The Fast Orbit Feedback (FOFB)

In the SLS storage ring a properly chosen BPM/corrector layout leads to an ``inverted'' response matrix where only the diagonal and their adjacent coefficients have non zero values. Thus, corrector settings are only determined by position readings from nearby BPMs [8]. This feature allows to run the FOFB decentralized, integrated in the 12 BPM stations of the storage ring where each of the stations handles 6 BPMs and correctors. The BPM data are transmitted over fiber optic links between adjacent stations. In order to provide well defined starting conditions for the FOFB the SOFB corrects the orbit to $<$ 5 $\mu$m rms. The FOFB gets initialized and started through the SOFB which downloads the ``Golden Orbit'', 6$\times$6 sub-matrices of the ``inverted'' 72$\times$72 response matrix, 1$\times$6 sub-matrices of the ``inverted'' 72$\times$1 off-energy response matrix and the FOFB loop PID parameters to the BPM stations (see Fig. 3). The SOFB continues to run in a ``watchdog'' like passive mode without touching any corrector other than the RF frequency. But it keeps monitoring BPM and corrector values. Whenever a BPM is switched off, declared faulty or a corrector is close to saturation the FOFB is stopped and restarted with adapted settings. The off-energy content of the horizontal orbit is taken into account by the FOFB but corrected by the SOFB.