Experimental study of laser penetration in overdense plasmas at relativistic intensities.: I:: Hole boring through preformed plasmas layers

Propagation of a high-contrast subpicosecond (400-600 fs) relativistic (I lambda(2) up to 2x10(19) W.cm(-2).mu m(2)) laser pulse through long preformed overdense plasmas is experimentally studied. Transmission values up to 10% are measured in plasmas with initial peak densities above 10 x n(c). We compare these results with one-dimensional (1D) analytical calculations of the hole boring effect. It is shown that the ponderomotive force of the laser beam can push forward the overdense plasma, create a channel, and lead to transmission levels similar to the experimental ones. (C) 1999 American Institute of Physics. [S1070-664X(99)00906-4].