Fluid Model Analysis of the Distribution of the Negative Ion Density before the Extraction from a Tandem Type of a Plasma Source

Unified description of the expansion (through a magnetic filter) and extraction regions, including also the driver, of volume-production based sources of negative hydrogen ions is presented in the study. The model is one-dimensional, developed within the fluid plasma theory. It covers description of both the second chamber of the tandem sources and a single inductively-driven (with a planar coil) discharge of a matrix source. Four parameters have been varied in the analysis of the results: type, position and magnitude of the filter field and rf power applied for the discharge maintenance. The obtained results for the spatial distribution of the plasma parameters as well as for the electronegativity, for the negative ion beam current and its ratio to the electron current at the position of the extraction are discussed regarding optimum conditions for the source operation. The conclusion is that a magnetic field extended over a large region till the extraction favors locality in the discharge behavior and, respectively, the local production of the negative ions. This leads to high density of the extracted negative ion beam current at reasonable values of its ratio to the electron current density.