Prof Peter Norreys FInstP;

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.

Abstract:

Laser-driven MeV proton beams are highly suitable for quantitative diagnosis of density profiles in dense matter by employing them as a particle probe in a point-projection imaging scheme. Via differential scattering and stopping, the technique allows to detect density modulations in dense compressed matter with intrinsic high spatial and temporal resolutions. The technique offers a viable alternative/complementary route to more established radiographic methods. A Monte-Carlo simulation package, MPRM, has been developed in order to quantify the density profile of the probed object from the experimentally obtained proton radiographs. A discussion of recent progress in this area is presented on the basis of analysis of experimental data, which has been supported by MPRM simulation. © 2008 Elsevier B.V. All rights reserved.

Abstract:

Experiments were performed in which intense laser pulses (up to 9×1019 W/cm2) were used to irradiate very thin (submicron) mass-limited aluminum foil targets. Such interactions generated high-order harmonic radiation (greater than the 25th order) which was detected at the rear of the target and which was significantly broadened, modulated, and depolarized because of passage through the dense relativistic plasma. The spectral modifications are shown to be due to the laser absorption into hot electrons and the subsequent sharply increasing relativistic electron component within the dense plasma. © 2008 The American Physical Society.