Speaker
Atsushi Kojima
(Fusion Research and Development Directorate)
Description
Acceleration of high-power-density negative ion beams of ~180 MW/m22 have been achieved up to 60 s for the first time. Because the achieved power density was comparable to ITER accelerator, and accelerated energy density of 10800 MJ/m22 is much higher than that for JT-60SA of 6500 MJ/m22, this achievement is one of promising results to overcome common issues for the heating neutral beams on JT-60SA and ITER having same accelerator concept.
Formerly, even after voltage holding capability and beam steering were improved by adjusting the gap length and compensating the beam deflections, the pulse length of over 100 MW/m22 beams was limited below 1 s due to breakdowns at acceleration grids. The breakdown was induced by impacts of negative ions and/or secondary electrons to the grids. In order to suppress this, one of remaining issues was the configuration of apertures on the grids, since the impact of the negative ions on the top surface of the grids had been minimized by the optimization of the beam steering.
This time, the acceptance of the negative ions to the downstream side was improved. At first, the thickness of the grids has been reduced from 20mm to 10mm, and the shape of the apertures has been changed from straight to conical type. Moreover, the diameter of the apertures was enlarged from 14mm to 16mm for the downstream grids which accelerated high energy beam.
After these modifications, because of the reduction of breakdowns with beam, acceleration of 1 MeV beam has been realized in shorter conditioning time of 4 days with cesium operation than the previous one of 10 days, and finally long-pulse acceleration of 60 s was obtained without any breakdown. So far, no degradation has been observed in terms of the negative ion acceleration during long pulse.
Co-authors
Atsushi Kojima
(Fusion Research and Development Directorate, Japan Atomic Energy Agency, Naka, Japan)
Hiroyuki Tobari
(Fusion Research and Development Directorate, Japan Atomic Energy Agency, Naka, Japan)
Junichi Hiratsuka
(Fusion Research and Development Directorate, Japan Atomic Energy Agency, Naka, Japan)
Kazuhiro Watanabe
(Fusion Research and Development Directorate, Japan Atomic Energy Agency, Naka, Japan)
Larry R Grisham
(Princeton Plasma Physics Laboratory, NJ 08543, United States)
Masafumi Yoshida
(Fusion Research and Development Directorate, Japan Atomic Energy Agency, Naka, Japan)
Masahiro Ichikawa
(Fusion Research and Development Directorate, Japan Atomic Energy Agency, Naka, Japan)
Masaya Hanada
(Fusion Research and Development Directorate, Japan Atomic Energy Agency, Naka, Japan)
Mieko Kashiwagi
(Fusion Research and Development Directorate, Japan Atomic Energy Agency, Naka, Japan)
Naotaka Umeda
(Fusion Research and Development Directorate, Japan Atomic Energy Agency, Naka, Japan)
Ryo Nishikiori
(Fusion Research and Development Directorate, Japan Atomic Energy Agency, Naka, Japan)
group the NB Heating Technology
(Fusion Research and Development Directorate, Japan Atomic Energy Agency, Naka, Japan)
