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Nishikiori, Ryo; Kojima, Atsushi; Hanada, Masaya; Kashiwagi, Mieko; Watanabe, Kazuhiro; Umeda, Naotaka; Tobari, Hiroyuki; Yoshida, Masafumi; Ichikawa, Masahiro; Hiratsuka, Junichi; et al.
Plasma and Fusion Research (Internet), 11, p.2401014_1 - 2401014_4, 2016/03
One of critical issues for high-energy high-current beam acceleration in ITER and JT-60SA is the high voltage holding which is dominated by vacuum discharges. The past results suggest that vacuum discharge occurs beyond the threshold of the dark current. The dark current can be derived from F-N theory where electric field enhancement factor beta is included. Though, beta could only be evaluated from the experiment previously. Therefore, the method to decide beta without experiment is required. This time dark currents were measured at three different areas to compare beta in different electric field. As a result, the effective electric field 
E, where E is average electric field, were found to be almost constant for different areas although the beta is largely different. By applying E, beta can be evaluated analytically, leading to the analytical prediction of the dark current and voltage holding capability without the measurements.
Kojima, Atsushi; Hanada, Masaya; Yoshida, Masafumi; Umeda, Naotaka; Hiratsuka, Junichi; Kashiwagi, Mieko; Tobari, Hiroyuki; Watanabe, Kazuhiro; Grisham, L. R.*; NB Heating Technology Group
AIP Conference Proceedings 1655, p.060002_1 - 060002_10, 2015/04
Times Cited Count：6 Percentile：87.25(Physics, Applied)In this paper, the recent activities on the new test stand are reported toward demonstration of the long pulse production for 22A, 100s negative ion beams. As for the temperature control of the plasma grid, a prototype of the grid with cooling/heating by circulating a high-temperature fluorinated fluid has been improved to cover the full extraction area by using 5 segments of the PG. These grids were found to have a capability to control the temperature with a time constant of 10s as well as the prototype grid. As a result, 15A negative ion beams for 100s have been achieved.
Kojima, Atsushi; Hanada, Masaya; Inoue, Takashi; NB Heating Technology Group; Yamano, Yasushi*; Kobayashi, Shinichi*
Journal of the Vacuum Society of Japan, 56(12), p.502 - 506, 2013/12
Voltage holding capability of a large negative ion source for fusion application is experimentally examined, which is characterized by multiple-stage acceleration with multiple-apertures over 1000 on large-area grids of 2 m
for the multiple-beamlet accelerations. From the observation of the vacuum discharge between the grids, it was found that the aperture generated 10 times larger dark current than the flat region and initiated the vacuum discharge associated with the breakdown. As a result, it was found that the sustainable voltages were dominated by not only the surface area but also the number of the apertures. Because these effects were originated in the area effects by weak and strong electric field profiles, these results implied the surface integration of the electric field were the key parameter for the vacuum insulation.
Kojima, Atsushi; Hanada, Masaya; Yoshida, Masafumi; Inoue, Takashi; Kashiwagi, Mieko; NB Heating Technology Group
no journal, ,
In this paper, the recent activities on the new test stand are reported toward demonstration of the long pulse production. As for the improvement of uniform beam current profile, an estimation from trajectory calculations of primary electrons with the symmetric magnetic field configuration showed that the primary electrons were distributed uniformly in a longitudinal direction. As for the temperature control of the plasma grid, a prototype of the grid with cooling/heating by circulating a high-temperature fluorinated fluid has been developed. This grid was found to have a capability to control the temperature with a time constant of 10 s by considering the physical properties of the fluid. In addition, the un-uniform plasma profile has been observed in the preriminary experiments on the new test stand, which agrees with the calculation results.
Kojima, Atsushi; Hanada, Masaya; Yoshida, Masafumi; Kashiwagi, Mieko; Tobari, Hiroyuki; Umeda, Naotaka; Watanabe, Kazuhiro; NB Heating Technology Group
no journal, ,
In this paper, the recent activities on the new test stand are reported toward demonstration of the long pulse production for 22A, 100s negative ion beams. As for the temperature control of the plasma grid, a prototype of the grid with cooling/heating by circulating a high-temperature fluorinated fluid has been developed. This grid was found to have a capability to control the temperature with a time constant of 10 s by considering the physical properties of the fluid. As a result, 120 A/m beams for 100 s have been achieved, which is about 90 percent of the rated value for JT-60SA.
Kojima, Atsushi; Hanada, Masaya; Yoshida, Masafumi; Kashiwagi, Mieko; Tobari, Hiroyuki; Umeda, Naotaka; Watanabe, Kazuhiro; NB Heating Technology Group
no journal, ,
Recent activities on the new test stand are reported toward demonstration of the long pulse production for 22A, 100s negative ion beams. As for the temperature control of the plasma grid, a prototype of the grid with cooling/heating by circulating a high-temperature fluorinated fluid has been developed. This grid was found to have a capability to control the temperature with a time constant of 10s by considering the physical properties of the fluid. Based on this result, new PG temp-control system has been developed with all extraction area. In the preliminary results, 22A beam for 10s has been achieved.
Kojima, Atsushi; Hanada, Masaya; Kashiwagi, Mieko; Watanabe, Kazuhiro; Umeda, Naotaka; Tobari, Hiroyuki; Yoshida, Masafumi; Hiratsuka, Junichi; NB Heating Technology Group
no journal, ,
no abstracts in English
Kojima, Atsushi; NB Heating Technology Group
no journal, ,
In JAEA, R&D for negative ion accelerators for JT-60SA and ITER is going on in order to obtain the long pulse acceleration of negative ions over 60 s. Up to now, the critical issues were the low voltage holding capability and high heat load of the accelerators. As for the voltage holding capability, the multi-aperture large-area grid has been investigated in terms of the area, multi-aperture and multi-grid effect on the voltage holding. As a result, high energy accelerations of 500 keV and 1 MeV for JT-60SA and ITER-prototype accelerators have been achieved. As for the heat load, compensation techniques toward beam deflection due to magnetic field in the accelerator has been developed by using 3D beam calculation. By combining the extraction grid having high cooling capability to this compensation technique, low heat load of 12% has been achieved, and long pulse acceleration of 700 keV beams has been realized.
