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Journal Articles

Neural-net predictor for beta limit disruptions in JT-60U

Yoshino, Ryuji

Nuclear Fusion, 45(11), p.1232 - 1246, 2005/11

 Times Cited Count:35 Percentile:73.97(Physics, Fluids & Plasmas)

Prediction of major disruptions observed at the $$beta$$-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 $$beta$$$$_{N}$$ limit every 2 ms. The target $$beta$$$$_{N}$$ limit is artificially set by the operator in the first step training and is modified in the second step training using the output $$beta$$$$_{N}$$ limit from the trained network. To improve the prediction performance further, the difference between the estimated $$beta$$$$_{N}$$ limit and the measured $$beta$$$$_{N}$$ 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.

Journal Articles

Role of low order rational $$q$$-values in the ITB events in JT-60U reverse shear plasmas

Neudatchin, S. V.*; Takizuka, Tomonori; Hayashi, Nobuhiko; Isayama, Akihiko; Shirai, Hiroshi; Fujita, Takaaki; Kamada, Yutaka; Koide, Yoshihiko; Suzuki, Takahiro

Nuclear Fusion, 44(9), p.945 - 953, 2004/09

The formation of internal transport barriers (ITBs) near q=3 surfaces in normal shear (NrS) discharges of JT-60U is known. In reverse shear (RS) JT-60U plasmas, the role of q minimum (qmin) equal to 3.5,3,2.5,2 was not obvious for ITB formation. ITB-events (non-local confinement bifurcations inside and around ITB) are found in various NrS and RS plasmas. Under sufficient heating power, ITB-events are seen at rational and not rational values of qmin. The space-time evolution of Te and Ti is similar, suggesting the same mechanism of Te and Ti transport. The temporal formation of strong ITB under passing of qmin=3 (after periodical improvements and degradations via ITB-events) in RS mode is presented. Under smaller power, ITB-events are observed only at rational values of qmin. In a weak RS shot, abrupt rise of Te is seen at qmin=3.5, while more cases of Ti rise are observed. The difference of Te and Ti evolutions seen regularly under the low power, suggests decoupling of Te and Ti transport.

Journal Articles

Role of low order rational ${it q}$-values in the ITB events in JT-60U reverse shear plasmas

Neudatchin, S. V.*; Takizuka, Tomonori; Hayashi, Nobuhiko; Isayama, Akihiko; Shirai, Hiroshi; Fujita, Takaaki; Kamada, Yutaka; Koide, Yoshihiko; Suzuki, Takahiro

Nuclear Fusion, 44(9), p.945 - 953, 2004/09

 Times Cited Count:14 Percentile:45.23(Physics, Fluids & Plasmas)

The formation of internal transport barriers (ITBs) near ${it q}$ =2,3 surfaces in normal shear (NrS) discharges of JT-60U and JET is known. In reverse shear (RS) JT-60U plasmas, the role of ${it q}$ minimum (${it q}$$$_{min}$$) equal to 3.5,3,2.5,2 is not obvious for ITB formation. In this paper, the effect of low rational ${it q}$ values on ITB-events in JT-60U NrS and RS plasmas is investigated. Under sufficient neutral beam power (${it P}$$$_{nbi}$$$$>$$8MW for 1.2-1.5MA/3.8T pulses), ITB-events are seen at rational and non-rational values of ${it q}$$$_{min}$$. The space-time evolutions of ${it T}$$$_{e}$$ and ${it T}$$$_{i}$$ are similar, suggesting the same mechanism for ${it T}$$$_{e}$$ and ${it T}$$$_{i}$$ transports. The temporal formation of stronger ITB under passing ${it q}$$$_{min}$$=3 (after periodical improvements and degradations via ITB-events) in RS H-mode with ${it P}$$$_{nbi}$$=8MW is presented. Under smaller power, ITB-events are observed only at rational ${it q}$$$_{min}$$ values. In a weak RS shot with ${it P}$$$_{nbi}$$=4MW, the abrupt rise of ${it T}$$$_{e}$$ is seen at ${it q}$$$_{min}$$=3.5, while more cases of ${it T}$$$_{i}$$ rise are observed. The difference between the ${it T}$$$_{e}$$ and ${it T}$$$_{i}$$ evolutions for the low power suggests the decoupling of ${it T}$$$_{e}$$ and ${it T}$$$_{i}$$ transports.

Journal Articles

Theory and simulation of self-organized critical transport driven by toroidal ITG turbulence in tokamak plasmas

Kishimoto, Yasuaki; J.Y.Kim*; Tajima, Toshiki*; W.Horton*; J.Furnish*

Theory of Fusion Plasmas, ISPP17, p.213 - 226, 1997/00

no abstracts in English

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