A horizontal and vertical bunch by bunch system has also been developed. The interaction of the beam with the vacuum chamber can cause unstable oscillations of the beam in the vertical or horizontal plane. These instabilities are currently avoided by tuning the sextupoles to obtain a positive value of the machine chromaticity. However, large chromaticity values have detrimental effects on beam lifetime and the injection efficiency, so it is desirable to use a transverse feedback system to allow operation with a reduced value for the chromaticity. The effect of the spurious oscillations of the beam caused by the imperfect closure of the injection bump will also be reduced by the horizontal feedback, which is also beneficial for operation with frequent topping up. These transverse feedback sytems are based on the scheme shown in Figure 149. We made initial tests of the vertical feedback system with encouraging results and have already obtained a damping time of the vertical oscillations of 100 µs (or 30 turns) instead of the natural betatron damping time of 7 ms. This fast damping is obtained without spoiling the vertical emittance, showing that the noise of the electronics detecting the position error signal is low. We tested a first application of the vertical feedback on a 200 mA beam stored with uniform filling. With such a filling pattern the emittance is usually spoiled by small vertical oscillations due to the interaction of the beam with positive ions trapped in the vicinity of the beam orbit. The application of the feedback reduced the vertical emittance from 27 pm to 21 pm.


Fig. 149: Bunch-by-bunch feedback system for the damping of coupled bunch beam instabilities (from: www.i-tech.si).