An ADS 650 MHz medium beta cavity study and designLIU Zhen-Chao( ) GAO Jie( ) LI Zhong-Quan( ) ZHAI Ji-Yuan( ) ZHAO Tong-Xian( ) LI Da-Zhang( ) LIU Yi-Lin( ) SUN Yi( )

Abstract:In this paper we will show a cavity and higher order mode (HOM) coupler designing scheme for the Circular ElectronPositron Collider (CEPC) main ring. The cavity radio frequency (RF) design parameters are showed in this paper. The HOM power is calculated based on the beam parameters in the Preliminary Conceptual Design Report (Pre-CDR). The damping results of the higher order modes (HOMs) and same order modes (SOMs) show that they are reached the damping requirements for beam stability. The circumference of the main ring is 54.7 km with 2 beams in one ring. The synchrotron radiation power for one beam is 51.7 MW. Table 1 shows the main parameters of the CEPC main ring [2]. The radio frequency (RF) system accelerates the electron and positron beams, compensates the synchrotron radiation loss and provide sufficient RF voltage for energy acceptance and the required bunch length in the collider. Superconducting Radio Frequency (SRF) cavities are used because they have much higher continuous wave (CW) gradient and energy efficiency as well as large beam aperture compared to normal conducting cavities. CEPC will use 384 five-cell 650 MHz cavities for the main ring. The cavity design and HOM power analysis are shown in this paper. Keywords Cavity designThe geometry of the superconducting cavity is shown in Fig. 1. The meaning and the effects of the cavity parameters are summarized in the following: 1) Riris is the radius of the iris. Fig. 2 (a) shows that large Riris is good for the cell-to-cell coupling. However, large Riris leads to large Ep/Eacc and Hp/Eacc value. Large Riris value also decreases the impedance of the fundamental mode. 2) D is the radius of the equator. It is used for the frequency tuning. 3) L is the half length of the cavity cell. It is determined by the fundamental frequency and the cavity geometry beta. 4) The equator ellipse ration (B/A) is ruled by the mechanical considerations. 5) The iris ellipse ration (b/a) is determined by the local optimization of the peak electric field. Fig. 2 (b) shows that the iris ellipse ration is sensitive to Ep/Eacc and cell-to-cell coupling. It has a little effect on Hp/ Eacc and R/Q. 6) alpha is the side wall inclination. Fig. 2 (c) shows that the angle has a little effect on R/Q. Small alpha decreases the Hp/Eacc value while increases the cellto-cell coupling. However, small alpha increases the Ep/Eacc value. The higher angle is better for the cavity chemistry and cleaning procedures. 7) d is the wall distance parameter. Fig. 2 (d) shows when d increases, Ep/Eacc will decrease while Hp/Eacc will increase. The parameter d is used to balance the Ep/Eacc and Hp/Eacc value. However, smaller d also increases the cell-to-cell coupling.