Results from a double Vlasov model for negative ion extraction from volume sources

A new negative ion source-extraction model has been formulated and implemented which explicitly considers the motion of positive ions and the volume generation of negative ions. It is found that: (1) for high-beam currents, the beam current is limited by a transverse space-charge limit, not an emission limit; (2) there is a saddle point with a concomitant potential barrier preventing most volume produced negative ions from being extracted (the combination of 1 and 2 indicates that in some interesting cases there is the opportunity to increase extraction currents above values presently observed); (3) introduction of cesium may cause an actual increase in the transverse space-charge limit by accumulation of positive ions of cesium in the presheath.