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Takechi, Manabu; Matsunaga, Go; Sakurai, Shinji; Sasajima, Tadayuki; Yagyu, Junichi; Hoshi, Ryo*; Kawamata, Yoichi; Kurihara, Kenichi; JT-60SA Team; Nishikawa, T.*; et al.
Fusion Engineering and Design, 96-97, p.985 - 988, 2015/10
Times Cited Count：10 Percentile：64.63(Nuclear Science & Technology)Tsuda, Takashi; Kurita, Genichi; Fujita, Takaaki
Journal of Plasma Physics, 72(6), p.1149 - 1152, 2006/12
Times Cited Count：3 Percentile：10.78(Physics, Fluids & Plasmas)The current density is expected to be negative in the central region of the tokamak when the amplitude of bootstrap current or off-axis current drive is large enough and a negative one-turn voltage exists. However, a flat current profile with almost zero value has been observed in experiments. The current profile with Current Hole has a tendency to be unstable to double tearing mode and we investigate the role of double tearing mode on the formation of Current Hole with RMHD simulation.
Aiba, Nobuyuki; Tokuda, Shinji; Ishizawa, Tomoko*
Journal of Plasma Physics, 72(6), p.1127 - 1131, 2006/12
Times Cited Count：1 Percentile：3.54(Physics, Fluids & Plasmas)We develop the method for the stability analysis of a ideal external magnetohydrodynamic (MHD) mode by solving the eigenvalue problem associated with the two-dimensional Newcomb equation, the inertia free linear ideal MHD equation. This eigenvalue problem can be expected to provide a powerful tool for not only a low-n external MHD mode but also a high-n mode, where n is a toroidal mode number. With this method, we analyze an effect of the aspect ratio on the stability of middle-n (1
n10) external MHD modes in tokamaks; this gets attention for the design research of a high performance tokamak. As the result of this work, we study that external MHD modes become stable as the aspect ratio decreases, and also find that the stability of middle-n external modes becomes important because an effect of a conducting wall is enhanced by reducing the aspect ratio.
Tsuda, Takashi; Kurita, Genichi; Fujita, Takaaki
Journal of the Korean Physical Society, 49, p.S83 - S86, 2006/12
The current density is expected to be negative in the central region of the tokamak when the amplitude of bootstrap current or off-axis current drive is large enough. However, a flat current profile with almost zero value has been observed in experiments even under the situation with negative one-turn voltage exists in the central region of plasma. Double tearing mode (DTM) can be unstable for the current profile with a "current hole" and some MHD activities are observed in JET before the formation of the current hole. On the contrary, no MHD activity is observed in the JT-60 experiment. Here, we study the condition of appearance of DTM and investigate the stability of DTM and the interaction between DTM (n=1 perturbation) and "Current Hole" with resistive RMHD simulations.
La Haye, R. J.*; Prater, R.*; Buttery, R. J.*; Hayashi, Nobuhiko; Isayama, Akihiko; Maraschek, M. E.*; Urso, L.*; Zohm, H.*
Nuclear Fusion, 46(4), p.451 - 461, 2006/04
Times Cited Count：151 Percentile：97.55(Physics, Fluids & Plasmas)no abstracts in English
Hoshino, Katsumichi; Yamamoto, Takumi; Tamai, Hiroshi; Oasa, Kazumi; Kawashima, Hisato; Miura, Yukitoshi; Ogawa, Toshihide; Shoji, Teruaki*; Shibata, Takatoshi; Kikuchi, Kazuo; et al.
Fusion Science and Technology, 49(2), p.139 - 167, 2006/02
Times Cited Count：2 Percentile：17.18(Nuclear Science & Technology)The main results obtained by the various heating and current drive systems, external coil system and divertor bias system are reviewed from the viewpoint of the advanced active control of the tokamak plasma. Also, the features of each system are described. The contribution of the JFT-2M in these areas are summarized.
