Faculty Profiles - KOJIMA Hiroki

Toroidal Dependence of Hydrogen Recycling Property with Helical Magnetic Perturbation in Long Time Discharge International journal

H. Kojima, S. Takamura

Europhysics Conference Abstracts Vol. 25A page： 2129-2132 2001.12

Effects of Dynamic Island Formations on Rotating Helical Magnetic Field Penetration into the Tokamak Plasma International journal

Y. Kikuchi, H. Kojima, T. Taniyama, M. Toyoda, Y. Uesugi, S. Takamura

Europhysics Conference Abstracts Vol. 25A page： 277-280 2001.12

Characteristics of Local Helical Coil System for Dynamic Ergodic Divertor Reviewed

Y. Kikuchi, H. Kojima, T. Taniyama, M. Toyoda, Y. Uesugi, S. Takamura

IEEJ Transactions on Fundamentals and Materials Vol. 121 ( 3 ) page： 249 - 255 2001.3

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：The Institute of Electrical Engineers of Japan

Heat and particle flux control is one of the most important and difficult problems in steady state fusion devices. Dynamic Ergodic Divertor (DED) is the advanced concept that a rotational perturbed magnetic field is applied to make tokamak edge magnetic structure stochastic so that it may decrease the heat and particle flux onto the local target in the conventional divertor. This rotational perturbed field is created by two sets of local helical coils (LHC) with the phase difference of 90 degrees in supplied AC currents. For the DED experiment, it is important to know the perturbed magnetic field structure due to this coil currents. We obtain the mode structure of the rotating helical field using a new Fourier analysis so that we may improve the mode quality by employing a new coil configuration which has an addtional series of return windings so as to avoid unnecessary mode and decrease the coil self-inductance.

DOI： 10.1541/ieejfms1990.121.3_249

CiNii Research

Penetration Process of the Externally Applied Rotating Helical Field into the Tokamak Plasma International journal

M. Kobayashi, H. Kojima, S. Takamura

Europhysics Conference Abstracts Vol. 24B page： 1404-1407 2001.1

Research Tokamak System with Multimode Discharges Using Inverter Power Supply Invited International journal

H. Kojima, M. Kobayashi, K. Tashiro, M. Takagi, S. Takamura

Electrical Engineering in Japan Vol. 133 ( 4 ) page： 8 - 18 2000.12

Authorship：Lead author,　Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：Wiley Periodicals, LLC.

In the Current Sustaining Tokamak of the Nagoya University (CSTN)-IV research tokamak system, using a compact 40-kHz pulse width modulation (PWM) inverter power supply, which is controlled through the LabVIEW program, we construct a new tokamak discharge system with multimode including a stable alternating current discharge and a high-repetition high-duty one. These discharge modes can be operated continuously for as long as 60 seconds. The continuous discharge of long duration is able to simulate the important physical and chemical processes of long discharges in fusion devices, in which the heat load to the wall and the particle balance in the plasma-wall system are crucial topics in order to realize a long pulse fusion reactor, like ITER. An ergodic divertor is one of the tools used to control the particle balance and the heat load to the wall. In addition, we installed another inverter power supply to generate a rotating magnetic perturbation for a dynamic ergodic divertor (DED) with the appropriate measurement system so that we could carry out experiments on heat and particle control with DED in prolonged operation.

DOI： 10.1002/1520-6416(200012)133:4<8::AID-EEJ2>3.3.CO;2-9

DOI： 10.1002/1520-6416(200012)133:4<8::AID-EEJ2>3.0.CO;2-I

Frequency Dependence of Tokamak Plasma Response to the Externally Applied Rotating Helical Field Reviewed International journal

M. Kobayashi, H. Kojima, K. Zhai, S. Takamura

Physics of Plasmas Vol. 7 ( 8 ) page： 3288 - 3300 2000.8

Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Publishing

The frequency dependence of the tokamak plasma response to the externally applied rotating helical field was investigated by measuring the radial profile of the perturbation field with small magnetic probes which were inserted in the plasma. The experimental results are discussed, directly comparing with the generally accepted linear theory, and taking into account the EXB drift and diamagnetic effect of the plasma. It was found that the experimental results are in good agreement with the simple linear analysis based on the single helicity approximation in a cylindrical geometry, except for the ergodic region, which comes from the overlapping of several sideband components of the perturbation. It is also found that the radial component of the perturbation field is still amplified inside the resonance surface (r<rs), even when a partial shielding takes place at r ≈rs. © 2000 American Institute of Physics.

DOI： 10.1063/1.874194

Electrostatic Fluctuations in the Current Sustained Torus IV at Nagoya University Reviewed International journal

K. Zhai, M. Kobayashi, K. Tashiro, H. Kojima, M. Takagi, S. Takamura

Physics of Plasmas Vol. 7 ( 6 ) page： 2449 - 2455 2000.6

Language：English Publishing type：Research paper (scientific journal) Publisher：AIP Publishing

A detailed experimental investigation of plasma fluctuation and its modification due to radial electric field effects, both its shear and curvature, has been carried out on the Current Sustained Torus IV at Nagoya University [Takamura et al., J. Plasma Fusion Res. 74, 38 (1998)] through measurement with an array of Langmuir probes. The observed statistical dispersion relation indicates a drift-wave-type turbulence. An examination of the radial electric field shear and curvature effects on the fluctuations as well as comparisons to theoretical predictions are presented. The decorrelation of turbulence is found to be influenced by the radial electric field shear in our experiment. A modification of the poloidal correlation length and the two-poloidal-point coherency due to the radial electric field has also been identified. Nonlinear analysis of quadratic mode coupling indicates that the radial electric field can affect the fluctuation amplitudes by modifying the coupling between different spectral components of the fluctuations. © 2000 American Institute of Physics.

