An Investigation of some of the Physical Processes Occuring in High Current Ion Sources

Abstract

Experimental and theoretical work relating to three high current plasma ion sources, namely the monopigatron, the circular magnetic multipole and the rectangular magnetic multipole sources are described. The development of the magnetic multipole source from the duopigatron is described together with a discussion of other types of existing high current ion sources. Theoretical models of ion source discharges are reviewed and classified according to the type of electrons responsible for ionisation. A model, based upon ionisation by monoenergetic primary electrons and conservation of the plasma particles is extended to include an analysis of plasma uniformity and potential profiles in the ion extraction plane. A movable Langmuir probe has been used to determine electron temperatures, plasma uniformity and potential profiles for the monopigatron and circular magnetic multipole sources. The circular magnetic multipole source produces a larger area of uniform plasma than the monopigatron. This plasma is also more quiescent and is generated at a comparable electrical efficiency. An electronic scanning unit has been developed and used in conjunction with a two-dimensional wall probe array mounted in the extraction electrode and to determine the uniformity profile for the rectangular multipole source. This source is found to produce a larger area of uniform plasma where the 90% density contour is very similar to the 0.015T magnetic field contour. Measurements of the electron energy distribution in the rectangular multipole source, using an electrostatic energy analyser show that the distribution of the primary electrons extends from the thermal population up to energies of about 30% greater than the cathode-anode potential.