LINEAR AND NONLINEAR OPTIMIZATIONS OF COMBINED 7BA-6BA LATTICES FOR THE FUTURE UPGRADE OF SOLEIL A. Loulergue, H.-C. Chao, P. Brunelle, L. S. Nadolski, M.-A. Tordeux, R. Nagaoka, A. Nadji, Synchrotron SOLEIL, Gif-sur-Yvette, France Abstract Previous MBA studies converged to a combination of 7BA and 6BA structures, with the target horizontal emittance in the range below 300 pm.rad, where the effect of anti-bends, dipole field values, and straight section lengths were investigated. Inspired by the successful lattice designs adopting the interleaved sextupole scheme with dispersion bumps originally developed at the ESRF, the 7BA-6BA structures adopting this scheme are studied in details in parallel to those without it. The former aims at the horizontal emittance in the 200-300 pm.rad range with on- and off-momentum dynamic acceptances sufficiently large for off-axis injection and good Touschek lifetime. The latter pursues the lower bound of the reachable horizontal emittance with quadrupole and sextupole strengths in the feasible range with maximum dynamic acceptance. The option of non-standard on-axis injection such as displacing the injected beam longitudinally is envisaged for the latter solutions. INTRODUCTION SOLEIL is the French third generation light source routinely operated for users since 2007 with a low electron beam emittance of 4 nm.rad at an energy of 2.75 GeV in high intensity multibunch (up to 500 mA) and temporal structure (e.g. 1 and 8 bunches) modes. After nearly 11 years of successful operation, a series of feasibility study is launched towards a possible future upgrade of the lattice with a significantly lower emittance. The approach taken is to employ whatever useful methods in lowering the emittance by fully respecting the geometric constraints such as the circumference of the ring and the available straight sections, so as not to impact the existing insertion device based beamlines. SOLUTIONS ALLOWING OFF-AXIS INJECTION The SOLEIL synchrotron storage ring is a fourfold symmetry lattice composed 16 double bend (DB) cells. It provides four long straight sections (SDL) of 12 m and twelve medium sections (SDM) of 7 m. In addition, a set of eight short sections (SDC) of 3.8 m have been inserted in half of the DB cells. Since 2012, one of the SDL has been split into two low vertical beta sections for two long canted beamlines by means of an additional quadrupole triplet [1]. To reach a much lower emittance, the number of dipoles must be increased. A first combination of 5BA- 4BA cells [2, 3] giving an emittance of 440 pm.rad including longitudinal gradient bends (LGB) was studied. The nonlinear beam dynamics analysis [4], although not fully optimized, exhibited a rather limited on-momentum acceptance for off-axis injection. In the same philosophy, a 7BA-6BA lattice was studied giving a much lower emittance of 160 pm.rad including LGB [5]. In this latter case, the dynamic acceptance is reduced even further more. Progressing further in the investigation of much lower emittance lattices having a large enough dynamic acceptance compatible with off-axis injection, a combination of hybrid 7BA-6BA cells based on the ESRF-EBS design [6] has been studied. As conceived elsewhere, removing the central bend of a 7BA cell, allows the 6BA cells to host the SDC [7]. The paring of the sextupoles in the dispersion bump to odd multiples of  (close to 3 in H and  in V) is very effective in compensating the driving terms as well as producing large on-momentum dynamic apertures. The optical functions producing an emittance of 220 pm.rad are plotted in Figure 1. This lattice keeps the present symmetry of the ring, but with shorter straights (sees Table 1). The circumference ratio dedicated to straights is noticeably reduced from 45 % to 32 %. Table 1: Straight Section Length Comparison Actual Upgrade Emittance (2.75 GeV) 4 nm.rad 220 pm.rad Circumference 354.1 m 354.1 m Number of cells 16 16 Long straight (SDL) 12 m 9 m Medium straight (SDM) 7 m 5 m Short straight (SDC) 3.8 m 2.8 m Circumference ratio dedicated to straights 45 % 32 % The dipole fields are rather low with 0.6 T combined with a transverse gradient of 30 T/m. The maximum gradient of quadrupoles reaches 70 T/m and the three sextupole family gradients are limited to 700 T/m 2 over 200 mm bore length. The beam dynamics optimization (by means of simple fits) on each of the individual cells (7BA or 6BA) with only 3 sextupoles families exhibits on-momentum horizontal aperture of ±15 mm (@x=8 m) together with an off-momentum acceptance of ±4%. The use of anti-bend [8] in the dispersion bump region improves the off-momentum optimization. Once merged together to build the full super-cell lattice, the on- momentum acceptance is reduced to about ±12 mm (Fig. Proceedings of IPAC2017, Copenhagen, Denmark MOPIK059 05 Beam Dynamics and Electromagnetic Fields D01 Beam Optics - Lattices, Correction Schemes, Transport ISBN 978-3-95450-182-3 659 Copyright © 2017 CC-BY-3.0 and by the respective authors