The negative-ion-based neutral beam injection (NNBI) system is a reliable plasma heating and current drive method for large-scale magnetic confinement fusion devices. In the negative ion source, the problem of stray particles is a serious and unavoidable issue. These stray particles are mainly produced through particle–gas interactions in the accelerator region and can be subsequently accelerated, leading to high-voltage breakdown and substantial thermal deposition on components. The gas pressure plays a key role in determining the frequency of particle–gas collisions and simultaneously influences plasma and beam parameters, which in turn affect the generation and transport of stray particles. An NNBI test facility has been constructed in the Comprehensive Research Facility for Fusion Technology (CRAFT) in China. To investigate the dependence of stray particle behavior on gas pressure, three sets of experiments have been conducted in the CRAFT NNBI test facility. As the gas pressure increased, the power load on the ground grid first decreased and then increased, exhibiting an unexpected minimum value in the pressure range of 0.40 ∼ 0.45 Pa. A similar trend was observed for the vertical beam divergence, whereas the horizontal beam divergence increased monotonically with gas pressure. In contrast, the ejecting electron power, stripping loss, and backstreaming positive ion power were positively correlated with gas pressure. These results provide experimental insight into the complex role of gas pressure in stray particle generation and beam quality in NNBI systems.
負イオン源中性粒子ビーム入射(NNBI)システムは、大規模磁気閉じ込め核融合装置における信頼性の高いプラズマ加熱・電流駆動手法である。負イオン源において、迷走粒子の問題は深刻かつ不可避な課題である。これらの迷走粒子は主に加速器領域における粒子-ガス相互作用によって生成され、その後加速されることで、高電圧破壊や構成部品への多大な熱堆積を引き起こす可能性がある。ガス圧は、粒子-ガス衝突の頻度を決定する上で重要な役割を果たし、同時にプラズマおよびビームパラメータに影響を与え、これらが迷走粒子の生成と輸送に影響を及ぼす。中国の核融合技術総合研究施設(CRAFT)にNNBI試験施設が建設された。迷走粒子の挙動のガス圧依存性を調べるため、CRAFT NNBI試験施設において3組の実験が実施された。ガス圧が増加するにつれて、接地グリッドへのパワー負荷は最初に減少し、その後増加し、0.40 ∼ 0.45 Paの圧力範囲で予期しない最小値を示した。垂直ビーム発散についても同様の傾向が観察された一方、水平ビ