FEL-2: Wavelength range 20 nm – 4 nm
If you are looking for the standard photon-beam parameters FERMI FEL-2 can offer during user experiments, please look here. If you are interested in a more general overview of FEL properties and/or in the peak performance we can reach during dedicated machine experiments, please read this page.
FEL-2 is an externally seeded FEL source, characterized by a high stability of the output central wavelength (fluctuations <10-4 rms, typically). FEL-2 is based on a double-stage setup, with a double harmonic conversion. As in the case of FEL-1, the best energy performance and spectral quality are obtained when the FEL is seeded using the third harmonic of the Ti:Sapphire amplifier (around 261 nm). In this case, only the integer harmonics of the seed are available. A limited tuning range of the seed of ±0.2% may be available for user experiments, upon specific request.
The FEL is instead continuously tunable when the seed is generated by an OPA. In this case the tuning range is still limited by the beam energy. At fixed beam energy, i.e. during a specific user beam time, the fine tuning range around a given harmonic is approximately limited to ±10-20%, depending on the requested polarization.
FEL-2 is available in four polarization states: linear horizontal, linear vertical, circular right and circulat left.
The following plots show the expected range of pulse duration and spectral bandwidth at the source in the whole tuning range.
The spectral bandwidth is estimated from the pulse duration at the Fourier limit (black-dashed line). More typical values providing the highest pulse energy (and accounting for presence of pedestals) are represented by the black-solid line.
The following plots show the estimated pulse energy at the source, at given wavelengths and electron-beam energies, for different available polarizations.
The OPA seed in the range 240-260 nm gives several prohibited intervals, which are indicated in the plots. The shot to shot energy stability is typically better than 40% (rms). The calculation was performed using the following parameters: seed pulse duration: 70 fs, electron-beam peak current: 600 A, electron-beam relative slice energy spread: 10-4.
An indication of the FEL performance at sample position, i.e., after photon-beam transport, can be obtained by using the plots above, and taking into account a beamline transmission of about 70% in the spectral range 20-10 nm, a transmission of about 20-70% in the range 10-6 nm, a transmission of about 20-40% in the range 6-5 nm, and a transmission of about 10-25% in the range 5-4 nm. Moreover, filters are required to remove unwanted radiation (due to seed laser, FEL harmonics, etc.). Typical filter transmissions are shown here.
References
[1] Finetti Paola et al., Pulse duration of seeded free-electron lasers, Physical Review X, Vol. 7 - 2, pp. 021043 (2017)
doi: 10.1103/PhysRevX.7.021043
