 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).