Positive ion influence on the beamlet optics for neutral beam injectors

Abstract Neutral beam injectors are based on the neutralization of ion beams accelerated at desired energy. In case ITER heating and diagnostic neutral beams, target power translates into stringent requirements acceptable beamlet divergence aiming to allow reach fusion plasma. The beamlets composing experimentally found feature a transverse velocity distribution exhibiting two Gaussian components: well-focused one is referred as core component while rest beam, halo, describes particles with much worse optics. codes that simulate extraction acceleration usually assume negative ions move towards plasma meniscus laminar flow (no velocity) or can be modelled Maxwellian current density uniformly illuminating meniscus; under such approximations, presence highly divergent components cannot explained. this work, we develop simple test-particle tracing code Monte Carlo collisions, named ICARO (for Ions Coming Around), study transport in region derive spatial (i.e. boundary where extracted). particular, origin halo its dependence source parameters discussed, highlighting key parameter energy positive