We have proposed a new method for detecting unusual local conditions in a high-temperature superconducting coil. In this method, two kinds of pick up coils for measuring electric fields and magnetic fields are used to obtain Poynting's vectors around the coil. This is a noncontact method using pick-up coils around the vessel for cooling the coil. The method has the following benefits: safe detection, easy handling, high sensitivity, and high usability. The purpose of this paper is to clarify the relation of measured signals to the conditions of the windings. When a temperature rise has been produced locally in the Bi-2223 solenoidal coil, Poynting's vectors around the coil and the temperature of windings were measured. It was found that the measured signals rapidly decrease as the temperature of the winding achieved a critical level, making this a reliable method to estimate the conditions in the windings.

Improved Poynting's vector method without movements was proposed to measure ac losses of HTS tapes, formed into a short straight or a solenoidal coil, which were carrying ac transport currents under transverse ac magnetic fields. In the previous Poynting's vector methods, for a short straight case two sets of both potential lead pairs and pickup coils were rotated around tapes in order to measure local energy flow densities, i.e., the distribution of Poynting's vector; for a solenoidal coil case two sets of both potential lead coils and spiral pickup coils were shifted to the coil axis direction by one winding pitch of the solenoidal coil. During moving these sets at low temperature, the data acquisition system was working and the value of ac losses was finally obtained. These previous methods, therefore, have inconveniences such as much time to get the value, much space needed for moving sets, or frequent troubles of movements due to low temperature deformation. The present method based on the following idea can resolve them: as for pillar parts of potential lead lines which are standing on the tape surface, only one plays a substitute role for all as long as all lines are connected together. As a result, immovable arrays of sets can be rigidly arranged, so that the signal measured at each position can be simultaneously. Moreover, information enough to clarify the electromagnetic behavior inside the tape can be obtained, because the sets can put more closely to the tape surface. The validity of the present method was confirmed by preliminary experiments using Bi-2223 multifilamentary tapes in liquid nitrogen.

The MgB2 coil wound with a parallel conductor composed of two MgB2 tapes with insulation has been developed. The tape is obtained by the rolling process from the round wire which is fabricated by in-situ method. The performances of tapes, such as critical current density and ac losses, are higher than that of round wires in case that the transverse magnetic fields are applied to the tape in direction of the parallel to the flat face of the tape. We proposed as a candidate of applying the tape to a conductor with large current capacity that the parallel conductor is composed of insulated tapes. We fabricated a tape with aspect ratio of two from a round wire with diameter of 0.8 mm as a strand for the conductor. The coil is composed of two coils that are jointed each other in series. At the joint, two strands are transposed to be identical the inductances of two strands. The inner and outer diameters of the coil are 65 mm and 107 mm, respectively. The length of the coil is 53 mm. The total turn number is 540 turn. The magnetic flux density at the center in the coil is 1.31 T when the transport current is 200 A. In this paper, the design and the test results of the MgB2 coil wound with the conductor are shown and the critical current properties and ac loss properties of the coil are discussed.

The MgB2 coil wound with a parallel conductor composed of two MgB2 tapes with insulation has been developed. The tape is obtained by the rolling process from the round wire which is fabricated by in-situ method. The performances of tapes, such as critical current density and ac losses, are higher than that of round wires in case that the transverse magnetic fields are applied to the tape in direction of the parallel to the flat face of the tape. We proposed as a candidate of applying the tape to a conductor with large current capacity that the parallel conductor is composed of insulated tapes. We fabricated a tape with aspect ratio of two from a round wire with diameter of 0.8 mm as a strand for the conductor. The coil is composed of two coils that are jointed each other in series. At the joint, two strands are transposed to be identical the inductances of two strands. The inner and outer diameters of the coil are 65 mm and 107 mm, respectively. The length of the coil is 53 mm. The total turn number is 540 turn. The magnetic flux density at the center in the coil is 1.31 T when the transport current is 200 A. In this paper, the design and the test results of the MgB2 coil wound with the conductor are shown and the critical current properties and ac loss properties of the coil are discussed.

We have developed a new monitoring system to detect an unusual event in the superconducting coils without direct contact on the coils, using Poynting’s vector method. In this system, the potential leads and pickup coils are set around the superconducting coils to measure local electric and magnetic fields, respectively. By measuring the sets of magnetic and electric fields, the Poynting’s vectors around the coil can be obtained. An unusual event in the coil can be detected as the result of the change of the Poynting’s vector. This system has no risk of the voltage breakdown which may happen with the balance voltage method, because there is no need of direct contacts on the coil windings. In a previous paper, we have demonstrated that our system can detect the normal transitions in the Bi-2223 coil without direct contact on the coil windings by using a small test system. For our system to be applied to practical devices, it is necessary for the early detection of an unusual event in the coils to be able to detect local normal transitions in the coils. The signal voltages of the small sensors to measure local magnetic and electric fields are small. Although the increase in signals of the pickup coils is attained easily by an increase in the number of turns of the pickup coils, an increase in the signals of the potential lead is not easily attained. In this paper, a new method to amplify the signal of local electric fields around the coil is proposed. The validity of the method has been confirmed by measuring local electric fields around the Bi-2223 coil.

