Formation of an atomic hydrogen beam by a hot capillary

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Abstract
A beam of atomic hydrogen is produced by passing hydrogen gas through a tungsten capillary heated to 2600 K by electron impact. By means of a differentially pumped quadrupole mass analyzer the angular distribution of the atomic hydrogen is measured for different hydrogen feed pressures. The angular distribution is considerably more peaked than a cosine distribution and is narrower the lower the feed pressure is. Based on the theory of beam formation by molecular flow a simple analytical model is developed which assumes a spatially limited transparent flow at the end of the capillary. The measured angular distribution can be described by the model using the length of transparent flow as the only fitting parameter.