Showing papers by "Hiroaki Tsutsui published in 2003"

High performance tokamak experiments with a ferritic steel wall on JFT-2M

Abstract: Compatibility between the plasma and low activation ferritic steel, which is a candidate material for fusion demonstration reactors, has been investigated step by step in the JFT-2M tokamak. We have entered the third stage of the Advanced Material Tokamak EXperiment (AMTEX), where the inside of the vacuum vessel wall is completely covered with ferritic steel plates ferritic inside wall (FIW). The effects of a FIW on the plasma production, impurity release, the operation region, and H-mode characteristics have been investigated. No negative effect has been observed up to now. A high normalized beta plasma of βN ~ 3, having both an internal transport barrier and a steady H-mode edge was obtained. A remarkable reduction in ripple trapped loss from 0.26 MW m−2 (without ferritic steel) to less than 0.01 MW m−2 was demonstrated by the optimization of the thickness profile of FIW. A code to calculate fast ion losses, taking into account the full three-dimensional magnetic structure was developed, and values obtained using the code showed good agreement with experimental results. Thus, encouraging results are obtained for the use of this material in the demo-reactor.

Demonstration of the stress-minimized force-balanced coil concept for SMES

Abstract: Strong electromagnetic force caused by high magnetic field and coil current is a serious problem in SMES systems. In facing this problem, we proposed the concept of Force-Balanced Coil (FBC) which is a helically wound toroidal coil. Based on the virial theorem, the FBC can minimize structure requirements for energy storage by selecting an optimal number of poloidal turns. We designed and fabricated a small experimental device which is composed of inner and outer helical coils mutually wound in opposite toroidal directions using NbTi superconductors. The distribution of the working stresses in this device can be changed by selecting the optimal current ratios between inner and outer coil currents. From the experimental results, we demonstrated the validity of the FBC concept.

Distribution of stress in force-balanced coils on virial theorem

Abstract: Distributions of stress on a device with helical coils for the superconducting magnetic energy storage (SMES) are analyzed experimentally and numerically. In the previous work, we showed a theoretical limit of stored energy in a toroidal SMES system by the virial theorem. To verify the theoretical limit, we designed and construct a small device which is composed of two sets of helical superconducting coils and a toroidal frame of aluminum alloy. Strains by electromagnetic force on the frame were obtained experimentally. Distributions of stress on a toroidal shell with a similar configuration to the device are numerically calculated. Comparing the results of the experiments and the numerical calculations, a qualitative agreement of stress distribution between the calculation and the experiment is obtained in the toroidal direction, while discrepancies of stress in the poloidal direction are not negligible.

Design of compact high field tokamak based on virial theorem

Abstract: A novel tokamak with a new type of toroidal field (TF) coils and a central solenoid (CS) whose stress is much reduced to a theoretical limit determined by the virial theorem is designed and constructed. According to the virial theorem, the best TF coil to produce the strongest magnetic field under the weakest stress requires equal averaged principal stresses in all directions. Therefore, the pitch number of a helical coil is determined to satisfy the uniform stress condition. Moreover, the helical winding is modulated in such a way that poloidal field exists only outside of a torus, because the poloidal field in the torus prevents the breakdown of plasma and causes the torsional force. In the case of the aspect ratio A =2, our optimal coil theoretically increases the magnetic field to 1.4 times larger than the conventional TF coil.

Plasma production in a small high field force-balanced coil tokamak based on virial theorem

Abstract: The plasma production and confinement experiments in a novel tokamak device with a new type of toroidal field (TF) coils and a central solenoid (CS) whose stress is reduced to a theoretical limit determined by the virial theorem are presented. According to the virial theorem, the best TF coil to produce the strongest magnetic field under the weakest stress requires equal averaged principal stresses in all directions. Therefore, the pitch number of a helical coil is determined to satisfy the uniform stress condition. Moreover, the helical winding is modulated in such a way that poloidal field exists only outside of a torus, because the poloidal field in the torus prevents the breakdown of plasma and causes the torsional force. In the case of the aspect ratio A = 2, our optimal coil theoretically increases the magnetic field to 1.4 times larger than the conventional TF coil. In order to prove the advantage of the VLC concept, we manufactured a small VLC pulsed tokamak "Todoroki-II" with a major radius of 0.3 m, a minor radius of 0.08 m, toroidal magnetic field strengths of B/sub T/ < 1.5 T and plasma currents of I/sub p/ < 40 kA. External vertical field increased both the plasma pulse length and current to 1 ms and 11 kA, respectively, while they were restricted because of no vertical field control. Using a Cauchy-condition surface (CCS) method, the shape and displacement of plasma boundary was reconstructed, and the validity of CCS under large eddy currents was verified.