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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

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

Transition between internal transport barriers with different temperature-profile curvatures in JT-60U tokamak plasmas

Ida, Katsumi; Sakamoto, Yoshiteru; Takenaga, Hidenobu; Oyama, Naoyuki; Ito, Kimitaka*; Yoshinuma, Mikiro*; Inagaki, Shigeru*; Kobuchi, Takashi*; Isayama, Akihiko; Suzuki, Takahiro; et al.

Physical Review Letters, 101(5), p.055003_1 - 055003_4, 2008/08

 Times Cited Count:29 Percentile:20.96(Physics, Multidisciplinary)

A spontaneous transition phenomena between two meta-stable states of plasmas with internal transport barrier (ITB), that are characterized by different radial profiles of second derivative of ion temperature inside the ITB region where the ion temperature gradient is large, is observed in the steady-state phase of magnetic shear in the negative magnetic shear plasma in JT-60U tokamak. The curvature asymmetry factor evaluated from the radial profile of second derivative of ion temperature profiles changes from 0.08 (symmetric curvature ITB) to -0.43 (asymmetric curvature ITB) during transition phase.

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

Measurement of derivative of ion temperature using high spatial resolution charge exchange spectroscopy with space modulation optics

Ida, Katsumi; Sakamoto, Yoshiteru; Yoshinuma, Mikiro*; Inagaki, Shigeru*; Kobuchi, Takashi*; Matsunaga, Go; Koide, Yoshihiko

Review of Scientific Instruments, 79(5), p.053506_1 - 053506_6, 2008/05

 Times Cited Count:18 Percentile:35.26(Instruments & Instrumentation)

A new technique to measure the first and second derivative of the ion temperature profile has been developed using a charge exchange spectroscopy system with space modulation optics. The space observed is scanned up to $$pm$$3 cm with a cosine wave modulation frequency up to 30 Hz by shifting the object lens in front of the optical fiber bundle by 0.5 mm with a piezoelement. The first and second derivatives of ion temperature are derived from the modulation component of the ion temperature measured using Fourier analysis.

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

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

Comparison of electron internal transport barriers in the large helical device and JT-60U plasmas

Ida, Katsumi*; Fujita, Takaaki; Fukuda, Takeshi*; Sakamoto, Yoshiteru; Ide, Shunsuke; Toi, Kazuo*; Inagaki, Shigeru*; Shimozuma, Takashi*; Kubo, Shin*; Idei, Hiroshi*; et al.

Plasma Physics and Controlled Fusion, 46(5A), p.A45 - A50, 2004/05

 Times Cited Count:19 Percentile:43.57(Physics, Fluids & Plasmas)

no abstracts in English

Oral presentation

Development of fast charge exchange recombination spectroscopy diagnostic, and behaviors of ion temperature and plasma rotation at transport barrier in JT-60U

Sakamoto, Yoshiteru; Ida, Katsumi*; Yoshinuma, Mikiro*; Inagaki, Shigeru*; Kobuchi, Takashi*; Matsunaga, Go; Koide, Yoshihiko; Yoshida, Maiko; Takenaga, Hidenobu

no journal, , 

no abstracts in English

Oral presentation

Structure of internal transport barriers measured by modulation CXRS

Sakamoto, Yoshiteru; Ida, Katsumi*; Yoshinuma, Mikiro*; Inagaki, Shigeru*; Kobuchi, Takashi*; Matsunaga, Go; Koide, Yoshihiko; Fujita, Takaaki; Takenaga, Hidenobu

no journal, , 

no abstracts in English

Oral presentation

Electron density profile and turbulence in toroidal plasmas

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

no journal, , 

In order to understand mechanisms for determining density profiles in toroidal plasmas, density profiles were compared in JT-60U tokamak and LHD helical plasmas. Transport theory indicates that neoclassical transport is enhanced in helical plasmas with low collisionality due to helical ripple. In JT-60U plasmas, density peaking increased with decreasing the collisionality. In LHD plasmas for magnetic axis (Rax) of 3.5m with small effective helical ripple, density peaking slightly increased with decreasing the collisionality as similar to that in tokamak plasmas. On the other hand, in LHD plasmas for Rax$$>$$3.6m with relatively large effective helical ripple, density profile became hollow as the collisionality decreased. Different turbulence structures are observed for Rax=3.5m and Rax=3.6m in LHD plasmas. Turbulence propagated towards electron diamagnetic direction for Rax=3.5m and towards ion diamagnetic direction for Rax=3.6m. This difference could be related to the difference of density profiles, as well as difference of neoclassical transport. Furthermore, when density decreased in the core region due to increase of electron temperature, it was found that turbulence was first modified in the edge region and then in the core region.

Oral presentation

Response of turbulence under change of density profiles in toroidal devices

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

no journal, , 

Comparative studies were carried out in LHD heliotron and JT-60U tokamak plasmas to elucidate effects of turbulence transport on 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$$_{ax}$$) of 3.6 m. On the other hand, in JT-60U, the density peaking factor clearly increased with a decrease of the collisionality. For R$$_{ax}$$=3.6 m in LHD, the increase of the fluctuation power with an increase in P$$_{NB}$$ was observed for a hollow density profile suggesting an increase on diffusion due to anomalous processes. In JT-60U, the increase of the radial coherence was observed with higher density peaking profile suggesting enhanced diffusion and inward directed pinch. The effects of curvature pinch on density profiles were also investigated in both devices. The curvature pinch produces a peaked density profile in JT-60U and a hollow density profile in LHD depending on their magnetic shear. However, these effects were too small to explain the density profiles observed in both devices.

Oral presentation

Response of turbulence associated with the change of density profiles in LHD heliotron and JT-60U tokamak

Tanaka, Kenji*; Takenaga, Hidenobu; Muraoka, Katsunori*; Yoshida, Maiko; Michael, C.*; Vyacheslavov, L. N.*; Mikkelsen, D. R.*; Yokoyama, Masayuki*; Oyama, Naoyuki; Urano, Hajime; et al.

no journal, , 

Density profile and turbulence was compared in JT-60U tokamak and LHD heliotron. Density peaking increases with decrease of collisionality in JT-60U. Density gradient predicted from zero flux condition agrees within factor 2 for Te/Ti=1, but large discrepancies are found for Te/Ti $$<$$ =0.5. In LHD, peaked profile and increase of density peaking with decrease of collisionality are found in strong magnetic hill configuration (Rax = 3.5 m). Hollowed-peaked density profile and increase of density peaking with increase of collisionality are found in weak magnetic hill configuration (Rax = 3.6 m). Fluctuation is localized in core gradient region and edge gradient region. Density gradient predicted from zero flux condition are compared. Then both cases agrees the sign and absolute values within factor 2.

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