Lin, L.Z.

Turowski, P.

Lin, L. Z.

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KfK 3240 April1982 The Stability of Nb Ti Composite Superconductors of High Cu/SC-Ratio P. Turowski, L. Z. lin Institut für Technisch.e Physik / Kernforschungszentrum Karlsruhe KERNFORSCHUNGSZENTRUM KARLSRUHE Institut für Technische Physik KfK 3240 The Stability of NbTi Cornposite Superconductors of High Cu/SC - Ratio P. Turowski, L.Z. Lin+) Kernforschungszentrum Karlsruhe GmbH, 7500 Karlsruhe +) Institute of Electrical Engineering, Acadernia Sinica, Peking, China Als Manuskript vervielfältigt Für diesen Bericht behalten wir uns alle Rechte vor Kernforschungszentrum Karlsruhe GmbH ISSN 0303-4003 Abstract Investigations with respect to stability behaviour were made on NbTi composite multifilamentary conductors of a Cu/SC ratio of 10 in magnetic field from 7 T up to 10 T. The recovery cur-rents in dependence on the magnetic field were determined under bath cooling conditions at 4.2 K for a heat disturbance of 0.6 Joule. The current sharing regime turned out to be very ex-tended due to the high copper content and consequently offered a safety margin in magnets in case of transient overcurrents. The copper stabilizer was attached to the superconductor in different ways but the stability behaviour was not changed. Zusammenfassung Die Stabilität von NbTi Verbundsupraleitern mit hohem Cu/SL-Verhältnis Untersuchungen des Stabilitätsverhalten wurden an NbTi-Multi-filament-Verbundleitern mit einem Cu/SL-Verhältnis von 10 im Feldbereich zwischen 7 und 10 T gemacht. Es wurden die Recovery-Ströme in Abhängigkeit vom Magnetfeld bei Kühlung im Heliumbad von 4·2 K nach einem Wärmepuls mit 0,6 Joule be-stimmt. Wegen des hohen Anteils an Kupfer im Leiter war der "current sharing" Bereich sehr ausgedehnt und bot damit bei transienten Stromüberschreitungen eine Sicherheitsreserve. Das stabilisierende Kupfer war am Supraleiter auf verschiedene Wei-sen angebracht, aber das Stabilitätsverhalten war davon nicht beeinträchtigt. - 1 -1. Introduction For a large magnet project in China a NbTi Superconductor with a copper ~matrix of Cu/SC ratio of 10 and a size of 3.6 x 7 mm2 was proposed. To determine the most economical manufacturing process, three types of conductors were com-pared: one monolithic conductor and two composite conductors, where NbTi-conductors of small size and low copper content were soldered into U shaped copper strips. This was the largest superconductor made in China up to now. A 35 cm diameter bore superconducting solenoid generating a magnetic field of 3.5 T was built in 1979 by the Institute of Electrical Engineering of Academia Sinica using these different con-ductor configurations in a pancake structure. It was of particular interest to compare the performance of these conductors with respect to critical current and the stability behaviour in high magnetic ·fields. The critical current, the take-off current, the recovery current and the current sharing behaviour of these conductors were determined using the equipment and the technique 1 ) at the ITP (Institute of Technical Physics) of Kernforschungszen-trum Karlsruhe. 2. The conductor configurations The main parameters of the conductor are given in the following: Size Filament diameter Nurober of filaments Twist pitch Cu/SC ratio 3.6 X 7 ~ 70 ]Jm 432 120 mm 10 2 mm Fig. 1 shows the schematic cross section of the monolithic conductor and the arrangement of the grooves provided for cooling channels. The grooves are rolled into the surface of the conductor and have therefore wide tolerances with respect to the nominal depth of 0.5 mm. Fig. 2 shows the schematic cross sections of the two other conductor configurations. The arrangement of grooves is the same as above. - 2 - .5 Fig. 1 The schematic cross section of the mono-lithic conductor SC Cu -l1 ~2~ a -j1 ~2~ b Fig. 2 The schematic cross sections of the two composite conductors In case a, the superconductor soldered into the U-shaped copper strip has a size of 1.5 x 2.8 mm2 . 432 filaments of 70 ~m dia-meter are embedded in a copper matrix of Cu/SC = 1 and are twis-ted with a pitch of 12 cm. In case b, the U shaped copper strip consists of three parts which are soldered tagether with the same superconductor applied by lead-tin solder. The stabilizing copper should have OFHC quality, indicated by the manufacturer, but the residual resistivity ratio (RRR) has only a value of 70. A certain reduction in the residual resisti-vity ratio may be caused by the cold working on the conductor. The magnetoresistance of the copper matrix at different magnetic fields could be measured when the superconductor was completely in the normal conducting stage. Slightly above the recovery cur-rent the superconductor could be transfered into a steady normal conducting stage by a heat pulse. Variation of the transport current in this situation gives an Ohrnie increase of voltage with a straight U-I-line that may be extrapolated through the origin. Due to the negligible temperature dependence of copper resistivity below about 20 K, the variation in conductor tempe-rature due to changing transport current does not influence the measurement of magnetoresistance. Fig. 3 illustrates the magneto-resistance effect of the copper used compared to one of RRR 154. The difference in the absolute values of resistivity is prac-tically given by the initial values, whereas the linear increase in the magnetic field is the same in both ca·ses, giving a slope of 0.45 x 1o-10 nm/T. 8 ~ 6 "j 0 .... 