Progress in long-pulse production of powerful negative ion beams for JT-60SA and ITER

Kojima, Atsushi; Umeda, Naotaka; Hanada, Masaya; Yoshida, Masafumi; Kashiwagi, Mieko; Tobari, Hiroyuki; Watanabe, Kazuhiro; Akino, Noboru; Komata, Masao; Mogaki, Kazuhiko; et al.

Nuclear Fusion, 55(6), p.063006_1 - 063006_9, 2015/06

https://doi.org/10.1088/0029-5515/55/6/063006

Significant progresses in the extension of pulse durations of powerful negative ion beams have been made to realize the neutral beam injectors for JT-60SA and ITER. In order to overcome common issues of the long pulse production/acceleration of negative ion beams in JT-60SA and ITER, the new technologies have been developed in the JT-60SA ion source and the MeV accelerator in Japan Atomic Energy Agency. As for the long pulse production of high-current negative ions for JT-60SA ion source, the pulse durations have been successfully increased from 30 s at 13 A on JT-60U to 100 s at 15 A by modifying the JT-60SA ion source, which satisfies the required pulse duration of 100 s and 70% of the rated beam current for JT-60SA. This progress was based on the R&D efforts for the temperature control of the plasma grid and uniform negative ion productions with the modified tent-shaped filter field configuration. Moreover, the each parameter of the required beam energy, current and pulse has been achieved individually by these R&D efforts. The developed techniques are useful to design the ITER ion source because the sustainment of the cesium coverage in large extraction area is one of the common issues between JT-60SA and ITER. As for the long pulse acceleration of high power density beams in the MeV accelerator for ITER, the pulse duration of MeV-class negative ion beams has been extended by more than 2 orders of magnitude by modifying the extraction grid with a high cooling capability and a high-transmission of negative ions. A long pulse acceleration of 60 s has been achieved at 70 MW/m (683 keV, 100 A/m) which has reached to the power density of JT-60SA level of 65 MW/m.

Improvement of uniformity of the negative ion beams by Tent-shaped magnetic field in the JT-60 negative ion source

Yoshida, Masafumi; Hanada, Masaya; Kojima, Atsushi; Kashiwagi, Mieko; Grisham, L. R.*; Akino, Noboru; Endo, Yasuei; Komata, Masao; Mogaki, Kazuhiko; Nemoto, Shuji; et al.

Review of Scientific Instruments, 85(2), p.02B314_1 - 02B314_4, 2014/02

https://doi.org/10.1063/1.4830365

Non-uniformity of the negative ion beams in the JT-60 negative ion source was improved by modifying an external magnetic field to a tent-shaped magnetic field for reduction of the local heat loads in the source. Distributions of the source plasmas (H ions and H atoms) of the parents of H ions converted on the cesium covered plasma grids were measured by Langmuir probes and emission spectroscopy. Beam intensities of the H ions extracted from the plasma grids were measured by IR camera from the back of the beam target plate. The tent-shaped magnetic field prevented the source plasmas to be localized by B grad B drift of the primary electrons emitted from the filaments in the arc chamber. As a result, standard derivation of the H ions beams was reduced from 14% (the external magnetic field) to 10% (the tent-shaped magnetic field) without reduction of an activity of the H ion production.

Progress in development and design of the neutral beam injector for JT-60SA

Hanada, Masaya; Kojima, Atsushi; Tanaka, Yutaka; Inoue, Takashi; Watanabe, Kazuhiro; Taniguchi, Masaki; Kashiwagi, Mieko; Tobari, Hiroyuki; Umeda, Naotaka; Akino, Noboru; et al.

Fusion Engineering and Design, 86(6-8), p.835 - 838, 2011/10

https://doi.org/10.1016/j.fusengdes.2011.04.068

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 30 MW D beams in total for 100 s. Each of the positive-ion-based NB injector is designed to inject 1.7 MW for 100s at 85 keV. 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 500 keV, 10 MW D beams for 100s is required, R&Ds of the negative ion source have been carried out. High-energy negative ion beams of 490-500 keV have been successfully produced at a beam current of 1-2.8 A 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 1 A to 500 keV. The design of the power supplies and the beamline is also in progress. The procurement of the acceleration power supply starts in 2010.

Achievement of 500 keV negative ion beam acceleration on JT-60U negative-ion-based neutral beam injector

Kojima, Atsushi; Hanada, Masaya; Tanaka, Yutaka*; Kawai, Mikito*; Akino, Noboru; Kazawa, Minoru; Komata, Masao; Mogaki, Kazuhiko; Usui, Katsutomi; Sasaki, Shunichi; et al.

