Abstract

The need for high brightness neutral beams for neutral beam heating systems has lead to extensive research into low pressure, high power negative ion sources. Negative ion sources at present have low gas and power efficiencies and to realise the current densities for the future H'/D' based neutral injectors needed for the next generation of fusion tokamaks will necessitate continued efforts into improving the design of negative ion sources. In this thesis a new approach to the production of negative ions in volume ion sources is investigated. For effective negative ion production large densities of vibrationally excited molecules, v>5, are required. These vibrationally excited molecules are effectively produced in volume ion sources by high energy electron collisions, e>20eV, with the background gas. The negative ions are subsequently produced by dissociative attachment of electrons to these vibrationally excited levels. However a major loss mechanism for negative ions is collisional detachment by fast electrons. Tandem sources divide the source into driver and extraction regions by a magnetic filter. When a plasma is switched off the fast electron density decays away in less the l|is , while the electron density and vibrationally excited states can survive well into the post discharge. Thus by pulsing the discharge current there results two distinct plasma regimes, similar to the driver and extraction regions of a tandem source. Here we use such a temporal filter to separate the driver and extraction regions in time rather than in space. High frequency modulation of the source can result in an increase by a factor of four in the time averaged negative ion density with a simultaneous reduction in the extracted electron current. Temporally and spatially resolved photodetachment and langmuir probe measurements are presented which show the enhancements in the H density to be consistent with modulation of a source in which E-V/DA production and CD loss mechanisms are dominant.