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FEL-1: Wavelength range 100 nm – 20 nm

Wavelength Tuneability & Stability

FEL-1 is an externally seeded free-electron laser (FEL) source, characterized by a high degree of longitudinal and transverse coherence and high wavelength stability (fluctuations <10-4 rms, typically). When the seed is generated by an optical parametric laser amplifier (OPA), the FEL is continuously tunable. However, covering the whole FEL tuning range (100-20 nm) requires the variation of the electron-beam energy, which is not possible during the same user beam time. Typical tuning ranges available for a given experiment are 65-20 nm or 100-30 nm.
The best performance in terms of FEL power and spectro-temporal quality is obtained when the FEL is seeded using the third harmonic of the Ti:Sapphire amplifier (around 261 nm). In this case, the FEL light is produced at the integer harmonics of the seed (see table below), and the relative spectral width is typically of the order of 1x10-3 (fwhm). In this operation mode, a limited tuning range of the seed of ±0.2% may be available for user experiments, upon specific request.



The FEL is available in four polarization states:
  • 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 tunability is available in linear horizontal polarization (LH). As an example, for an electron-beam energy of  1.3 GeV 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

The typical energy per pulse is in the range 20-100 μJ. The upper limit depends on the operation wavelength and polarization 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 duration. Therefore, an increase in energy does not necessarily imply an increase in peak power.
The FEL light at a given wavelength is typically mixed to 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).

Ultima modifica il Martedì, 14 Maggio 2019 16:54