The first 500 MHz superconducting accelerator module of the High Energy Photon Source (HEPS), a major national science and technology infrastructure project in China, completed a high-power horizontal test at cryogenic temperature on March 26, 2024. The results exceeded the facility’s design specifications, marking the successful development of the 500 MHz cryomodule and establishing a strong basis for the commissioning and operation of the HEPS accelerator.

The 500 MHz cryomodule is an active third-harmonic RF for electron bunch lengthening at the HEPS storage ring. In the early commissioning phase, it will also serve as the main accelerating RF to provide energy to the electron beam in the storage ring for vacuum cleaning. It is a key piece of equipment for HEPS. The 500 MHz superconducting cavity, high-power coupler, higher-order-mode absorber, tuner, beamline components of the cavity string, and cryostat have been developed by the HEPS Radio Frequency (RF) team led by Dr. ZHANG Pei and the Cryogenics team led by Dr. GE Rui.

The 500 MHz superconducting cavity, high-power coupler, and cavity string components were manufactured by Beijing HE-Racing Technology Co., Ltd., and the cryostat was manufactured by Hefei Juneng Electro Physics High-tech Development Co., Ltd. The design, post-processing, system integration, and high-power tests of the cryomodule were conducted by physicists and engineers of the HEPS RF and Cryogenics teams. Strong support has been received from the Platform of Advanced Photon Source Technology R&D (PAPS).

At 4 K, when the accelerating voltage reached the HEPS operating goal of 1.75 MV, the quality factor of the 500 MHz cryomodule reached 2.6E+9. The static heat load of the module was measured to be 20.3 W, while the dynamic heat load of the module was 12.8 W, with no field emission or multipacting inside the cavity being observed. Limited by the available power of the transmitter during the test, the maximum accelerating voltage was measured to be 1.81 MV, corresponding to a continuous-wave power of 155 kW for the power coupler. The field-emission-induced radiation was measured to be less than 0.1 μSv/h, equivalent to the bare cavities during vertical tests.

Two 500 MHz cryomodules (third-harmonic RF) and six 166 MHz cryomodules (fundamental RF) will be eventually installed in the HEPS storage ring. The first 166 MHz cryomodule has been successfully developed in November 2023. The production, assembly, and test of the subsequent module have been launched, in preparation for the installation and commissioning of the HEPS storage ring. It also established a strong basis for the development of the core equipment, the 500 MHz cryomodule, for the BEPCII upgrade.

HEPS 500 MHz cavity string clean assembly (left) and the completed cryomodule (right)

HEPS 500 MHz horizontal test results at 4 K