  Some internal transport barriers in tokamaks have been related to the
vicinity of extrema of the plasma equilibrium profiles. This effect is
numerically investigated by considering the guiding-center trajectories of
plasma particles undergoing ExB drift motion, considering that the electric
field has a stationary nonmonotonic radial profile and an electrostatic
fluctuation. In addition, the equilibrium configuration has a nonmonotonic
safety factor profile. The numerical integration of the equations of motion
yields a symplectic map with shearless barriers. By changing the parameters of
the safety factor profile, the appearance, and breakup of these shearless
curves are observed. The successive shearless curves breakup and recovering is
explained using concepts from bifurcation theory. We also present bifurcation
sequences associated to the creation of multiple shearless curves. Physical
consequences of scenarios with multiple shearless curves are discussed.
