Damping Bunch Shape Oscillations in the Brookhaven AGS

Abstract
Bunch shape oscillations are present in the AGS both before and after the transition energy (¿8 BeV) is reached. At transition, they can be excited by timing, phase and radial misadjustments, and beyond by rapid large radial position changes used for targeting or by rapid large (> 20%) targeting losses. They can also be excited before transition by misadjusting the low level RF control system. In addition, excitation before transition can occur when 2fs = 720 Hz, the magnet ripple frequency, or when 2fs = 360 Hz, the ripple frequency on the RF cavity tuning servo systems. An analysis of the RF phase control systems including second order effectsl indicates that it should damp these oscillations but at a rate~fs/ß¿ except in the vicinity of transition. At 200 ms, E ¿ 6.5 BeV, ¿ ¿ 6.9, fs 200 Hz, and the damping rate is ¿ 0.46 s-1. Thus adequate inherent damping is present only early in the acceleration cycle. This, plus the fact that the sensitivity of the phase control electronics to bunch shape variations can at times produce antidamping, has led to the design and testing of various feedback damping systems to control these oscillations. An analysis of RF phase and amplitude feedback, using a peak detected signal, as applied to the RF control system, will be given. Operational results using both types of feedback will be described.

This publication has 0 references indexed in Scilit: