Isotope effect and cesium dependence of negative ion production in volume H- and D- ion sources

Isotope effect of H- and D- productions with and without cesium vapor injection is studied theoretically as a function of plasma parameters. Model calculation in a tandem volume source is performed by solving a set of particle balance equations for steady-state hydrogen and deuterium discharge plasmas. In a pure volume case, as a whole, H- production is higher than D- production for various plasma conditions. This isotope effect is caused by mainly atomic collision processes, i.e., collisional cooling of vibrational molecules and collisional destruction of negative ions. Therefore, density of atoms plays an important role for deciding negative ion density. On the other hand, in a cesium case, there is no remarkable isotope effect on H-/D- production. Considering H-/D- surface production caused by both atoms and positive ions, H-/D- production is enhanced by a large factor and then isotope effect observed in pure volume case seems to be almost masked. (C) 1998 American Institute of Physics.