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ABSTRACT: The radiation losses and the global power balance for Ohmic and neutral beam heated plasmas have been investigated in different JT-60 configurations. Discharges with a TiC coated molybdenum wall and with a graphite wall, with limiter, outer and lower X-point configurations have been studied by bolometric measurements, thermocouples and an infrared TV camera. In neutral beam heated outer X-point discharges with a TiC coated molybdenum first wall, the radiation loss of the main plasma was very low (10% of the absorbed power). The radiation loss due to oxygen was dominant in this case. On the contrary, in discharges with TiC coated molybdenum limiters the radiation loss was very high (>60% of the absorbed power). In the discharges with a graphite wall the radiated power from the main plasma was 20-25% for both limiter and lower X-point configurations. In lower X-point discharges the main contributor to the radiation loss was oxygen, whereas in limiter discharges the loss due to carbon was equal to the loss due to oxygen. The radiation loss from the lower X-point divertor increased with increasing electron density of the main plasma.
Nuclear Fusion 01/2011; 30(6):1095. · 4.09 Impact Factor
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ABSTRACT: In the JT-60 tokamak with nearly perpendicular neutral hydrogen beam injection of up to ~ 21 MW, some regimes of plasma species and beam directions have been observed in which the sawtooth periods are significantly extended. It is found that the use of helium as working gas together with co-injection of neutral beams can have an important influence on the occurrence of extended sawtooth periods. Beam induced stabilizing effects on sawteeth and internal kink modes are also discussed.
Nuclear Fusion 01/2011; 30(3):552. · 4.09 Impact Factor
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ABSTRACT: Particle transport in JT-60 was investigated with a simple perturbation method. From an analysis of the sawtooth induced density perturbations, it was found that the particle diffusivity has an inverse dependence on the electron density and that the measured diffusion coefficients correspond to the values obtained from the analysis of pellet induced perturbations. Also, from an analysis of the density and the heat pulse propagations, the ratio of the electron thermal diffusivity and the particle diffusivity, χe/D, was estimated to be about four. These experimental results were compared with the drift wave transport models.
Nuclear Fusion 01/2011; 30(11):2367. · 4.09 Impact Factor
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H. Nakamura,
S. Tsuji,
M. Nagami,
T. Ozeki,
S. Ishida,
M. Azumi,
M. Akiba,
T. Ando,
T. Fujii,
T. Fukuda, [......],
M. Saigusa,
M. Sato,
K. Shimizu,
H. Shirai,
T. Sugie,
H. Takeuchi,
K. Uehara,
K. Ushigusa,
H. Yoshida,
R. Yoshino
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ABSTRACT: In JT-60, H-mode experiments with outer and lower divertors have been performed. In the outer divertor discharge, an H-mode similar to the modes observed in the lower/upper divertor discharges is obtained. Its threshold absorbed power and electron density are 16 MW and 1.8 × 1019 m−3. In the two schemes of combined heating with NB + ICRF and NB + LHRF, H-mode discharges are also obtained. Moreover, in the new configuration with the JT-60 lower divertor, H-mode phases with and without edge localized modes are obtained. The improvement in the energy confinement time in both divertor configurations is limited to values within 10%. The paper mainly presents the H-mode results of the outer divertor discharges. Also, typical results of the lower divertor discharges are shown for a comparison of the H-mode characteristics of the two configurations.
Nuclear Fusion 01/2011; 30(2):235. · 4.09 Impact Factor
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H. Kubo,
T. Sugie,
A. Sakasai,
Y. Koide,
N. Nishino,
T. Hirayama,
T. Nishitani, K. Nagashima,
N. Akaoka,
H. Takeuchi,
Team
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ABSTRACT: Impurities in JT-60 were studied by visible and VUV spectroscopy over two periods of operation. The two periods were distinguished by the use of different first wall materials: from April 1985 to March 1987, TiC coated molybdenum was used as limiter material; from June to October 1987, graphite was used. Quantitative spectroscopic measurements of Zeff, impurity concentrations and radiated power losses were made for ohmically heated and neutral beam heated discharges with limiter and divertor configurations. In the first phase with metallic first wall material, oxygen, carbon and titanium were identified as the main plasma impurities. In neutral beam heated, diverted discharges, Zeff was 1.6 at e = 4 × 1019 m−3. The concentrations of oxygen, carbon and titanium were 1%, 0.1% and 0.006% of ne, respectively. In the second phase with graphite material, the metallic impurities were reduced, and the contribution of metallic impurities to the radiated power loss was less than 1%. However, Zeff increased up to 3 in neutral beam heated discharges. In limited plasmas, the concentrations of oxygen and carbon were 1% and 5%, respectively, at e = 4 × 1019 m−3; in diverted plasmas, these concentrations were 2% and 0.4% at the same e. The radiated power loss from the main plasma was 20–40% of the input power in neutral beam heated, limited discharges, and 7–25% in diverted discharges. The contributions of oxygen and carbon to the radiated power in limited discharges were comparable, and in diverted discharges the contribution of oxygen was dominant.
