IA Guang-Ming et al: Studies on high order mode of bell-shaped prototype cavities 1015 The problem of the lowest dipole modes is a surement are in Fig. 5. They are as expected, but lemma of the"single-mode" cavity. Because they do not exactly correspond to the calculation result don't propagate out the cell even the beam pipe's ra- The reasons are due to the bead's vibration during the dius is increased. Since the dipole modes may deflect measurement and the errors of positioning the bead. electrons off the synchronous orbit, it's an important The R/Q calculation method needs to be studied fur- Issue In the experiment, we found the two modes, each ne has two degenerate modes. Their electric fields d Er component along the axis are shown in Fig 4 800 MHz bell-shaped HOM These two modes cant propogate along the beam 60 pipe so that they can't be damped by the ferrite HOm calculation O measurement (higher order modes)damper. In order to damp these two modes, special treatment of the beam pipe need 」。。 to be applied, e. g. to enlarge the beam pipe like KeK B cavity, or to flute the beam pipe like CESR cavity 6 frequency/ GHz Fig. 5. Monopole mode results 3.2 800 MHz bell-shaped aluminum cavity We also performed a test of the 80 4 Conclusion and discussion aped aluminum cavity. The following steps were taken to measure the hom characteristics The field mapping system has been a 1)Clean the cavity parts using ethanol and ace- The results of the cavities tests demonstrate that ts main functions have beer 2)Make bead, i.e. perturbation object, and scale kinds of cavities are measured by using this system. the perturbation object's shape factor. The shape The system can be well used to identify the differ factor scaling is performed like this: we use Super- ent modes in the cavities. It roughly provides the Fish or MAFIA to calculate the fundamental mode's monopole modes'R/Q values. More work will be R/Q first, and then adjust the factor until the funda- done to improve the systen mental's measurement results are consistent with the calculation results The authors would like to thank Prof. TONG 3) Perform the "bead-pull "experiment, identify Dechun, as well as TAO Xiaokui, from Tsinghua Uni each mode. see Fig 3: versity for their enthusiastic help. The rF group col 4) Measure the mode's r/Q value. leagues of ssrf are also acknowledged for their sup- The results of 1* bell-shaped cavity R/Q mea- port References 3 LIANG Y Z. Research on HOM Coup monic Cavity. Master thesis, at El partment, Beijing: Tsinghua Univer 1 MA G, ZHAO Z, LIU J. Chinese Physics C, 2008, 32(4): 4 Maier L C, Jr, Slater J C. Journal of Applied Physics, 1952,23:68 2 TAO X K, CHEN H B, TONG D C. Study of HOM Cou- 5 XIAO L, LIANG Y Z, TONG D et al. NIM A, 2001, 463 plers on an 800 MHz Prototype Cavity. In: Proceedings of 8693 he Asian Particle Accelerator Conference. Beijing, China, 6 Padamsee, Barnes P et al. Particle Accelerators, 1992, 40 2001,746-748No. 12 MA Guang-Ming et alµStudies on high order mode of bell-shaped prototype cavities 1015 The problem of the lowest dipole modes is a dilemma of the “single-mode” cavity. Because they don’t propagate out the cell even the beam pipe’s ra￾dius is increased. Since the dipole modes may deflect electrons off the synchronous orbit, it’s an important issue. In the experiment, we found the two modes, each one has two degenerate modes. Their electric fields and Er component along the axis are shown in Fig. 4. These two modes can’t propogate along the beam pipe so that they can’t be damped by the ferrite HOM (higher order modes) damper. In order to damp these two modes, special treatment of the beam pipe need to be applied, e.g. to enlarge the beam pipe like KEK￾B cavity, or to flute the beam pipe like CESR cavity [6] . 3.2 800 MHz bell-shaped aluminum cavity We also performed a test of the 800 MHz bell￾shaped aluminum cavity. The following steps were taken to measure the HOM characteristics: 1) Clean the cavity parts using ethanol and ace￾tone; 2) Make bead, i.e. perturbation object, and scale the perturbation object’s shape factor. The shape factor scaling is performed like this: we use Super￾Fish or MAFIA to calculate the fundamental mode’s R/Q first, and then adjust the factor until the funda￾mental’s measurement results are consistent with the calculation results. 3) Perform the “bead-pull” experiment, identify each mode, see Fig. 3; 4) Measure the mode’s R/Q value. The results of 1# bell-shaped cavity R/Q mea￾surement are in Fig. 5. They are as expected, but do not exactly correspond to the calculation results. The reasons are due to the bead’s vibration during the measurement and the errors of positioning the bead. The R/Q calculation method needs to be studied fur￾ther. Fig. 5. Monopole mode results. 4 Conclusion and discussion The field mapping system has been developed. The results of the cavities tests demonstrate that its main functions have been realized. Two different kinds of cavities are measured by using this system. The system can be well used to identify the differ￾ent modes in the cavities. It roughly provides the monopole modes’ R/Q values. More work will be done to improve the system. The authors would like to thank Prof. TONG Dechun, as well as TAO Xiaokui, from Tsinghua Uni￾versity for their enthusiastic help. The RF group col￾leagues of SSRF are also acknowledged for their sup￾port. References 1 MA G, ZHAO Z, LIU J. Chinese Physics C, 2008, 32(4): 275 2 TAO X K, CHEN H B, TONG D C. Study of HOM Cou￾plers on an 800 MHz Prototype Cavity. In: Proceedings of the Asian Particle Accelerator Conference. Beijing, China, 2001, 746—748 3 LIANG Y Z. Research on HOM Coupler for Higher Har￾monic Cavity. Master thesis, at Engineering Physics De￾partment, Beijing: Tsinghua University, 2003 4 Maier L C, Jr., Slater J C. Journal of Applied Physics, 1952, 23: 68 5 XIAO L, LIANG Y Z, TONG D et al. NIM A, 2001, 463: 86—93 6 Padamsee, Barnes P et al. Particle Accelerators, 1992, 40: 17