USER OPERATION

The Fast Orbit Feedback (FOFB) is running in user operation since November 2003. The overall reliability of hardware and software has been the main focus during the first phase of operation. Therefore, the FOFB has been operated at moderate PID loop gains and DBPM filter settings, providing a regulation bandwidth of $\approx$60 Hz [5]. In order to avoid beam loss due to possibly malfunctioning FOFB components, a low level software security package has been implemented which monitors the parameters of the FOFB and the performance of its subsystems. If corrector magnets exceed predefined dead bands after initialization by a SOFB based orbit correction to RMS difference orbits of $<$5 $\mu$m, the FOFB is automatically stopped and resumed only on operator intervention. The FOFB has shown an overall availability of $\approx$98%. Although some interruptions of the FOFB have been caused by malfunctioning DBPM electronics ($\approx$1 failure/month), corrector magnet power supplies and the control system including the operator (1$-$3 failures/month), a growing number of FOFB interruptions is caused by users which perform beamline commissioning work during user operation.

Frequencies up to $\approx$100 Hz in both transverse planes are attenuated by the FOFB in its present mode of operation [6]. Although the overall loop delay of $\approx$1.5 ms could not be reduced by a recent DBPM firmware upgrade, the global synchronization of the DBPM electronics over the 12 sectors allowed the use of a more stringent set of PID parameters, which finally led to a operation mode of the FOFB close to its design parameters [3]. In this mode orbit perturbations up to 95 Hz (0 dB point) are attenuated , while noise sources between 95 Hz and 300 Hz are moderately excited. Damping factors of $\approx$100 at 2 Hz (most critical for ID gap induced motion) and $\approx$4$-$5 at 50 Hz have been achieved. It should be mentioned that the orbit excitation spectrum in the frequency range 80$-$100 Hz does not contain any dominant lines. A remaining problem are intermittent ``RF glitches'' of $\approx$30 ms length which have been observed on RF frequency changes introduced via the IEEE interface of the RF generator. The resulting off-energy orbits of $\approx$300 $\mu$m at DBPMs with large horizontal dispersion are picked up by the FOFB. Fortunately corrections occur rarely ($<$1/h) whenever the SOFB predicted frequency change exceeds 5 Hz.

Table 1: Integrated beam position temporal RMS values with FOFB off and on at the tune BPM normalized to the beta function $\beta _{x/y}\approx$12/17 m for fixed ID gaps.

	horizontal		vertical
FOFB	off	on	off	on
1$-$100 Hz	0.83 $\mu$m	0.38 $\mu$m	0.40 $\mu$m	0.27 $\mu$m
100$-$150 Hz	0.08 $\mu$m	0.17 $\mu$m	0.06 $\mu$m	0.11 $\mu$m
1$-$150 Hz	0.83 $\mu$m	0.41 $\mu$m	0.41 $\mu$m	0.29 $\mu$m

Tab. 1 summarizes the improvements on beam stability at the SLS with the FOFB running compared to the situation without feedback. The values still contain the noise contribution of the DBPM system, which has been measured to be $<$0.13 $\mu$m within the FOFB bandwidth. Orbit distortions caused by ID gap changes, which lead to severe orbit perturbations when the FOFB is not active, are not included. The temporal RMS values are integrated over the active range of the FOFB (up to 100 Hz) and up to 150 Hz, where significant noise contributions could still be observed. The RMS values at a location $s$ in the storage ring are obtained from the table by multiplication with $\sqrt{\beta_{x/y}(s)}$. At the source points of the low gap IDs in the short straight sections with $\beta_y\approx$0.9 m this translates to vertical RMS values of $\sigma_y$ = 0.25 $\mu$m integrated up to 100 Hz. Apart from the improved integrated beam stability up to 100 Hz, the FOFB allows autonomous changes of ID gaps as well as transparent optimization of all beamlines at the same time since the effect of rapidly moving ID gaps and correctors in the vicinity of these IDs has been proven to be invisible to the other beamlines if the FOFB is running. Since the FOFB uses the SVD-inverted 72x72 corrector/BPM response matrix without any eigenvalue cut-off for correction, it can follow ``arbitrary'' reference orbit changes within the limits of the available corrector strength. In this case orbit steering for a beamline translates to a change of the orbit references of two DBPMs upstream and downstream of the ID. Due to the large FOFB damping factors at low frequencies operator induced reference modifications appear to be established instantaneously. It also allows the simple implementation of slow (up to a few Hz) XBPM based feedbacks which are discussed in the following section.