Magnetic chicane

[3][4][5] A free-electron laser depends upon a beam of tightly bunched electrons.

As the bunch is accelerated, the electrons gain mass and quickly approach the speed of light.

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.

[6] Because the beam is traveling at almost the speed of light, the trailing electrons gain mass, rather than velocity.

A chicane consists of four dipole magnets with the following roles: In practice, bunch compression cannot be done a single step.