Energy scaling of high-order harmonics in soft X-ray region

The advent of ultrashort high-peak power lasers enables us to investigate a variety of phenomena of high-field interaction with atom or ions. High-order harmonic generation (HHG) is one of such interesting phenomena. HHG can produce ultrashort coherent extreme ultraviolet (EUV) pulses from a compact laser system. This emission source is useful for EUV nonlinear optics and attosecond physics, and EUV spectroscopy. We investigated the energy scaling of the HHG. The efficiency and the spatial quality of high-order harmonics under phase-matched conditions strongly depend on the target gas density, the interaction length, and the pump laser focusing geometry. Based on our model calculation, a focusing geometry was designed to deliver several tens mJ pump pulses into a 100-mm-long interaction cell.