Chinese Physics C(HEP NP) Vol.32.No.12.Dec..2008 Studies on high order mode of bell-shaped prototype cavities MA Guang-Ming(马广明)2 ZHAO Zhen-Tang(赵振堂)1 (Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied CAS, Shanghai 201800, China) 2(Graduate University of Chinese Academy of Sciences, Abstract Aluminium and copper prototype cavities were designed to study higher order modes(HOM). An automatic field mapping system was developed with LabviEW to measure the radiofrequency(RF) charac- teristics, such as resonant frequency, Q-value, shunt impedance and electromagnetic field distribution of the higher-order modes in a model RF cavity. Two kinds of the bell-shaped cavities were measured using the field mapping system, their frequencies are 1.5 GHz and 800 MHz respectively. The fields'distributions of the monopole modes and dipole modes, as well the r/Q values, were measured Key words perturbation method, higher order modes, LabVIEW, superconducting RF PACs29.20.db,29.27.Bd 1 Introduction Here we set up a new system of HOM field map- ping, R/Q measurement, etc. This system is used to The superconducting RF cavity, which is widely study the bell-shaped cavity, the common shape of sed in particle accelerators of the world, has the ad- the superconducting cavities vantages of low RF power consumption on the cav- The prototype shown in Fig. 1(a)is the newly de- ity wall and high cavity shunt impedance. However, veloped 1.5 GHz higher harmonic cavity for the SSRF to ensure stability of the storage ring, Higher Or Some copper prototypes were also der Modes(HOM) of the cavity have to be heavily made to study the RF modes'characteristics.The other one, Fig. 1(b)is an 800 MHz cavity. Previous measurements have been done at Tsinghua Univer sity, including HOM measurement, LHC type HOM coupler 2) and CWct type HOM coupler), etc 2 Field mapping system The measurement system is a computer aided au- omatic system based on a perturbation technique, which is employed to evaluate higher order mode and shunt impedance characteristics of RF cavities. A small object is suspended on a nylon thread down the central beam pipe of the cavity and is drawn along its Fig. 1. Two kinds of prototype cavities. entire length, perturbing the electromagnetic fields (a) The 1.5 GHz single cell copper harmonic cavity shape;(b)800 MHz aluminum cavity then calculated from the resultant shift in resonant frequency. Received 10 March 2008 Supported by Science and Technology Commission of Shanghai Municipality(026505027, 036505011) 1)E-mail: zhaozt@ ssrcac c 1012-1015Chinese Physics C (HEP & NP) Vol. 32, No. 12, Dec., 2008 Studies on high order mode of bell-shaped prototype cavities * MA Guang-Ming(ê2²) 1,2 ZHAO Zhen-Tang(ë,) 1;1) 1 (Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, CAS, Shanghai 201800, China) 2 (Graduate University of Chinese Academy of Sciences, Beijing 100049, China) Abstract Aluminium and copper prototype cavities were designed to study higher order modes (HOM). An automatic field mapping system was developed with LabVIEW to measure the radiofrequency (RF) charac￾teristics, such as resonant frequency, Q-value, shunt impedance and electromagnetic field distribution of the higher-order modes in a model RF cavity. Two kinds of the bell-shaped cavities were measured using the field mapping system, their frequencies are 1.5 GHz and 800 MHz respectively. The fields’ distributions of the monopole modes and dipole modes, as well the R/Q values, were measured. Key words perturbation method, higher order modes, LabVIEW, superconducting RF PACS 29.20.db, 29.27.Bd 1 Introduction The superconducting RF cavity, which is widely used in particle accelerators of the world, has the ad￾vantages of low RF power consumption on the cav￾ity wall and high cavity shunt impedance. However, to ensure stability of the storage ring, Higher Or￾der Modes (HOM) of the cavity have to be heavily damped. Fig. 1. Two kinds of prototype cavities. (a) The 1.5 GHz single cell copper harmonic cavity shape; (b) 800 MHz aluminum cavity shape. Here we set up a new system of HOM field map￾ping, R/Q measurement, etc. This system is used to study the bell-shaped cavity, the common shape of the superconducting cavities. The prototype shown in Fig. 1(a) is the newly de￾veloped 1.5 GHz higher harmonic cavity for the SSRF storage ring[1] . Some copper prototypes were also made to study the RF modes’ characteristics. The other one, Fig. 1(b) is an 800 MHz cavity. Previous measurements have been done at Tsinghua Univer￾sity, including HOM measurement, LHC type HOM coupler[2] and CWCT type HOM coupler[3] , etc. 2 Field mapping system The measurement system is a computer aided au￾tomatic system based on a perturbation technique, which is employed to evaluate higher order mode and shunt impedance characteristics of RF cavities. A small object is suspended on a nylon thread down the central beam pipe of the cavity and is drawn along its entire length, perturbing the electromagnetic fields. The longitudinal and transverse mode impedances are then calculated from the resultant shift in resonant frequency. Received 10 March 2008 * Supported by Science and Technology Commission of Shanghai Municipality (026505027, 036505011) 1)E-mail: zhaozt@ssrc.ac.cn 1012 — 1015