KeV x-ray emission produced by a sub-picosecond laser interacting with a controlled preformed plasma

The efficiency of the conversion from laser energy into aluminium K-shell line energy hhs been measured for different interaction regimes of a sub-picosecond laser pulse with controlled preformed plasmas. We compare the cases of thermal plasmas for which transient ionization effects are dominant to non-thermal plasmas in which both transient and non-Maxwellian effects are present. In a first experiment, a low-intensity green laser pulse (400 fs, 10(16) W cm(-2)) is normally incident on a preformed plasma previously generated by a similar laser pulse. In a second experiment a high-intensity p-polarized green pulse (400 fs, 5 x 10(17) W cm(-2)) is sent with a large incidence angle on the preformed plasma. We have observed in these experiments that the conversion efficiency (He-alpha or Li-like lines from aluminium) scales as tau(X)(3), where tau(X) is the duration of the x-ray emission measured at the full width half maximum, and that this scaling was not affected by non-Maxwellian effects.