A low beamlet divergence is crucial for the efficiency of the ITER-NBI systems, since it affects the transmission of the beam through the duct. There is a requirement of 7 mrad e-folding divergence for the ITER Heating Neutral Beam. Significantly higher divergences (10–15 mrad) have been observed in RF-source based experiments albeit at low beam energy. This could be the consequence of a broad perpendicular velocity distribution of the H−/D− particles before extraction. This paper explores this hypothesis and its implications for ITER. To estimate H−/D− perpendicular temperatures in the RF-driven BATMAN Upgrade test facility, spatially resolved measurements of the beam power density are compared with IBSimu calculations. The estimated perpendicular temperatures show a strong dependence on the source filling pressure, decreasing from approximately 4 eV at 0.3 Pa to 2 eV at 0.4 Pa. Ion-optics calculations of the ITER-HNB grid system are performed to evaluate whether the temperatures estimated in the BATMAN Upgrade test facility are tolerable in view of beam-grid interaction and beamline transmission. The beamline transmission is fairly insensitive to the perpendicular temperature, but the heat loads at the downstream grids increase with the perpendicular temperature.
This paper investigates the beam divergence of RF-driven ion sources, which is crucial for the efficiency of ITER's Neutral Beam Injection (NBI) systems. It explores the relationship between the perpendicular velocity distribution of H-/D- particles and the observed high beam divergence, and evaluates the impact on the ITER-HNB grid system.