Spatial characteristics of Kalpha x-ray emission from relativistic femtosecond laser plasmas.

The spatial structure of the Kalpha emission from Ti targets irradiated with a high intensity femtosecond laser has been studied using a two-dimensional monochromatic imaging technique. For laser intensities I<5 x 10(17) W/cm(2), the observed spatial structure of the Kalpha emission can be explained by the scattering of the hot electrons inside the solid with the help of a hybrid particle-in-cell/Monte Carlo model. By contrast, at the maximum laser intensity I=7 x 10(18) W/cm(2) the half-width of the Kalpha emission was 70 microm compared to a laser-focus half-width of 3 microm. Moreover, the main Kalpha peak was surrounded by a halo of weak Kalpha emission with a diameter of 400 microm and the Kalpha intensity at the source center did not increase with increasing laser intensity. These three features point to the existence of strong self-induced fields, which redirect the hot electrons over the target surface.