Speaker: Stefania Papadopoulou (BE-ABP-HSI)

Bunch characteristics evolution for lepton and hadron rings under the influence of Intra-beam scattering

The physical parameter quantifying particle events' production and thereby the performance of a collider is the luminosity. High luminosities are reached by increasing the beam brightness, i.e. the intensity on a specific phase space volume (emittance). In this respect, the luminosity of hadron and lepton rings is limited by a combination of collective effects causing particle losses and emittance growth. In particular, Intra-beam scattering (IBS) impacts beam quality, through emittance growth. Several IBS theoretical models and their approximations exist, all assuming Gaussian beams. In this seminar, the optimization of the lattice design for a lepton ring and the study of the bunch characteristics evolution for a hadron ring, under the influence of IBS, are discussed. Taking into account IBS, based on analytical results and numerical simulations, an alternative optics design is proposed for the damping rings (DRs) of the Compact Linear Collider (CLIC). Specifically, aiming to reduce the betatron emittance of the DRs, dipoles whose magnetic field varies longitudinally are used together with high-field SC wigglers. Based on measurements and Monte-Carlo simulations, the interplay between IBS and radiation effects is also studied for the Large Hadron Collider (LHC), in view of understanding the bunch parameters evolution that determine the delivered luminosity. For the LHC bunch profiles which are observed to be non-Gaussian along the LHC energy cycle, appropriate fitting functions are used in order to describe accurately the distributions. In addition, the impact of the non-Gaussian distributions on the estimation of the beam size and thus, of the luminosity is presented. The importance to develop analytical formulas and simulation tools that calculate IBS for any distribution is underlined. 

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