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Development and research in high magnetic fields at ASIPP
HIYSICA ELSEVIER
Physica B 216 (1996) 166 170
Development and research in high magnetic fields at ASIPP B.J. Gao, J.L. Chen*, S.L. Yuan, W.F. Yuan, F.T. Wang, L.R. Ding, X.N. Liu, Z.M. Liu, Z. Huang, Z.Y. Chen, S. Wang High Magnetic Field Laboratory. Institute o/" Plasma Physics..4cademia Sinica. Hefei 230031. China
Abstract
Steady magnetic fields up to 20 T have been generated at the Institute of Plasma Physics, Academia Sinica (ASIPP), by a hybrid magnet consisting of an outer NbTi superconducting (SC) coil and an inner water-cooled (WC) Bitter coil consuming 3.0 MW. The laboratory has also installed WC magnets at two magnet sites with fields up to 14 T and some SC magnets capable of producing 8.0 T in different bores. The research programs in high magnetic fields are in progress and extensive studies on superconductors, magnetic materials, low dimensional organic conductors as well as biological effects of high magnetic fields have been performed. Based on the use of the 40 MW fly-wheel generators the next project for constructing a 45 T, 2 s flat-top quasi-steady magnet has been proposed for creating new scientific opportunities at higher fields.
1. Introduction
ASIPP is a research institute mainly for high temperature plasma physics and fusion technology. It now runs the HT-7 tokamak which has a SC toroidal field system with a magnetic energy of 15 MJ and a cold mass of 1.4 x 105 kg. There exist also the research activities on superconductivity and high field magnet technology at the Institute since 1980. In the area of magnet technology we have developed the pulsed resistive magnets (40 T/20 ms/20 mm bore and 30 T/26 ms/32 mm bore) [1], small and mid-size general purpose SC magnets [2], a SC wiggler magnet for synchrotron radiation light source [3], WC magnets and a 20 T hybrid magnet. The 20 T hybrid magnet project was initiated in 1984 and completed in May 1992 [4]. With the 20 T hybrid magnet and other magnet facilities
*Corresponding author.
becoming available for the scientific experiments, the research groups of superconductivity at the Institute has joined the High Magnetic Field Laboratory to exploit the potentials of the high field facilities. The laboratory is also open to visiting scientists in all areas of science. Now we publish about 30 papers each year on high field research in current journals and conference proceedings. Recently, three institutions (Chinese University of Science and Technology, Institute of Solid State Physics, and ASIPP) have run jointly the Hefei Physical Research Center under Extreme Experimental Conditions with the aim of promoting physical research in high magnetic fields, high pressure, low temperature, etc. on a nationwide scale. Many experiments, for example the experimental observations of the de Haas-van Alphen effects in some superconductors, need higher fields than those achieved. The next project of the laboratory for more intense fields has been considered. The hybrid magnet is no doubt the most important approach for higher steady fields, but it is costly and sophisticated. For a hybrid system higher
0921-4526/96/$15.(X) ( 1996 Elsevier Science B.V. All rights reserved SSDI 092 1 -4526195 i 0 0 4 6 4 - "~
167
B.J. Gao et al, 'Phvsica B 216 (1996) 166 170
than the present 20 T, we have to upgrade the power and water-cooling systems. The 10 MW power supply which was transferred to the Institute from Grenoble in 1990 seems to provide an opportunity for constructing a 30 T hybrid magnet, but the mains in the area where our institute located are limited. The 10 MW power supply cannot be fully fed and the timetable for improving the situation is uncertain. On the other hand, the existing four huge fly-wheel generators at ASIPP with 140 MJ stored energy each can be the excellent power supply for generating high quasi-steady magnetic fields. With this inertial energy storage unit we can generate a field much higher than that by a hybrid system consuming 10 MW DC power and it is believed that most steady field experiments can be adapted to the quasi-steady fields with the pulse duration in 1 s order in accordance with the progress in experimental techniques. The conceptual design of a 45 T quasi-steady magnet with a fiat-top pulse of 2 s is outlined in the latter section of the paper.
