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Journal Articles

Investigations of impurity seeding and radiation control for long-pulse and high-density H-mode plasmas in JT-60U

Asakura, Nobuyuki; Nakano, Tomohide; Oyama, Naoyuki; Sakamoto, Yoshiteru; Matsunaga, Go; Itami, Kiyoshi

Nuclear Fusion, 49(11), p.115010_1 - 115010_8, 2009/11

 Times Cited Count:54 Percentile:9.31(Physics, Fluids & Plasmas)

Impurity gas seeding is one of techniques to decrease peak heat flux to the divertor both in steady-state and transient phases. Power handling by large radiation power loss has been studied in the ELMy H-mode plasmas on JT-60U with argon (Ar) gas seeding since good confinement ($$H$$$$_{rm H}$$ $$>$$ 0.85) was obtained up to high density ($$n$$$$_{rm e}$$/$$n$$$$^{rm GW}$$ = 0.8-0.9, $$n$$$$^{rm GW}$$ is the Greenwald density). On the other hand, it was not clearly understood to sustain the good confinement plasma with the large radiation power under the wall saturated condition where particle recycling flux changes during the long discharge. In this paper, control of the large radiation in the good energy confinement plasma was, for the first time, investigated. Total radiation fraction of $$P$$$$_{rm rad}$$/$$P$$$$_{rm abs}$$ = 0.7-0.95 was maintained for the cases of Ar gas puffing and combination of Ar and Ne puffing. Control of radiation power in the main plasma has been investigated using feedback control of gas puff rate during outgasing condition from the PFCs.

Journal Articles

Compact DEMO, SlimCS; Design progress and issues

Tobita, Kenji; Nishio, Satoshi; Enoeda, Mikio; Kawashima, Hisato; Kurita, Genichi; Tanigawa, Hiroyasu; Nakamura, Hirofumi; Honda, Mitsuru; Saito, Ai*; Sato, Satoshi; et al.

Nuclear Fusion, 49(7), p.075029_1 - 075029_10, 2009/07

 Times Cited Count:123 Percentile:1.62(Physics, Fluids & Plasmas)

Recent design study on SlimCS focused mainly on the torus configuration including blanket, divertor, materials and maintenance scheme. For vertical stability of elongated plasma and high beta access, a sector-wide conducting shell is arranged in between replaceable and permanent blanket. The reactor adopts pressurized-water-cooled solid breeding blanket. Compared with the previous advanced concept with supercritical water, the design options satisfying tritium self-sufficiency are relatively scarce. Considered divertor technology and materials, an allowable heat load to the divertor plate should be 8 MW/m$$^{2}$$ or lower, which can be a critical constraint for determining a handling power of DEMO (a combination of alpha heating power and external input power for current drive).

Journal Articles

Statistical analysis of fluctuation characteristics at high- and low-field sides in L-mode SOL plasmas of JT-60U

Tanaka, Hirohiko*; Ono, Noriyasu*; Asakura, Nobuyuki; Tsuji, Yoshiyuki*; Kawashima, Hisato; Takamura, Shuichi*; Uesugi, Yoshihiko*; JT-60U Team

Nuclear Fusion, 49(6), p.065017_1 - 065017_7, 2009/06

 Times Cited Count:27 Percentile:24.94(Physics, Fluids & Plasmas)

Comparison between fluctuation characteristics at high-field-side (HFS) and low-field-side (LFS) scrape off layers (SOLs) has been made, for the first time, in the L mode plasma of the JT-60U tokamak using reciprocating Langumuir probes. Statistical analysis based on probability distribution function (PDF) was employed to describe intermittent (non-diffusion) transport in SOL plasma fluctuations. The positive bursty events appeared most frequently at LFS midplane associated with blobby plasma transport, then the PDF is strongly skewed positively, while the PDF in HFS SOL is close to Gaussian distribution. Conditional averaging analysis of the positive bursty events at LFS midplane indicates the intermittent feature with a rapid increase and a slow decay is similar to that of plasma blobs theoretically predicted. Statistical self-similarity was also investigated with Fourier power spectrum and statistics of waiting-time and duration-time of the fluctuation.

