Benchmark of 3D Multi-Beamlet Numerical Models for the Optics Design of Negative Ion Accelerators

Beam optics is a key requirement in multi-stage multi-beamlet negative ion accelerator for fusion applications, such as the high power Heating Neutral Beam injectors for ITER and JT-60SA. In particular, the efficiency of beam neutralization and transport to the tokamak plasma to be heated is crucially dependent on the divergence and deflection angle of the single beamlets with respect to their ideal trajectories. In 2016 an extensive collaboration between Consorzio RFX (Padova, Italy) and QST (Naka, Japan) was established in order to experimentally test some ITER-like multi-beamlet configurations on the Negative Ion Test Stand (NITS) in Naka. This campaign allowed a detailed comparison of the experimental results with respect to the estimations obtained by numerical models during the design phase. Although the numerical models were based on well-established commercial codes (OPERA-3D and COMSOL), some discrepancies were evidenced both between the two numerical models and between the models and the experimental results. A detailed benchmark has therefore been carried out in order to better identify the origin of these discrepancies, so as to obtain a fully reliable numerical description of the experiments. The present contribution describes the results of such an extensive comparison.