Isayama, Akihiko; JT-60 Team
Plasma Science and Technology, 8(1), p.36 - 40, 2006/01
Times Cited Count：4 Percentile：14.66(Physics, Fluids & Plasmas)no abstracts in English
Isayama, Akihiko; Oyama, Naoyuki; Urano, Hajime; JT-60 Team
Proceedings of 21st IEEE/NPSS Symposium on Fusion Engineering (SOFE 2005) (CD-ROM), 4 Pages, 2006/00
no abstracts in English
Takechi, Manabu; Fujita, Takaaki; Ishii, Yasutomo; Ozeki, Takahisa; Suzuki, Takahiro; Isayama, Akihiko; JT-60 Team
Nuclear Fusion, 45(12), p.1694 - 1699, 2005/12
Times Cited Count：7 Percentile：24.62(Physics, Fluids & Plasmas)High performance reversed shear discharges with strong internal transport barrier (ITB) and flat pressure profile in the plasma core region disrupt frequently even with low beta. We analyzed MHD instabilities leading to low beta disruption with measuring fluctuations and current profile with MSE measurement. We mainly observed two type of disruptions. One is the disruption without precursor at 
. The other is the disruption with n=1 precursor of 
100 ms. To explain these characteristics of disruption, we introduce the simple model such as, disruption occurs when the both MHD instabilities at plasma surface and at safety factor being equal to surface mode are unstable. This simple model can explain almost all observed disruption by two process. One is the surface mode triggered disruption, which occurs when qeff change, corresponding q surface at ITB layer changes discretely. The other is the internal mode triggered disruption, which occurs when corresponding q surface become unstable gradually.
Yoshino, Ryuji
Nuclear Fusion, 45(11), p.1232 - 1246, 2005/11
Times Cited Count：37 Percentile：74.07(Physics, Fluids & Plasmas)Prediction of major disruptions observed at the 
-limit for tokamak plasmas has been investigated in JT-60U with developing neural networks. A sub-neural network is trained to output a value of the 
limit every 2 ms. The target limit is artificially set by the operator in the first step training and is modified in the second step training using the output limit from the trained network. To improve the prediction performance further, the difference between the estimated limit and the measured and the other 11 parameters are inputted to a main neural network to calculate the stability level. Major disruptions have been predicted with a prediction success rate of 80% at 10 ms prior to the disruption while the false alarm rate is lower than 4%. This 80% is much higher than about 10% previously obtained. A prediction success rate of 90% has been also obtained with a false alarm rate of 12% at 10 ms prior to the disruption. This 12% is about a half of previously obtained one.
Matsumoto, Taro; Naito, Hiroshi*; Tokuda, Shinji; Kishimoto, Yasuaki*
Nuclear Fusion, 45(11), p.1264 - 1270, 2005/11
Times Cited Count：14 Percentile：43.63(Physics, Fluids & Plasmas)A gyrokinetic particle simulation is executed to clarify the effect of the electron inertia on the MHD phenomena in the reversed shear configuration (RSC) of a cylindrical tokamak plasma. It is found that the collisionless (kinetic) double tearing modes grow up at the Alfv
n time scale, and nonlinearly induce the internal collapse when the helical flux at the magnetic axis is less than that at the outer resonant surface. After the internal collapse, the secondary reconnection is induced by the current concentration due to the 
convective flow. It is also clarified that a nonlinear dynamics accompanied with the elementary processes caused by the flow can generate a new RSC with resonant surfaces. In the presence of the density gradient, after the full reconnection induced by the mode, the radial electric field is found to be generated due to the difference of the 
motion between ions and electrons. However, the intensity of the radial field is not so large as that induced by the collisionless kink mode.
Masnavi, M.*; Nakajima, Mitsuo*; Sasaki, Akira; Hotta, Eiki*; Horioka, Kazuhiko*
Applied Physics Letters, 87(11), p.111502_1 - 111502_3, 2005/09
Times Cited Count：4 Percentile：17.87(Physics, Applied)no abstracts in English
Matsumoto, Taro; Naito, Hiroshi*; Tokuda, Shinji; Kishimoto, Yasuaki
Physics of Plasmas, 12(9), p.092505_1 - 092505_7, 2005/09
Times Cited Count：2 Percentile：6.86(Physics, Fluids & Plasmas)The behavior of the collisionless magnetohydrodynamics modes is investigated by the gyro-kinetic particle simulation in a cylindrical tokamak plasma in the parameter region where the effects of electron inertia and electron parallel compressibility are competitive for magnetic reconnection. Although the linear growth of the 
internal kink-tearing mode is dominated by the electron inertia, it is found that the growth rate can be nonlinearly accelerated due to the electron parallel compressibility proportional to the ion sound Larmor radius 
. It is also found that, as decreasing the electron skin depth 
, the maximum growth rate before the internal collapse saturates independently of the microscopic scales such as and . The acceleration of growth rate is also observed in the nonlinear phase of the double tearing mode.
n eigenmodes in reversed shear plasmas in JT-60U negative-ion-based neutral beam injection dischargesTakechi, Manabu; Fukuyama, Atsushi*; Ishikawa, Masao; Cheng, C. Z.*; Shinohara, Koji; Ozeki, Takahisa; Kusama, Yoshinori; Takeji, Satoru*; Fujita, Takaaki; Oikawa, Toshihiro; et al.
