Reverse-bend FODO lattice applied to damping ring for SuperKEKB

SuperKEKB, which is a plan for upgrading KEKB to a higher luminosity of 5×1035cm2s-1, requires a beam currents of 9.4 A for LER (3.5 GeV-electrons) and 4.1 A for HER (8 GeV-positrons). In order to supply HER with a positron beam, which is currently injected to LER, the field gradient of the injector linac has to be increased. To meet this requirement, the S-band accelerating structures in the linac are to be replaced with C-band structures. A damping ring (DR) is indispensable, since the aperture of the C-band structure is much smaller than the beam emittance. In this paper, we describe the lattice design of DR. We adopt a new cell structure: FODO cell with alternating bends, where one of two bends in a cell is reversed. One of advantages of the proposed lattice is that a very small, even negative, momentum compaction factor is easily achieved by properly choosing the bend-angle ratio of the reverse bend to the main bend. A tracking simulation for the proposed DR has shown that it has a very large dynamic aperture in both the transverse and longitudinal phase space, for a very wide tune space.