2. Magnet facilities Table I lists the magnets of the laboratory which are in active service. The hybrid magnet H M-1 was firstly tested in May 1992 producing 20.2 T. It can run stably to 20 T in the routine operation. Its inner WC coil WM-1 is of the Bitter type which produces 13 T with a power consumption of 3.0MW. AI20 3 dispersion-strengthenedcopper was used for the conductor. In the new versions of WM-1 designed, bi-Bitter and polyhelix configurations are employed. The bi-Bitter design reduces the stress level of the coils so that the less costly material, hard copper, instead of AlzO3-dispersioned-copper can be used for the conductor. The polyhelix magnet developed will be in the place of WM-1 and makes HM-1 produce 23T. The cryogenic system attached to HM-1 precools SC coil SM-1 and its cryostat down to temperatures lower
than 20 K in about 16 h. Then the cryostat is filled with liquid helium and the magnets are charged. For keeping the SM-1 and its cryostat cold, the helium refrigeratorliquefier (Model 1430, Koch, USA) with a capacity of 34 1/h or 92 W at 4.6 K should be run as refrigerator and liquefier simultaneously. That is actually hard to handle for a long-term operation regime. So we are planning to equip HM-1 with a modest capacity cryogenerator and save the model 1430 to serve as a general purpose helium liquefier. Liquid helium required by SM-1 will be supplied by a movable tank. Now the helium liquefiers at the institute have a total capacity of 200 l/h and with many 5001 or 10001 liquid helium tanks, so we have no problem with liquid helium supply for high field experiments. The hybrid HM-1 provides the laboratory's highest fields but the WC magnet WM-2, WM-3 and the SC magnets are used more frequently because of their easier operation. For effective running of the high field magnets, instrumentation has been developed and emphasized. Helium insert cryostats were equipped in hybrid and WC magnets. The sample temperature can be varied from 1.5 K to 300 K. General measuring instruments for magneto-transport properties are available and efforts are made to increase the S/N ratio when the signal level is very low. A rotating sample holder with an angular resolution of 0.1 ° has been used for studying the magnetic anisotropy of high-Tc superconductors (such as the field-orientation-dependent critical current and resistivity-broadening, etc.). The sample vibrating magnetometers are being developed and will be available soon. The optical equipment and high pressure cells etc. are being planned.
3. Research in high magnetic fields Extensive experimental research has been made on the field-induced broadening of resistive transition, magnetic
Table 1 List of high field magnets at ASIPP Magnet
Type
Effective bore (mm)
Center field IT)
Power (MW)
HM-1 SM-1 WM-1 WM-2 WM-3 SM-2 SM-3
Hybrid SC Bitter Bitter Bitter SC SC
32 266 32 50 45 100 54
20 8.0 13 15 14 7.0 8.0
3.0 3.0 3.9 4.3
Current (A)
Homogeneity in 1 cm DSV
724 10941 13 900 14300 580 52
8 × 10 -4 1.4 × 10-4 1.1 × 10-3 2.0 × 10- 3 2.5 x 10 3 6.8 x 10-4 5.0 x 10-4
168
B.J Gao et al
Phvsica B 216 (1996) 166 170
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Fig. 1. The magnetic field dependence of transport critical current in a Ag-clad Bi-2212 tape,
anisotropy and its relation to superconductivity, and the effect of magnetic field on transport critical current density for various high-To oxide superconductors. A high transport critical current, 5 × 104 A/cm z, at 77 K in selffield has been obtained at our laboratory by a physical deposition method following the powder-in-tube technique [5] in Ag-clad Bi-based tapes. As an example, the effect of magnetic field on the transport critical current at different temperatures is shown in Fig. 1 for a Ag-clad Bi-2212 tape. It can be found that at lower temperatures (e.g,, 30.5 K) there is no significant effect of magnetic field up to 16 T on the critical current. This proposes a potential possibility in future applications at temperatures close to the hydrogen boiling point in the presence of a strong magnetic field. At higher temperatures, however, the critical current is significantly decreased with increasing field strength. An attempt to improve the critical current in magnetic fields is in progress. Other research is also being conducted, such as the magnetoresistance of conducting polypyrrole films and the mesoscopic quantum effect for magnetic multilayered films of [81NiFe/Cr]2 and [Co/Cu]1,~. In order to take concerted action on experimental research in magnetic fields, a theoretical study is also performed on the effect of magnetic field on the transport properties of low-dimensional systems ~such as mesoscopic system, low-dimensional organic conductors, the layered high-Tc superconductors, etc). Significant progress has been made in flux vortex dynamics, magneticfield-induced energy dissipation, and the physical origin of the irreversibility line in highly anisotropic superconductors. A phenomenological model [6] for the out-ofplane resistive dissipation has been developed for highTc superconductors, which has quantitatively provided a consistent explanation [7, 8] on the Lorentz-force-independent resistive dissipation generally reported for a configuration of H II1 c in almost the whole transition region for various high-To superconductors.
