  A one dimensional model of the magnetic multipole volume plasma source has
been developed for application to intense ion/neutral atom beam injectors. The
model uses plasma transport coefficients for particle and energy flow to create
a detailed description of the plasma parameters along an axis parallel to that
of the extracted beam. In this paper the isotopic modelling of positive
hydrogenic ions is considered and compared with experimental data from the
neutral beam injectors of the Joint European Torus. The use of the code to gain
insights into the processes contributing to the ratios of the ionic species is
demonstrated and the conclusion is drawn that 75% of the atomic ion species
arises from ionization of dissociated molecules and 25% from dissociation of
the molecular ions. However whilst the former process is independent of the
filter field, the latter is sensitive to the change in distribution of fast and
thermal electrons produced by the magnetic filter field and an optimum
combination of field strength and depth exists. Finally the code is used to
predict the species ratios for the JET source operating in tritium and hence
the neutral beam power injected into the plasma in the JET tritium campaign
planned for 2016.
