Physics aspects of negative ion sources

The physical phenomena important in volume and surface production negative hydrogen ion sources are reviewed. The phenomenon, designated volume production, was attributed to dissociative electron attachment of low energy electrons to rovibrationally excited molecules. The experimental verification in 2005 of the reality of this mechanism is reported. Recent results obtained by measuring the negative ion temperature in a filament discharge ion source indicate that two negative ion populations are present. Understanding the physics of volume negative ion sources operating in accelerators with H- ion density in the 100 mA cm-2 range opens the prospect of creating volume caesium-free sources for fusion. The evolution of physics of surface production is also described. A measurement effected near the plasma electrode (PE) using two-laser beam photodetachment showed the presence of a directed negative ion flow. The ratio of the negative ion and electron currents extracted through the transverse magnetic filter field near the PE can be predicted from a collisional flow model. A specific for two negative species plasmas transport mechanism in the weak transverse magnetic filter field is described