2 2 - 3 -"' 4 COPPER USED ,/ IRRR70) )/' "' ... "' ...... 6 ..... RRR 154 8 10 Btn Fig. 3 The magnetoresistance effect of the copper used 3. Experimental set-up The background superconducting ma~net used for this ex~eriment was a NbTi/Nb3Sn solenoid magnet system with a clear bore of 100 mm diameter. The maximum field at the sample position (radius r = 45 mm) was 10.5 T. The conductor samples were wound spirally onto a coil former of 90 mm diameter and 30 mm length. A heater of 0.1 mm diameter manganin wire was lAI'Ound bifilarly areund the conductor and then thermally insulated by a layer of epoxy resin. With several potential taps along the conduc-tor, growth and collapse of a normal conducting zone could be observed. From the voltage amplitude, the extension of the normal zone could be determined. Cooling channels at the upper and lower surfaces of the conductors guaranteed a good thermal contact to the liquid helium. 4. Stability behaviour of the conductors Fig. 4 shovrs the critical current Ic, take-off current It.o. and recovery current IR of the monolithic conductor in dependence on the magnetic field from 7 T to 10 T. The critical current Ic was defined for a voltage drop of 0.2 ~V per cm conductor length. The take-off current It is that current iri the current sharing • 0. 800 600 400 200 0 7 8 9 - 4 -10 Fig. 4 Critical current Ic, take-off current It.o. and recovery current IR of monolithic con-ductor in dependence on the magnetic field B. regime, at which the superconducting stage breaks down completely by lack of cooling power or internal processes in the suoercon-ductor. The measurements on recovery current IR were performed in the following way: A heat pulse of 10 msec duration was applied to the conductor by the heater and the pulse current generator. The input energy is about 0.6 J resulting in an estimated tem-perature of the conductor of about 30 K over a lengbh of 2-3mm. The transport current was raised after each heat oulse till the superconductor turned normal conducting. The recovery current is the limit of current at which the suner-· conducting stage is restered after an excursion into normal conductivi ty. The experimen~have shown that the recovery current IRis smal-ler than the critical current in the magnetic fields below 7.5 T. The recovery current is found in the reqion between critical cur-rent and take-off current in magnetic fields higher than 8 T. - 5 -The Maddock stability criterion3 ) {'(8)·1 2 A ·'l·P is used as a measure of stability. q is the effective heat e flow rate, W/cm2 , p (B) is the resistivity of the Cu-stabi-lized conductor, nm, A is the cross section of the stabi-lizer, cm2 , n (= 0.56 for the monolithic conductor and 0.64 for the composite) is the liquid helium wetted fraction of the surface and P is the perimeter of the conductor, cm. The effec-tive heat flow rate q can be calculated if the current I is e replaced by the recovery current IR. In table 1, the calcula-ted values of the effective heat flow rate q of the composite e conductor are given in different magnetic fields. The values of the effective heat flow are somewhat smaller than from other measurements with the same nominal cooling channel geometry_4 ). It must be mentioned again that the cooling channels in the present case have certain deviations in the depth due to the manufacturing process resulting in an average depth some-what smaller than 0.5 mm. This may be the explanation for the deviation from usual values. B 7.5 8 l T ] qe 1Wcm2J 0328 0.315 8.5 9 0.309 0.286 9.5 0.263 Table 1: The effective heat flow rates qe in dependence on the mag-netic field B. As in other cases, the effective heat flow increases with de-creasing magnetic field even though the magnetoresistance effect is taken into account. Table 2 shows that there is nearly no difference between dyna-mically and steadily measured recovery currents. The dynamic recovery currents IR are those which are determined by apply-BITJ 8 8.5 15._ IAJ 1130 1080 I " lAI 1110 1070 9 1000 1020 9.5 940 960 Table 2: The steady and the dynamic reco-.very currents IsR, IR in dependence on the magnetic field B. - 6 -ing a local heat pulse to introduce a transient normal conduct-ing zone; they are shown in Fig. 4. Recovery of a completely normal conducting sample by reducing the transport current slowly is called stationary recovery. The stationary recovery currents ISR are determined in the way, that normal conducti-vity in the total sample was introduced by a heat pulse at a transport current just above the dynamic recovery current. The current was then reduced to that point where the sample returns into the superconducting stage. In general, this stationary recovery current is somewhat smaller than the dy-namic recovery current. The Fig. 5 shows the critical current I , take-off current c It and recovery current IR in dependence on the magnetic • 0. l~)r---.---.----.---.----.-~ 600 400 1400 200 8 BrrJ 10 7 8 Fig. 5 The characteristics of both soldered composite conductors 9 BrTJ field for the both soldered composite conductors. There is nearly no difference in the performance ceropared to the mono-lithic conductor. 