The effect of high magnetic field on the living body is initially studied on Wistar rats and frogs in cooperation with the University of Anhui Medico-Science. In the Wistar rat experiments, a group of the experimental Wistar rats (including 12 young rats) are exposed to 10 T steady field two times, each time for 30 rain at the 10th day and the 20th day after their birth. By comparing the experimental group with the control group (12 rats also), the anatomical, histological and physiological changes of the Wistar rats were observed and studied. The results suggest that a strong field results in the stress state of the rats and increases the leucocyte count in blood but has significant effect on neither the growth and development nor the function of learning and memory. Further studies on this effect are still in progress.
4. Design of 45 T quasi-steady magnet
The hybrid magnets which will produce 40 45 T steady magnetic fields are being designed and constructed at N H M F L , NRIM and Grenoble [9, 10]. It is difficult to produce continuously more intense fields at the present level of technology because of the severe power limitations and relatively low critical fields of the practical superconductors. Quasi-steady fields are less restricted by power and, therefore, higher fields can be obtained in this way. The 40 T 0.1 s fiat-top quasi-steady magnet has been in operation successfully at the University of Amsterdam for years [11] and various new versions of this kind of liquid nitrogen (neon) cooled magnet with a 60 T fiat-top pulse for 0.1 s are under development at several magnet laboratories, including N H M F L , University of Amsterdam, Princeton University and Grenoble [12]. The pulse duration of the maximum fields of these magnets is limited by the adiabatic heating during the field generation. A design of the 45 T 2 s fiat-top quasi-steady magnet is proposed at ASIPP based on the use of the 40 MW fly-wheel generator power supply. The characteristics of this magnet are as follows: (at it is a full cooling resistive magnet with fields up to the state-of-the-art of the hybrid magnet and a configuration simpler than the hybrid magnet; (b) its pulse duration of the maximum field is an order of magnitude longer than that of the quasi-steady magnet of the University of Amsterdam. The design parameters of the magnet are listed in Table 2. The magnet is of a poly-Bitter configuration consisting of a set of 10 concentric Bitter coils. The current distribution and geometry of the coils are optimized under the constraints of stress level and power density to maximize the field output at the given power of 40 MW. The stress in the inner coils reaches 800 MPa and the high strength C u - A g alloy with relatively high
169
B.J. Gao et al. Phvsica B 216 (1996) 166 170
Table 2 Design parameters of 45 T quasi-steady magnet Total central field (T) Working bore (mm) Inner diameter (mmt Outer diameter (mm) Total number of coils Total power in magnet (MW) Total water flow rate (l/s) Coil unit Number of coils Inner radius (mm) Outer radius (mm) Coil height (average) {mini Conductor material Conductivity (%IACS) Power (MW) Maximum power density (W mm3i Inlet water temp. (~C) Pressure drop (bar) Flow rate (l/s) Average temp. (°C) Maximum temp. (CI Current (kA) Voltage (V) Maximum stress (MPa) Field (T)
45.3 32 38 1000 10 40 324 Inner 1 4 19 61.4 186.6 Cu Ag 75 5.8 9.84 10 28 43 61 85 100 57.7 796 14.5
conducti:vity is chosen as the conductor, it is the recent development of the materials for conductors that makes it possible to design such a high stress quasi-steady magnet [13]. At A S I P P we have four huge fly-wheel generators rated at a D C power of 80 M W and a stored energy of more than 500 MJ. Table 3 lists the parameters for each generator set. The quasi-steady magnet will be driven to 40 M W (100 kA at 400 V) and will produce 45 T by two generators connected in parallel. There exist the bus bars between the fly-wheel generator hall and the high magnetic field experimental hall, which are in a copper section of 120 x 10 x 8 mm 2. enough to pass 100 kA during several seconds. The coils of the magnet are specially designed and grouped into four units to match the output characteristics of the generator set. The inner part of the magnet consists of two units, each unit has four coils. The outer two units are composed of two coils, one per unit. The four units are connected to the generator set in series. Fig. 2 is the equivalent circuit of the magnet and its generator power supply. The fly-wheel generators are taken as an electrical capacitance and the electrical parameters are given. Fig. 3 shows the characteristics of current, voltage, energy and power of the magnet versus time. The constant current is obtained during 2 s by controlling the excitation voltage of the generator set.