Nuclear Fusion, 51(8), p.083049_1 - 083049_8, 2011/08

https://doi.org/10.1088/0029-5515/51/8/083049

Hydrogen negative ion beams of 490 keV, 3 A and 510 keV, 1 A have been successfully produced in the JT-60 negative ion source with three acceleration stages. These successful productions of the high-energy beams at high current have been achieved by overcoming the most critical issue, i.e., a poor voltage holding of the large negative ion sources with the grids of 2 m for JT-60SA and ITER. To improve voltage holding capability, the breakdown voltages for the large grids was examined for the first time. It was found that a vacuum insulation distance for the large grids was 6-7 times longer than that for the small-area grid (0.02 m). From this result, the gap lengths between the grids were tuned in the JT-60 negative ion source. The modification of the ion source also realized a significant stabilization of voltage holding and a short conditioning time. These results suggest a practical use of the large negative ion sources in JT-60SA and ITER.

Development and design of the negative-ion-based NBI for JT-60 Super Advanced

Hanada, Masaya; Akino, Noboru; Endo, Yasuei; Inoue, Takashi; Kawai, Mikito; Kazawa, Minoru; Kikuchi, Katsumi; Komata, Masao; Kojima, Atsushi; Mogaki, Kazuhiko; et al.

Journal of Plasma and Fusion Research SERIES, Vol.9, p.208 - 213, 2010/08

http://www.jspf.or.jp/JPFRS/PDF/Vol9/jpfrs2010_09-207.pdf

A large negative ion source with an ion extraction area of 110 cm 45 cm has been developed to produce 500 keV, 22 A D ion beams required for JT-60 Super Advanced. To realize the JT-60SA negative ion source, the JT-60 negative ion source has been modified and tested on the negative-ion-based neutral beam injector on JT-60U. A 500 keV H ion beam has been produced at 3 A without a significant degradation of beam optics. This is the first demonstration of a high energy negative ion acceleration of more than one-ampere to 500 keV in the world. The beam current density of 90 A/m is being increased to meet 130 A/m of the design value for JT-60SA by tuning the operation parameters. A long pulse injection of 30 s has been achieved at a injection D power of 3 MW. The injection energy, defined as the product of the injection time and power, reaches 80 MJ by neutralizing a 340 keV, 27 A D ion beam produced with two negative ion sources.

Recent R&D activities of negative-ion-based ion source for JT-60SA

Ikeda, Yoshitaka; Hanada, Masaya; Kamada, Masaki; Kobayashi, Kaoru; Umeda, Naotaka; Akino, Noboru; Ebisawa, Noboru; Inoue, Takashi; Honda, Atsushi; Kawai, Mikito; et al.

IEEE Transactions on Plasma Science, 36(4), p.1519 - 1529, 2008/08

https://doi.org/10.1109/TPS.2008.927382

The JT-60SA N-NBI system is required to inject 10 MW for 100 s at 500 keV. Three key issues should be solved for the JT-60SA N-NBI ion source. One is to improve the voltage holding capability. Recent R&D tests suggested that the accelerator with a large area of grids may need a high margin in the design of electric field and a long time for conditioning. The second issue is to reduce the grid power loading. It was found that some beamlets were strongly deflected due to beamlet-beamlet interaction and strike on the grounded grid. The grids are to be designed by taking account of beamlet-beamlet interaction in three-dimensional simulation. Third is to maintain the D- production for 100 s. A simple cooling structure is proposed for the active cooled plasma grid, where a key is the temperature gradient on the plasma grid for uniform D- production. The modified N-NBI ion source will start on JT-60SA in 2015.

Technical design of NBI system for JT-60SA

Ikeda, Yoshitaka; Akino, Noboru; Ebisawa, Noboru; Hanada, Masaya; Inoue, Takashi; Honda, Atsushi; Kamada, Masaki; Kawai, Mikito; Kazawa, Minoru; Kikuchi, Katsumi; et al.

Fusion Engineering and Design, 82(5-14), p.791 - 797, 2007/10

https://doi.org/10.1016/j.fusengdes.2007.05.077

Modification of JT-60U to a superconducting device (so called JT-60SA) has been planned to contribute to ITER and DEMO. The NBI system is required to inject 34 MW for 100 s. The upgraded NBI system consists of twelve positive ion based NBI (P-NBI) units and one negative ion based NBI (N-NBI) unit. The injection power of the P-NBI units are 2 MW each at 85 keV, and the N-NBI unit will be 10 MW at 500 keV, respectively. On JT-60U, the long pulse operation of 30 s at 2 MW (85 keV) and 20 s at 3.2 MW (320 keV) have been achieved on P-NBI and N-NBI units, respectively. Since the temperature increase of the cooling water in both ion sources is saturated within 20 s, further pulse extension up to 100 s is expected to mainly modify the power supply systems in addition to modification of the N-NBI ion source for high acceleration voltage. The detailed technical design of the NBI system for JT-60SA is presented.

