  Bright Ar K-shell x-ray with very little background has been generated using
an Ar clustering gas jet target irradiated with an 800 mJ, 30 fs ultra-high
contrast laser, with the measured flux of 1.1 x 10^4 photons/mrad^2/pulse. This
intense x-ray source critically depends on the laser contrast and the laser
energy and the optimization of this source with interaction is addressed.
Electron driven by laser electric field directly via nonlinear resonant is
proved in simulation, resulting in effective electron heating and the
enhancement of x-ray emission. The x-ray pulse duration is demonstrated to be
only 10 fs, as well as a source size of 20 um, posing great potential
application for single-shot ultrafast x-ray imaging.
