Oxford Centre for High Energy Density Science (OxCHEDS)

Abstract:

We have studied the picosecond lattice dynamics of optically pumped hexagonal LuMnO3 by using ultrafast x-ray diffraction. The results show a shift and broadening of the diffraction curve due to the stimulated lattice expansion. To understand the transient response of the lattice, the measured time- and angle-resolved diffraction curves are compared to a theoretical calculation based on the dynamical diffraction theory of coherent phonon propagation modified for the hexagonal crystal structure of LuMnO3. Our simulations reveal that a large coupling coefficient (c13) between the a-b plane and the c axis is required to fit the data. Though we interpret the transient response within the framework of thermal coherent phonons, we do not exclude the possibility of strong nonthermal coupling of the electronic excitation to the atomic framework. We compare this result to our previous coherent phonon studies of LuMnO3 in which we used optical pump-probe spectroscopy. © 2008 The American Physical Society.

Abstract:

The heating of solid targets irradiated by 5 x 10(20) W cm(-2), 0.8 ps, 1.05 microm wavelength laser light is studied by x-ray spectroscopy of the K-shell emission from thin layers of Ni, Mo, and V. A surface layer is heated to approximately 5 keV with an axial temperature gradient of 0.6 microm scale length. Images of Ni Ly(alpha) show the hot region has 100 G bar light pressure compresses the preformed plasma and drives a shock into the solid, heating a thin layer.

Abstract:

The heating of plane solid targets by the Vulcan petawatt laser at powers of 0.32-0.73 PW and intensities of up to 4 × 1020W cm -2 has been diagnosed with a temporal resolution of 17 ps and a spatial resolution of 30 μm, by measuring optical emission from the opposite side of the target to the laser with a streak camera. Second harmonic emission was filtered out and the target viewed at an angle to eliminate optical transition radiation. Spatial resolution was obtained by imaging the emission onto a bundle of fibre optics, arranged into a one-dimensional array at the camera entrance. The results show that a region 160 μm in diameter can be heated to a temperature of ∼107 K (kT/e ∼ keV) in solid targets from 10 to 20 μm thick and that this temperature is maintained for at least 20 ps, confirming the utility of PW lasers in the study of high energy density physics. Hybrid code modelling shows that magnetic field generation prevents increased target heating by electron refluxing above a certain target thickness and that the absorption of laser energy into electrons entering the solid target was between 15-30%, and tends to increase with laser energy. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

Abstract:

The heating of solid targets irradiated by 5×1020Wcm-2, 0.8 ps, 1.05μm wavelength laser light is studied by x-ray spectroscopy of the K-shell emission from thin layers of Ni, Mo, and V. A surface layer is heated to ∼5keV with an axial temperature gradient of 0.6μm scale length. Images of Ni Lyα show the hot region has ≤25μm diameter. These data are consistent with collisional particle-in-cell simulations using preformed plasma density profiles from hydrodynamic modeling which show that the >100Gbar light pressure compresses the preformed plasma and drives a shock into the solid, heating a thin layer. © 2008 The American Physical Society.