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Negative ion measurements in RF and DC temporally modulated hydrogen volume ion sources

Dublin City University. School of Physical Sciences
Publication Date
  • Nuclear Physics
  • Physics
  • Ion Sources
  • Anions
  • Measurement
  • Negative Ions
  • Medicine


Negative ion research is stimulated by the need for high power and high density neutral beams for neutral beam heating systems to be used in proposed nuclear fusion reactors. Extensive research has been carried out on the enhancement of the production of negative ions from hydrogen/deuterium discharges. The negative ion sources used at present are being investigated in order to optimise current densities of H7D' for future fusion machines. In this thesis low pressure radio frequency (RF) hydrogen plasmas are investigated and compared to filament driven hydrogen plasmas to understand further the physics in the two different modes, and to investigate the proposal of the utilisation of an RF volume ion source in he application of neutral beam injection (NBI) systems for the proposed nuclear fusion reactors of the future. Tuned Langmuir probes are used as a diagnostic method to measure different plasma parameters as a function of pressure and power. The spatial variations of these parameters are examined within the bulk plasma, and the effect of an applied magnetic field investigated. The temporal filter concept is examined with a view to enhancing negative ion generation in the afterglow of an RF temporally modulated discharge. It is proposed that by modulating the RF discharge, the dominant destructive mechanism, collisional detachment (CD), will become negligible and the dominant production mechanism, dissociative attachment (DA), will be optimised, hence allowing optimum production of negative ions in the volume ion source. A laser photodetachment technique was used to determine negative ion densities in temporally resolved filament generated and radio frequency powered hydrogen plasmas, and experimental data compared to theoretically modelled results. In an effort to enhance negative ion production, the addition of argon to the hydrogen discharge was examined. All plasma parameters were investigated and an increase in negative ion density observed. A model of the reaction rates for production and destruction in the discharge lead to a possible explanation for the increase detected. Langmuir probe measurements and laser photodetachment results are presented which show that the enhanced production of H' density which is achieved with a temporally modulated filament driven discharge is also achievable in a temporally modulated radio frequency hydrogen plasma.

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