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Co-traveling ion beams interact with one another along the beam transport path primarily through space-charge (SC) forces (coulomb), which can significantly influence beam propagation and power transport efficiency of multi-beam group-based ion sources. Previous studies in the ROBIN source have shown the role of SC interactions during transport, and their correlation with the accelerator parameters [1]. However, during transport, the charge state of the beam species evolves due to collisions with the background neutral gas, accompanied by the generation of secondary charged particles through beam-induced ionization processes [2]. These mechanisms lead to a partial neutralization of the beam space charge, thereby mitigating inter-beam Coulomb interactions. For extended transport lengths (~ 1 - 10 m), the background gas pressure is spatially non-uniform due to the location of pumps and different beamline components, and follows a characteristic pressure profile. In the present work, the neutral gas density profile along the transport line is computed using Molflow+, a dedicated Monte Carlo–based code designed for pressure calculations inside a vacuum chamber in the molecular flow regime [3]. The resulting pressure profile is subsequently employed to evaluate beam-induced ionization rates and to model the evolution of beam species and secondary charged particles along the transport path [4]. These studies are aimed at understanding and optimizing space-charge effects in high-power ion beam transport and contribute to improving the power transport performance of large-area neutral beam sources developed for fusion reactor applications.
1. Dash, Sidharth, et al. "Probing into space charge interactions of negative ion beams through imaging diagnostics." Journal of Instrumentation 20.07 (2025): C07054.
2. Tabata, Tatsuo, and Toshizo Shirai. "Analytic cross sections for collisions of H+, H2+, H3+, H, H2, and H− with hydrogen molecules." Atomic Data and Nuclear Data Tables 76.1 (2000): 1-25.
3. Kersevan, Roberto, and Marton Ady. "Recent developments of Monte-Carlo codes molflow+ and synrad+." 10th International Particle Accelerator Conference. Vol. 6. 2019.
4. Surrey, Elizabeth. "Modelling the neutraliser plasma of ITER negative ion beams." Negative ion-based neutral beam injectors. Proceedings of a technical meeting. 2006.