Andrew V. Gavrilin, Victor E. Keilin, Ivan A. Kovalev,. Sergei L. Kruglov, and Vladimir I. Shcherbakov. Kurchatov Institute, 1 Kurchatov's Sq., 123 182 Moscow, ...
531
IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 9, NO. 2, JUNE 1999
Optimized HTS Current Leads Andrew V. Gavrilin, Victor E. Keilin, Ivan A. Kovalev, Sergei L. Kruglov, and Vladimir I. Shcherbakov Kurchatov Institute, 1 Kurchatov's Sq., 123182 Moscow, Russia Igor I. Akimov, Dmitry K. Rakov, and Alexander K. Shikov Bochvar Institute, 5 Rogov St., 123060 Moscow, Russia
--
Abstract The problem of optimizing HTS current leads by varying their cross-section along the length is investigated both experimentally and numerically at 500 A current level. Bi-2223based HTS multi-filament composite tapes were used with two types of matrices: pure Ag and (Ag + 1 at. % Au) alloy. The warm ends of the HTS parts of the current leads were cooled with liquid nitrogen. Very low evaporation rates in the case of Au-doped matrix and rather long time constants to reach thermal equilibrium were observed.
I. INTRODUCTION The first practical impact of high-temperature superconductivity on low-temperature superconducting (LTS) magnets operating in the liquid helium (LHe) temperature range are just high-temperature superconducting (HTS) current leads with substantially reduced heat input to the LHe bath compared to that for conventional normal metal current leads. Since heat inleakage at 4.2 K due to current leads for large LTS magnets can amount to a substantial part of the total heat load, the possibility to reduce the former by an order of magnitude looks very tempting. Both bulk Bi-2212 tubes and Bi-2223 tapes are successfully applied to HTS current leads [l]. The evident advantages of the latter are: intrinsic shunting provided by normal metal (usually Agbased) matrix, reduced mechanical sensitivity and design flexibility. As for the high thermal conductivity of Ag matrix, it can be considerably reduced by doping (say, with small amount of Au). Therefore, we decided to centre our efforts on HTS current leads made of Bi-2223/Ag-Au composite tapes. Our first experiments were performed using standard HTS tapes with a pure Ag matrix developed and manufactured by Bochvar Institute [2]. Meanwhile, Bochvar Institute developed a technology of alloying Ag matrix with Au (1 at. % and 10 at. % Au). To make choice between these two alloys easier, their electrical resistivity at room, liquid nitrogen (LN2) and LHe temperatures was measured. The results are given in Table I. It is reasonable to assume, at least as a first approximation, that the matrix metal thermal conductivity and electrical resistivity obey the WiedemannFranz-Lorentz law. So the thermal conductivity of tapes can
be obtained from extremely simple measurements of the resistivity. Later, the correctness of this assumption was confirmed by comparing our values of the thermal conductivity, derived from the resistivity measurements, with those published in [3]; the discrepancy is within 15%. A. Choice of Matrix of HTS Tapes
As may be seen from Table I, alloying the silver matrix with Au (1 at. %) results in a noticeable increase of the resistivity and in a corresponding decrease of the thermal conductivity. Further increase of Au content up to 10 at. % looks helpful from this standpoint. However, our decision was in favor of (Ag + 1 at. % Au) matrix for the following reasons: - though (Ag + 1 at, % Au) matrix heat conductance is higher than that of (Ag + 10 at. % Au) matrix, it is sufficiently low, as preliminary estimates showed, to make very low-loss HTS current leads; moreover, the lower electrical resistivity of (Ag + 1 at. 96Au) alloy is preferable from a self-protection standpoint; - (Ag + 10 at. % Au) alloy is more expensive because of the relatively high content of gold; - there are some technical difficulties in making and handling of HTS tapes with (Ag + 10 at. % Au) alloy matrix due to Ag Au Cu eutectic formation. The HTS tapes (both with Ag and (Ag + 1 at. % A u ) matrices), which we used in our experiments, were produced by Bochvar Institute, Each tape, 4.0 mm wide and 0.3 mm thick, has 61 superconducting filaments. B ~ I . ~ P ~ O . ~ S ~precursor ~ , O C Uwas ~ .used ~ C toU ~produce .~~~ 100 m long HTS tape by the customary "powder-in-tube'' method. The main stages of this method are: filling matrix tubes with powder precursor, drawing, tape rolling and heat treatment with intermediate deformations between annealing steps. The matrix-to-superconductor ratio is equal to 3.
