Development of a New Vacuum Furnace and Its Application for 650 MHz Superconducting Cavities

A new vacuum furnace has been developed at the Institute of High Energy Physics, Chinese Academy of Sciences (IHEP), which was dedicated to processing superconducting radio-frequency (SRF) cavities. It was integrated and commissioned in 2021, and then used for high-temperature annealing, nitrogen doping, and medium-temperature (mid-T) furnace baking, et c. The nitrogen doping of two 650 MHz SRF cavities was conducted at this vacuum furnace, followed by light electro-polishing (EP). Then, the two cavities nitrogen doped both demonstrated a higher intrinsic quality factor (
Q
₀
) than baseline during the vertical test (@ 2.0 K), which reached 6.5 × 10
¹⁰
and 7.2 × 10
¹⁰
at 20 MV/m, respectively. However, the maximum accelerating gradient (
E
_acc
) was reduced considerably for both 650 MHz SRF cavities. Thus, the process of nitrogen doping could enhance the
Q
₀
of 650 MHz SRF cavities at the expense of
E
_acc
. In general, nitrogen doping may be advocated for continuous-wave (CW) accelerators, which are usually operated at a medium gradient.