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Journal Articles

Characterization of edge radial electric field structures in the large helical device and their viability for determining the location of the plasma boundary

Kamiya, Kensaku; Ida, Katsumi*; Yoshinuma, Mikiro*; Suzuki, Chihiro*; Suzuki, Yasuhiro*; Yokoyama, Masayuki*; LHD Experimental Group*

Nuclear Fusion, 53(1), p.013003_1 - 013003_9, 2013/01

 Times Cited Count:15 Percentile:35.31(Physics, Fluids & Plasmas)

This paper provides and proposes a new technique to determine the location of the LCFS that is based on a characterization of the $$E$$$$_{r}$$ structure derived from CXS measurements in the LHD. We found that the spatial derivative in the $$E$$$$_{r}$$ structure had the local maximum value at the region very near, or possibly outside the vacuum LCFS location of vacuum magnetic field at the outer midplane in the low $$beta$$ plasma.

Journal Articles

Investigation of the helical divertor function and the future plan of a closed divertor for efficient particle control in the LHD plasma periphery

Shoji, Mamoru*; Masuzaki, Suguru*; Kobayashi, Masahiro*; Goto, Motoshi*; Morisaki, Tomohiro*; Yamada, Hiroshi*; Komori, Akio*; Iwamae, Atsushi; Sakaue, Atsushi*; LHD Experimental Group*

Fusion Science and Technology, 58(1), p.208 - 218, 2010/07

 Times Cited Count:9 Percentile:41.83(Nuclear Science & Technology)

Journal Articles

Particle transport of LHD

Tanaka, Kenji*; Kawahata, Kazuo*; Tokuzawa, Tokihiko*; Akiyama, Tsuyoshi*; Yokoyama, Masayuki*; Shoji, Mamoru*; Michael, C. A.*; Vyacheslavov, L. N.*; Murakami, Sadayoshi*; Wakasa, Arimitsu*; et al.

Fusion Science and Technology, 58(1), p.70 - 90, 2010/07

 Times Cited Count:16 Percentile:22.36(Nuclear Science & Technology)

Particle confinement processes were studied in detail on LHD. Diffusion coefficients (D) and convection velocities (V) were estimated from density modulation experiments. The magnetic configuration and collisionality were widely scanned in order to investigate parameter dependences of D and V. In order to study the effect of the magnetic configuration, magnetic axis positions (R$$_{ax}$$) were scanned from 3.5 m to 3.9 m. This scan changed the magnetic ripples quite significantly, enabling the effects of neoclassical properties on measured values to be widely elucidated. Dependences of electron temperature (T$$_{e}$$) and helically trapped normalized collsionality ($$nu$$$$_{h}^{*}$$), where $$nu$$$$_{h}^{*}$$=1 indicates a rough boundary between the 1/$$nu$$ and plateau regimes, were examined using the heating power scan of neutral beam injection (NBI). It was found out that generally larger (or smaller) contributions of neoclassical transport resulted in more hollow (or peaked) density profiles. The larger neoclassical contribution was found to be situated at a more outwardly shifted R$$_{ax}$$ for the same T$$_{e}$$, and higher T$$_{e}$$ or lower $$nu$$$$_{h}^{*}$$ at each R$$_{ax}$$. However, it is to be noted that R$$_{ax}$$=3.5 m showed different characteristics from these trends in that this case showed a more peaked density profile at higher T$$_{e}$$.

Journal Articles

Dynamics of ion internal transport barrier in LHD heliotron and JT-60U tokamak plasmas

Ida, Katsumi*; Sakamoto, Yoshiteru; Yoshinuma, Mikiro*; Takenaga, Hidenobu; Nagaoka, Kenichi*; Hayashi, Nobuhiko; Oyama, Naoyuki; Osakabe, Masaki*; Yokoyama, Masayuki*; Funaba, Hisamichi*; et al.

Nuclear Fusion, 49(9), p.095024_1 - 095024_9, 2009/09

 Times Cited Count:26 Percentile:26.42(Physics, Fluids & Plasmas)

Dynamics of ion internal transport barrier (ITB) formation and impurity transport both in the Large Helical Device (LHD) heliotron and JT-60U tokamak are described. Significant differences between heliotron and tokamak plasmas are observed. The location of the ITB moves outward during the ITB formation regardless of the sign of magnetic shear in JT-60U and the ITB becomes more localized in the plasma with negative magnetic shear. In LHD, the low Te/Ti ratio ($$<$$ 1) of the target plasma for the high power heating is found to be necessary condition to achieve the ITB plasma and the ITB location tends to expand outward or inward depending on the condition of the target plasmas. Associated with the formation of ITB, the carbon density tends to be peaked due to inward convection in JT-60U, while the carbon density becomes hollow due to outward convection in LHD. The outward convection observed in LHD contradicts the prediction by neoclassical theory.

