Aerodynamic focusing of clusters into a high intensity and low divergence supersonic beam

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 helium seeded by carbon clusters. The laminar flow of the helium-clusters mixture through a focalizing nozzle assembly has been numerically simulated and compared to the experiments. A three-dimensional steady compressible flow model has been considered for the simulation. Carbon clusters have been modeled by rigid spheres with uniform density. The trajectories of the particles are calculated during their travel through the nozzle. The simulations show that the effect of the focalizing nozzle is to divert the particles from their streamlines towards the center of the beam, thus narrowing the spatial and velocity cluster distribution. The dependence of these effects on the nozzle geometry and on the beam parameters is reproduced by the simulations in good agreement with the experimental findings.