Novel diagnostic techniques for negative ion sources
The development of negative hydrogen (H-) ion sources with performances exceeding those achieved today is a key requirement for the next generation of high power proton accelerators. Although widely used in accelerators for charge-exchange injection from linear to circular machines, negative ions have a variety of applications beyond high energy physics. Negative hydrogen ion sources are a key element in the formation of intense beams of energetic neutrals for fusion plasma heating. In general, negative hydrogen ions are produced in low pressure hydrogen plasmas, but the demands on the source differ substantially. The ultimate aim of the HP NIS network was to assemble all the competence within the European Union to respond to this technological challenge. For promoting the optimisation of existing negative ion sources in European research institutes, a better understanding of the source operation was sought. The Plasma Research Laboratory at Dublin City University was the project partner most involved in plasma modelling. The complexity of this task was addressed with the simultaneous development of several codes. A two-dimensional (2D) Particle-In-Cell code with Monte-Carlo-Collisions calculations provided a solution to the non-linear plasma kinetics. By carefully linking the code with global chemistry models, scientists drew on their strengths in one area to address the weaknesses of the other. To test the validity of the new numerical scheme coupling the two models, expected behavioural trends such as increased ion density at lower pressures were compared with experimental data. For this purpose, a novel alternative to Cavity Ring Down spectroscopy was introduced which had not yet been applied to high power, caesium (Cs) seeded negative ion sources. This laser-based absorption spectroscopy technique was applied to measure the negative hydrogen ion density of dilute or weakly absorbing species. Scaling laws were derived from the analytical models, which would be difficult to derive from experiments and could be applied to reference ion sources for the ITER fusion project.
