Search

In order to reduce the tritium inventory in a fuel circulation system in fusion plants, a possibility of deuterium-tritium (DT) mixed neutral beam injectors with DT negative ion beams has been investigated by analyzing the beam optics of the DT-mixed negative ion beams for the first time. In a two-dimensional beam analysis, DT-mixed ion beams could be accelerated regardless of the DT current mixing ratio (T/D) from the identical acceleration geometry with the identical applied voltages by adjusting total space charge for the D and T components. However, in a three-dimensional beam analysis including magnetic fields, the DT-mixed ion beams were separated due to a mass dependence of the beam deflection by orthogonal magnetic fields of an electron suppression and transverse filter fields for negative ion production. In the case of the beamline for ITER, which requires beam deflection angle within ±1 mrad, the operational range for the DT-mixed ion beams was found to be the DT current mixing ratio of 0.7-1.5 and beam energy of 0.9-1.0 MeV. Although the operational range was not wide, the DT-mixed beam was found to be feasible in terms of the beam acceleration. These analyses also contribute to the study of the acceleration of other mixed ion beams such as impurity or heavy ions, which are useful techniques for diagnostic beams. [ABSTRACT FROM AUTHOR]

Abstract: This paper reports R&D progress of the MeV class accelerator, beam focussing technique and a high voltage (HV) bushing toward the ITER Neutral Beam system at JAEA. A H− ion beam of 320mA (current density: 140A/m2) was successfully accelerated up to 796keV by fixing air leak in the ion source chamber caused by heat loading due to backstream positive ions. From analyses of a three-dimensional beam trajectory, a countermeasure against beamlet deflection due to space charge repulsion among beamlets was identified to compensate the deflection by proper aperture offset in the electron suppression grid. For development of the HV bushing, a half-size (Ø800mm) mockup single stage bushing was manufactured and tested, that demonstrated the dc voltage holding capability of up to 220kV in a condition with vacuum inside. Electrostatic analyses and design of the HV bushing are in progress, in which some design improvements are suggested to reduce electric field strength on the surface of ceramic insulator. [Copyright &y& Elsevier]

Abstract: The paper reports recent progress of R&D towards the beam source and HV bushing of the ITER Neutral Beam System. A new magnetic configuration with a “tent” magnetic filter was tested to suppress localization of fast electrons in a large negative ion source. The result was a drastic improvement of the negative ion uniformity with a beam intensity deviation less than ±4%. This fulfills the requirement of ITER (deviation ≤±5%). In a 1MeV accelerator development, experiment of high current beam acceleration is in progress. The H− ion beams of 0.24A has been obtained so far, at beam energy of 800keV. Concerning the insulator rings of the HV bushing, a new cold-isostatic press method was developed to press alumina powder to the ring form. Formation and sintering of a full-scale (1.56m in diameter) ceramic insulator ring was successfully completed without damages or cracks in the ceramic body. [Copyright &y& Elsevier]

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]

• 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]

A common problem of the multi-beamlet, negative-ion based Heating Neutral Beam (HNB) Injectors presently used in fusion research, is the undesired deflection of negative ions due the permanent magnets embedded in the Extraction Grid (EG) for the suppression of the unwanted co-extracted electrons. A new solution has been recently developed at Consorzio RFX, which makes use of an additional set of permanent magnets, also embedded in the EG, with a series of advantages in terms of performances and versatility with respect to the traditional electrostatic compensation. This solution is now the new standard for ITER. After a thorough validation of the design by different numerical models, it was decided to experimentally test this solution for the first time on the Negative Ion Test Stand (NITS) at National Institute for Quantum and Radiological science and technology (QST), in Japan, within the framework of a scientific cooperation agreement between QST and Consorzio RFX. To this purpose, an EG having an ITER-like profile and compatible with NITS accelerator was designed and constructed at Consorzio RFX. This paper describes the design solutions adopted for this grid and its related permanent magnets, the construction phase, and a brief report on the first experimental results. [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]

Abstract: The HV bushing has a function of a bulk head between insulation gas (SF6) of 0.6MPa and vacuum, which is needed to insulate −1MV in vacuum in ITER heating neutral beam (NB) injection system. Mechanical analyses were carried out to verify the mechanical integrity of the HV bushing composed of five-stage double-layered insulator columns with large brazed ceramic ring and fiber reinforced plastic (FRP) ring of which non-metallic material is required to satisfy a safety factor of ≥3.5. As for the FRP ring required sustaining the pressure load, seismic load and dead weight, it was confirmed that isotropic fiber cloth FRP rings having sufficient strength against shear stress should be used. The shape and fixation area of the Kovar sleeve were modified to lower the stress at the brazing area having vacuum leak tightness and pressure tightness against the air filled at 0.6MPa. As a result, a design of the insulator for the HV bushing was established satisfying the requirement. [Copyright &y& Elsevier]

