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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, meaning that an increase in negative ion density at the extraction sheath will result in a lower beam current (this result is universally observed at high beam currents); (2) there is a saddle point with a potential barrier preventing the most volume produced negative ions from being extracted (the combination of 1 and 2 indicates that most of the negative ions being created do not find their way into the beam); (3) introduction of cesium may cause, most importantly, an increase in the transverse space charge limit (there is an abundance of experimental data supporting this effect); (4) cesium may also result in an increase in the fraction of volume produced negative ions which are extracted; (5) cesium may also result in a reduction of extracted electrons by dint of a less negative bias on the plasma electrode with respect to the adjacent plasma, thus allowing the transverse space charge limit budget to be taken up wholly by the ions. (The combination of 3-5 represents the way an actual increase in the beam current can be achieved); (6) a strong ion time scale sheath instability due to violation of Bohm criteria produces an anomalous ion temperature that increases with beam current routinely seen in experiments; and (7) introduction of cesium may result in a reduction in this instability. These insights may lead to improvements in volume negative ion sources, and the most important finding of an increased space charge limit has apparently been verified experimentally by reinterpretation of the findings of Bacal, Allison and Leung.

First results from a double Vlasov model for negative ion extraction from volume sources—The possibility of an enhanced transverse space charge limit