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Stabilization of a Nb3Sn persistent current switch
Fusion Engineering and Design 20 (1993) 409-414 North-Holland
409
Stabilization of a Nb3Sn persistent current switch M. U r a t a a, H. M a e d a a, S. N a k a y a m a ~, E. Y o n e d a a, y . O d a b, T. K u m a n o b, N. A o k i b, T. T o m i s a k i c a n d S. K a b a s h i m a c a Toshiba R&D Center, 4-1 Ukishima, Kawasaki 210, Japan b Showa Electric" Wire and Cable Co. ltd., 2-1-1, Odasakae, Kawasaki, Japan c Tokyo Institute of Technology, 4259, Nagatsuda, Midori-ku, Yokohama, Japan
A 2000 A class Nb3Sn persistent current switch has been successfully fabricated in the Toshiba R&D Center. The Nb tube processed conductor with Cu-10wt.%Ni matrix has been developed for the switch in the Showa Electric Wire and Cable Co. Ltd. The magnetic instability which was observed in the previous 35 l-I Nb3Sn persistent current switch was improved in the present switch. The problem of quench current degradation and flux jump on magnetization, emerged in the previous switch, were confirmed to be solved. In the fast ramp, however, the switch degrades from the calculated results assuming the self field ac loss. In the Nb3Sn reaction process, Sn in the bronze diffuses into the Nb tube, which decreases the switch resistance. It was observed by a computer aided micro analysis (CMA) that Ni in the CuNi matrix precipitated on the Nb tube, which slightly reduced the switch resistance.
1. Introduction
2. Conductor and the switch
Persistent current switches (PCS) are typical devices for superconducting magnet applications. Using thermal PCS, persistent currents with a smaller than 0.1 p p m / h decay rate have already been in use such as M R I or N M R spectrometer magnets. Using a high resistivity C u - N i alloy matrix, however, PCS involves inevitable an instability problem; i.e., PCS suffers unexpected quenches at far below the designed current. This problem occurs especially in low fields and at high current. No stable PCSs are obtained which can be operated at higher than 1000 A up to now. Trials for parallel connection of thinner conductors were not always satisfactory due to the current unbalance between strands. Nb3Sn PCS is a promising device which enables the high-current persistent-current systems such as magneto-hydrodynamic propulsion ships or SMES, with using its larger temperature margin. Moreover, Nb3Sn PCS is one of indispensable components to develop low-decay Nb3Sn persistent magnets for N M R use. This paper presents (a) the performance of a Nb tube processed Nb3Sn conductor with a CuNi matrix and the switch wound with the conductor, and (b) results of stability for the conductor. The conductor resistivity is also discussed in relation with the conductor current density.
2.J. Conductor
0920-3796/93/$06.00
The tube processed Nb3Sn conductor with Cu10wt.%Ni matrix has been developed for a Nb3Sn PCS at the Showa Electric Wire and Cable Co. LTD. The parameters for the two types of conductor are listed in table 1. These two types of conductor have almost the same parameters except for the filament outer diameter. The performance for conductor (A) and the 35 U/ switch wound with it was presented elsewhere [1]. The quench current degradation for conductor (A), occurred at lower than a 1.5 T magnetic field, was supposed to he caused by the relatively large filament diameter or Nb3Sn layer thickness. Therefore, the filament diameter for the new conductor (B) was set 21.7 Table 1 Nb3Sn persistent current switch conductor parameters
Outer diameter Nb-tube number Nb-tube diameter Sn content in Cu Matrix/super ratio Matrix material
Conductor (A)
Conductor (B)
0.87 mm 258 38 0~m 25% 1.02 Cu-10wt.%Ni
0.87 mm 738 21.7 ~m 30% 1.18 Cu-10wt.%Ni
© 1993 - E l s e v i e r S c i e n c e P u b l i s h e r s B.V. A l l r i g h t s r e s e r v e d
M. Urata et al. / A Nb~Sn persistent current switch
41{)
detect any flux c h a n g e in the PCS winding. Scvcral voltage taps were soldered o n t o the c o n d u c t o r at the coil end flange after the PCS was i m p r e g n a t e d . A n acoustic emission ( A E ) sensor was a t t a c h e d onto the PCS b o t t o m flange (see fig. 2) to detect any mechanical disturbances such as epoxy cracking or debonding. T h e PCS voltages, search coil signal, coil c u r r e n t and the A E signal were r e c o r d e d a n d analyzed with a multic h a n n e l t h e r m a l recorder.
