# Electron Beam Characterization of REBCO-Coated Conductors at Cryogenic Conditions
## 3 Results and Discussion  
### Secondary Electron Yield  
The secondary electron yield measurements reported in Figure 3 provide a few important findings. First, there are no charging effects, even at ambient temperatures, despite the thick RESCO layer. This is essential since charging effects (often on dielectric surfaces) are detrimental in many technical applications, including applications in beam screens or RF devices. Secondly, similar to technical-grade metals, the SEY of a RESCO-CC sample decreases under 300 eV of electron irradiation, a phenomenon known as electron conditioning. As most UHV-cleaned metal surfaces exhibit high SEY in their as-received air-exposed states [36; 37], they typically condition during electron irradiation to reach SEY values at around 1, which are low enough to prevent electron multipacting. The first stage of the SEY decrease is dominated by the ESD process, while graphitization of carbon-containing surface-bound contaminants dominates in the later stages, as proven independently by [PERSON] [38], [PERSON] [39], and [PERSON] [40], who linked the SEY decrease to graphitization. The conditioning process is mostly done at a dose of a few mC\(\cdot\)mm\({}^{-2}\) and further electron irradiation at 300 eV does not lead to a significant SEY decrease. The SEY of the RCE-deposited sample conditions to an acceptable value of \(\delta_{max}\) = 1.2 at 1 mC\(\cdot\)mm\({}^{-2}\) dose. Conversely, the PLD-deposited sample only conditions to around \(\delta_{max}\) = 2. The XPS analysis reveals two factors that explain the significant difference in SEY. Firstly, barium, which is present in BaO oxide form, is known to have a high SEY. Secondly, carbon contamination, when graphitized under electron irradiation, can reduce the SEY of surfaces close to unity. The RCE-deposited sample showed a higher atomic concentration ratio of carbon to barium than the PLD-deposited sample. This may partially account for the lower SEY observed after irradiation of the RCE-deposited sample. However, even after conditioning, the SEY of the PLD-deposited sample is too high for technical applications, and further SEY-reducing treatment is necessary to prevent multipacting. [PERSON] et al. [26] and [PERSON] et al. [41] also reported high as-received SEY values of \(\delta_{max}\approx 2.9\), as measured at ambient temperatures. The shapes of the SEY curves also closely resemble classical technical metals, as described by the \(E_{max}\) and \(\delta_{max}\) values. This is somewhat unexpected given the surface roughness observed in the SEM images that could point toward a more flat energy dependence that is typical for rough surfaces. The energy of maximum SEY \(\delta_{max}\), the \(E_{max}\), is important for the following reason. The typical energy spectrum of an electron cloud developed in an RF device consists of a high-energy peak at hundreds to thousands of eV [15; 18; 21; 22]. The multipacting conditions can be better mitigated if the high-energy peak of EC is offset from the SEY peak position \(E_{max}\), as is visible in the simulations done in [21]. Similar to the other technical-grade metals characterized in this setup, such as Cu, Al, and steel, the SEY of the studied RESCO coatings is temperature-invariant within the experimental precision and conditions in the investigated 12-300 K temperature and 0-1.4 keV energy ranges. Hence, the secondary electron emission seems to be agnostic to cryogenic temperatures, including around the superconducting transition around 90 K. The tentative explanation for the SEY stability in HTS under electron irradiation lies in the fact that electronic excitations have an energy range of units to tens of eV, while the binding energy of superconducting pairs is on the order of tens of meV, as theoretically calculated for HTS cuprates [44]. Recent measurements conducted at cryogenic temperatures [28] and further reasoning in [45] show that high-energy synchrotron radiation does not impair the superconducting properties in any way.