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The outlook for a 20 Tesla superconducting magnet
1084 THE OUTLOOK FOR A 20 TESLA SUPERCONDUCTING MAGNET
P.S. SWARTZ, W.D. MARKIEWICZ and C.H. ROSNER lntermagnetics General Corporation, P.O. Box 566, Guilderland, New York 12084, USA In this paper we address the question of whether a 20 T superconducting magnet can now be undertaken, using V,Ga tape and Nb3Sn tape. In the discussion, we assume that the magnet will be of duplex configuration.
1. Introduction
The first commercial 10 Tesla superconducting magnet was delivered to Bell Telephone Laboratory nine years ago, soon after Nb3Sn tape became available. High field magnet technology has advanced steadily since then. An Nb3Sn tape-wound magnet delivered to Oxford University in 1974 established a new record for superconducting magnets producing 15.8T at 4.2 K [1]. Because the critical current density of Nb3Sn decreases very rapidly between 16 and 17 T, a field level of 16.5 T probably represents a practical upper limit to the maximum field achievable at 4.2 K using Nb3Sn. The only superconductor available in long lengths with superconducting properties exceeding those of Nb3Sn at high fields is VaGa. The first significant magnet utilizing V3Ga superconductor was designed and manufactured by IGC and commissioned at the Japanese National Research Institute for Metals (JNRIM) early in 1976 [2]. This magnet is of duplex construction. Its outer section uses Nb3Sn tape and produces 13.5 T in a 160 mm bore. The inner section is wound from V3Ga tape manufactured by Vacuum Metallurgical Co., Ltd. of Tokyo and produces an incremental 4 T in a 31 mm diameter bore. 2. General design considerations
Several key factors are operative in the design of each of the inner and outer magnets, independent of the contribution of each magnet to the central 20T field. Since the winding thickness, R2-RI, of the solenoid is roughly proportional to the product of central field and average current density Jave in the winding space, it is important to maximize the current-carrying capability of the superconductor. Because the current density of superconductors decreases with increasing field, the winding thickness in general increases faster than linearly with design field. Physica 86-88B (1977) 1084--1086 © North-Holland
The stress tr in a turn of conductor of radius R caused by the field-current interaction in a field B is given by tr = RJB. Hence, as the size of a magnet, measured by R and B, increases, so will the stress. To limit the associated strain in brittle A15 compounds, stainless steel is generally included in the windings of high field Nb3Sn and V3Ga magnets and thus the average current density in the windings will decrease with increasing field. The energy stored in the magnetic field is proportional to the square of the field, integrated over all space. In the event that a portion of the windings undergoes a transition to the non-superconducting state (i.e. a magnet "quench"), the stored magnetic energy is transformed into Joule heating, J2pcu, in the copper substrate with which the superconductor is clad. During quench, the magnet must develop sutliciently high internal resistance to rapidly dissipate the stored energy; yet the j2p Joule heating must be distributed broadly within the winding space to prevent local overheating. These opposing conditions require that additional copper be incorporated into the conductor as the stored energy increases, leading again to a decrease in the average current density in the winding space as magnet energy increases. The conclusion reached from this discussion is that from several independent design considerations the average current density in the winding space decreases with design field, and consequently the physical dimensions and weight of a high field magnet will generally increase much faster than linearly with field. 3. Critical current density and high magnetic fields
An absolute prerequisite for a 20T superconducting magnet (of practical size) is the availability in long lengths of a superconductor with a current density of at least 5 × 104 A/cm 2 in
1085 the superconductor itself at this field. As shown in fig. 1, Nb3Sn fails this criterion; the currentcarrying capability of material commercially available from Vacuum Metallurgical Co., Ltd. however, significantly exceeds that of Nb3Sn. This material was used to produce 17.5T at 4.2 K. The load line of that V3Ga inner magnet is also shown. It has recently been demonstrated by Tachikawa that the current-carrying capability of V3Ga can be increased at high fields with small additions of AI and, most notably, Mg [3-5]. Using laboratory processes that lend themselves to the production of long lengths, current densities of 105 A/cm 2 have now been achieved at 2 0 T [5].