3. The switch performance 3.1. Performance for conductor and switch T h e PCS c o n d u c t o r and the switch p e r f o r m a n c e s are shown in fig. 3. A short sample of the c o n d u c t o r carried 2650 A at 0 T a n d 2000 A at 1.5 T. T h e q u e n c h c u r r e n t s for the switch at 20 A / s are 1960 A at 0 T a n d
Fig. 1. Nb3Sn conductor cross section.
~ m to improve the m a g n e t i c instability. T h e cross-sectional view of the c o n d u c t o r after heat t r e a t m e n t is shown in fig. 1.
2.2. The switch fabrication Nb3Sn PCS was fabricated with the wind and react process [1]. T h e i n n e r a n d o u t e r d i a m e t e r of the switch are 18 m m a n d 60 ram, respectively. T h e coil height is 44 mm. 55 m of the Nb3Sn c o n d u c t o r was w o u n d non-inductively. A f t e r the heat t r e a t m e n t , the switch was i m p r e g n a t e d with epoxy resin. T h e switch was m o u n t e d on a test p r o b e as shown in fig. 2 and was tested in the b o r e of a 7 T magnet.
2.3. Experimental procedure T h e c o n d u c t o r p e r f o r m a n c e was m e a s u r e d in a 15 T magnet. T h e PCS was tested in the b o r e of a 7 T NbTi magnet. A search coil was w o u n d a r o u n d the PCS to
Fig. 2. Outer view of the switch.
M. Urata et al. / A Nb3Sn persistent current switch I
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Fig. 3. The Quench current for the switch and is conductor: (a) 38/xm filament, (b) 21.7/zm filament. 1560 A at 1.5 T. The heat treatment time for the conductor and the switch is 40 h at about 700°C. Q u e n c h current degradation at lower than 1.5 T observed in conductor (A) and its switch are drastically improved, almost no degradation is observed in the newly developed conductor (B) and the switch. The conductor critical current density without matrix, 2430 A / m m 2 at 7 T, is higher than 1520 A / m m 2 at 7 T for conductor (A).
3.2. Magnetization curries Magnetization curves are compared in fig. 4 for conductors (A) and (B). Magnetization curve fluctua-
tion observed for conductor (A) in fig. 4a diminished for conductor (B) as can be seen in fig. 4b. It corresponds well to the I¢-B characteristics for each conductor shown in fig. 3.
3.3. Transient property of the switch The quench current for the switch was measured at various current ramp rates up to a 7 T magnetic field. Figure 5 shows the quench current as a function of the current ramp rate at 0 T, 3 T and 5 T. The solid lines in the fig. 5 are the calculated quench currents assuming the conductor temperature rise due to self field ac loss. In the range of current sweep rates lower than b
~ ¢'~. ¢-I0
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--I
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Fig. 4. Magnetization curves for O 0.87 mm Nb3Sn peristent current switch conductor: (a) 38 gm filament, (b) 21.7 gm filament.
