A hydrodynamic model of the negative ion source

Unlike atomic and molecular processes in ion sources, the macroscopic properties have received little attention. Here, open-quote hydrodynamic close-quote equations are used to investigate the electron, positive and negative ion properties of a bucket source. The source is assumed to be two-dimensional with the filter magnetic field (- 30G) in the ignorable direction. The momentum equations include elastic collision processes between neutrals, electrons, and both species of ions. Negative ions are produced by dissociative attachment, and destroyed by electron detachment and mutual neutralisation. Estimates based on charge exchange suggest that the positive ion and neutral temperatures are similar, whilst ion-ion Coulomb collisions indicate that the negative ion temperature is also of a similar order. Results from the Culham source suggest that the electron density along the axis falls exponentially. Assuming that the positive ion temperature also falls exponentially, then analysis leads to exponential forms for the electron temperature and electrostatic potential - in good agreement with experiment, as is the calculated axial profile for the negative ion density