5. The current sharing behaviour Because the investigated superconductors have a high Cu/Sc ratio, they show a distinct current sharing behaviour in parti-- 7 -cular in high magnetic fields, i.e. at small current densities. The Fig. 6 demonstrates the behaviour for the monolithic NbTi-conductor at ~ T. The transition into the complete normal con-ducting stage at the take-off point is not determined by the limited cooling power, because the recovery into the supercon-10 8 6 4 2 Fig. 6 The quench and recovery pro-cess of the monolithic NbTi-conductor in 1 turn ducting sta~e sets in at a higher power level during reduction of the transport current. Rather it must be supposed, that intrin-sic processes depending on the current density are finally re-sponsible for the change from the superconducting to the normal conducting stage. In a very high magnetic field a nearly smooth transition from the superconducting to the normal conducting stage can be observed as Fig. 7 shows. This behaviour has been observed beforeS). As is well known the current sharing is caused by the appear-ance .of resistivity in the superconductor, i.e. the NbTi, due to flux flow when the transport current is exceeding the criti-cal current. In that case the superconductor behaves like a nor-mal conductor and a current sharing with the normal conducting matrix sets in. By means of the volta~e-current characteristic and the resistance of the matrix it is possible to calculate the partial curren~in the matrix and the superconducto~ and consequently also the resistivity of the superconductor can be determined. - 8 -u ~--~----r----.----.----.---. [mv) 10 8 6 4 2 200 Fig. 7 B= 9.5 T 800 1000 I[A] The transition between normal and superconduc-ting stages for the mo-nolithic NbTi-conductor at B = 9. 5 T l[AJ 80 600 Fig. 8 B=as r Current distribu-tion in monolithic con-ductor between the NbTi and the copper a t B = 8 • 5 T In Fig. 8 the current distribution in the monolithic conductor between the NbTi and the copper matrix is given versus the trans-port current up to the take-off point. The current in the matrix increases continuously, whilst the current in the superconductor passes a maximum value. The decrease of the current in the NbTi can be explained by a temperature enhancement in the whole con-ductor due to Joule dissipation. In Table 3 the current distribution between NbTi and copper for different magnetic fields is given at the take-of~ point. BIT I 8 8S ItQ(Al 1340 1160 IeulAI 'Zl3 410 l 5clAI 1067 750 9 1060 806 254 9.5 980 857 123 Table 3: Current distribution bet-ween copper and supercon-ductor at the take-off point for various magne-tic fields. 1 -14 E Cl 4 - 9 -With decreasing magnetic field, i.e. with increasing critical current density in the NbTi, the bypass current in the matrix becomes smaller. That means the resistivity of the NbTi in the resistive stage depends on the current density and achieves in low magnetic fields at high current densities only smaller values before the·superconducting stage collapses, as it is shown in Fig. 9. 8 BIT] Fig. 9 The resistivity of NbTi in the resistive stage A comparison of the current sharing behaviour at 8 T of the three investigated composite conductors is demonstrated in Fig. 10. The monolithic conductor shows a very smooth increase of voltage in the current sharing regime, whereas the current sharing of the two other composites, where the real supercon-8=8T I I Ic! a c -I Fig. 10 The current sharing be-haviour of the three investigated conductor types - 10 -ductor was soldered into a U-shaped cop?er stri~ is character-ized by a peculiarly shaped curvature. It can be assumed, that the change in ,curvature is caused by the band resistance between the superconductor and the matrix. 6. Summary The experiments have shown that the monolithic and the soldered composite NbTi conductors have the same recovery currents for applied pulsed heat disturbances.· The cooling channels at the surface of the conductors allow an effective heat flow rate of. more than 0.3 W/cm2 in fields less than 8.5 T. At a field of 7.5 T the recovery current equals the critical current, i.e. at that field the conductor behaves cryogenically stable. Because of the high copper content there is an extended current sharinq regime which offers· a marginal safety in case of overcurrents. The current sharing regime reveals certain differences in the current ·sharing behaviour of a monolithic and of soldered com-posite conductors. - 11 -7. References 1) Turowski, P., The stability of Nb 3Sn-composite conductors cooled by nor-mal and superfluid helium, Cryogenics 21, 533 {1981). 2) Turowski, P., Lin, L.Z., Seibt, E., Cu- and Al-stabilized Nb3 sn~multifilamentary conductors and their stability against heat pulses under bath cooling con-ditions, IEEE, MAG-17, 2047 {1981). 3) Maddock, B.J. et al., Superconductive composites: Heat Transfer and steady state stabilization, Cryogenics 9, 261 {1969). 4) Turowski, P., A Review on stability experiments with superconductors in liquid helium under pool boiling conditions, to be published by Int. Inst. of Refrigeration. 5) Gauster, W.F., Steady-state performance of multistrand superconducting compound conductors, J·. Appl. Phys. 40, 2060 {1969). 

The stability of NbTi composite superconductors of high Cu/SC-ratio

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The stability of NbTi composite superconductors of high Cu/SC-ratio

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