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Photon Beam Parameters

FEL-1: Wavelength range 100 nm – 20 nm

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 FEL is available in four polarization states: linear horizontal, linear vertical, circular right and circulat left. 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. In this operation mode, a limited tuning range of the seed of ±0.2% may be available for user experiments, upon specific request.
The following plots show the expected range of pulse duration and spectral bandwidth at the source in the whole tuning range.

The pulse duration is estimated according to  [1], starting with a seed of 100 fs.  The duration is in the (red) range between the two (black) lines. The upper (black-bold) line represents the typical value providing the highest power/energy. The lower (black dashed) line is the theoretical lower limit (at very low energy). 
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 20% (rms). The calculation was performed using the following parameters: seed pulse duration: 100 fs, electron-beam peak current: 500 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 in the range 60-10% in the spectral range100-26 nm, and a transmission of about 60% in the range  26-17 nm. Moreover, filters are required to remove unwanted radiation (due to seed laser, FEL harmonics, etc.). Typical filter transmissions are shown here here.


[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
Last Updated on Wednesday, 15 January 2020 08:57