Showing papers by "Hiroaki Tsutsui published in 2002"

Optimization of SMES coil by using virial theorem

Abstract: The coil for the superconducting magnetic energy storage (SMES) is optimized by use of the virial theorem with stored energy and stress. In this work, we show the theoretical limit of stored energy with the maximum stress. To achieve the ideal limit, we propose the toroidal coil with helical winding. It is a hybrid coil of a toroidal field (TF) coil and a solenoidal coil helically wound on a torus. The winding is modulated in such a way that the toroidal field is created in the torus whereas the poloidal field is only out of the torus. In this case, the electromagnetic force is represented by the difference in the poloidal and the toroidal magnetic pressure. The virial theorem in the magnet is the relation of the magnetic energy and the averaged stress, and shows that the best coil to store the magnetic energy under the weakest averaged stress requires equal averaged principal stresses in all directions, which determines the ratio of the poloidal and toroidal current of our toroidal coil. The coil increases the magnetic energy to 4 times the conventional TF coil with the same maximum stress.

Variations of force-balanced coils for SMES

Abstract: Strong electromagnetic force caused by high magnetic fields and coil current is a serious problem in superconducting magnetic energy storage (SMES) systems. In facing this problem, we proposed the concept of the force-balanced coil (FBC) which is a helically wound toroidal coil applied to SMES. This paper shows the variations of the helical windings in order to reduce structure requirements. We discuss the relationship between the shape of SMES coils and the structure requirements based on the virial theorem. From this result, the FBC can minimize the working stresses by selecting the optimal number of poloidal turns.

Optimization of helical hybrid coil for high field tokamak

Abstract: A new toroidal field (TF) coil whose stress is much reduced is proposed for tokamaks by use of the virial theorem. It is a hybrid coil of a conventional TF coil and an ohmic heating (OH) coil helically wound on a torus. The winding is modulated in such a way that the poloidal field exists only outside of the torus, because the poloidal field in the torus prevents the breakdown of plasma and causes the torsional force. According to the virial theorem, the best TF coil to produce the strongest magnetic field under the weakest averaged stress requires equal averaged principal stresses of all directions. Therefore, the pitch number of the helical coil is determined to satisfy the uniform stress condition. The enhancement factor of the magnetic field in our TF coil increases as the aspect ratio A decreases where the distribution of the stress by the toroidal effect is important. In the case of A = 2, our optimal coil increases the magnetic field to 1.4 times larger than the conventional TF coil. As concerning stress, it is reduced by a factor of 2 in comparison with the conventional TF coil.

Bench testing of magneto-optic polarimeters for long-pulsed fusion devices

Abstract: We have developed magneto-optic polarimeters as magnetic field sensors for long-pulsed fusion devices. In this paper, results of bench testing of a polarimeter using a pair of photoelastic modulators (PEMs) and a superluminescent diode (SLD) with a center wavelength of 822 nm are described. Angle resolutions of 0.004/spl deg/ and 0.002/spl deg/ with response times of 1 ms and 10 ms, respectively were achieved at an incident polarization angle of about 0/spl deg/ while those at 21/spl deg/ were 0.07/spl deg/ independently on the response time. When a 40-mm-long ZnSe sensing rod was used, the resolution of 0.07/spl deg/ corresponds to a magnetic field resolution of 7 gauss. The long-time continuous measurement demonstrated good stability and little dependence on the room temperature in comparison with a polarimeter by the polarization rotation method.

Experimental study on a high-temperature superconducting helical coil

Abstract: High temperature superconductors (HTS) are expected to improve small-sized superconducting magnetic energy storage (SMES) systems. On the other hand, HTS conductors are extremely brittle so that the SMES with HTS coils requires special structural considerations to limit tensile stresses. We propose the concept of the force-balanced coil (FBC) which is a helically wound toroidal coil applied to SMES. The FBC can minimize the working stresses and reduce the mass of the structure for energy storage. However, the winding of the FBC is a three-dimensional complex shape so that it may be difficult to manufacture the helical windings without a decrease in the critical current of HTS conductors. To estimate the helical winding technique problems, we designed and fabricated a small helical coil using 340 m of Ag sheathed Bi-2223 HTS tapes. This paper describes the experimental results with liquid nitrogen cooling and a solution to the helical winding technique problems in order to prevent a drop in the critical current of HTS conductors.

Mathematical approach to current sharing problem of superconducting triple strands

Abstract: Current sharing between insulated strands in a superconducting cable is one of the important problems for its utilization. From the view points of the inverse problem, the sensitivity of current sharing between the insulated strands is determined by the condition number of the inductance matrix. For triple strands with self similar structure, we derive the analytic form of the inductance matrix which only includes two parameters; the self inductance of a unit wire and the ratio of mutual to self inductance for unit wires. Since the matrix elements also have self similar structure, we can analytically obtain the eigenvalues, eigenvectors and condition number, which is the ratio of maximum and minimum eigenvalues. Next, we derive the formula to estimate the sensitivity of the current distribution against the displacement of inductance from the ideal case by use of the condition number. This formula shows that the sensitivity is inversely proportional to the difference of self and mutual inductances of unit wires. Moreover, we estimate the condition number of very thin wire to check our formula. Finally, we verify our analytic form by numerical calculations.

Development of Helically Winding Machine for Force-Balanced Coil and Winding Technique

Abstract: Strong electromagnetic force caused by high magnetic fields and coil current is a serious problem in Superconducting Magnetic Energy Storage (SMES) systems and magnetically confined fusion devices. In facing this problem, we proposed the concept of Force-Balanced Coil (FBC) applied to SMES and fusion devices. The FBC is a helically wound toroidal coil which balances the centering force with the hoop force. The helical winding of the FBC is modulated in order to reduce the torsional force. However, the winding of the FBC is a three-dimensional complex shape so that it may be difficult to manufacture the modulated helical windings. To overcome this difficulty, we developed a helically winding machine. In this paper, we discuss the winding techniques by using the winding machine and improve the shape of the helical windings from the view point of the reduction of the time consumed in winding the coil.