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Persistent Current HTS Magnet Cooled by Cryocooler (1) -Project OverviewMotohiro Igarashi, Hiroyuki Nakao, Motoaki Terai, Toru Kuriyama, Satoshi Hanai, Tomohisa Yamashita and Mutsuhiko Yamaji
Abstract— This paper describes a project overview for a persistent current HTS magnet, which has been in development for Maglev trains since 1999. The HTS magnet operates with a very small current decay rate of 0.44%/day and can be cooled by a cryocooler below 20 K. The HTS coil consists of 12 single-pancake coils, which were wound with 4 parallel Ag-sheathed Bi2223 tapes. In order to minimize the magnetic field decay rate during persistent current operation, we have made efforts not to decrease the high Tc superconductor characteristics during the winding of the single-pancake coils. The HTS coil is connected with a persistent current switch made of a YBCO thin film, and cooled by a GM type two -stage pulse tube cryocooler. Detachable current leads were used to reduce heat leakage to the 1st stage of the cryocooler. Index Terms—Superconducting magnets, High-temperature superconductors, Superconducting tapes, Linear synchronous motors
I. INTRODUCTION
Ｃ
JAPAN RAILWAY COMPANY ( JR CENTRAL ) has been in development of a Superconducting Maglev system. The study of this Maglev system began in 1962 by The Japanese National Railways. In 1997, the Yamanashi Maglev Test Line, operated by JR Central and Railway Technical Research Institute (RTRI), was constructed and running tests were initiated. A manned world speed record of 581 km/h was attained on Dec 2nd, 2003. At present, the cumulative travel distance traveled by this Test Line is a approximately 400,000 km, and the total passengers carried have exceeded 80,000. In the Yamanashi MAGLEV system, low temperature superconducting (LTS) coils, using niobium titanium (Nb-Ti) wire wound in a racetrack configuration are installed in the ENTRAL
Manuscript received October 5, 2004. This work was partly supported by the New Energy and Industrial Technology Development Organization (NEDO) through the International Superconductivity Technology Center (ISTEC) as Collaborative Research and Development of Fundamental Technologies for Superconductivity Applications. Motohiro Igarashi, M. Terai are with Central Japan Railway Company , JR Central Shinagawa Bldg.-A wing 2-1-85 Konan, Minato-ku Tokyo, 108-8204, Japan (phone: +81-3-6711-9556; fax: +81-3-6716-1605; e-mail:
[email protected]). Toru Kuriyama, Satoshi Hanai are with Toshiba Corporation, 2-4 Suehiro-cho, Yokohama, 230-0045, Japan. Tomohisa Yamashita is with Toshiba Corporation, 1,Toshiba-cho, Futyu, 183-8511, Japan Hiroyuki Nakao, Mutsuhiko Yamaji are with Toshiba Corporation, 1-1-1 Minato-ku Tokyo,105-8001, Japan
superconducting magnets (SCM). These superconducting coils are cooled by liquid helium (LHe) using a pool cooling method as shown in Fig. 1. They are energized at the train depot and are operated in a persistent current mode (PC mode) such that they do not require any on-board energizing power units. Recent development of high temperature superconductor (HTS) wire has resulted in their potential use in various applications within the power industry. Research & Development of HTS electric power cables, HTS SMES, HTS motors/generators and other applications have recently been observed. The R & D of these HTS applications has continued to advance towards commercialization, with the use of BSCCO wires (Ag sheathed Bi2223 tapes). A point in common between these HTS R&D applications, was their required continuous energization using a power supply. That is, these systems were operated in a “non-persistent current mode”. This paper describes a project overview for a persistent current HTS magnet cooled by a cryocooler using a conducting cooling design. This HTS magnet has been developed for the Maglev system to be used in place of the existing LTS magnets.
Fig. 1. Four LTS coils installed in a SCM of Yamanashi Maglev system use Nb-Ti wire.
