Dimerized island chains in tokamaks

Magnetic surfaces are studied on MHD equilibrium plasmas with non-monotonic safety factor radial profiles, confined on large aspect-ratio tokamaks. Perturbing helical windings lead to twin Poincaré-Bhirkhoff chains and reconnection phenomenon, giving rise to dimerized island chains. Evolution of radial rotation number profiles, together with the radial progression of equilibrium points clarify the reconnection process. Toroidal perturbation leads to reconnection for values of the parameter Ih much lower than those in the integrable case. Then, for higher Ih, we have overlap of secondary islands and the onset of chaotic regions from the edge region to the center.