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Description
One of the indexes of ion source performances is a ratio of beam current to discharge power (discharge power efficiency). In some cases, one ion source is utilized for several kinds of ion beams depending on the application. The discharge power efficiency can change by ion species. In particular, hydrogen and deuterium ion beams are utilized in the ion source of a Neutral Beam Injector (NBI) for nuclear fusion experiments. In the NBIs on the Large Helical Device (LHD) project, both of hydrogen and deuterium ion beam operations started in positive and negative ion sources. Ion source plasmas are generated by filament arc discharge in these ion sources. The same discharge power efficiencies were observed in hydrogen and deuterium operation in the positive ion source, while the discharge power efficiency in the deuterium operation was 50 - 70 % of that in the hydrogen operation in the negative ion source. In positive ion source, positive hydrogen and deuterium ion fluxes to the plasma grid correspond while ion density and velocity increases and decreases by a factor of square root, respectively. In the negative ion source, the extracted negative ions as the beam are mainly produced on the cesium covered plasma grid surface from parent particles which are atoms and/or positive ions. A negative ion production yield on the plasma grid depends on the parent particle speed perpendicular to the plasma grid surface. Slower parent particle speed by a factor of square root induces lower negative ion production yield in deuterium operation comparing with hydrogen. A ratio of beam current to arc discharge current (arc current efficiency) related to the discharge power efficiency is useful to connect ion source plasma parameters to extracted beam current. We discuss causes of similarity and difference of the discharge power and arc current efficiencies of hydrogen and deuterium operations in positive and negative ion sources. Additionally, from the causes, we estimate how much electron current increases in negative-deuterium-ion beam extraction comparing with hydrogen.
