Interplay of Touschek Scattering, Intrabeam Scattering, and RF Cavities in Ultralow-emittance Storage Rings

When it goes into operation in 2016, the MAX IV 3 GeV storage ring will be the first ultralow-emittance storage ring based on a multibend achromat lattice. Such ultralow-emittance rings make use of a large number of weak bending magnets which leads to low radiation losses in the dipoles compared to power radiated from insertion devices. Therefore, the emittance in such rings depends strongly on the insertion devices and gap settings. The large stored current along with the low transverse emittance lead to strong intrabeam scattering which blows up the beam's 6D emittance. The Touschek lifetime depends on the 6D emittance: it grows with increasing longitudinal emittance which makes bunch lengthening cavities attractive. On the other hand, in the ultralow-emittance regime, reducing the transverse emittance actually increases the Touschek lifetime. Damping wigglers and insertion devices reduce the transverse emittance, but they can also increase the Touschek lifetime by reducing the available cavity overvoltage and thus increasing the bunch length. 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 ring as well as its implications for brightness optimization.