Seminars Archive

Beam Coupling Impedences and Wake Fields - They are forty, but they don\'t look it

Abstract
The concepts of Coupling Impedance (C.I.) and Wake Fields (W.F.) was born in the middle of the sixties in connection with the design of high intensity beams with small energy spread. These concepts were originally introduced in connection with the interaction of a coasting beam with the accelerating cavities and with clearing electrodes in Intersecting Storage Ring (ISR) at CERN. They were progressively extended to include bunched beams and interaction with various device. More generally these concepts are applicable whenever one must define a sort of transfer function between a perturbation in the beam current (cause) and the field produced by the perturbation (effect). Indeed, knowledge of the electromagnetic interaction between a beam and all the equipment in the surrounding vacuum chamber is necessary in order to optimize the accelerator performance in terms of stored current. Many instability phenomena may occur in the machine because of the fields produced by the beam and acting back on itself just as in a feedback device. Basically, these fields produce an extra energy gain (), affecting the longitudinal dynamics, and a transverse momentum kick which deflects the beam.