Journal Articles

Dynamic transport study of the plasmas with transport improvement in LHD and JT-60U

Ida, Katsumi*; Sakamoto, Yoshiteru; Inagaki, Shigeru*; Takenaga, Hidenobu; Isayama, Akihiko; Matsunaga, Go; Sakamoto, Ryuichi*; Tanaka, Kenji*; Ide, Shunsuke; Fujita, Takaaki; et al.

Nuclear Fusion, 49(1), p.015005_1 - 015005_7, 2009/01

 Times Cited Count:13 Percentile:49.72(Physics, Fluids & Plasmas)

Transport analysis during the transient phase of heating (a dynamic transport study) applied to the plasma with internal transport barriers (ITBs) in the Large Helical Device (LHD) heliotron and the JT-60U tokamak is described. In the dynamic transport study the time of transition from the L-mode plasma to the ITB plasma is clearly determined by the onset of flattening of the temperature profile in the core region and a spontaneous phase transition from a zero curvature ITB (hyperbolic tangent shaped ITB) or a positive curvature ITB (concaved shaped ITB) to a negative curvature ITB (convex shaped ITB) and its back-transition are observed. The flattening of the core region of the ITB transition and the back-transition between a zero curvature ITB and a convex ITB suggest the strong interaction of turbulent transport in space.

Journal Articles

Experimental investigation of particle pinch associated with turbulence in LHD heliotron and JT-60U tokamak plasmas

Tanaka, Kenji*; Takenaga, Hidenobu; Muraoka, Katsunori*; Michael, C.*; Vyacheslavov, L. N.*; Yokoyama, Masayuki*; Yamada, Hiroshi*; Oyama, Naoyuki; Urano, Hajime; Kamada, Yutaka; et al.

Proceedings of 22nd IAEA Fusion Energy Conference (FEC 2008) (CD-ROM), 8 Pages, 2008/10

Comparative studies were carried out in LHD heliotron and JT-60U tokamak plasmas to elucidate the most essential parameter(s) for control of density profiles in toroidal systems. A difference in the collisionality dependence was found between the two devices. In LHD, the density peaking factor decreased with decrease of the collisionality at the magnetic axis position (R$$_{rm au}$$) 3.6 m, while the density peaking factor gradually increased with a decreased of collisionality at R$$_{rm au}$$ = 3.5 m. On the other hand, in JT-60U, the density peaking factor clearly increased with a decrease of the collisionality. The difference in the collisionality dependence between R$$_{rm au}$$ = 3.5 and R$$_{rm au}$$ = 3.6 m is likely due to the contribution of the anomalous transport. At R$$_{rm au}$$ = 3.5 m, larger anomalous transport caused a similar collisionality dependence. Change of the fluctuation property was observed with different density profiles in the plasma core region on both devices. In JT-60U, the increase of the radial coherence was observed with higher density peaking profile suggesting enhanced diffusion and inward directed pinch. For a magnetic axis positions (R$$_{rm au}$$) at 3.6 m in LHD, the increase of the fluctuation power with an increase in P$$_{rm NB}$$ was observed for a hollow density profile suggesting an increase on diffusion due to anomalous processes. Change of density profiles from peaked to hollow indicates change in the convection direction. This is due to increase in neoclassical processes. The reduction of the density peaking factor with increase of P$$_{rm NB}$$ in LHD is partly due to the neoclassical effect and partly due to the anomalous effect.

Journal Articles

Particle transport and fluctuation characteristics around the neoclassically optimized configurations in LHD

Tanaka, Kenji*; Michael, C.*; Vyacheslavov, L. N.*; Yokoyama, Masayuki*; Murakami, Sadayoshi*; Wakasa, Arimitsu*; Takenaga, Hidenobu; Muraoka, Katsunori*; Kawahata, Kazuo*; Tokuzawa, Tokihiko*; et al.

