Evaluation of beam coupling impedance by generating a dedicated database in an ultra-low emittance storage ring

Abstract

The evaluation and accurate understanding of the cause of collective effects and the related instabilities using corrective measures, which may limit the achievable current per bunch, is significant for a prosperous design of charged particle accelerators. We describe in detail two dominant collective effects, i.e. space-charge and wakefield effects for a multi-bend achromat (MBA) storage ring lattice under design. We have developed a detailed impedance model, which is used to study the beam dynamics, including the wakefield effects. The beam coupling impedances due to the wakefields were calculated to construct a coupling impedance database, with the wakefields of each vacuum component being maintained in a standard format. The Self-Describing Data Sets (SDDS) toolkit was used to calculate the wake potentials for all coupling impedance contributing vacuum components, to be used for an ultra-low emittance lattice design. The ELEGANT code was utilized for the particle tracking in an element-by-element procedure along the lattice. A bunch with the Gaussian distribution was assumed to calculate the model for a distributed resistive-wall impedance and the impedance generated by geometrical discontinuities for the various accelerator elements. Moreover, we evaluated the potential-well distortion effect due to longitudinal phase space motion for a system of electron bunches. The collective effects in multi-particle systems have been evaluated for the estimation of the beam instability with more accurate results, defined by their sources in the designed lattice and describing their effects on beam instabilities with remarkable consequences like energy loss and betatron tune shift.