Abstract
Harmonic cavities have been used in storage rings to lengthen bunches and increase beam lifetimes dominated by Touschek scattering. Transient beam loading in the harmonic cavities generated by asymmetries in the fill pattern causes significant variation of the bunch synchronous phase and bunch length along the bunch train when the longitudinal restoring force has been reduced. This results in a significant reduction in the mean bunch lengthening and potential lifetime increase. We describe how beam current modulations give rise to transient effects much larger than expected from the linear model of the beam cavity interaction. We also develop a tracking simulation to predict results and apply this simulation to an analysis of the beam loading transients for the case of passive and active normal and superconducting third harmonic rf systems using Advanced Light Source parameters.
Highlights
The beam lifetime in low-medium energy third generation synchrotron light sources is typically dominated by large-angle intrabeam (Touschek) scattering
We have observed in the Advanced Light Source (ALS) that gaps in the fill pattern create transient beam loading effects along the bunch train which significantly degrade the total amount of bunch lengthening and the lifetime improvement
We evaluate the dependence of the phase transient and bunch lengthening on the cavity frequency
Summary
The beam lifetime in low-medium energy third generation synchrotron light sources is typically dominated by large-angle intrabeam (Touschek) scattering. We have observed in the ALS that gaps in the fill pattern create transient beam loading effects along the bunch train which significantly degrade the total amount of bunch lengthening and the lifetime improvement. We believe that the transient behavior is not specific to the ALS and presents a serious limitation to the lifetime improvement which can be achieved with harmonic systems in third generation light sources, regardless of whether the system is active or passive, normal or superconducting. We believe these results may apply to the operation of such systems which are in either the design or the construction stage at light sources around the world.
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