412
34. Urata et al. / A Nb3Sn persistent current switch
400 A / s , the measured quench current agreed well to the calculated results, while as the ramp rate rises up to larger than 800 A / s , the measured quench current degradation became worse apart from the calculation. The quench current at about 10000 A / s was only 100 A. In the transient signals of conductor voltage, A E and search coil voltage no irregular signal was observed during the current ramp or magnetic field sweep and just before the quench, such as voltage spikes or A E bursts.
o 5 vx
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u-
w
oNi
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3.4. Switch resistance and CMA analysis
The resistance for the Nb3Sn PCS at the 5.86 W heater power was 5.65 1). The resistance sensitively depends on the heater power even under the thermally insulated condition because of the high critical temperature for Nb3Sn composite. The resistance shrinks to 0.18 ~ when the heater power is 2 W. This decrease shows that the equilibrium switch temperature at 2 W heating is near the conductor critical temperature, which was measured as 17 K. Normal state resistivities measured with the short sample conductor agreed well to the switch resistance. To determine the optimum heat treatment time for the same temperature, conductor resistivities were measured together with the critical current. In the Nb3Sn reaction process, the Sn inside the Nb tube diffuses into the Nb tube, which reduces the bronze resistivity. The Sn content in the bronze was analyzed quasi-quantitively with C M A (Computer aided Micro Analysis). Resistivity calculated with using the Sn con-
n
I000
b,x,~
Calculation
r
i
50
Sn Content [wf%)
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~r
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20
Fig. 6. Resistivity change inside the Nb tube after the heat treatment for Nb3Sn reaction.
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20
G 0
t
t
t
I
200
400
600
800
I
I000 1200
Current Sweep Rate ( A / s )
Fig. 5. Quench current degradation dependence on the current ramp rate.
~9
I0
20
30
40
50
Position
6(3
70
80
90
o ioo
(ffm)
Fig. 7. The results of the CMA analysis; (a) conductor crosssectional view for Ni (red), Nb (green) and Sn (blue); (b) Ni, Sn and Nb distribution observed by a CMA line analysis.
M. Urata et al. / A Nb~Sn persistent current switch tent inside Nb tube obtained by CMA are shown in fig. 6. The resistivity of the C u - S n alloy becomes small as the reaction time becomes long. The current density in the Nb3Sn layer was constant (6 × 109 A / m 2 at 7 T) for these reaction time. The forty-hour treatment reduced the C u - S n resistivity to 0.93 × 10 7 ~,~m. This is lower than the Cu-10wt.%Ni matrix resistivity, 1.43 x 10 - 7 t i m . On the contrary, the conductor critical current became larger as the Sn content decreased. However, I c began to saturate for over the 40 h heat treatment. The reaction time was thus determined as 40 h for optimum critical current and resistivity values. Ni precipitation in the heat treatment process was also observed by the CMA analysis. The conductor cross section of weight content analyzed for Ni (red), Nb (green), Sn (blue) with CMA is shown in fig. 7a. Ni precipitates on the Nb tube (yellow) at the filaments in the boundary of inner and outer C u - N i matrix. Ni precipitation suggests that it might be some composite. Results of the line analysis for the three atoms are shown in fig. 7b. With this quasi-quantitative analysis, the weight content for N i : N b was obtained as 30:50. This content ratio corresponds to the atomic content ratio of Ni : Nb = 1 : 1.05. If the Ni precipitation at the Nb t u b e / C u - N i boundary is a composite, it should be NiNb. The Ni content in the matrix decreases from 10wt.% (compared to copper) to 3 wt.% at 15 ~m outside of the N b / C u - N i boundary, which slightly decreases the conductor resistivity.
4. Discussion
4.1. Performance o f Nb3Sn PCS A conductor current of 2560 A in a single strand with a copper-nickel matrix is the value that a Cu-Ni based NbTi conductor never reaches. This development shows the possibility for high-current persitentcurrent switches with reasonable stability. The conductor critical current density without matrix (including copper pipe and tin core in the Nb tube) at 1.5 T is 7190 A / r a m 2. This current density is higher than 4200 A / m m 2 at 1.5 T for NbTi. This fact shows the possibility for a high-current persistent-current switch with a higher temperature margin and increased conductor current density.
