-
[show abstract]
[hide abstract]
ABSTRACT: From the intensity ratios of the three He I lines measured at 20 kHz, the temporal evolutions of the electron temperature and density during and after the power and the particle flow into the divertor plasma caused by edge localized modes are determined. The electron temperature increases from 70 eV to 80 eV with increasing Dα intensity. Then, at the peak of Dα intensity, the electron temperature starts decreasing down to 60 eV. The electron density increases from 0.1 × 1019 m−3 to 0.3 × 1019 m−3 with increasing Dα intensity, and then starts to decrease more gradually compared with the electron temperature after the peak of Dα intensity. It is interpreted that the increase of the electron temperature is ascribed to the power and the particle flow into the divertor plasma, and that the decrease of the electron temperature and the increase of the electron density are ascribed to the ionization of the recycled neutrals, which consumes the electron energy and produces electrons.
Journal of Physics B Atomic Molecular and Optical Physics 07/2010; 43(14):144014. · 1.88 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We observe the vacuum ultraviolet spectra of highly charged tungsten ions in plasmas at electron temperatures of 8 keV and 14 keV. The plasmas are produced by a tokamak JT-60U. The observed emission lines in a wavelength range of 2.6–3.2 nm for the plasma at 8 keV are identified to be the 3p3/2–3d5/2 transitions of W47 +–W54 +. The observed emission lines at 2.0 nm and 2.3 nm at 14 keV are identified to be the 3p1/2–3d3/2 transitions of W55 +–W61 + and 3s1/2–3p3/2 transitions of W60 +–W63 +, respectively. The identification is obtained with the collisional-radiative calculation. Then the observed spectra are compared with the synthesized spectra with a prediction of the ionization balance. The emission lines identified here can be used for the tungsten density measurement in ITER plasmas.
Journal of Physics B Atomic Molecular and Optical Physics 07/2010; 43(14):144013. · 1.88 Impact Factor
-
Y. Shibata,
K.Y. Watanabe,
M. Okamoto,
N. Ohno,
A. Isayama,
K. Kurihara, T. Nakano,
N. Oyama,
Y. Kawano,
G. Matsunaga,
S. Sakakibara,
M. Sugihara,
Y. Kamada,
the team
[show abstract]
[hide abstract]
ABSTRACT: An L/R model that predicts the current decay time from the circuit equation is essentially used for the design of the International Thermonuclear Experimental Reactor. In order to verify the validity of the L/R model in the determination of current decay time during disruption, the plasma current decay time in the JT-60U tokamak is studied using experimental plasma resistance and inductance. The plasma resistance during the initial phase of current quench is estimated from the electron temperature profile measured using the electron cyclotron emission diagnostic system and by measuring the He I line emission intensity ratios and plasma inductance is estimated by the Cauchy-Condition surface method using magnetic sensor signals. Further, the radiation-induced disruptive plasma discharges with massive neon gas puffing are also analysed. The observed area-normalized current decay times have a weak dependence on the electron temperature, particularly in a small decay time region (5–10 ms m−2). The observed decay times are lesser by one order of magnitude than the decay times estimated by the L/R model. However, a novel model for decay time prediction, which takes into account the time derivative of the plasma inductance, wields results that are extremely consistent with the experimental decay time.
Nuclear Fusion 01/2010; 50(2):025015. · 4.09 Impact Factor
-
S I Krasheninnikov,
A Yu Pigarov,
R D Smirnov,
M Rosenberg,
Y Tanaka,
D J Benson,
T K Soboleva,
T D Rognlien,
D A Mendis,
B D Bray, [......],
S Masuzaki,
N Ashikawa,
M Shiratani,
M Tokitani,
R Kumazawa,
N Asakura, T Nakano,
A M Litnovsky,
R Maqueda,
the LHD Experimental Group
[show abstract]
[hide abstract]
ABSTRACT: It has been known for a long time that microscopic dust appears in plasmas in fusion devices. Recently it was shown that dust can be responsible for the termination of long- discharges. Also, in ITER-scale experiments dust can pose safety problems related to its chemical activity, tritium retention and radioactive content. In particular, the presence of dust in the vacuum chamber of ITER is one of the main concerns of the ITER licensing process. Here we review recent progress in the understanding of different experimental and theoretical aspects of the physics of dust dynamics and transport in fusion plasmas and discuss the remaining issues.
