Propagation of a randomized 600-ps laser beam in a helium gas jet over long scale lengths - art. no. 026404

The propagation and interaction of a randomized 600-ps laser with a helium gas jet were studied experimentally for laser intensities of 10(14) W/cm(2) . Such a study is of interest for the indirectly driven inertial confinemen t fusion scheme, where a randomized laser beam propagates into a gas-filled cavity over a distance of a few millimeters. The dynamics of ionization wa s studied using time resolved interferometry. Maps of electronic density n( e)(z,t) were retrieved from time resolved interferograms. The plasma temper ature was studied using Thomson scattering. The results show that the laser diffracts while propagating, leading to a decrease in laser intensity and causing ionization to occur later in time. An ionization front, moving at a velocity of about upsilon (f)similar or equal to2.8X 10(6) m/s, was observ ed. Beam diffraction also causes a nonhomogeneous heating of the plasma: th e entrance of the plasma is hotter than the exit. A one-dimensional model w as used to fit the results. It takes into account collisional ionization an d heating by inverse bremsstrahlung. The model shows very good agreement wi th the experiment.