Fig. 1. HGHG seeding scheme at FERMI: (a) single stage, FEL1; (b) fresh-bunch double stage, FEL2. DS: dispersive section.

Fig. 2. Schematic layout of the seed laser table. THG, third harmonic generation setup; SLU, pulse sent to the user stations; IR-SSXC, IR single-shot cross-correlator; UV-AC, UV autocorrelator; UV-XCORR, UV cross-correlator; COMPR, UV grating compressor.

Fig. 3. OPA1 output pulse energy versus wavelength for (a) second harmonic of the sum frequency of signal (SH-SFS) process and (b) fourth harmonic of signal (FHS) process.

Fig. 4. FEL1 wavelength tunability and estimated pulse duration (from Equation (2)) at harmonic orders 3–15 for (a) seed range 232–267 nm and (b) seed range 295–360 nm.

Fig. 5. FROG trace of the compressed seed pulse at 325 nm: (a) measured and (b) retrieved FROG trace with a G error of G = 0.0077; (c) retrieved temporal intensity and phase; (d) retrieved spectral intensity (red) and phase with the independently measured spectrum (blue). The retrieved temporal and spectral intensity FWHM values are 49 fs and 2.73 nm, respectively.

Fig. 6. FEL2 pulse duration with OPA and THG seed, with first stage harmonic orders 3–15 and second stage harmonic orders 2–7.

Fig. 7. OPA1 output central wavelength stability measured over 4 hours.

Fig. 8. Schematic layout of the seed beam insertion breadboard (IBB) of FEL1. SLR, beam from seed laser room; CCD BT, CCD VU1, CCD VU2, cameras for monitoring the beam after the optical transport and in two virtual planes corresponding to the undulator entrance and exit, respectively; EM, energy meter.

Fig. 9. Pointing stability of the FEL1 seed beam position in the horizontal (upper trace) and vertical (lower trace) planes, measured on CCD VU2 over 8 hours of continuous operation at 50 Hz with transverse feedback ON; one shot is registered every second.

Fig. 10. Schematic layout of the seed laser synchronization setup. TMU Rf, coarse synchronization unit; LNA, low-noise amplifiers; PI, proportional integral; TMU, timing control unit; Ti:Sa, titanium sapphire; pzt, piezo-based actuator.

Fig. 11. (a) Phase noise and timing jitter of the seed oscillator. Left axis, integrated timing jitter; right axis, phase noise spectral density. (b) Timing stability of the seed laser oscillator, measured out of loop using a second balanced optical cross correlator (BOCC).

Fig. 12. Schematic layout of the seed laser for users (SLU) system. SLR, beam from the seed laser room; Low Vacuum BT, optical beam transport in chambers and tubes at 1–10 mbar vacuum level; CDP, common distribution point of the SLU pulse in the FERMI Experimental Hall; CCD BT, camera for beam position control; COMPR, grating compressor.

Fig. 13. Expected tunability of FEL1 in echo enabled harmonic generation (EEHG) mode by using harmonic order n = −1 and harmonic order m = 13–26.

Fig. 14. Planned seed laser system layout after the upgrade. RG1, RG2, RG3, regenerative amplifiers; IBB1 FEL1 and IBB2 FEL2 are the insertion breadboards of the corresponding seed pulse to the FEL lines; SH, second harmonic; EOS FEL2, beam provided for electro-optical sampling setup; the solid blocks and arrows indicate existing units and beam transport lines while the dashed blocks and lines indicate planned additions.