Plasma Physics and Controlled Fusion 11/2008; 50(12):124054. · 2.42 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: In JT-60U, the filament structure of Edge Localized Modes (ELMs) is measured at three poloidal locations, and their temporal evolution is investigated at the inner (high magnetic-field-side, HFS) SOL as well as outer (low magnetic-field-side, LFS) SOL. At the inner SOL, filament structure with 7-8 temporal multi-peaks in ion-saturation probe signals and other diagnostics is, for the first time, measured close to the sepatratrix. The delay of the first peak after the start of MHD activity is faster than the characteristic time of the parallel convection from the outer midplane, and the Mach numbers of the plasma parallel flow reach ion sonic levels. These results show that ELM filaments extend from the outer to the inner plasma edge, and that a part of the filaments are ejected into the inner SOL. The toroidal mode numbers (n) and poloidal width of the filaments (δz) are evaluated from the interval of the multi-peaks and duration of each peak, yielding n ~ 18-44 and δz ~ 2-6 cm on the HFS SOL, while n and δz are comparable on the LFS SOL. After the multi-peaks appear, flow reversal of the SOL plasma toward is observed over a wide region of the inner SOL.
Journal of Physics Conference Series 08/2008; 123(1):012009.
-
N. Oyama,
A. Isayama,
T. Suzuki,
Y. Koide,
H. Takenaga,
S. Ide, T. Nakano,
N. Asakura,
H. Kubo,
M. Takechi,
Y. Sakamoto,
Y. Kamada,
H. Urano,
M. Yoshida,
K. Tsuzuki,
G. Matsunaga,
C. Gormezano,
the Team
[show abstract]
[hide abstract]
ABSTRACT: After installation of ferritic steel tiles, fast ion losses due to toroidal field ripple have been reduced by 1/2–1/3. The increase in absorbed power at same injection power can reduce the required number of neutral beam injector (NBI) units to sustain a given normalized beta, βN, resulting in a better flexibility of torque input by increasing the available combination of tangential NBI units. By making use of these advantages to sustain an internal transport barrier (ITB), the performance of long-pulse ELMy H-mode plasmas was improved in terms of sustained duration time for both high βN and high thermal confinement enhancement factor (HH98(y,2)). High βN > 2.3 together with HH98(y,2) ~ 1 was sustained for 23.1 s (~12τR, where τR is the current diffusion time) at q95 ~ 3.3, which also provide high βNHH98(y,2) ≥ 2.2 and a bootstrap current fraction of ≥40%. βNHH98(y,2) of 2.0 was sustained for 28.6 s, which is limited by the maximum injection period of 30s for NBI system. These long-pulse plasmas are possible candidates for ITER hybrid operation scenario. Improved confinement is characterized by the larger thermal components at a given density maintained by lower heating power than in previous experiments. The strength of the ITB depends on the pedestal temperature, which varies with edge density while keeping constant the edge pressure (limited by type I ELMs). The fact that co-toroidal rotation as a result of reduced fast ion losses provides better quality of Te-ITB also contributes the improvement of thermal plasma confinement. These long-pulse plasmas indicate that further investigation to establish high performance plasmas longer than the time scale of wall saturation (τW) with active particle control is essential to establish the operational scenarios for next step devices, where the wall pumping does not work.
Nuclear Fusion 06/2007; 47(7):689. · 4.09 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The chemical sputtering yields of CH4/CD4 and C2Hx/C2Dx have been measured at the divertor plates of JT-60U. Spectroscopic measurements for CH/CD and C2 spectral bands are applied to estimate the CH4/CD4 and the C2Hx/C2Dx flux. At the surface temperatures of 380, 440 and 560 K, the CH4 yield is, respectively, ~0.8%, 1-2% and 2-3%, the C2Hx yield 1-2%, 3-4% and 4-5%, and the total sputtering yield by hydrogen ions 3-4%, ~8% and ~10%. With increasing ion flux to the divertor plates (Γion), the sputtering yields (Y) decrease, i.e. YΓion(-0.05 to -0.40). With increasing electron temperature (Te), the sputtering yields increase, i.e. YTe0.5. It is concluded from the result of regression analysis of YTe0.5 that the negative dependence of the yields on the ion flux is attributed to the incident ion energies to the carbon plates. The ratio of the sputtering yields by deuterium ions and hydrogen ions is estimated to be ≥1.5 based on the ion flux measurement by Hα/Dα intensity. The C2Hx/C2Dx sputtering yield accounts for ~80% of the total number of sputtered carbon atoms.
