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

The spatial structure of the Kα 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<5×10¹⁷W/cm², the observed spatial structure of the Kα 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=7×10¹⁸W/cm² the half-width of the Kα emission was 70μm compared to a laser-focus half-width of 3μm. Moreover, the main Kα peak was surrounded by a halo of weak Kα emission with a diameter of 400μm and the Kα 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.