LATTICE DESIGN FOR FUTURE PLAN OF UVSOR

Abstract

INTRODUCTION
UVSOR is a low energy synchrotron light source, which had been operated since 1983. After two major upgrades [1-4], now it is called UVSOR-III. The circumference of the storage ring is 53 m and the electron beam energy 750 MeV. It has 8 straight sections and six of them are occupied with undulators of various kinds. It has a moderately small emittance of about 17 nm and provides vacuum ultraviolet light of high brightness. Nowadays, to meet the demand of diffraction limited light beam in the vacuum ultraviolet and x-ray range from scientific community, several synchrotron light sources, which have exceedingly small emittance less than 1nm, are under consideration, construction, or in operation [5-7]. In such a situation, we have started considering a future plan for UVSOR with an emittance smaller than at least a few nm to provide diffraction-limited light in the vacuum ultra- violet range. As the first step of the investigation, we have analyzed the present magnetic lattice of UVSOR and found that a few optics with emittance around 10 nm may be re- alized [8]. To reach the lower emittance around a few nm, we have started designing a totally new storage ring. The storage ring has a higher electron energy, l GeV and a larger circumference, 82.5m. The lattice consists of twelve double bend cells. Among twelve straight sections, two sections will be used for the injection and RF cavity, and ten sections will be used for insertion devices. In this paper, we will describe the design of the new lat- tice for UVSOR-IV and the related beam dynamics studies.
LATTICE DESIGN
The lattice has been designed based on a compact double bend achromat cell (DBA), which consists of two bending magnets and four focusing magnets, all of which are com¬ bined function magnets. Two sextupole families are located † clham@ims.ac.jp in between two combined dipoles for the chromaticity cor- rection and two harmonic sextupole families are also em- ployed to correct the high order geometric aberrations. This lattice has twelve DBA cells with six long straight sections about 4 m and six short straight sections around 1.5 m long. These lengths are same as those of UVSOR-III. This may enable us to use the undulators at UVSOR  III in the new ring. It is noted that this lattice has a flexibility on the disper- sion function at the straight sections, which enables to re- alize various operation modes such as of achromatic, lower emittance, or isochronous. A tune survey was performed to find the linear optics with a low emittance and appropriate optical functions. EL- EGANT [9] was used for the calculations. The emittance is around 4 nm in the achromatic condition, which becomes lower in the non-achromatic condition. Figure 1 shows the lattice functions in the achromatic condition. The major pa- rameters are listed in Table 1 and are compared with those of UVSOR-III.
sections with 2T multipole wigglers in symmetry, the
PASJ2022 FRP020
energy loss due to the radiation in the wigglers enhances the damping effect and the  emittance reduces to 3.7 nm. These wigglers can provides tender X-ray, which are hardly accessible at the present UVSOR-III.
DYNAMIC APERTURE
We use ELEGANT [9] to optimize the dynamic aperture with two family sextupoles. The particle tracking has been without considering multipole errors and misalignment er- rors. Figure 2 shows the dynamic aperture for on-energy particles at the straight section with two family sextupoles whose strengths are determined so as to make the linear chromaticity zero. The horizontal aperture for the on-mo- mentum electron is about -19 to 10 mm and vertical aper- ture is about 6 mm. To realize a larger dynamic aperture, two harmonic sextuple families have been added to the lat- tice and their strengths are optimized using Bayesian method for maximizing the dynamic aperture. The details of the calculation will be described in a future paper. Figure 3 shows an example of the dynamic aperture of an opti- mized optic.
CONCLUSION
A new 1 GeV storage ring lattice with a circumference of 82.5 m has been proposed to upgrade the UVSOR syn- chrotron facility. The lattice has been designed based on DBA cell to have a low emittance. Optimising the magnet arrangement and the working point, a small natural emit- tance 4.2 nm is obtained in the achromatic condition. The lattice has twelve straight sections, ten of them are availa- ble for insertion devices. Occupying 3 short straight sec- tions with 2T multipole wigglers can reduce the natural emittance to 3.6 nm-rad. Moreover, these wigglers provide intense tender X-rays. Dynamic aperture was studied without machine errors. We used Bayesian method to optimize the strength of the harmonic sextupole families for maximizing the dynamic aperture. Our simulations show that the optics has suffi- ciently large dynamic aperture.
ACKNOWLEDGEMENTS
The authors thank Prof. S. Kera, the facility director, for his support and other facility staffs for their cooperation.