In order to improve the spatial uniformity of the negative ion beam and to produce high current negative ion beams in a large negative ion source, a magnetic field configuration is modified from an original transverse filter to a tent-shaped filter, in combination with reducing the magnetic field strength in the JT-60 negative ion source. As a result, the beam uniformity is improved from 68% to 83% over an area of the whole extraction area of 450 × 1100 mm<sup>2</sup>. The improvement of the beam uniformity leads to the production of 32 A H<sup>-</sup> ion beams with the whole extraction area. The obtained beam current fulfills the requirement for JT-60SA.
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