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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]

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]

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]