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).

(a) 在长度为1 mm的钻孔式气室中充入390 mbar氦气，测量软X射线（Soft X-Ray，SXR）光谱随峰值光强的变化关系。通过在离轴抛物面反射镜前加装光阑，将激光功率以1 W为步长从4 W调节至20 W。
(b) 采用开口宽度为0.03 mm的开放式喷嘴，在峰值光强为2×10^15 W/cm²的条件下，测量氦气环境中软X射线光谱随三阶色散（Third-order Dispersion，TOD）的变化关系。三张内嵌插图展示了利用频率分辨光学门（Frequency-Resolved Optical Gating）测得的三种不同TOD取值（-4000 fs³、0 fs³和4000 fs³）下的驱动激光时域波形。