| 摘要 |
PROBLEM TO BE SOLVED: To enhance the ion extraction efficiency by restraining the divergence of an ion beam even if the strength of electrode material is increased through an increase in the thickness. SOLUTION: During irradiation of a negative ion beam 6, a boundary area S between the surface of a plasma electrode 1 and a plasma in a discharge chamber 4 is shifted in a beam extracting direction because the inclined surface 1b of the plasma electrode 1 is provided, and the distance from the boundary face B of the plasma 5 and the ion beam 6 to an extraction electrode is decreased at d2, so that an electric field for extracting the negative ion beam 6 can be intensified, with the result that even if the thickness of the electrode 1 is increased, the divergent angle of the negative ion beam 6 is restrained from increasing to make it possible to prevent the negative ion beam 6 from colliding against the next extraction electrode and an accelerating electrode. Moreover, when the inclined face 1b is provided at the peripheral edge of an extraction hole 1a, negative ions N produced on a surface are ejected in the direction of the beam extraction hole 1a and positively extracted from the extraction hole 1a of the plasma electrode 1 whereby the efficiency of extracting the negative ions N is enhanced and the development of the negative ion beam 6 into a large current can be achieved accordingly.
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