Some aspects of the design study on a fourth-generation-like VUV/SXR light source

摘要

Some aspects of the design study on a fourth-generation-like vacuum ultraviolet/soft X-ray (VUV/SXR) light source are summarized. A storage ring lattice adopting a seven-bend achromat (SBA) structure is presented. High brightness and coherence of SR light are emphasized. The natural electron beam emittance reaches as low as 0.25 nm.rad at 2 GeV. The ring circumference is 473.68 m. Eight 7-m long and four 18-m long dispersion-free straights are available for installation of insertion devices (IDs) and other kinds of equipment. Sources based on long undulators placed in long straights would be able to provide SR spectra with a photon beam brightness up to 10~(21) photons/s/mrad~2/mm~2/0.1%BW. The Touschek lifetime, which is reputed as one of the most crucial issues for design of such low energy (2-4 GeV), low emittance (in the 10~(-10) m.rad range) light sources, has been studied by taking into account the bunch lengthening and intra-beam scattering effects. We find that at lower energies (around 2 GeV) both the bunch length and momentum spread increase by a factor of 2.7 due to the longitudinal microwave instability, and the emittance blows up by factors of 2.7-4.4, corresponding to coupling ratios of 10%-1%, caused by the intra-beam scattering; however, at the higher energies (around 4 GeV), there are no remarkable changes for these parameters (an average bunch current of 1 mA is assumed throughout the calculation). Thus, the energy dependence of the Touschek lifetime is modified correspondingly, and the Touschek lifetime issue becomes less serious at the cost of the bunch lengthening and widening and the emittance blowup at the low energy range.