Nuclear Fusion 01/2011; 29(4):571. · 4.09 Impact Factor
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Y. Koide,
T. Hirayama,
T. Sugie,
A. Sakasai,
H. Kubo,
N. Akaoka,
T. Nishitani, K. Nagashima,
H. Shirai,
H. Takeuchi,
Team
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ABSTRACT: The impurity transport characteristics of ohmically and neutral beam heated diverted plasmas in JT-60 have been studied. In sawtoothfree, ohmically heated plasmas with Ip = 1.5 MA, e = (2−3) × 1019 m−3, q(a) = 3.2, no radiation buildup is observed. The temporal evolution of injected titanium has been measured by VUV and X-ray crystal spectrometers and PIN diodes. The spectral lines of Ti XX (259.3 Å) and Ti XXI (2.6097 Å) are compared with one-dimensional diffusive/convective impurity transport calculations. The transport calculations for Ohmic discharges with Ip = 1.5−2 MA, e = (1–3) × 1019 m–3, q(a) = 3−3.7, show that DA =(1−2) m2s−1 and that the inward convective velocity is small. In neutral beam heated plasmas with Ip = 1.5 MA, e = 4 × 1019 m–3, q(a)= 3.6, Pb = 8 MW, no difference is observed in the global time evolution of the Ti XXI line emissions between co- and counter-injected discharges. The experimental observations are well represented by choosing DA = (1–2) m2s−1 and a small inward velocity.
Nuclear Fusion 01/2011; 28(10):1835. · 4.09 Impact Factor
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ABSTRACT: The characteristics of particle confinement and recycling of ohmically and neutral beam (NB) heated hydrogen plasmas in the JT-60 tokamak are investigated by using the global particle balance equation. The total number of ionization events is estimated by employing a simple formula for the Hα radiation profile, and the gas fuelling efficiency is determined from the time derivatives of the electron density before and after gas feed termination. The global particle confinement time in the Ohmic heating divertor plasma decreases nearly inversely proportionally to the electron density, because of the enhancement of recycling in the plasma periphery. NB injection deteriorates the particle confinement time by a factor of about two. The effective particle confinement time closely related to the density control increases proportionally to the electron density in the Ohmic plasma. In contrast, the effective particle confinement time of the NB heated plasma is independent of the electron density although it decreases with the NB injection power. These results show that the plasma density can be controlled by the gas puff in both ohmically and NB heated hydrogen discharges.
Nuclear Fusion 01/2011; 27(8):1203. · 4.09 Impact Factor
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ABSTRACT: Stability against ideal infinite-n ballooning and low-n internal kink modes is analysed for pellet fuelled plasmas in JT-60, which have strongly peaked pressure profiles within the q = 1 resonance surface. The observed pressure gradient in pellet fuelled plasmas locally reaches the marginal value for the infinite-n ballooning mode limit just inside the q = 1 surface. After reaching the marginal value on the magnetic flux surface, the marginally stable region grows inside the q = 1 surface. However, the poloidal beta within the q = 1 surface, βp1, does not reach the limit imposed by the ideal ballooning mode. The results of the analysis of the low-n kink mode using experimental observations show that the maximum βp1 attained is consistent with the beta limit imposed by the n = 1 internal kink mode.
Nuclear Fusion 01/2011; 31(1):51. · 4.09 Impact Factor
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ABSTRACT: High temperature bulk superconductors have significant potential for various engineering applications such as a flywheel energy storage system. This system is expected to decrease the energy loss by using bulk superconductors for the bearing. Recently, the authors have developed a new superconducting magnet to realize the large levitation force. The magnet has two coils on the same axis and each coil's current flows to the opposite direction. In this system, the axial component of magnetic field is canceled each other but the radial component of magnetic field is expected to be enhanced. In this paper, the authors measured the levitation force and its time relaxation at different bulk positions in both cases of one and two bulks by using this new magnet. From the results, it was made clear that the levitation force of two bulks was twice as large as that of one bulk and its time relaxation was decreased. In addition, Hall sensors were attached on the sample surface to measure the surface magnetic field distribution. The behavior of the magnetic field was discussed.
