Anomalous attenuation at low temperatures in high-intensity helium beam sources

Abstract

High-intensity supersonic helium beam sources are a critical component in modern thermal energy helium atom scattering apparatuses. In common with sources developed elsewhere, we have observed beam attenuation at low temperature as the helium flow through the nozzle is increased. A detailed analysis of the measurements demonstrates that direct backscattering in the nozzle-skimmer region is likely to be the major attenuation source. A numerical model, which describes the principle sources of scattering in the region between source and skimmer, is shown to provide a good description of the experimental data for a range of flow conditions and for stagnation temperatures between 37 and 300 K. The model includes partial accommodation of atoms scattered from surfaces of the skimmer and chamber, together with a realistic scattering potential. We use the model to predict the effect of various geometric alterations, aimed at improving the ultimate beam intensity. © 2005 American Institute of Physics.