LATTICE DESIGN AND BEAM LIFETIME STUDY FOR HLS STORAGE RING UPGRADE PROJECT

HLS (Hefei Light Source) is a dedicated synchrotron radiation research facility, whose emittance is relatively large. In order to improve performance of the machine, especially getting higher brilliance synchrotron radiation and increasing the number of straight sections for insertion devices, an upgrade project is on going. A new low emittance lattice, which keeps the circumference of the ring no changing, has been studied and presented in this paper. For the upgrade project, a new ring will be installed on current ground settlement of HLS and all of the magnets will be reconstructed. After optimization, two operation modes have been chosen for different users. Nonlinear dynamics shows that dynamic aperture for onmomentum and off-momentum particle is large enough. Beam lifetime has also been studied. Calculation results proves that expected beam lifetime about 8.5 hours can be obtained with a fourth harmonic cavity operation. INTRODUCTION HLS is a typical 2nd generation synchrotron radiation light source. A 200MeV electron linear accelerator, an 800MeV electron storage ring and a beam transport line are the main component parts about the machine. Phase I project of HLS was finished in 1989 [1], which marks that the first dedicated synchrotron radiation light source has been fulfilled in china. From 1999 to end of 2004, Phase II Upgrade Project [2] of HLS was carried out in order to improve the machine reliability and install more beamlines for users. For the operating machine, beam emittance of the lattice is 160nm⋅rad. The magnet lattice is a TBA structure with 4 super periods and transverse tunes are 3.54 and 2.60 respectively, which means only 4 straight sections can be used to install insertion devices or injection system. With the current situation of HLS, an upgrade project was brought forward one year ago, which main purpose is to improve the machine’s performance. Comparing with 2nd generation synchrotron light sources in the world, there are three considering aspects for this project. First, a new lattice should be found to reduce beam emittance to get higher brilliance synchrotron radiation for users. Second, more straight sections are needed for installing wiggler or undulators. Third, beam orbit stability should be improved to guarantee the radiation’s quality. Some systems such as magnet system will be rebuilt to meet the project requirement. Considering time limitation of the project, the new ring will be installed on current ground settlement of HLS, which means circumference of the ring will keep 66.13m as current situation. LINEAR OPTICS Several lattices have been studied over the past years. As work progressed, it became clear that DBA lattice could meet the emittance purpose while providing an increased number of insertion device straight sections. In this way, a DBA with 4 periods was chosen as the lattice structure. There are 8 straight sections along the ring. Four short straight sections about 2.3m in chromatic arcs can be used besides four 4.0m long achromatic straight sections. There are eight quadrupoles in each cell. 4 quadrupoles are located in chromatic arcs and others in long straight section which can be used to adjust tunes and β function. In order to lengthen straight sections, combined function sextupoles are chosen to save space along the ring. Sextupoles can not only compensate chromaticity and harmonics, but correct closed orbit distortion and transverse coupling by providing dipole and skew quadrupole field components. The magnet layout of the ring is shown in Figure 1. Figure 1: Magnet layout of HLS II storage ring. Two operation modes (mode A and mode B) have been optimized. Mode A is an achromatic mode, whose dispersion in the long straight section is zero. Mode B is a distributed dispersion mode, whose emittance is smaller than that of mode A. Table1 shows list of the linear lattice about the two modes in half cell. β and dispersion functions of the two modes are shown in Figure2 and 3. Table2 gives main parameters of the storage ring. From above results, one can see that there are same tunes in the two modes although their beam emittances are quite ___________________________________________ *Work supported by NSFC (10905054 and 10979045) fenggy@ustc.edu. cn Proceedings of IPAC’10, Kyoto, Japan WEPEA042 02 Synchrotron Light Sources and FELs A05 Synchrotron Radiation Facilities 2585 different. This means one can easy get Mode B from Mode A by lattice tuning. Table 1: List of The Linear Lattice (Half cell)