More about PERLE

The PERLE accelerator complex is organized in a racetrack configuration, initially featuring one cryomodule [4, 5]. The 350 keV electron beam is produced by a phtotgun, which provides a CW (continuous wave) bunched beam at 40 MHz. In order to reach the ambitious design value of 500 pC per bunch, multi-alkali photocathodes of high quantum efficiency (QE) will be used and optically pumped by a green laser. High bunch charge will limit the lifetime of the photocathode: a dedicated photocathode preparation facility is directly connected to the photogun under vacuum for a fast photocathode exchange to maximize the accelerator availability.

The bunch length is adjusted by a warm buncher cavity and accelerated to 7 MeV by an SRF booster linac ( single-cell cavities at 801.58 MHz). The beam is injected into the ERL loop via a merging section and accelerated by an SRF linac. The cryomodule houses four 5-cell cavities that operate at the same frequency. The cryomodule provides an energy change of up to 82 MeV to the high average current electron beam (20 mA). The beam is circulated in the loop by a vertical stack of three recirculating arcs on each side, as well as spreaders and recombiners, including matching sections. The spreaders are positioned directly after each linac to separate beams of different energies and route them to their respective arcs. In contrast, recombiners serve to merge beams of varying energies into a single trajectory.
The path length of each arc is designed to be an integer multiple of the RF wavelength, except for the highest energy pass, arc 6, which is extended by half of the RF wavelength. This means that the beam will gain energy incrementally by 3 successive accelerations through the linac. Consequently, over three turns, an energy increase of 246 MeV is achieved. When adding the initial injection energy of 7 MeV, the total energy reaches 253 MeV.
Then the highest energy beam will be decelerated by traveling through the linac on its deceleration phase, 3 times incrementally down to its injection energy.
The beam can be used to generate X-rays with an optical Fabry-Perot cavity at the output of the cryomodule or for elastic scattering off radioactive nuclei in the straight section.
Concerning the ERL operation, as the beam is decelerated in the cavity, it excites the cavity and transfers its power (thus energy) back to the accelerating structure. Thanks to the fast RF regulation system of the cavity, the power feeding the cavity is tuned down: the energy recovery linac thus allows to accelerate continuously injected bunches with minimal RF power into the cavity. After use and full deceleration of the beam, the remaining bunches are directed to a dump at their injection energy.
The main beam parameters of the PERLE facility are summarized in the following Table