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Photon Beam Parameters

FEL-1: Wavelength range 100 nm – 20 nm

Wavelength Tuneability & Stability

FEL-1 is an externally seeded FEL source, characterized by a high stability of the output central wavelength (typically <10-4 rms). When the seed is generated by the Optical Parametric Laser Amplifier (OPA mode), the FEL is continuously tunable. However, the FEL tuning range of 100-20 nm is covered with different machine operation setups (the OPA non-linear process and/or electron beam energy). Therefore the full tuning range cannot be provided at the same user beamtime/machine run. Typical tuning ranges available are 65-20 nm or 100-30 nm and, with additional restrictions, at long wavelengths, depending on the OPA tuning range (e.g. 230 nm-260 nm) and harmonic order.

The best energy performances and spectral quality are available when the FEL is seeded at fixed wavelength, at the third harmonic of the Ti:Sapphire amplifier (261.6 nm – THG mode). In this case the integer harmonics of the seed are available (see table below) and the spectral width is typically Δλ/λ ≈ 1x10-3 (fwhm). A limited tuning range of ±0.2% may be considered upon specific request with the seed in THG mode.


The light from FERMI is characterized by a high degree of polarization (>90%). The FEL is available in four states of polarizaton:
  • Linear Horizontal (LH)
  • Linear Vertical (LV)
  • Circular Right (CR)
  • Circulat Left (CL)

The highest energy per pulse is available in circular polarization (CR or CL), while the widest wavelength tuneability is available in linear horizontal polarization (LH). As an example (at 1.3 GeV of electron beam energy) during the same user beamtime the range 20-65 nm can be typically covered in LH, the range reduces to 20-53 nm in CR and CL and to 43 nm-20 nm in LV.

Energy per pulse

Typical energy per pulse in the range 20-100 μJ is available. The upper limit depends on the operation wavelength and polarizartion state. A factor of two or more energy may be available by relaxing the condition on spectral purity. Note that an increase of pulse energy at the expense of spectral purity may be associated to an increase of the pulse length. Therefore, an increase in energy does not necessarily imply an increase in peak power.
The FEL light at the selected wavelength is typically mixed with a small fraction of the seed light reaching the experimental chamber and may contain a fraction (10-2/10-3) of energy at higher harmonics. Filtering these components may affect the effective pulse energy at the sample.

Energy stability

The shot to shot energy stability is typically better than 20% (rms).

Pulse length

The expected pulse length depends on the seed pulse properties and on the harmonic order. With a seed of 130 fs (fwhm) an FEL pulse length shorter than 100 fs is expected.


E. Allaria et al., Nat. Photonics 6, 699 (2012)
E. Allaria et al., New J. Phys. 14, 113009 (2012).
E. Allaria et al., Nat. Photonics 7, 913 (2013).
E. Allaria et al., Phys. Rev. X 4, 041040 (2014).

Last Updated on Monday, 12 December 2016 18:47