Search

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.

MgB2 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 MgB2 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 MgB2/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.

A new quench monitoring system by using the Poynting vector method was proposed. This new system can indirectly detect normal transitions which are suddenly induced during an operation of superconducting coils. In this system, the superconducting coil is surrounded by the thin metallic cylinder composed of several pieces, where the cylinder is electrically insulated from the coil; potential leads to measure local electric fields are directly attached to this cylinder surface and pickup coils to measure local magnetic fields are put on the surface. The Poynting vector distribution around the coil can be obtained by using many sets of both the potential lead and the pickup coil. Since sudden changes of energy flow profiles around the superconducting coil are induced in the event of local normal transitions of the coil winding, such an event of coils can be detected by the change of the Poynting vector. This system has no risk of the destructive voltage breakdown which may happen in case of the existing balance voltage method, because there is no need of direct contacts on the coil windings. In addition, this method is expected to be higher reliability and sensitivity than the acoustic emission method. A demonstration test was carried out of monitoring the quench for the Bi-2223 coil partially immersed in liquid nitrogen, and a sign of normal transitions before the quench was successfully detected by this method.

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 77 K. 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.

A 1 MW, 1 s UPS-SMES is being developed for a protection from a momentary voltage drop and an instant power failure. As a key technology of the UPS-SMES, we developed a prototype LTS pulse coil with a stored energy of 100 kJ and conducted cooling and excitation tests in 2005. The operation test of the prototype UPS-SMES using this 100 kJ coil with power converters have been performed in 2006. A 1 MJ coil was designed before the fabrication of the 100 kJ prototype coil. The superconductor, the electric insulation technique, the winding method, and the cooling structure used for the 100 kJ coil were based upon the 1 MJ coil design. The successful performance test results of the prototype 100 kJ coil validated the design concept and fabrication technique of the 1 MJ coil. According to the achievement of the prototype 100 kJ UPS-SMES, the 1 MJ conduction-cooled LTS pulse coil has been fabricated successfully. The successful experimental results of the 100 kJ prototype coil with power converters and the fabrication procedure of the 1 MJ full size coil are described.

The stability margin of a conduction-cooled LTS pulse coil with stored energy of 1 MJ, which is used for UPS-SMES as protection from momentary voltage drops, is estimated from thermal analyses using the two-dimensional finite elementary method. How much the 1 MJ coil is able to be downsized is discussed. The winding conductor of this coil is a NbTi/Cu Rutherford cable, which is extruded with aluminum. Dyneema FRP and Litz wires are used as the spacers of this coil. In the design of this 1 MJ coil, the effects of spacers were neglected. In order to estimate the stability margin, thermal analyses was carried out by taking into account the thermal effects of spacers, for the coil during overload operation. And it was checked whether the maximum temperature The results showed that the 1 MJ coil has a large stability margin of 8 times the rated ac loss. In addition, new coils with a stability margin of about two times were designed. Possibilities for reducing the size and the cost of a 1 MJ LTS pulse coil are also shown.

Several kinds of NbTi/Cu multifilamentary tapes with high aspect-ratio cross-sections were fabricated in order to develop new conductors used for windings of conduction-cooled LTS pulse- coils with both low ac losses and high stability. These tapes were obtained by the cold rolling process from normal multifilamentary wires. For the edge-on transverse magnetic field at 4.2 K, about 20% increases in critical current densities at 6 T were observed for the tape with the highest aspect ratio of 7.4, and its hysteresis losses estimated from the observed magnetization were reduced to approximately one-half of those in the original wire. In this edge-on filed condition, the observed coupling losses in the tape were also decreased to about 1/17 times as large as those in the original wire. These experimental results showed that the NbTi/Cu multifilamentary tape had strong anisotropy of electromagnetic properties such as critical currents, hysteresis losses, and coupling losses and that good performances were obtained for edge-on transverse magnetic fields.

