Characteristics of a Multipole Volume H-Plasma

Abstract

An experimental study of a filtered magnetic-multipole ion source has been carried out, in order to examine the characteristics of a plasma in which negative hydrogen ions are formed by volume processes. The ultimate object was to aid in understanding the mechanisms of volume H⁻ production, so enabling the output of this type of device to be optimised for use in the fusion programme. The study entailed the design and construction of one such source, and diagnostic systems for both this and another. A major portion of the results were obtained using planar Langmuir probes. To facilitate the use of these, a computer-based control and analysis system was built. Empirical corrections were obtained for both electron screening by a magnetic field and ion sheath expansion, and justified theoretically. An attempt was also made to construct a mass spectrometer small enough to use within the source. Using the probes, a detailed spatial and parametric survey of particle densities and energies within the filtered multipole plasma was performed. The results are shown mostly to fit simple models of plasma production and loss. The H⁻ density and fraction were obtained from the same dataset, and compared with data acquired using beam extraction. The results are discussed in relation to models of volume H⁻ production in a multipole source, based on both atomic and plasma processes. A balance of atomic processes predicted H⁻ ion densities within 20% of those measured experimentally. The spatial survey located a maximum of H⁻ density at the peak of the filter field, confirmed by beam-extraction data, which may be explained by the atomic balance. The parametric variation of H⁻ density was compared with an appropriate parametric model. This was successful, agreement being obtained from both sources.