413
jump criterion for the slab model is given [3] as, (aFj)2 _ 3~c(oc
- Oo)
tz0jc:
(1)
where a~j is half width of the slab where flux jumps appear. Using the following parameters; enthalpy C = 0.21 J / k g K , density y = 5 . 4 × 1 0 3 k g / m 3, 0c = 1 8 K, and J~: = 3.6 × 101° A / m 2 [1], aFj was obtained as 5.1 p~m. This is near the Nb tube thickness for conductor (A), 5.9 p,m. Therefore, the Nb tube thickness was set at a little lower value, 3.3 p~m. Then, the filament outer diameter was set at 21.9 t~m. The quench current degradation and flux jump signal recorded on magnetization curve was drastically improved by reducing the Nb tube thickness based on the slab model criterion. From these results, the instability of conductor (A) was solved not by reducing the filament outer diameter but by reducing the Nb tube thickness. To design the Nb3Sn switch conductor, the slab model criterion can be a rule of thumb. If the filament is not a tube as bronze route conductor, the thickness might correspond to the radius of the filament.
4.3. Degradation at high ramp rate Quench current degradation at high current ramp rate has been also studied for the NbTi persistent current switch [3]. The quench current of the present switch degrades similarly as the NbTi switch. The calculated degradation saturates because, at the high ramp rate, temperature rise is determined by the adiabatic condition for the equal mass of self field ac loss. However, the degradation at a current ramp higher than 1000 A / s becomes more serious in the Nb3Sn switch. Though the magnetic stability is accomplished in the static states, dynamic instability may exist. The instability is determined not by the conductor outer diameter, because the NbTi switch wound with diam 0.9 mm conductor can carry about 1/3 of I c even at a higher ramp than 1000 A / s . The dynamic instability for the switch is supposed to be determined by the Nb tube thickness. Further improvement of Nb tube size will possibly rise the quench current of the switch at high ramp rates to the NbTi level.
4.4. CMA analysis 4.2. Magnetic stability of Nb3Sn PCS In conductor (B) tt~e filament diameter was determined based on the slab model as follows. The flux
The peristent-current switch conductor length is necessary only for the resistance. Thus, the normal state resistivity is as important as its Ic. The Sn content
414
M. Urata et al. / A Nb~Sn persistent current switch
effect on conductor resistivity decrease, by changing the heat treatment time from 0 to 96 h, is 48%. Heat treatment for 40 h is not desirable from the stand point of higher resistivity. From fig. 6 it follows that more than 11 wt.%Sn inside the Nb tube is desirable to keep the resistivity of the bronze higher than that for Cu10wt.%Ni. The Ni content effect on resistivity calculated based on CMA results is 6%. Therfore, the Ni precipitation effect is not so much a problem. The Ni precipitation on the Nb tube is supposed to be some compositc. If it were not a composite, the Ni diffusion might have resulted in the gradual change of the Ni content on the Nb tube. The phase diagram of Ni-Nb is similar to that for Nb-Sn. Thus, Ni3Nb was the first candidate for the composite. However, the CMA results tell that the composite is probably NiNb.
5. Conclusions
The Nb3Sn peristent current switch with 5.6 1~ normal resistance has been successfully fabricated. It
attained 2000 A at 0 T and 1560 A under 1.5 T. The quench current degradation was improved in the low field by reducing the Nb tube thickness which was calculated based on the flux jump criterion of the adiabatic slab model. The instability at a high ramp rate was caused by the dynamic instability due to the still thicker Nb tube. The Sn content reduction inside the Nb tube filament decreases the resistivity. The Ni precipitation on the surface of the Nb tube is probably NiNb which slightly reduces the matrix resistivity.
References
[1] M. Urata, H. Maeda, M. Tanaka, S. Murase, Y. Oda, S. Nakamura, E. Suzuki, M. Kageyama and S. Kabashima, Nb3Sn persistent current switch, MT-11 (1990)443. [2] M.N. Wilson, Superconducting magnets (Clarendon Press, Oxford, 1983). [3] H. Maeda, M. Urata, Y. Oda, M. Kageyama and S. Kabashima, Instabilities of persistent current switch, IEEE Trans. Magn. 27 (1991) 2124.
