R. Gaillard

Intense laser pulse propagation and channel formation through plasmas relevant for the fast ignitor scheme

Measurements of self-channeling of picosecond laser pulses due to relativistic and ponderomotive expulsion effects have been obtained in preformed plasmas at laser irradiances between 5–9×1018 Wcm−2. The self-focused channel was surrounded by a multi-megagauss magnetic field. The orientation of the field was consistent with a forward going relativistic electron beam propagating along the laser pulse. Self-channeling and magnetic field generation mechanisms were modeled by multidimensional particle-in-cell (PIC) simulations and good agreement was obtained with the experimental observations. Measurements of the channel expansion after the interaction were obtained and the rate of expansion was consistent with a blast wave solution. The level of transmission of an intense laser pulse through such performed density channels was observed to increase significantly compared to the case without a channel. High levels of transmission of an intense laser pulse through microtubes were also observed. The relevance of...

Inertial confinement fusion and fast ignitor studies

Laser imprinting has been studied and, in particular, saturation of areal density perturbations induced by near single mode laser imprinting was observed. Several issues important for the foam buffered direct drive scheme have been investigated. These studies included measurements of the absolute levels of stimulated Brillouin and Raman scattering observed from laser irradiated low density foam targets, either bare or overcoated with a thin layer of gold. A novel scheme is proposed to increase the pressure in indirectly driven targets. By heating a foam supersonically that is attached to a solid target the pressure generated is not only the ablation pressure but also the combined pressure due to ablation at the foam-foil interface and the heated foam material. Planar brominated plastic foil targets overcoated with a low density foam were irradiated by a soft X ray pulse. The pressure was obtained by comparing the rear side trajectory of the driven target observed by soft X ray radiography with one dimensional radiation hydrodynamic simulations. Observations were also carried out of the transition from supersonic to subsonic propagation of an ionization front in low density chlorinated foam targets irradiated by an intense soft X ray pulse. The diagnostic for these measurements was K shell point projection absorption spectroscopy. In the fast ignitor area the channelling and guiding of picosecond laser pulses through underdense plasmas, preformed density channels and microtubes were investigated. It was observed that a large fraction of the incident laser energy can be propagated. Megagauss magnetic fields were measured, with a polarimetric technique, during and after propagation of intense picosecond pulses in preionized plasmas. Two types of toroidal fields, of opposite orientation, were detected. In addition, the production and propagation of an electron beam through solid glass targets irradiated at intensities above 1019W/cm2 were observed using optical probing techniques.