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.

Abstract:In cavities, there exists not only the fundamental mode which is used to accelerate the beam but also higher order modes (HOMs). The higher order modes excited by beam can seriously affect beam quality, especially for the higher R/Q modes. This paper reports on measured results of higher order modes in the IHEP02 1.3GHz low-loss 9-cell superconducting cavity.Using different methods, Q e of the dangerous modes passbands have been got. The results are compared with TESLA cavity results. R/Q of the first three passbands have also been got by simulation and compared with the results of TESLA cavity.Keywords: higher order modes, Q e , experimental measurements PACS: 29.20.Ej IntroductionThe layout of IHEP02 1.3GHz low-loss 9-cell superconducting cavity [1] is shown in Fig.1. In order to damp HOMs, two HOM couplers were mounted respectively at the upstream and downstream beam tube. Distances from the HOM couplers to end cells were 65mm and 50mm, which were different from TESLA cavity [2]. Length of upstream beam tube was also different from downstream. Simulation results of R/QThe ratio R/Q is a very important quantity related to the interaction of the beam and cavity. The higher value of R/Q stands for larger energy change between beam and cavity. For monopole HOMs, beam energy loss will be more. For dipole modes, the transverse displacement of the beam will be larger, which will causes emittance larger and even worse it will leads beam loss. It becomes very important to measure modes with large R/Q.An important parameter describing the transverse beam-cavity interaction is transverse shunt impedance. From the Panofsky-Wenzel theorem [3], the transverse momentum change of the particle passing through a cavity excited in a single mode is proportional to a parameter ( / ) R Q ⊥ . In this paper, for dipole modes,