The 166.6 MHz superconducting radio frequency cavities have been proposed for High Energy Photon Source (HEPS) storage ring. HEPS is a 6 GeV, 200 mA diffraction-limited synchrotron light source to be built in Beijing. Higher-order modes (HOMs) excited in 166.6 MHz SRF cavities have to be sufficiently damped in order to prevent coupled-bunch instabilities and to limit parasitic mode losses.
A hybrid HOM damping scheme has been proposed. An enlarged beam pipe allows HOMs above beam pipe cutoff frequencies to propagate and subsequently be absorbed by dampers installed on the downstream beam pipe, while a petal-shaped coaxial structure extracts HOMs below beam pipe cutoff frequencies and subsequently damped by the specially designed coaxial filter.
HOM damping in cavities has been demonstrated by 3D simulations. The impedance of HOMs has been successfully reduced to be lower than the coupled-bunch instability threshold. In addition, no hard barrier multipaction has been observed and acceptable heat loss is obtained.
In this paper, the hybrid HOM damping scheme has been demonstrated. Based on these studies, a prototype petal–loop structure and its associated filter are being fabricated.