Interstrand contact resistances of Bi-2212 Rutherford cables for SMES coils were evaluated from a comparison between measured data and 2D-FEM analyses on interstrand coupling losses in these cables. The cables were composed of 30 non-twisted Bi-2212 strands with a diameter of 0.81 mm and a cable twist pitch of 90 mm. Three samples were measured; one of them had NiCr cores and the others had no cores. One of the latter two samples repeatedly experienced bending. The interstrand coupling losses were measured in liquid helium for the straight samples under transverse ac ripple magnetic fields superposed on dc bias magnetic fields. The transverse magnetic field was applied to the samples in directions both perpendicular and parallel to the flat face of the cable. The effect of the bending on the interstrand coupling losses could be neglected for the non-cored samples. The interstrand coupling losses of NiCr cored sample decreased by about 30% compared with the non-cored samples, in case the direction of the transverse magnetic fields applied to the cable is perpendicular to the flat face of the cable. Using these results and 2D-FEM analyses, taking into account that interstrand contact conditions vary from the center to the edge in the cross-section of cables, gave us the conclusion that the between side-by-side strands contact with metallurgical bond only in both edges of the cables.

Inter-strand coupling loss measurements were carried out in liquid helium for Bi-2212 Rutherford cables composed of 30 non-twisted strands. Short, straight samples composed of three stuck cables were set in a uniform field space inside the NbTi magnet of race-track and split type. Frequency characteristics of coupling losses were observed for transverse ac magnetic field, both perpendicular and parallel to the flat face of cables. An additional measurement of ac losses was carried out in order to clarify properties of hysteresis losses and intra-strand coupling losses. The obtained effective filament diameter describing the hysteresis loss property indicated that almost all filaments in strands were strongly connected to each other. From both this result and the frequency characteristics of coupling losses for the perpendicular field case, the inter-strand coupling time-constant was given in the order of 10 m. Moreover, the effective contact resistance between crossover strands in the cable was also given in the order of 10-10 Omegam2, which was the same order as that for the NbTi Rutherford cable composed of Cu matrix strands

We have been developing a 1 MW, 1 sec UPS-SMES for the 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. A conduction cooled prototype LTS pulse coil of 100 kJ class was developed as a key component of the UPS-SMES. The prototype coil has demonstrated excellent thermal characteristics during cooling and exciting tests. In this paper, measurements of the temperature in the coil during experiments and thermal analysis by using two-dimensional finite element methods were compared to clarify the high heat transfer properties of this prototype coil. This coil was wound with a NbTi/Cu Rutherford cable, which is extruded with aluminum. In order to realize the conduction cooled LTS pulse coil, FRP with polyethylene fibers (Dyneema FRP) and Litz wires were used as spacers. Dyneema FRP improves the heat transfer from layer to layer in the windings. Litz wires increase the heat transfer from turn to turn in the windings and enable conduction cooling of the coil by attaching the end of the Litz wires directly to the cold heads of the cryocoolers. It was clarified that these spacers were very effective and the coil has a large stability margin in terms of the design values

A conduction-cooled low temperature superconducting (LTS) pulse coil has been developed as a key technology for UPS-SMES. We have been developing a 1 MW, 1 s UPS-SMES for a protection from a momentary voltage drop and an instant power failure. A conduction-cooled LTS pulse coil has excellent characteristics, which are adequate for a short-time uninterruptible power supply (UPS). The LTS coil has better cost performance over the HTS coil at present and the conduction cooling has higher reliability and easier operation than the conventional cooling schemes such as pool boiling with liquid helium or forced flow of supercritical helium. To demonstrate the high performances of the LTS pulse coil, we have fabricated a prototype coil with stored energy of 100 kJ and have conducted cooling and excitation tests. The successful performance test results including current shut-off test with a time constant of 1.3 s and repeated excitation of a triangular waveform with high ramp rate are reported

We are planning to develop a 1 MW, 1 sec UPS-SMES for a protection from a momentary voltage drop and an instant power failure. As the first step, we have been developing a 100 kJ class prototype UPS-SMES, using a low temperature superconducting coil because of its better cost and performance over the high temperature superconducting coil. However, the difficulty to utilize a pool-boiling LTS pulse coil is the reliability of operation. To solve this problem, a conduction-cooled LTS pulse coil has been designed and fabricated as a key component of the UPS-SMES. The reduction of AC loss and high stability are required for the SC conductor for the conduction-cooled coil because of a limited cooling capacity. The SC conductor of a NbTi/Cu compacted strand cable extruded with an aluminum is designed to have the anisotropic AC loss properties to minimize the coupling loss under the specified orientation of the time varying magnetic field. The coil was wound with a new twist-winding method in which the variation of twist angle of the conductor was controlled with the winding machine designed specifically for this purpose. The fabrication technique and performance of a conduction-cooled prototype LTS pulse coil are described.

