Principle
A DC photoelectron gun generates short low emittance electron bunches with the length of several ps and accelerates them to a modest 350keV. The nominal bunch charge on ALICE is 80pC. The bunches are produced in trains lasting from ~10ns to 100ms and the train repetition frequency can vary from 1 to 20 Hz. Within the train, the bunches are separated by 12.3ns that corresponds to the laser pulse repetition frequency of 81.25 MHz.
The electron beam is then injected into the superconductive linac (booster), accelerated to the energy of 8.35MeV and transported to the main linac that increases the beam energy to 35MeV. Both superconductive linacs 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 back 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
Read more about this topic: ALICE (accelerator)
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