A one-dimensional model of a negative ion source

Abstract

A theoretical model is presented which uses one-dimensional plasma transport to describe a hydrogen discharge in which negative ions are formed. The model is based on discharges which are confined by a magnetic multipole field on the anode surface (multipole or `bucket' sources) and includes the effects of weak transverse magnetic fields within the plasma to cool the electron temperature in the discharge to a level at which dissociative attachment collisions allow significant fractional densities of ions to form. The transport equations are based on the moments of the Boltzmann equation and these are coupled with the plasma collision processes associated with ionization and negative ion production. The presence of negative ions in the plasma allows the electron transport across the magnetic fields to be anomalously high because each electron spends part of its time as a negative ion which is only weakly affected by the field. The model is tested on two multipole sources which have produced negative current densities up to around and have extensive measurements of the electron and ion densities, electron temperature and plasma potential as functions of position, arc current, voltage and pressure. The model uses the source geometry and arc parameters to calculate the plasma properties and obtains good agreement with experiment in both cases. In the companion paper the model is tested on a third source chamber with different diagnostics.