  It is demonstrated by single-atom simulations that X-ray signals in the 3.4
to 4 keV region from an 8 micron laser driven high harmonic generation can be
increased by more than two orders of magnitude when a single-cycle pulse
centered at 800 nm is added. The ionization probability of a helium atom by the
two-pulse field is set to 4.56x10^-5, which is needed for balancing the index
of refraction of free electrons with that of neutral helium atoms to achieve
phase matching.
