Aerodynamic Lens System for Producing Particle Beams at Stratospheric Pressures

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Abstract
An aerodynamic lens has been developed and tested to produce a narrow particle beam of aerosols that are sampled from air at pressures between 20 and 75 mbar. The development of such a lens was motivated by polar ozone research to obtain a composition analysis of polar stratospheric cloud particles (PSCs). Although these particles play a key role in the ozone destruction process, their composition never has been measured exactly. The lens for stratospheric research is an extension development of a device operating at much lower pressures. The lens consists of seven single orifices that are separated by spacers. The orifice diameters range from 0.925 to 0.52 mm, and the entrance and exit of the lens contain openings of 1.40 and 0.45 mm, respectively. Laminar gas flow produces a particle beam that is directed through a skimmer into a vacuum chamber for determining beam widths and transport efficiencies. The lens was tested with spherical monodisperse DOS particles (0.1–0.9 μm in diameter) as well as with nonspherícal solid NaBr particles (0.05–0.25 μm) simulating larger water-rich crystals with lower density. The results show that spherical particles from 0.1 to 0.9 μm in diameter suspended in the pressure range of 20 to 75 mbar are well focused to a beam smaller than 1 mm in diameter, measured at a distance of 9.5 cm downstream from the lens. Nonspherical particle beams are broader, with diameters typically between 2 and 3 mm. This fact will permit a distinction between liquid spherical and frozen nonspherical aerosols in a polar stratosphere balloon experiment. The transmission efficiency was also determined to be 70–90% for spherical and 50–90% for nonspherical particles.