High Efficiency Low-Pressure Ion Source

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Abstract
A magnetically confined, oscillating electron bombardment source has been designed which delivers high currents of noble gas ions in the 1‐ to 10‐keV energy range. The energy spread of the beam was 2±1 eV using an anode potential of 45 V. Pressure in the source chamber was measured during source operation so that accurate gas efficiency measurements could be made. The optimum gas efficiency obtained for argon at 5‐kV extraction potential and an operating pressure of 4×10−4 Torr was 59%. Mass analysis of source output showed that when the source was operated with research grade argon (99.999% pure) impurity ions in the beam consisted of only 0.50% N2+, 0.75% O2+, 0.75% H2O+, and 0.70% Ar++. Filament life is extremely long, well over 750 h. Analysis of ion production rate and plasma density shows that a large contribution to electron emission is made by the secondary electrons produced at the tungsten filament due to positive ion bombardment. The secondary electron emission current was found to be as much as 30% of the true thermionic emission current and is not negligible as previous investigators have assumed.

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