  We present a numerical study of charged particle motion in a time-independent
magnetic field in cylindrical geometry. The magnetic field model consists of an
unperturbed reversed-shear helical part and a perturbation consisting of a
superposition of modes. Contrary to most of the previous studies, the particle
trajectories are computed by directly solving the full Lorentz force equations
of motion in a six-dimensional phase space using a sixth-order, implicit,
symplectic Gauss-Legendre method. The level of stochasticity in the particle
orbits is diagnosed using averaged, effective Poincare sections. It is shown
that when only one mode is present the particle orbits can be stochastic even
though the magnetic field line orbits are not stochastic. The lack of
integrability of the particle orbits in this case is related to separatrix
crossing and the breakdown of the global conservation of the magnetic moment.
Some perturbation consisting of two modes creates resonance overlapping,
leading to Hamiltonian chaos in magnetic field lines. Then, the particle orbits
exhibit a nontrivial dynamics depending on their energy and pitch angle. It is
shown that the regions where the particle motion is stochastic decrease as the
energy increases. The non-monotonicity of the $q$-profile implies the existence
of magnetic ITBs which correspond to shearless flux surfaces located in the
vicinity of the $q$-profile minimum. It is shown that depending on the energy,
these magnetic ITBs might or might not confine particles. That is, magnetic
ITBs act as an energy-dependent particle confinement filter. Magnetic field
lines in reversed-shear configurations exhibit topological bifurcations due to
separatrix reconnection. We show that a similar but more complex scenario
appears in the case of particle orbits that depends in a non-trivial way on the
energy and pitch angle of the particles.
