Fig. 1. Schematic diagram of the ELI-NP HPLS. Marked with green is the stretcher that was updated during the activities reported here.

Fig. 2. Schematic diagram of the Offner-type stretcher implemented at the ELI-NP HPLS.

Fig. 3. Roughness measurements for the old mirror (top row) and new, highly polished mirror (bottom row) performed on three separate locations.

Fig. 4. Schematic diagram of the new reflective Martinez-type stretcher implemented at the ELI-NP HPLS.

Fig. 5. Contrast measurement curves for the three cases: grey curve, Offner-type stretcher using a regular convex mirror; blue curve, Offner-type stretcher using an ultrahigh-quality convex mirror; orange curve, reflective Martinez-type stretcher. The peak at approximately 9 ps is generated by a bulk dispersion compensator. The inset provides a clearer representation of the position of the approximately 9 ps pre-pulses, thereby revealing the sampling rate of the measuring device and the peaks with similar relative positions.

Fig. 6. Contrast measurement at 1 shot per minute, showing similar contrast obtained with AMP 3 running at nominal parameters to the measurements presented in Figure 5: blue cross, similar conditions to that in the Figure 5 blue curve (no pump on AMP 3); red rhomb, similar to the previous and with AMP 3 pumped; grey circle, reproduction of the blue curve from Figure 5 for comparison. The difficulty of optimizing the alignment at a low repetition rate leads to a lower dynamic range obtained for these measurements in comparison to the one presented in Figure 5. The inset plots show details of the measurement of the –100 ps region (a), of the pre-pulse at approximately 9 ps (b) and of the main pulse (c).