4. The inner V3Ga magnet We now address the question of the specific design of a 20 T superconducting solenoid. We consider a magnet of duplex configuration with an inner section wound from V3Ga tape having a current density of 105 A/cm 2 at 20 T, and an outer section wound from Nb3Sn tape. We assume that the outer Nb3Sn solenoid contributes 14 T and the inner V3Ga section contributes 6 T. A design optimized to the shape of the critical current curve will use double laminate V3Ga in the winding space between - 1 7 and 2 0 T , and single laminate V3Ga between 14 and - 1 7 T . The calculated load line of the high field section of the inner magnet is shown in fig. 1 and the specifications of the inner magnet are summarized in table I. Table I 20 T Magnet design specifications
500- ~ M COMMF.RClANb3SII L TAPE
I©
TAPE
ZOO]J_,~I_x 0 ~ _
VSG6
Clear Bore Outer Diameter Winding Weight Operating Current Average Current Density Self Energy Field Increment
Inner V~Ga Magnet
Outer Nb3Sn Magnet
Unit
32 170 26 300
170 480 385 350
mm mm kg Amps
10.0 × 103 11.6× 10~ Amps/cmz -30× 10~ 2.25× 106 Joule 6.0 14.0 Tesla
5. The outer Nb3Sn section
io
iii
12 I
13 i
"
14 I"
i 15 16 B(TESLA) i
ITi
i 16
~ t9
I 20
21 f"
r 21
Fig. 1. Critical current (lc) vs. field (B) for high field materials. A future candidate for the production of 20 T fields is Nb3Ge, reported to have a critical temperature approaching 25 K and an He2 in the range of 33 T at 4.2 K, significantly exceeding the critical temperature and critical field of both Nb3Sn and V3Ga. Although the critical current density of Nb3Ge is not yet optimized at high fields, extrapolation of data from lower fields suggests a value in the range of 5 x 104 A/cm 2 at 20 T [6].
Because of its large bore and high magnetic field, the outer Nb3Sn magnet represents a much more difficult design challenge than the inner V3Ga section. As discussed earlier, the design current density in the winding space decreases both with increasing magnetic field and bore size. To illustrate this point, it is instructive to examine the " A c h i e v e m e n t B o u n d a r y " of superconducting tape-wound solenoids produced to date (fig. 2). In this graph, several IGC solenoids encompassing a broad range of field and bore size are plotted, demonstrating a rather good correlation between stored energy and the average winding current density. This Achievement Boundary may, therefore, be used as input to the design of large bore, high field superconducting magnets.
P.S. SWARTZ, W.D. MARKIEWICZ and C.H. ROSNER lntermagnetics General Corporation, P.O. Box 566, Guilderland, New York 12084, USA In this paper we address the question of whether a 20 T superconducting magnet can now be undertaken, using V,Ga tape and Nb3Sn tape. In the discussion, we assume that the magnet will be of duplex configuration.
1. Introduction
The first commercial 10 Tesla superconducting magnet was delivered to Bell Telephone Laboratory nine years ago, soon after Nb3Sn tape became available. High field magnet technology has advanced steadily since then. An Nb3Sn tape-wound magnet delivered to Oxford University in 1974 established a new record for superconducting magnets producing 15.8T at 4.2 K [1]. Because the critical current density of Nb3Sn decreases very rapidly between 16 and 17 T, a field level of 16.5 T probably represents a practical upper limit to the maximum field achievable at 4.2 K using Nb3Sn. The only superconductor available in long lengths with superconducting properties exceeding those of Nb3Sn at high fields is VaGa. The first significant magnet utilizing V3Ga superconductor was designed and manufactured by IGC and commissioned at the Japanese National Research Institute for Metals (JNRIM) early in 1976 [2]. This magnet is of duplex construction. Its outer section uses Nb3Sn tape and produces 13.5 T in a 160 mm bore. The inner section is wound from V3Ga tape manufactured by Vacuum Metallurgical Co., Ltd. of Tokyo and produces an incremental 4 T in a 31 mm diameter bore. 2. General design considerations
Several key factors are operative in the design of each of the inner and outer magnets, independent of the contribution of each magnet to the central 20T field. Since the winding thickness, R2-RI, of the solenoid is roughly proportional to the product of central field and average current density Jave in the winding space, it is important to maximize the current-carrying capability of the superconductor. Because the current density of superconductors decreases with increasing field, the winding thickness in general increases faster than linearly with design field. Physica 86-88B (1977) 1084--1086 © North-Holland
