Fast neutral atom beams show significant application potential in fields such as space environment simulation and micro-nano device processing. To obtain high-quality fast neutral atom beams with a wide tunable parameter range, this paper proposes a low-energy ion beam neutralization scheme based on high-aspect-ratio rectangular channels. A two-dimensional-in-space and three-dimensional-in-velocity (2D3V) particle-in-cell (PIC) model is established to simulate the transport and neutralization processes of a He⁺ beam inside a rectangular channel with atomically flat metal walls. The effects of three key parameters—incident ion drift energy (10-100 eV), channel length (6-30 mm, aspect ratio up to 300), and grazing angle (0°-5°)—on neutralization efficiency and outgoing beam characteristics are systematically investigated.
The simulation results reveal that the neutralization efficiency is governed primarily by the number of collisions. It decreases nonlinearly from 0.78 to 0.43 as the drift energy increases from 10 eV to 100 eV, because higher drift energy shortens the ion residence time in the channel and improves beam collimation, thereby reducing the number of collisions. In contrast, increasing the channel length from 6 mm to 30 mm raises the neutralization efficiency from 0.80 to 0.96 and tends to saturate, as more collisions occur. Similarly, increasing the grazing angle from 0° to 5° enhances the number of collisions and achieves a neutralization efficiency above 0.97 when the angle exceeds 4°.
Regarding the beam quality, the exit angular distribution of non-neutralized ions depends only on the channel length: longer channels impose stronger geometric constraints and produce smaller divergence angles. The angular and velocity distributions of the outgoing neutral atoms are directly inherited from the incident ions. Notably, as the grazing angle increases, the angular distribution of the outgoing atoms broadens and develops multiple peaks due to the spatial superposition of different reflection paths, while the velocity distribution remains almost unchanged. Furthermore, the velocity distribution of the outgoing atoms shifts toward higher speeds with increasing drift energy but is unaffected by channel length or grazing angle.
This study reveals the key physical mechanisms governing low-energy ion beam neutralization in high-aspect-ratio rectangular channels. It also provides quantitative parameter-regulation rules for the design and optimization of fast neutral beam sources with high efficiency, low divergence, and wide tunability. Future work will incorporate energy loss, secondary electron emission, and three-dimensional geometric effects, along with experimental validation.