Formation of H− by scattering H+ on a cesiated polycrystalline tungsten surface

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Abstract
We present measurements on the charge transfer and reflection of H+ ions which are scattered from a cesiated polycrystalline tungsten surface. The particle energy ranges from 400 to 2 keV, the angle of incidence with respect to the surface normal is varied between 65 and 90°. The measured values are compared with data obtained earlier for cesiated monocrystalline tungsten (110). The maximum differential H− fraction of scattered particles in the case of cesiated polycrystalline tungsten is 25%. This value is roughly a factor of 2 lower than that of cesiated monocrystalline tungsten (110). The maximum total conversion efficiency, that is the reflected H− current divided by the incident positive ion current, is 12%. This value is about a factor of 3 lower than that obtained for monocrystalline tungsten (110). The different behavior of the polycrystalline with respect to monocrystalline material cannot be explained theoretically by the difference in work function. Calculated values are a factor of 1.7 higher than the measured ones.