Swiss Light Source upgrade lattice design

Following the spectacular success of the third-generation light source over the past decades, a few new-generation light sources based on the multibend achromat (MBA) scheme have come into operation. Other such facilities are also under construction while existing ones are being upgraded. Likewise, the Swiss Light Source, which has been in operation for more than 20 years at the Paul Scherrer Institute, is to be upgraded with the present storage ring being replaced by an MBA low-emittance ring. A natural emittance of 158 pm at a beam energy of 2.7 GeV is achieved for a storage ring of comparatively short circumference, 288 m, providing a gross straight section of 83.6 m. The objectives of the upgrade were met under tight constraints by the application of two novel concepts: the introduction of both a longitudinal gradient and a reverse bend into the unit cell and pushing the superperiodicity to the number of arcs, effectively with ``pseudosymmetry.'' A detailed account of the MBA design and its expected performance is presented and the experience gained is highlighted with the aim to facilitate next-generation light source lattice design.