LOCAL MODIFICATION OF LATTICE OF A LONG STRAIGHT SECTION FOR INSTALLING SMALL GAP IN-VACUUM UNDULATORS AT SPring-8

In the SPring-8 storage ring there are four magnet-free long straight sections (LSS’s) of about 30m. Recently we locally modified one of these sections by installing two sets of the quadrupole-triplet and divided it into three subsections. The vertical betatron function at the middle of each sub-section was lowered to 2.5m so that small gap in-vacuum undulators with a short period can be installed. By this lattice modification the symmetry of the ring was lowered but we could keep sufficient dynamic aperture and momentum acceptance by taking optics matching into account and rearranging sextupole magnets. The beam commissioning of the new lattice has successfully beenfinished and from September 2011 it is used in user operation. We review our method of realizing a storage ring lattice having a very low symmetry and report the beam performance of the modified lattice.