## 4 Experimental Results
### Testing the Cross-Sectional Area of HTS Tapes  
The Nanosurf Flex Axiom, atomic force microscope was used to assess the cross-sectional morphology of the sample of HTS tapes. Measurements were made in a non-contact mode using the NCLR probe by NanoWorld and the VHX-7100 optical microscope by Keyence.  
In order to conduct a microscopic examination, samples of the tested tapes were embedded in epoxy resin and then polished (Figure 20).  
The results obtained with the use of an optical microscope indicate the presence of local distortions in the form of bubbles in the superconducting layer and on the surface of the tape from the superconductor side. Figure 21b shows one of several defects in the cross-section of a superconducting tape embedded in epoxy resin. The possibility of distortions during the sample preparation process should be ruled out as no such distortions were observed in the control samples (Figure 21a).  
Figure 19: Decrease in the value of the critical current of the HTS 2G tape due to its transition from the superconducting state with test current impulses after 3, 10, and 25 transitions for the HTS tape SF12100-CF (2 \(\upmu\)m silver) without electrical insulation.  
Figure 20: THS tape sample intended for microscopic examination.  
For a more detailed analysis of defects in the samples subjected to the surge currents, measurements were carried out using the AFM technique, which is shown in Figure 22. In the topography image (Figure 22a), the profiles are marked with red lines, the curves of which are shown in Figure 22b. Profile lines guided through two areas within the defect. Due to different thermal expansion coefficients, the superconducting layer is below the Hastelloy, silver, and resin layers in both cross-sections. However, it is noteworthy that there are specific depressions in a part of the superconducting layer (marked with a blue arrow), which may be material losses in the superconducting layer.  
The microscopic examinations showed the existence of bubble-like changes formed on the HTS tapes due to the impact of currents in the layers of silver and superconductor. They were observed in all samples subjected to the surge current. Control samples showed no such changes. At the same time, no differences were found at the superconductor-Hastelloy boundary, and no changes in the silver layer and the silver-Hastelloy boundary on the other side of the tape. This may indicate that the reasons for the changes are processes taking place in the superconductor, which require further research and analysis.  
Figure 21: The images show cross-sections of tape samples obtained with an optical microscope: (**a**) control sample (Lens: E500: \(\times 1500\)) and (**b**) sample subjected to surge currents (Lens: E500: \(\times 700\)). The corresponding topographic maps are presented below (**c**,**d**).  
The bubble-type microdamage may be areas that initiate damage to the THS tape, affecting the change in the value of the critical current of the tape and the need for a faster replacement of superconducting elements in the SFCL or the entire device. Determining the degradation mechanisms of HTS tapes is essential from the point of view of the correct operation and lifetime of the SFCL and requires further research.