FEL-1: Wavelength range 100 nm – 20 nm
If you are looking for the standard photon-beam parameters FERMI FEL-1 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-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 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.
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
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
Optical laser available for pump-probe experiments
The currently available pump-probe setups at all FERMI end-stations are based on the use of the infrared (IR) laser pulse generated by one of the FERMI seed laser Ti:Sapphire amplifiers, propagated to the experimental hall by a high-stability optical beam transport. This IR laser beam, referred to as SLU, is then delivered to a dedicated insertion breadboard at each end-station (one at a time), where the final beam manipulation, including polarization state adjustment, harmonic conversion, pulse compression, beam steering and focusing, as well as diagnostics and pointing stabilization, is performed.The typical values of the main pulse parameters available are summarized in the following table:
| Central wavelength, nm | 794(1) | 392 | 261 |
| Max Pulse Energy , mJ | 2(2) | 0.1-0.5 | 0.04-0.1 |
| Compressed Pulse Duration , fs (FWHM) (3) | 55-65 | 60-90 | 100-170 |
| Typical jitter with respect to FEL pulse, fs (RMS) | < 8 | ||
| Min Beam Diameter on sample, μm (1/e2 ) | 50-80 | ||
| Stability of beam position on sample, μm (RMS) | < 3 | ||
| Maximum Laser intensity on sample , W/cm2 | 1-2 x 1014 | ||
Tunable pulses in the ranges 740-780 nm and 800-880 nm with an energy per pulse in the range 50-120 µJ can be made available, however need to be requested at the stage of proposal submission.
(1) On request can be adjusted to other values in the range 784-800 nm (and respectively the harmonics). When FEL1 is operated, the SLU wavelength can be remotely tuned in real time in this range.
(2) Measured at the beamline optical breadboard.
(3) Fundamental (IR) pulses with desired chirp value and sign can be provided. The harmonic pulse duration can be optimized for the needs of the experiment by the choice of nonlinear crystal length combination, within the limits imposed by the required harmonic pulse energy.
As described here, both FEL-1 and FEL-2 allow one to implement pump-probe schemes in which both the pump and the probe are FEL pulses. The performance of these schemes is quite flexible and can be tailored based on user requirements. For more information and specific questions, users are invited to send a message to the address
Options providing an extension of the above parameters towards higher energy and shorter pulses, as well as the addition of new wavelength ranges by an OPA at some of the end stations, are expected to be available in the near future.
