Fermi Machine Description

The FERMI single-pass linac-based FEL at the Elettra Laboratory is an international user facility located in Italy for scientific investigations of ultra-fast and ultra-high resolution processes in material science and physical biosciences with ultra high brilliance X-ray pulses. With a peak brightness of about 6 orders of magnitude higher than third generation sources, full transverse coherence, (close to) transform limited bandwidth, pulse lengths in the range between 100 fs and 10 fs, variable polarization and energy tunability, the FERMI source is a powerful tool for scientific exploration in a wide spectrum of disciplines.

The seed laser provides a reference signal throughout the FERMI facility (including the experimental beamlines) to facilitate the femtosecond level precision timing and synchronization of all systems.

The main machine characteristics are:

  • High peak power (~ GW) optical pulses with synchronization to external laser sources. Generation of shorter pulses (sub fs) will also be explored.
  • APPLE II type undulators to enable flexible tuning of both photon wavelength and polarization.
  • Implementation of seeded harmonic cascade FEL schemes for tunable and controlled short-wavelength photon pulse production.
  • Advanced feedback and feed-forward systems to improve output stability.


A general layout of the facility is shown in the above Figure. The accelerator and FEL complex comprises the following parts: a photoinjector and two short linac sections (LINAC 0) generating a bright ~ 100 MeV electron beam (first 10 meters), the main linear accelerator (LINAC 1-4) in which the beam is time-compressed and accelerated up to ~ 1.5 GeV (170 meters), the system to transport the beam to the undulators (SPREADER), the undulator complex generating the FEL radiation (270 meters), the photon beamlines taking the radiation from the undulator to the experimental area (320 meters) and the experimental area itself.

After leaving the undulators, while the FEL radiation is transported to the experimental areas, the electron beam is brought to a beam dump by a sequence of bending magnets. The FEL radiation transport system, designed to handle the high peak power of up to 10 GW in the sub-ps long pulse, includes a differentially pumped windowless vacuum system and low-Z material beam-line components operating at grazing incidence angles. The photon beam transport system incorporates all provisions and equipment necessary to ensure pulse length and energy resolution preservation, monochromatization, 

source shift compensation, beam splitting and focusing into the experimental chambers.

As above mentioned, the FERMI facility comprises two separate coherent radiation sources, FEL-1 and FEL-2. FEL-1 operates in the wavelength range between 100 and 20 nm via a single cascade harmonic generation, while the FEL-2 line operates at shorter wavelengths (20-4 nm) via a double cascade mechanism.

The Photo-injector

The present photoinjector is based on the proven 1.6 cell electron gun developed at BNL/SLAC/UCLA and it was delivered by RadiaBeam Technologies  in the 2013. The FERMI photoinjector has demonstrated during the commissioning to be able to achieve the project requirements. It can produce a 10 ps long pulse with up to 1 nC charge and a rms normalized transverse emittance of 1.2 mm-mrad at 100 MeV.  However, the standard operational parameters are 700 pC with an emittance of about 1.0 mm-mrad at 100 MeV. The repetition rate is switchable between 10 Hz and 50 Hz, depending from operation requirements. Following standard layout schemes, the design includes a solenoid for emittance compensation and acceleration to 100 MeV with two S-band rf sections (LINAC 0).
An UV laser pulse (@262 nm) provides temporal and spatial bunch shaping. The same laser system provides also the optical pulse used in the Laser Heater device.
Simulations were performed using the GPT and Astra codes and indicate that the electron beam performance objectives for injection into the main linac at ~ 100 MeV are attainable. The timing and charge stability, – 0.5 ps and 1% respectively, are challenging but within present state of the art.

1.6 cell gun cavity section

LINAC: Acceleration, Compression and Transport to the Undulators

At the exit of the photoinjector, the ~ 100 MeV electrons enter the L1 linac (four S-band, i.e. 3 GHz, accelerating sections) where they are accelerated to ~ 250 MeV. Acceleration off-crest creates the correlated energy spread along the bunch needed to compress it in the first compressor, BC1. An X-band rf structure tuned at the 4th harmonic of the main (3 GHz) linac frequency is placed half-way between the four sections of L1. The function of the structure is to provide the non-linear quadratic and, when operated off-crest, cubic corrections of the correlated momentum distribution along the bunch in presence of the photoinjector and the magnetic compressors non-linearities and of longitudinal wakefields.

