Shearless bifurcation on symplectic maps of magnetic field lines in tokamaks with reversed current

• Symplectic maps model magnetic field lines in tokamaks with reversed currents.
• Symmetry breaking creates shearless invariant curves.
• Symmetry breaking introduces islands and chaos near a rotationless invariant curve.

We introduce two-dimensional symplectic maps to describe the Poincaré maps of magnetic field lines in large aspect ratio tokamak equilibria with reversed non-monotonic plasma current density profiles. For these maps, we investigate the effect of the symmetry breaking due to the toroidal correction with a peculiar invariant, namely, a magnetic surface with a null rotation number, enclosing a vanishing current. We find that this rotationless invariant surface is surrounded by many small island chains. Furthermore, near such invariant, the symmetry breaking gives rise to two magnetic shearless invariants surrounded by twin island chains. We also find chaotic lines adjacent to all the observed islands created by the considered structurally unstable equilibria.