Within the next few years X-ray free-electron lasers (XFELs) now
under construction are expected to generate highly collimated XUV
pulses with 1013 photons and a duration of 100 fs. Focusing
this radiation to a spot some 10 μm in diameter generates
intensities of up to 1016 W/cm2. Such pump
intensities make feasible the investigation of photopumped XUV lasers
using this radiation. We present simulations taking into account two
different mechanisms generating the gain: (1) photoionization with
subsequent three-body recombination, which takes advantage of the
monochromaticity of the pump radiation to generate very cold electrons;
(2) inner-shell ionization in which transient inversion is obtained by
generating a hole in an otherwise completely filled shell. The
simulations show that under appropriate conditions both mechanisms
generate very high gain. However, a number of further issues must be
considered, such as the propagation of the pump pulse in the medium to
be pumped.