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Space charge compensation and residual-field effects in negative-ion NBI beam transport through the drift section
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Space charge compensation and residual-field effects in negative-ion NBI beam transport through the drift section
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Journal Article
Space charge compensation and residual-field effects in negative-ion NBI beam transport through the drift section
2026
Overview
The beam transport process plays a crucial role in the design and realization of negative-ion-based neutral beam injection (N-NBI) systems for thermonuclear fusion. In particular, space charge compensation (SCC), where positive ions generated by collisions between negative ions and background gas mitigate space charge effects, serves as a critical mechanism influencing beam divergence and envelope evolution. In this work, the SCC process in N-NBI systems and its impact on beam transport are investigated using a 3D simulation model based on a Vlasov beam-tracing algorithm. Systematic simulations show that SCC evolution and downstream beam optics are jointly controlled by background gas pressure and beam energy, with higher pressure and higher energy strengthening SCC and mitigating divergence and envelope growth downstream of the extraction system. A key physical observation is that collisionless phase mixing drives the transverse velocity distribution of secondary ions toward a Maxwellian distribution, enabling an effective transverse ion temperature that quantitatively correlates with SCC degree. The residual electric field leaking downstream of the extraction system locally weakens SCC over a finite extent, producing negligible beam optics impact for the ITER-relevant 1 MeV case at pressures of 5×10−3 Pa and 1×10−2 Pa, but non-negligible transport sensitivity at 200 keV under the same pressure range. These results provide new insight into the interplay between SCC, secondary-ion dynamics, and beam transport, with direct relevance to the modeling and optimization of high-energy NBI beamlines.
Publisher
IOP Publishing