Characteristics of voltage holding and light emission on the accelerator of JT-60U N-NBI ion source

Kikuchi, Katsumi; Akino, Noboru; Hanada, Masaya; Ikeda, Yoshitaka; Kamada, Masaki; Kawai, Mikito; Mogaki, Kazuhiko; Noto, Katsuya; Usui, Katsutomi

JAEA-Technology 2007-027, 17 Pages, 2007/03

JAEA-Technology-2007-027.pdf:2.3MB

Voltage holding capability of the 500 kV accelerator in the JT-60 negative ion source that is one of the key issues for high performance of the JT-60 negative-ion-based NBI system was investigated. The achieved voltage holding capabilities with and without the beam acceleration were 400 kV and 455 kV, respectively. To understand a poor voltage holding capability of the negative ion source, correlation between the voltage holding capability and the light emitted inside the ion source was carefully examined. The acceleration voltage was stably applied at 400kV, where the light intensity was almost zero. Increasing the acceleration voltage beyond 400 kV, the voltage holding become very unstable where the light intensity increases in proportion to the acceleration voltage. The spectroscopy measurement showed that the light spectrum was a broad wavelength of 360 - 500 nm peaked at 420 nm. There was no line spectrum due to the gas discharge such as hydrogen, oxygen, carbon. From these results, it is seemed that the origin of the light emission is a cathode luminescence from the FRP (Fiberglass Reinforced Plastic) insulator in JT-60 negative ion source due to the electron impact. Moreover, breakdown phenomena at inside and outside of the ion source were examined by using photo-multipliers with fast data acquisition system. When the breakdown occurred inside the ion source, the breakdowns sequentially occurred at the spark gap switches outside of the ion source, which protect the FRP insulator from the flashover on its surface. Once the spark gap was turned on after the breakdown inside the ion source, the breakdowns at the spark gap occurred at lower voltage than the normal set value when the high voltage was applied again after 70 ms interval. This result indicates that the voltage holding capability was limited by the spark gap switches in this operational sequence.

Preliminary thermal analyses of the beamline components in JT-60SA neutral beam injection system

Komata, Masao; Mogaki, Kazuhiko; Kazawa, Minoru; Hanada, Masaya; Ikeda, Yoshitaka

JAEA-Technology 2007-023, 41 Pages, 2007/03

JAEA-Technology-2007-023.pdf:10.41MB

In JT-60 Super Advanced (JT-60 SA) where the first plasma will be in 2014, D beams of 10 MW are designed to be injected for 100 s. The negative-ion-based neutral beam injection (N-NBI) system in JT-60U will be upgraded from existing JT-60U N-NBI system while the modification of existing components should be minimized. The feasibility on the further long pulse operation of the existing beamline components in the JT-60U N-NBI system, which has successfully injected 3.2 MW for 21 s, has been studied. The thermal characteristic of the beamline components during long pulse operation was estimated by a three-dimension analysis code (AMPS). As the result, it is found that most of the beamline components except for the 4th beam limiter without water cooling and a plasma grid and acceleration grids in the negative ion source are to be available without modification in the JT-60 SA N-NBI system. For the 4th beam limiter, the water cooling is required to withstand the power loading. For the acceleration grids, the power loading of the grounded grid should be reduced to a half of the present value to realize a 10 MW injection for 100 s.

Characteristics of voltage holding and outgassing on the accelerator of JT-60 N-NBI ion source

Kikuchi, Katsumi; Akino, Noboru; Ikeda, Yoshitaka; Usui, Katsutomi; Umeda, Naotaka; Oga, Tokumichi; Kawai, Mikito; Mogaki, Kazuhiko

JAEA-Technology 2006-016, 25 Pages, 2006/03

JAEA-Technology-2006-016.pdf:2.54MB

The 500 keV negative-ion based neutral beam injector (NBI) has been operated to heat plasma and drive plasma current on JT-60U since 1996. The ion source was designed to accelerate the negative ions up to 500 keV. During the last 10 years, the accelerated voltage of the negative ion beam has been limited to 400 keV by breakdowns in the accelerator. To understand the breakdown phenomena, the characteristics of the voltage holding of the ion source were studied without beam extraction. Outgassing with the main species of m/e=28 was observed when high voltage was applied even without breakdowns. It was noticed that the fraction of the main species at breakdown was almost the same as without breakdowns. Conditioning reduced the outgassing and resulted in improvement of the voltage holding capability. Inside the ion source, a brightening was observed even without breakdown. The brightening intensity was suppressed by increasing the D pressure in the accelerator in the range of 10 Pa to 0.5 Pa. Since the voltage holding was also improved with D pressure, breakdowns seemed to correlate with the brightening phenomena in the accelerator. This report gives the preliminary results of outgassing and brightening measurements when the high voltage was applied on the accelerator without beam extraction.