  We report a novel quasi-phase matching technique for high-order harmonic
generation in low-density gases. Numerical simulations show that in few-optical
cycle pulsed Bessel beams it is possible to control the pulse envelope and
phase velocities which in turn allows to control the carrier-envelope phase
during propagation. The resulting oscillations in the peak intensity allow to
phase-match the high-order harmonic generation process with a nearly two decade
enhancement in the XUV power spectrum.
