Nsls2 Transverse Feedback System Design

A diffraction-limited storage ring like NSLS2 requires stringent beam stability. Even with careful design of vacuum component to limit the geometric impedance, due to resistive wall impedance and fast-ion effect, transverse instabilities will happen at low current (~15mA). An active transverse feedback system has been designed to cure the coupled bunch instability. The system will have less than 200 micron seconds damping time at 500mA to suppress the fast-ion instabilities, which is severe in vertical plane due to small beam size. INTRODUCTION Bunch-by-bunch feedback systems have been proved effective to cure the coupled bunch instabilities. Third generation light sources and particle colliders need to run at high beam current to get the desired brightness and luminosity. Even with careful design of the vacuum chamber and other methods to cure the instabilities, there may be residual betatron oscillation or synchrotron oscillation. An active feedback system is straight forward to achieve the design current and required stability. Those methods to increase the tune spread by introduce nonlinear field (like Chromaticity or Octupole magnets) may limit the dynamic aperture, which is not suitable especially at top-off operation. Active feedback has no side effect on the dynamic aperture and typically helps to suppress the injection transient. Feedback systems are generally composed of three parts: RF front end to detect the transverse position error signal or longitudinal phase information; digitizer to sample and filter the error signals, as well as feedback gain control, phase adjustment etc; corrector module including high power amplifier and kickers. Shunt impedance of the kicker should be sufficient to kick the beam effectively with reasonable power. With the recent development of digital technology, especially the FPGAs (Field Programmable Gate Array), there is no reason to select digital feedback processors other than FPGA based. In a digital feedback loop, one can easily get rid of the DC offset as well as revolution harmonics by using digital filters. The digital delay adjustment avoids the use of long cables to do one-turn delay. The feedback digitizer itself is a powerful diagnostic tool with large memory to store the bunch-by-bunch position/phase information. Bunch oscillation and unstable modes can be revealed by transient measurements. The digitizer can be used to measure the tune of each individual bunch, or to clean unwanted bunches. Treating the bunch as a rigid particle, the central motion of beam position/phase can be written as damped harmonic oscillation with external force. Eq (1) gives the oscillations in longitudinal plane, transverse plane is similar.