Charged Particle Beam

A charged particle beam is a spatially localized group of electrically charged particles that have approximately the same position, kinetic energy (resulting in the same velocity), and direction. The kinetic energies of the particles are much larger than the energies of particles at ambient temperature. The high energy and directionality of charged particle beams make them useful for applications (see Particle beam#Usage and Electron beam technology).

Such beams can be split into two main classes:

• unbunched beam (or coasting beam), which has no longitudinal substructure.
• bunched beam, in which the particles are distributed into several pulses (bunches) of particles. The bunched beam is most common in modern facilities, since the most modern accelerator concepts require bunched beams for acceleration.

Assuming a Gaussian distribution of particle positions and impulses, a charged particle beam (or a bunch of the beam) is characterized by

• the species of particle, e.g. electrons, alpha particles, or hydrogen ions
• the mean energy of the particles, often expressed in Electronvolt (typically: keV to GeV)
• the (average) particle current, often expressed in Ampere,
• the beam size (position variance), often using the so-called β-function
• the beam emittance, a measure of the degree to which the particle trajectories are non-laminar.

These parameters can be expressed in various ways. For example, the current and beam size can be combined into the current density, and the current and energy (or beam voltage V) can be combined into the perveance K = I V -3/2.

The charged particle beams that can be manipulated in particle accelerators can be subdivided into electron beams (sometimes called cathode rays), ion beams and proton beams.