Inner 2 4 64.4 134.4 247.2 Cu Ag 75 15.2 2.44 10 28 109 58 83 100 153.4 799 14.4
Outer 1 1 146.4 264.7 404.3 Hard copper 97.97 11.9 0.41 10 28 108 46 64 100 119.4 362 10.2
Outer 2 1 276.7 500.0 576.8 Copper 97.97 7.1 0.04 10 28 63 44 62 100 70.8 29 5.87
Table 3 Main parameters of the fly-wheel generator Power rating Voltage Current Speed rating Energy at full speed Current repetition rate Current non-uniformity Drive system rating Fly-wheel torque Weight
20 MW 500 V 50 kA 368 rpm 140 MJ 10 rain 1% 0.63 MW 7.5 x 105 kgm 2.24 x 105 kg
For a 40 M W full cooling resistive magnet, a deionized water cooling system with a flow rate of 400 l/s is needed in principle. We now have only a water cooling system of 40 l/s installed for the 20 T hybrid magnet. To cope with this problem, a system shown schematically in Fig. 4 is conceived. A water tank with a capacity of, for example, 5 m 3, controlled to a constant pressure of 28 bar, will deliver the required flow rate to the magnet in a time period during the charging time of the magnet. The detailed design is to be made. We expect that the realization of the 45 T quasi-steady magnet will provide an important research tool by virtue
170
B.J. Gao et al. Physica B 216 (1996) 166 170
B
)
LM
RM
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F 1 "~--
v
Fig. 4. Schematic drawing of the water cooling system that delivers a water flow of 400 l/s during the charging time of the magnet: high pressure gas tank (A), pressure-controlling valve (B), high pressure water tank (C), water tank (D), heat exchanger {E), magnet (M) and valves (F1 F4).
1 LM
-C
R,M
Fig. 2. Equivalent circuit of the 45 T quasi-steady magnet with its generator power supply, where C = 2226 F, LM = 2.64 mH and RM = 4 m~.
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recording systems now available, will permit most of the steady-field experiments to be accommodated to the quasi-steady fields. For example, the de H a a s - v a n AIphen experiments can be easily handled in these fields. Also, with this difficult but exciting task for constructing such a 45 T and 2 s flat-top magnet, we will make our contributions to the development of high field magnet technology.
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Acknowledgement One of the authors (B.J. Gao) would like to acknowledge the National High Magnetic Field Laboratory in the USA where the calculation of the 45 T quasi-steady magnet was completed and is grateful to Dr. Hans J. Schneider-Muntau for his valuable discussions and help while pursuing this study.
References
II
20
10 0
iii
0.5
1
1.5 2 time (s)
2.5
a~
0
3
Fig. 3. Characteristics of current, voltage, energy and power of the magnet versus time. The constant current is obtained during 2 s by controlling the excitation voltage of the generator set.
