Extraction of Negative Hydrogen Ions from Radio Frequency Driven Plasma Sources for Fusion Plasma Heating

概要

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Microwave power at 14 GHz frequency coupled to a magnetic field aligned parallel to the beam extraction axis successfully excited a plasma in a compact negative hydrogen (H−) ion source. A volume of low electron temperature plasma was formed based on the idea of "tent filter" magnetic field geometry by placing the extraction aperture at a recess made inside of a magnetic material. The amount of H− current increased by enlarging a volume of low electron temperature plasma, which seemed consistent with the two step model of H− production via vibrational excitation of hydrogen molecules. The depth of the recess was made shallower expecting the H−current with the magnitude halfway between the one without the volume of low electron temperature plasma and the one with the volume. The result has indicated the expected correlation between the size of the volume and the H− current at low H2 pressure. Meanwhile, the beam current density has abruptly increased as the microwave discharge power was raised above 60 W, indicating a change of discharge mode around that input power.
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