- -
TABLE I RESISTIVITIES O F A ~ (Ag , + 1 at. % Au) AND (Ag + 10 at. ?%Au) MATRICES. Matrix
metal
Manuscript received September 14, 1998. This work was supported in part by the Russian Ministry of Science and Technologies (Grant No. 96-15-98230) and the Russian Ministry of Atomic Energy.
1051-8223/99$10.00
Ag A g + I at. % Au Ag + IO at. 70 Au
0 1999 IEEE
290K 16 18.5 40
p @Ohm. m) 78K 4.2K 2.8 6.4 28
0.05 3.8 20
P?')OK/PlXK
5.9 2.9 I .4
P?YO$P4.?K
320 4.9 2
532
B. HTS Current Leads Optimizing
Since the critical current of HTS tapes is strongly temperature dependent, a considerable gain in the total amount of superconducting composite material (as well as in heat input to LHe bath) can be obtained if the cross-section of the HTS current lead decreases along its length from warm end (say, at 78 K) to the cold end (say, at 4.2 K). The ideal geometry of the lead corresponds to the case where its crosssection A is varied smoothly along the length, A=A(x), such that the value of critical current Z,(T) at any point x, i.e., at any temperature T(x), is the same (0 Cx IL, x - the coordinate along the lead, L - the HTS lead length). Such a hypothetical case was considered numerically (on the assumption that the critical current density j,(T) linearly increases as the temperature decreases from 78 K to 4.2 K and jc(4.2 K)/j,(78 K) = 5) for two cases: perfect self-gascooling and in absence gas-cooling. The resulted ideal geometry are shown in Fig.1. In the same figure, the geometry of graded current leads made of tapes with (Ag + 1 at. % Au) matrix used in our tests is given. Detailed numerical calculations show that the “smooth” optimization (Fig. 1, curves 1 & 2) decreases heat input to LHe bath (compared to that for the current leads of uniform cross-section) almost 2.8 times in the case of perfect gascooling and nearly 2.2 times in the case of gas-cooling absence. As can be seen from Fig. 1, the HTS material saving is, in both cases, close to 50% (for perfect gas-cooling somewhat higher than in the case where no gas-cooling). It is seen also from this figure that our three-stage grading results in approximately 35% saving in the HTS composite material. 11. EXPERIMENT
A. Experimental Device
The experimental device is shown in Fig. 2. The HTS 1.0
0.8
Fig. 2. Sketch of experimental device.
tapes were mounted onto two glass-fiber strips, 35 mm wide and 2 mm thick each. The total length of each pole between LN2 and LHe temperatures was 600 111111. In the upper part, the tapes were soft soldered to copper rods; in the lower part, two poles were joined through massive stabilized LTS conductor. The number of tapes was graded along the lead’s length according to Table 11. Each part of the current leads was equipped with voltage taps. In the upper part and in the middle part of one of the poles, carbon resistance thermometers were attached t o the lead surface. In the upper part, each copper plate was soft soldered to two 7 mm diameter copper rods. These rods comprised normal parts of current leads, and they were cooled with liquid nitrogen that was contained in a Dewar vessel. The narrow lower portion of the latter was inserted into the neck of a 250 liter LHe transport storage vessel. The copper rods were extended out of the lower portion of the LN2 vessel through a glass-fiber plug hermetically sealed to the bottom neck of the LN2 vessel and to the rods with the cryogenic glue “Cryoseal” [4]. The liquid nitrogen could be pumped to decrease its boiling-point. In order to improve the heat transfer from the HTS tapes to the evaporated helium, a 54 mm ID stainless steel tube was used to direct evaporated helium along the HTS current leads. TABLE I1 NUMBEROF HTS COMPOSITE TAPESALONG LEN) LENGTH
0:0
0:2
0.6
0.8
1.0
Length (mm)
Dimentionless cross-section Fig. I . HTS current leads optimization. 1 & 2 - Ideal geometry of optimized HTS current leads: I - perfect self-gas-cooling, 2 - without gas-cooling. 3 - Real geometry of tested current leads with (Ag + 1 at. % Au) matrix.