Plasma and Fusion Research (Internet), 3, p.S1069_1 - S1069_7, 2008/08

Density profiles in LHD were measured and particle transport coefficients were estimated from density modulation experiments in LHD. The dataset of different magnetic axis, toroidal magnetic filed and heating power provided data set of widely scanned neoclassical transport. At minimized neoclassical transport configuration ($$R$$$$_{rm ax}$$ = 3.5 m, $$B$$$$_{rm t}$$ = 2.8 T) showed peaked density profile. Its peaking factor increased gradually with decrease of collisional frequency. This is a similar result observed in JT-60U. At other configuration, peaking factor reduced with decrease of collsional frequency. Data set showed that larger contribution of neoclassical transport produced hollowed density profile. Comparison between neoclassical and estimated particle diffusivity showed different minimum condition. Clear difference of spatial profile of turbulence was observed between hollowed and peaked density profiles. Major part of fluctuation existed in the unstable region of ion temperature gradient mode.

Journal Articles

Application of tomographic imaging to multi-pixel bolometric measurements

Liu, Y.*; Tamura, Naoki*; Peterson, B. J.*; Iwama, Naofumi*; Konoshima, Shigeru; LHD Experimental Group*; JT-60 Team

Plasma and Fusion Research (Internet), 2, p.S1124_1 - S1124_4, 2007/11

Two improved tomographic algorithms, including a linear (Tikhonov-Phillips) and a nonlinear (maximum entropy) regularisation method, have been employed for multi-pixel bolometric measurements in order to get as much information as possible while keeping the assumptions to a minimum. The most important features of these improved methods are the capability of reconstructing radiation distributions without any symmetry assumptions, built-in smoothing, and useful reconstructions with relatively few detectors. Furthermore, the effects of finite detector size have been taken into account with a full three dimensional treatment of the detector geometry. The application of tomographic imaging was implemented to a two-array X-ray camera on the Large Helical Device (LHD) and a two-dimensional infrared imaging bolometric pinhole camera on JT-60U. Pertinent examples of the results are presented both to illustrate the analysis techniques and to demonstrate the wealth of physics which can be studied.

Journal Articles

Extended steady-state and high-beta regimes of net-current free heliotron plasmas in the Large Helical Device

Motojima, Osamu*; Yamada, Hiroshi*; Komori, Akio*; Oyabu, Nobuyoshi*; Muto, Takashi*; Kaneko, Osamu*; Kawahata, Kazuo*; Mito, Toshiyuki*; Ida, Katsumi*; Imagawa, Shinsaku*; et al.

Nuclear Fusion, 47(10), p.S668 - S676, 2007/10

 Times Cited Count:34 Percentile:22.29(Physics, Fluids & Plasmas)

The performance of net-current free heliotron plasmas has been developed by findings of innovative operational scenarios in conjunction with an upgrade of the heating power and the pumping/fuelling capability in the Large Helical Device (LHD). Consequently, the operational regime has been extended, in particular, with regard to high density, long pulse length and high beta. Diversified studies in LHD have elucidated the advantages of net-current free heliotron plasmas. In particular, an internal diffusion barrier (IDB) by a combination of efficient pumping of the local island divertor function and core fuelling by pellet injection has realized a super dense core as high as 5$$times$$10$$^{20}$$ m$$^{-3}$$, which stimulates an attractive super dense core reactor. Achievements of a volume averaged beta of 4.5% and a discharge duration of 54 min with a total input energy of 1.6 GJ (490 kW on average) are also highlighted. The progress of LHD experiments in these two years is overviewed by highlighting IDB, high-beta and long pulse.

Journal Articles

Comparison of boronized wall in LHD and JT-60U

Ashikawa, Naoko*; Kizu, Kaname; Yagyu, Junichi; Nakahata, Toshihiko*; Nobuta, Yuji; Nishimura, Kiyohiko*; Yoshikawa, Akira*; Ishimoto, Yuki*; Oya, Yasuhisa*; Okuno, Kenji*; et al.

Journal of Nuclear Materials, 363-365, p.1352 - 1357, 2007/06

 Times Cited Count:9 Percentile:41.44(Materials Science, Multidisciplinary)

no abstracts in English

Journal Articles

Extended steady-state and high-beta regimes of net-current free heliotron plasmas in the large helical device

Motojima, Osamu*; Yamada, Hiroshi*; Komori, Akio*; Oyabu, Nobuyoshi*; Kaneko, Osamu*; Kawahata, Kazuo*; Mito, Toshiyuki*; Muto, Takashi*; Ida, Katsumi*; Imagawa, Shinsaku*; et al.

