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Journal Articles

Turbulent transport reduction induced by transition on radial electric field shear and curvature through amplitude and cross-phase in torus plasma

Kobayashi, Tatsuya*; Ito, Kimitaka*; Ido, Takeshi*; Kamiya, Kensaku*; Ito, Sanae*; Miura, Yukitoshi; Nagashima, Yoshihiko*; Fujisawa, Akihide*; Inagaki, Shigeru*; Ida, Katsumi*

Scientific Reports (Internet), 7(1), p.14971_1 - 14971_8, 2017/11

 Times Cited Count:20 Percentile:81.1(Multidisciplinary Sciences)

Journal Articles

Experimental identification of electric field excitation mechanisms in a structural transition of tokamak plasmas

Kobayashi, Tatsuya*; Ito, Kimitaka*; Ido, Takeshi*; Kamiya, Kensaku; Ito, Sanae*; Miura, Yukitoshi; Nagashima, Yoshihiko*; Fujisawa, Akihide*; Inagaki, Shigeru*; Ida, Katsumi*; et al.

Scientific Reports (Internet), 6, p.30720_1 - 30720_7, 2016/08

 Times Cited Count:13 Percentile:67.28(Multidisciplinary Sciences)

In this Letter, we report model verifications for a key physics for the L-H transition. Radial electric field excitation, using discharges having a two-step transition. Examining time derivative of Poissons equation, it is found that the sum of the loss cone loss current and neoclassical bulk viscosity current agrees the experimentally observed current in the order of magnitude at the first transition. Following second transition and an ambipolar condition in the L-mode cannot be explained by the models.

Journal Articles

Dynamics of edge limit cycle oscillation in the JFT-2M tokamak

Kobayashi, Tatsuya*; Ito, Kimitaka*; Ido, Takeshi*; Kamiya, Kensaku; Ito, Sanae*; Miura, Yukitoshi; Nagashima, Yoshihiko*; Fujisawa, Akihide*; Inagaki, Shigeru*; Ida, Katsumi*; et al.

Nuclear Fusion, 54(7), p.073017_1 - 073017_14, 2014/07

 Times Cited Count:26 Percentile:79.9(Physics, Fluids & Plasmas)

In the JFT-2M tokamak, limit-cycle oscillation among several variables is observed before L-to-H transition. Spatiotemporal dynamics of the LCO is analyzed in detail. Zonal fows are not seen, and modulation in edge-localized poloidal flow and density gradient is observed. Modulation is also seen in Reynolds stress, caused by that in turbulence intensity and turbulence wavenumber. However, flow acceleration is not able to be explained by the modulation in the Reynolds stress. Rapid inward propagation in density gradient and turbulence packet is also observed. Characteristics of the propagation are veried by means of turbulence spreading theory and diffusion theory.

Journal Articles

Spatiotemporal structures of edge limit-cycle oscillation before L-to-H transition in the JFT-2M tokamak

Kobayashi, Tatsuya*; Ito, Kimitaka*; Ido, Takeshi*; Kamiya, Kensaku; Ito, Sanae*; Miura, Yukitoshi; Nagashima, Yoshihiko*; Fujisawa, Akihide*; Inagaki, Shigeru*; Ida, Katsumi*; et al.

Physical Review Letters, 111(3), p.035002_1 - 035002_5, 2013/07

 Times Cited Count:90 Percentile:94.52(Physics, Multidisciplinary)

In this Letter, we report analyses of spatiotemporal dynamics of turbulence and structure in the limit cycle oscillation that precedes L-to-H transition. Zonal flows are not observed during LCO, and the oscillation is the periodic generations/decays of barrier with edge localized mean flow. Oscillatory Reynolds stress is found too small to accelerate the LCO flow. Changes of the density gradient and turbulence amplitude are found to propagate into the core in a ballistic manner, suggesting a cause of fast change of core transport after H-mode transition.

Journal Articles

Experimental progress on zonal flow physics in toroidal plasmas

Fujisawa, Akihide*; Ido, Takeshi*; Shimizu, Akihiro*; Okamura, Shoichi*; Matsuoka, Keisuke*; Iguchi, Harukazu*; Hamada, Yasuji*; Nakano, Haruhisa*; Oshima, Shinsuke*; Ito, Kimitaka*; et al.

