NONLINEAR SATURATION OF IDEAL INTERCHANGE MODES IN A SHEARED MAGNETIC-FIELD

Pressure-driven ideal modes cannot completely interchange flux tubes of a sheared magnetic field; instead, they saturate, forming new helical equilibria. These equilibria are studied both analytically and numerically with reduced magnetohydrodynamic equations in a flux-conserving Lagrangian representation. For unstable localized modes, the structure of the nonlinear layer generated around the resonant flux surface depends on the value of Mercier parameter D(M). The shape of magnetic surfaces in the vicinity of resonance is changed significantly even close to the instability threshold. However, the radial width of the affected layer becomes exponentially small near the threshold. The appearance of sheet currents and islandlike structures along the resonant flux surface may be of interest for the description of forced reconnection in models with finite resistivity. This study also includes the case of ballooning instability by representing nonlocal driving terms through the matching parameter DELTA', which defines the outer boundary conditions for the interchange layer.