## 4 Experimental Results
for an unused sample (\(I_{C0}\)) and then after 3 applications of the test current impulses and after applications 10 and 25. According to the manufacturer of HTS tapes, the values of the critical current (\(I_{C0}\)) of new samples of HTS tapes may slightly differ, as was described in [33]. The results of critical current measurements for HTS SF12100-CF tape samples with 4 \(\upmu\)m and 2 \(\upmu\)m silver as a function of the number of test current impulses activations for 3, 10, and 25 transitions from the superconducting state are shown in Figures 16 and 17. With the increase in the number of transitions from the superconducting state in the tested samples, the value of the critical current decreased. The rate of degradation of the critical current of HTS tapes depends on the value of the test current. For one of the samples of HTS tape (with an energy of 206.99 J and a voltage drop of 0.77 V/cm), after 25 applications of the test current impulse, the \(I_{C}\) value dropped to 174 A. During the tests of changes in the value of the critical current, the voltage drop values were also recorded, and the value of the energy released in the HTS tape during the test pulse was determined. The results are shown in Figures 18 and 19. For the tested HTS tapes, a decrease in the value of the critical current was observed, increasing with the number of exits of the tapes from the superconducting state. For the HTS tape with a 4 \(\upmu\)m silver layer, slight decreases in the value of the critical current were observed for the test current pulses, for which the energy released on the sample did not exceed 158.47 J (\(I_{C}\) drop below 3% for 3, 10, and 25 activations of the test current pulses). For the HTS tape with a 2 \(\upmu\)m silver layer, similar \(I_{C}\) changes were observed at energies of 90.73 J. For higher energy values, the degradation process was more noticeable. After 10 leads, the difference in the decrease in the critical current depending on the thickness of the silver layer was clearly marked. At the highest energies, after 10 leads, the 4 \(\upmu\)m silver tape had a 7.55% decrease in \(I_{C}\) and 4.08% for the 2 \(\upmu\)m silver tape, and the 4 \(\upmu\)m silver tape showed a 45.28% decrease in \(I_{C}\) after 25 leads (it should be assumed that the tape has been damaged), while the 2 \(\upmu\)m silver tape only decreased by 7.48%. For low values of prospective current (energy dissipated in the HTS tape), changes in the value of the critical current are negligible. This parameter degrades faster with the number of current test operations for higher energy values, which means the progressing process of  
Figure 8:2 Dependences of the critical current value of the SF12100-CF tape with 4 \(\upmu\)m silver as a function of the number of test current pulse activations for 3, 10, and 25 transitions from the superconducting state. thermal aging of the HTS tapes. The process is much more intensive for the HTS tape with a 4 \(\upmu\)m silver layer due to the lower resistance value of the silver layer. Based on the test results, it is possible to determine the permissible value of voltage drops for HTS tapes, at which \(I_{C}\) degradation does not occur or occurs only slowly. Paper [41] presents a numerical model of the HTS tape SF12100, based on which the permissible value of voltage drops for a current pulse of 0.2 s duration is 0.47 V/cm. The permissible value of voltage drops is an important design parameter that determines, among other things, the minimum length of the tape used in the SFCL while ensuring the safe operation of the device. In the paper [42], from the point of view of design assumptions, the permissible value of the voltage drop was 1 V/cm. Figure 8:3 Dependences of the critical current value of the SF12100-CF tape with 2 \(\upmu\)m silver as a function of the number of test current pulse activations for 3, 10, and 25 transitions from the superconducting state.