• T. Mastorides, J.D. Fox, C.H. Rivetta, D. Van Winkle
  SLAC, Menlo Park, California

The growth rates of the longitudinal higher-order impedance-driven modes beam have greatly increased since the initial PEP-II design and commissioning. This increase is attributed to the addition of 8 1.2 MW RF klystrons and 12 accelerating cavities, as well as to operations at twice the design current and almost four times the luminosity. As a result, the requirement for the longitudinal feedback has greatly increased since the design, and the feedback filters and control schemes have evolved during PEP-II operations. In this paper growth and damping rate data for the high-order cavity driven beam modes are presented and compared with historical estimates and measurements. The available margins for the 4 A LER and 2.2 HER ring case at PEP-II are estimated, and suggestions for reliable operation are also presented. The effect of noise in the feedback processing channel is also studied. Data for different gain configurations of the filter from PEP and other accelerators are presented. These data show the tradeoff between instability control and higher noise levels.

• T. Obina, J.W. Flanagan, T. Honda, T.M. Mitsuhashi, M. Tadano, M. Tobiyama
  KEK, Ibaraki
• W.X. Cheng, J.D. Fox
  SLAC, Menlo Park, California
• D. Teytelman
  Dimtel, San Jose

A bunch-by-bunch feedback system has been developed to suppress longitudinal coupled-bunch instabilities at the KEK-PF. A generalpurpose signal processor, called iGp, has been developed by a collaboration among KEK, SLAC and INFN-LNF. A longitudinal kicker based on the DAΦNE-type over-damped cavity was designed and installed in the ring. The whole feedback loop was closed at the end of June 2007. The longitudinal dipole-mode instabilities are successfully suppressed up to 430 mA. The performance and the details of the system will be presented in this paper.

• J. Xu, J.D. Fox, D. Van Winkle
  SLAC, Menlo Park, California

Bunched beam signals from button-style Beam-Position Monitor (BPM) electrodes can have spectral content up to 20-30 GHz and time-domain structure of narrow impulsive trains. Multi-bunch feedback systems require receivers to process such beam signals and generate ΔX, ΔY, and ΔZ beam motion signals. To realistically test these receivers, we have developed a 4-bunch programmable impulse generator, which mimics the signals from a multi-bunch beam. Based on step-recovering diode techniques, this simulator produces modulated 100-ps impulse signals. The programmable nature of the system allows us to mimic Betatron and Synchrotron signals from 4 independent bunches with adjustable beam spacing from 1 to 8 ns. Moreover, we can observe nonlinear effects and study the noise floor and the resolution of the receiver. This paper presents the design of the system and shows typical achieved results.

J. Xu, J.D. Fox, D. Van Winkle
Stanford Linear Accelerator Center
Stanford, CA 94309, U.S.A.