A discharge excited supersonic source of metastable rare gas atoms

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Abstract
The performance of a discharge excited supersonic source of metastable atoms is described in terms of the virtual source model and the model of skimmer interaction, which give a good description of undisturbed supersonic expansions, i.e. without discharge. It is shown that these models can be also used for the description of the velocity distribution and centre line intensity of the beam of metastable rare gas atoms from a discharge excited source. The authors have obtained optimum centre line intensities of 7.3, 2.0, and 1.4*1013 s-1 sr-1 for He*, Ne*, and Ar*, respectively, using a pumping speed of 1101 s-1. The discharge is sustained through the nozzle orifice in an insulating material between a cathode in the high-pressure region upstream of the nozzle and a skimmer which separates the expansion chamber from the experimental vacuum chamber. Stable operation of the discharge requires a minimum pressure of typically 0.1 to 0.2 Pa in the expansion chamber between nozzle and skimmer.