of its higher fields and longer flat-top duration. The slower field rise time eliminates the eddy current heating and disequilibrium in the sample. The flat-top field of 2 s, combined with the advanced instruments and data
[-1] J.B. Shi et al., Chin. J. Nucl. Sci. Eng. 4 (1984) 163. [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
Z.K. Jiao et al., Cryogenics 27 (1987) 416. QL. Wang et al., Chin. J. Low Temp. Phys. 13 (1991) 391. J.L. Chen, B.J. Gao et al., Physica B 201 (1994) 526. G.C. Han et al., Cryogenics 34 (1994) 613. S.L. Yuan, Z.J. Yang, Phys. Lett. A 196 (1994) 259. S.L Yuan, Appl. Phys. Lett. 64 (1994) 1033. S.L. Yuan, Acta Phys. Sinica 44 (1995) 1268. B.L. Brandt et al., Physica B 201 (1994) 504. K. Inoue et al., Physica B 177 (1992) 7. J.J.M. Franse et al., in: Physical Phenomena at High Magnetic fields (Addison-Wesley, New York, 1992) p. 539. [12] L.J Campell et al., Physica B 201 (1994) 538. [13] Y. Sakai et al., Appl. Phys. Lett. 59 (1991) 2965.
Physica B 216 (1996) 166 170
Development and research in high magnetic fields at ASIPP B.J. Gao, J.L. Chen*, S.L. Yuan, W.F. Yuan, F.T. Wang, L.R. Ding, X.N. Liu, Z.M. Liu, Z. Huang, Z.Y. Chen, S. Wang High Magnetic Field Laboratory. Institute o/" Plasma Physics..4cademia Sinica. Hefei 230031. China
Abstract
Steady magnetic fields up to 20 T have been generated at the Institute of Plasma Physics, Academia Sinica (ASIPP), by a hybrid magnet consisting of an outer NbTi superconducting (SC) coil and an inner water-cooled (WC) Bitter coil consuming 3.0 MW. The laboratory has also installed WC magnets at two magnet sites with fields up to 14 T and some SC magnets capable of producing 8.0 T in different bores. The research programs in high magnetic fields are in progress and extensive studies on superconductors, magnetic materials, low dimensional organic conductors as well as biological effects of high magnetic fields have been performed. Based on the use of the 40 MW fly-wheel generators the next project for constructing a 45 T, 2 s flat-top quasi-steady magnet has been proposed for creating new scientific opportunities at higher fields.
1. Introduction
ASIPP is a research institute mainly for high temperature plasma physics and fusion technology. It now runs the HT-7 tokamak which has a SC toroidal field system with a magnetic energy of 15 MJ and a cold mass of 1.4 x 105 kg. There exist also the research activities on superconductivity and high field magnet technology at the Institute since 1980. In the area of magnet technology we have developed the pulsed resistive magnets (40 T/20 ms/20 mm bore and 30 T/26 ms/32 mm bore) [1], small and mid-size general purpose SC magnets [2], a SC wiggler magnet for synchrotron radiation light source [3], WC magnets and a 20 T hybrid magnet. The 20 T hybrid magnet project was initiated in 1984 and completed in May 1992 [4]. With the 20 T hybrid magnet and other magnet facilities
*Corresponding author.
becoming available for the scientific experiments, the research groups of superconductivity at the Institute has joined the High Magnetic Field Laboratory to exploit the potentials of the high field facilities. The laboratory is also open to visiting scientists in all areas of science. Now we publish about 30 papers each year on high field research in current journals and conference proceedings. Recently, three institutions (Chinese University of Science and Technology, Institute of Solid State Physics, and ASIPP) have run jointly the Hefei Physical Research Center under Extreme Experimental Conditions with the aim of promoting physical research in high magnetic fields, high pressure, low temperature, etc. on a nationwide scale. Many experiments, for example the experimental observations of the de Haas-van Alphen effects in some superconductors, need higher fields than those achieved. The next project of the laboratory for more intense fields has been considered. The hybrid magnet is no doubt the most important approach for higher steady fields, but it is costly and sophisticated. For a hybrid system higher
0921-4526/96/$15.(X) ( 1996 Elsevier Science B.V. All rights reserved SSDI 092 1 -4526195 i 0 0 4 6 4 - "~
167
B.J. Gao et al, 'Phvsica B 216 (1996) 166 170
than the present 20 T, we have to upgrade the power and water-cooling systems. The 10 MW power supply which was transferred to the Institute from Grenoble in 1990 seems to provide an opportunity for constructing a 30 T hybrid magnet, but the mains in the area where our institute located are limited. The 10 MW power supply cannot be fully fed and the timetable for improving the situation is uncertain. On the other hand, the existing four huge fly-wheel generators at ASIPP with 140 MJ stored energy each can be the excellent power supply for generating high quasi-steady magnetic fields. With this inertial energy storage unit we can generate a field much higher than that by a hybrid system consuming 10 MW DC power and it is believed that most steady field experiments can be adapted to the quasi-steady fields with the pulse duration in 1 s order in accordance with the progress in experimental techniques. The conceptual design of a 45 T quasi-steady magnet with a fiat-top pulse of 2 s is outlined in the latter section of the paper.
