Prof Peter Norreys FInstP;

Abstract:

Cone-guided fast ignition laser fusion depends critically on details of the interaction of an intense laser pulse with the inside tip of a cone. Generation of relativistic electrons in the laser plasma interaction (LPI) with a gold cone and their subsequent transport into a copper wire have been studied using a kJ-class intense laser pulse, OMEGA EP (850 J, 10 ps). Weobserved that the laser-pulse-energy-normalized copper K signal from the Cu wire attached to the Au cone is significantly reduced (by a factor of 5) as compared to that from identical targets using the Titan laser (150 J, 0.7 ps) with 60 × less energy in the prepulse. We conclude that the decreased coupling is due to increased prepulse energy rather than 10 ps pulse duration, for which this effect has not been previously explored. The collisional particle-in-cell code PICLS demonstrates that the preformed plasma has a significant impact on generation of electrons and their transport. In particular, a longer scale length preplasma significantly reduces the energy coupling from the intense laser to the wire due to the larger offset distance between the relativistic critical density surface and the cone tip as well as a wider divergence of source electrons. We also observed that laser-driven plasma ionization increase in the LPI region can potentially alter the electron density profile during the laser interaction, forcing the electron source to be moved farther away from the cone tip which contributes to the reduction of energy coupling. © IOP Publishing and Deutsche Physikalische Gesellschaft.

Abstract:

Laser pulses of 0.9 kJ/1 ns/1053 nm were focused onto low-Z plastic targets in both spherical and planar geometry. The uniformity of the resulting plasma production was studied using x-ray pinhole imaging. Evidence is provided suggesting that thermal filamentation starts to occur for irradiances on the target of Iλ2 1014 W cm-2 μm 2, even on deployment of phase plates to improve the focal spot spatial uniformity. The experiments are supported by both analytical modelling and two-dimensional particle-in-cell simulations. The implications for the applications of laser-plasma interactions that require high degrees of uniform irradiation are discussed. © IOP Publishing and Deutsche Physikalische Gesellschaft.

Abstract:

A review of the proton radiography technique will be presented. This technique employs laser-accelerated laminar bunches of protons to diagnose the temporal and spatial characteristic of the electric and magnetic fields generated during high-intensity laser-plasma interactions. The remarkable temporal and spatial resolution that this technique can achieve (of the order of a picosecond and a few microns respectively) candidates this technique as the preferrable one, if compared to other techniques, to probe high intensity laser-matterinteractions. © 2012 American Institute of Physics.