Lattice design of a quasi-isochronous ring for a storage-ring FEL

Design work for a Quasi-Isochronous Ring (QI-Ring) dedicated to Storage Ring FELs in Electrotechnical Laboratory has been completed. The motivation for this work is to shorten the electron bunch length in order to get a high peak current in a compact Storage-Ring (SR). By placing an inverted dipole field in a location where the energy dispersion function is relatively large, one can reduce the momentum compaction factor (α) and shorten a bunch length in a SR. The main requirements for the QI-Ring are: 1.5GeV maximum beam energy; 80m circumference; two 10m-long dispersion free straight sections for insertion devices. A few meters dispersion free straight sections for RF cavities and injection bumpers; and a wide tune ability in betatron functions and momentum compaction factor (α). As shown in figure 1, the lattice includes two 49 degree, 3 T superconducting bending magnets to reduce the circumference of the ring, a -8 degree normal inverted dipole magnet (ID), 4 families quadrupole magnets (QF, QD, QFA, QDA), and 3 families sextupole magnets. Each quadrupole family has a specific function: QF ampersand QD control the betatron tunes, and QFA ampersand QDA control the α and suppress the energy dispersion in a straight section. In this type of ring it is important to compensate the second order momentum compaction factor (α2), so at least three families of sextupoles are required