# Electron Beam Characterization of REBCO-Coated Conductors at Cryogenic Conditions
## 4 Summary and Conclusions  
This electron beam-based laboratory investigation of REBCO-CCs aims to characterize surface properties related to electron clouds in accelerators, including the secondary electron yield (SEY), which is a determining factor for electron multipacting, and electron stimulated desorption (ESD), which links dynamic pressure rise to EC activity during operation. The presented results are applicable to other types of RF devices that use HTS, such as radio-frequency waveguides and cavities, which may also suffer from electron cloud or multipacting effects. We first identified the surface morphology and composition using SEM imaging with an EDS capability. SEM imaging showed a flat surface structure covered with segregated nanocrystalline precipitates with characteristic sizes in the micrometer range. The ESD conditioning rate and the shape of measured SEY curves generally resemble those of bare technical-grade metals, which supports the SEM observation of a flat, non-porous, surface structure covered with micrometer-sized precipitates. The EDS measurement is bulk-sensitive at the micrometer range, and considering the characteristic micrometer-sized skin depths, the EDS results are relevant to the HTS properties. Meanwhile, the XPS measurement with a probe-depth in the nanometer range makes it highly surface sensitive,  
Figure 5: ESD dose dependence measured at 300 eV for PLD–deposited (**left**) and RCE–deposited (**right**) REBCO samples held at 12 K starting from an as–received state. Best–fit parameters for Equation (5) from [49] is listed in the legend. More details are in the text. rendering the XPS results relevant to the SEY properties. Both EDS and XPS analyses could identify the main chemical constituents at the surface of the Gd-Ba-Cu-O film. Knowing the morphology and composition of the samples and that the REBCO was directly exposed on the surface, we held the samples at 12 K and studied their response to electron irradiation. The energy range of 0-1.4 keV was chosen to cover the entire relevant EC spectrum [13; 15; 21]. Meanwhile, 300 eV irradiation was selected for electron conditioning studies, as it represents the high-energy part of the EC, which is responsible for the conditioning effect [38], i.e., the decrease of SEY and ESD yields during electron irradiation commonly observed during beam-scrubbing of an accelerator [20; 23]. We measured the SEY energy dependence in the 0-1.4 keV range both before and after the 300 eV irradiation with a clear conclusion that the SEY decreases in the entire studied range. The SEY of the RCE-deposited sample decreased to a value of \(\delta_{max}=1.2\) at 1 mC-mm\({}^{-2}\) electron dose, which is a value acceptable for many applications. Conversely, the PLD-deposited sample SEY only conditioned to \(\delta_{max}\approx 2\) at a 4.5 mC-mm\({}^{-2}\) dose. The reason for this is unclear, but based on the XPS analysis, it can be hypothesized that the etched PLD-deposited sample lacks surface-bound carbon contaminants that would graphitize into a sufficiently thick graphitic overlayer with intrinsically low SEY values. The XPS spectrum seconds this hypothesis since the RCE-deposited sample had 49% carbon content whilst the PLD-deposited sample only had 33%, resulting in a smaller SEY reduction. The electron conditioning was studied up to a few mC-mm\({}^{-2}\), which is an electron dose rapidly achievable during an accelerator operation. The RCE-deposited case represents a proof-of-concept capability of REBCO-type coatings to electron condition to SEY values sufficiently low not to trigger electron multipacting, resulting in EC build-up in the beam screen. Parallel studies by [PERSON] et al. [26] also consider depositing a thin, anti-multipacting, low-SEY layer of amorphous carbon coating atop the REBCO-CCs to alleviate the need for beam scrubbing. Therefore, future SEY and ESD studies could focus on characterizing carbon-coated REBCO-CCs at cryogenic conditions.