Medium-temperature furnace baking of 1.3 GHz 9-cell superconducting cavities at IHEP

Abstract

Recently, heat treatment between 250 °C and 500 °C has been attempted to improve quality factor (Q) of superconducting radio-frequency cavities at FNAL and KEK. Experiments of such medium temperature (mid-T) bake with furnaces have also been carried out at IHEP. Firstly, over ten 1.3 GHz 1-cell cavities were treated with different temperatures at a small furnace, which all demonstrated improvement of Q and anti-Q-slope phenomenon. The average quality factor has reached 3.6×10 10 when the gradient is 16 MV/m，while the highest Q is 4.9×10 10 @16MV/m; the maximum gradients of these 1-cell cavities are between 25.1 and 36.9 MV/m. Then, the recipe of mid-T furnace bake at 300 °C for 3 hours has been applied to six 1.3 GHz 9-cell cavities at a new big furnace, which have all shown higher Q and anti-Q-slope at medium field (16~24 MV/m). The average quality factor has reached 3.8×10 10 when the gradient is 16 MV/m. The maximum gradients of the 9-cell cavities are between 22.7 and 26.5 MV/m.

Higgs-Energy LEptoN (HELEN) Collider based on advanced superconducting radio frequency technology

“…Surface analysis of similarly treated samples show a nitrogen peak at a few nm below the surface, suggesting that nitrogen is naturally present at the surface and has diffused into the Nb to give the N-doping effect. IHEP in Beijing, China followed up on these encouraging results with several 9-cell TESLA cavities with similar exciting results [23]. After mid-T (300 • C) furnace bake, and HPR, all the 9-cell cavities demonstrate high Q in the range of 3.5 − 4.4 × 10 10 at the gradient between 16-24 MV/m, as shown in Figure 4.…”

Quality Factor Enhancement of 650 MHz Superconducting Radio-Frequency Cavity for CEPC

“…R BCS was 2 nΩ constantly at low fields, and slightly increased as a function of accelerating gradient above 20 MV/m in Figure 5. The decrease in R BCS as a function of accelerating gradient for 1.3 GHz cavity [1][2][3] did not appear for 650 MHz cavity. It confirms that mid-T furnace baking could not reverse R BCS at the frequency of 650 MHz.…”

“…There was no field emission during the test, and the cavity quenched at 31.2 MV/m (B peak = 131 mT) finally. Furthermore, there was no anti-Q-slope behavior, which is usual for 1.3 GHz cavities after mid-T baking [1][2][3].…”

“…Then, the cavity was exposed to air for 3 days in cleanroom (oxidation). Finally, it received mid-T furnace baking at 300 • 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.…”

“…Here, R S is the electrical resistance of surface layer to current, and inversely proportional to Q 0 of SRF cavity. Recently, the technology of medium-temperature (mid-T, 250-400 • C) baking is used to increase Q 0 of 1.3 GHz cavities successfully, which dissolves the oxides in the surfaces of niobium [1][2][3]. It is more convenient and easier than nitrogen doping/infusion [4][5][6].…”