DOI： 10.1063/1.874084

Interaction of Externally Applied Rotating Helical Field with Tokamak Plasma Reviewed International journal

M. Kobayashi, T. Tuda, K. Tashiro, H. Kojima, K. Zhai, S. Takamura

Nuclear Fusion Vol. 40 ( 2 ) page： 181 - 193 2000.2

Language：English Publishing type：Research paper (scientific journal) Publisher：IOP Publishing

The penetration process of a rotating magnetic perturbation into a rotating tokamak plasma has been investigated, taking account of the mode structure of the perturbations, and using a small magnetic probe in the small research tokamak CSTN-IV. It was observed that the radial component of the perturbation was amplified in the plasma when magnetic islands are formed, while the poloidal component was attenuated but, deep inside the plasma, amplified. Using a resistive MHD code, such modifications of external perturbation were found to be caused by a redistribution of plasma current due to the formation of magnetic islands. Considering the phase velocity of both the perturbation and the intrinsic plasma rotation, it was also found that the sideband component substantially affects the penetration process with magnetic islands forming. Therefore, in dynamic ergodic divertor operation, it is necessary to put the emphasis not only on the frequency of the perturbation fields but also on their direction, mode structure and the radial profile of the intrinsic plasma rotation at the same time.

DOI： 10.1088/0029-5515/40/2/304

Research Tokamak System with Multi-Mode Discharges Using Inverter Power Supply Reviewed

H. Kojima, M. Kobayashi, K. Tashiro, M. Takagi, S. Takamura

IEEJ Transactions on Fundamentals and Materials Vol. 119 ( 12 ) page： 1436 - 1444 1999.12

Authorship：Lead author,　Corresponding author Language：Japanese Publishing type：Research paper (scientific journal) Publisher：The Institute of Electrical Engineers of Japan

In Current Sustaining Tokamak in Nagoya university (CSTN)-IV research tokamak system using a compact 40kHz pulse width modulation (PWM) inverter power supply, which is controlled through LabVIEW program, we construct a new tokamak discharge system with multi-mode including a stable alternating current discharge and a high-repetition high-duty one. These discharge modes can be operated continuously for as long as 60sec. The continuous discharge with long duration is able to simulate the important physical and chemical processes of long time discharges in fusion devices, in which the heat load to the wall and the particle balance in the plasma-wall system are crucial topics in order to realize a long pulse fusion reactor, like ITER. Employing ergodic divertor (ED) is one of tools to control the particle balance and the heat load to the wall. In addition, we installed another inverter power supply to generate a rotating magnetic perturbation for dynamic ergodic divertor (DED) with the appropriate measurement system so that we may carry out experiments on heat and particle control with DED at long time operation.

DOI： 10.1541/ieejfms1990.119.12_1436

CiNii Research

Experimental Investigation on Penetration of Rotating Magnetic Perturbations into the Tokamak Plasma International journal

M. Kobayashi, T. Tuda, Z. Kan, H. Kojima, K. Tashiro, S. Takamura

Europhysics Conference Abstracts Vol. 23J page： 437-440 1999.7

Two-dimensional Structure of the Detached Recombining Helium Plasma Associated with Molecular Activated Recombination Reviewed International coauthorship International journal

D. Nishijima, N. Ezumi, H. Kojima, N. Ohno, S. Takamura, S. I. Krasheninnikov, A. Yu. Pigarov

Journal of Nuclear Materials Vol. 266 page： 1161 - 1166 1999.3

Language：English Publishing type：Research paper (scientific journal) Publisher：Elsevier B.V.

The structural change of the detached plasmas associated with the molecular activated recombination (MAR) has been studied in a high density helium plasma with helium or hydrogen puff in the linear divertor plasma simulator, NAGDIS-II. The ion particle flux with the hydrogen gas puff starts to decrease in the upstream region close to the plasma source, compared to that with the helium puff. The reduction of the ion particle flux along the magnetic field is found to depend on the plasma density. With the hydrogen gas puff, the attenuation of the ion particle flux is getting smaller as increasing plasma density, which is opposite tendency in pure helium detached plasma where the conventional electron-ion recombination (EIR) is dominating.

DOI： 10.1016/S0022-3115(98)00565-0

Contribution of Molecular Activated Recombination to Hydrogen Plasma Detachment in the Divertor Plasma Simulator NAGDIS-II Reviewed International coauthorship International journal

N. Ezumi, D. Nishijima, H. Kojima, N. Ohno, S. Takamura, S. I. Krasheninnikov, A. Yu. Pigarov

Journal of Nuclear Materials Vol. 266 page： 337 - 342 1999.3

Language：English Publishing type：Research paper (scientific journal) Publisher：Elsevier B.V.

We have performed experiments on detached hydrogen plasmas associated with molecular activated recombination (MAR) in a linear divertor plasma simulator. Reductions of the ion particle flux and heat load to the target plate were clearly observed in hydrogen plasmas with the hydrogen gas puff. Detailed analysis of Balmer series spectra with the Collisional-Radiative Atomic-Molecular data (CRAMD) code shows that MAR mainly appears as a weak dependence of the Balmer series emission intensities on experimental conditions in detached hydrogen plasmas. The plasma conditions necessary to obtain plasma detachment through MAR or EIR (electron ion recombination including the radiative and three-body recombination) in tokamak divertors are also discussed.

DOI： 10.1016/S0022-3115(98)00533-9