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.

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A low-frequency superconducting cavity is needed in main accelerators for storage ring light sources with ultralow emittance. A compact 166.6 MHz superconducting proof-of-principle cavity was designed adopting a quarter-wave β = 1 geometry for a High Energy Photon Source (HEPS). It is a 6 GeV diffraction-limited synchrotron light source currently being developed at the Institute of High Energy Physics. The cavity is exceedingly compact in size yet possessing a low resonant frequency. The nearest higher order mode is largely separated from the fundamental, making the cavity an attractive geometry for effective damping of these modes in high current accelerators such as HEPS. The achieved accelerating voltage of 3.0 MV is well beyond the designed 1.5 MV and required 1.2 MV for HEPS operation. High surface electromagnetic fields were reached with excellent rf and mechanical performances, and multipacting barriers were easily processed. This constitutes the first demonstration of a compact low-frequency β = 1 superconducting cavity for HEPS. The design, fabrication, surface preparation, and cryogenic tests of the cavity are presented.

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A tuning system for BEPC * MI Zheng-Hui( ) 1,2;1) SUN Yi() 1,2 ZHANG Juan( ) 1,2 PENG Ying-Hua( ) 1,2

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