A new type of cable-in-conduit conductor composed of HTS tapes was proposed as a winding for high-performance HTS coils with high current capacities and low AC losses. In the fabrication of the conductor, the twist of stacked tapes around their axes was made before inserting them inside conduits. The twist angle should be changed continuously along the axis to reduce face-on oriented magnetic fields applied to the tape in the winding conductor during coil operation. In order to confirm the high current capacity of this type conductor, two single-layered solenoidal coils wound with sample conductors composed of five stacked Bi-2223 tapes with bias angles of 20 and 0° were fabricated and tested in liquid nitrogen. A copper magnet system was used in this experiment to generate the spread magnetic field with a spread angle of 20° from the coil axis to the radial direction, which simulates the profile of magnetic fields near the edge windings of practical coils. A large improvement on critical-current degradation affected by spread magnetic fields was successfully observed for the test coil wound with the 20° bias conductor. © 2005 Wiley Periodicals, Inc. Electr Eng Jpn, 153(4): 12–19, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20218

A new method for designing compact stranded superconducting conductors is proposed as a solution to the dilemma that low loss and high stability cannot be simultaneously attained in the commonly used conductors. In our design, the twist directions of the conductor and those of the sub-cables in it are the same. In addition, the twist pitch of the sub-cables is relatively longer than that of the conductor. The sub-cables crossover each other in the conductor. Under the changing transverse magnetic fields oriented perpendicular to the broad face of the conductor, the induced voltages between the above-mentioned crossover sub-cables become small, so inter-sub-cable coupling losses are decreased. As a result, not only the total coupling loss in the conductor is decreased, but also high stability is maintained due to the low contact resistance between the sub-cables. Our method theoretically indicates such high performance as attaining both low ac loss and high stability. An example of our design is shown for a large-scale compact stranded superconducting conductor.

Electrical loss-measurements were carried out in liquid nitrogen for long Bi-2223 tapes wound into a solenoidal-coil under various external conditions of ac transport currents and ac magnetic fields. In this experiment a new measuring-system, which is an extension of our original measuring-system applicable to short and straight HTS tapes, was developed. In this new system, the Poynting vectors at the outer and the inner surfaces of the sample coil were measured to get ac loss values by using two movable sets consisting of both a potential lead pair and a pick-up coil on each surface of the coil; The moving direction is the coil axis and its distance is one pitch of coil windings. Several samples were prepared in order to clarify fundamental electromagnetic-properties of Bi-2223 coils with various windings. The obvious difference of loss properties among these samples was observed for some external conditions of ac transport currents and ac magnetic fields.

In order to develop the 100 kJ class UPS-SMES as a protection from momentary voltage drops, design of the conduction cooled LTS pulse coil was carried out and special winding machine has been developed. Such coil is required to simultaneously attain low AC loss and high stability and the distributions of temperature in the coil are sensitively controlled. For this purpose, an aluminum stabilized conductor with circular cross-section composed of a Cu stabilized NbTi Rutherford cable was used as the winding conductor, and in the winding process the twist angle of the conductor around its axis was controlled to adjust the direction of edge-on orientation to the Rutherford cable to direction of local transverse magnetic fields applied to the conductor in winding area of the coil. The developed winding machine is used for this winding method. As a result, conduction cooled LTS pulse coil can be expected to operate stably in adequate temperature margin.

We have been developing the UPS-SMES as a protection from momentary voltage drop and power failure. The superconducting system is suitable as electric power storage for large energy extraction in a short time. The most important feature of superconducting coil system for the UPS-SMES is easy handling and maintenance-free operation. We have selected low temperature superconducting (LTS) coils instead of high temperature superconducting (HTS) coils from the viewpoint of cost and performance. However, it is difficult for the conventional LTS coils to fulfill maintenance-free operation since the cooling methods are either pool boiling with liquid helium or forced flow of supercritical helium. Thus, a conduction cooled LTS pulse coil has been designed as a key component of the UPS-SMES. The development program of 1 MW, 1 sec UPS-SMES is explained.