The L2 and L3 linac structures, located between the first and second bunch compressors, accelerate the beam from ~ 250 MeV to ~ 650 MeV. They also provide the residual momentum chirp needed for the second compressor, BC2. After BC2 the beam is accelerated to its final FEL-1 ~1.2 GeV energy in the L4 structure (FEL-2 energy is 1.5 GeV). The rf phases of the linac sections following BC1 are chosen to provide the necessary momentum spread for compression and also to cancel the linear part of the longitudinal wakes. The non-linear correlated momentum spread at the end of the linac is fine-tuned by acting on the amplitude and phase of the x-band structure.

Magnetic Bunch Compressor 3D model

The linac focusing system is designed to minimize transverse emittance dilution due to transverse wakefields, momentum dispersion and coherent synchrotron radiation in bends. Two transfer lines, one assigned to FEL-1 and the other to FEL-2, transport the electron beam from the linac end to the undulators. This system, called the “Spreader”, starts with two, three degree bending magnets that deflect the beam away from the linac. In the line that leads to the FEL-2 undulator, two more, three degree bend dipoles of opposite polarity bring the beam back parallel to the linac at a distance from it of 1 m.
When operating the FEL-1 line, one of the afore-mentioned dipoles is switched off and the beam proceeds to a second pair of dipoles that again bend the beam parallel to the linac and displaced from it by 2 m. The two undulator lines are thus parallel and separated by 1 m. The electron optics is designed to cancel any emittance blow up due to the emission of coherent synchrotron radiation in the bends by a suitable choice of the (small) bending angles and of the phase advances between dipoles. The lattice of the spreader is flexible, and allows to switch from the configuration for photon delivery to a configuration less suitable for operation but optimized for electron beam diagnostics purposes.

The Undulators and the FEL Process

One undulator around the vacuum chamber

FEL-1 and FEL-2 are required to provide, at all wavelengths, continuously tunable beam polarization ranging from linear-horizontal to circular to linear-vertical. The FEL-1 radiator and the final radiator in FEL-2 have therefore been chosen to be of the APPLE-II, pure permanent magnets type. For the modulator a simple, linearly-polarized configuration is best, due to both its simplicity and because the input radiation seed can be linearly polarized. The wavelength will be tuned by changing the undulator gap at constant electron beam energy. The FEL-1 and FEL-2 radiators consist of 6 and 8 undulator magnets. The magnetic lengths of the individual magnets are 2.34 m (containing 36 periods) for the FEL-1 and first FEL-2 radiators and 2.40 m (48 periods) for the second FEL-2 radiator, respectively. Electromagnetic quadrupoles, high quality beam position monitors, and quadrupole movers are installed in between undulators to correct the electron trajectory.
The accelerator and FEL parameters were defined based on theoretical studies and simulations. A cornerstone has been provided by “start-to-end” simulations, in which the electron beam is tracked from the photocathode, through the linac and all the way through the FEL process. The exhaustive studies carried out included foreseen random perturbations and jitters of accelerator and FEL parameters.
The consequence of orbit displacements from the ideal trajectory in the undulators were simulated. At the wavelength of 10 nm, the FEL process requires the straightness of the electron trajectory in the undulators to stay within 10 µm (rms value over the undulators length). While this requirement is beyond the state-of-the-art of present surveying techniques, realistic simulations show that a combination of the latter and of beam-based-alignment procedures (tested at the Stanford Linear Collider and proposed for the LCLS) will achieve the desired performance.

Photo gallery

2009-07-15 CONTROL ROOM
2010-08-12 BEAM DUMP
2010-08-12 Linac FERMI
2010-08-12 Linac FERMI-2
2010-08-12 Linac FERMI-3
2010-08-12 Linac FERMI-4
2010-08-12 Linac FERMI-5
2010-08-12 Linac Quadrupoles
2010-09-27 Dump Dipole
2010-09-27 Sala Ondulatori
2010-09-27 Sala Ondulatori-2
2010-09-27 Sala Ondulatori-3
2010-09-27 Sala Ondulatori-4
2010-09-27 Sala Ondulatori-5
2010-09-27 Sala Ondulatori-6
2010-09-27 Sala Ondulatori-7
2010-10-28 Edifici Fermi-1
2010-10-28 Edifici Fermi-2
2010-10-28 Edifici Fermi-3
2010-10-28 Edifici Fermi-4
2010-10-28 Fermi building
2010-10-28_Experimental Hall
start stop bwd fwd

Facility 3D rendering

DiProI beamline


Last Updated on Thursday, 09 January 2020 18:54