Abstract
Both the production of negative hydrogen ions on a barium surface and surface sputtering under low energy (∼ 200 eV) intense particle bombardment is studied in the FOM Surface Conversion Experiment. We have observed a decrease in barium sputter-yield when a barium surface is exposed to an intense positive hydrogen or deuterium ion flux, extracted from a plasma. This effect is attributed to the loading of hydrogen into barium, causing enhanced preferential sputtering of hydrogen. The sputtering process is observed by means of optical emission spectroscopy. Whenever the surface is saturated with hydrogen, the barium line intensity is constant in time, and is proportional to a calculated solid-body sputter-coefficient. The scaling with calculated sputter-coefficient is a feature which is also found during argon ion bombardment. The effects of dynamic hydrogen loading of a barium converter surface are observed with three different diagnostic techniques, and shown to be consistent. In a high pressure experiment it is indicated that it might be possible to determine the sputtered barium ion flux with a Langmuir probe. The steady state negative ion output of our surface conversion negative source amounts to 17 mA/cm 2 of H − and 12 mA/cm 2 of D − at an operating pressure of 4 × 10 −3 mbar.
Published Version
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