The FERMI single pass Free Electron Laser (FEL) lightsource at Elettra - Sincrotrone Trieste is one of the major FEL projects in Europe. It is the international user facility for scientific investigations in Italy employing ultra high brilliance X-ray pulses. The FEL design is based on an external seeding scheme that improves the output pulse coherence, central wavelength control and spectral bandwidth. The FEL output is tunable in power, wavelength, temporal duration, and polarization.
The seeding scheme by which FEL-1 line produces coherent output radiation down to 20 nm wavelength, is the following: An initial seed signal, provided by a conventional, high peak power, pulsed laser operating at 260 nm at the electron bunches repetition frequency, is temporally synchronized to overlap the electron beam, produced by the linac accelerator, in a first undulator section called the modulator (MOD). The laser field modulates the transversely wiggling electron bunch energy at its own frequency. This energy modulation is then converted to bunch charge spatial density modulation by passing the electrons through a chromatic dispersive section.
The resultant density modulation contains higher harmonics of the seed laser wavelength. A subsequent sect of undulator sections, named the radiators (RAD), is tuned in magnetic strength so that intense, coherent FEL radiation at a wavelength corresponding to one of these harmonics is emitted and then amplified to a high peak power level. The temporal duration of the FEL radiation is approximately that of the seed laser and the polarization properties of the radiator undulators.
The benefits of a Seeded FEL come from the control by the seed laser of the electron distribution within a bunch:
In order to be able to reach a final shorter wavelength range (from 20 to ~4 nm) but still starting from a seed laser at UV wavelength, FEL-2 is based on a double cascade of high gain harmonic generation (HGHG). The first stage is very similar to the FEL-1 configuration (but with a shorter radiator) while the second stage modulator is tuned to use output from the first stage radiator, again modulate the electron beam in energy and then density, and then produce coherent radiation in the second stage radiator at a wavelength tuned to a harmonic of the first stages output. Altogether then, the output wavelength of the second stage has been upshifted twice (once in each stage) from the wavelength of the input, external laser seed.FEL-2 also utilizes a magnetic delay line between its two stages in order to improve the FEL performance of the second stage by using the so-called "fresh bunch" technique.
|Photon Beam Parameters||FEL-1||FEL-2|
|Photon energy range / wavelength range||20 - 100 [nm]||4 - 20 [nm]|
|Photon energy/wavelength fluctuations||1 * 10-2 [%, RMS]||1 * 10-2 [%, RMS]|
|Spectral Bandwidth FWHM||50 - 100 [meV]||50 - 100 [meV]|
|Tunability||1 [%]||1 [%]|
|Other beam shape||TEM_00||TEM_00|
|Pulse duration FWHM||50 [fs]||50 [fs]|
|Polarisation||Linear Horizontal, Linear Vertical, Circular||Linear Horizontal, Linear Vertical, Circular|
|Pulse repetition rate||50 [Hz]||50 [Hz]|
|Maximum pulse energy||0.1 [mJ]||0.1 [mJ]|
|Pulse energy fluctuations||0.1 [sigma]||0.25 [sigma]|
|Peak power||1 * 109 [W]||1 * 109 [W]|
|Source size (spot)||X = 290 [um], Y = 290 [um]||X = 140 [um], Y = 140 [um]|
|Pointing stability at the source||5 [urad]||5 [urad]|