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Hoshino, Katsumichi; Suzuki, Takahiro; Isayama, Akihiko; Ide, Shunsuke; Takenaga, Hidenobu; Kubo, Hirotaka; Fujita, Takaaki; Kamada, Yutaka; Fujii, Tsuneyuki; Tsuda, Takashi; et al.
Fusion Science and Technology, 53(1), p.114 - 129, 2008/01
Times Cited Count：2 Percentile：16.99(Nuclear Science & Technology)We report on the applications of the ECH to the JT-60U tokamak. The ECH contributes to the advanced tokamak study in JT-60U in such respects as (1)NTM suppression, (2)Study on the internal transport barrier, (3)Impurity exhaust, (4)Tokamak start-up without center solenoid, (5)Current hole study, (6)Confinement and transport study, and (7)Start-up and shut-down assist. These contributions comes from the good local coupling of the ECH to the tokamak plasma.
Sakamoto, Yoshiteru; Takenaga, Hidenobu; Fujita, Takaaki; Ide, Shunsuke; Suzuki, Takahiro; Takechi, Manabu; Kamada, Yutaka; Oyama, Naoyuki; Isayama, Akihiko; Koide, Yoshihiko; et al.
Nuclear Fusion, 47(11), p.1506 - 1511, 2007/11
Times Cited Count：7 Percentile：25.84(Physics, Fluids & Plasmas)Controllability of plasmas with a large bootstrap current fraction (
) has been investigated in JT-60U. Dynamic change in current profile, especially 
, which was induced by change in pressure profile at the internal transport barrier (ITB) through rotation control, was observed, indicating the strong linkage among the profiles. Response of pressure and current profiles to off-axis neutral beam injection is investigated and found that the alignment of peaks of total current profile and beam driven current profile affects to location, and the change in toroidal rotation induces the movement of ITB location. The current profile in strong reversed shear plasma without current hole is largely varied in the core region by electron cyclotron current drive, while the ITB structure is not affected.
Ide, Shunsuke; Takenaga, Hidenobu; Isayama, Akihiko; Sakamoto, Yoshiteru; Yoshida, Maiko; Gormezano, C.*
Nuclear Fusion, 47(11), p.1499 - 1505, 2007/11
Times Cited Count：9 Percentile：32.07(Physics, Fluids & Plasmas)Impact of the electron cyclotron range of frequency wave (ECRF) on the internal transport barriers (ITBs) in a weak shear (WS) plasma has been investigated in JT-60U. It is observed that the ion temperature (
) ITB in a WS plasma can be degraded by ECRF. It is clarified for the first time that the degradation depends increasingly on the EC power (
) but decreasingly on the plasma current (
). Moreover it is confirmed that ECRF affects the toroidal rotation (
) indirectly and results in flattening of 
profiles regardless of the direction of the target . Furthermore, it is newly found that and in the whole ITB region are affected with almost no delay from the EC onset even with off-axis EC deposition. These results indicate that the EC injection unveiled a semi-global structure that characterizes ITB in a WS plasma.
Sato, Masayasu; Isayama, Akihiko
Plasma and Fusion Research (Internet), 2, p.S1029_1 - S1029_5, 2007/11
Using the extended Trubnikov's expression for the fully relativistic Maxwellian in the case of oblique propagation to the magnetic field (
), electron cyclotron emission (ECE) spectra are calculated in a high temperature (
) tokamak plasma. We investigate the ECE by changing the angles between the sight line and the equatorial plane or direction. Feature of ECE spectra can be interpreted from the viewpoints of relativistic, Doppler and absorption effects. The downshift frequency variation due to the relativistic effect in the high field side (HFS) observation is bigger than that in the low field side observation. Absorption at the HFS plasma results in the deep dip at the HFS of fundamental, second and third harmonics in the ECE spectra. For the vertical observation, in the case of optically thin case, ECE spectra are similar to the emissivity profile, and when electron density (
) is higher, ECE spectra are modified due to the absorption in plasma. Since the , , increase in the case of the SlimCS DEMO reactor, the ECE spectra expands to high frequency emission (2000 GHz). So, ECE detector in the case of Fourier transform spectrometer system should be modified from present liquid He cooled InSb detector because of the detection of high frequency emission.
