1 MV Holding and Beam Optics in a Multi-Aperture Multi-Grid Accelerator for ITER NBI

Full text: In a negative ion accelerator utilized in the ITER neutral beam (NB) system, generation of 1 MeV, 40 A (200 A/m2) deuterium negative ion (D-) beams is required for 3600 s. To meet the requirement, a negative ion accelerator, called a 'MeV accelerator', has been developed at Japan Atomic Energy Agency (JAEA). The MeV accelerator is a five stage Multi-Aperture Multi-Grid (MAMuG) accelerator, which was chosen as a baseline accelerator design for the ITER NB. A target of the MeV accelerator is to accelerate 0.5 A (200 A/m2)H- ion beam at the energy of 1 MeV for several tens seconds. After successes of beam acceleration at 836 keV, 140 A/m2, 0.2 s presented at the previous conference of this series, present R and D are dedicated to increase the pulse length. For stable MeV-class beam acceleration, voltage holding studies were performed. Although structure and grid area are different, voltage holding capability of the MeV accelerator and JT-60U accelerator shows the same dependence on the gap length. Moreover, sustainable voltage of these accelerators at JAEA is a half of that in the small electrode used for design of these accelerators. The result indicates that the gap lengths in the present MeV accelerator (minimum gap; 72 mm) are considered to be marginal for stable voltage holding. Then, the gap length was extended to 100 mm so as to hold 200 kV stably (1 MV for five stages) including the margin of 20%. As a result, the MeV accelerator achieved stable voltage holding of 1 MV without gas feeding (base pressure; 2 x 10-4 Pa) for 4000 s. In the beam acceleration test, it was observed that relatively high heat loads on acceleration grids were occurred. So as to investigate the cause of the heat loads, a 3D multi beamlet analysis was applied to investigate the beam deflection due to magnetic field and space charge repulsion. (author)