Free-electron lasers are permitting rapid progress in the study of matter by providing new sources of ultra-short (femtoseconds), high-brightness, coherent pulses spanning the vacuum ultraviolet to X-ray wavelength range. Figure 1 illustrates the peak brightness versus pulse duration of several genres of short wavelength light sources. Recently, a few, new FEL-based machines have been providing light to users, many others are coming on line, and many more are planned. Some examples include DESY’s FLASH soft X-ray source that has been in operation since 2005, the Linac Coherent Light Source at the SLAC Accelerator Center that is in its commissioning stages and has observed lasing at 1.5 Å. The SCSS at Spring-8 Japan is in operation, FERMI in Trieste, Italy is set to produce light in 2010, and the European XFEL in Hamburg, Germany is scheduled to go on-line in 2015.
FERMI@Elettra is a single-pass FEL user-facility covering the wavelength range from
100 nm (12 eV) to 3 nm (300 eV) with the potential to provide a significant number of photons down to 1 nm (1.2 keV) via use of harmonics. FERMI is located next to the third-generation synchrotron radiation facility ELETTRA in Trieste, Italy and will be operated by Sincrotrone Trieste S.C.p.A. It employs a normal-conducting, 1.7-GeV, 3-GHz (S-band) linear accelerator
with beam supplied by a laser-driven photocathode radio-frequency (RF) electron gun. The linac uses two magnetic bunch compressors to increase the peak electron bunch current as well as one so-called laser heater device, recently demonstrated at the LCLS, that serves to reduce the micro-bunching instability. The FERMI@Elettra linear accelerator layout is illustrated in Figure 2. Actual installation is presently ongoing.
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(a)
(b) |
Figure 1. High brightness light source comparisons: peak brightness versus pulse duration. |
Figure 2. FERMI@Elettra linear accelerator layout: (a) photocathode-RF gun to first bunch compressor and (b) second bunch compressor to the linac’s end. |
The FERMI@Elettra FEL will make use of two undulator magnet strings or FEL lines designed to deliver photons to multiple experimental beamlines that serve three genres of scientific disciplines. The two FEL lines utilize the high-gain harmonic generation (HGHG) FEL process developed at Brookhaven National Laboratory and first demonstrated at 5.3-µm in a joint Brookhaven and Argonne National Laboratory collaboration. In HGHG a fully coherent (transverse and temporal) seed laser at a longer wavelength than the eventual FEL output is used to imprint the full coherence properties on the radiating electron beam and ultimately produce a short wavelength FEL pulse. The properties of the two FEL lines are listed in Table 1. Note that FEL-1 utilizes one HGHG FEL module while FEL-2 utilizes a cascade of two HGHG modules. The entire site layout of the FERMI@Elettra, co-located with the third-generation machine Elettra, is illustrated in Figure 3.
Table 1. FERMI@Elettra FEL-1 and FEL-2 parameters. |
Parameter
HGHG Stages
Fundamental Wavelength range [nm]
Output pulse length (rms) [fs]
Bandwidth (rms) [meV]
Polarization
Repetition rate [Hz]
Peak power [GW]
Harmonic peak power [GW]
Photons per pulse
Pulse-to-pulse stability
Pointing stability [µrad]
Virtual waist size [µm]
Divergence (rms, intensity) [µrad] |
FEL-1
1
100 to 20
< 100
17 (at 40 nm)
Fully Variable
50
1 to > 5
(% of fundamental) ~2
1014 (at 40 nm)
≥30 %
<20
250 (at 40 nm)
50 (at 40 nm) |
FEL-2
2 (“fresh bunch” in 2nd stage)
20 to 3 (1 at 3rd harm.)
20 – 100 (<10 future goal)
100 (at 4.2 nm)
Fully Variable
50
0.5 to 2
~0.2 (at 4.2 nm)
2 ×1012 (at 4.2 nm)
~40 %
<20
120
10 (at 4.2 nm) |
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| Figure 3. FERMI@Elettra and Elettra overview. |
Following generation the FEL photons are delivered to the experimental stations via the PADReS (Photon Analysis, Delivery, and Reduction System) beamline system. For each FEL line PADReS includes (from FEL toward experimental hall) a Front-End section (x-ray slits, shutter and radiation stopper), a Gas Monitor Detector 1 (GMD1) (combination ionization chamber and photon beam position monitor), a Gas Attenuator (GA), a Gas Monitor Detector 2 (GMD2), a Radiation Absorption Mirror (RAM), an X-ray spectrometer and a common Switching Chamber (SC) that directs
the light to the various beamlines leading to the experimental stations.
Three separate user end stations are dedicated to the following scientific research areas: Low Density Matter (LDM), Elastic and Inelastic Scattering (EIS) and Diffraction and Projection Imaging (DiProI). The LDM and EIS beamlines share the monochromatization system and are divided by a Refocusing Switching Mirror (RSM), while the DiProI beamline stands separate from the other two.
The advent of femtosecond lasers has revolutionized many areas of science from solid state physics to biology. FELs continue to extend this new ultra-short research frontier by providing similar high-power, ultra-short, coherent photon pulses into wavelengths from VUV to X-ray. The FERMI@Elettra is of this new generation of synchrotron light sources that will enable science by providing femtosecond, high peak power (~GW) pulses with femotosecond synchronization to external laser sources. Also, because of its use of undulators capable of switching over the full range of polarization, planar through circular, it can and will enable new science in areas such as magnetic dynamics. And finally, due to its implementation of a seeded harmonic cascade FEL scheme it should provide a stable, fully coherent pulse over the entire wavelength range to be explored. The FERMI@Elettra team looks forward to updating the IEEE NPSS once first photons are achieved.
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