Experiments have shown that highly intense and collimated cluster beams can
be produced by a simple aerodynamic lens coupled to the nozzle of a pulsed
microplasma cluster source. The mechanism of the observed cluster focusing
is here presented. We discuss, as a case example, a supersonic beam of hel
ium seeded by carbon clusters. The laminar ow of the helium-clusters mixtur
e through a focalizing nozzle assembly has been numerically simulated and c
ompared to the experiments. A three-dimensional steady compressible ow mode
l has been considered for the simulation. Carbon clusters have been modeled
by rigid spheres with uniform density. The trajectories of the particles a
re calculated during their travel through the nozzle. The simulations show
that the effect of the focalizing nozzle is to divert the particles from th
eir streamlines towards the center of the beam, thus narrowing the spatial
and velocity cluster distribution. The dependence of these effects on the n
ozzle geometry and on the beam parameters is reproduced by the simulations
in good agreement with the experimental findings.