Physics of Plasmas, 12(8), p.082509_1 - 082509_7, 2005/08
Times Cited Count：34 Percentile：71.2(Physics, Fluids & Plasmas)Rapid frequency sweeping modes observed in reversed magnetic shear (RS) plasmas on the Japan Atomic Energy Research Institute Tokamak 60 Upgrade (JT-60U) have been identified as Reversed-Shear-induced Alfvn Eigenmodes (RSAEs), which are ideal MHD Alfvn eigenmodes (AEs) localized to the region of minimum safety factor, 
, and are excited by negative-ion-based neutral beam injection. The chirping and subsequent saturation of the mode frequency are consistent with theoretical predictions for the transition from RSAEs to Toroidal Alfv
n eigenmodes (TAEs). The previously observed rapid frequency sweeping modes in ion cyclotron wave heated plasmas in JT-60U can also be similarly explained. The observed AE amplitude is largest during the transition from RSAEs to TAEs, and fast ion loss is observed when the AE amplitude is largest at this transition. It is preferable to operate outside the transition range of 
, e.g., 2.4 2.7 for the n = 1 AE to avoid substantial fast ion loss in RS plasmas.
Tokuda, Shinji
Denki Gakkai Kenkyukai Shiryo, Purazuma Kenkyukai (PST-05-20
22
2435), p.35 - 38, 2005/03
The Newcomb equation arising in magnetohydrodynamic stability problems for a tokamak plasma has regular singular poins that are rational surfaces. The Frobenius solution around the regular singular point plays important roles in various MHD problems. A numerical method is proposed which extracts, by using Legendre polynomials, the singular solution included in a solution for the eigenvalue problem associated with the Newcomb equation.
Kawano, Yasunori; Kawahata, Kazuo*; Kusama, Yoshinori; Sasao, Mamiko*; Sugie, Tatsuo; Mase, Atsushi*; Asakura, Nobuyuki; Kato, Takako*; Takamura, Shuichi*; Tanabe, Tetsuo*; et al.
Purazuma, Kaku Yugo Gakkai-Shi, 81(2), p.128 - 130, 2005/02
no abstracts in English
Satake, Shinichi*; Kunugi, Tomoaki*; Naito, Norihito*; Takase, Kazuyuki; Ose, Yasuo*
Dai-18-Kai Suchi Ryutai Rikigaku Shimpojiumu Koen Yoshishu (CD-ROM), 4 Pages, 2004/12
no abstracts in English
Aiba, Nobuyuki*; Tokuda, Shinji; Ishizawa, Tomoko*; Okamoto, Masao*
Plasma Physics and Controlled Fusion, 46(11), p.1699 - 1721, 2004/11
Times Cited Count：3 Percentile：9.99(Physics, Fluids & Plasmas)The theory of the Newcomb equation has been applied to low-n external modes in a tokamak and a method has been developed to compute the stability matrix that gives the change of plasma potential energy due to external modes in terms of the surface values of the perturbations. By using this method, the spectral properties of the ideal external modes has been elucidated, such as coupling between external modes and internal modes, and the difference of the stability properties between a normal shear tokamak and a reversed shear tokamak. These results will be also useful in the stability analysis of resistive wall modes.
Nagasaki, Kazunobu*; Isayama, Akihiko; Hayashi, Nobuhiko; Ozeki, Takahisa; Takechi, Manabu; Oyama, Naoyuki; Ide, Shunsuke; Yamamoto, Satoshi*; JT-60 Team
Proceedings of 20th IAEA Fusion Energy Conference (FEC 2004) (CD-ROM), 8 Pages, 2004/11
no abstracts in English
Konovalov, S. V.; Mikhailovskii, A. B.*; Kovalishen, E. A.*; Kamenets, F. F.*; Ozeki, Takahisa; Shirokov, M. S.*; Takizuka, Tomonori; Tsypin, V. S.*
Doklady Physics, 49(7), p.405 - 408, 2004/07
Times Cited Count：1 Percentile：11.79(Mechanics)Effect of feedback system, placed outside the vacuum vessel, on neoclassical tearing modes (NTMs) in tokamaks is analytically studied. External matching parameters for magnetic island chain are derived for the simplest case of step profile of equilibrium parallel current. The structure of dynamical equations for the island width and island rotation frequency in terms of matching parameters is explained. It is shown that such a feedback system can lead to essential shift of the island rotation, in particular, for stabilization of NTMs by the polarization current effect.
