M. Urata

Thermal Stability of Conduction-Cooled YBCO Pancake Coil

When a high-temperature superconducting (HTS) coil is operated using a conduction-cooling technique, the coil should be impregnated with epoxy resin because the coil is not sufficiently cooled without it. On the other hand, it is important that the coil should have no damaged area in the winding because the damaged area would generate heat locally and the coil would eventually quench. However, whenever we fabricated an impregnated HTS coil wound with YBCO tapes and evaluated its V-I characteristics, the n-value of the coil was much lower than we expected, indicating that the critical currents of some areas in the winding drastically decreased. Therefore, we have started to improve the performance of an impregnated HTS coil wound with YBCO tapes. In this paper, we investigated the cause of the degradation and found that the degradation did not occur when we decreased the radial tensile stress in the windings. Then we fabricated four single-pancake coils, stacked them, and tested them in a conduction-cooled condition. The measured V-I curves were in good agreement with the calculated ones, suggesting that we successfully developed a technique of fabricating an impregnated HTS coil wound with YBCO tapes with no degradation. We also measured thermal runaway currents of a conduction-cooled HTS coil composed of two single-pancake coils wound with YBCO tapes and numerically simulated the thermal properties by using a three-dimensional heat conduction equation in order to study the thermal stability of the YBCO coil. The measured thermal runaway currents were in good agreement with the calculated ones.

Stabilization of superconducting dry solenoids

Premature quenches in superconducting solenoids wound with Formvar-coated NbTi conductors have been studied. Some model coils wound with various tensions were tested. The experimental results are discussed with attention given to the stress distribution for coil winding, cool-down to liquid helium temperature, and energization at 4.2 K. Some mechanisms of premature quench are classified according to the winding tension, and some stabilization methods are presented on the basis of these quench mechanisms. It is found that if a solenoid is wound loosely, macroscopic slips occur in inner layers due to low frictional force. Although such motions are removed if the coil is wound tightly, shear stress concentration at the interface between the winding and the bore tube then causes quenches by microslips. Inserted polymer films affect the sliding behavior of the conductor, which can improve the coil performance. >

Relation between radial stress and quench current for tightly wound dry solenoids

Premature quenches in superconducting solenoids, wound by Formvar coated NbTi conductors, has been studied. Radial compression between winding layers is supposed to be essential to suppress conductor motions; the relation between radial stress distribution in the winding and quench current is investigated by experiment for a model magnet. The result is discussed based on the stress calculation and frictional characteristics of the Formvar coated conductor.

Compact 17 T epoxy-impregnated magnet without bore tube

Abstract A compact 17 T magnet was successfully fabricated and energized using a flexibly supported epoxy-impregnated coil without a bore tube. This coil structure was developed with a magnet stabilization study on small NbTi and Nb 3 Sn coils. The 17 T magnet is composed of two sets of Nb 3 Sn coils and an NbTi coil graded into two sections. The magnet successfully attained the designed field, 17 T, in the 33 mm bore. The stabilization contributed to the performance of each coil. No quench occurred in the inner Nb 3 Sn coil. The stabilization enabled a current density as high as 260 A mm −2 , generating 8.8 T, to be achieved for the NbTi coil in a compact system operated in a 300 mm diameter cryostat.

Cryocooler directly cooled 6 T NbTi superconducting magnet system with 180 mmroom temperature bore

Abstract This paper describes a cryocooler cooled NbTi superconducting magnet system. The technical features of this magnet system are a 4K-Gifford-McMahon (GM) refrigerator using magnetic regenerator material and a high-Tc Bi 2 Sr 2 CaCu 2 O y superconducting current lead. The NbTi coil was directly cooled by the 4K-GM refrigerator without liquid helium and it took about 21 hours for the NbTI coil to be cooled from room temperature to below 4 K. The stable magnetic field of 6 T at 3.5 K and the maximum magnetic field of 6.45 T were obtained in the 180 mm room temperature bore.

Design and Test Results of a Fault Current Limiter Coil Wound With Stacked YBCO Tapes

This paper describes the design and test results of a 6.6 kV-class superconducting fault current limiter (FCL) coil wound with YBCO tapes. YBCO tapes, with stainless-steel lamination, were prepared for an experimental FCL coil. The main feature of this FCL coil is winding of multiple YBCO tapes in parallel in order to increase the rated current. To obtain a rated current up to several hundred amps, four tapes, electrically insulated, were wound in parallel to form a non-inductive coil. The coil specifications, such as winding pitch, number of turns, and transpositions, were carefully designed by numerical simulation to equalize the current distribution in each tape. The fault current limiting performance of the FCL coil was evaluated through over-current tests using a capacitor bank. The FCL coil successfully suppressed a fault current of 10.4 kA to below 2.1 kA.

