Studies of the normal zone in high-temperature superconducting wires are extremely important for power applications, such as fault current limiters, motors, cables etc. We studied the temperature distribution and normal domain propagation in high-temperature superconducting YBCO tape with highly resistive substrate.
For applied voltages exceeding a certain threshold value the normal domain was found to become unstable and started to propagate along the tape.
The normal domain in superconducting tape with highly resistive substrate appears when voltage is applied to the sample.
At voltages greater than the threshold value, the domain starts to move.
This motion enables us to find the domain temperature and potential profile.
Nucleation and propagation of normal zone are of great importance for high-temperature superconductors (HTS) power applications. In previous experimental studies of the normal zone structure in HTS tapes, it was generated e.g. in vacuum by means of an external heater (Daibo et al., 2011, Pelegrin et al., 2011) or by pulsed current (Mader et al., 2011). These conditions are different from those in the HTS power devices where normal zone is generated in liquid N2 due to overcritical AC current. In the present paper we generate a normal zone by transport current in the sample. The normal zone appears to be restricted in the finite volume, i.e. we deal with normal domain (ND). Using the time resolved thermocouple and potential probe measurements we can explore the ND spatial structure.
Methods and results
The sample of YBCO tape wire SuperPower SF12100 was tested in liquid N2. The cross-section of the 12 mm width wire has the following structure: 100 μm Hastelloy substrate, 1 μm YBCO, 1.5 μm Ag. Both nominal and measured critical currents of the wire are about 300 A, critical temperature is about 91K. Figure 1 shows the geometry of the sample with the thermocouple (TC) and potential probes.
The procedure of our AC-measurements is described in detail in Ref. (Fleishman et al., 2010). A constant amplitude 50 Hz AC voltage V0 is applied across the current leads to the HTS sample at the moment t0.
Then, current through the sample I, temperature of the TC T, and voltage in the middle part of the sample V2 are being measured with 1 ms time resolution within 40 s interval. ND is generated by local overcritical current Joule heating in a specific weak tape segment due to sample inhomogeneity. Location of this weak place (5 mm from the right current lead) was determined by eye from liqud nitrogen boiling in the preliminary experiment. TC was soldered near this point.
Figure 2 shows a typical temperature T versus time plot after an application of V0 = 0.76 V AC voltage. The data indicate that the temperature achieves its maximum in 3 s and almost does not change until switching the voltage off. During this period the voltage V2 between the potential probes is approximately zero.
The data in the range 0.3 V < V0 < 0.76 V demonstrate qualitatively similar behavior: once normal domain is formed it stays in the same place and its parameters remain unchanged. It means that for V0 < 0.76 V the normal zone is located between the potential probe and the right current lead, and TC measures the ND temperature.
The temperature versus time dependence changes drastically when the voltage step V0 is increased to 0.79 V (Figure 3). The temperature reaches almost the same maximum value as for lower V0 value but instead of being constant decreases afterwards. The rate of temperature drop changes at points t4*, t3*, t2*, t1*. At the same time the voltage V2≠0 appears between the potential probes (Figure 4) exhibiting irregularities at points t1, t2, t3, t4. The data in Figures 3 and 4 suggest that ND begins to move as a whole from the sample end towards its center. The irregularities in the T(t) and V2(t) dependences correspond to the motion of specific ND regions across the TC and voltage probe respectively. TC traces the outgoing right edge of the domain. Passing of the left edge is traced by V2, e.g. from t1 to t2 a foremost portion crosses a probe. This portion is cooled by film boiling and has temperature slightly above 77K. From t2 to t3 the left edge moves into the segment between the potential probes. From t3 to t5 the ND top part passes the probe and the temperature is maximal at t4. The interval from t4 to t5 corresponds to the TC measurement interval from t4* to t3*. At the period from t3* to t2* the right edge passes TC and after t2* the outermost portion crosses TC position. All the processes occur at the voltage of V0 = 0.79 V. Current achieves a stable I = 50 A level in 3 s after t0. The ND velocity estimated from the rate of the resistance change is about 1 mm/s. To obtain temperature and potential profiles of the ND as function of distance one should multiply t-axis in Figures 3 and 4 by scale factor 1 mm/s. For higher V0 values ND also drifts towards sample center and both T(t) and V2(t) dependencies demonstrate the similar behavior.
Figure 5 shows the maximum temperature measured by TC as a function of V0 value. When V0 < 0.3 V the ND is so small that TC is out of it and does not respond to heating.
We have shown that for voltages greater than 0.79 V the domain moves and the TC measures its temperature profile. In the interval 0.3 V < V0 < 0.79 V the domain stays within the weak place and TC measures the temperature near its top. In this case the ND maximum temperature TM (in K) as a function of applied voltage V0 (in V) is approximated by: TM = 623 (V0-0.3) + 90. This fit allows one to estimate the upper boundary of the voltage (V ~ 1 V), for which the tape survives (T ~ 500 K).
To summarize, the above experimental data indicate that the normal domain in superconducting tape with highly resistive substrate appears when voltage is applied to the sample. At voltages greater than 0.79 V the domain starts to move. This motion enables us to find the domain temperature and potential profile from the simultaneous measurements with a thermocouple and potential probes.
Daibo M, et al.: Evaluation of the normal-zone propagation characteristics of REBCO coated conductors with laminated Cu tape. IEEE Trans Appl Supercond 2011, 21(3):2428-2431.
This work was supported by Russian Foundation for Basic Research (Grant 14-08-00418-a and Grant 12-08-31415-mol-a), Programs of Russian Academy of Sciences, Russian Ministry of Education and Sciences (Grant 8203), Russian science support foundation and using facilities of the Shared Research Equipment Center at LPI.
Authors and Affiliations
Lebedev Physical Institute RAS, 119991, Moscow, Russia
Andrey V Malginov, Alexander Yu Kuntsevich & Vladimir A Malginov
Moscow Institute of Physics and Technology (State University), 141700, Dolgoprudny, Russia
Alexander Yu Kuntsevich
Krzhizhanovsky Power Engineering Institute, 119991, Moscow, Russia
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