# Electron Beam Characterization of REBCO-Coated Conductors at Cryogenic Conditions
## 1 Introduction
The electric field wake created by passing bunches repeatedly accelerates electrons and results in avalanche-like electron multiplication and the formation of a stable and self-sustaining electron cloud (EC). The precondition for EC emergence in a BS, or another RF device, is a sufficiently high value of a secondary electron yield (SEY), defined as the number of secondary electrons leaving a surface after the incoming primary electron  
Figure 1: (**a**) Comparison of the critical surface phase space for LTS and HTS formed by the three interdependent characteristic parameters of a superconductor, i.e., the critical temperature, critical magnetic field, and critical current density. Superconductivity can only occur below the critical surfaces, which are unique for each material. The figure is inspired by [8]. (**b**) Structure of a complex–layered REBCO HTS–coated conductor tape. Note the substrate and buffer layers residing under the REBCO coating, the silver and copper stabilizing, and protecting overlayers. impact. If this ratio is above unity, the electrons can multiply, while a surface with a SEY below unity acts as an electron absorber, therefore, hindering EC build-up. The EC is a phenomenon commonly observed during the operation of storage rings [14; 15; 16; 17] and RF devices [18; 19] and is detrimental to the accelerator operation in many regards, such as EC-induced beam instabilities, leading to emittance growth [14], EC-induced heat-load deposited into the cryogenically cooled BS [12], and electron-induced gas desorption [20] that deteriorates the ultra-high vacuum (UHV) conditions in the BS. The EC-induced gas load is characterized by the electron-stimulated desorption (ESD) yield. The EC has a characteristic energy spectrum known from calculations, in situ measurements, and simulations [13; 15; 18; 21; 22], mostly consisting of low-energy electrons below 20 eV, and a smaller population of electrons accelerated by the electric field to hundreds of electron volts. Fortunately, both SEY and ESD yields decrease during electron irradiation to acceptable levels. This effect is leveraged in so-called beam-scrubbing [17; 23], i.e., in situ electron and photon conditioning by circulating a beam when gradual decreases in EC activity and the dynamic vacuum effect are observed during the accelerator operation. These effects mainly pertain to accelerator storage rings, which aim to circulate bunched, positively charged beams at high intensity and high energy while maintaining a low emittance to generate high luminosity for physics research [24], such as the Future Circular Hadron Collider (FCC-hh) led by CERN [10] and the Super Proton-Proton Collider (SPPC) led by IHEP and CAS in China [25]. Previous studies [26; 27; 28; 29] done on REBCO-CC samples (of the same type and provider studied here) produced experimental evidence for compatibility from the RF and superconducting perspectives across the necessary temperatures, fields, and current densities. This makes them potential candidates that are currently being investigated for BS construction to be used in future accelerators [27]. Hence, this manuscript investigates the compatibility of REBCO-CC surface properties linked to the dynamic vacuum effect and electron cloud at conditions representative of an accelerator application. This includes cryogenic temperatures and electron irradiation similar to that of an actual EC, i.e., with a focus on the entire 0-1 keV energy region. The following measurements use various electron beam-based methods to infer the sample's response to electron irradiation. First, the surface morphology and composition were investigated to verify that the REBCO coating was actually exposed and matched the manufacturer's specifications. The analytical methods we used were X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM), conducted on a device equipped with an energy-dispersive X-ray spectroscopy (EDS) function. Second, the SEY was investigated as the main EC-driving property.