INTRODUCTION

Orbit stability and reproducibility of the electron beam at the location of the radiation source points is a crucial requirement at the Swiss Light Source. It is desirable to suppress the photon beam fluctuations by at least one order of magnitude below the spot size at the experiments. At the SLS this translates into electron beam angular stability along the insertion device straights below 1 $\mu$rad and into beam position stability below 1/10th of the vertical beam size which corresponds to $\approx$ 1 $\mu$m. A Slow Orbit Feedback (SOFB) working with less than 1 Hz correction rate is in operation since August 2001 and succeeded to stabilize the orbit in both planes to 1 $\mu$m RMS [2]. At the low beta short straight insertion devices the electron beam oscillations could even be reduced to a sub-micron level of $\sigma_{x/y} \approx 0.5~\mu$m. Measurements of the power spectral densities at the tune BPM have shown that the main contributions to orbit perturbations lie in a frequency range below 100 Hz (see Tab. 1).

Frequency	Noise	RMS contribution
	Source	horizontal	vertical
3 Hz	booster ramp	0.5 $\mu$m	0.35 $\mu$m
20-35 Hz	girder eigen	0.85 $\mu$m	1.4 $\mu$m
	frequencies
50 Hz	line frequency,	1.2 $\mu$m	0.45 $\mu$m
	vacuum pumps
	(async.)
85 Hz	unidentified		0.35 $\mu$m

Table 1: Main contributions to orbit oscillations at SLS without feedback measured at the location of the tune BPM with $\beta_x~\simeq~11$ m and $\beta_y~\simeq~18$ m. The quadratic sum of the contributions yields a RMS value of 1.6 $\mu$m horizontally and 1.55 $\mu$m vertically.

Integration up to 100 Hz results in beam motions of 0.5 ${\mu\mbox{m}}/{\sqrt{\mbox{m}}}$ horizontally and 0.4 ${\mu\mbox{m}}/{\sqrt{\mbox{m}}}$ vertically (normalized to the beta function) without any fast feedback. The average machine beta values for the SLS storage ring are $\beta_{x/y}$ $\approx$ 10 m at the locations of the BPMs. The residual beam motion caused by sources given in Tab. 1 as well as additional orbit perturbations introduced by an increasing number of insertion devices (ID) and experimental stations require stabilization by a fast orbit feedback.