Normal domain temperature profile in second generation HTS tape wire
© Malginov et al.; licensee Springer. 2013
Received: 16 May 2013
Accepted: 18 September 2013
Published: 17 October 2013
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 N 2 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 procedure of our AC-measurements is described in detail in Ref. (Fleishman et al., 2010). A constant amplitude 50 Hz AC voltage V 0 is applied across the current leads to the HTS sample at the moment t 0.
Then, current through the sample I, temperature of the TC T, and voltage in the middle part of the sample V 2 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.
The data in the range 0.3 V < V 0 < 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 V 0 < 0.76 V the normal zone is located between the potential probe and the right current lead, and TC measures the ND temperature.
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 < V 0 < 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 T M (in K) as a function of applied voltage V 0 (in V) is approximated by: T M = 623 (V 0-0.3) + 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.
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.
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