  The ongoing development of intense high-harmonic generation (HHG) sources has
recently enabled highly nonlinear ionization of atoms by the absorption of at
least 10 extreme-ultraviolet (XUV) photons within a single atom [Senfftleben
\textit{et al.}, arXiv1911.01375]. Here we investigate the role that reshaping
of the fundamental, few-cycle, near-infrared (NIR) driving laser within the
30-cm-long HHG Xe medium plays in the generation of the intense HHG pulses.
Using an incident NIR intensity that is higher than what is required for
phase-matched HHG, signatures of reshaping are found by measuring the NIR
blueshift and the fluorescence from the HHG medium along the propagation axis.
These results are well reproduced by numerical calculations that show temporal
compression of the NIR pulses in the HHG medium. The simulations predict that
after refocusing an XUV beam waist radius of 320 nm and a clean attosecond
pulse train can be obtained in the focal plane, with an estimated XUV peak
intensity of 9x10^15 W/cm^2. Our results show that XUV intensities that were
previously only available at large-scale facilities can now be obtained using
moderately powerful table-top light sources.
