  Atomic-size vortex beams have great potential in probing materials' magnetic
moment at atomic scales. However, the limited depth of field of vortex beams
constrains the probing depth in which the helical phase front is preserved. On
the other hand, electron channeling in crystals can counteract beam divergence
and extend the vortex beam without disrupting its topological charge.
Specifically, in this paper, we report atomic vortex beams with topological
charge \pm1 can be coupled to the 2p columnar bound states and propagate for
more 50 nm without being dispersed and losing its helical phase front. We gave
numerical solutions to the 2p columnar orbitals and tabulated the
characteristic size of the 2p states of two typical elements, Co and Dy, for
various incident beam energies and various atomic densities. The tabulated
numbers allow estimates of the optimal convergence angle for maximal coupling
to 2p columnar orbital. We also have developed analytic formulae for beam
energy, convergence-angle, and hologram dependent scaling for various
characteristic sizes. These length scales are useful for the design of
pitch-fork apertures and operations of microscopes in the vortex-beam imaging
mode.
