A prototype of in-air undulator vacuum chamber coated with non-evaporable getter films

The inner wall of the in-air undulator (IAU) vacuum chamber at the High Energy Photon Source (HEPS) is planned to be coated with a non-evaporable getter (NEG) film. After activation, the specified vacuum environment can be achieved inside the storage ring beam pipe, meeting the requirements for beam lifetime. The current challenge for depositing the IAU vacuum chamber is to maintain cathode alignment and ensuring stable plasma discharge across the entire area during the magnetron sputtering coating process. This research concentrates on the particle-in-cell/monte carlo collision (PIC/MCC) method to model and simulate the magnetron sputtering discharge process. Through analysis of plasma discharge states at various positions within the IAU vacuum chamber, a stable discharge region inside the solenoid was identified. Furthermore, a specialized segmented coating method was proposed to counteract the influence of the solenoid magnetic field's edge effects on discharge stability, facilitating the uniform deposition of the NEG film across the entire axial extent of the vacuum chamber. After activation of the NEG film for the IAU vacuum chamber (180 °C for 48 h), the ultimate vacuum level is capable of reaching 3.7 × 10⁻⁸ Pa. This coating method has been proved to be feasible and ensures the stability of the discharge and the reliability of the NEG film quality, which satisfy the stringent engineering requirements of HEPS. This study may also offer a reference for similar vacuum chamber coating applications.