Zhang, X.Y. Dai, J. Guo, L. Ma, Q. Meng, F. Zhang, P. Zheng, H.J. JACoW Publishing Geneva, Switzerland 2673-5520 978-3-95450-250-9 10.18429/JACoW-MEDSI2023-WEOAM04 English 129-132 cavity HOM cryomodule photon superconducting-cavity Contribution to a conference proceedings 2024 2024-07 https://doi.org/10.18429/JACoW-MEDSI2023-WEOAM04 https://jacow.org/medsi2023/papers/weoam04.pdf A low-frequency superconducting cavity is one of the most critical devices in the High Energy Photon Source (HEPS), a 6 GeV diffraction-limited synchrotron light source under construction in Beijing. A higher-order-mode (HOM) damped 166.6 MHz ß=1 quarter-wave superconducting cavity, first of its kind in the world, has been designed by the Institute of High Energy Physics. Compact structure, excellent electromagnetic and mechanical properties and manufacturability were realized. Mounted with a forward power coupler, a tuner, two thermal break beam tubes, a collimating taper transition, two gate valves and some shielded bellows, the dressed cavity was then assembled into a cryomodule. Two cryomodules were later required to fit into HEPS straight sections with a length limitation of 6 meters, which posed a significant challenge for the design of the cavity string. The success of the horizontal test also verifies the design of the cavity string. This article presents the design, fabrication, post-processing, system integration, and cryogenic tests of the first HOM-damped compact 166.6 MHz superconducting cavity module.