## 2 Experimental Descriptions
### Superconducting Cavities Treatment
#### 2.2.3 Surface Treatment outside the NS03 Cavity  
The relatively low \(E_{acc}\) achieved in NS04 cavity tests was primarily attributed to insufficient thermal stability and thermal uniformity, which is presented in Section 3.1. To solve this problem, a cold spraying technique was used to deposit a 3.5 mm thick Cu layer on the outer surface of NS03 cavity. Cold spraying is capable of creating a bond between Nb and Cu with a strength exceeding 40 MPa. In order to guarantee the bonding and to avoid deformation of the cavity, instead of sandblasting, we first cold sprayed a Cu base layer of 300 \(\upmu\)m with He gas. Subsequently, a Cu layer of about 3.2 mm was deposited in eight passes using N\({}_{2}\), and three flanges were sprayed at the equator and near the irises to facilitate the connection with the Al plate. The powder used for spraying was 99.95% pure, spherical Cu powder with a particle size of 15-40 \(\upmu\)m. Prior to spraying, the cavity was sealed with an adhesive ring and the interior was filled with 0.12 MPa Ar gas to protect the Nb\({}_{3}\)Sn film. During the spraying process, we used N\({}_{2}\) to cool the cavity, in order not to exceed 100 \({}^{\circ}\)C so we could minimize the oxidation of the Cu layer and the effect on the Nb\({}_{3}\)Sn film. All deposition processes were finished in 1 day. The cavity was then machined; Figure 6 shows the deposition and machining processes. In the NS03 cavity test, we used high-purity Al (purity > 99.999%, SN) plates with a high thermal conductivity instead of OFC straps. Four Al plates were used to connect the cavity to the cryocoolers and were bent in a C-shape to reduce vibration transfer from the cryocoolers. Two 5N Al rings were assembled on both sides of the equatorial Cu flange to enhance the circumferential heat conduction of the superconducting cavity, and the equatorial flange was also connected to the beam tube flanges with 5N Al plates to increase the axial heat conduction. The connections were all connected using 316 L screws, spacers, and silicon bronze nuts, and 0.1 mm thick indium foils were padded on the contact surface. The preloading force of the bolts was 10 N\(\cdot\)m. Apiezon N grease was applied to the contact  
Figure 5: (**a**) the NS04 cavity is placed on a G-10 base, and the ellipsoid is clamped to the beam tube with Cu hoops. (**b**) OFC hoops, straps and cavity actually installed. Figure 6: (**a**) Pictures taken during the deposition and machining processes: sealed cavity, (**b**) sprayed base layer, (**c**) sprayed 3.2 mm Cu layer and 3 flanges, (**d**) machining process. surface between the Al plates and the cryocoolers, and indium foils were not used to enable easy disassembly. After the machining process, the cavity was ultrasonically cleaned with anhydrous ethanol, and subjected to HPR in a class 100 clean room. Indium wire with a 2 mm diameter was used to seal the cavity with 316 L flanges, which was then pumped to 8 \(\times\) 10\({}^{-5}\) Pa vacuum. The installation of the 5N Al thermal links was then completed in the clean room. The cavity was then mounted in the cryostat for a horizontal test. In this cryostat, a horizontal test allows for shorter thermal links, which facilitates enhanced thermal conduction. Figure 7 shows the installation of the cavity in a clean room and its placement in the cryostat. Eleven thermometers were mounted on the cavity, thermal links, and 2 nd stage of one cryocooler. Three mutually orthogonal cryogenic flux-gate magnetometers were arranged at different positions on the equator of the cavity. Four heaters (metal film resistors) were installed on the thermal connections. Figure 8 shows the approximate mounting location of the thermometers, magnetometers, and heaters. We tested the mechanical properties and thermal conductivity of cold sprayed Cu. We fabricated three samples with a 2 mm thick Cu layer cold sprayed with N\({}_{2}\) on a Nb substrate and tested the bond strength between Nb and Cu using a tensile method.