To understand the plasma characteristics in the extraction region of negative H- sources is very important for the optimization of H - extraction from the sources. The profile of plasma density and electrostatic potential in the extraction region with and without extraction grid voltage are analyzed with a 2D particle in cell modeling of the NIFS-RD H- sources. The simulation results make clear the physical process forming a double ion plasma layer (which consists only of positive H+ and negative H- ions) recently observed in the Cs-seeded experiments of the NIFS-RD source in the vicinity of the extraction hole and the plasma grid. The results also give a useful insight into the formation mechanism of the plasma meniscus and the H- extraction process for such double ion plasma.
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