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Recent evidences show that heteroclinic bifurcations in magnetic islands maybe caused by the amplitude variation of resonant magnetic perturbations intokamaks. To investigate the onset of these bifurcations, we consider a largeaspect ratio tokamak with an ergodic limiter composed of two pairs of ringsthat create external primary perturbations with two sets of wave numbers. Anindividual pair produces hyperbolic and elliptic periodic points, and itsassociated islands, that are consistent with the Poincar\'e-Birkhoff fixedpoint theorem. However, for two pairs producing external perturbations resonanton the same rational surface, we show that different configurations ofisochronous island chains may appear on phase space according to the amplitudeof the electric currents in each pair of the ergodic limiter. When one of theelectric currents increases, isochronous bifurcations take place and newislands are created with the same winding number as the preceding islands. Wepresent examples of bifurcation sequences displaying (a) direct transitionsfrom the island chain configuration generated by one of the pairs to theconfiguration produced by the other pair, and (b) transitions with intermediateconfigurations produced by the limiter pairs coupling. Furthermore, we identifyshearless bifurcations inside some isochronous islands, originatingnonmonotonic local winding number profiles with associated shearless invariantcurves.

Isochronous island bifurcations driven by resonant magnetic perturbations in Tokamaks