IEEE Transactions on Appiled Superconductivity 07/2009; · 1.04 Impact Factor
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ABSTRACT: The authors conducted a study on superconducting magnetic bearing, which consists of superconducting rotor and stator to apply the flywheel energy-storage system for railways. In this study, high temperature bulk superconductor (HTS bulk) was combined with superconducting coils to increase the load capacity of the bearing. In the first step of the study, the thrust rolling bearing was selected for application by using liquid nitrogen cooled HTS bulk. 60mm-diameter HTS bulks and superconducting coil which generated a high gradient of magnetic field by cusp field were adopted as a rotor and a stator for superconducting magnetic bearing, respectively. The results of the static load test and the rotation test, creep of the electromagnetic forces caused by static flux penetration and AC loss due to eccentric rotation were decreased to the level without any problems in substantial use by using two HTS bulks. In the result of verification of static load capacity, levitation force (thrust load) of 8900N or more was supportable, and stable static load capacity was obtainable when weight of 460kg was levitated.
Journal of Physics Conference Series 03/2008; 97(1):012101.
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ABSTRACT: In this research, we proposed a similarity-based image retrieval by self-organizing map with refractoriness. In the self-organizing map with refractoriness, the plural neurons in the map layer corresponding to the input can fire sequentially because of the refractoriness. The image retrieval system using the self-organizing map with refractoriness makes use of this property in order to retrieve plural similar images. In this image retrieval system, as the image feature, not only color information but also spectrum, impression words and key words are employed. We carried out a series of computer experiments and confirmed that the effectiveness of the proposed system.
Neural Networks, 2007. IJCNN 2007. International Joint Conference on; 09/2007
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ABSTRACT: For the development of a magnetic field compensation system suited for the open end of the cylindrical magnetic shell, magnetic field distributions were characterized on the axis of the magnetic shell for the externally impressed low-frequency AC magnetic field and for the compensating magnetic field generated by a set of ring coils. Based on this observation, a system for actively compensating the magnetic field leakage through the opening is proposed. A cylindrical magnetic shield of desk-top size was constructed by combining an axial shell made of 2 in wide Metglas 2705M amorphous tapes encircling the outer surface of a cylindrical body in its axial direction, with a magnetic shaking coil, and then a proposed compensation system was built on top of the structure. The axial shielding factor was improved with this active compensation system by 55 times for a 2 Hz impressed magnetic field, 20 times for a 10 Hz impressed field, ten times for a 30 Hz impressed field, and five times for a 50 Hz impressed field. An interesting phenomenon observed during the characterization of the magnetic fields is that the magnetic field in the space inside the axial magnetic shell leads in phase with respect to that of the impressed magnetic field, unlike the case with a helical shell where the magnetic field is lagging in phase. A model for explaining this phenomenon is also given.
IEEE Transactions on Magnetics 10/2003; · 1.36 Impact Factor
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ABSTRACT: This article shows a demonstration of magnetoencephalography measurement in a cylindrical magnetic shield made of cobalt-based amorphous tape with magnetic shaking. The noise levels of the first-order superconducting quantum interference device gradiometers that operated in the shield were reduced to as low as 40 fT /√ Hz at 10 Hz by surrounding it with a simple rf shield made of conductive cloth. We observed spontaneous alpha rhythms from a human brain in this shielding system. Alpha rhythms and their suppression caused by opening the eyes were clearly found, which was also confirmed by electroencephalography measurement from the same volunteer under similar conditions. © 2003 American Institute of Physics.
Journal of Applied Physics 06/2003; · 2.17 Impact Factor
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ABSTRACT: The stability of toroidal Alfven eigenmodes (TAEs) is presented for steady state plasmas in the JT-60 Super Upgrade (JT-60SU). Studies are carried out by using the ACCOME code for calculating self-consistent MHD equilibria and by using the NOVA-K code for analysing the TAE stability. Characteristics of the TAE stability are obtained for non-inductive steady state plasmas (3 MA/3 T) with a large bootstrap current and a 500 keV neutral beam (NB) current. Above the density value corresponding to VhVA~1, the TAE becomes unstable owing to the large pressure gradient, ΔPh, and the large β of the hot particles, (βh). Here, VhVA is the ratio of the hot particle velocity to the Alfven velocity. As the density and the temperature increase, the bootstrap current increases so that the NB power required for the current drive decreases. Consequently, both ΔPh and (βh) decrease and the TAEs are stabilized by ion Landau damping. In the high current and high toroidal field plasma (10 MA/6.25 T) case, because ΔPh is small, owing to high density, and (βh) is small, owing to high toroidal field, the TAE is stable for low n to medium n (≤15)
Nuclear Fusion 11/2002; 35(12):1553. · 4.09 Impact Factor
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S. Tsuji,
N. Nosogane,
K. Itami,
H. Kubo,
T. Nishitani,
M. Shimada,
Y. Koide,
N. Nishino,
T. Sugie, K. Nagashima,
Y. Kawano,
O. Naito,
H. Nakamura,
