  Characteristics of the intense ultrafast quazi-monochromatic X-ray source at
17.4 keV generated by a multi-TW femtosecond Ti:Sa-laser beam ($\lambda$=800
nm) tightly focused on a thick molybdenum target are investigated within a wide
range of laser pulse peak intensities ($10^{16}$ - $10^{19}$ W/cm$^2$) and
temporal contrast ($10^6$ - $10^{10}$). Spatial and absolute
spectrally-resolved energetic parameters of produced pulsed X ray emission are
measured. Fluxes of Mo K$\alpha$ line radiation reaching $2\times 10^9$
photons/sr per laser shot and energy conversion efficiency as high as
$3.2\times 10^{-5} sr^{-1}$ are obtained at high laser contrast of $>10^9$. The
interplay of resonance absorption, vacuum heating and ponderomotive (JxB)
heating mechanisms of the laser-plasma interaction is clearly demonstrated by
different K$\alpha$ conversion efficiency dependencies at different laser pulse
intensity and contrast parameters. The results are confirmed by examining
dependencies on the laser incidence angle. In the relativistic intensity
regime, the K$\alpha$ efficiency shows similar values within the whole examined
range of the laser temporal contrast, which varies over 4 orders of magnitude
resulting in the electron density scale length L of the pre-formed plasma
changing from L<<$\lambda$ to L>$\lambda$. It confirms the dominant role of the
JxB mechanism accompanied by the effect of plasma interface steepening under
laser radiation pressure in the relativistic interaction regime.