Matrix type Ag
Upper part Middle part Lower part
240 170 I90
13 10
8
Ag
+
I c/o at. Ail
33 19 12
533
1.6
9 1.2 E
W Q)
-2 0.8 0
->E
3
Q)
> 0.4
0
0
100
200
300
400
500
600
700
800
900
0
1000 1100
200
0
Current (A) Fig. 3. V-I characteristics of current leads at different cooling conditions. I - current leads fully immersed into LN2. 2 - warm end @ 77K, charging rate 1 Ns. 3 - warm end @ 77K, steady state, fixed current. 4 - warm end @ 63K, charging rate 1 Ns. 5 - warm end @ 63K, charging rate 20 N S .
400
600
800
1000
1100
Time (s) Fig. 4.Time dependence of overall voltage across current leads
In some tests, this tube was withdrawn. Some amount of gaseous helium corresponding to the background evaporation, i.e., to that due to the storage vessel in the absence of the current leads (approx. 60 liters of gaseous helium @ STP per hour), went outside the tube (to ensure the current leads cooled only with the gas produced by the leads themselves “self-gas-cooling”) through a special flowmeter. The evaporation rates were measured by two drum type
respectively). The relatively small difference in take-off current at different LN2 temperatures is due presumably to the influence of approx. 70mm long uncooled portions of the copper rods connecting the superconducting parts with LN2 cooled portions of these rods through the glass-fiber plug. It is worthwhile to mention that a procedure to find steady-state current values schematically reflected in Fig. 4 could be used as a definition of minimum propagation current of HTS leads (by analogy with the minimum propagation current in LTS windings). It is seen that time constants to reach equilibrium
flowmeters.
turned out to be surprisingly long (many hundreds of seconds)
B. Experimental Results and Discussion The HTS poles prior to being tested as current leads were completely immersed into liquid nitrogen and their critical currents and V-I characteristics were measured. For the tapes used, the results are given in Fig. 3 (curve 1) and in Table 111. Then, the device was inserted into the LHe storage vessel and tested in current leads mode. It is notable that the V-I characteristics are very sensitive to charging rate (see Fig. 3 , curves 2 & 3, 5 & 4, respectively). Curve 2 corresponds to continuous charging at 77 K warm end temperature, while curve 3 corresponds to the equilibrium voltages measured during a long-time stop at a fixed current). Curves 4 & 5 were measured, when the LN2 temperature was reduced by pumping, at different charging rates (1 A / s and 20 A/s, TABLE 111 CRITICAL CURRENTS I N AMPERS (1 pV/cm criterion) FOR DIFFERENT PARTSOF CURRENT L E ~ AT S DIFFERENT COOLING CONDITIONS
which is helpful to ensure tape HTS current leads safety in emergency cases as well as such leads ability to stand overloading. Very long time constants associated with the thermal equilibrium of HTS current leads can be also illustrated by the next example (see Fig. 5). We undertook measurements of 1 c
F
4
o 0
Critical current @ LN2 Total Total length Upper pan Middle pan Lower pan
I60 410 248 I49
Per 1 tape
12.4 13.1 12.4
Crhcal current @ LHe Total 560 485 840
800
Per 1 taDe
14.6 44.2 66.7
-V>I
r
!
. 100
!
.
! 200
.
!
.
300
! 400
,
1 500
. 600
Current (A) Fig. 5. HTS lead temperature versus current. I - upper T, warm end @ 77K, soon after immersion into the storage vessel. 2 - upper T, warm end @ 77K, in about one day after immersion. 3 - upper T, warm end @ 63K, in about one day after immersion. 4 - middle T, warmend @ 77K, soon after immersion into the storage vessel. 5 - middle T, warm end @ 77K, in about one day after immersion.
