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Journal Articles

Roles of the double tearing mode on the formation of a current hole

Tsuda, Takashi; Kurita, Genichi; Fujita, Takaaki

Journal of Plasma Physics, 72(6), p.1149 - 1152, 2006/12

 Times Cited Count:3 Percentile:11.73(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.

Journal Articles

Stability of double tearing mode in current hole configuration

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.

Journal Articles

Profile formation and sustainment of autonomous tokamak plasma with current hole configuration

Hayashi, Nobuhiko; Takizuka, Tomonori; Ozeki, Takahisa

Nuclear Fusion, 45(8), p.933 - 941, 2005/08

 Times Cited Count:13 Percentile:43.77(Physics, Fluids & Plasmas)

We have investigated profile formation and sustainment of current hole (CH) plasma by 1.5D transport simulations with current limit model inside CH based on Axisymmetric Tri-Magnetic-Islands equilibrium. Sharp reduction of anomalous transport in RS region can reproduce JT-60U experiments. The transport becomes neoclassical-level in RS region, which results in autonomous profile formation of ITB and CH through large bootstrap current. ITB width determined by neoclassical-level transport agrees with that in JT-60U. Energy confinement inside ITB agrees with JT-60U scaling. The scaling means that core plasma inside ITB is governed by MHD equilibrium limit, i.e., autonomous limitation of energy confinement. The plasma with large CH is sustained with full current drive by bootstrap current. The plasma with small CH and small bootstrap current fraction shrinks due to penetration of inductive current. This shrink is prevented and CH size can be controlled by appropriate external CD. CH plasma is found to respond autonomically to external CD. Application of CH plasma to reactor is discussed.

Journal Articles

Current clamp at zero level in JT-60U current hole plasmas

Fujita, Takaaki; Suzuki, Takahiro; Oikawa, Toshihiro; Isayama, Akihiko; Hatae, Takaki; Naito, Osamu; Sakamoto, Yoshiteru; Hayashi, Nobuhiko; Hamamatsu, Kiyotaka; Ide, Shunsuke; et al.

Physical Review Letters, 95(7), p.075001_1 - 075001_4, 2005/08

 Times Cited Count:14 Percentile:62.24(Physics, Multidisciplinary)

We found that no current can be driven in a central region of high-temperature, magnetically-confined, axi-symmetric torus plasma once the central current density becomes nearly zero ("current hole"), in spite of high electric conductivity. The current clamp was observed against current drive by a toroidal electric field and a radio-frequency wave in experiments on the JT-60U tokamak. This is a new, stiff, self-organized structure of magnetic field in an axi-symmetric torus plasma.

Journal Articles

Profile formation and sustainment of autonomous tokamak plasma with current hole configuration

Hayashi, Nobuhiko; Takizuka, Tomonori; Ozeki, Takahisa

Proceedings of 20th IAEA Fusion Energy Conference (FEC 2004) (CD-ROM), 8 Pages, 2004/11

Profile formation and sustainment of tokamak plasmas with current hole (CH) have been investigated by using 1.5D transport simulations. A current limit model inside CH based on the Axisymmetric Tri-Magnetic-Islands equilibrium is introduced. We found that the sharp reduction of anomalous transport in the reversed-shear (RS) region can reproduce JT-60U experiment. The transport becomes neoclassical-level in RS region, which results in the autonomous formation of internal transport barrier (ITB) and CH through large bootstrap current. ITB width determined by neoclassical-level transport agrees with that in JT-60U. Energy confinement inside ITB agrees with JT-60U scaling, which means autonomous limitation of energy confinement. The plasma with large CH is sustained with the full current drive by bootstrap current. The plasma with small CH and small bootstrap current fraction shrinks due to inductive current penetration. This shrink is prevented and the CH size can be controlled by appropriate external current drive (CD). The CH plasma is found to respond autonomically to external CD.

