Toward a realistic and tractable model for negative ion extraction from volume sources

The physics of negative ion formation is fundamentally different than that for positive ion formation. For example, with negatively biased plasma electrodes, relative to the plasma, it can be shown that the Bohm sheath stability criteria is not generally satisfied and that, in addition, the potential structure near the extraction region is a saddle point. Traditional modeling of volume negative ion sources treats them in a manner similar to positive ions with a monatomic downhill run of the negative ions from source plasma to extraction and acceleration. This method (for positive ions) consists of finding self consistent solutions to a multidimensional Poisson equation, with the plasma electrons represented as a Boltzmann distribution, and ions as a Vlasov equation simulated by a suitably large number of trajectories with space charge embeddings