合肥先进光源储存环磁聚焦结构选择研究

合肥先进光源（Hefei Advanced Light Facility, HALF）是我国最近立项建设的一台软X射线与真空紫外波段的衍射极限储存环光源，其束流能量为2.2 GeV，发射度目标小于100 pm·rad。磁聚焦结构（lattice）设计是储存环物理设计的核心，lattice结构对储存环的性能表现与采用的相关技术至关重要。本文首先设计了一个分布式色品校正的6BA（six-bend achromat）lattice，周长为441.6 m，具有18个周期，实现了74 pm·rad的超低自然发射度，水平动力学孔径达6 mm左右；然后简要介绍了为HALF设计的两个混合型MBA lattice（hybrid MBA, HMBA），包括20个周期的H7BA lattice与HALF当前采用的变版H6BA lattice；最后对比研究了HALF采用这3个lattice设计的结果。研究表明，HALF当前采用的变版H6BA lattice的综合性能最好，是相对更好的结构方案。

Study on Selection of Storage Ring Lattice for Hefei Advanced Light Facility

The Hefei Advanced Light Facility (HALF), recently approved by the Chinese government, is a soft X-ray and VUV light source based on diffraction-limited storage ring (DLSR), which has beam energy of 2.2 GeV and emittance goal of less than 100 pm·rad. The lattice design is the core of the storage ring physics design, and the selection of lattice is crucial to the storage ring performance and the related technology that will be used. The emittances of DLSRs are as low as hundreds or even tens of pm·rad and the insertion device radiation brightness is about 1.2 orders of magnitude higher than that of the third-generation light sources. Generally, increasing the number of bends is the most effective method to reduce the emittance and thus multi-bend achromat (MBA) lattices are used for designing DLSRs. In this paper, firstly, a conventional 6BA (six-bend achromat) lattice with distributed chromaticity correction was designed for the HALF storage ring, which has a circumference of 441.6 m with 18 identical cells. In this lattice design, the horizontal and vertical phase advances of each bend unit cell are about (0.4, 0.1)×2π, so that main nonlinear effects can be effectively cancelled over five identical unit cells based on higher-order achromat. The lattice has an ultra-low emittance of 74 pm·rad and a horizontal dynamic aperture (DA) of about 6 mm. Then two hybrid MBA lattices (HMBA) designed for HALF with 20 cells were briefly introduced, including an ESRF-EBS type H7BA lattice and a modified H6BA lattice, which have horizontal DAs larger than 10 mm. The latter is the present baseline lattice of HALF, which has a long straight section and a short one in each lattice cell. And the two central bend cells of the baseline lattice use longitudinal gradient bends and reverse bends to reduce the emittance and damping times. The H7BA lattice has a circumference of 441.6 m and a natural emittance of 84 pm·rad, and the circumference and natural emittance of the H6BA lattice are 480 m and 86 pm·rad. Compared to the conventional 6BA lattice designed in this paper, two HMBA lattices have larger DAs which can allow off-axis injection. The conventional 6BA lattice and H6BA lattice have shorter damping times than that of the H7BA lattice, which is beneficial for suppressing emittance increase due to intra-beam scattering. Besides, the H6BA lattice has much more straight sections than the other two lattices. So the present modified H6BA lattice is a better option for the HALF storage ring.