Editing Accelerator physics (section)

==Acceleration and interaction of particles with RF structures==
{{See also|Microwave cavity|Shunt impedance|Superconducting Radio Frequency|Reciprocity (electromagnetism)}}

[[File:Desy tesla cavity01.jpg|thumb|Superconducting [[niobium]] [[microwave cavity|cavity]] for acceleration of ultrarelativistic particles from the TESLA project]]

While it is possible to accelerate charged particles using electrostatic fields, like in a [[Cockcroft-Walton voltage multiplier]], this method has limits given by [[electrical breakdown]] at high voltages. Furthermore, due to electrostatic fields being conservative, the maximum voltage limits the kinetic energy that is applicable to the particles.

To circumvent this problem, [[linear particle accelerator]]s operate using time-varying fields. To control this fields using hollow macroscopic structures through which the particles are passing (wavelength restrictions), the frequency of such acceleration fields is located in the [[radio frequency]] region of the electromagnetic spectrum.

The space around a particle beam is evacuated to prevent scattering with gas atoms, requiring it to be enclosed in a vacuum chamber (or ''beam pipe''). Due to the strong [[electromagnetic field]]s that follow the beam, it is possible for it to interact with any electrical impedance in the walls of the beam pipe. This may be in the form of a resistive impedance (i.e., the finite resistivity of the beam pipe material) or an inductive/capacitive impedance (due to the geometric changes in the beam pipe's cross section).

These impedances will induce ''wakefields'' (a strong warping of the electromagnetic field of the beam) that can interact with later particles. Since this interaction may have negative effects, it is studied to determine its magnitude, and to determine any actions that may be taken to mitigate it.