Sato, Masayasu; Isayama, Akihiko
Fusion Science and Technology, 52(2), p.169 - 175, 2007/08
Times Cited Count：1 Percentile：11.32(Nuclear Science & Technology)no abstracts in English
Isayama, Akihiko; Oyama, Naoyuki; Urano, Hajime; Suzuki, Takahiro; Takechi, Manabu; Hayashi, Nobuhiko; Nagasaki, Kazunobu*; Kamada, Yutaka; Ide, Shunsuke; Ozeki, Takahisa
Nuclear Fusion, 47(8), p.773 - 782, 2007/08
Times Cited Count：29 Percentile：69.18(Physics, Fluids & Plasmas)Results from active control of neoclassical tearing modes (NTMs) by electron cyclotron current drive (ECCD) in 
are described. Evolution of an NTM with the poloidal mode number 
and the toroidal mode number 
has been suppressed by ECCD inside the sawtooth inversion radius in the co-direction, showing the possibility of the coexistence of sawtooth oscillations and a small-amplitude 
NTM without large confinement degradation. Stabilization of an 
NTM by ECCD at the mode rational surface has been demonstrated with a small value of the ratio of the current density driven by electron cyclotron (EC) wave to the local bootstrap current density (
). Also, dependence of the stabilization effect on ECCD location has been investigated in detail, and it has been clarified that an NTM can be completely stabilized with the misalignment of the ECCD location less than about half of the island width, and that the NTM is destabilized with the misalignment comparable to the island width. Time-dependent, self-consistent simulation of magnetic island evolution using the TOPICS code has shown that the above stabilization and destabilization of an NTM are well reproduced with the same set of coefficients of the modified Rutherford equation. The TOPICS simulation has also clarified that EC wave power required for complete stabilization can be significantly reduced by narrowing the ECCD width.
Oyama, Naoyuki; Isayama, Akihiko; Suzuki, Takahiro; Koide, Yoshihiko; Takenaga, Hidenobu; Ide, Shunsuke; Nakano, Tomohide; Asakura, Nobuyuki; Kubo, Hirotaka; Takechi, Manabu; et al.
Nuclear Fusion, 47(7), p.689 - 697, 2007/07
Times Cited Count：15 Percentile：45.18(Physics, Fluids & Plasmas)After the installation of FSTs in JT-60U, ripple induced fast ion losses have been reduced by 1/2
1/3. The increase in the absorbed power at the same injection power can reduce the required number of NB units to sustain a given 
. The smaller recycling level and edge density, which is suitable condition to get higher edge temperature, can be kept longer. The higher edge temperature is essential in keeping the peaked pressure/temperature profile. In addition to this, smaller fast ion losses reduce the effective torque for ctr-rotation and increase in better flexibility of torque input. Then, better quality of 
-ITB was obtained in the plasmas with co-rotation. By making use of these advantages to sustain an ITB, the performance of long-pulse ELMy H-mode plasmas was improved in terms of sustained duration time for both high and high thermal confinement enhancement factor (
). High 
2.3 together with 1 was sustained for 23.1s (
, where 
is the current diffusion time.), which also provide high 
. These long-pulse plasmas are possible candidates for ITER hybrid operation scenario.
Prater, R.*; La Haye, R. J.*; Luce, T. C.*; Petty, C. C.*; Strait, E. J.*; Ferron, J. R.*; Humphreys, D. A.*; Isayama, Akihiko; Lohr, J.*; Nagasaki, Kazunobu*; et al.
Nuclear Fusion, 47(5), p.371 - 377, 2007/05
Times Cited Count：58 Percentile：87.07(Physics, Fluids & Plasmas)The 
/
neoclassical tearing mode (NTM) has been observed to strongly degrade confinement and frequently lead to a disruption in high discharges in DIII-D if allowed to grow to large size. Stabilization of grown NTMs by application of highly localized electron cyclotron current drive (ECCD) at the island location has led to operation at increased plasma pressure, up to the no-wall kink limit. After the NTM is stabilized by the ECCD, the correct location for the current drive is maintained using information from real-time equilibrium reconstructions which include measurements from the motional Stark effect diagnostic. This same process is used alternatively to prevent the mode from ever growing, leading to performance at the pressure limit in high performance hybrid discharges with above 4%. Modeling using the modified Rutherford equation shows that the required power is in close agreement with the experimental threshold for prevention of the 2/1 NTM.
Takenaga, Hidenobu; Oyama, Naoyuki; Isayama, Akihiko; Takizuka, Tomonori; Fujita, Takaaki
Plasma Physics and Controlled Fusion, 49(4), p.525 - 534, 2007/04
Times Cited Count：2 Percentile：7.53(Physics, Fluids & Plasmas)no abstracts in English
Oyama, Naoyuki; Kamada, Yutaka; Isayama, Akihiko; Urano, Hajime; Koide, Yoshihiko; Sakamoto, Yoshiteru; Takechi, Manabu; Asakura, Nobuyuki; JT-60 Team
Plasma Physics and Controlled Fusion, 49(3), p.249 - 259, 2007/03
Times Cited Count：32 Percentile：71.26(Physics, Fluids & Plasmas)no abstracts in English
Isayama, Akihiko; Oyama, Naoyuki; Urano, Hajime; Suzuki, Takahiro; Takechi, Manabu; Hayashi, Nobuhiko; Nagasaki, Kazunobu*; Kamada, Yutaka; Ide, Shunsuke; Ozeki, Takahisa; et al.