II. P ROJECT OBJECTIVE The objectives of this project are the possibilities of cost reduction and improved reliability over the existing LTS-SCM for the Maglev. To initiate this project, the first necessary step was to evaluate the availability and capability of an HTS-SCM. III. EVALUATION T ARGET OF T HIS PROJECT During the initial evaluation, the project was focused towards ensuring that an HTS-SCM could replace an LTS-SCM. This evaluation was done through the fabrication of an HTS one-coil magnet system. The main target for the evaluation was to obtain a current decay rate of 10%/day or less under PC mode operation.
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The HTS one-coil magnet R&D project can be divided broadly into two periods. These periods are denoted as the “R&D by JR Central period” and the “National project Period”. A. R&D by JR Central 1999-2001(Japanese fiscal year) - Checking HTS ability and Component R&D In fy99, JR Central began its study of HTS magnet technology for the MAGLEV, concurrently with the observed growth in the manufacturing technology of Bi2223 tapes. The solid lines in Fig. 2 represent R&D flow from the project’s initiation to the development of a trial coil that was 1/2 scale of the actual coil used in the MAGLEV SCM. In order to save time, the indispensable components for persistent current operation were developed simultaneously. B. R&D as National Project 2001-2003(Japanese fy) - HTS magnet operated under Persistent Current mode In fy2001, JR Central was entrusted with the “development of a non-circular HTS coil magnet for general industries” by NEDO through ISTEC. Through this program,JR Central took steps towards the d evelopment of a racetrack shaped HTS coil magnet for potential adaptation into the SCM for the MAGLEV. The dotted lines in Fig. 2 show the progress of the R&D as a national project. In fy2003, the HTS one-coil magnet, including the HTS-coil, indispensable components, software and tests was completed and all development results were obtained. The results from the coil showed that the HTS magnet operated with a current decay rate of about 0.44% per day. This was a very low decay rate for a HTS coil and indicated virtually persistent current operation. V. REMARKABLE DEVELOPMENT AND SUMMARY A. Feasibility Study TABLE I and Fig. 3 show specifications of an actual racetrack coil for the Yamanashi MAGLEV SCM. The LTS coil specifications provided the preconditions for the development of HTS coil. Factors important for the successful compatibility between LTS coils and HTS coils included dimensions, robustness to magnetomotive force and operating current. The results from the feasibility study determined the necessary specifications required for the HTS coil as shown in TABLE II, as well as the schematic view of the HTS coil as shown in Fig. 4. Table III shows the specifications of Bi2223 tape used in the feasibility study. TABLE I S PECIFICATIONS OF ACTUAL RACETRACK COIL FOR MAGLEV Diameter of arc part 500mm Length of straight line part 570mm Superconductor Nb-Ti Number of winding turns 1000-1500 Operating temperature LHe (4.2K) Magnetomotive force 700-750kA Operating current 500A Cooling method Pool cooling method
1) Feasibility Study of HTS wire applicability to MAGLEV SCM
2) Checking characteristics of simple HTS wire 6) Development of GM type two-stage pulse tube cryocooler 3) Establishment of HTS wire winding technique with checking coil characteristics 7) Development of persistent current switch made of YBCO thin film 4) Checking HTS coil ability; a. Mechanical endurance test b. Heat spot thermal conduction test etc.
5) Making 1/2 scale race-track shaped HTS coil for MAGLEV SCM 8) Development of detachable current lead
Persistent current HTS one coil magnet – National Project period Fig. 2. R&D flow from feasibility study in 1999 to fabrication of HTS one coil magnet in 2003. 570mm
500mm
Fig. 3. Typical dimensions of coil schematically. Bi2223 tapes
Coil case
Insulated wire
Racetrack coil
Cross section of coil
Cross section of pancake coil
Fig. 4. Schematic view of the HTS racetrack coil.
