I.12 — Collective effects

Particle accelerators use external electromagnetic fields to guide and accelerate charged particles. In a real machine, however, there is another source of electromagnetic fields, the beam itself, which, interacting with the accelerators’ devices, produces additional self-induced fields which perturb the particle’s motion leading to the so-called collective effects.The self-induced fields are commonly divided into space charge fields, generated directly by the charge distribution and including the image currents circulating on the walls of a smooth, perfectly conducting pipe, and the wakefields, produced by the finite conductivity of the walls, resonant de- vices, or any geometrical variation of the beam pipe.Collective effects in particle accelerators are one of the key constituents for determining the ultimate particle accelerator performance. Their role is becoming increasingly important as particle accelerators are being pushed to ever higher intensity and beam brightness.