Cavity fundamental mode and beam interaction in CEPC main ring

Abstract

CEPC is a 100-kilometer-long electron-position collider, aiming to produce Higgs, W and Z-pole. Double ring (DR) is the baseline design of its main ring, and advanced partial double ring (APDR) is the alternative one. The purpose of this paper is to study the beam loading effects and the corresponding longitudinal beam dynamics of CEPC DR and APDR. The phase shift of the bunches modulated by the bunch gap is calculated with the approximation formulas and simulated with the program. All these methods are compared, and the application conditions of them are also elaborated. In addition, the longitudinal coupled-bunch instability in CEPC main ring is calculated by analytical formulas. In CEPC DR high-lumi Z, the phase shift can be reduced to 0.51 deg( $$^\circ $$ ). Besides, the total number of unstable longitudinal modes is 15 in CEPC DR high-lumi Z when no feedback system is added. The phase shift of the bunches in CEPC can be reduced to an acceptable value if the optimal filling pattern is chosen. For CEPC APDR, the RF parameters are calculated and the beam loading effects are tolerable.