534
t
E
@ I SO
-
.
5
I ~
440
460
480
500
520
540
560
580
Cunent (A) Fig. 7. Specific heat flux to LHe (per I current lead) versus current. I - upper part @ 77K, soon after immersion into the storage vessel. 2 - upper part 0 77K, in about one day after immersion. 3 - upper part (3 63K, in about one day after immersion.
heat inputs - at least an order of magnitude lower than those due to optimum conventional normal metal leads. the evaporation rate of our current leads during several hours just after they were inserted into the LHe storage vessel. The time-average evaporation rate corresponded to approx. 140 I/h of gaseous helium. Then, the tests were stopped for about 15 hours (nighttime) while the current leads remained inserted into the storage vessel. The next morning, the evaporation rate measurements corresponded to considerably lower values (approx. 90 I/h of gaseous He). It is interesting to note also that during a substantial span of this time interval the warm ends of the HTS current leads were not cooled with LN2, as the latter evaporated in a few hours. Nevertheless, it did not result i n an unacceptably high LHe evaporation rate. Before the LN2 vessel was refilled, the LHe evaporation rate was approx. 200 Ih. This suggest that a heat station on the normal-superconducting junction of HTS current leads is essential only when they are charged (in order to prevent normal zone propagation). Typical temperature and evaporation rate measurements are summarized in Fig. 5 and Fig. 6. As may be inferred from Fig. 5, next day, the temperature, especially in the middle part of the current leads, turned out to be considerably lower. Horizontal line 4 in Fig. 6 (83 I/h) corresponds to the numerically calculated evaporation rate of our leads in the case of perfect gascooling and warm end at 77 K. It is seen that the real evaporation rate (curve 2 in Fig. 6) is less than 10% higher. Horizontal line 5 in Fig. 6 (53 I/h) corresponds to the numerically calculated evaporation rate of the current leads of ideal geometry and the same length (600 mm) for the case of perfect gas-cooling (curve 1 in Fig. 1) and for the warm end at 77 K. This value represents the theoretically attainable minimum of steady state evaporation for the given material at ZC(77K) = 485 A. In Fig. 7, the specific heat flux to LHe per one lead vs the current is given; it corresponds to very low
111. CONCLUSIONS
1. HTS tapes based on Bi-2223 in (Ag + 1 at. % Au) matrix are quite suited for low-loss current leads. The matrix represents a reasonable technological, cost and heat and conductance compromise between pure Ag (Ag + 10 at. % Au) matrices. 2. According to our experimental results, heat removal at the warm ends of HTS current leads is essential only when they are charged with current. If no current, a cooling intercept is of secondary importance, because its absence does not lead to a considerable increase in evaporation rate. 3. Time constants to reach thermal equilibrium of HTStape current leads are surprisingly long. This empiric fact looks favorable to insure the leads safety and is worth further investigation. ACKNOWLEDGMENT
The authors would like to express their gratitude to Alexei Nikolaev for his active participation in the tests and in the leads’ material properties analysis.
REFERENCES [I] R.Heller et al., “Development program of 60 kA current lead using high temperature superconductors,” IEEE Trans. A,@. Supercond., June 1997, vol. 7, NO 2, pp. 692-695. [2] A.D. Nikulin, “Material science aspect of HTS technical superconductors,” in Physics arid Material Science of High-Tc Superconductors IV, NATOASJ Series, 1996, vol. 26, pp. 129-150. [3] H. Fujishiro et al., “Thermal and electrical properties of Ag-Au and AgCu alloy tapes for metal stabilizers of oxide superconductors,” Cryogenics, 1993, vol. 33, NO 11, pp. 1086-1090. [4] O.P. Anashkin, V.E. Keilin, “The use of VT-200 adhesive for lowtemperature vacuum joints,” Cryogenics, 1974, vol. 14, pp. 406 - 407.
