Preliminary Change Request for the Chromatic Sextupoles and Trim Quadrupole Elements of the SNS Accumulator Ring

In view of the adopted modi ed design for the 1.3 GeV compatible accumulator ring of the SNS [1], complementary studies have been undertaken, in order to justify the need of higheld chromatic sextupoles for chromaticity correction. Furthermore, as these chromatic sextupoles introduce a rst order quadrupole \feed-down" e ect, the trim quadrupole elements speci cations had to be reviewed in order to accommodate this change. 1 The SNS accumulator ring lattice The SNS accumulator ring has a hybrid lattice design consisting of four identical arcs and straight sections. The arc has a FODO structure, with four cells (eight quadrupoles and dipoles) plus a quadrupole that matches the arcs with the straight sections. The later contains two quadrupole doublets, setting the total number of quadrupoles at fty-two. The magnetic elements are placed and powered in a way to preserve a four-fold symmetry. A schematic presentation of one super-period is presented in Fig. 1. The lattice has been matched for four working points: (6.3,5.8), (6.3,5.27), (5.82,4.8) and (5.82,5.8). The optical functions for the nominal working point (6.3,5.8) are shown in Fig. 2. In the proposed lattice design, ve chromatic sextupoles are placed downstream to the arc quadrupoles, at areas of large dispersion (Figs. 1 and 2). These \strong" sextupoles have been foreseen for adjusting the chromaticities to desired values, minimizing o -momentum optical mismatch and control resonance e ects and instabilities [2]. large quadrupole large narrow quadrupole dipole dipole corrector multipole corrector S.S.: doublets regular sextupole arc straight section Four-fold symmetry Arc: four FODO cells regular quadrupole large sextupole Figure 1: Schematic layout showing dipole, quadrupole, sextupole, and corrector magnets of one lattice super-period. The chromatic sextupoles are shown in green. In order to correct any type of quadrupole perturbation and to allow a robust tuning of the SNS ring, trim quadrupole windings will be mounted in the bodies of the lattice quadrupoles. A detailed study for the evaluation of this e ect was performed for the original all FODO-type