Abstract
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. (Less)
Highlights
Already proposed in the 1990s [1,2,3,4,5], multibend achromat (MBA) lattices have only recently started to appear in storage ring-based light source designs [6,7,8,9,10,11]
When the MAX IV facility [13,14] goes into operation in 2016, its 3 GeV storage ring will become the first ultralow-emittance storage ring based on a MBA [6,15]
Modern ultralow-emittance storage rings at medium energy are characterized by strong intrabeam scattering (IBS) and potentially large Touschek lifetime if sufficient momentum acceptance (MA) is provided by the lattice and rf system
Summary
Already proposed in the 1990s [1,2,3,4,5], multibend achromat (MBA) lattices have only recently started to appear in storage ring-based light source designs [6,7,8,9,10,11]. As is typical for such ultralow-emittance lattices, the radiative losses in the dipoles (364 keV=turn) are low compared to what can be expected (roughly 1 MeV=turn) once the ring is fully equipped with insertion devices (IDs) and/or damping wigglers (DWs) Damping wigglers and IDs reduce the transverse emittance and can increase the Touschek lifetime in ultralow-emittance storage rings They achieve this in another way: because their added losses reduce the available cavity overvoltage, they can lengthen the bunches which increases Touschek lifetime. If they increase the energy spread in the bunch, the emittance blowup from IBS is reduced, which in turn affects the resulting Touschek lifetime.
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