Contribution of Optical Aberrations to Spot-size Increase with Bunch Intensity at Atf2

A primary goal of ATF2 (Accelerator Test Facility 2) at KEK is to demonstrate an unprecedentedly low vertical beam size at the interactateck ion point (IP) of about 37 nm. Measurements over the past years indicate that the ATF2 vertical beam size, achieved after tuning, strongly rises with bunch intensity. This increase could have several different origins. For example, it could be due to wake fields occurring between the ATF damping ring and the IP, and/or be due to changes in the transverse emittances and energy spread in the damping ring, which are known to increase with intensity as a result of intrabeam scattering (IBS). In this paper we address the second possibility. Past measurements and simulations of the IBS effects in the ATF damping ring and in the extraction line are used to model the intensity-dependent initial emittances and energy spread at the entrance of the final focus. Using this model, particle tracking simulations predict the IP vertical beam size growth expected from the known optical aberrations for initial beam parameters corresponding to varying bunch intensities. We consider both the nominal final-focus optics and a relaxed optics with ten times increased horizontal IP beta function, which has also been used in operation. Comparing simulation results with emittance measurements at different locations allows us to draw some conclusions about the impact of IBS in the damping ring on the IP spot size, and about possible single-bunch wakefields in the ATF2.