Bunch compression using a magnetic chicane

The ELBE accelerator beamline contains a magnetic chicane after the first accelerator cryomodule. The chicane consists of four dipole magnets all of equal field strength and magnetic length. The amount of deflection can be adjusted by altering the field (the current) of the magnets.

When an electron beam is injected off-crest into an accelerating structure the particle energy is modulated depending of the actual pahse of the particle. If the bunch is injected before the field reaches its maximum the first particles in the bunch experience less acceleration than the last particles. The bunch length, however, is not affected.
The amount of deflection of a charged particle in a magnetic field depends on its momentum. Therefore, the higher the energy of an electron the less it is deflected in the dipoles of a magnetic chicane. However, if all four deflection angles are equal, the electrons leave the chicane on the same trajectory independant of their energy.
Electrons of differing energy take flight paths of different length through the chicane. Electrons with higher energy appear advanced, electrons with lower energy delayed at the exit. This can lead to a bunch compression in time if the bunch was appropriately energy-time correlated.