obtained. It is shown how the convective transport is enhanced by nonlinear interaction between modes with different rational surfaces compared with the single-helicity case. We confirm that theoretical results are in good agreement with direct numerical simulations.