Nuclear Fusion, 47(10), p.S718 - S726, 2007/10

 Times Cited Count:99 Percentile:95.85(Physics, Fluids & Plasmas)

Present status of experiments on zonal flows is overviewed. Innovative use of modern diagnostics has revealed the existence of zonal flows, their spatio-temporal characteristics, their relationship with turbulence, and their effects on confinement. Particularly a number of observations have been accumulated on the oscillatory branch of the zonal flow, dubbed as geodesic acoustic modes suggesting necessity of theories to give their proper description. Several new methods have elucidated the zonal flow generation processes from the turbulence. Further investigation of relationship between the zonal flows and confinement is strongly encouraged as cross-device activity.

Journal Articles

In search of zonal flows by using direct density fluctuation measurements

Nagashima, Yoshihiko*; Ito, Kimitaka*; Ito, Sanae*; Fujisawa, Akihide*; Yagi, Masatoshi*; Hoshino, Katsumichi; Shinohara, Koji; Ejiri, Akira*; Takase, Yuichi*; Ido, Takeshi*; et al.

Plasma Physics and Controlled Fusion, 49(10), p.1611 - 1625, 2007/10

 Times Cited Count:42 Percentile:80.85(Physics, Fluids & Plasmas)

It was found that the envelope of the ambient density fluctuation of the drift wave turbulence has spectral peaks aroound zonal flow frequencies. A method to detect the zonal flow using this characteristic was considered using the JFT-2M density fluctuation data. There is a possibility that the zonal flow will be able to be detected in the future burning plasmas by using the density fluctuations measured by some density fluctuation measurement by such as the microwave reflectometer and so on.

Journal Articles

Measurement and analysis of the fluctuations and poloidal flow on JFT-2M tokamak

Hoshino, Katsumichi; Ido, Takeshi*; Nagashima, Yoshihiko*; Shinohara, Koji; Ogawa, Hiroaki; Kamiya, Kensaku; Kawashima, Hisato; Tsuzuki, Kazuhiro*; Kusama, Yoshinori; Oasa, Kazumi; et al.

Proceedings of 21st IAEA Fusion Energy Conference (FEC 2006) (CD-ROM), 8 Pages, 2007/03

We report on the potential/density fluctuations in the JFT-2M tokamak. We identified a geodesic acoustic mode (GAM) and its electric field structure. The GAM interacts with the background turbulence in consistent with a drift wave - zonal flow theory. The GAM modulates the turbulence, and the suppression depends on the direction and gradient of the flow velocity in consistent with a theory. During the H-mode, the GAM disappears presumably due to the suppression of the turbulence. We analysed the low frequency potential oscillation during the H-mode. A wavelet analysis showed a low frequency characteristic potential fluctuations (several handred kHz), which disappears in the L-mode and at the occurence of ELM. Observed poloidal flow is about 20 times as large as that of the GAM during the H-mode. If the potential has a spatial inhomogineity, there is a possibility that this low frequency fluctuation comes from the EXB flow or the zonal flow.

Journal Articles

Experimental progress on zonal flow physics in toroidal plasmas

Fujisawa, Akihide*; Ido, Takeshi*; Shimizu, Akihiro*; Okamura, Shoichi*; Matsuoka, Keisuke*; Hamada, Yasuji*; Hoshino, Katsumichi; Nagashima, Yoshihiko*; Shinohara, Koji; Nakano, Haruhisa*; et al.

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

Present status of experiments on zonal flows is overviewed. Innovative use of traditional and modern diagnostics has revealed unambiguously the existense of the zonal flows, their spatio-temporal caracteristics, their relationship with turbulence, and their effects on confinement. Particularly, a number of observations have been accumulated on the oscillatory branch of zonal flows, dubbed geodesic acoustic modes, suggesting necessity of theories to give their proper description. Furthur investigation of relationship between zonal flows and confinement is strongly encouraged as cross-device activity.

Journal Articles

Advanced studies by using the geodesic acoustic mode measurements; Experimental identification of the separatrix location and search for zonal flows from the envelope of turbulent density fluctuations

Nagashima, Yoshihiko*; Ito, Kimitaka*; Fujisawa, Akihide*; Ito, Sanae*; Yagi, Masatoshi*; Hoshino, Katsumichi; Shinohara, Koji; Ido, Takeshi*; Miura, Yukitoshi; Ejiri, Akira*; et al.

