  Cyclotron autoresonance acceleration has been recently advanced as a
potential mechanism for accelerating nuclei to ZeV energies (1 ZeV = $10^{21}$
eV). All results have been based on single- and many-particle calculations
employing analytic solutions to the relativistic equations of motion in the
combined magnetic and radiation fields, excluding effects related to the
particle-particle interactions. Here, results from many-particle calculations
and Particle-In-Cell (PIC) simulations, are presented which lend support to the
single-particle investigations. Each single-particle result is found to lie
well within one standard deviation about the ensemble average obtained from the
corresponding many-particle calculation. The PIC simulations show that, even
for number densities far exceeding those employed in the non-interacting case,
the energy gain drops markedly due to the particle-particle interactions, over
the first $\sim 8~ mm$ of the acceleration length. Together with the
substantial attenuation, this finding supports the conclusion that the
particle-particle interaction effects can be negligibly small over acceleration
lengths of typically many kilometers.