Nuclear Fusion 06/2002; 42(6):689. · 4.09 Impact Factor
-
J. Roth,
R. Preuss,
W. Bohmeyer,
S. Brezinsek,
A. Cambe,
E. Gasarotto,
R. Doerner,
E. Gauthier,
G. Federici,
S. Higashijima, [......],
H Kubo,
J. M. Layet, T Nakano,
V. Philipps,
A. Pospieszczyk,
R. Pugno,
R. Ruggiéri,
B. Schweer,
G. Sergienko,
M. Stamp
-
H. Akasaka,
N. Akino,
K. Anno,
M. Arai,
T. Arai,
N. Asakura,
N. Ashikawa,
M. Azumi,
P. E. Bak,
M. Bakhtiari, [......],
J. Yagyu,
H. Yamada,
T. Tamamoto,
Y. Yamashita,
H. Yamazaki,
K. Yamazaki,
Y. Yang,
K. Yokokura,
I. Yonekawa,
H. Yoshida
Review of JT-60U experimental results in 2001 and 2002.
-
T. Fujita,
H. Akasaka,
N. Akino,
K. Anno,
M. Arai,
T. Arai,
N. Asakura,
N. Ashikawa,
M. Azumi,
P. E. Bak, [......],
H. Yamada,
T. Yamamoto,
Y. Yamashita,
H. Yamazaki,
K. Yamazaki,
Y. Yang,
K. Yokokura,
I. Yonekawa,
H. Yoshida,
Jt-60 Team
[show abstract]
[hide abstract]
ABSTRACT: Recent JT-60U results leading to high integrated performance are reported with emphasis on the projection to the reactor-relevant regime. Negative-ion-based neutral beam (NB) and electron cyclotron (EC power increased up to 6.2 MW and 3 MW, respectively. A high beta(p) H-mode plasma with full non-inductive current drive has been obtained at 1.8 MA, and the fusion triple product reached 3.1 x 10(20) m(-3) keV s. A high beta with beta(N) = 2.7 was maintained for 7.4 s. Neoclassical tearing mode suppression with EC was accomplished using a real-time feedback control system, and improvement in beta(N) was obtained. The stable existence of a current hole was observed. A high DT-equivalent fusion gain of 0.8 was maintained for 0.55 s in a plasma with a current hole. Current profile control in high bootstrap current reversed shear plasmas was demonstrated using N-NB and LH. A new operation scenario has been established in which a plasma with a high bootstrap current fraction and internal transport barriers (ITBs) is produced without the use of an OH coil. An ECCD study was undertaken in a reactor-relevant high T-e regime. A new type of Alfven eigenmode mode has been proposed and has been found to explain the observed frequency chirp quite well. High confinement reversed shear plasmas with T-e > T-i were obtained. Ar exhaust with EC heating was obtained in a high beta(p) mode plasma. Impurity accumulation related to strong ITBs in a reversed shear plasma and degradation of ITB by ECH in a weak positive shear plasma have been found. Dedicated measurement of edge localized mode dynamics and scrape-off-layer plasma flow have advanced understanding of the physics. N-NB heating in an Ar-seed plasma extended the density region to 95% of the Greenwald density, with HHy2 = 0.9. Enhancement of pedestal pressure was obtained with an increase in beta(p) in a high triangularity configuration.
Nuclear Fusion. 43(12):1527-1539.
-
N. Asakura,
A. Loarte,
G. Porter,
V. Philipps,
B. Lipschultz,
A. Kallenbach,
G Matthews,
G. Federici,
A. Kukushkin,
A. Mahdavi,
A. W. Leonard,
D. Whyte,
K. Itami,
H. Takenaga,
A. V. Chankin,
S. Higashijima, T Nakano,
A Herrmann,
T. Eich,
B. LaBombard
[show abstract]
[hide abstract]
ABSTRACT: Three important physics issues for the ITER divertor design and operation are summarized based on the experimental and numerical work from multi-machine database (JET, JT-60U, ASDEX Upgrade, DIII-D, Alcator C-Mod and TEXTOR). (i) The energy load associated with Type-I ELMs is of great concern for the lifetime of the ITER divertor target. In order to understand the physics base of the scaling models[1], the ELM heat and particle transport from the edge pedestal to the divertor is investigated. Convective transport during ELMs plays an important role in heat transport to the divertor. (ii) Determination of the SOL flow pattern and the driving mechanism has progressed experimentally and numerically. Influences of the drift effects on the SOL and divertor plasma transport were discussed. (iii) Carbon erosion and redeposition are of great importance in particular for tritium retention via codeposition. Characteristics of chemical yield at two different deposited carbon surfaces, i.e. erosion- and redeposition-dominated areas, have been studied. Progress in the understanding of the chemical erosion is reviewed.