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 ITER [1] fusion device is expected to demonstrate the feasibility of magnetically confined deuterium–tritium plasma as an energy source which might one day lead to practical power plants. Injection of energetic beams of neutral atoms (up to 1MeV D0 or up to 870keV H0) will be one of the primary methods used for heating the plasma, and for driving toroidal electrical current within it, the latter being essential in producing the required magnetic confinement field configuration. The design calls for each beamline to inject up to 16.5MW of power through the duct into the tokamak, with an initial complement of two beamlines injecting parallel to the direction of the current arising from the tokamak transformer effect, and with the possibility of eventually adding a third beamline, also in the co-current direction. The general design of the beamlines has taken shape over the past 17 years [2], and is now predicated upon an RF-driven negative ion source based upon the line of sources developed by the Institute for Plasma Physics (IPP) at Garching during recent decades [3–5], and a multiple-aperture multiple-grid electrostatic accelerator derived from negative ion accelerators developed by the Japan Atomic Energy Agency (JAEA) across a similar span of time [6–8]. During the past years, the basic concept of the beam system has been further refined and developed, and assessment of suitable fabrication techniques has begun. While many design details which will be important to the installation and implementation of the ITER beams have been worked out during this time, this paper focuses upon those changes to the overall design concept which might be of general interest within the technical community. [Copyright &y& Elsevier]

Abstract: Neutral beam (NB) injectors for JT-60 Super Advanced (JT-60SA) have been designed and developed. Twelve positive-ion-based and one negative-ion-based NB injectors are allocated to inject 30MW D0 beams in total for 100s. Each of the positive-ion-based NB injector is designed to inject 1.7MW for 100s at 85keV. A part of the power supplies and magnetic shield utilized on JT-60U are upgraded and reused on JT-60SA. To realize the negative-ion-based NB injector for JT-60SA where the injection of 500keV, 10MW D0 beams for 100s is required, R&Ds of the negative ion source have been carried out. High-energy negative ion beams of 490–500keV have been successfully produced at a beam current of 1–2.8A through 20% of the total ion extraction area, by improving voltage holding capability of the ion source. This is the first demonstration of a high-current negative ion acceleration of >1A to 500keV. The design of the power supplies and the beamline is also in progress. The procurement of the acceleration power supply starts in 2010. [Copyright &y& Elsevier]

Abstract: IRFM (CEA Cadarache) and JAEA Naka have entered into a collaboration in order to test a Single Gap, Single Aperture (SINGAP) [H.P.L. de Esch, R.S. Hemsworth, P. Massmann, Updated physics design ITER-SINGAP accelerator, Fusion Eng. Des. 73 (2005) 329.] accelerator at the JAEA Megavolt Test Facility (MTF) at Naka, Japan. Whereas at the CEA testbed the acceleration current was limited to 0.1A, at JAEA 0.5A is available. This allows the acceleration of 15 H− beamlets in SINGAP to be tested and a direct comparison between SINGAP and Multi Aperture, Multi Grid (MAMuG) [M. Taniguchi, T. Inoue, N. Umeda, M. Kashiwagi, K. Watanabe, H. Tobari, M. Dairaku, K. Sakamoto, Acceleration of ampere class H− ion beam by MeV accelerator, Rev. Sci. Instrum. 79 (2008) 02C110.] to be made. High voltage conditioning in the SINGAP configuration has been quite slow, with 581kV in vacuum achieved after 140h of conditioning. With 0.25Pa of H2 gas present in the accelerator 787kV could be achieved. The conditioning curve for MAMuG is 200kV higher. SINGAP beam optics appears in agreement with calculation results. A beamlet divergence better than 5mrad was obtained. SINGAP accelerates electrons to a higher energy than MAMuG. Measurements of the power intercepted on one of the electron dumps have been compared with EAMCC code [G. Fubiani, H.P.L. de Esch, A. Simonin, R.S. Hemsworth, Modeling of secondary emission processes in the negative ion based electrostatic accelerator of the International Thermonuclear Experimental Reactor, Phys. Rev. ST Accel. Beams 11 (2008) 014202.] calculations. Based on the experiments described here, electron production by a SINGAP accelerator scaled up to ITER size was estimated to be too high for comfort. [Copyright &y& Elsevier]

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]