Proceedings of 21st IAEA Fusion Energy Conference (FEC 2006) (CD-ROM), 8 Pages, 2007/03
no abstracts in English
Oyama, Naoyuki; Isayama, Akihiko; Suzuki, Takahiro; Koide, Yoshihiko; Takenaga, Hidenobu; Ide, Shunsuke; Nakano, Tomohide; Asakura, Nobuyuki; Kubo, Hirotaka; Takechi, Manabu; et al.
Proceedings of 21st IAEA Fusion Energy Conference (FEC 2006) (CD-ROM), 8 Pages, 2007/03
After installation of ferritic steel tiles (FSTs) which reduced fast ion losses due to toroidal field ripple, the performance of long-pulse ELMy H-mode plasmas was improved in terms of sustained duration time for both high normalized beta (
) and high thermal confinement enhancement factor (
). High 2.3 together with 1 was sustained for 23.1s (12
) 
, which also provide high 
, which is higher than ITER reference scenario. These long-pulse plasmas are possible candidates for ITER hybrid operation scenario.
Ide, Shunsuke; Takenaga, Hidenobu; Isayama, Akihiko; Sakamoto, Yoshiteru; Yoshida, Maiko; Gormezano, C.*; JT-60 Team
Proceedings of 21st IAEA Fusion Energy Conference (FEC 2006) (CD-ROM), 8 Pages, 2007/03
Impact of an electron cyclotron range of frequency (ECRF) wave on the internal transport barriers (ITBs) in a weak shear (WS) plasma has been investigated in JT-60U. It has been observed that the ITB in the ion temperature (Ti ITB) in a WS plasma can be degraded by an EC injection. It is clarified for the first time that the degradation effect depends increasingly on the EC power (PEC) but decreasingly with plasma current (Ip). Moreover it was confirmed that an EC injection affects the toroidal rotation profile (Vt (r)) indirectly and results in flattening of Vt (r) regardless to the direction of the target Vt (r) peaking, co- (relative to the Ip direction) or counter. Furthermore, it was newly found that the central Ti and Vt are affected with no delay from the EC onset even by an off-axis EC injection. These results suggest that an EC injection acts on background mechanism that has special structure, and EC deposition in any location in this structure gives instantaneous effect.
La Haye, R. J.*; Prater, R.*; Buttery, R. J.*; Hayashi, Nobuhiko; Isayama, Akihiko; Maraschek, M. E.*; Urso, L.*; Zohm, H.*
Proceedings of 21st IAEA Fusion Energy Conference (FEC 2006) (CD-ROM), 8 Pages, 2007/03
Resistive neoclassical tearing mode (NTM) islands will be the principal limit on stability and performance in ITER as beta is well below the ideal kink limit. NTM island control in ITER is predicted to be challenging both because of the relatively narrower marginal island widths and the relatively broader electron cyclotron current drive (ECCD). Measurements from ASDEX Upgrade, DIII-D, and JET in beta rampdown experiments are used to determine the marginal island size for m/n=3/2 NTM removal. This is compared to data from ASDEX Upgrade, DIII-D and JT-60U with elimination of the m/n=3/2 island by continuous ECCD at near constant beta. The empirical marginal island size is consistent in both sets of removal experiments and found to be about twice the ion banana width. A common methodology is developed for fitting the saturated m/n=3/2 island before (or without) ECCD in all four experimental devices. To this is added (and model tested to experiments) the effect of unmodulated co-ECCD on island stabilization including both replacing the missing bootstrap current and making the classical tearing stability index more negative. The experimentally benchmarked model is then used to evaluate ITER. The ITER ECCD upper launcher with up to 20 MW of injected power is appraised with or without modulation for both the m/n=3/2 mode and the m/n=2/1 NTM (which can lock to the resistive wall and induce disruption). An m/n=2/1 rotating island model with drag from eddy current induced in the resistive wall is used to predict the necessary ECCD to keep the island from locking as a function of the rotation in ITER. The planned relatively wide ECCD should be capable of regulating the island width to avoid mode locking with the anticipated rotation in ITER but there is little margin available for inevitable misalignment. Narrower ECCD of more power and/or more rotation in ITER would increase confidence in island control and successful operation.
Takechi, Manabu; Matsunaga, Go; Ozeki, Takahisa; Aiba, Nobuyuki; Kurita, Genichi; Isayama, Akihiko; Koide, Yoshihiko; Sakamoto, Yoshiteru; Fujita, Takaaki; Kamada, Yutaka; et al.
