In 2006, bunch by bunch feedback systems were installed and commissioning was begun. These feedback systems operate in each of the three orthogonal directions (longitudinal, horizontal and vertical). The feedback loop monitors the position selectively bunch by bunch (from any one of the maximum 992 bunches present along the ring circumference), then computes and applies a kick to the corresponding bunch in order to stabilise its motion. The loop makes use of fast digital electronics based on field programmable gate arrays (FPGA).

The goal of the longitudinal feedback system is to dampen the instability driven by the beam in the higher order mode (HOM) of the radio frequency cavities, above 200 mA. The first successful test occurred in December 2006 and testing will continue during 2008. The horizontal and vertical feedback systems allow the damping of resistive wall as well as ion driven instabilities which are at their most severe in uniform filling mode at high current.

In the future, it is planned to raise the current from 200 to 300 mA in uniform or 7/8 +1 filling modes only. In such filling modes, the ion induced instability may severely increase the vertical emittance. The associated beam blow-up can be significantly reduced by an increase in the vertical chromaticity of the ring magnet lattice, however the latter has a detrimental effect on the lifetime. The vertical bunch by bunch feedback system is an alternative means to combat the ion induced instability.

The efficiency of both horizontal and vertical bunch by bunch feedback systems has been demonstrated through several experiments where the feedback systems were proven effective in combating various instabilities. It was first used to store a 200 mA ring current in uniform filling mode with zero horizontal and vertical chromaticities and unchanged horizontal and vertical emittances. Later, during tests at 300 mA there was evidence that the feedback systems efficiently damped the ion induced instability. In this test, the beam required the vertical feedback to keep a low vertical size to avoid triggering a vacuum chamber protection interlock.

In November and December 2007, the vertical feedback was operated regularly in user time (USM) in uniform filling mode. Figure 145 presents the image of the beam as seen through a pinhole camera without (left hand-side) and with (right hand-side) feedback. The effect of the feedback systems is particularly noticeable immediately after the shutdown during which the opening of a number of vacuum vessels to atmospheric pressure results in poor vacuum and a strong influence from ions.

Fig. 145: The electron beam in a bending magnet afforded by a pinhole camera system. a) No vertical feedback; b) Vertical feedback system in use.


From these experiments, it appears that both the increase in chromaticity and the transverse feedback are complementary methods to ensure the stabilisation of the beam at high ring current. A precise study of the advantages and drawbacks of each technique will be undertaken in the year to come.