physics.atm-clus

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# Title: Centre line intensity of a supersonic helium beam

Abstract: Supersonic atom beams are used in a range of applications, for example scattering experiments and, most recently, microscopy. The high ionization potential of neutral helium atoms makes it difficult to build efficient detectors. Therefore, it is important to develop beam sources with a high centre line intensity. Several approaches for predicting this intensity exist, with the quitting surface model incorporating the largest amount of physical dependencies in a single equation. However, until now only a limited amount of experimental data has been available. We present a study where we compare the quitting surface model with an extensive set of experimental data. In the quitting surface model the source is described as a spherical surface from where the particles leave in a molecular flow determined by Maxwellian statistics. We use numerical solutions of the Boltzmann equation to determine the properties of the expansion. The centre line intensity is then calculated using an analytical integral which can be reduced to two cases; one which assumes a continuously expanding beam until the skimmer aperture, and another which assumes a quitting surface placed before the aperture. We compare the two cases to experiments with a nozzle diameter of 10 $\upmu\mathrm{m}$, skimmer diameters from 4 $\upmu\mathrm{m}$ to 390 $\upmu\mathrm{m}$, a source pressure range from 2 to 190 bar, and nozzle-skimmer distances between 17.3 mm and 5.3 mm. To further support the two analytical approaches, we present equivalent ray tracing simulations. We conclude that the quitting surface model predicts the centre line intensity of helium beams well for skimmers with a diameter larger than $120\;\upmu\mathrm{m}$ when using a continuously expanding beam until the skimmer aperture. For the case of smaller skimmers the trend is correct, but the absolute agreement not as good. We propose several explanations for this.
 Subjects: Atomic and Molecular Clusters (physics.atm-clus) Cite as: arXiv:1806.00527 [physics.atm-clus] (or arXiv:1806.00527v1 [physics.atm-clus] for this version)
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