Damping Coherent Oscillations in the AGS

In order to further study the vertical instability in the Brookhaven AGS a narrow band feedback damping system has been developed. The damping force is obtained from a pair of 6 ft long coils located in one of the straight sections. The damping signal is provided by a pair of pickup electrodes at an angle ¿ = 10.7° upstream from the coils. Since ¿V ¿ 8.8, ¿v ¿ ¿ ¿/2. The damping is proportional to sin [n¿ - (n-¿) 2¿ foT] where n is the mode number, fo the rotation frequency and T the time delay between the pickup and damping elements. The system is designed to work over a range in fo from 200 kc to 370 kc. In this range, with T = .14 ¿sec, and for n = 7-11, the angle in brackets varies ± 19° from ¿/2 if ¿ changes about ± 1%. With a simple filter in the feedback loop it is possible to provide damping for the lower order modes n = 9,8,10, without exciting those for n = 7,11 etc. under normal operating conditions.