A one dimensional model of the magnetic multipole volume plasma source has been developed for use in 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. Primarily constructed for applications to neutral beam injection systems on fusion devices, the model concentrates on the hydrogenic isotopes but can be extended to any gas by substitution of the relevant masses, cross sections and rate coefficients. The model considers the flow of fast ionizing electrons that create the ratios of the three hydrogenic isotope ion species, H + , H 2 + , H 3 + (and similarly for deuterium and tritium) as they flow towards the beam extraction electrode, together with the production of negative hydrogenic ions through volume processes. The use of detailed energy balance in the discharge allows the determination of the fraction of the gas density that is in an atomic state and also the gas temperature as well as the electron temperatures and plasma potential. Comparisons are made between the results of the model and experimental measurements in deuterium from a number of different filament driven sources used on beam heating facilities.