Compensation of beam deflection due to the magnetic field using beam steering by aperture displacement technique in the multibeamlet negative ion source

Abstract

A beam steering technique using aperture displacement was examined to correct the negative ion beam deflection due to the magnetic field for electron suppression in a large-area multibeamlet H− source. The total deflection angle was estimated, including the effect of the deflection by the electron suppression magnetic field and the beam steering by the aperture displacement, both by linear optics theory and by three-dimensional beam trajectory simulation. Two methods were compared; one used the displacement both of the grounded grid (GG) apertures and of the exit part of the extraction grid (EG exit) apertures, and the other used the displacement only of the EG exit. The beam steering experiments were performed using a large-area multibeamlet H− source with both displacement methods, and the results were compared with the theoretical estimations. As a result, both methods were effective to correct the beam deflection. In particular, the displacement of only the EG exit with a simplified displacement structure achieved a large steering angle by a small displacement. The steering angle in the experiment was a few mrad smaller than the estimations. Based on these results, the aperture displacement of the EG exit was applied to the 1/5 segment of a H− source in the Large-Helical-Device neutral beam injector, where the GG apertures are displaced only focusing of the large-area multibeamlet. In this case, 1.0 mm of the displacement is concluded to be proper to sufficiently compensate the beam deflection at 180 keV, from extrapolation of the beam energy characteristics at 100–140 kev.

Demonstration of Beam Optics Optimization Using Plasma Grid Bias in a Negative Ion Source

“…2 (b), the beam width broadens for both hydrogen and deuterium operations. Note that the beam width is wider in deuterium operations than that in hydrogen operations because negative ion beamlets are steered by the aperture displacement technique applied on the SG in order to compensate the beam deflection due to the magnetic field generated by electron deflection magnets in the EG and the steering angle is optimized for the H − beam [6]. From an engineering point of view, the current ratio is another important parameter to be considered, which is shown in Fig.…”

Ion beam transport: modelling and experimental measurements on a large negative ion source in view of the ITER heating neutral beam

“…The experimental data used as benchmark for the numerical codes in the present paper were measured at the RNIS (Research and development Negative Ion Source) test stand, hosted at NIFS institute. The test stand is based on an arc-driven plasma source and accelerator, and is very similar by design to the ion beam sources used in Neutral Beam Injectors of the Large Helical Device experiment [34] [35]. The features of the RNIS source and acceleration system are listed below.…”

Neutralisation and transport of negative ion beams: physics and diagnostics

“…The deflection angle induced by the combined effect of CESM and EG lens steering, in fact, can be large (several mrad) and depends on beam energy, magnet size and distance between CESM and plasma. In existing NBIs the straight ion trajectories are recovered by adding a dedicated electrode at the exit of the EG [41,42], whose aperture axis displacement is tuned so as to compensate for the CESM-induced deflection, according to equation (7). As already mentioned, in the case of SPIDER, MITICA and NIO1 a different approach was adopted, based on tailoring the magnetic field profile along the accelerator so as to compensate for the unwanted beam deflection, which itself has a magnetic origin.…”