Fusion Energy 2002, International Atomic Energy Agency (2003).
-
H. Takenaga,
N. Asakura,
S. Higashijima, T. Nakano,
H. Kubo,
S. Konoshima,
N. Oyama,
A. Isayama,
S. Ide,
T. Fujita,
Y. Miura
[show abstract]
[hide abstract]
ABSTRACT: Long time scale variation of plasma-wall interactions and its impact on particle balance, main plasma performance and particle behavior have been investigated in ELMy H-mode plasmas by extending the discharge pulse and the neutral beam heating pulse to 65 s and 30 s, respectively. The wall pumping rate starts to decrease in the latter phase by repeating the long-pulse discharges with 60% of Greenwald density sustained by gas-puffing. After several discharges, the wall inventory is saturated in the latter phase and, consequently, the density increases with neutral beam fuelling only. The edge pressure in the main plasma is reduced and ELMs are close to the type III regime under conditions of wall saturation. The intensities of C II emission near the X-point and CD band emission in the inner divertor start to increase before the wall saturates and continue to increase after the wall is saturated.
Journal of Nuclear Materials.
-
[show abstract]
[hide abstract]
ABSTRACT: In ELMy H-mode plasmas of Japan Atomic Energy Research Institute Tokamak-60 Upgrade, extension of a high confinement regime toward high density and high radiation-loss-power fraction has been investigated by Ar injection. With injecting Ar and putting the outer strike point on the top of divertor dome, confinement improvement, electron density, and radiation-loss-power fraction reached the ITER relevant regime; the thermal energy confinement time was as high as that the International Tokamak Experimental Reactor ELMy H-mode scaling IPB98(y,2), the electron density is ∼80% of the Greenwald density, and the radiation-loss-power fraction is ∼0.8. Although electron density profile is slightly peaked with Ar injection, the confinement improvement is mainly attributed to the improvement of ion transport. Moreover, the maximum divertor heat load due to ELMs has been reduced by a factor of 3–5 with keeping high confinement. ELM activity can be controlled by injecting Ar and changing the strike point position.
Journal of Nuclear Materials.
-
A.V. Chankin,
N. Asakura,
T. Fukuda,
A. Isayama,
K. Itami,
Y. Kamada,
H. Kubo,
Y. Miura, T. Nakano,
N. Oyama,
S. Takeji,
H. Takenaga
[show abstract]
[hide abstract]
ABSTRACT: In JT-60U, extensive plasma–neutral interaction during type I edge localized modes (ELMs) leads to a transient increase in the edge plasma density, seen as spikes of a few ms duration on the inner vertical interferometer channel (FIR1). The spikes can reach up to ∼40% of the pre-ELM level of the FIR1 signal, in the case of giant ELMs. Fast edge measurements revealed that these density spikes are caused by ionisation of neutrals circulating between the plasma and the wall, as a result of the ELM particle and heat load onto the target. The increase in the edge density is more or less equally divided between the scrape-off layer (SOL) and outer core region, but is outside of the top of the H-mode pedestal. Prompt ionisation of neutrals and the increase in the plasma density around the separatrix position may affect the edge MHD stability, as often manifested by the occurrence of a second, satellite ELM triggered at lower pedestal temperature and pressure, clusters of type III ELMs or periods of L-mode. Both the magnitude of the density spikes and the observed changes in the ELM behaviour were found to depend on the wall conditions.
Journal of Nuclear Materials.
-
-
-
K Ohya,
T Tanabe,
N. Asakura,
H Kubo,
K. Simizu,
T. Takizuka,
H. Takenaga, T Nakano,
S. Higashijima,
A Itoh, [......],
T Ono,
J. Kawata,
A. Hatakeyama,
K Sawada,
M. Shouji,
Y Tomita,
H Nakamura,
S. Masuzaki,
D. Kato,
T Kato
-
-
-
H. Zushi,
K. Uchino,
A. Mase,
M. Shiratani,
S Inagaki,
K Hanada,
M Sakamoto,
H. Idei,
K. Nagasaki,
T. Shikama, [......],
K Tanaka,
K. Nagaoka,
M. Osakabe,
M. Tokitani,
T Yoshinaga, T Nakano,
S. Kado,
A. Ejiri,
Y Nakashima,
M Miyamoto
