Investigation of the effects of neutral particles on beam-ion losses in the EAST tokamak

Efficient control of beam-ion losses is crucial for sustaining long-pulse, high-performance operation in future fusion devices. In this work, EAST tokamak experiments combined with three-dimensional modelling are used to systematically assess the effects of localized background density enhancement on the initial deposition and loss behaviour of beam ions generated by neutral beam injection. The localized density enhancement is produced by supersonic molecular beam injection (SMBI), a plasma fuelling method. Three-dimensional background plasma and neutral parameter distributions for different SMBI injection locations are reconstructed with EMC3–EIRENE and implemented in ASCOT5 orbit-following simulations. The results show that SMBI3, the SMBI system located at the A port on EAST, substantially modifies the initial beam deposition, increasing the scrape-off layer deposition fraction by ∼50% and thereby significantly reducing core beam-ion deposition and heating efficiency. A two-stage simulation workflow is employed to quantitatively evaluate beam-ion losses, fast-neutral yields, and wall power loading. Beam-ion losses occur predominantly within the first poloidal transit, and their spatial distribution is highly sensitive to the toroidal location of SMBI. When the locally enhanced neutral-density region overlaps trapped beam-ion trajectories, charge-exchange reactions are strongly intensified, generating abundant fast neutrals and markedly increasing both localized wall power loading and beam-ion losses.The simulated loss levels and velocity-space characteristics are in good agreement with fast-ion loss detector measurements, indicating that non-axisymmetric background particle distributions are essential for elucidating charge-exchange-driven beam-ion loss mechanisms. These results show that SMBI-induced three-dimensional particle distributions and their relative geometry to the neutral beam critically govern beam-ion loss behaviour, providing guidance for optimizing fuelling–heating system layouts and mitigating wall power loading in future fusion devices.