2. Magnet facilities Table I lists the magnets of the laboratory which are in active service. The hybrid magnet H M-1 was firstly tested in May 1992 producing 20.2 T. It can run stably to 20 T in the routine operation. Its inner WC coil WM-1 is of the Bitter type which produces 13 T with a power consumption of 3.0MW. AI20 3 dispersion-strengthenedcopper was used for the conductor. In the new versions of WM-1 designed, bi-Bitter and polyhelix configurations are employed. The bi-Bitter design reduces the stress level of the coils so that the less costly material, hard copper, instead of AlzO3-dispersioned-copper can be used for the conductor. The polyhelix magnet developed will be in the place of WM-1 and makes HM-1 produce 23T. The cryogenic system attached to HM-1 precools SC coil SM-1 and its cryostat down to temperatures lower
than 20 K in about 16 h. Then the cryostat is filled with liquid helium and the magnets are charged. For keeping the SM-1 and its cryostat cold, the helium refrigeratorliquefier (Model 1430, Koch, USA) with a capacity of 34 1/h or 92 W at 4.6 K should be run as refrigerator and liquefier simultaneously. That is actually hard to handle for a long-term operation regime. So we are planning to equip HM-1 with a modest capacity cryogenerator and save the model 1430 to serve as a general purpose helium liquefier. Liquid helium required by SM-1 will be supplied by a movable tank. Now the helium liquefiers at the institute have a total capacity of 200 l/h and with many 5001 or 10001 liquid helium tanks, so we have no problem with liquid helium supply for high field experiments. The hybrid HM-1 provides the laboratory's highest fields but the WC magnet WM-2, WM-3 and the SC magnets are used more frequently because of their easier operation. For effective running of the high field magnets, instrumentation has been developed and emphasized. Helium insert cryostats were equipped in hybrid and WC magnets. The sample temperature can be varied from 1.5 K to 300 K. General measuring instruments for magneto-transport properties are available and efforts are made to increase the S/N ratio when the signal level is very low. A rotating sample holder with an angular resolution of 0.1 ° has been used for studying the magnetic anisotropy of high-Tc superconductors (such as the field-orientation-dependent critical current and resistivity-broadening, etc.). The sample vibrating magnetometers are being developed and will be available soon. The optical equipment and high pressure cells etc. are being planned.
