  We report the experimental observation of an enhancement in the yield of
high-order harmonics using an array of gas jets as the source medium. By
comparing the experimental outcome for jet arrays of different spacings with
the predicted harmonic intensity in the case of slit sources of equivalent
lengths, we clearly show how the periodic modulation of the gas density can
improve the harmonic yield. This behavior may attributed to a
quasi-phase-matching effect which increases the length of coherent harmonic
build-up during propagation by partially counteracting the dephasing induced by
free electrons.
