An ion species model for positive ion sources: I. Description of the model

Abstract

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.

A 1D ion species model for an RF driven negative ion source

“…The gas density and temperature depend on the interaction between the three positive ion species and electrons with the gas, with the only major difference being the fact that thermal electron rates must be used as opposed to the fast primary electron rates as used in [1,2,3]. The division into atomic or molecular gas depends on recombination at the walls and this is assumed to be the same as in the DC model.…”

“…As in [1,2,3], it is assumed that the main source of energy is the electron population which heats the ion population by Coulomb energy transfer. These ions are then assumed to have the same temperature as the gas atoms or molecules because of the very high collisionality, and the latter then accumulate on the source walls.…”

“…A successful model of an arc discharge source had been previously developed which was able to describe accurately the properties of both negative [1] and positive ion sources [2,3]. In such sources the ionisation takes place by primary electrons emitted by filaments according to a rate determined by the local electron temperature and density.…”

“…It makes extensive use of the work of Gudmundsson and Lieberman on inductive discharges [5] but also deviates from this approach by keeping the classical plasma transport equations in place. The gas temperature and division of the gas into atomic and molecular gas densities is almost identical to the theory of DC sources [2], and this part of the model is incorporated to obtain the density of negative ions as well as positive species fractions.…”

Negative ion research at the Culham Centre for Fusion Energy (CCFE)

“…In order to do this a one-dimensional model of an RF driven negative ion source has been developed. Previously a very successful model of arc discharge sources has been developed which is able to describe accurately the properties of both negative [31] and positive ion sources [32,33] using arc discharges. In these sources ionization takes place by primary electrons emitted by filaments according to an ionization rate which is determined by the local electron temperature and density.…”