409
Stabilization of a Nb3Sn persistent current switch M. U r a t a a, H. M a e d a a, S. N a k a y a m a ~, E. Y o n e d a a, y . O d a b, T. K u m a n o b, N. A o k i b, T. T o m i s a k i c a n d S. K a b a s h i m a c a Toshiba R&D Center, 4-1 Ukishima, Kawasaki 210, Japan b Showa Electric" Wire and Cable Co. ltd., 2-1-1, Odasakae, Kawasaki, Japan c Tokyo Institute of Technology, 4259, Nagatsuda, Midori-ku, Yokohama, Japan
A 2000 A class Nb3Sn persistent current switch has been successfully fabricated in the Toshiba R&D Center. The Nb tube processed conductor with Cu-10wt.%Ni matrix has been developed for the switch in the Showa Electric Wire and Cable Co. Ltd. The magnetic instability which was observed in the previous 35 l-I Nb3Sn persistent current switch was improved in the present switch. The problem of quench current degradation and flux jump on magnetization, emerged in the previous switch, were confirmed to be solved. In the fast ramp, however, the switch degrades from the calculated results assuming the self field ac loss. In the Nb3Sn reaction process, Sn in the bronze diffuses into the Nb tube, which decreases the switch resistance. It was observed by a computer aided micro analysis (CMA) that Ni in the CuNi matrix precipitated on the Nb tube, which slightly reduced the switch resistance.
1. Introduction
2. Conductor and the switch
Persistent current switches (PCS) are typical devices for superconducting magnet applications. Using thermal PCS, persistent currents with a smaller than 0.1 p p m / h decay rate have already been in use such as M R I or N M R spectrometer magnets. Using a high resistivity C u - N i alloy matrix, however, PCS involves inevitable an instability problem; i.e., PCS suffers unexpected quenches at far below the designed current. This problem occurs especially in low fields and at high current. No stable PCSs are obtained which can be operated at higher than 1000 A up to now. Trials for parallel connection of thinner conductors were not always satisfactory due to the current unbalance between strands. Nb3Sn PCS is a promising device which enables the high-current persistent-current systems such as magneto-hydrodynamic propulsion ships or SMES, with using its larger temperature margin. Moreover, Nb3Sn PCS is one of indispensable components to develop low-decay Nb3Sn persistent magnets for N M R use. This paper presents (a) the performance of a Nb tube processed Nb3Sn conductor with a CuNi matrix and the switch wound with the conductor, and (b) results of stability for the conductor. The conductor resistivity is also discussed in relation with the conductor current density.
2.J. Conductor
0920-3796/93/$06.00
The tube processed Nb3Sn conductor with Cu10wt.%Ni matrix has been developed for a Nb3Sn PCS at the Showa Electric Wire and Cable Co. LTD. The parameters for the two types of conductor are listed in table 1. These two types of conductor have almost the same parameters except for the filament outer diameter. The performance for conductor (A) and the 35 U/ switch wound with it was presented elsewhere [1]. The quench current degradation for conductor (A), occurred at lower than a 1.5 T magnetic field, was supposed to he caused by the relatively large filament diameter or Nb3Sn layer thickness. Therefore, the filament diameter for the new conductor (B) was set 21.7 Table 1 Nb3Sn persistent current switch conductor parameters
Outer diameter Nb-tube number Nb-tube diameter Sn content in Cu Matrix/super ratio Matrix material
Conductor (A)
Conductor (B)
0.87 mm 258 38 0~m 25% 1.02 Cu-10wt.%Ni
0.87 mm 738 21.7 ~m 30% 1.18 Cu-10wt.%Ni
© 1993 - E l s e v i e r S c i e n c e P u b l i s h e r s B.V. A l l r i g h t s r e s e r v e d
M. Urata et al. / A Nb~Sn persistent current switch
41{)
detect any flux c h a n g e in the PCS winding. Scvcral voltage taps were soldered o n t o the c o n d u c t o r at the coil end flange after the PCS was i m p r e g n a t e d . A n acoustic emission ( A E ) sensor was a t t a c h e d onto the PCS b o t t o m flange (see fig. 2) to detect any mechanical disturbances such as epoxy cracking or debonding. T h e PCS voltages, search coil signal, coil c u r r e n t and the A E signal were r e c o r d e d a n d analyzed with a multic h a n n e l t h e r m a l recorder.
