# Electron Beam Characterization of REBCO-Coated Conductors at Cryogenic Conditions
## 1 Introduction  
High-temperature superconductors (HTSs) constitute a class of highly correlated anisotropic systems that theoretically have proven to be extremely difficult to address. Since their discovery in 1986 [1], numerous studies have been conducted to describe these materials, as well as to produce them at a scale suitable for large-scale technical applications. The general interest lies in the fact that the critical surface of HTS is significantly higher than that of low-temperature superconductors (LTSs), as visible in Figure 1a. Consequently, work gradually proceeded on technical applications, leading to large-scale HTS-based prototype devices over the years, and demonstrating the possible performance advantages over devices based on conventional LTS [2] or even normal-conducting technology. The most promising industry-grade method available for a large-scale HTS application is by coating a thin layer of rare-earth barium copper oxide (REBCO) material on flexible metal tapes, in a complex-layered structure containing buffering metal oxide underlayers and stabilizing and protecting Cu and Ag overlayers. This technology is known as a second-generation coated conductor (CC), as depicted in Figure 1b, and is already commercially available in kilometer-lengths from several manufacturers. These second-generation REBCO-CC tapes promise high-performance properties, such as high current densitiesand compatibility with high magnetic field applications, and their matured technology and mass production make them an affordable and viable engineering solution. Aside from the imminent application in high-temperature superconducting magnets, in the next-generation, particle accelerators [3; 4], passive microwave devices (not generating power) will be equally strong candidates for eventual HTS-coating commercialization. A HTS can have lower microwave surface impedance compared to pure metal [5]. Subsequently, this could allow the design of high-performance RF devices, such as high-quality factor (Q-factor) resonators, narrow-band filters, and antennas with improved functioning over conventional normal-conducting devices. Additionally, such complex microwave signal transmission and signal-processing systems could be designed to be more compact and more efficient [2; 6]. For instance, a potential application lies within future space-based communication systems with a more effective strategy by producing fewer and lighter high-performance satellites with the reduced weight of cavities and filters. Furthermore, the feasibility of coating curved walls with REBCO-CCs enables manufacturing high Q-factor RF cavities, such as the one developed within the relic axion detector exploratory setup (RADES) [7]. Here, the detector sensitivity scales linearly with the Q-factor of the 9 GHz resonant cavity held at 4.2 K, and REBCO is the essential technology enabling operation in a strong magnetic field of 11 T while maintaining a high Q-factor. In particle accelerators with magnets operating at cryogenic temperatures, a beam screen (BS) inserted into the cryomagnet apertures is used to intercept the heat and radiation generated by the circulating particle beam before it can reach the superconducting magnets. The BS can equally be considered an RF device due to the time-variant electric field induced by the passing bunched charged particle beam. Taking the BS in the Large Hadron Collider (LHC) as an example, its steel structure is co-laminated with Cu to reduce the surface impedance at its operating temperatures of 5-20 K [9]. The flat geometry makes REBCO-CC a feasible alternative coating material for BS to potentially reduce the resistive-wall impedance at operating temperatures of \(\sim\)50 K [10]. Decreasing the impedance reduces the beam instabilities [11] and heat load to the BS [12]. Electron multipacting can occur inside the BS when certain resonant operating conditions are met [13].