A new winding method was proposed to reduce ac losses in stable LTS pulse coils. The suitable conductor for this winding method is a multi-layer type conductor composed of stacked Rutherford type cables with low resistive contact between strands. In this winding method, the twist angle around axis of the conductor is controlled during winding process to adjust the direction of edge-on orientation to stacked cables to direction of local magnetic fields applied to the conductor in winding areas. Inter-strand coupling losses in this coil are expected to be small in spite of low resistive contact between strands in the wound conductor. In order to clarify the effect of this winding method, firstly, a test conductor was fabricated and ac losses in short samples of the conductor were measured. This was an aluminum stabilized conductor, and a Rutherford cable composed of 8 Cu/Nb-Ti strands was used as the core of this conductor. The loss is measured with the transverse magnetic fields applied either perpendicular or parallel to the broad cable face, the "face-on" (FO) and "edge-on" (EO) orientations, respectively. From observed data, it is found that EO loss is 0.12 times FO loss . Secondly, ac losses in test coils wound with this conductor were calculated. The result showed that this winding method was very effective.

A new electrical system was developed for measuring ac losses of long HTS tapes or wires wound into a solenoidal-coil. This coil-loss measuring-system is an extension of our original ac-loss measuring system that is applicable to short, straight HTS tapes under various external conditions of transport currents and transverse magnetic fields, and is useful for fundamental studies of various windings of practical multi-layered coils. In this new system, the Poynting vectors at the outer and the inner surfaces of the coil were measured to get ac loss values by using two movable sets consisting of both a potential lead pair and a pick-up coil on each surface of the coil. The moving direction is the coil axis and its distance is one pitch of coil windings. The value of ac losses can be obtained by summation of the Poynting vector at many positions during the movement. The applicability of this system was confirmed by ac loss measurements on a solenoidal-coil wound loosely with Ag-sheathed, multifilamentary Bi-2223 tapes.

In our previous paper, we proposed a new method for designing compact stranded superconducting conductors as a solution to the dilemma that low loss and high stability could not be simultaneously attained in commonly used conductors. By adjusting the twist pitches and directions of the sub-cables in the conductor, inter-sub-cable coupling losses in it are decreased. As a result, not only the total coupling loss in the conductor is decreased, but also high stability is maintained due to the low contact resistance between the sub-cables. The fundamental performance of the conductors designed using our method has been confirmed through measurements of the coupling loss and the minimum quench energies in the conductor. Our measurements are, in this case, carried out using Rutherford cables with strands instead of the conductors with sub-cables. The results obtained successfully show the validity of our new design method.

Measurements of minimum quench energy (MQE) of a new type Rutherford cable were carried out. This cable has low resistive paths located near each edge of cable's cross-section, enabling quick transfer of transport currents from one strand to other strands when a part of normal state appears locally in the strand. In addition, this cable has relatively longer strand twist pitches in relation to the cable twist pitches so as to decrease coupling losses in the cable under changing transverse magnetic fields with face-on orientation. In order to evaluate the effect of current transfer on cable stability, measured MQE were compared with numerically calculated values for a single strand. Measured MQE was greater than 18 times the calculated single strand value (18 strands in our cable). This indicates that current transfer between strands effects MQE of our cable. And our cable has almost the same MQE per unit volume as standard Rutherford cable without stainless steel cores. Thus we confirmed that our Rutherford cable has high stability.

A new winding method was proposed for fabrication of high performance coils wound with Bi-2223 tapes, which have anisotropy of both critical-current and magnetization-loss properties depending on direction of applied magnetic fields. In the winding process, continuous control of twist angles around tape axes is adopted to reduce face-on oriented magnetic fields applied to the wound tape in coil operation. For an example of large-scale solenoid coils cooled at 77, 66 or 20 K, great improvement of the current capacity was theoretically predicted. This result implies that compared to the old coil, and under identical operating currents, the new type coil would allow a great reduction in the amount of Bi-2223 tapes needed. It was also shown theoretically that a one-order reduction of magnetization losses could be achieved. A preliminary experiment was carried out on the fundamental problem of the strain effect of twisting and bending tapes on the critical current, and the observed data suggested that the effect can be neglected.

New superconducting wires with transposed Bi-2223 filaments were fabricated and tested to confirm both uniform transport-current distribution and loss reduction. Sample wires, 1 m length, were made of four multifilamentary pre-tapes, which were wrapped spirally, without any insulation, into two layers around a silver core wire. The wrapping direction and the pitch of the two layers can be changed easily, allowing both `opposite-' and `same-direction' wires to be prepared and tested. The sample parameters for the opposite-direction wire were chosen to achieve uniform distribution of transport currents. The wire obtained after heat treatment had relatively high critical currents of over 120 A at liquid nitrogen temperature with no external magnetic field. Where the current is greater than half the critical current, the observed hysteresis loss due to AC transport currents in the opposite-direction wire is about 60% as large as that in the same-direction wire. The experimental results for loss reduction and uniform current distribution in the opposite-direction wire agree closely with our theoretical predictions.

