# Quality Factor Enhancement of 650 MHz Superconducting Radio-Frequency Cavity for CEPC
## 2 The Process of Mid-T Furnace Baking  
The cavity investigated in this paper is a 650 MHz single-cell \(\beta\) = 1 SRF cavity for CEPC, which is made of fine-grain niobium with high residual-resistivity-ratio (RRR \(>\) 300). The ratio of the peak surface magnetic field (\(B_{\text{peak}}\)) to accelerating gradient (\(E_{\text{acc}}\)) is approximately 4.2. Firstly, the 650 MHz cavity received bulk electro-polishing (EP) process with a total removal of \(\sim\)315 \(\upmu\)m, where the temperature was below 25 \({}^{\circ}\)C all the time. There was no high-temperature annealing for hydrogen outgassing before or after EP, because we wanted to simplify the recipe of mid-T furnace baking, which combines high-temperature annealing and mid-T furnace baking at a later stage. Therefore, the 650 MHz cavity received high-pressure rinsing (HPR), assembly, low-temperature baking (120 \({}^{\circ}\)C for 48 h) and vertical test directly after bulk EP [1]. In the vertical test of EP baseline (blue icon), the 650 MHz cavity quenched at a very high gradient of 37.5 MV/m (\(B_{\text{peak}}\) = 158 mT) with \(Q_{0}\) of 1.4 \(\times\) 10\({}^{10}\) in Figure 1, which indicated smooth and defect-free surfaces of the cavity. However, the Q value was slightly lower than the CEPC vertical test (VT) spec of 4.0 \(\times\) 10\({}^{10}\) at 22 MV/m. The behavior of \(Q\)-slope may result from hydrogen diffused in the cavity during EP process. The radiation began to increase at 33 MV/m, so field emission was low during the vertical test. \(R_{\text{S}}\) of the 650 MHz cavity (blue icon) increased exponentially along with the accelerating gradient in Figure 2, which is typical for cavity EP processed [7; 9].  
Hence, the cavity was considered qualified to receive mid-T furnace baking.  
Afterwards, the 650 MHz cavity was subjected to high-temperature annealing at 900 \({}^{\circ}\)C for 3 h (hydrogen degassing) [11; 12], and the following light EP was cancelled. Then, the cavity was exposed to air for 3 days in cleanroom (oxidation). Finally, it received mid-T furnace baking at 300 \({}^{\circ}\)C for 3 h [13] as well as 1.3 GHz nine-cell cavity [3], which is shown in Figure 3. During the entire process, both cavity flanges were covered with niobium foils, which had been chemically polished, to keep the cavity inner surface from dust. The process of mid-T furnace baking was key to improve \(Q_{0}\) of 650 MHz cavity. Prior to mid-T furnace baking, the furnace was pumped adequately to obtain ultimate vacuum (\(<\)5 \(\times\) 10\({}^{-6}\) Pa). The highest pressure of furnace vacuum was about 3.5 \(\times\) 10\({}^{-5}\) Pa during the process of mid-T furnace baking (Figure 4), which was maintained with two cryopumps.  
Figure 1: Vertical test results of 650 MHz SRF cavity (Blue: processed with EP; Red: processed with mid-T furnace baking).  
After cooling down, the vacuum pressure was about 2.5 \(\times\) 10\({}^{-6}\) Pa, and the cavity was taken out.  
Figure 3: Setup of the mid-T furnace baking for 650 MHz cavity.  
Figure 2: Surface resistance (\(R_{\rm S}\)) of 650 MHz SRF cavity (Blue: processed with EP; Red: processed with mid-T furnace baking).