only to the scattering processes in the capillary, this focusing factor
is written by 1 + Ns/Np, where Ns is the number of scattered particles
and Np is the number of particles passing through the capillary
without any collision. By using an originally developed Monte-Carlo
code, we then evaluate the beam-focusing factor based on the
above definition. Finally, we discuss the ion transport mechanism
in the capillary and limiting factors related to the performance of
the tapered capillary as a focusing device.