A new type of high temperature superconducting wire with transposed filaments is fabricated and tested to confirm reduction of transport current losses. Sample wires of 1 meter long are made of four Bi2223 multifilamentary pre-tapes which are wrapped spirally into two layers around the silver core wire without any insulation. The wrapping direction and pitch for the two layers can be changed easily so that both the opposite and the same direction wires are prepared for the present experiment. The sample parameters are chosen to achieve uniform distribution of transport currents inside the opposite-direction wire. These wires, obtained after heat treatment, have relatively high critical currents of 99-122 amperes in liquid nitrogen in no external magnetic fields. In the range of currents higher than a half of the critical current, the observed hysteresis loss due to ac transport currents in the opposite-direction wire is about 60% as large as that in the same-direction one. This experimental result for the loss reduction agrees closely with our theoretical prediction.

The in-situ measurements of pulse losses are carried out for the total system of ESK (Experimental SMES (superconducting magnetic energy storage) of Kyushu Electric Power Co.) with a capacity of 1 kWh/1 MW which was completed in 1998. ESK has six toroidal coils wound by Rutherford cables, each coil being contained in a fiber reinforced plastic (FRP) vessel set up in a Dewar. The loss was calculated by using a method of measuring the decrement of liquid helium (LHe) level in each vessel. The form of current variation for the measurement is a repeated triangular waveform. The charge-discharge rate was changed from 10 A/s to 140 A/s. Although there are considerable errors in the measurements, agreement between the measured results and theoretical calculations is satisfactory in view of the limited conditions of the in-situ measurement for the SMES coils of this scale. The origins of the loss are discussed in detail.

A new system was developed for measuring losses electrically in short, straight HTS tapes under various external conditions of transport currents and transverse magnetic fields. In this method, Poynting vectors around the straight sample were measured to get loss values by using two movable sets consisting of both a potential lead pair and a miniature pick-up coil on each flat face of the tape. In order to confirm validity of this system, loss measurements on short samples of Ag-sheathed Bi2223 multifilamentary tapes were carried out in liquid nitrogen for three cases as simultaneous sweep of ac transport currents and transverse ac magnetic fields and two single sweep cases of one of them. The results obtained show that this is extremely useful for measuring these ac losses as long as longitudinal change in the magnetic field can be neglected.

Basic studies are carried out on causes of the increase of inter-strand coupling losses in cable-in-conduit conductors used for windings of large scale magnets, such as a single inner vertical (IV) coil in the Large Helical Device. In this paper, the effect of AC transverse magnetic fields periodic changing along a two-strand cable axis on loss properties of the cable is considered as a candidate of the cause. The evidence that the periodic changes produce additional inter-strand coupling losses characterized by another long coupling time constant in addition to the intrinsic one is theoretically shown. We confirmed properties by measurements on two kinds of sample wires, using inter-filament coupling in a strand instead of inter-strand coupling in cable.

A full system of 1 kWh/1 MW superconducting magnetic energy storage (SMES) has been completed early this year. This SMES is the first step to the realization of practical SMES system for power line stabilization. Main points in its design are two module type arrangement of six coils having three coils and one converter as one unit of module, modified-D-shaped coils with mechanical supports, liquid helium vessel type cooling of coils, and high-temperature superconducting current leads. The first test experiment was carried out on the site recently. The above design points were examined. A preliminary test for power line control was also made in the distribution line at the site. Satisfactory results were obtained.

In order to get Ag-BSCCO tapes or wires with small losses, we propose a new fabrication technique named the 'wrapping method.' This is not only an alternative technique for fabrication of tapes and wires with very small twist pitches and well-aligned filaments instead of the conventional twisting process but also a new method of producing tapes and wires with transposed filaments. Especially for the latter, it Is theoretically confirmed that a uniform distribution of transport currents in the new two-layered multifilamentary wire can be easily obtained by optimizing wire parameters. As a result transport current produced hysteresis losses can be reduced to the desired level in the multi-layered wire with transposed filaments.

