The characteristics of a large negative-ion source with external filter is investigated for the neutral-beam injection system in a large helical device. The magnetic cusp field is set up at almost the same strength as that in our well optimized 1/3 ion source with rod-type magnetic filter. An H- ion current of 5.2 A is extracted from the ion source with Cs seeding. The H- current corresponds to 19.7 mA/cm2, and the value is comparable to the current density obtained using the 1/3 scaled ion source with a rod-type magnetic filter at the same extraction voltage. In low operating pressure, the ion source with the external filter has a better performance on the extraction of H- ions.
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