The stress tr in a turn of conductor of radius R caused by the field-current interaction in a field B is given by tr = RJB. Hence, as the size of a magnet, measured by R and B, increases, so will the stress. To limit the associated strain in brittle A15 compounds, stainless steel is generally included in the windings of high field Nb3Sn and V3Ga magnets and thus the average current density in the windings will decrease with increasing field. The energy stored in the magnetic field is proportional to the square of the field, integrated over all space. In the event that a portion of the windings undergoes a transition to the non-superconducting state (i.e. a magnet "quench"), the stored magnetic energy is transformed into Joule heating, J2pcu, in the copper substrate with which the superconductor is clad. During quench, the magnet must develop sutliciently high internal resistance to rapidly dissipate the stored energy; yet the j2p Joule heating must be distributed broadly within the winding space to prevent local overheating. These opposing conditions require that additional copper be incorporated into the conductor as the stored energy increases, leading again to a decrease in the average current density in the winding space as magnet energy increases. The conclusion reached from this discussion is that from several independent design considerations the average current density in the winding space decreases with design field, and consequently the physical dimensions and weight of a high field magnet will generally increase much faster than linearly with field. 3. Critical current density and high magnetic fields
An absolute prerequisite for a 20T superconducting magnet (of practical size) is the availability in long lengths of a superconductor with a current density of at least 5 × 104 A/cm 2 in
1085 the superconductor itself at this field. As shown in fig. 1, Nb3Sn fails this criterion; the currentcarrying capability of material commercially available from Vacuum Metallurgical Co., Ltd. however, significantly exceeds that of Nb3Sn. This material was used to produce 17.5T at 4.2 K. The load line of that V3Ga inner magnet is also shown. It has recently been demonstrated by Tachikawa that the current-carrying capability of V3Ga can be increased at high fields with small additions of AI and, most notably, Mg [3-5]. Using laboratory processes that lend themselves to the production of long lengths, current densities of 105 A/cm 2 have now been achieved at 2 0 T [5].
4. The inner V3Ga magnet We now address the question of the specific design of a 20 T superconducting solenoid. We consider a magnet of duplex configuration with an inner section wound from V3Ga tape having a current density of 105 A/cm 2 at 20 T, and an outer section wound from Nb3Sn tape. We assume that the outer Nb3Sn solenoid contributes 14 T and the inner V3Ga section contributes 6 T. A design optimized to the shape of the critical current curve will use double laminate V3Ga in the winding space between - 1 7 and 2 0 T , and single laminate V3Ga between 14 and - 1 7 T . The calculated load line of the high field section of the inner magnet is shown in fig. 1 and the specifications of the inner magnet are summarized in table I. Table I 20 T Magnet design specifications
500- ~ M COMMF.RClANb3SII L TAPE
I©
TAPE
ZOO]J_,~I_x 0 ~ _
VSG6
Clear Bore Outer Diameter Winding Weight Operating Current Average Current Density Self Energy Field Increment
Inner V~Ga Magnet
Outer Nb3Sn Magnet
Unit
32 170 26 300
170 480 385 350
mm mm kg Amps
10.0 × 103 11.6× 10~ Amps/cmz -30× 10~ 2.25× 106 Joule 6.0 14.0 Tesla
5. The outer Nb3Sn section
io
iii
12 I
13 i
"
14 I"
i 15 16 B(TESLA) i
ITi
i 16
~ t9
I 20
21 f"
r 21
Fig. 1. Critical current (lc) vs. field (B) for high field materials. A future candidate for the production of 20 T fields is Nb3Ge, reported to have a critical temperature approaching 25 K and an He2 in the range of 33 T at 4.2 K, significantly exceeding the critical temperature and critical field of both Nb3Sn and V3Ga. Although the critical current density of Nb3Ge is not yet optimized at high fields, extrapolation of data from lower fields suggests a value in the range of 5 x 104 A/cm 2 at 20 T [6].
Because of its large bore and high magnetic field, the outer Nb3Sn magnet represents a much more difficult design challenge than the inner V3Ga section. As discussed earlier, the design current density in the winding space decreases both with increasing magnetic field and bore size. To illustrate this point, it is instructive to examine the " A c h i e v e m e n t B o u n d a r y " of superconducting tape-wound solenoids produced to date (fig. 2). In this graph, several IGC solenoids encompassing a broad range of field and bore size are plotted, demonstrating a rather good correlation between stored energy and the average winding current density. This Achievement Boundary may, therefore, be used as input to the design of large bore, high field superconducting magnets.