Uniform H ion beam extraction in a large negative ion source with a tent-shaped magnetic filter

Tobari, Hiroyuki; Hanada, Masaya; Kashiwagi, Mieko; Taniguchi, Masaki; Umeda, Naotaka; Watanabe, Kazuhiro; Inoue, Takashi; Sakamoto, Keishi; Takado, Naoyuki*

In order to improve the spatial uniformity of negative ion beam, a novel magnetic filter, the tent-shaped filter was applied in the JAEA 10 ampere negative ion source as a replacement to the conventional external magnetic filter. The cusp magnetic field configuration for plasma confinement was changed to form a symmetric cusp field commonly used in many of positive ion sources for high proton yield. This configuration together with the tent filter allows fast electrons emitted from filaments drift azimuthally around the beam axis. This is a major advantage of the current configuration to prevent localization of fast electrons due to B grad B drift in the external filter field. As the result, the standard deviation of H ion beam profile extracted from a wide area of 170 mm 340 mm decreased from 16% with the external filter to 7.9 % with the tent filter. Thus the ion source with the tent filter demonstrated the uniform beam extraction that fulfilled the requirement for ITER NBI.