Prof Peter Norreys FInstP;

Abstract:

The sensitivity of inertial confinement fusion implosions, of the type performed on the National Ignition Facility (NIF), to low-mode flux asymmetries is investigated numerically. It is shown that large-amplitude, low-order mode shapes (Legendre polynomial P4), resulting from low-order flux asymmetries, cause spatial variations in capsule and fuel momentum that prevent the deuterium and tritium (DT) "ice" layer from being decelerated uniformly by the hot spot pressure. This reduces the transfer of implosion kinetic energy to internal energy of the central hot spot, thus reducing the neutron yield. Furthermore, synthetic gated x-ray images of the hot spot self-emission indicate that P4 shapes may be unquantifiable for DT layered capsules. Instead the positive P4 asymmetry "aliases" itself as an oblate P2 in the x-ray images. Correction of this apparent P2 distortion can further distort the implosion while creating a round x-ray image. Long wavelength asymmetries may be playing a significant role in the observed yield reduction of NIF DT implosions relative to detailed postshot two-dimensional simulations. © 2013 American Physical Society.

Abstract:

Experiments were performed on the Omega EP laser facility to study laser pulse propagation, channeling phenomena and electron acceleration from high-intensity, high-power laser interactions with underdense plasma. A CH plasma plume was used as the underdense target and the interaction of the laser pulse channeling through the plasma was imaged using proton radiography. High-energy electron spectra were measured for different experimental laser parameters. Structures observed along the channel walls are interpreted as having developed from surface waves, which are likely to serve as an injection mechanism of electrons into the cavitated channel for acceleration via direct laser acceleration mechanisms. Two-dimensional particle-in-cell simulations give good agreement with these channeling and electron acceleration phenomena. © IOP Publishing and Deutsche Physikalische Gesellschaft.

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.