Generating Particle Beams of Controlled Dimensions and Divergence: II. Experimental Evaluation of Particle Motion in Aerodynamic Lenses and Nozzle Expansions

TLDR

The authors developed a particle‑beam‑forming apparatus to produce narrow particle beams, building on the theory presented in paper I of the series. The system comprises a variable number of aerodynamic lenses (3.5–7.0 mm diameters) followed by a 3 mm accelerating nozzle, and was tested with monodisperse DOS and NaCl particles (0.02–0.24 µm) at ~1 torr, with the resulting beams focused through a 1 mm skimmer into a 10⁻⁴–10⁻⁵ torr chamber where beam widths, velocities and transport efficiencies were measured. Adding lenses progressively narrowed the particle beams to asymptotic minima that matched the Brownian limit for spherical DOS but exceeded it for nonspherical NaCl due to lift forces, while transport efficiencies surpassed 90 % for 0.03–0.24 µm particles and measured velocities agreed with calculations.

Abstract

A particle-beam-forming apparatus for producing narrow particle beams was developed based on the theory discussed in paper I of this series. It consists of a variable number of aerodynamic lenses (short capillaries and/or thin-plate orifices with diameters ranging from 3.5 to 7.0 mm) followed by an accelerating nozzle (3 mm). It was evaluated using monodisperse DOS and NaCl particles (0.02–0.24 μm) at upstream pressures on the order of 1 torr. The particle beams produced by the lens-nozzle system were focused through a skimmer (1 mm) into a high vacuum chamber (10−4–10−5 torr) where the beam widths, velocities and transport efficiencies were measured. The experiments showed that as more lenses were added the particle beam widths were reduced asymptotically to the minimum values. For spherical particles (DOS) these minimum values are in good agreement with the Brownian limit derived in paper I. For nonspherical particles (NaCl) these minimum widths are much larger than the Brownian limit, indicating that beam broadening is dominated by lift forces (see paper I). The particle transport efficiencies through the lens-nozzle-skimmer system exceed 90% for particle sizes from 0.03 to 0.24 μm. The measured beam velocities are also in good agreement with the calculated values.