Experimental study of laser penetration in overdense plasmas at relativistic intensities. II: Explosion of thin foils by laser driven fast electrons

Propagation of a high-contrast frequency-doubled subpicosecond (300 fs) rel ativistic (I lambda(2) up to 5x10(18) W.cm(-2).mu m(2)) laser pulse through thin and initially solid foils is studied. Transmission values up to 10% a re measured through targets with initial near solid densities. The strong i ntensity threshold observed for the transmitted energy is correlated with c lear modifications of the transmitted and reflected spectra, electron gener ation, and beam imaging. Two-dimensional Cartesian particle-in-cell (PIC) s imulations that qualitatively reproduce the experimental results suggest sp ecific rapid heating of the thin targets by fast electrons, plasma expansio n, and density decrease to relativistically transmissive conditions during the pulse. (C) 1999 American Institute of Physics. [S1070-664X(99)01006-X].