Journal Articles

Evaluation on failure resistance to develop design basis for quasi-ductile silicon carbide composites for fusion application

Nozawa, Takashi; Hinoki, Tatsuya*; Koyama, Akira*; Tanigawa, Hiroyasu

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

Silicon carbide composites (SiC/SiC) are promising candidate materials for fusion reactors with advanced features such as high thermal efficiency. This paper will provide a present status in development of design basis for quasi-ductile SiC/SiC as a structural application. For that purpose, failure behavior, i.e., matrix cracking behavior, of a new class of radiation-resistant SiC/SiC, e.g., a nano-infiltration transient-eutectic phase (NITE) SiC/SiC, was evaluated. With a fact of the notch-insensitivity and very minor size effect on the failure behavior, a stress criterion is suggested in failure of NITE-SiC/SiC.

Journal Articles

-1 MV DC UHV power supply for ITER NBI

Watanabe, Kazuhiro; Yamamoto, Masanori; Takemoto, Jumpei; Yamashita, Yasuo*; Dairaku, Masayuki; Kashiwagi, Mieko; Taniguchi, Masaki; Tobari, Hiroyuki; Umeda, Naotaka; Sakamoto, Keishi; et al.

Proceedings of 22nd IAEA Fusion Energy Conference (FEC 2008) (CD-ROM), 7 Pages, 2008/10

The JADA contributes procurement of DC -1 MV ultra-high voltage (UHV) components such as a -1 MV DC generator, a transmission line and a -1 MV insulating transformer. The DC UHV insulation has been carefully analyzed since DC long pulse insulation is quite different from conventional AC insulation. The insulation structure has been designed and the overall dimensions of the DC UHV components have been finalized. The JADA contributes to provide an effective surge suppression system composed of core snubbers and resistors. Input energy into the accelerator from the power supply can be reduced to about 20 J which satisfies the design criteria of the total 50 J at -1 MV breakdown. From these studies, major technical issues were considered and the functional technical specifications of the UHV components have been developed for the procurement by the JADA.

Journal Articles

Effects of a toroidal rotation on the stability boundary of the MHD modes in the tokamak edge pedestal

Aiba, Nobuyuki; Tokuda, Shinji; Furukawa, Masaru*; Oyama, Naoyuki; Ozeki, Takahisa

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

Effects of a toroidal rotation are investigated numerically on the stability of the MHD modes in the edge pedestal, which relate to the type-I edge-localized mode (ELM). A new linear MHD stability code MINERVA is developed for solving the Frieman-Rotenberg equation, which is the linear ideal MHD equation with flow. As the result of the stability analysis, it is revealed that the sheared toroidal rotation destabilizes the edge localized MHD modes. The change of the safety factor profile affects this destabilizing effect. This is because the rotation effect on the edge MHD stability becomes stronger as the toroidal mode number of the unstable MHD mode increases, and this toroidal mode number strongly depends on the safety factor profile.

Journal Articles

Pedestal stability comparison and ITER pedestal prediction

Snyder, P. B.*; Aiba, Nobuyuki; Beurskens, M.*; Groebner, R. J.*; Horton, L. D.*; Hubbard, A. E.*; Hughes, J. W.*; Huysmans, G. T. A.*; Kamada, Yutaka; Kirk, A.*; et al.

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

Investigation of intermediate wavelength MHD modes has led to improved understanding of important constraints on the pedestal height and the mechanism for ELMs. The combination of high resolution pedestal diagnostics and a suite of highly efficient stability codes, has made edge stability analysis routine on several major tokamaks, contributing both to understanding, and to experimental planning and performance optimization. Here we present extensive comparisons of observations to predicted edge stability boundaries on several tokamaks, both for the standard ELM regime, and for small ELM and ELM-free regimes. We further use the stability constraint on pedestal height to test models of the pedestal width, and self-consistently combine a simple width model with MHD stability calculations to develop a new predictive model (EPED1) for the pedestal height and width. This model is tested against experimental measurements, and used in initial predictions of the pedestal height for ITER.