3. The switch performance 3.1. Performance for conductor and switch T h e PCS c o n d u c t o r and the switch p e r f o r m a n c e s are shown in fig. 3. A short sample of the c o n d u c t o r carried 2650 A at 0 T a n d 2000 A at 1.5 T. T h e q u e n c h c u r r e n t s for the switch at 20 A / s are 1960 A at 0 T a n d
Fig. 1. Nb3Sn conductor cross section.
~ m to improve the m a g n e t i c instability. T h e cross-sectional view of the c o n d u c t o r after heat t r e a t m e n t is shown in fig. 1.
2.2. The switch fabrication Nb3Sn PCS was fabricated with the wind and react process [1]. T h e i n n e r a n d o u t e r d i a m e t e r of the switch are 18 m m a n d 60 ram, respectively. T h e coil height is 44 mm. 55 m of the Nb3Sn c o n d u c t o r was w o u n d non-inductively. A f t e r the heat t r e a t m e n t , the switch was i m p r e g n a t e d with epoxy resin. T h e switch was m o u n t e d on a test p r o b e as shown in fig. 2 and was tested in the b o r e of a 7 T magnet.
2.3. Experimental procedure T h e c o n d u c t o r p e r f o r m a n c e was m e a s u r e d in a 15 T magnet. T h e PCS was tested in the b o r e of a 7 T NbTi magnet. A search coil was w o u n d a r o u n d the PCS to
Fig. 2. Outer view of the switch.
M. Urata et al. / A Nb3Sn persistent current switch I
a
I
I
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1500 g
sample
Herical
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b
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I
,
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4
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1000
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0
6
Magnetic field (T)
I
i
i
i
f
2 4 6 Magnetic Field (T)
Fig. 3. The Quench current for the switch and is conductor: (a) 38/xm filament, (b) 21.7/zm filament. 1560 A at 1.5 T. The heat treatment time for the conductor and the switch is 40 h at about 700°C. Q u e n c h current degradation at lower than 1.5 T observed in conductor (A) and its switch are drastically improved, almost no degradation is observed in the newly developed conductor (B) and the switch. The conductor critical current density without matrix, 2430 A / m m 2 at 7 T, is higher than 1520 A / m m 2 at 7 T for conductor (A).
3.2. Magnetization curries Magnetization curves are compared in fig. 4 for conductors (A) and (B). Magnetization curve fluctua-
tion observed for conductor (A) in fig. 4a diminished for conductor (B) as can be seen in fig. 4b. It corresponds well to the I¢-B characteristics for each conductor shown in fig. 3.
3.3. Transient property of the switch The quench current for the switch was measured at various current ramp rates up to a 7 T magnetic field. Figure 5 shows the quench current as a function of the current ramp rate at 0 T, 3 T and 5 T. The solid lines in the fig. 5 are the calculated quench currents assuming the conductor temperature rise due to self field ac loss. In the range of current sweep rates lower than b
~ ¢'~. ¢-I0
-,:o/
-o:o
-
Mognetic Field ?
o
1.5
--I
/
(TLI~
I-
Fig. 4. Magnetization curves for O 0.87 mm Nb3Sn peristent current switch conductor: (a) 38 gm filament, (b) 21.7 gm filament.
