  One of the main difficulties to efficiently generating high-order harmonics
in long neutral-gas targets is to reach the phase-matching conditions. One
issue is that the medium cannot be sufficiently ionized by the driving laser
due to plasma defocusing. We propose a method to improve the phase-matching by
pre-ionizing the gas using a weak capillary discharge. We have demonstrated
this mechanism, for the first time, in absorption-limited XUV generation by an
800 nm femtosecond laser in argon and krypton. The phase-mismatch control
ability of our method is confirmed by an analytical model and numerical
simulation of the complete generation process. Our method allows increasing the
efficiency of the harmonic generation significantly, paving the way towards
photon-hungry applications of these short-wavelength compact sources.