Europhysics Conference Abstracts (CD-ROM), 31F, 4 Pages, 2007/00

The recent results on the identification of the outer most magnetic surface and on the identification of the characteristic of the zonal flow are presented by using the measured data of the zonal flow which is a plasma flow in the tokamak plasma. The positon of the outermost magnetic surface was identified using the characteristic of the oscillating zonal flow called as the geodesic acoutic mode (GAM) that the mode exists only in the closed magnetic surface, and the position of the surface was in agreement with that by the magnetic equilibrium reconstruction. Some coincidence with the drift wave - zonal flow theory was obtained by using the finding that the envelope of the density fluctuation of the background turbulence is modulated by the GAM.

Journal Articles

Compatibility of reduced activation ferritic steel wall with high performance plasma on JFT-2M

Tsuzuki, Kazuhiro; Kamiya, Kensaku; Shinohara, Koji; Bakhtiari, M.*; Ogawa, Hiroaki; Kurita, Genichi; Takechi, Manabu; Kasai, Satoshi; Sato, Masayasu; Kawashima, Hisato; et al.

Nuclear Fusion, 46(11), p.966 - 971, 2006/11

 Times Cited Count:16 Percentile:49.57(Physics, Fluids & Plasmas)

no abstracts in English

Journal Articles

Experimental study of nonlinear processes in edge turbulence of toroidal plasmas

Nagashima, Yoshihiko*; Hoshino, Katsumichi; Nagaoka, Kenichi*; Shinohara, Koji; Fujisawa, Akihide*; Uehara, Kazuya; Kusama, Yoshinori; Ida, Katsumi*; Yoshimura, Yasuo*; Okamura, Shoichi*; et al.

Plasma and Fusion Research (Internet), 1(9), p.041_1 - 041_15, 2006/09

Nonlinear processes between meso-scale structures and turbulent fluctuations in the edge region of the toroidal plasmas were studied using the bispectrum analysis. In the JFT-2M tokamak, divertor plasmas were investigated as well as the limiter plasmas, and the amplitude and phase of the bispectrum function of the potential fluctuation were found to be consistent with an analysis of the drift wave-zonal flow system. In the CHS machine, the edge plasmas during the H-mode were investigated. It was found that the edge potential begins to decrease before the decrease of the intensity of the deuterium line (by about 30ms), and the edge fluctuations are found to be composed of low frequency MHD fluctuations, drift wave like broad band electrostatic fluctuations and high frequency electromagnetic fluctuations. Nonlinear coupling between the MHD fluctuations and the electrostatic fluctuations or the electromagnetic fluctuations were found for the first time.

Journal Articles

Direct identification of magnetic surface by a differential double probe in JFT-2M

Uehara, Kazuya; Tsuzuki, Kazuhiro; Amemiya, Hiroshi*; Nagashima, Yoshihiko*; Hoshino, Katsumichi; Sadamoto, Yoshiro*

Japanese Journal of Applied Physics, Part 2, 45(24), p.L630 - L633, 2006/07

 Times Cited Count:3 Percentile:13.2(Physics, Applied)

Data for detecting the magnetic flux surfaces at the edge plasma in tokamak are presented. When the magnetic surfaces are moved by the plasma jog, it is observed that a current increment appears in the two set of double probe whose front surfaces are shifted by a small distance and that the difference of these two currents through the differential amplifier giving a signal of the arrival of the separatrix also appears, where we refer the separatrix point as a typical position of the magnetic flux surfaces. It is confirmed that magnetic flux surfaces are located by a few cm outward from that calculated by the eqfit-code (EQFIT) and the absolute value of its deviation is identified. This identification is supported by the result of observation of the correlation of the floating potential fluctuation between two different points using one probe pin of the same differential double probe. The ion temperature and the space potential are observed by the another use of the differential double probe are modified following the estimated real magnetic surfaces positions.

Journal Articles

Observation of the interaction between the geodesic acoustic mode and ambient fluctuation in the JFT-2M tokamak

Ido, Takeshi*; Miura, Yukitoshi; Hoshino, Katsumichi; Kamiya, Kensaku; Hamada, Yasuji*; Nishizawa, Akimitsu*; Kawasumi, Yoshiaki*; Ogawa, Hiroaki; Nagashima, Yoshihiko*; Shinohara, Koji; et al.

Nuclear Fusion, 46(5), p.512 - 520, 2006/05

 Times Cited Count:85 Percentile:93.24(Physics, Fluids & Plasmas)

The electrostatic fluctuation spectrum and the fluctuation-induced particle flux of the JFT-2M tokamak were estimated using the hevy ion beam probe (HIBP) measurement. A geodesic acoustic mode (GAM) of the frequency about15kHz was identified in the L-mode. The GAM has its peak at about 3cm inside of the separatrix with the electric field of about 1.4kV/m. The estimated turbulent particle flux is found to be intermittent. It is found that the density fluctuation is modulated by the GAM in the wide frequency range and the coherence analysis shows that the flucruation-induced particle flux is partially contributed by the GAM. In the H-mode the GAM disappears and the fluctuation and the flux is much decreased.The mechanism of the large burst-like flux in the L-mode is not understood yet and left as the future problem.

