Compensations of beamlet deflections for 1 MeV accelerator of ITER NBI

Kashiwagi, Mieko; Taniguchi, Masaki; Umeda, Naotaka; Dairaku, Masayuki; Tobari, Hiroyuki; Yamanaka, Haruhiko; Watanabe, Kazuhiro; Inoue, Takashi; DeEsch, H. P. L.*; Grisham, L. R.*; Boilson, D.*; Hemsworth, R. S.*; Tanaka, Masanobu*

In a five stage multi-aperture multi-grid (MAMuG) accelerator for the ITER neutral beam injector (NBI), 1 MeV, 40 A D ion beam is required for 1 hour. However, beamlets are deflected due to (1) magnetic field for electron suppression and (2) space charge repulsion between beamlets, and consequently, cause excess grid heat load. A three dimensional beam analysis has been carried out to compensate the beamlet deflections. This paper shows that the beamlet deflections due to (1) and (2) are compensated by an aperture offset of only 0.6 mm applied to the aperture of 17 mm in diameter in the extractor and by a metal bar attached around aperture area beneath the extractor, respectively. When the metal bar is increased to 3 mm in thickness and installed 30 mm away from the aperture area, the beamlet is steered gently by the weaker electric field distortion. The beam optics was confirmed not deteriorated by those compensations. The presentation also discusses application of these compensation techniques to the ITER design.