B. HTS coil fatigue test Mechanical performance must be taken into account in the design of a superconducting magnet in order to preclude degradation during operation. TABLE II S PECIFICATIONS OF F ULL- SIZED RACETRACK COIL Diameter of arc part 500mm Length of straight line part 570mm Number of strands 4 Number of pancake coils 12 Operating temperature 20K Magnetomotive force 750kA Operating current 521A Cooling method Conduction cooling method
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3 TABLE III S PECIFICATIONS OF WIRE
Superconducting wire Wire width Wire thickness Ic@10-6V/cm N-value Tensile stress Tensile strain
Ag-sheathed Bi2223 tapes 4.1mm 0.21mm 120A＠77K,s.f. 15@10-6V/cm 95% Ic retention @ 75Mpa 95% Ic retention @ 0.15%
Repetitive hoop stress was applied to the test coil as shown in Fig. 5. The peak current was set to 80% of the statistically deteriorating current as shown in Fig. 6. In this work, the mechanical performance of HTS coils with epoxy impregnation was investigated [1]. The I-V curves showed that the coil did not deteriorate after 104 times loading of 80% of the critical hoop stress as shown in Fig. 7. Stress∝Current ID x 80%
TABLE IV S PECIFICATIONS OF HALF- SIZE RACE- TRACK COIL Diameter of arc part 250mm Length of straight line part 285mm Number of strands 4 Number of pancake coils 6 Operating temperature 20K Magnetomotive force 180kA Operating current 500A
JR Central acquired knowledge that n-value at the 10-9V/cm range was an important requirement for persistent operation. Thus, it became important to evaluate the initial performance of the wire to 10-9V/cm, prior to coil fabrication. This is evident from the results Fig. 8 that show early tapes to have variable performance below 10-6V/cm. Recent tapes in the coil show consistent and satisfactory performance down to 10-9V/cm. Thus the development and test of the 1/2 scale coil shown in Fig. 9 provided important knowledge towards the development requirements for the HTS SCM. Electrical field (V/cm)
1.E-05
time
Fig. 6. Repetitive hoop stress applying. Cryostat
Strand Strand Strand Strand
1.E-06 1.E-07
#1 #2 #3 #4
1.E-08 1.E-09 1.E-10 10
Strand current (A)
100
Fig. 8. Photograph of the single-pancake coil and I-V characteristics at 77K of a single-pancake coil.
LHe
Backup Coil (7.25T)
Fig. 5. Schematic drawing of the experimental setup in liquid helium and tested coil. 1
Fig. 9. The trial 1/2 scale HTS coil.
Initial After 10 Voltage , mV/m
After 10
3 4
time loading time loading
0.1
Ic
0.01
0.001 10
100 Current , A
Fig. 7. The V-I curves at the initial, after 103 times loading and after 104 times loading experiment.
C. 1/2 scale coil In the 1/2 scale coil R&D, one of the important requirements was to investigate any characteristics of the coil, which could not be deduced from the wire characteristics. Table IV shows the coil specifications. The four tapes used in the 1/2 scale single-pancake coil is similar to those required for the full-scale racetrack coil. An early trial single-pancake coil is shown in Fig. 8. Figure 8 also shows the I-V characteristics of each strand in a single-pancake coil at 77 K. Until 1999, discussions of n-value at low electrical field, such as 10-9V/cm as a general specification, had not been thoroughly explored. Instead the common definition of electrical field for Ic and n-value was usually given in voltage range of 10-6V/cm.
D. Persistent Current Switch (PCS) A PCS is one of the most important components required for successful PC mode operation. In order to obtain higher resistance during non-persistent operation, YBCO thin film was adopted for the PCS conductors [2]. Figure 10 shows the schematic of the structure of the PCS adopted with a thermal switch. PC mode operation tests were successfully carried out with this PCS design (see Fig. 11). Ag-Sheathed Bi2223 wire YBCO thin film patterned by wet etching Splice
Voltage taps Sapphire substrate Thermal switch Thermal conductor
Fig. 10. Schematic drawing of a YBCO thin film on a sapphire substrate spliced with Ag-sheathed Bi2223 tapes and connected with a thermal switch and photograph of the 500 A class PCS.