We have proposed MgB2 multifilamentary tapes with high critical current densities and low ac losses under external magnetic fields which is parallel to the tape face. The purpose of this proposal is to develop the MgB2 new conductors composed of plural tapes as windings of superconducting coils with both large current capacities and low losses for fusion reactors, superconducting magnetic energy storage systems, and so on. Our MgB2 multifilamentary tapes are fabricated by rolling process from round wires. The tapes are fabricated by using in-situ method. The purpose of this paper is to clarify the effects of deformation to tape shape on the electromagnetic properties of MgB2 filaments in multifilamentary wires. For the purpose, measurements of both critical currents and magnetizations have been carried out on MgB2 fine single filament wires which have simulated a filament in multifilamentary wires. Measurements on wires with large diameter have been also carried out, in order to clarify the effect of diameters of original round wires. We prepared pure and SiC-added MgB2/Nb/Cu composite wires as the original round wires before rolling process. The diameters of round wires have been varied and then the aspect ratios of tapes have been varied. The observed critical current densities of the tapes are about 7 times larger than that of the original round wire at a maximum. The ratios of Jc of wires to tapes don't have dependence of the diameter of the original round wire. In addition, the results of measurements of magnetizations have shown that magnetization losses in MgB2 tapes decrease to 1/3 compared with original round wires.

MgB 2 multifilamentary tapes with 6 twisted filaments were fabricated and it was experimentally proven that both an increase in critical currents and a decrease in ac losses in the tapes due to deformation of the tapes with high aspect ratio can be simultaneously attained. This will improve the performance of MgB 2 wires for the new conductors as windings of superconducting coils with both low losses and high stability for fusion reactors, superconducting magnetic energy storage systems, and so on. There are three kinds of prepared samples in which one sample is a round wire of 1.02 mm in diameter and other samples are tapes obtained by the rolling process from the round wire. The aspect ratios of these tapes are 3.3 and 6.6. These wires are composed of MgB 2 /Nb/CuNi . The 6 filaments are twisted with twist pitches of 50 mm. The measurements of critical currents and coupling losses were carried out at 4.2 K. The observed critical current densities of the tapes are two times larger than that of the round wire under the external magnetic fields of 2 T to 7 T in a direction parallel to the flat faces of the tapes. The observed coupling losses in the tape with aspect ratio of 6.6 were 1/10 smaller than that of the round wires, where the parallel magnetic fields were applied to the flat faces of the tapes.

We have strategically promoted R&D activity for the development of elemental technologies in order to put into practical HTS power transformers applying YBa2Cu3O7 (YBCO) coated conductors, focused on AC loss reduction in HTS coils. This paper shows AC loss reduction in model coils of YBCO tapes divided into five filaments by laser ablation. At first experiment, magnetization loss of YBCO tape with five filaments and YBCO tape with non-divided were firstly measured using a pick-up coil technology. Then, AC loss measurement is performed using two types of model coils of YBCO tapes with five filaments and non-divided, by two different technologies: an electric technique (i.e. 4-probe) and a calorimetric technique (i.e. boil-off). The results of the experiment show that the AC loss in the model coil of YBCO tapes with five filaments was reduced to one-fifth when compared with model coil of YBCO tapes with non-divided.

It is important to evaluate the characteristics of the current distributions in the HTS tapes and conductors in order to realize HTS power devices. In this study, we have developed a simple measurement method to evaluate the current distributions in HTS tapes. In this method, the current distributions are indirectly found from the results of the field distributions measured by an array of precise pickup coils arranged on the upper surface of the sample tape. To confirm the accuracy and sensitivity of this method, current distributions in a test conductor made of copper wires are measured. Numerical and measured current distributions are compared, and it is found that our method is accurate enough to measure the current distributions in HTS tapes. Using this method, the current distributions in a multifilamentary Ag-alloy sheathed Bi-2223 tape are measured at 77K. In addition, the effectiveness of our method as detection method of degradation areas of the HTS tapes is examined.

In order to realize HTS power devices with high performances, it is important to evaluate the characteristics of HTS conductors theoretically and experimentally. In the experimental evaluation, an ac power supply with a large current capacity and few ripples is necessary. To this end, we developed a compact HTS current transformer for evaluating the characteristics of HTS conductors. This transformer is wound in an elliptical shape with using Bi-2223 tape. The secondary current of the current transformer can be induced at about 1 kA at 77 K when the primary current is 40 A. The transformer was fabricated very compactly, and the externals size is 70 mm in the diameter, and 110 mm in length. To confirm the practicality of the transformer, the current was actually energized to an HTS conductor composed of 16 Bi-2223 tapes. The results of the performance test of the current transformer and the method of increasing the current capacity of the current transformer are also discussed.

AC losses in the conduction-cooled low temperature superconducting (LTS) pulse coil with stored energy of 1 MJ are estimated. The 1 MJ coil is a superconducting pulse coil for 1 MW, 1 sec UPS-SMES. UPS-SMES is an uninterruptible power supply (UPS) with superconducting magnetic energy storage (SMES) for protection of production lines of an industrial plant or large-scale experimental devices such as a fusion device, from a momentary voltage drop and an instant power failure. The winding conductor for the 1 MJ coil is a NbTi/Cu Rutherford cable, which is extruded with aluminum. The 1 MJ coil was wound by a new twist winding method. A 1 MJ coil was fabricated and cooling and excitation tests were carried out. In this paper, two methods to estimate ac losses in conduction-cooled LTS coils are proposed. One method estimates ac losses in the coil under steady-state conditions. The other method estimates ac losses in the coil under transient-state conditions. For estimation of ac losses in the 1 MJ coil, measurement of temperature in the coil during tests and thermal analysis using the two-dimensional finite element method are compared. These procedures clarified that the 1 MJ coil has low losses.