Beam commissioning and achieved performance of SPring-8 storage ring phase-2 lattice with four magnet-free long straight sections

We had installed four magnet-free long straight sections (LSSs) of 27 m in the SPring-8 storage ring in the summer 2000 in spite that a lattice modification breaks high symmetry of the ring optics. This is because by the potentiality of the LSSs we can not only increase the brilliance of photon beams but also develop innovative light sources. The most serious problem for introducing the LSSs was that the lattice modification markedly deteriorates the stability of electron beams at a large amplitude or a large momentum deviation. To overcome this, a concept of “Quasi-Transparent Matching of sextupole fields” was proposed. This concept simultaneously manages both the symmetry restoration of sextupole fields and the local compensation of chromaticities. Owing to the reasonable lattice design and to the preparation with the precise ring model, the beam commissioning was successfully accomplished for about 3 weeks, achieving the beam performance which almost equals to that in the phase-1 lattice without the LSSs. A long undulator was also installed and it has been used in routine user operation. This success is the first step to challenge the new ideas growing up into future synchrotron radiation sources in the SPring-8 storage ring.