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• Joint experimental campaigns carried out in QST lab in Naka, Ibaraki, Japan. • Full compensation of magnetic criss-cross deflection of beamlets obtained. • Non-uniform extraction assumed for interpreting the experimental results. • Numerical simulations can reproduce the experimental results with good accuracy. In the framework of the collaboration between Consorzio RFX (Padova, Italy) and QST (Naka, Japan) two experimental campaigns have been organized on the Negative Ion Test Stand (NITS) in Naka, employing an ITER-like multi-beamlet configuration. These campaigns were aimed at compensating for the undesired magnetic deflection of the ion beam. The experiments have clarified the influence of operating parameters on the optical properties of the ion beam. The physical phenomena acting on the optics have been investigated, in particular the effect of an asymmetry of the ion current density at meniscus. Comparisons with simulations have been made and the modeling has been improved consequently. During the second experimental campaign, a perfect compensation of the ion deflection has been achieved. [ABSTRACT FROM AUTHOR]

The long-pulse acceleration of the high-power positive ion beam has been demonstrated with the JT-60 positive ion source in the joint experiment among Japan Atomic Energy Agency (JAEA), Korea Atomic Energy Research Institute (KAERI) and National Fusion Research Institute (NFRI) under the collaboration program for the development of plasma heating and current drive systems. In this joint experiment, the increase of the heat load and the breakdowns induced by the degradation of the beam optics due to the gas accumulation was one of the critical issues for the long-pulse acceleration. As a result of development of the long-pulse operation techniques of the ion source and facilities of the neutral beam test stand in KAERI, 2 MW 100 s beam has been achieved for the first time. The achieved beam performance satisfies the JT-60SA requirement which is designed to be a 1.94 MW ion beam power from an ion source corresponding to total neutral beam power of 20 MW with 24 ion sources. Therefore, it was found that the JT-60 positive ion sources were applicable in the JT-60SA neutral beam injectors. Moreover, because this ion source is planned to be a backup ion source for KSTAR, the operational region and characteristic has been clarified to apply to the KSTAR neutral beam injector. [ABSTRACT FROM AUTHOR]

• Experimental campaign on MTF carried out in QST lab in Naka, Ibaraki, Japan. • Opera, SLACCAD, COMSOL, EAMCC3D correctly predict beam divergence. • Semi-empirical Opera model with current density asymmetry correctly predicts the beam deflection. • COMSOL and EAMCC3D slightly underestimate heat loads on the grids. It is a widely recognized phenomenon that, in negative ion sources for NBI, the magnetic fields in the source cause an asymmetry of the current density at the meniscus, which affects the beam optics. In order to achieve a more realistic estimation of the negative ion beam optics, the ion current density profile at the beamlet meniscus is sometimes assumed to be non-symmetric with respect to the beamlet axis. The new hypothesis tested in this paper is an empirical law obtained by recent investigations on a similar Kamaboko ion source, which relates this asymmetry to the magnetic field strength at the meniscus and to the power injected in the ion source. For the first time this hypothesis is used to directly and correctly predict the value of the beamlet deflection. The MeV ion source Test Facility (MTF) is a high voltage negative ion beam accelerator dedicated to fusion experiments in QST, Naka, Ibaraki, Japan. MTF can be operated as a three- or five-stage multi-aperture accelerator with a configuration and beam energy comparable to the future ITER NBI. In 2019, experiments were conducted on MTF aiming at accelerating a H- beam with high energy and good optics for a long duration of the beam (around 100 s). These experiments allowed 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. The predictions of several beam simulation codes (Opera, COMSOL, SLACCAD, EAMCC3D) have been compared to the experimental measurements, which allows to obtain a good agreement in terms of beam optics and a fair agreement in terms of heat loads. [ABSTRACT FROM AUTHOR]