Shearless Transport Barriers *

We discuss properties of shearless transport barriers associated with extrema of non-monotonic plasma profiles. Initially, we introduce shearless transport barriers in nontwist maps, systems with nonmonotonic rotation number profiles. We show how this barrriers existence depends on the map control parameters. Next, we introduce a symplectic nontwist map to describe the magnetic field lines in tokamaks. For this map, we present the scenarios of the onset and break up of shearless transport barriers as the control parameters, namely, the plasma current and the amplitude of the perturbing resonant field, are varied.