Characterisation of a small negative ion source

Abstract

A small volume source capable of delivering ∼1 mA of H - ions at 20 kV has been characterised. The results have been shown to be compatible with a rigid plasma boundary below 0.75 kV. Above this value of extraction voltage, the boundary is not fixed although the flexible boundary model employed did not adequately explain the results in this region. Considerable cooling of the H - ions has been demonstrated at high gas flow rates. The ion temperature was reduced by a factor of 8 (to 0.17 eV) by an 18-fold increase in source pressure. This result, which was predicted by the energy balance equation of Green and Holmes (private communication), is of importance where high brightness beams are required. The decrease in ion temperature is achieved at the cost of negative ion current density. There is evidence to suggest that the loss of beam current may be due to a decrease in positive ion density as a result of plasma loss to the walls. Therefore if the confinement of the source was improved it may be possible to recoup some of the loss of beam current.