Design and Test Results of 18.1 T Cryocooled Superconducting Magnet With Bi2223 Insert

An 18 T cryocooled superconducting magnet (18T-CSM) with a 52-mm room temperature bore has been successfully constructed. The magnet consists of an outer NbTi coil, which generates 6.2 T, four Nb3Sn coils, which generate 9.3 T and a Bi2223 high-Tc insert coil, which generates 2.5 T. The magnet system is cooled by a GM/JT cryocooler and two single-stage GM cryocoolers. The initial cool down takes 11.5 days. The ramp time up to 18 T is 60 minutes. The innermost layer of the Nb3Sn coils employs an internal-tin processed Nb3Sn wire due to its high critical current density in high magnetic fields. High mechanical strength is required for other three layers because of the large hoop stress of 234 MPa at 18 T. With this view, bronze-processed high-strength Nb3Sn wires reinforced with Cu-NbTi intermetallic compound are employed. The Bi2223 high-Tc insert coil is composed of 25 double pancake coils using Ag-alloy sheathed Bi2223 superconducting tape with stainless steel tape reinforcement. The reinforcement co-winding reduces the hoop stress down to 48 MPa, which is sufficiently applicable to the Bi2223 tape. The magnet is successfully operated up to 18.1 T.

Design and Test Results of 66 kV High-Tc Superconducting Fault Current Limiter Magnet

This paper describes the design of a 66 kV high-Tc superconducting fault current limiter (FCL) magnet and the test results obtained. The magnet mainly consists of a vacuum vessel, a nitrogen bath, a pair of current leads, cryocoolers, and six sets of coils wound with Bi2223 tape. The rated current is 750 A. The insulation voltage of the magnet is of the 66 kV class. In the final year of the project, all six sets of the coils are set connected in the cryostat and evaluation tests have been implemented. In the cooling test, sub-cooled nitrogen of 65 K was obtained, with homogenous temperature distribution in the cryogen. The rated current of 750 A was successfully obtained for both direct and alternating current tests. In addition, the magnet passed the simultaneous current and voltage application test. The dielectric test results showed that the magnet satisfied the insulation for 66 kV apparatus in the Japanese Electrotechnical Committee Standard. Finally, the magnet was combined with a rectifier bridge circuit to form an FCL. In a fault current limiting test with a short-circuit generator, the FCL successfully limited short circuit current up to 20 kV applied voltage. The obtained result was in good agreement with the design and the Electro-Magnetic Transients Program (EMTP) analysis

Racetrack coil instability resulting from friction heat generation at fixtures

A series of experiments on the instability resulting from mechanical disturbance at the coil surface of a small racetrack coil is described, along with a preventive measure against its instability. Epoxy-impregnated racetrack coils sometimes experience premature quenches due to frictional heat produced by coil slides at fixtures. The first experiment confirmed coil slides during coil charging. These slides were about 10 mu m, an equivalent of 20 mJ in fractional heat generation. One effective preventive measure against this mechanical disturbance is the utilization of a thermal barrier method. The thermal barrier is an insulation layer at the interface between the coil and the fixtures. The second experiment examined the thermal barrier effect on the stability margin on the racetrack coil. A thicker insulation layer substantially increased the coil stability margin. The margin increased from 105 mJ to 200 mJ by thickening the insulation layer from 0.36 mm to 1.00 mm. The two experiments showed that the racetrack coil was stabilized if the thickness of the insulation layer exceeded 0.20 mm. Charging a racetrack coil with a 0.36 mm-thick insulation layer confirmed this criterion.<<ETX>>

Design and Experimental Results of Three-Phase Superconducting Fault Current Limiter Using Highly-Resistive YBCO Tapes

As one of the programs in the Ministry of Economy, Trade and Industry (METI) R&D project on coated conductors, we developed a three-phase 6.6 kV superconducting fault current limiter (SFCL) and conducted some evaluation tests. The developed SFCL mainly comprised a set of three-phase current-limiting coils installed in a sub-cooled nitrogen cryostat with a Gifford-McMahon (GM) cryocooler, circuit breakers, and a sequence control circuit. Two tapes were wound in parallel in each limiting coil to obtain a rated current of 72 A rms. AC characteristics of each coil were measured, and relevant performance metrics were obtained. The whole system was installed in a cubicle. Short circuit experiments were then conducted with a short circuit generator. In a three-line ground fault test, the SFCL successfully restricted a short circuit current of over 1.56 kA to about 840 A with an applied voltage of 6.6 kV. The system integration ability and the obtained data show the promise of this approach for practical implementation. The SFCL was ready for user field tests.