Dispersion and transport of energetic particles created during the interaction of intense laser pulses with overdense plasma

We present the results of a parametric study of the interaction of intense laser pulse of intensity varying from 10 18 W/cm 2 up to 10 21 W/cm 2 with plasmas of density ranging from 10 times up to 160 times critical density. This study has been performed with the help of large 2D and 3D PIC simulations. It gives clear evidences that the divergence of the beam is an intrinsic property of the interaction of a laser pulse with a sharp density gradient. This divergence is related to the excitation of large static magnetic fields in the layer of interaction. The effective spread is a complex balance between the intensity of the beam and the plasma density.