Journal Articles

Long pulse/high power ECRF system development in JT-60U

Moriyama, Shinichi; Kobayashi, Takayuki; Isayama, Akihiko; Terakado, Masayuki; Sawahata, Masayuki; Suzuki, Sadaaki; Yokokura, Kenji; Shimono, Mitsugu; Hasegawa, Koichi; Hiranai, Shinichi; et al.

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

In the gyrotron development in JT-60U ECRF system, output power of 1.5 MW for 1 s has been achieved at 110 GHz. It is the world highest power oscillation $$>$$ 1 s. In addition to the carefully designed cavity and collector in view of thermal stress, an RF shield for the adjustment bellows, and a low-dielectric-loss DC break enabled this achievement. Power modulation technique by anode voltage control was improved to obtain high modulation frequency and 5 kHz has been achieved for NTM stabilizing experiments. Long pulse demonstration of 0.4 MW, 30 s injection to the plasma has been achieved with real time control of anode/cathode-heater. It has been confirmed that the temperature of cooled components were saturated and no evidence of damage were found. An innovative antenna having wide range of beam steering capability with linearly-moving-mirror concept has been designed for long pulse. Beam profile and mechanical strength analyses shows the feasibility of the antenna.

Journal Articles

Integrated simulation of ELM energy loss and cycle in improved H-mode plasmas

Hayashi, Nobuhiko; Takizuka, Tomonori; Aiba, Nobuyuki; Oyama, Naoyuki; Ozeki, Takahisa; Wiesen, S.*; Parail, V.*

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

Journal Articles

Conservative global gyrokinetic toroidal full-$$f$$ 5D Vlasov simulation

Idomura, Yasuhiro; Tokuda, Shinji; Aiba, Nobuyuki; Urano, Hajime

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

A new global gyrokinetic toroidal full-$$f$$ five dimensional Vlasov simulation GT5D is developed based on the gyrokinetic theory and the linear Fokker-Planck collision operator. In a normal shear tokamak, source driven ion temperature gradient turbulence simulations are performed, and turbulent transport and profile evolutions are evolved over a collision time. It is found that a significant part of the heat flux is carried by avalanches in source free regions, where stiff temperature profiles are sustained. In this region, the equilibrium radial electric field $$E_r$$ becomes significantly larger than the neoclassical level, and its shear strongly affects the directions of the avalanche propagation and the momentum flux. A non-diffusive momentum flux due to the $$E_r$$ shear is observed and non-zero (intrinsic) toroidal rotation is sustained without momentum input near the axis.

Journal Articles

Dynamics and stability of resistive wall mode in the JT-60U high-$$beta$$ plasmas

Matsunaga, Go; Sakamoto, Yoshiteru; Aiba, Nobuyuki; Shinohara, Koji; Takechi, Manabu; Suzuki, Takahiro; Fujita, Takaaki; Isayama, Akihiko; Oyama, Naoyuki; Asakura, Nobuyuki; et al.

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

Journal Articles

Heat transport and pedestal structure of H-mode in the variation of current density profiles in JT-60U

Urano, Hajime; Sakamoto, Yoshiteru; Suzuki, Takahiro; Fujita, Takaaki; Kamiya, Kensaku; Isayama, Akihiko; Kamada, Yutaka; Takenaga, Hidenobu; Oyama, Naoyuki; Matsunaga, Go; et al.

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

Dependence of the H-mode confinement and pedestal structure on the plasma current is investigated. Higher energy confinement is obtained in higher $$l$$$$_{rm i}$$ H-mode. The profiles of the core $$n$$$$_{rm e}$$ and $$T$$$$_{rm e}$$ are tends to be peaked. The H-factor for the core plasma depends strongly on $$l$$$$_{rm i}$$. Higher energy confinement obtained at higher $$l$$$$_{rm i}$$ is attributed to the core confinement improvement. The edge pedestal pressure is not significantly changed by core current density profile. In both $$I$$$$_{rm p}$$ ramp and $$I$$$$_{rm p}$$ scan experiments, the core energy confinement is improved by the increased core $$B$$$$_{rm p}$$ value. The effect of magnetic shear on the confinement should be investigated in next step study.

