A magnetic chicane also called a bunch compressor helps form dense bunches of electrons in a

free-electron laser A free-electron laser (FEL) is a (fourth generation) light source producing extremely brilliant and short pulses of radiation. An FEL functions and behaves in many ways like a laser, but instead of using stimulated emission from atomic or molecula ...

. A magnetic chicane makes electrons detour slightly from their otherwise straight bath, and in that way is similar to a

chicane A chicane () is a serpentine curve in a road, added by design rather than dictated by geography. Chicanes add extra turns and are used both in motor racing and on roads and streets to slow traffic for safety. For example, one form of chicane is ...

on a road. A magnetic chicane consists of four

dipole In physics, a dipole () is an electromagnetic phenomenon which occurs in two ways: *An electric dipole deals with the separation of the positive and negative electric charges found in any electromagnetic system. A simple example of this system i ...

magnets, giving electrons at the beginning of a bunch a longer path than electrons at the end of the bunch, thereby allowing the laging electrons to catch up.

Free-electron laser

A free-electron laser depends upon a beam of tightly bunched electrons. Short bunches of electrons are produced by a

photoinjector A photoinjector is a type of source for intense electron beams which relies on the photoelectric effect. A laser pulse incident onto the cathode of a photoinjector drives electrons out of it, and into the accelerating field of the electron gun. In ...

, but they quickly grow, because electrons have negative charge and little mass, causing the bunch to expand. As the bunch is accelerated, the electrons gain mass and quickly approach the speed of light. After that, electrons at the end of the bunch cannot go any faster to catch up with electrons at the beginning of the bunch.

Chirp

This problem is solved by adjusting the phase of the driving electric field to more strongly add energy and mass to electrons at the trailing end of the bunch. This is called ''negative energy

chirp A chirp is a signal in which the frequency increases (''up-chirp'') or decreases (''down-chirp'') with time. In some sources, the term ''chirp'' is used interchangeably with sweep signal. It is commonly applied to sonar, radar, and laser system ...

'', meaning the energy decreases along the direction of beam travel. Because the beam is traveling at almost the speed of light, the trailing electrons gain mass, rather than velocity. This results in a correlation between mass and position in the bunch.

Chicane

The chicane gives lagging electrons time to catch up. More massive electrons are deflected less by the magnetic field than lighter electrons, and therefor take a shorter path through the chicane, resulting in a shorter bunch. A chicane consists of four dipole magnets with the following roles: # Deflects the beam slightly away from the central axis of the accelerator, with lighter electrons deflected more than more massive electrons. # Deflects the beam in the opposite direction, making it parallel to the central axis, but with an offset. The offset is greatest for lighter electrons. # Deflects the beam back towards the central axis. # Deflects the beam back in the direction of the central axis.

Limitations

In practice, bunch compression cannot be done a single step. To avoid

beam emittance In accelerator physics, emittance is a property of a charged particle beam. It refers to the area occupied by the beam in a position-and-momentum phase space. Each particle in a beam can be described by its position and momentum along each of ...

blowup, beam compression is usually done by using two chicanes.

References

External links

RF and Space Charge Emittance in Guns
a basic definition of emittance
Space Charge Induced Beam Emittance Growth and Halo Formation
Electron beam Free-electron lasers Accelerator physics {{Lasers