The latest generation of storage ring-based light sources employs multibend achromat lattices to achieve ultralow emittance. These lattices make use of a large number of weak bending magnets which considerably reduces the amount of power radiated in the dipoles in comparison to power radiated from insertion devices. Therefore, in such storage rings, parameters such as emittance, energy spread, and radiated power are—unlike 3rd generation storage rings—no longer constant during a typical user shift. Instead, they depend on several varying parameters such as insertion device gap settings, bunch charge, bunch length, etc. Since the charge per bunch is usually high, intrabeam scattering in medium-energy storage rings with ultralow emittance becomes very strong. This creates a dependence of emittance on stored current. Furthermore, since the bunch length is adjusted with rf cavities but is also varied as insertion device gaps change, the emittance blowup from intrabeam scattering is not constant either. Therefore, the emittance, bunch length, and hence the resulting Touschek lifetime have to be calculated in a self-consistent fashion with 6D tracking taking into account not only the bare lattice and rf cavity settings, but also momentary bunch charge and gap settings. Using the MAX IV 3 GeV storage ring as an example, this paper demonstrates the intricate interplay between transverse emittance (insertion devices, emittance coupling), longitudinal emittance (tuning of main cavities as well as harmonic cavities), and choice of stored current in an ultralow-emittance storage ring as well as some implications for brightness optimization.

• Received 11 March 2014

DOI:https://doi.org/10.1103/PhysRevSTAB.17.050705

This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society