412
34. Urata et al. / A Nb3Sn persistent current switch
400 A / s , the measured quench current agreed well to the calculated results, while as the ramp rate rises up to larger than 800 A / s , the measured quench current degradation became worse apart from the calculation. The quench current at about 10000 A / s was only 100 A. In the transient signals of conductor voltage, A E and search coil voltage no irregular signal was observed during the current ramp or magnetic field sweep and just before the quench, such as voltage spikes or A E bursts.
o 5 vx
2 /*
5h
u-
w
oNi
/ o. °
40h
96h i
3.4. Switch resistance and CMA analysis
The resistance for the Nb3Sn PCS at the 5.86 W heater power was 5.65 1). The resistance sensitively depends on the heater power even under the thermally insulated condition because of the high critical temperature for Nb3Sn composite. The resistance shrinks to 0.18 ~ when the heater power is 2 W. This decrease shows that the equilibrium switch temperature at 2 W heating is near the conductor critical temperature, which was measured as 17 K. Normal state resistivities measured with the short sample conductor agreed well to the switch resistance. To determine the optimum heat treatment time for the same temperature, conductor resistivities were measured together with the critical current. In the Nb3Sn reaction process, the Sn inside the Nb tube diffuses into the Nb tube, which reduces the bronze resistivity. The Sn content in the bronze was analyzed quasi-quantitively with C M A (Computer aided Micro Analysis). Resistivity calculated with using the Sn con-
n
I000
b,x,~
Calculation
r
i
50
Sn Content [wf%)
O
o~
60
~tL
0
~r
5O
I
14o izo :20
4o
o
5T
bib .....
80
3T
o
E
I
20
Fig. 6. Resistivity change inside the Nb tube after the heat treatment for Nb3Sn reaction.
100
-g
i
I
o~. o
i
I0
•
!1o
20
G 0
t
t
t
I
200
400
600
800
I
I000 1200
Current Sweep Rate ( A / s )
Fig. 5. Quench current degradation dependence on the current ramp rate.
~9
I0
20
30
40
50
Position
6(3
70
80
90
o ioo
(ffm)
Fig. 7. The results of the CMA analysis; (a) conductor crosssectional view for Ni (red), Nb (green) and Sn (blue); (b) Ni, Sn and Nb distribution observed by a CMA line analysis.
M. Urata et al. / A Nb~Sn persistent current switch tent inside Nb tube obtained by CMA are shown in fig. 6. The resistivity of the C u - S n alloy becomes small as the reaction time becomes long. The current density in the Nb3Sn layer was constant (6 × 109 A / m 2 at 7 T) for these reaction time. The forty-hour treatment reduced the C u - S n resistivity to 0.93 × 10 7 ~,~m. This is lower than the Cu-10wt.%Ni matrix resistivity, 1.43 x 10 - 7 t i m . On the contrary, the conductor critical current became larger as the Sn content decreased. However, I c began to saturate for over the 40 h heat treatment. The reaction time was thus determined as 40 h for optimum critical current and resistivity values. Ni precipitation in the heat treatment process was also observed by the CMA analysis. The conductor cross section of weight content analyzed for Ni (red), Nb (green), Sn (blue) with CMA is shown in fig. 7a. Ni precipitates on the Nb tube (yellow) at the filaments in the boundary of inner and outer C u - N i matrix. Ni precipitation suggests that it might be some composite. Results of the line analysis for the three atoms are shown in fig. 7b. With this quasi-quantitative analysis, the weight content for N i : N b was obtained as 30:50. This content ratio corresponds to the atomic content ratio of Ni : Nb = 1 : 1.05. If the Ni precipitation at the Nb t u b e / C u - N i boundary is a composite, it should be NiNb. The Ni content in the matrix decreases from 10wt.% (compared to copper) to 3 wt.% at 15 ~m outside of the N b / C u - N i boundary, which slightly decreases the conductor resistivity.
4. Discussion
4.1. Performance o f Nb3Sn PCS A conductor current of 2560 A in a single strand with a copper-nickel matrix is the value that a Cu-Ni based NbTi conductor never reaches. This development shows the possibility for high-current persitentcurrent switches with reasonable stability. The conductor critical current density without matrix (including copper pipe and tin core in the Nb tube) at 1.5 T is 7190 A / r a m 2. This current density is higher than 4200 A / m m 2 at 1.5 T for NbTi. This fact shows the possibility for a high-current persistent-current switch with a higher temperature margin and increased conductor current density.