Journal Articles

Impurity accumulation in the main plasma and radiation processes in the divetor plasma of JT-60U

Nakano, Tomohide; Kubo, Hirotaka; Asakura, Nobuyuki; Shimizu, Katsuhiro; Higashijima, Satoru

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

It has been found that tungsten accumulation becomes more significant with increasing plasma rotation velocity against the plasma current direction, where negative radial electric field is formed. In a high density plasma at 62% of the Greenwald density, the tungsten accumulation levels decreased significantly, suggesting a shielding effect of the peripheral plasma. It has been found that $$mbox{C}^{3+}$$ is produced by the volume recombination of $$mbox{C}^{4+}$$ and the ionization of $$mbox{C}^{2+}$$ comparably. In contrast, the volume recombination of $$mbox{C}^{3+}$$ is not detected, and the ionization flux of $$mbox{C}^{3+}$$ is less than 1% of the $$mbox{C}^{3+}$$ generation flux. Thus, this result suggests that another loss mechanism of $$mbox{C}^{3+}$$ such as transport loss from the ${sf X}$-point is significant.

Journal Articles

Localized tungsten deposition in divertor region in JT-60U

Ueda, Yoshio*; Fukumoto, Masakatsu*; Watanabe, Jun*; Otsuka, Yusuke*; Arai, Takashi; Asakura, Nobuyuki; Nobuta, Yuji*; Sato, Masayasu; Nakano, Tomohide; Yagyu, Junichi; et al.

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

Deposition profiles of tungsten released from the outer divertor were studied in JT-60U. A neutron activation method was used for the first time to accurately measure deposited tungsten. Surface density of tungsten in the thick carbon deposition layer can be measured by this method. Tungsten was mainly deposited on the inner divertor (around inner strike points) and on the outer wing of the dome. Toroidal distribution of the W deposition was significantly localized near the tungsten released position, while other metallic impurities such as Fe, Cr, Ni were distributed more uniformly. These data indicate that inward drift in the divertor region played a significant role in tungsten transport in JT-60U.

Journal Articles

Characteristics of internal transport barrier under reactor relevant condition in JT-60U weak shear plasmas

Takenaga, Hidenobu; Oyama, Naoyuki; Urano, Hajime; Sakamoto, Yoshiteru; Kamiya, Kensaku; Miyo, Yasuhiko; Nishiyama, Tomokazu; Sasajima, Tadayuki; Masaki, Kei; Kaminaga, Atsushi; et al.

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

Characteristics of internal transport barrier (ITB) have been investigated under reactor relevant condition with edge fuelling and electron heating in JT-60U weak shear plasmas. High confinement was sustained at high density with edge fuelling by shallow pellet injection or supersonic molecular beam injection (SMBI). The ion temperature ($$T_{i}$$) in the central region decreased even with edge fuelling. The $${it T}_{i}$$ decrease with edge fuelling was larger inside the ITB than that outside the ITB, which can be described by cold pulse propagation using the ion thermal diffusivity ($$chi_{i}$$) estimated from power balance analysis in the SMBI case. By optimizing the injection frequency and the penetration depth, the decreased $$T_{i}$$ was recovered and good ITB was sustained with enhanced pedestal pressure. The $$T_{i}$$-ITB also degraded significantly when stiffness feature was strong in the electron temperature ($$T_{e}$$) profile against electron cyclotron heating (ECH). The value of $$chi_{i}$$ in the ITB region increased with the electron thermal diffusivity ($${it chi}_{e}$$), indicating existence of clear relation between ion and electron thermal transport. On the other hand, $$T_{i}$$-ITB unchanged or even grew, when stiffness feature was weak in the $$T_{e}$$ profile. Density fluctuation level seemed to be unchanged during ECH, however, correlation length became longer in the $${it T}_{i}$$-ITB degradation case and shorter in the $$T_{i}$$-ITB unchanging case.