Journal Articles

Observation of coherent bicoherence and biphase in potential fluctuations around geodesic acoustic mode frequency on JFT-2M

Nagashima, Yoshihiko*; Ito, Kimitaka*; Ito, Sanae*; Hoshino, Katsumichi; Fujisawa, Akihide*; Ejiri, Akira*; Takase, Yuichi*; Yagi, Masatoshi*; Shinohara, Koji; Uehara, Kazuya; et al.

Plasma Physics and Controlled Fusion, 48(5A), p.A377 - A386, 2006/05

 Times Cited Count:24 Percentile:63.2(Physics, Fluids & Plasmas)

A bispector analysis and envelope moduration analysis of the potential oscillations of a low frequency (1kHz) oscillation, a high frequency (10-15kHz) oscillation and background turbulence are presented. The low frequency mode and the high frequency mode are considered to be a low frequency zonal flow and a geodesic acoustic mode respectively. We could found that the low frequency oscillation has a three wave interaction with the background turbulence as well as the high frequency oscillation does from the bicoherence (nonlinear coupling coefficient), biphase and envelope moduration relation. Apparently, the modulation of the high frequency mode by the low frequency mode was suggested, but it was found that the bicoherence between them is small and the modulation relation was not concluded. Therefore, it is concluded that the nonlinear interaction between the zonal flow - zonal flow system does not exist or the interaction of them is linear if any. Thus it was found that the marked nonlinear interaction occurs only in the turbulence - zonal flow system and not in the zonal flow - zonal flow system.

Journal Articles

Bispectral analysis applied to coherent floating potential fluctuations obtained in the edge plasmas on JFT-2M

Nagashima, Yoshihiko*; Ito, Kimitaka*; Ito, Sanae*; Fujisawa, Akihide*; Hoshino, Katsumichi; Takase, Yuichi*; Yagi, Masatoshi*; Ejiri, Akira*; Ida, Katsumi*; Shinohara, Koji; et al.

Plasma Physics and Controlled Fusion, 48(4), p.S1 - S15, 2006/04

 Times Cited Count:35 Percentile:74.9(Physics, Fluids & Plasmas)

This paper presents the results of bispectral analysis of floating potential fluctuations in the edge region of ohmically heated plasmas in the JFT-2M tokamak. Inside of the outermost magnetic surface,coherent modes were observed around the frequency of geodesic acoustic mode which is a kind of the zonal flow. The squared bicoherence shows significant nonlinear couplings between the coherent fluctuations and the background fluctuations (which are likely to contain drift wave turbulent fluctuations). The experimental results that the total bicoherence is proportional to the squared amplitude of the coherent fluctuation, and that the biphase of the coherent modes localizes around a constant value $$pi$$, are consistent with the theoretical prediction on the drift wave - zonal flow systems based on the Hasegawa-Mima model.

Journal Articles

Convergence study of bispectral analysis in experiments of high temperature plasmas

Nagashima, Yoshihiko*; Ito, Sanae*; Yagi, Masatoshi*; Ito, Kimitaka*; Fujisawa, Akihide*; Hoshino, Katsumichi; Shinohara, Koji; Uehara, Kazuya; Kusama, Yoshinori; Ejiri, Akira*; et al.

Review of Scientific Instruments, 77(4), p.045110_1 - 045110_6, 2006/04

 Times Cited Count:10 Percentile:47.97(Instruments & Instrumentation)

A convergence study of bispector non-linear analysis applied to the fluctuation of toroidal plasmas is studied. A theoretical study of the convergence is presented. And it was found that the convergence property of the total bicoherence and bispector of the fluctuations of drift wave or zonal flow observed in the JFT-2M tokamak plasma is consistent with the theoretical analysis, and also found that the realization number for the convergence is fairly large, for example about 1000 for the JFT-2M experimental fluctuation data. As stated, we made a base for such fluctuation analysis.

Journal Articles

Plasma diagnostics in JFT-2M

Kasai, Satoshi*; Kamiya, Kensaku; Shinohara, Koji; Kawashima, Hisato; Ogawa, Hiroaki; Uehara, Kazuya; Miura, Yukitoshi; Okano, Fuminori; Suzuki, Sadaaki; Hoshino, Katsumichi; et al.

