Vacuum system of the HEPS storage ring

HEPS (High Energy Photon Source) is a 4th generation ring-based light source with a beam energy of 6 GeV and a beam current intensity of 200 mA. The HEPS accelerator consists of a linac, a booster, a storage ring, and three transport lines connecting the linac, booster, and storage ring. The storage ring serves as the core region of HEPS. In this paper, the characteristics of the storage ring vacuum system are reviewed, and the designs and fabrication approaches for some key vacuum components are detailed. The main challenges include the extrusion of thin-walled CuCrZr vacuum chambers and the application of NEG (Non Evaporable Getters) coatings on their inner surfaces. These are used to mitigate substantial heat loads induced by synchrotron radiation along the vacuum chamber walls and provide an effective pumping speed for conductance-limited vacuum pipes with an inner diameter of 22 mm. The installation of the storage ring vacuum system started in November 2023, and finished in July 2024. Vacuum sectors were in-situ baked and activated for NEG films in the vacuum chambers, an average static pressure of 3 × 10⁻⁸Pa has been reached, which is better than the specification. These results confirm the feasibility of the storage ring vacuum system, covering aspects from the design and fabrication of components such as vacuum chambers, RF shielding bellows, and photon absorbers to processes like NEG coating via magnetron sputtering, installation, and in-situ activation of NEG films. After more than 20 days of conditioning, the beam current of the HEPS storage ring reached 12 mA, marking a milestone progress in the HEPS accelerator. At present, the accumulated beam dose is about 30 Ah (Jan.2025), the vacuum system performs well, the dynamic pressure of the storage ring is expected to decrease with the increase of accumulated beam dose.