Proceedings of 21st IAEA Fusion Energy Conference (FEC 2006) (CD-ROM), 12 Pages, 2007/03

The performance of net-current free Heliotron plasmas has been developed by findings of innovative operational scenarios in conjunction with an upgrade of the heating power and the pumping/fueling capability in the Large Helical Device (LHD). Consequently, the operational regime has been extended, in particular, with regard to high density, long pulse length and high beta. Diversified studies in LHD have elucidated the advantages of net-current free heliotron plasmas. In particular, an Internal Diffusion Barrier (IDB) by combination of efficient pumping of the local island divertor function and core fueling by pellet injection has realized a super dense core as high as 5$$times$$10$$^{20}$$m$$^{-3}$$, which stimulates an attractive super dense core reactor. Achievements of a volume averaged beta of 4.5 % and a discharge duration of 54-min. with a total input energy of 1.6 GJ (490 kW in average) are also highlighted. The progress of LHD experiments in these two years is overviewed with highlighting IDB, high $$beta$$ and long pulse.

Journal Articles

Observation of localized oscillations at ${it m/n}$=2/1 rational surface during counter neutral beam injection in the large helical device

Isayama, Akihiko; Inagaki, Shigeru*; Watanabe, Kiyomasa*; Narushima, Yoshiro*; Sakakibara, Satoru*; Funaba, Hisamichi*; Ida, Katsumi*; Nagayama, Yoshio*; Yamada, Hiroshi*; Kawahata, Kazuo*; et al.

Plasma Physics and Controlled Fusion, 48(1), p.L45 - L55, 2006/04

 Times Cited Count:12 Percentile:57.45(Physics, Fluids & Plasmas)

no abstracts in English

Journal Articles

Comparison of transient electron heat transport in LHD helical and JT-60U tokamak plasmas

Inagaki, Shigeru*; Takenaga, Hidenobu; Ida, Katsumi*; Isayama, Akihiko; Tamura, Naoki*; Takizuka, Tomonori; Shimozuma, Takashi*; Kamada, Yutaka; Kubo, Shin*; Miura, Yukitoshi; et al.

Nuclear Fusion, 46(1), p.133 - 141, 2006/01

 Times Cited Count:52 Percentile:12.61(Physics, Fluids & Plasmas)

no abstracts in English

Journal Articles

Comparison of transient electron heat transport in LHD helical and JT-60U tokamak plasmas

Inagaki, Shigeru*; Takenaga, Hidenobu; Ida, Katsumi*; Isayama, Akihiko; Tamura, Naoki*; Takizuka, Tomonori; Shimozuma, Takashi*; Kamada, Yutaka; Kubo, Shin*; Miura, Yukitoshi; et al.

Proceedings of 20th IAEA Fusion Energy Conference (FEC 2004) (CD-ROM), 8 Pages, 2004/11

no abstracts in English

Oral presentation

Mapping of flux quantities in the high beta heliotron plasmas

Watanabe, Kiyomasa*; Yamaguchi, Taiki; Narushima, Yoshiro*; Sakakibara, Satoru*; Suzuki, Yasuhiro*; Nakamura, Yuji*; Funaba, Hisamichi*; Nakajima, Noriyoshi*; LHD Experimental Group*

no journal, , 

no abstracts in English

Oral presentation

Particle transport in LHD and comparisons with tokamaks

Tanaka, Kenji*; Takenaga, Hidenobu; Muraoka, Katsunori*; Urano, Hajime; Michael, C.*; Vyacheslavov, L. N.*; Yokoyama, Masayuki*; Yamagishi, Osamu*; Murakami, Sadayoshi*; Wakasa, Arimitsu*; et al.

no journal, , 

In order to understand particle transport systematically in toroidal plasmas, a density profile was compared in LHD helical and JT-60U tokamak plasmas. In large tokamak devices such as JT-60U, the density profile is always peaked and the peaked density profile can be explained based on outward diffusion flux and inward convection flux driven by microinstability. A peaking factor of the density profile was increased with decreasing collisionality. On the other hand, the density profile was changed from peaked one to hollow one depending on discharge conditions in LHD. The hollow density profile can be explained based on outward convection flux driven by neoclassical transport and inward diffusion flux driven by microinstability. In the configuration with a small helical ripple, where the neoclassical transport is reduced, the density profile tended to be peaked and dependence of the peaking factor on the collisionality was similar to that in tokamak plasmas. These results indicated that magnetic field ripple and microinstability are some of the main mechanisms determining the density profile.

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