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8 results on '"Nuga, H."'

1. Charge-exchange measurements of high-energy fast ions in LHD using negative-ion neutral beam injection

Author

Hayashi, W. H. J., Heidbrink, W. W., Muscatello, C. M., Lin, D. J., Osakabe, M., Ogawa, K., Kawamoto, Y., Yamaguchi, H., Seki, R., Nuga, H., Isobe, M., Fujiwara, Y., and Kamio, S.

Subjects
Physics - Plasma Physics
Abstract

A new sightline geometry for the fast-ion D-alpha (FIDA) diagnostic on the Large Helical Device (LHD) has been confirmed to measure signals for high-energy fast ions produced by negative-ion neutral beam injection. The newly installed sightline uses a 180 keV tangential negative-ion neutral beamline as the active source. Due to the small angle between the beamline and FIDA sightline, the relative velocity between fast ions and injected neutrals is small. This allows for high-energy fast ions just below the beam injection energy to produce measurable Doppler-shifted FIDA emission. Experiments were conducted at LHD in order to compare the new sightline, which views a high-energy negative-ion tangential beamline, and the old sightline, which views a low-energy perpendicular positive-ion neutral beamline. The measured FIDA signal is validated against predictions from the synthetic fast-ion diagnostic code FIDASIM with a distribution function modelled by the 5D transport code GNET. The results of the experiment confirm that reducing the viewing angle with a tangential active beam allows FIDA diagnostic to view high-energy fast ions with a sufficient signal-to-noise ratio., Comment: 12 pages, 6 figures, 1 table

Published
2024

3. Core density profile control by energetic ion anisotropy in LHD

Author

Nishiura, M., Shimizu, A., Ido, T., Satake, S., Yoshinuma, M., Yanai, R., Nunami, M., Yamaguchi, H., Nuga, H., Seki, R., Fujita, K., Salewski, M., Nishiura, M., Shimizu, A., Ido, T., Satake, S., Yoshinuma, M., Yanai, R., Nunami, M., Yamaguchi, H., Nuga, H., Seki, R., Fujita, K., and Salewski, M.

Abstract

Electron and impurity ion density profiles have been controlled by using tangential and perpendicular neutral beams for plasma heating in a stellarator/heliotron for the first time. Reduced anisotropy of stored energy for energetic ion En?/Enjj has resulted in an inward electron and impurity transport, forming a core electron density peaking. Increased anisotropy leads to a flat or hollowed electron density profile with an impurity exhaust in a core region [Yoshinuma et al., Nucl. Fusion 49, 062002 (2009)]. A high confinement state of particles in LHD has yet to be achieved, except for a temporal state of an electron density peaking created by a pellet injection. As a pioneering and crucial research result, the operation of energetic ion anisotropy by neutral beams has newly demonstrated that the direction of the radial transport of bulk and impurity ions can be controlled. At the same time, the overall plasma performance rises in neutron flux and stored energy. On the other hand, the increase in the anisotropy flattens the density profile. This new finding holds promise for a control knob of nuclear fusion reactors to enhance fusion power output.

Published
2024

5. Degradation of fast-ion confinement depending on the neutral beam power in MHD quiescent LHD plasmas.

Author

Nuga, H., Seki, R., Ogawa, K., Yamaguchi, H., Kamio, S., Fujiwara, Y., Kawamoto, Y., Yoshinuma, M., Kobayashi, T., Takemura, Y., Isobe, M., Osakabe, M., and Yokoyama, M.

Subjects
*PLASMA beam injection heating, *NEUTRAL beams, *QUIESCENT plasmas, *NEUTRON emission, *ELECTRON density, *ELECTRON temperature
Abstract

We investigated the degradation of neutral beam (NB) fast-ion confinement depending on the NB power without magnetohydrodynamics instabilities in the Large Helical Device (LHD). In the LHD deuterium experiment, the neutron emission rate per NB power decreased by up to 20% with increasing injected NBs during a single discharge. Because there were no significant variations in the electron temperature and density, the NB shine-through rate, or the magnetic fluctuation due to the change in NB power, the reduction in the neutron emission rate indicates the degradation of the fast-ion confinement. In this paper, we formulated this degradation depending on the NB power and quantitatively estimated the degraded effective confinement time. In addition, we performed neutron emission rate simulations using the obtained effective confinement time. The simulation and experimental results were in good agreement, suggesting that the degraded effective confinement time is valid. [ABSTRACT FROM AUTHOR]

Published
2024
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6. Characteristics of neutron emission profile from neutral beam heated plasmas of the Large Helical Device at various magnetic field strengths

Author

Ogawa, K, Isobe, M, Shota, Sugiyama, A Spong, D, Sangaroon, S, Seki, R, Nuga, H, Yamaguchi, H, Kamio, S, Fujiwara, Y, I Kobayashi, M, Jo, J, Osakabe, M, Ogawa, K, Isobe, M, Shota, Sugiyama, A Spong, D, Sangaroon, S, Seki, R, Nuga, H, Yamaguchi, H, Kamio, S, Fujiwara, Y, I Kobayashi, M, Jo, J, and Osakabe, M

Abstract

The neutron emission profile of deuterium plasma in the Large Helical Device was measured with a multi-sightline vertical neutron camera under various magnetic field strength conditions. It was found that the line-integrated neutron emission profile shifts outward in the co-neutral beam (NB) case and inward in the counter NB case. Here, co- and counter directions correspond to enhance and reduce the poloidal magnetic field directions, respectively. The shift becomes more significant when the magnetic field decreased in strength. The experimentally obtained neutron emission profile was compared with the orbit-following models simulated through the DELTA5D code. The calculated neutron emission profiles vary according to the magnetic field strength because of the change of beam ion orbit and the slowing down due to the plasma parameter changes. Although a relatively narrow profile was obtained in the calculations at the inboard side for the co-NB case in the relatively low field condition, the profiles obtained through calculation and experiment were almost qualitatively aligned.

Published
2021

8. First application of data assimilation-based control to fusion plasma.

Author

Morishita Y, Murakami S, Kenmochi N, Funaba H, Yamada I, Mizuno Y, Nagahara K, Nuga H, Seki R, Yokoyama M, Ueno G, and Osakabe M

Abstract

Magnetic fusion plasmas, which are complex systems comprising numerous interacting elements, have large uncertainties. Therefore, future fusion reactors require prediction-based advanced control systems with an adaptive system model and control estimation robust to uncertainties in the model and observations. To address this challenge, we introduced a control approach based on data assimilation (DA), which describes the system model adaptation and control estimation based on the state probability distribution. The first implementation of a DA-based control system was achieved at the Large Helical Device to control the high temperature plasma. The experimental results indicate that the control system enhanced the predictive capability using real-time observations and adjusted the electron cyclotron heating power for a target temperature. The DA-based control system provides a flexible platform for advanced control in future fusion reactors., (© 2024. The Author(s).)

Published
2024
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