413
jump criterion for the slab model is given [3] as, (aFj)2 _ 3~c(oc
- Oo)
tz0jc:
(1)
where a~j is half width of the slab where flux jumps appear. Using the following parameters; enthalpy C = 0.21 J / k g K , density y = 5 . 4 × 1 0 3 k g / m 3, 0c = 1 8 K, and J~: = 3.6 × 101° A / m 2 [1], aFj was obtained as 5.1 p~m. This is near the Nb tube thickness for conductor (A), 5.9 p,m. Therefore, the Nb tube thickness was set at a little lower value, 3.3 p~m. Then, the filament outer diameter was set at 21.9 t~m. The quench current degradation and flux jump signal recorded on magnetization curve was drastically improved by reducing the Nb tube thickness based on the slab model criterion. From these results, the instability of conductor (A) was solved not by reducing the filament outer diameter but by reducing the Nb tube thickness. To design the Nb3Sn switch conductor, the slab model criterion can be a rule of thumb. If the filament is not a tube as bronze route conductor, the thickness might correspond to the radius of the filament.
4.3. Degradation at high ramp rate Quench current degradation at high current ramp rate has been also studied for the NbTi persistent current switch [3]. The quench current of the present switch degrades similarly as the NbTi switch. The calculated degradation saturates because, at the high ramp rate, temperature rise is determined by the adiabatic condition for the equal mass of self field ac loss. However, the degradation at a current ramp higher than 1000 A / s becomes more serious in the Nb3Sn switch. Though the magnetic stability is accomplished in the static states, dynamic instability may exist. The instability is determined not by the conductor outer diameter, because the NbTi switch wound with diam 0.9 mm conductor can carry about 1/3 of I c even at a higher ramp than 1000 A / s . The dynamic instability for the switch is supposed to be determined by the Nb tube thickness. Further improvement of Nb tube size will possibly rise the quench current of the switch at high ramp rates to the NbTi level.
4.4. CMA analysis 4.2. Magnetic stability of Nb3Sn PCS In conductor (B) tt~e filament diameter was determined based on the slab model as follows. The flux
The peristent-current switch conductor length is necessary only for the resistance. Thus, the normal state resistivity is as important as its Ic. The Sn content
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effect on conductor resistivity decrease, by changing the heat treatment time from 0 to 96 h, is 48%. Heat treatment for 40 h is not desirable from the stand point of higher resistivity. From fig. 6 it follows that more than 11 wt.%Sn inside the Nb tube is desirable to keep the resistivity of the bronze higher than that for Cu10wt.%Ni. The Ni content effect on resistivity calculated based on CMA results is 6%. Therfore, the Ni precipitation effect is not so much a problem. The Ni precipitation on the Nb tube is supposed to be some compositc. If it were not a composite, the Ni diffusion might have resulted in the gradual change of the Ni content on the Nb tube. The phase diagram of Ni-Nb is similar to that for Nb-Sn. Thus, Ni3Nb was the first candidate for the composite. However, the CMA results tell that the composite is probably NiNb.
5. Conclusions
The Nb3Sn peristent current switch with 5.6 1~ normal resistance has been successfully fabricated. It
attained 2000 A at 0 T and 1560 A under 1.5 T. The quench current degradation was improved in the low field by reducing the Nb tube thickness which was calculated based on the flux jump criterion of the adiabatic slab model. The instability at a high ramp rate was caused by the dynamic instability due to the still thicker Nb tube. The Sn content reduction inside the Nb tube filament decreases the resistivity. The Ni precipitation on the surface of the Nb tube is probably NiNb which slightly reduces the matrix resistivity.
References
[1] M. Urata, H. Maeda, M. Tanaka, S. Murase, Y. Oda, S. Nakamura, E. Suzuki, M. Kageyama and S. Kabashima, Nb3Sn persistent current switch, MT-11 (1990)443. [2] M.N. Wilson, Superconducting magnets (Clarendon Press, Oxford, 1983). [3] H. Maeda, M. Urata, Y. Oda, M. Kageyama and S. Kabashima, Instabilities of persistent current switch, IEEE Trans. Magn. 27 (1991) 2124.