Benchmarking of beam acceleration codes on a high voltage negative ion accelerator for fusion with a new hypothesis on the beam meniscus

The predictions obtained using newly developed numerical models, based on consolidated beam acceleration codes (Opera, COMSOL, SLACCAD, EAMCC3D), have been compared to the experimental measurements obtained on the MeV ion source Test Facility (MTF). MTF is a high voltage negative ion beam accelerator, dedicated to fusion experiments in QST, Naka, Ibaraki, Japan. During 2019, MTF has been operated in conditions comparable to those of the future ITER NBI, for a long duration of the beam (around 100 s), in a three- or five-stage multi-aperture accelerator configuration, with high accelerating energy and good optics. These experiments allowed us to measure the beamlet divergence and deflection as well as the heat loads due to particle impinging on the accelerator grids, over a wide range of operating parameters. In the calculations, a new hypothesis on the ion current density profile at the beamlet meniscus is assumed, as indicated by recent studies that correlate the non-uniform extraction of negative ions to the strength of the applied transverse magnetic field. As a result, when the ion current distribution is non-symmetric with respect to the beamlet axis, the beamlet trajectory changes considerably. The good agreement obtained between measurements and beam optics simulations strengthens the credibility of this hypothesis. The heat loads on the accelerator grids predicted by simulations are also compared to the heat loads measured by water calorimetry during experiments. The differences still existing concerning the heat loads are then discussed in detail.