Electric measurements of coupling losses in five-layer stacks of Nb/sub 3/Sn Rutherford cables with a stainless steel (SS) core are made with the applied transverse magnetic-field both normal and parallel to the plane of the cable. For the former (face-on, FO) field case, a one-order reduction of the FO inter-strand coupling loss due to the existence of the resistive core is clearly observed. The measured loss for the latter (edge on, EO) field case is very small, that is, about 1/5000 times as large as the reduced loss. In order to explain these loss properties, two dimensional FEM analysis is carried out taking into account the details of the cable cross-section. The theoretical results show that the FO inter-strand coupling loss in the SS-cored cable is determined mainly by the thickness and conductivity of not only the SS core but also of the outermost bronze region, and that the negligible EO loss corresponds to the intra-strand coupling loss produced in isolated strands due to high surface-resistance of the side-by-side contact.

Improved Rutherford cables have been proposed for magnets for experiments of high energy physics. They have either a sheet of stainless-steel or a composite metal sheet as a core put in the usual Rutherford cable. The cable with this composite core sheet in it is a candidate for a new cable with both low losses and high stability. In order to estimate the interstrand coupling losses produced in this cable, three dimensional finite element method (3D FEM) analysis is carried out. The results obtained show that the existence of 3D shielding current in it determines the profile of loss properties under changing magnetic fields of face-on orientation. The effect of spatial period of the core structure along the cable axis on the loss is also elucidated. This will be a useful aid for the design of the cable.

We have strategically promoted R&D activity for the development of elemental technologies in order to put into practical HTS power transformers applying YBa 2 Cu 3 O 7 (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.

The nitrogen boil-off method, which is categorized as one of the calorimetric methods, has been developed as a practical method for measuring AC losses in high temperature superconducting (HTS) coils. The boil-off method offers direct and convenient measurement compared to the electric method. The measurement sensitivity of about 0.5 W was confirmed by the improvement of sample vessel structure and the control of the heater power. This paper reports the results of a comparison of the AC losses in superconducting coils using YBCO tapes measured with the boil-off method and those obtained by the electric method and by analysis.

Extra AC losses were observed during the Experiments on a Single Inner Vertical coil (EXISV). The Inner Vertical (IV) coils are the smallest poloidal coils for the Large Helical Device (LHD) and their inner and outer diameters are 3.2 m and 4.2 m, respectively. The coil consists of 16 pancake coils wound with cable-in-conduit conductor (CICC) whose strands are NbTi/Cu without any surface coating. Many causes for the extra AC losses were considered, such as the decrease of a contact resistance between strands due to the large electromagnetic force in the conductor or due to the stress during the coil winding process, etc. and possibilities were investigated from the experimental data. Finally, we found that a coupling current with a very long time constant of 124 s caused the AC loss increase. The coupling current with such a long time constant cannot be explained from the symmetric twisting configuration of the CICC but can be explained as a local loop current corresponding to a cyclic change of the non-uniform current distributions in the cable. The non-uniform current distribution could be induced by an asymmetry of the strand transposition in the cable. To verify the above reasoning, we did fundamental experiments on a two-strands-cable, which has an intended asymmetry in the cable twisting. Extra AC losses were also observed for an asymmetric two-strands-cable, and it was demonstrated that the non-uniform current distribution causes an increase of AC losses.

A new-type structure of multifilamentary oxide-superconducting wire and tape is proposed. It is expected that this structure will be effective in reducing transport losses produced when they carry changing currents. The fabrication method of such a wire or tape with low transport losses has more flexibility in the design of its parameters, such as the pitch and the direction of twisting or the number of filament layers, than the existing method does. Theoretical analyses of transport current distributions and resultant transport losses are made for a cylindrical two-layer wire with a twist in the same or the opposite direction for each layer. The theoretical result shows that uniformity in transport current distribution is achieved by optimizing the wire parameters regardless of the twist direction. Moreover, the possibility that the obtained uniformity reduces transport losses by one order compared with the existing type, which has strongly localized current distribution, is theoretically shown for a new multilayer wire or tape.

We have developed a novel apparatus for applying a nitrogen boil-off method as a mean of making calorimetric measurements and allowing for simple measuring and evaluating of AC losses in HTS coils at liquid nitrogen temperature. With the ability to measure AC loss generated in superconductors directly, this method produces more reliable data than that obtained by general, electro-magnetic measurements. A sensitivity of about 0.1 W was achieved by improving sensitivity in measuring gas evaporation via the following three steps: (1) use of nonmetallic vessel, heat-insulated housing in which the sample is located; (2) optimization for sample vessel thickness and tube size; and (3) stabilization of evaporating gas emitted by the heater. It is widely expected that the proposed apparatus may be used for individual measurement of the magnetization loss and the transport current loss in HTS coils, as well as their total losses, in addition to the AC loss in irregularly shaped samples