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1.5 1 0.5 0 -0.1
0
0.1 0.2 0.3 Time [hours]
0.4
6.3
6.4
6.5 6.6 6.7 Time [hours]
Temperature [K]
60 Fig. 11. 1 Persistent current operation of the HTS magnet.
PCS temperature PCS heat input
PCS temperature PCS heat input
140 120 100 80 60 40 20 0 -0.1
900 800 700 600 500 400 300 200 100 0
Cryocooler Thermal conductor
Themal shield Cryocooler
Demountable Detachablejoint current leads
6.8
80 70 60 50 40 30 20 10 0
E. Detachable current lead In the case of magnet cooled via pool cooling method, the current lead could be cooled by the gas of the coolant. In case of a magnet using conduction method, it is reasonable that 0 a0.1 0.2 0.3 0.4 cooling 6.3 6.4 6.5 6.6 6.7 6.8 Time [hours] Time [hours] its current lead should be cooled via conduction cooling. In order to reduce heat load, a detachable current lead design was developed and installed between the thermal shield and the vacuum vessel [3]. HTS coil Cryocooler
Protection resistor
HTS lead
Current [A]
Magnetic field Power supply current
Magnetic field Power supply current
Heat input [W]
Magnetic field [T]
2
PCS
Thermal switch
HTS coil Vacuum vessel
Thermal shield
Fig. 14. Schematic configuration of the persistent current HTS magnet.
Ultrasonic motor
Vacuum vessel Detachable current lead HTS lead
Fig. 15. Outside view of persistent current HTS magnet and HTS coil.
Thermal Anchor
HTS Coil
Ｉ
Ｍ
Joints
3
150
Flexible lines
Plug
Ultrasonic rotator
Connector
Fig. 12. Schematic configuration of the detachable current lead. 600
250
500
200
400
150
300
100
200
50
100
0 5
10 15 20 Time [minute]
25
1.5
75
Power supply current PCS Temperature
1
50
Coil temperature
0.5
25
0
Socket
2
4
6
8
10
12
Time [ｈ]
Brass lead (Cold side) Thermal anchor
0 0
Brass lead (Warm side)
Fig. 16. 10 hours’ persistent current operation and the magnetic field transition.
Shield coldhead Current
0 0
100 Magnetic field
Plug
Current [A]
Temperature [K]
300
125
2
Temperature [K]
Socket
Magnetic field [T]， Current [kA]
Brass lead
0.44%/day
2.5
Power supply Thermal Shield
30
Fig. 13. Temperature of component in the detachable current lead during a energizing test.
Figure 12 shows a schematic configuration and photo of the detachable current lead. Figure 13 shows the temperature variation of a component in the detachable current lead during an energizing test. The result indicates that the detachable current lead works favorably for a 500A class HTS magnet. F. Persistent Current HTS Magnet The full scale HTS one-coil magnet was developed based on the study shown in TABLE II, with all of its developed components (including a GM type two-stage pulse tube cryocooler which is not discussed because of space limit). Figure 14 provides a schematic configuration of this magnet, while the external view of this magnet is shown in Fig. 15. PC mode operation test result showed the current decay rate to be a very small value of 0.44%/day as shown in Fig.16. This small decay rate was observed during PC mode operation tests performed in conjunction with electromagnetic vibration tests. Moreover the mechanical vibration tests of +-150m/s 2 on the HTS coil did not result in any unusual occurrences.
VI. CONCLUSION During the R&D by JR Central, it became clear that there were problems that needed to be solved when applying Bi2223 tapes towards PC mode operation for the SCM developed for the Yamanashi MAGLEV Test Line. As this report shows, JR Central has succeeded in solving these problems. The HTS one-coil magnet achieved a current decay rate of 0.44%/day under PC mode operation in the national project. This result certifies that the development program is proceeding down the correct path.
ACKNOWLEDGMENT Motohiro Igarashi thanks American Superconductor Corp., Sumitomo Electric Industries, Ltd., Aisin Seiki co.,Ltd..
REFERENCES [1] [2]
[3]
T. Yazawa et al., “Fatigue Tests of HTS Coils,” IEEE. on Applied Superconductivity, vol.14, No.2,pp.1214-1217, June 2004 T. Tosaka et al., “Development of Persistent Current Switch for HTS Magnets,” IEEE. on Applied Superconductivity, vol.14, No.2, pp.1218-1221, June 2004 K. Nemoto et al., “Development of a Low Heat Leak Current-Lead System,” IEEE. on Applied Superconductivity, vol.14, No.2, pp.1222-1224, June 2004
