  The issue of magnetic confinement in magnetic fusion devices is addressed
within a purely magnetic approach. Using some Hamiltonian models for the
magnetic field lines, the dual impact of low magnetic shear is shown in a
unified way. Away from resonances, it induces a drastic enhancement of magnetic
confinement that favors robust internal transport barriers (ITBs) and
stochastic transport reduction. When low-shear occurs for values of the winding
of the magnetic field lines close to low-order rationals, the amplitude
thresholds of the resonant modes that break internal transport barriers by
allowing a radial stochastic transport of the magnetic field lines may be quite
low. The approach can be applied to assess the robustness versus magnetic
perturbations of general (almost) integrable magnetic steady states, including
non-axisymmetric ones such as the important single helicity steady states. This
analysis puts a constraint on the tolerable mode amplitudes compatible with
ITBs and may be proposed as a possible explanation of diverse experimental and
numerical signatures of their collapses.
