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Superconducting 166.6 MHz β = 1 cavities of quarter-wave geometry have been chosen for a high energy photon source, a 6 GeV diffraction-limited synchrotron light source currently under construction in Beijing. Five cavities will provide the required 5.4 MV radio frequency (rf) voltage and 900 kW beam power. Each cavity will be equipped with one fundamental power coupler (FPC), delivering a minimum rf power of 180 kW to the beam. A 50 Ω coaxial structure with one planar warm window was employed. Its location was carefully selected to avoid electron bombardments on the ceramic window due to potential cavity field emission while preserving the required strong coupling. Focusing on optimized heat loads, a compact geometry was pursued to allow assembly with the cavity in a class 10 clean room, thus minimizing contamination. Two prototype FPCs have been fabricated and examined with high power on room-temperature test stands. The couplers were tested up to 50 kW continuous wave (cw) limited by the available solid-state amplifier. The rf conditioning was conducted initially in travelling-wave mode and later in a standing-wave setup with the variable phase of the reverse wave. All rf surfaces were thus exposed to a high field equivalent to 200 kW cw traveling wave. Being the most critical component, the window-inner-conductor assembly was conditioned up to 150 kW at 650 MHz on a hybrid test stand. Multipacting barriers were encountered as predicted and can be processed by rf conditioning. The design, fabrication, and high-power tests of the first 166.6 MHz FPCs are presented.

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.