Improving a Negative Ion Accelerator for next generation of Neutral Beam Injectors: results of QST-Consorzio RFX collaborative experiments

and transport to the tokamak plasma strongly depends on the divergence and the deflection angle of each single beamlet with respect to its ideal trajectory. In fact, a very narrow window is available for the particle beam to pass through the neutralizer panels and the duct reaching the tokamak plasma. For this reason, beam optics quality is one of the key requirements in multi-stage multi-beamlet negative ion accelerator, such as the high power Heating Neutral Beam injectors for ITER and JT-60SA. In the framework of the collaboration established between Consorzio RFX (Padova, Italy) and QST (Naka, Japan) experimental campaigns have been organized on the Negative Ion Test Stand (NITS) in Naka employing an ITER-like multi-beamlet configuration [1]. These campaigns were intended to: o Test and optimize new solutions for cancelling the undesired ion deflection caused by the transverse magnetic field required for suppressing the co-extracted electrons [2]. o Better understand the physics underlying the extraction of single beamlets in the initial part of their trajectories, close to the ,,meniscus" region. o Validate and improve the numerical codes used for the accelerator design Although it was not possible to explore all the intended parameter space due to power supply limitations, the experiments carried out in 2016 and 2017 allowed to successfully achieve the first most important (engineering) objective and to provide bases for the achievement of the other two (physics) objectives, which will be completed during new already planned activities and collaborations.