An Introduction to Wake Fields and Impedances

Martin Dohlus, Rainer Wanzenberg


The concepts of wake fields and impedance is introduced to describe the electromagnetic interaction of a bunch of charged particles with its environment in an particle accelerator. The various components of the environment are the vacuum chamber, cavities, bellows, dielectric coated pipes, and all other kinds of obstacles the beam has to pass on its passage through the accelerator. The wake fields can act back on the beam and can lead to instabilities, which limit either the achievable current per bunch or the total current or even both. Starting from some typical examples the wake function and the basic properties are explained. The wake fields in cavities and resonant structures are studied in detail. Finally the frequency domain view of the wake field or the impedance is explained. The basic properties of the impedance are derived.


Wake field; impedance; modes in a cavity.

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