Journal Articles

Analysis of pedestal characteristics in JT-60U H-mode plasmas based on Monte-Carlo neutral transport simulation

Nakashima, Yosuke*; Higashizono, Yuta*; Kawano, Hirokazu*; Takenaga, Hidenobu; Asakura, Nobuyuki; Oyama, Naoyuki; Kamada, Yutaka

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

This paper describes the results of pedestal characteristics in H-mode plasmas of JT-60U based on neutral transport simulation using the DEGAS Monte-Carlo code. A three-dimensional (3-D) mesh structure including isolated dome, baffle and divertor plates is precisely built-up and the simulation space is extended from SOL to core plasma region. In this simulation background plasma parameters and the intensity of particle flux on the divertor plates are determined from the UEDGE plasma code. The dependence of plasma density on the neutral penetration and ionization area were investigated in two cases of ELMy H-mode discharge with gas puffing and without gas puffing. The simulation results showed noticeable differences in 1/e scale length in the neutral penetration and in ionization width near edge transport barrier region. Dependence of the plasma density from the pedestal edge to SOL region on the ionization region and the 3-D effect of local gas puffing are discussed in terms of neutral transport.

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

High energy, high current accelerator development for ITER NBI at JADA

Kashiwagi, Mieko; Taniguchi, Masaki; Dairaku, Masayuki; DeEsch, H. P. L.*; Grisham, L. R.*; Svensson, L.*; Tobari, Hiroyuki; Umeda, Naotaka; Watanabe, Kazuhiro; Sakamoto, Keishi; et al.

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

At JAEA, beam parameters achieved by the MAMuG (multi-aperture multi-grid) accelerator were increased to 0.32 A H$$^{-}$$ at the ion current density of 140 A/m$$^{2}$$ and 796 keV at the beam energy. This was achieved as the result of countermeasures to handle unexpected heat load by backstream positive ions. A test of SINGAP (single-aperture single-gap) accelerator, which was performed at JAEA under an ITER R&D task agreement. The objective of this study was to compare the two different accelerator concepts (SINGAP and MAMuG) at the same test facility. As the result, it was concluded that the MAMuG accelerator was more suitable for the ITER NBI because of its better voltage holding and less electron acceleration. A simulation is in progress for space charge repulsion of beamlets in the JT-60U accelerator. An aperture offset steering was suggested effective at exit of extractor for compensation of the beamlet deflection due to its own space charge.

Journal Articles

Multi-scale transport dynamics dominated by multiple dissipation mechanisms near the critical gradient

Kishimoto, Yasuaki; Miki, Kazuhiro*; Li, J. Q.*; Miyato, Naoaki; Wang, Z. X.*; Anderson, J.*

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

A new class of transient transport near the critical gradient (CG) referred to as GAM growing intermittency due to the collision-less GAM damping has been found based on Landau-fluid simulation. Here, we present a new predator-prey model to understand the essential features of the growing intermittency. We have also extended the simulation model by taking into account the collisional zonal flow (ZF) damping. Due to the mixture of two kinds of damping mechanisms, i.e. the GAM damping and collisional damping, the growing intermittency is found to recursively appear accompanied with complex envelope modulation to ZFs over collisional (or transport) time scale. Furthermore, we have investigated the effect of zonal pressure (ZP) near the CG, which also works as a dissipation mechanism. The multiple dissipation mechanisms are found to synergetically couple each other and lead plasmas to complex dynamical transport over long time scale.

Oral presentation

Demonstration of 1 MW quasi-CW operation of 170 GHz gyrotron and progress of EC technology for ITER

Kasugai, Atsushi; Sakamoto, Keishi; Takahashi, Koji; Kajiwara, Ken; Oda, Yasuhisa; Kobayashi, Noriyuki

no journal, , 

In the last two years, significant progress has been obtained in the development of electron cyclotron heating and current drive (EC H and CD) technology for ITER. On the 170 GHz gyrotron, 1.02 MW power generation was attained for 800 s pulse duration with an efficiency of 55 percent for the first time in the world, which satisfies the ITER criteria of 1 MW, 500s, 50%, respectively. The total output of microwave energy from the gyrotron is 190 GJ without any trouble. A $$sim$$0.97 MW, quasi-CW power transmission was proved using the ITER relevant waveguide system. The basic design was completed for the equatorial launcher, and a mock-up of key components, such as a movable mirror, were fabricated. These give a clear prospect for the success and accomplishment of the EC H and CD system on the ITER.

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