Plasma and gas neutralisation of high energy H- and D- in an Argon plasma neutraliser

A beam driven plasma neutraliser (BDPN) has been proposed [1] as a relatively simple way to increase the efficiency of neutral beam injectors based on the acceleration and neutralisation of H^- or D^-. Initial calculations showed that sufficient levels of ionisation could be achieved with H₂ or D₂ as the initial gas target in the neutraliser if a plasma confinement time of the plasma in the neutraliser is >0.5 ms could be achieved. However, later calculations [2,3] show that the afore-mentioned results were very optimistic as the calculations did not take into account the recombination of molecular ions with electrons. Hence, it has been suggested [4] to use an argon (Ar) plasma as the molecular ion content in such a plasma will be negligible. The use of an Ar plasma neutraliser has been suggested previously [[5], [6], [7]], but few details have been given of the obtainable neutralisation efficiency, the necessary line density for that neutralisation efficiency, or of the calculations and cross sections used to deduce those quantities. This paper briefly discusses why Ar could be a good choice for a plasma neutraliser, then the reactions occurring between the H^-/D^- beam and the particles in the Ar plasma, and the available cross section data for those reactions. Subsequently, the differential equations for the change of the species in the beam as it traverses an Ar plasma are deduced and solved, and the solutions used to calculate the achievable neutralisation efficiency as a function of the line density in the neutraliser etc. Results are given of the neutralisation efficiency as a function of the degree of ionisation in the neutraliser, the required line density, and the species changes in the beam for accelerated H^- beams with 100, 500 and 870 keV energies.