The authors are planning to build a 1 kWh/1 MW (maximum stored energy/maximum power capability) module-type SMES (named ESK; experimental SMES of Kyushu Electric Power) as a first step towards the realization of practical SMESs for power line control. The main points of the design are those of: module-type coils for the development of SMES capacity scale-up; the choice of low loss stranded cables for reducing pulse operating loss; the choices of modified D shape coils and the reduction of stresses in the conductor-which become more serious in scaling-up and high-T/sub c/ superconductor (HTSC) current leads for covering weak points due to thermal loss in a module-type SMES which need many current leads. Some other points are also studied such as the design of the cooling system in which a single coil quench does not induce that of others, and harmonics suppression in the SMES power converter system.

The AC losses of the Inner Vertical (IV) coil have been measured during the Experiments on a Single Inner Vertical coil (EXSIV). The IV coils are the smallest poloidal coils of the Large Helical Device (LHD) and their inner and outer diameters are 3.2 m and 4.2 m, respectively. The coil consists of 16 pancake coils wound with the cable-in-conduit conductors (CICC) whose strands are NbTi/Cu without any surface coating. The AC losses were measured with a calorimetric method during excitation tests and current shut-off tests. In addition to the usual inter-strand coupling losses with short time constants, unexpected coupling losses were observed due to the coupling current of a long time constant.

Interstrand coupling losses of Rutherford cables are easily affected by curing conditions. We have measured the interstrand coupling losses and contact resistances for three kinds of Rutherford cable, made from strands with a Cu matrix, a CuMn barrier around a filamentary bundle and a CuMn matrix inside a Cu sheath. Furthermore we have observed the contact surfaces of the strands by using a scanning electron microscope. We have found that strands of cables with a Cu surface were bonded at crossover points cured at higher temperatures, and there were traces of diffusion bonding on the contact surfaces. We have also investigated the mechanical properties of the matrix materials and reviewed the fabrication process of magnets in which pressure is released after curing. We have concluded that the losses are determined by a balance between bonding forces and spring back of the strands.

In order to elucidate the dependence of the internal electromagnetic-forces on the interstrand coupling loss in superconducting cable-in-conduit conductors, we develop a new measuring system. This system enables us to measure the coupling loss of large scale conductors with transport current of 20 kA and the transverse magnetic field of 3.22 T bias in the wide frequency range of the superposed ripple field of 0.1-300 Hz. The data of the preliminary measurements show that the change of the coupling loss is only a little in the low frequency range to the extent of the electromagnetic-force of 0.44 MPa in the case of IS-conductor of the poloidal field coil for LHD.

The change of coupling losses in aluminum-stabilized superconductors due to the Hall effect is investigated experimentally and numerically. The Hall effect is here taken to be an interaction between the inter-strand coupling current and the external DC magnetic field. The loss measurement of the R&D conductor for the helical coil of the Large Helical Device is carried out under the coexistence of the transverse and the longitudinal magnetic fields. The measured coupling loss of the real conductor induced by the changing transverse magnetic field is increased by the existence of the longitudinal DC field. These loss features are compared with those by numerical analysis, based on the two dimensional finite element method, of a real conductor with complex cross-sectional structure. >

From the viewpoint of distribution at the steady state in conducting cable, we investigate the inter-strand coupling, which affects the fundamental properties of the conductor such as AC loss and current distribution. We introduce an approximate analytical expression for a uniform current distribution in the superconducting cable. Next, we examine the dependence of the change of the current distribution on the main factors of the inter-strand coupling such as the inter-strand contact resistance and the structure of the conductor. We obtain useful information on the conductor design so as to have a uniform current distribution. It is also shown that the intensity of the inter-strand coupling can be estimated by measuring the change of the current distribution. >

The frequency characteristic of AC losses in aluminum-stabilized superconducting conductors are investigated in order to elucidate their loss mechanism under changing transverse magnetic field. AC loss measurement is carried out for the short and straight sample conductor for the helical coil system of the Large Helical Device. The sample of about 50 cm in length is subjected to an AC ripple field with a frequency range from 0.05 to 337 Hz superimposed on the bias field. The experimental results show that the loss feature much depends both on direction of the transverse magnetic field and location of the stabilizer relative to the strand bundle. These results are quantitatively explained by the theoretical analysis. >