are cooled down to approximately 20 K with liquid helium. The accelerating phase of the main linac is chosen such that a specific energy chirp is introduced along the bunch so that it can be later compressed longitudinally in a magnetic chicane (bunch compressor). The beam reaches the chicane after being turned by 180° in the first triple bend achromat ARC1. After compression, the beam, consisting now of sub-picosecond bunches, enters the magnetic undulator that constitutes a major part of the mid-IR Free Electron Laser (FEL). This laser generates IR light with the wavelength of ~5mm. The spent electron beam is returned to the entrance of the main linac via the second ARC2 at a precise time when the RF phase is exactly opposite to the initial accelerating phase. This condition requires an accurate adjustment of the electron beam path length that is accomplished by moving the ARC1 as a whole. The beam is now decelerated thus giving its energy back to the electromagnetic field inside the linac RF cavities (energy recovery) and emerges from the linac having the original energy of 8.35MeV. This energy recovered beam is diverted to the beam dump ending its short but useful life Main parameters See also EMMA External links ERLP ASTeC project page Science and Technology Facilities Council Particle accelerators Research institutes in Cheshire Science and Technology Facilities Council