Journal Articles

Steady-state operation scenarios with a central current hole for JT-60SC

Tamai, Hiroshi; Ishida, Shinichi; Kurita, Genichi; Shirai, Hiroshi; Tsuchiya, Katsuhiko; Sakurai, Shinji; Matsukawa, Makoto; Sakasai, Akira

Fusion Science and Technology, 45(4), p.521 - 528, 2004/06

 Times Cited Count:0 Percentile:0.01(Nuclear Science & Technology)

The 1.5D time-dependent transport analysis has been carried out to investigate steady state operation scenarios with a central current hole by off-axis current drive schemes consistent with a high bootstrap current fraction for a large superconducting tokamak JT-60SC. A steady state operation scenario with HH$$_{y2}$$=1.4 and $$beta$$$$_{N}$$=3.7 has been obtained at I$$_{p}$$=1.5 MA, B$$_{t}$$=2 T and q$$_{95}$$=5 where non-inductive currents are developed during the discharge to form a current hole with beam driven currents by tangential off-axis beams in combination with bootstrap currents by additional on-axis perpendicular beams. The bootstrap fraction increases up to nearly 75% of the plasma current and the current hole region is enlarged up to about 30% of the minor radius at 35 s from the discharge initiation. The current hole is confirmed to be sustained afterward for a long duration of 60 s. The stability analysis shows that the beta limit with the conducting wall can be about $$beta$$$$_{N}$$=4.5, which is substantially above the no wall ideal MHD limit.

Journal Articles

Variation of current profiles in tokamaks; Formation mechanism and confinement property of current-hole configuration

Takizuka, Tomonori

Purazuma, Kaku Yugo Gakkai-Shi, 79(11), p.1123 - 1129, 2003/11

no abstracts in English

Journal Articles

Study of improved confinement modes with edge and/or internal transport barriers on the Japan Atomic Energy Research Institute Tokamak-60 Upgrade (JT-60U)

Miura, Yukitoshi; JT-60 Team

Physics of Plasmas, 10(5), p.1809 - 1815, 2003/05

 Times Cited Count:9 Percentile:30.38(Physics, Fluids & Plasmas)

no abstracts in English

Journal Articles

Studies of MHD behaviour in JT-60U

Ozeki, Takahisa; JT-60 Team

Plasma Physics and Controlled Fusion, 45(5), p.645 - 655, 2003/05

 Times Cited Count:23 Percentile:59.6(Physics, Fluids & Plasmas)

Plasmas of an advanced tokamak have a hollow current profile and a good confinement character, consistent with the large bootstrap current induced by a high beta. In an extreme situation of the hollow current profile, it was found for the first time in JT-60U that there is no plasma current in the central region, called Current Hole. The current hole formed by the growth of the bootstrap current exists stably for several seconds. In the region of the current hole, there is almost no gradient of the density, temperature and the toroidal rotation velocity, and no global MHD instability is obtained. In this paper, physics related to the current hole, i.e., possible equilibrium, instability and phenomena of energetic particles, are reviewed, and a prospect for steady-state fusion plasmas is discussed.

Journal Articles

Axisymmetric tri-magnetic-islands equilibrium of strongly-reversed-shear tokamak plasma; An Idea for the current hole

Takizuka, Tomonori

Purazuma, Kaku Yugo Gakkai-Shi, 78(12), p.1282 - 1284, 2002/12

An idea of a new equilibrium of a strongly-reversed-shear tokamak plasma with a current hole is proposed. This equilibrium configuration called "Axisymmetric Tri-Magnetic-Islands (ATMI) equilibrium" has three islands, a central-negative-current island and side-positive-current islands, along the R direction and two x-points along the Z direction. The equilibrium is stable with the elongation coils when the current in the ATMI region is limited small.

Journal Articles

Steady-state operation scenarios with a central current hole for JT-60SC

Tamai, Hiroshi; Ishida, Shinichi; Kurita, Genichi; Sakamoto, Yoshiteru; Fujita, Takaaki; Shirai, Hiroshi; Tsuchiya, Katsuhiko; Matsukawa, Makoto; Sakasai, Akira; Sakurai, Shinji; et al.

Proceedings of 29th European Physical Society Conference on Plasma Physics and Controlled Fusion, 4 Pages, 2002/00

This paper presents the feasibility and issues of steady state operation with a central current hole for JT-60SC, the superconducting tokamak to be modified from JT-60, from the view of reactor applicability of the current hole. An impact of the current hole on fusion engineering would stand in no necessity of central current drive leading to a remarkable reduction of neutral beam injection energy. The 1.5D time-dependent transport code analysis is made with using thermal and particle transport coefficients deduced from observations in JT-60U as a function of the magnetic shear. A steady state operation with HHy2~1.6 and beta_N~4 is obtained at Ip=1.5 MA, Bt=2 T and q95=4.5 by off-axis beams of 11.2 MW. The bootstrap fraction of ~75% of the plasma current and the current hole region of ~30% of the minor radius are sustained up to 70 s. The results suggest that a bootstrap current evolution near the current hole region and a relation between the location of ITB and the central current hole region are important to achieve a steady state plasma compatible with the current hole.

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