  Recent developments in the design of magnetic confinement fusion devices have
allowed the construction of exceptionally optimized stellarator configurations.
The near-axis expansion in particular has proven to enable the construction of
magnetic configurations with good confinement properties while taking only a
fraction of the usual computation time to generate optimized magnetic
equilibria. However, not much is known about the overall features of
fast-particle orbits computed in such analytical, yet simplified, equilibria
when compared to those originating from accurate equilibrium solutions. This
work aims to assess and demonstrate the potential of the near-axis expansion to
provide accurate information on particle orbits and to compute loss fractions
in moderate to high aspect ratios. The configurations used here are all scaled
to fusion-relevant parameters and approximate quasisymmetry in various degrees.
This allows us to understand how deviations from quasisymmetry affect particle
orbits and what are their effects on the estimation of the loss fraction.
Guiding-center trajectories of fusion-born alpha particles are traced using
gyronimo and SIMPLE codes under the NEAT framework, showing good numerical
agreement. Discrepancies between near-axis and MHD fields have minor effects on
passing particles but significant effects on trapped particles, especially in
quasihelically symmetric magnetic fields. Effective expressions were found for
estimating orbit widths and passing-trapped separatrix in quasisymmetric
near-axis fields. Loss fractions agree in the prompt losses regime but diverge
afterward.
