Low emittance lattice for the Duke storage ring soft X-ray FEL

In this paper, we present a possible lattice for the Duke storage ring with horizontal emittance of 1.4nm rad at an energy of 1 GeV. The new lattice was constrained to fit the layout of the existing storage ring and to re-use existing magnets. Within these constrains, the improvement of the emittance (from current 18nm rad) is possible due to the use in the arcs of combined function bending magnets with strong dipole, quadrupole and sextuple fields. We present the results of 2D and 3D simulations of magnetic fields for these magnets. We discuss the choice of the arc's lattice cell and the per-cell tune advance. The lattice is based on the concept of local compensation of the non-linear geometrical aberrations. Preliminary studies of the dynamic aperture for this lattice are very encouraging. We briefly discuss the possibility further emittance reduction using new magnets for the arcs.