Complex unit lattice cell for low-emittance storage ring light source

TL;DR

This paper introduces the complex unit lattice cell, a novel design for low-emittance storage rings that reduces magnet strengths and space requirements, demonstrated through a 17BA lattice example for a 3 GeV light source.

Contribution

The paper proposes the complex unit lattice cell concept, improving space efficiency and magnet strength reduction in multi-bend achromat storage ring designs.

Findings

The 17BA lattice with the complex unit cell achieves a natural emittance of 19.3 pm.rad.

Compared to conventional cells, the complex unit cell reduces magnet strengths and space.

The design introduces a new MBA lattice with semi-distributed chromatic correction.

Abstract

To achieve the true diffraction-limited emittance of a storage ring light source, such as ~10 pm.rad for medium-energy electron beams, within a limited circumference, it is generally necessary to increase the number of bending magnets in a multi-bend achromat (MBA) lattice, as in the future upgrade plan of MAX IV with a 19BA replacing the current 7BA. However, this comes with extremely strong quadrupole and sextupole magnets and very limited space. The former can result in very small vacuum chambers, increasing the coupling impedance and thus enhancing the beam instabilities, and the latter can pose significant challenges in accommodating the necessary diagnostics and vacuum components. Inspired by the hybrid MBA lattice concept, in this paper we propose a new unit lattice concept called the complex unit lattice cell, which can reduce the magnet strengths and also save space. The complex unit cell is numerically studied using a simplified model. Then as an example, a 17BA lattice based on the complex unit cell concept is designed for a 3 GeV storage ring light source with a circumference of 537.6 m, which has a natural emittance of 19.3 pm.rad. This 17BA lattice is also compared with the 17BA lattice designed with conventional unit cells to showcase the benefits of the complex unit cell concept. This 17BA lattice also suggests a new type of MBA lattice, which we call the MBA lattice with semi-distributed chromatic correction.