Figure 8: Effect of the peak intensity and the presence of pre-pulses on the HHG efficiency at high photon energies with a 0.8-µm driver

(a) Measurement of the SXR spectrum as a function of the peak intensity in 390 mbar of helium in a 1-mm long drilled gas cell. The power is adjusted from 4 to 20 W by steps of 1 W with an iris placed before the off-axis parabola. (b) Measurement of the SXR spectrum in helium as a function of TOD with a peak intensity of 2×10^(15) W/cm^2 with a 0.03-mm wide open-end nozzle. The 3 inserts show the driving laser temporal shape measured with frequency-resolved optical gating for three different values of TOD (-4000 fs^3, 0 fs^3 and 4000 fs^3).