  In our recent study [Commun. Phys. 3, 92 (2020)], we have developed an
approach for energy-scaling of high-order harmonic generation in water-window
region under neutral-medium condition. More specifically, we obtained
nanojoule-class water-window soft x-ray harmonic beam under phase match
condition. It has been achieved by combining a newly developed terawatt-class
mid-infrared femtosecond laser and a loose focusing geometry for high-order
harmonic generation. The generated beam is more than 100 times intense compared
to previously reported results. The experimental setup included two key parts:
terawatt mid-infrared femtosecond driving laser [ Sci. Rep. 8, 7692 (2018)] and
specially designed gas cell. Despite the dramatic drop in the optimal gas
pressure due to loose focusing geometry, it still reached 1 bar level for
helium. Moreover, faster leaking speed caused by larger pinhole size of the gas
cell made the use of a normal gas cell impossible. Thus, we have designed a
double-structured pulsed-gas cell with a differential pumping system, which
enabled providing sufficiently high gas pressure. Moreover, it allowed reducing
gas consumption significantly. Robust energy-scalable apparatus for high-order
harmonic generation developed in in this study will enable the generation of
over tens nanojoule water-window attosecond pulses in the nearest future.
