Challenging issues during ESRF storage ring commissioning

Abstract

Third-generation synchrotron light sources raise numerous challenges associated with the low emittance requirement. The 6 GeV ESRF storage ring is among the first of these new sources to successfully meet these challenges. This paper will emphazise some of the most significant outcomes achieved during the ten month commissioning. The experimental approach to obtain the large dynamic aperture needed to allow the injection-accumulation process and a long lifetime will be presented. The upgrading of the predicted current thresholds set by multibunch instabilities and transverse mode coupling will be discussed. We will as well report on photon beam stability which is one of the most stringent requirements for the high brilliance light sources. The ESRF storage ring is a 6 GeV low emittance ring based on a Chasman-Green type lattice. The low emittance is being achieved by low p, in a large number of dipoles, thus implying a very strong focusing and a large natural chromaticity. The need for strong sextupoles makes the machine highly sensitive to errors. A perfect correction of the orbit is a prerequisite for the obtention of a large dynamic aperture. The first injection trials into the storage ring started on February 17*, 1992. Progress was very successful since all target performances were achieved and even exceeded before the end of the ten months of scheduled commissioning [l], [2]. The issues of adequate dynamic aperture, beam instabilities, X-ray beam stability associated with the running-in of thB third genaration light source are discussed. 2. OBTENTION OF A LARGE DYNAMIC APERTURE The primary challenge in the early stage of commissioning arose directly from the extreme sensitivity of the lattice to imperfections due to the requirement for low emittance. Magnet positioning errors are greatly magnified by the beam in such a high focusing lattice, with amplification factors of 50 (horizontal) and 100 (vertical). Although the tolerance on quadrupole and sextupole alignment had been set to 1/10 mm, the probability of getting the beam transmitted over the entire circumference from the outset was very small. 2.1 First turns It was decided to immediately test the low emittance optics of the storage ring, and not a detuned version, since we believed that running a detuned version in order to reduce the sensitivity to errors would not bring any valuable information. The strategy used during the turns process was to inject the beam on-axis, with no sextupoles and no RF. The achievement of the first turns circulating in the ring as soon as the injection parameters were set proves that this strategy was very effective. The maximum amplitudes of the beam trajectory measured by the Beam Position Monitor (BPM) system operated in the first turn mode was 15 mm