H. Yoshida,
N. Miya,
M. Nagami
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ABSTRACT: Improvement of both energy and particle confinement was observed in beam heated hydrogen plasmas in JT-60 when the heating power and the line averaged density exceeded thresholds. The radiation power from the divertor plasma increased to as much as 50% of the heating power. This regime was termed improved divertor confinement (IDC). The discharges with improvement of the energy confinement by up to 20% and enhanced divertor radiation lasted for several seconds in a quasi-steady state without significant impurity accumulation in the core. The threshold line averaged electron density for the IDC regime increased linearly with power and dropped with increasing safety factor. The improvement was observed only when the ion Del B drift was towards the X-point. The behaviour of light impurities and in-out asymmetries in particle recycling changed with reversal of the toroidal magnetic field. These findings are consistent with the theoretical model in which collisional transport near the separatrix is connected with the onset of the IDC regime
Nuclear Fusion 10/2002; 32(8):1313. · 4.09 Impact Factor
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ABSTRACT: Toroidal momentum transport is examined experimentally by using on- and off-axis tangential neutral beam injections on the JT-60U tokamak. From a steady state momentum balance analysis-on the assumption that momentum flux is diffusive-it is found that the profiles of the momentum diffusivity are quite different in the two cases of on- and off-axis beam injections. In addition, transient toroidal momentum transport was examined by using a momentum source modulation experiment. On the assumption that the toroidal momentum flux consists of a diffusive and a convective flow term, it is found that there is non-diffusive inward flux of toroidal momentum whose absolute value is comparable to that of the diffusive flux
Nuclear Fusion 10/2002; 34(3):449. · 4.09 Impact Factor
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ABSTRACT: A high-performance bench-top cylindrical magnetic shield made of amorphous ribbons is presented. It has double active shells made of Metglas 2705M amorphous ribbon to which magnetic shaking enhancement is applied. Passive shells made of Metglas 2714A amorphous ribbons were installed to reduce the leakage field of magnetic shaking. With total weight 1.3 kg of the amorphous ribbons, sufficiently large shielding performance was measured against 10-μT, 10-Hz uniform magnetic field. The value of the axial shielding factor (ASF) was 700 and transverse shielding factor (TSF) was more than 20,000. The residual dc fields were less than 20 nT in the axial direction and 5 nT in the radial direction under the worst condition, where the shield axis is oriented along the horizontal component (∼ 32 μT) of the Earth's magnetic field.
IEEE Transactions on Magnetics 10/2002; · 1.36 Impact Factor
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ABSTRACT: Experimentally observed sawtooth activities have been investigated for ohmic discharges in divertor operation of the JT-60 Tokamak through a detailed analysis of soft X-ray emissions and line electron density. To characterize the sawtooth behavior, the sawtooth period and inversion radius were measured and compared between hydrogen and helium plasmas, including the scaling studies. For both plasmas, most of the observed sawteeth were roughly grouped into compound and single types of sawteeth without precursor oscillations prior to an internal disruption, but often associated with m=odd successor oscillations. A particular interest lies in the experimental observation that the hydrogen discharge is apt to give rise to compound sawteeth, but the helium discharge is not. Such different features are discussed on the basis of the resistive skin time and the characteristic time of the growing inversion radius. The results suggest that some reason besides skin current effect must be more incorporated into the interpretation of the different sawtooth activities.
Plasma Physics and Controlled Fusion 12/2000; 30(8):1069. · 2.42 Impact Factor
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ABSTRACT: Recent advancement in the bulk fabrication process has enabled the production of large single-grain bulk RE-Ba-Cu-O superconductors with high Jc values, which can trap fields much higher than those associated with conventional permanent magnets. However, a large electromagnetic force acts on the bulk superconductors when they trap a large field, which sometimes leads to fatal fracture. In this study, we measured mechanical properties of single-domain Y-Ba-Cu-O and Sm-Ba-Cu-O bulk superconductors from the output of strain gauges glued directly to the sample surface. The stress was applied to the samples by reducing the external fields from 10 to 0 T at 50 K. It was found that Ag addition is effective in improving the mechanical properties of melt processed Y-Ba-Cu-O and Sm-Ba-Cu-O.
Superconductor Science and Technology 05/2000; 13(6):816. · 2.66 Impact Factor
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ABSTRACT: Bulk superconductors can function as quasi-permanents with
generating magnetic fields much higher than those of conventional
permanent magnets. However, a large electromagnetic force acts on such
bulk superconductors, which sometimes leads to fracture. It is known
that Ag addition is effective in improving the fracture toughness of
bulk Y-Ba-Cu-O superconductors. Therefore, in this paper, we have
studied the effects of Ag addition both on trapped field capability and
mechanical properties of bulk RE-Ba-Cu-O (RE: Y, Sm) and have found that
Ag addition is indeed effective in improving the mechanical properties
of a large single-grain RE-Ba-Cu-O
IEEE Transactions on Appiled Superconductivity 07/1999; · 1.04 Impact Factor
