Evolution of the SpallationNeutron Source Ring Lattice l J. Wei*, N. Catalan-Lasheras*, A. Fedotov*, CJ. Gardner*, Y.Y. Lee*, Y. Papaphilippou*, D. Raparia*, N. Tsoupas* and J. Holmes 1 ^ *Brookhaven National Laboratory, Upton, NY 11973, USA ^ Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA Abstract. Requirements of minimum beam loss for hand-on maintenance and flexibility for future operations are essential for the lattice design of the Spallation Neutron Source (SNS) accumulator ring. During the past seven years, the lattice has evolved from an all-FODO to a FODO/doublet hybrid, the circumference has been increased to accommodate for a higher energy foreseen with a super-conducting RF linac, and the layout has evolved from an a- to an Q- geometry. Extensive studies are performed to determine working points that accommodate injection painting and minimize beam losses due to space charge and resonances. In this paper, we review the evolution of the SNS ring lattice and discuss the rationales. TABLE 1. Major lattice parameters of the SNS ring. Quantity Circumference Kinetic energy Repetition rate Number of protons per pulse Ring dipole field Unnormalized full emittance, H+V Betatron acceptance Momentum acceptance (full beam) Number of super-periods Nominal tunes v*, Vy Transition energy, J T Maximum fi x , Py Maximum ft, jfry in arc Maximum ratio Pmax/P min (H, V) Maximum dispersion D x Natural chromaticity (^, % y ) Value 248.0 1 60 1.6 0.792 240 480 ±2 4 6.23, 6.20 5.3 27.9, 15.7 12.9, 13.3 11.6,7.5 4.0 -7.9, -6.9 Unit m GeV Hz 10 14 T TTjUm TTjUm % m m m m movable fixed scraper collimators INTRODUCTION The SNS project is designed to reach an average beam power above 1.4 MW for pulsed neutron production [1,2]. The accumulator ring, operating at a fixed energy of 1 GeV, compresses at 60 Hz repetition rate 1 ms beam pulses containing 1.6 x 10 14 protons to 650 ns bunches for delivery onto the target. Table 1 lists major lattice parameters of the ring. DESIGN EVOLUTION As shown in Fig. 1, the ring presently has a four- fold lattice symmetry containing four dispersion-free 1 SNS is managed by UT-Battelle, LLC, under contract DE-AC05- OOOR22725 for the U.S. Department of Energy. SNS is a partnership of six national laboratories: Argonne, Brookhaven, Jefferson, Lawrence Berkeley, Los Alamos, and Oak Ridge. injection septum.!] & bumps extraction kickers extraction septum RF s / instrumentation FIGURE 1. Functions of the SNS accumulator ring. straight sections, each housing injection, collimation, radio-frequency (RF) system, and extraction [3]. Layout At pre-construction stage, the lattice's four-fold sym- metry [4] was chosen against a three-fold symmetry [5] for its separate, dedicated function of each straight sec- tion, reduced maximum dispersion, and fewer structure resonances. Since the start of construction in 1999, sev- eral iterations have been made to the general layout of the ring. The early a-geometry (Fig. 2) was later replaced by the Q-geometry to avoid the crossing of injection and extraction lines for easy tunnel access and maintenance. The circumference of the ring was also increased from 221 m to 248 m to accommodate the potential increase of injection energy up to 1.3 GeV (longer injection-chicane magnets with lower field to avoid H~ stripping), which is made possible by the adoption of superconducting RF linac, and to reduce beam density and foil traversals. CP642, High Intensity and High Brightness Hadron Beams: 20 th ICFA Advanced Beam Dynamics Workshop on High Intensity and High Brightness Hadron Beams, edited by W. Chou, Y. Mori, D. Neuffer, and J.-F. Ostiguy © 2002 American Institute of Physics 0-7354-0097-0/02/$ 19.00 157 injection septum & bumps movable collimators fixed beam beam gap kicker RF instrumentation scraper extraction septum extraction kickers