  Based on accurate representation of the He+-He angular differential
scattering cross sections consisting of both elastic and charge exchange
collisions, we performed detailed numerical simulations of the ion velocity
distribution functions (IVDF) by Monte Carlo collision method (MCC). The
results of simulations are validated by comparison with the experimental data
of the ion mobility and the transverse diffusion. The IVDF simulation study
shows that due to significant effect of scattering in elastic collisions IVDF
cannot be separated into product of two independent IVDFs in the transverse and
parallel to the electric field directions.
