We observed the increase of the conversion efficiency from laser energy to $K\alpha$ x-ray energy (${\eta }_K$) produced by a 60 fs frequency doubled high-contrast laser pulse focused on a Cu foil, compared to the case of the fundamental laser pulse. ${\eta }_K$ shows a strong dependence on the nonlinearly modified rising edge of the laser pulse. It reaches a maximum for a 100 fs negatively modified pulse. The hot electron efficient heating leads to the enhancement of ${\eta }_K$. This demonstrates that high-contrast lasers are an effective tool for optimizing ${\eta }_K$, via increasing the hot electrons by vacuum heating.

• Received 6 October 2006

DOI:https://doi.org/10.1103/PhysRevLett.100.045004