3. Research in high magnetic fields Extensive experimental research has been made on the field-induced broadening of resistive transition, magnetic
Table 1 List of high field magnets at ASIPP Magnet
Type
Effective bore (mm)
Center field IT)
Power (MW)
HM-1 SM-1 WM-1 WM-2 WM-3 SM-2 SM-3
Hybrid SC Bitter Bitter Bitter SC SC
32 266 32 50 45 100 54
20 8.0 13 15 14 7.0 8.0
3.0 3.0 3.9 4.3
Current (A)
Homogeneity in 1 cm DSV
724 10941 13 900 14300 580 52
8 × 10 -4 1.4 × 10-4 1.1 × 10-3 2.0 × 10- 3 2.5 x 10 3 6.8 x 10-4 5.0 x 10-4
168
B.J Gao et al
Phvsica B 216 (1996) 166 170
,,,ir,,l,,,i,,,i,,,i,,,i,,,i,,,
t~ - o
~0.1
i
60.3K '
ii • 70.6K ,
,
i
2
,
,
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4
,
,
,
i
6
,
,
,
ii
,
,
i1
,
i
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,
i
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,
8 10 12 14 16 H (T)
Fig. 1. The magnetic field dependence of transport critical current in a Ag-clad Bi-2212 tape,
anisotropy and its relation to superconductivity, and the effect of magnetic field on transport critical current density for various high-To oxide superconductors. A high transport critical current, 5 × 104 A/cm z, at 77 K in selffield has been obtained at our laboratory by a physical deposition method following the powder-in-tube technique [5] in Ag-clad Bi-based tapes. As an example, the effect of magnetic field on the transport critical current at different temperatures is shown in Fig. 1 for a Ag-clad Bi-2212 tape. It can be found that at lower temperatures (e.g,, 30.5 K) there is no significant effect of magnetic field up to 16 T on the critical current. This proposes a potential possibility in future applications at temperatures close to the hydrogen boiling point in the presence of a strong magnetic field. At higher temperatures, however, the critical current is significantly decreased with increasing field strength. An attempt to improve the critical current in magnetic fields is in progress. Other research is also being conducted, such as the magnetoresistance of conducting polypyrrole films and the mesoscopic quantum effect for magnetic multilayered films of [81NiFe/Cr]2 and [Co/Cu]1,~. In order to take concerted action on experimental research in magnetic fields, a theoretical study is also performed on the effect of magnetic field on the transport properties of low-dimensional systems ~such as mesoscopic system, low-dimensional organic conductors, the layered high-Tc superconductors, etc). Significant progress has been made in flux vortex dynamics, magneticfield-induced energy dissipation, and the physical origin of the irreversibility line in highly anisotropic superconductors. A phenomenological model [6] for the out-ofplane resistive dissipation has been developed for highTc superconductors, which has quantitatively provided a consistent explanation [7, 8] on the Lorentz-force-independent resistive dissipation generally reported for a configuration of H II1 c in almost the whole transition region for various high-To superconductors.
The effect of high magnetic field on the living body is initially studied on Wistar rats and frogs in cooperation with the University of Anhui Medico-Science. In the Wistar rat experiments, a group of the experimental Wistar rats (including 12 young rats) are exposed to 10 T steady field two times, each time for 30 rain at the 10th day and the 20th day after their birth. By comparing the experimental group with the control group (12 rats also), the anatomical, histological and physiological changes of the Wistar rats were observed and studied. The results suggest that a strong field results in the stress state of the rats and increases the leucocyte count in blood but has significant effect on neither the growth and development nor the function of learning and memory. Further studies on this effect are still in progress.
4. Design of 45 T quasi-steady magnet
The hybrid magnets which will produce 40 45 T steady magnetic fields are being designed and constructed at N H M F L , NRIM and Grenoble [9, 10]. It is difficult to produce continuously more intense fields at the present level of technology because of the severe power limitations and relatively low critical fields of the practical superconductors. Quasi-steady fields are less restricted by power and, therefore, higher fields can be obtained in this way. The 40 T 0.1 s fiat-top quasi-steady magnet has been in operation successfully at the University of Amsterdam for years [11] and various new versions of this kind of liquid nitrogen (neon) cooled magnet with a 60 T fiat-top pulse for 0.1 s are under development at several magnet laboratories, including N H M F L , University of Amsterdam, Princeton University and Grenoble [12]. The pulse duration of the maximum fields of these magnets is limited by the adiabatic heating during the field generation. A design of the 45 T 2 s fiat-top quasi-steady magnet is proposed at ASIPP based on the use of the 40 MW fly-wheel generator power supply. The characteristics of this magnet are as follows: (at it is a full cooling resistive magnet with fields up to the state-of-the-art of the hybrid magnet and a configuration simpler than the hybrid magnet; (b) its pulse duration of the maximum field is an order of magnitude longer than that of the quasi-steady magnet of the University of Amsterdam. The design parameters of the magnet are listed in Table 2. The magnet is of a poly-Bitter configuration consisting of a set of 10 concentric Bitter coils. The current distribution and geometry of the coils are optimized under the constraints of stress level and power density to maximize the field output at the given power of 40 MW. The stress in the inner coils reaches 800 MPa and the high strength C u - A g alloy with relatively high
