
90 Materials Processing by Cluster Ion Beams
nozzle. Figure4.5 shows the condensation rate in the conical
nozzle shown in Figure4.4b as calculated using the DSMC
analysis combined with classical condensation theory [9]. It
becomes clear that condensation of gases occurs most strongly
near the nozzle throat and then gradually decreases through
the conical expansion region.
After expansion of the gas out of a nozzle, a skimmer hav-
ing the shape of a truncated cone is usually employed to sepa-
rate the cluster- containing core of the gas stream from the
waste gas. Skimmers typically have entrance aperture edges
made as sharp as possible to avo