The High Energy Photon Source (HEPS), a 6 GeV green-field diffraction-limited storage ring light source, will be built in Beijing, China. The HEPS design has been evolving for about ten years, and is now mostly finished and ready for construction. The storage ring is based on a modified hybrid seven-bend achromat (7BA) design, where bending magnets with reverse bending angles and longitudinal gradients are adopted to reach an ultralow natural emittance of 34.2 pm with a circumference of 1360.4 m. The central slice of the dipole in the middle of the modified hybrid 7BA, with flexible magnetic field, is used as the source of the bending-magnet beamline. Moreover, alternating high- and low-beta sections are specially designed to generate and deliver X-ray synchrotron radiation with high brightness of 5 × 1022 photons s−1 mm−2 mrad−2 (0.1% bandwidth)−1. Here, the HEPS storage ring design and solutions to the challenges inherent in this ultralow-emittance design are presented.

The HEPS project

The High Energy Photon Source (HEPS) is an ultralowemittance, kilometer-scale storage ring light source to be built in China. In this paper we will introduce the progress of the physical design and studies on HEPS over the past one year, covering issues of storage lattice design and optimization, booster design, injection design, collective effects, error study, insertion device effects, beam lifetime, etc.

Progress of Lattice Design and Physics Studies on the High Energy Photon Source

For the High Energy Photon Source (HEPS), a green-field fourth-generation storage ring light source, the preliminary design report (PDR) was completed in 2018, when the accelerator physics design had been basically finished. During the subsequent hardware and engineering design of the HEPS storage ring based on the PDR design, a few problems and challenges emerged, calling for modifications of the lattice. In this paper, we will introduce the background and reasons for the modifications and present the linear optics and simulation results for the nonlinear performance of the modified lattice of the HEPS storage ring. The modified lattice satisfies the requirements from hardware and engineering design.

Modification and optimization of the storage ring lattice of the High Energy Photon Source

In the design of multi-bend achromat (MBA) lattices for diffraction-limited storage rings (DLSRs), usually, many magnet parameters need to be adjusted, and the constraints to be satisfied can be stringent. When using an evolutionary algorithm to optimize such a complicated lattice, there can be only few or even none of lattice solutions satisfying the constraints in the early evolution stage of the algorithm, which may significantly affect the optimization result. Therefore, the techniques for constraint handling in an evolutionary algorithm will be important for better optimizing MBA lattices. In this paper, three typical constraint handling methods that are incorporated into a multi-objective particle swarm optimization algorithm are used to optimize a hybrid 7BA lattice for a 2.2 GeV DLSR. In this lattice optimization, the impacts of the population size of the algorithm and the constraint condition on the optimization result are studied for these methods, and comparisons are also made among these methods. It is shown that, in general, the penalty function method is a better constraint handling method for optimizing MBA lattices. From the optimization result obtained by the penalty function method, a lattice with a natural emittance of about 84 pm ⋅ rad is selected, which has large dynamic aperture and momentum aperture.

Constraint handling in constrained optimization of a storage ring multi-bend-achromat lattice

Progress in the Design and Related Studies on the High Energy Photon Source

In the design of a diffraction-limited storage ring (DLSR) consisting of compact multi-bend achromats (MBAs), it is challenging to simultaneously achieve an ultralow emittance and a satisfactory nonlinear performance, due to extremely large nonlinearities and limited tuning ranges of the element parameters. Nevertheless, taking the High Energy Photon Source (HEPS) as an example, we demonstrate that the potential of a DLSR design can be explored with a successive and iterative implementation of the particle swarm optimization (PSO) and multi-objective genetic algorithm (MOGA). It turns out that with a hybrid MBA lattice, it is feasible for the HEPS to attain a natural emittance of about 50 pm.rad, and meanwhile, realize a sufficient ring acceptance for beam accumulation with an on-axis longitudinal injection scheme. Particularly, this study indicates that a rational combination of the PSO and MOGA is more effective than either of them alone in approaching the true global optima, for an explorative multi-objective problem with many optimizing variables and local optima.

Optimizing the lattice design for a diffraction-limited storage ring with a rational combination of particle swarm and genetic algorithms

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Optimizing the lattice design for a diffraction-limited storage ring with a rational combination of particle swarm and genetic algorithms