Design Optimization of a Mechanically Improved 499.8-MHz Single-Cell Superconducting Cavity for HEPS

Superconducting 499.8-MHz radio frequency (rf) cavities have been proposed for the High Energy Photon Source (HEPS), a 6-GeV diffraction-limited synchrotron light source currently under construction in Beijing. Being an active third-harmonic system, two cavities shall provide 3.5-MV rf voltage and 400 kW of beam power to enable a complex rf gymnastics required by a novel injection scheme. Adopting the veteran KEKB-type 500-MHz single-cell geometry, the cavity design has been focused on optimizing its mechanical properties. Various cavity wall thickness and external stiffening mechanisms were investigated to reinforce the cavity to ensure compliance to the pressure vessel codes under different working conditions from a bare cavity to a dressed cavity in the cryomodule. Small margins on operational pressure previously reported on this type of cavity have been largely improved with slightly reduced yet acceptable frequency tunability. Extensive design optimizations have been conducted aiming for lower stress, larger elastic buckling pressure, and higher frequencies of mechanical modes to cause microphonics, while the frequency tunability, pressure sensitivity, and Lorentz force detuning were being carefully monitored. In view of the HEPS's lifetime, cavity fatigue was examined. This constitutes a comprehensive design optimization of the 499.8-MHz single-cell superconducting cavity with improved mechanical properties.