Development and tests of the 499.8 MHz srf cryomodules for HEPS

Abstract A 499.8 MHz single-cell superconducting cavity has been proposed as the active third-harmonic cavity for the High Energy Photon Source (HEPS), a 6 GeV diffraction-limited synchrotron light source currently under construction in Beijing. Two 499.8 MHz cryomodules are required to provide a total of 3.5 MV of rf voltage and 400 kW of beam power. A mechanically enhanced cavity design, based on the original KEKB-type cavity, was developed to meet the specifications of HEPS. Three cryomodules have been assembled and subjected to horizontal tests. A comprehensive clean-assembly procedure was developed, resulting in excellent rf performance maintained from vertical tests to horizontal tests at cryogenic temperatures. The unloaded quality factors of all three modules exceeded 2.45×10 9 at 1.75 MV, significantly surpassing the design goal. No field emissions were observed throughout the tests. These are the highest-performing 500 MHz modules treated by buffered chemical polishing, also outperforming the average performance of their electropolished counterparts. Following dedicated analyses and multiple road tests, one cryomodule was successfully transported in its entirety from the test platform to the HEPS tunnel, with well-controlled accelerations on delicate components. Vacuum integrity and performance were well maintained. The module has been accelerating electron beams since August 2024 for the HEPS commissioning. This paper presents the design, fabrication, assembly, cryogenic tests, and transportation of the mechanically improved 499.8 MHz single-cell srf cryomodule.