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Description
A higher density of the negative ion in front of the plasma beam boundary is important to extract a higher beam current density in the negative hydrogen ion source. Negative ions are mainly produced on the surface of the plasma-facing grid electrode (PG) in cesium seeded negative ion source for fusion. The negative ions are produced from positive ions and atoms impinging to the PG surface by electron capture from the PG surface. Positive ion density in front of the PG is one of the key parameters to evaluate negative ion density. Relation between measured negative and positive ion density in Research and development Negative Ion Source at National Institute for Fusion Science (NIFS-RNIS) were investigated in hydrogen and deuterium operation with variations of various parameters. The negative hydrogen and deuterium ion density increased linearly with positive ion density when positive ion density was low, and gradually saturated in the higher positive ion density region. This relation in hydrogen operation was almost identical to that in deuterium operation at not only central higher positive ion density but also peripheral lower density regions. A Mutual Neutralization reaction (MN) is a dominant negative ion loss process in the present study. Taking into account parameter estimations of the MN frequency and fitting factors of the relation between negative and positive ion density, the negative ion density saturation and the the identical relation between hydrogen and deuterium operations can be explained by the loss process by the MN. Considering the variation of the positive ion flux and the constant atomic flux to the PG surface, dominant parent particle species will be discussed.
