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X Q Jia Research Institute of Superconductor Electronics (RISE), School of Electronic Science and Engineering, Nanjing University, Nanjing 210023, People’s Republic of China E-mail: [email protected] Radiofrequency (RF) cavities are the main accelerating components in a modern accelerator, which provides an accelerating electric field to convert RF energy into accelerated particle energy. Since it is first proposal in 1961 [1], compared with normal cavities, superconducting RF (SRF) cavities have played a crucial role in the field of accelerators because of their small surface resistance, high intrinsic quality factor, high acceleration gradient, and high efficiency. SRF cavities have gradually become paramount in promoting accelerator development in the fields of nuclear physics, high energy physics, materials science, and life science [2]. Nb3Sn is an intermetallic alloy with an A15 crystal structure. Its superconductivity was first discovered in 1954, with a superconducting transition temperature of ∼18 K [3]. At present, it is commonly used to fabricate cables for superconducting magnets [4]. Niobium (Nb) is a type II superconductor, which is the most widely used superconducting material in SRF cavities. Compared with Nb, Nb3Sn has a higher superconducting transition temperature. This means that, unlike Nb, Nb3Sn can be used for SRF cavities working in liquid helium or even higher temperature regions and can be cooled via cryocooler [5, 6], which will significantly reduce the operating cost. A higher transition temperature may also lead to a higher quality factor and lower RF loss, which will help reduce the cavity size and make the accelerator more compact. In addition, this material has a high thermodynamic critical field, which will help further improve the essential parameter, accelerating gradient Eac. This is further conducive to reducing the size of the accelerator. However, because the thermal conductivity of Nb3Sn is substantially lower than that of Nb, providing poor heat transfer through the cavity wall, it is susceptible to penetration of vortices and RF overheating, limiting its applicability in high fields [2]. Therefore, the realization of Nb3Sn SRF cavities usually does not use pure Nb3Sn directly, but it is prepared in the form of thin films on Nb or Copper (Cu) with higher thermal conductivity [7]. The attempt of an Nb3Sn RF cavity began in 1975 [8]. Since then, many groups have devoted themselves to this research and successfully demonstrated the actual system [5, 6]. At present, the main method for realizing an Nb3Sn RF cavity is the vapor diffusion process, i.e. reacting Nb RF cavity and Sn vapor at high temperatures, which usually involves five stages: degas, nucleation, ramp, coating, and annealing. The temperature of each stage is different, so it is difficult to control the complex preparation parameters [7]. Another method is to form a layer of Nb3Sn [9] on a Cu resonant cavity, which can be achieved using alloy target sputtering [10], cosputtering [11], preparing Nb and tin (Sn) layers and reacting them with each other [12], and using Nb and bronze reaction [13]. However, for the Nb and bronze layers prepared at room temperature, the post-heat treatment is required to complete the reaction, which is timeconsuming and may lead to the poor stoichiometric ratio of the Nb3Sn layer due to the evaporation or diffusion of Sn [14]. Withanage et al [15] proposed a method for the rapid growth of Nb3Sn thin films on heated bronze substrates. Nb was grown via DC magnetron sputtering on bronze substrates (Cu-15 wt.% Sn) heated above 700 ◦C. Nb reacted in situ with