Fusion Science and Technology, 49(2), p.225 - 240, 2006/02

 Times Cited Count:3 Percentile:24.8(Nuclear Science & Technology)

The diagnostic system of JFT-2M has consisted of about 30 individual diagnostic instruments,which were used to study the plasma production, control, equilibrium, stability, confinement, plasma heating by NBI and/or RF (LH, ICRF, ECH) and current drive by RF. In these instruments, the motional Stark effect (MSE) polarimeter, charge exchanged recombination spectroscopy (CXRS), heavy-ion beam probe (HIBP), time of flight (TOF) neutral particle analyzer, etc. have contributed to make clear the improved mechanism of confinement such as H-mode and High Recycling Steady (HRS) H-mode, and to search the operational region of these modes.The infrared TV camera system and lost ion probe have played a very important role to investigate the heat load onto the walls due to ripple loss particles and escaping ions from core plasma, respectively.

Journal Articles

Study of SOL/divertor plasmas in JFT-2M

Kawashima, Hisato; Sengoku, Seio; Uehara, Kazuya; Tamai, Hiroshi; Shoji, Teruaki*; Ogawa, Hiroaki; Shibata, Takatoshi; Yamamoto, Masahiro*; Miura, Yukitoshi; Kusama, Yoshinori; et al.

Fusion Science and Technology, 49(2), p.168 - 186, 2006/02

 Times Cited Count:2 Percentile:17.66(Nuclear Science & Technology)

Experimental efforts on JFT-2M have been devoted to understand SOL/Divertor plasmas and to investigate power and particle controllability. Open divertor configuration was used for the first decade of JFT-2M started in 1984. We found out the SOL/Divertor plasma properties such as in/out asymmetry, heat and particle diffusivities, and SOL current at ELMs. Handling of power and particle was demonstrated by active control methods such as local pumping, edge ergodization, divertor biasing, and edge heating. For improvement of power and particle control capability of divertor, it was modified to closed configuration in 1995, which demonstrated the baffling effects with narrower divertor throat. Dense and cold divertor state (n$$_{e}$$$$^{div}$$ = 4$$times$$10$$^{19}$$ m$$^{-3}$$ and T$$_{e}$$$$^{div}$$ = 4 eV), compatible with the improved confinement modes (e.g. H-mode), was realized by strong gas puffing. Being related with the core confinement at H-mode, the edge plasma fluctuations were identified by an electrostatic probe. These are reviewed in this paper.

Journal Articles

Characteristics of plasma operation with the ferritic inside wall and its compatibility with high-performance plasmas in JFT-2M

Tsuzuki, Kazuhiro*; Kimura, Haruyuki; Kusama, Yoshinori; Sato, Masayasu; Kawashima, Hisato; Kamiya, Kensaku; Shinohara, Koji; Ogawa, Hiroaki; Uehara, Kazuya; Kurita, Genichi; et al.

Fusion Science and Technology, 49(2), p.197 - 208, 2006/02

 Times Cited Count:11 Percentile:61.42(Nuclear Science & Technology)

no abstracts in English

Journal Articles

Studies of radial electric field formation and its effect on fluctuations and transport barriers in the JFT-2M tokamak

Ida, Katsumi*; Miura, Yukitoshi; Ido, Takeshi*; Nagashima, Yoshihiko*; Shinohara, Koji

Fusion Science and Technology, 49(2), p.122 - 138, 2006/02

 Times Cited Count:5 Percentile:37.35(Nuclear Science & Technology)

The mechanism of E$$times$$B flow formation, the effect of E$$times$$B flow on parallel flow, the reduction of fluctuation by the shearing effect of E$$times$$B flow, the relation between GAM and density fluctuation are discussed based on the experiments using various Er measurements on JFT-2. The experiments in the plasma with H-mode and counter-NB mode shows that the feedback loop of the E$$times$$B flow shear, fluctuation suppression and an increase of ion diamagnetic flow is a key of the formation of the transport barrier in toroidal plasmas. Two important effects of radial electric field are presented, one is fluctuation suppression by E$$times$$B flow shear and the other is a drive of parallel flow by radial electric field, which explain a driving mechanism of a spontaneous toroidal flow. The relation between GAM and density fluctuation is also discussed. The GAM is observed to be excited by the non-linear coupling of density fluctuations, while GAM itself affects the amplitude of density fluctuation.

49 (Records 1-20 displayed on this page)