Proceedings of 21st IAEA Fusion Energy Conference (FEC 2006) (CD-ROM), 8 Pages, 2007/03
This is the first experimental result which demonstrates the dependence of critical RWM onset of plasma rotation and beta on initial plasma rotation with the variation of the angular momentum input and without magnetic braking. In the JT-60U with newly ferretic wall installed, it is possible to produce high beta plasma tightly coupled with the wall (b/a=1.2) above 
. In near-zero plasma rotation, the RWM started to grow at = and with finite plasma rotation, the plasma pressure survives up to much higher level than that with small rotation. The observed critical beta onset 
and the RWM growth rate 
are discussed along with theoretical predictions using experimentally observed q-, pressure-, and rotation profiles. The critical rotation of RWM on JT-60U is less one-second times lower than previous report of other tokamaks.
Neudatchin, S. V.*; Takizuka, Tomonori; Sakamoto, Yoshiteru; Hayashi, Nobuhiko; Isayama, Akihiko; Kislov, A. Ya.*; Krylov, S. V.*; Pavlov, Yu. D.*; Shirai, Hiroshi; Borshegovskii, A. A.*
Proceedings of 21st IAEA Fusion Energy Conference (FEC 2006) (CD-ROM), 8 Pages, 2007/03
Non-local transport bifurcations inside and around ITB were found in various JT-60U reverse shear (RS) and normal shear (NrS) plasmas and called ITB-events. The abrupt reduction of transport in the central part of the plasma column often interrupts a slow diffusive inward cold pulse propagation (CPP) in T-10. CPP is created by a cut-off of the off-axis ECRH. This phenomenon may referred to ITB-events as well. In many cases these events occur, when the central safety factor q decreases below unity. In JT-60U low-power heated RS plasmas, ITB-events are observed at the crossing of minimum of q equal to 3.5, 3, 2.5 values. Internal MHD n=1 activity has been reported earlier as ITB-events trigger in JT-60U. The present paper shows that an ITB-event is triggered by a series of small internal disruptions probably associated with q=2.5 surface in RS plasmas. In a high-poloidal-beta NrS shot, an ITB-event occurs in correlation with the start of ELMs series. The total heat flux reduces abruptly in the zone 0.3
r/a0.7. This new finding of this ITB event suggests the possibility to control the ITB formation by inducing the ELM-like MHD activity.
Prater, R.*; La Haye, R. J.*; Luce, T. C.*; Petty, C. C.*; Strait, E. J.*; Ferron, J. R.*; Humphreys, D. A.*; Isayama, Akihiko; Lohr, J.*; Nagasaki, Kazunobu*; et al.
Proceedings of 21st IAEA Fusion Energy Conference (FEC 2006) (CD-ROM), 8 Pages, 2007/03
Onset of the m/n=2/1 neoclassical tearing mode (NTM) has been prevented in high-performance DIII-D discharges using localized electron cyclotron current drive (ECCD). Active tracking of the =2 surface location, using real-time equilibrium reconstructions with motional Stark effect data, allows the current drive to be maintained at the rational surface even in the absence of a detectable mode. With the application of this technique in DIII-D hybrid discharges, the 2/1 mode is avoided and good energy confinement is maintained for more than 1 second with at the estimated n=1 no-wall stability limit for ideal kink modes (
approximately equals 3.9 % and normalized beta 
approximately equals 3.2). The results can be understood through modeling using the modified Rutherford equation.
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
10
m
, 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 and long pulse.
Takechi, Manabu; Matsunaga, Go; Aiba, Nobuyuki; Fujita, Takaaki; Ozeki, Takahisa; Koide, Yoshihiko; Sakamoto, Yoshiteru; Kurita, Genichi; Isayama, Akihiko; Kamada, Yutaka; et al.
Physical Review Letters, 98(5), p.055002_1 - 055002_4, 2007/02
Times Cited Count：121 Percentile：94.54(Physics, Multidisciplinary)The plasma rotation necessary for stabilization of resistive wall modes (RWMs) is investigated by controlling the toroidal plasma rotation with external momentum input by injection of tangential neutral beams. The observed threshold is 0.3 % of the Alfv
n velocity and much smaller than the previous experimental results obtained with magnetic braking. This low critical rotation has a very weak dependence as the ideal wall limit is approached. These results indicate that for large plasmas such as in future fusion reactors with low rotation, the requirement of the additional feedback control system for stabilizing RWM is much reduced.
Kawahata, Kazuo*; Kawano, Yasunori; Kusama, Yoshinori; Mase, Atsushi*; Sasao, Mamiko*; Sugie, Tatsuo; Fujita, Takaaki; Fukuda, Takeshi*; Fukuyama, Atsushi*; Sakamoto, Yoshiteru; et al.
Purazuma, Kaku Yugo Gakkai-Shi, 83(2), p.195 - 198, 2007/02
no abstracts in English
