Beamline setup
Sketch of the optical layout of EIS-TIMER (top view)
The TIMER beamline is on an independent branch of the FERMI transport. The beam exiting from the Photon Analysis, Delivery and Reduction System (PADReS; not shown) is deflected towards the EIS-TIMER beamline by a switching plane mirror (SM). Then the beam impinges into a plane mirror (M1) working as a wavefront-division beam splitter, i.e. it intercepts just half of the beam diameter. One half of the beam is reflected upwards while the other half can propagate freely. The latter beam is further split, in the horizontal plane, by a second wavefront-division beamsplitter (M2). The beam reflected by M1 will be used as probe pulse, while the two FEL beams emerging from M2 are the pump pulses and are focused at the sample position by a pair of toroidal mirrors (FMA and FMB). Four pairs of such mirrors can be inserted and removed from the beam paths in order to change the crossing angle (2θ), and hence the transient grating vector:
k=(4π/λFEL)sin(2θ/2),
where λFEL is the pump wavelength. In the current configuration the possible values of 2θ are: 18.4o, 27.6o, 79o and 105.4o.
The optical layout is designed to provide focal spots of about 200x100 um2 and a spatial profile that somehow optimizes the overlap conditions (see sketch).
The combination of such (2θ,λFEL)-values permits to map the 0.03-1.1 nm-1 momentum range, as summarized in the table below.
The time delay between the crossed FEL pulses can be adjusted in the ~5 ps range (see bottom-left sketch). In case of double-pulse operation, this also allows to easily delay the two trains of pulses relatively to each other, in order to overlap the first pulse coming from one side with the second one coming from the other side. This enables, for example, to perform coherent Raman scattering (CRS) experiments where the two pumps have different wavelength.
The third FEL pulse, reflected upwards by M1 and made coplanar to the pump trajectories by two additional plane mirrors (M3 and M4, not shown), goes through a delay line equipped with 4 multilayer mirrors (ML) at 45o incidence. Currently, one can select one out of 4 sets of 4 narrow-band MLs (designed to reflect the FEL radiation at λpr=20.8, 16.5, 13.3 and 6.72 nm) that act as bandpass filters for the wavelength of the probe photons, with an ideal intensity reduction by a factor 10 to 100. Other wavelengths might be possible upon request or the users can provide their own set of ML mirrors. The maximum time delay range depends on the chosen value of 2θ and varies from ~3.5 ns (2θ=18.4o) to ~1.5 ns (2θ=105.4o). The probe pulse is focused onto the sample by other 4 (insertable) toroidal mirrors. The beamline is designed to meet the TG phase matching conditions (Bragg diffraction) for λFEL=3λpr. The TG signal can be measured both in transmission and in reflection.
2θ / λFEL
62.4
49.5
39.9
20
18.4°
0.03
0.04
0.05
0.1
27.6°
0.05
0.06
0.08
0.15
79°
0.14
0.17
0.21
0.4
105°
0.16
0.2
0.25
0.5
Important note
The full set of wavelengths cannot be exploited in a single beamtime, since 20.8 nm is in the range of FEL1 and 13.3 and 6.7 are in the range of FEL2. 16.5 nm is the only wavelength that is, under given circumstances, accessible with both sources.References:
Advances in instrumentation for FEL-based four-wave-mixing experiments
Riccardo Mincigrucci, L. Foglia, D. Naumenko, E. Pedersoli, A. Simoncig, R. Cucini, A. Gessini, M. Kiskinova, G. Kurdi, N. Mahne, M. Manfredda, I. P. Nikolov, E. Principi, L. Raimondi, M. Zangrando, C. Masciovecchio, F. Capotondi, F. Bencivenga
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, Vol. 907, pp. 132 - 148 (2018)
doi: 10.1016/j.nima.2018.03.051
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