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Abstract: Requirement and technical issues of the neutral beam (NB) system are discussed for a fusion DEMO plant. The NB system for DEMO should be characterized by high efficiency, high energy, high reliability in year-long steady operation, and low maintenance frequency. From the viewpoint of high efficiency, use of conventional electrostatic accelerator is realistic. Due to operation under radiation environment, vacuum insulation is essential in the accelerator. According to the insulation design guideline, it was clarified that a beam energy of 1.5–2MeV is possible in the accelerator. Development of filamentless, and cesium free ion source is required to achieve low maintenance frequency, based on the existing high current/high current density negative ion production technology. The gas neutralization is not applicable due to its low efficiency (60%). R&D on an advanced neutralization scheme such as plasma neutralization (efficiency: >80%) is required. Recently, development of cw high power semiconductor laser is in progress. The paper shows a conceptual design of a high efficiency laser neutralizer utilizing the new semiconductor laser array. [Copyright &y& Elsevier]

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]

Toward long-pulse productions of 22A negative ion beams for 100s in JT-60SA, the plasma generator of the negative ion source has been developed to achieve stable long-pulse generations of spatially uniform negative ions. To realize the long pulse production of the negative ions, an actively cooled plasma grid (PG) has been designed to keep a surface temperature of 250–300°C optimized for high negative ion production. The designed active cooling system has a potential to control the surface temperature of the PG to 250°C with a time constant of 10s by flowing the fluorinated fluid having the high boiling temperature of 270°C. Since the time constant is sufficiently shorter than that of the degradation of the negative ion production, the active cooling system is applicable to the long-pulse operation of the large-current negative ion source, such as JT-60SA and ITER ion sources. As for the high current beam production, spatial uniformity of the negative ion productions in the plasma generator is improved. The calculation results of the magnetically confined electron orbits emitted from filaments showed that longitudinal deviations of the primary electron distributions has been reduced from 27% to 6% by the application of the symmetric magnet arrangements, so called the tent-shaped filter. This reduction of the deviation could reduce the grid heat load, leading to the long pulse production of the negative ion beams. [ABSTRACT FROM AUTHOR]

Abstract: The origin of the beam non-uniformity, that is one of the key issues in JT-60 negative ion sources, was experimentally examined by measuring correlations between the intensity of the H− ion beam and plasma parameters, such as an electron temperature and plasma density in the JAERI 10 A negative ion source. The ion source was operated without cesium. Although a uniform filter field was applied, the electron temperature near plasma grid (PG) was higher than 1eV in the upper half region of the longitudinal direction, where the spatial beam intensity decreased rapidly. This indicates that the beam non-uniformity is caused by a higher electron temperature than 1eV, namely, by a leakage of the high-energy electrons toward the PG. To investigate the path of high-energy electrons leaking into the PG, a metal plate of 50mm×50mm was placed in the plasma. By placing the plate in a weak magnetic field of <30G leading to a driver region near the upper wall, the electron temperature near the PG was sufficiently reduced to 1eV in the upper half region, where the beam intensity was simultaneously increased. As a result, a 20% gain of the beam current was obtained. Further, the source of the high-energy electrons leaking toward the PG was examined by changing the relative filament position to the upper wall. It is found that the filaments closest to the upper wall, i.e., at 80mm from the upper wall is one of the sources of the high-energy electrons leaking into the PG. [Copyright &y& Elsevier]

High energy accelerator and high current ion source development have been carried out at JAERI for negative-ion based neutral beam (N-NB) systems of JT-60U and ITER. One of R&D issues on the ITER prototype 5-staged electrostatic accelerator was voltage holding capability of the accelerator insulator column. By installing a stress ring, which reduces electric field concentration at the triple junction (interface of metal flange, insulator, and vacuum), 300 kV is held at each stage instead of rated voltage of 200 kV. At present the accelerator insulator column sustains 900 kV stably with the rings in all five stages. In the JT-60U N-NB system, a beamlet deflection by distorted electric field was found at the bottom of the extractor. Correcting the distorted field, reduction in beam divergence was confirmed for the overall beam. As the result, the heat load on the NB port limiter of JT-60U, located about 20 m away from the accelerator, was reduced to less than a half of the previous value before the correction. Consequently, we have succeeded in 10 s continuous injection of H0 beam with the NB power of 2.6 MW at 355 keV. [Copyright &y& Elsevier]

• 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]