169
B.J. Gao et al. Phvsica B 216 (1996) 166 170
Table 2 Design parameters of 45 T quasi-steady magnet Total central field (T) Working bore (mm) Inner diameter (mmt Outer diameter (mm) Total number of coils Total power in magnet (MW) Total water flow rate (l/s) Coil unit Number of coils Inner radius (mm) Outer radius (mm) Coil height (average) {mini Conductor material Conductivity (%IACS) Power (MW) Maximum power density (W mm3i Inlet water temp. (~C) Pressure drop (bar) Flow rate (l/s) Average temp. (°C) Maximum temp. (CI Current (kA) Voltage (V) Maximum stress (MPa) Field (T)
45.3 32 38 1000 10 40 324 Inner 1 4 19 61.4 186.6 Cu Ag 75 5.8 9.84 10 28 43 61 85 100 57.7 796 14.5
conducti:vity is chosen as the conductor, it is the recent development of the materials for conductors that makes it possible to design such a high stress quasi-steady magnet [13]. At A S I P P we have four huge fly-wheel generators rated at a D C power of 80 M W and a stored energy of more than 500 MJ. Table 3 lists the parameters for each generator set. The quasi-steady magnet will be driven to 40 M W (100 kA at 400 V) and will produce 45 T by two generators connected in parallel. There exist the bus bars between the fly-wheel generator hall and the high magnetic field experimental hall, which are in a copper section of 120 x 10 x 8 mm 2. enough to pass 100 kA during several seconds. The coils of the magnet are specially designed and grouped into four units to match the output characteristics of the generator set. The inner part of the magnet consists of two units, each unit has four coils. The outer two units are composed of two coils, one per unit. The four units are connected to the generator set in series. Fig. 2 is the equivalent circuit of the magnet and its generator power supply. The fly-wheel generators are taken as an electrical capacitance and the electrical parameters are given. Fig. 3 shows the characteristics of current, voltage, energy and power of the magnet versus time. The constant current is obtained during 2 s by controlling the excitation voltage of the generator set.
Inner 2 4 64.4 134.4 247.2 Cu Ag 75 15.2 2.44 10 28 109 58 83 100 153.4 799 14.4
Outer 1 1 146.4 264.7 404.3 Hard copper 97.97 11.9 0.41 10 28 108 46 64 100 119.4 362 10.2
Outer 2 1 276.7 500.0 576.8 Copper 97.97 7.1 0.04 10 28 63 44 62 100 70.8 29 5.87
Table 3 Main parameters of the fly-wheel generator Power rating Voltage Current Speed rating Energy at full speed Current repetition rate Current non-uniformity Drive system rating Fly-wheel torque Weight
20 MW 500 V 50 kA 368 rpm 140 MJ 10 rain 1% 0.63 MW 7.5 x 105 kgm 2.24 x 105 kg
For a 40 M W full cooling resistive magnet, a deionized water cooling system with a flow rate of 400 l/s is needed in principle. We now have only a water cooling system of 40 l/s installed for the 20 T hybrid magnet. To cope with this problem, a system shown schematically in Fig. 4 is conceived. A water tank with a capacity of, for example, 5 m 3, controlled to a constant pressure of 28 bar, will deliver the required flow rate to the magnet in a time period during the charging time of the magnet. The detailed design is to be made. We expect that the realization of the 45 T quasi-steady magnet will provide an important research tool by virtue
170
B.J. Gao et al. Physica B 216 (1996) 166 170
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recording systems now available, will permit most of the steady-field experiments to be accommodated to the quasi-steady fields. For example, the de H a a s - v a n AIphen experiments can be easily handled in these fields. Also, with this difficult but exciting task for constructing such a 45 T and 2 s flat-top magnet, we will make our contributions to the development of high field magnet technology.
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Acknowledgement One of the authors (B.J. Gao) would like to acknowledge the National High Magnetic Field Laboratory in the USA where the calculation of the 45 T quasi-steady magnet was completed and is grateful to Dr. Hans J. Schneider-Muntau for his valuable discussions and help while pursuing this study.
References
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of its higher fields and longer flat-top duration. The slower field rise time eliminates the eddy current heating and disequilibrium in the sample. The flat-top field of 2 s, combined with the advanced instruments and data
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