The requirement for sources of hard X-rays suitable for high resolution radiography through large pR targets is prominent in many aspects of current laser-driven plasma physics research. In recent work using the OMEGA EP laser facility [L. J. Waxer, M. J. Guardalben, J. H. Kelly et al., CLEO/QELS, Optical Society of America, San Jose, CA, IEEE (2008)] at the Laboratory for Laser Energetics (LLE) in Rochester, NY, experiments have been performed to measure characteristics of 22-52 keV X-ray sources using high intensity short-pulse lasers. High quality point projection, two-dimensional radiography was demonstrated by irradiating microwire targets with laser intensities of 10(16)W cm(-2)-10(19) W cm(-2). Microwire targets were manufactured to dimensions of 10 mu m x 10 mu m x 300 mu m and were supported by a 100 mu m x 300 mu m x 6 mu m low-Z substrate. Measurements of the k-alpha conversion efficiency and X-ray source-size are discussed and, of particular importance for radiography, the spectral purity of the backlighter is characterized to assess the relative importance of the K alpha emission to bremsstrahlung background. (C) 2013 Published by Elsevier B.V.

Techniques to achieve uniform near-spherical symmetry of radiation drive on a capsule in a laser-heated hohlraum have received detailed attention in the context of inertial confinement fusion. However, much less attention has been paid to the understanding of the hohlraum physics in cases where the radiation drive departs significantly from spherical symmetry. A series of experiments has been carried out to study the implosion dynamics of a capsule irradiated by a deliberately asymmetric x-ray drive. The experimental data provide a sensitive test of radiation transport in hohlraums in which drive symmetry is modulated by asymmetric laser beam timing and the use of wall materials of different albedos. Data from foam ball and thin-shell capsule experiments are presented together with modeling using consecutively linked Lagrangian and Eulerian calculational schemes. The thin-shell capsules exhibit much stronger sensitivity to early-time asymmetry than do the foam balls, and this sensitivity results in the formation of a well-defined polar jet. These data are shown to challenge computational modeling in this highly asymmetric convergent regime. All of the experiments detailed were carried out at the OMEGA laser facility [J. M. Soures, R. L. McCrory, C. P. Verdon et al., Phys. Plasmas 3, 2108 (1996)] at the Laboratory for Laser Energetics in Rochester, NY.

Experiments have been conducted using laser-driven cylindrical hohlraums whose walls are machined from Ta2O5 foams of 100 mg/cc and 4 g/cc densities. Measurements of the radiation temperature demonstrate that the lower density walls produce higher radiation temperatures than the high density walls. This is the first experimental demonstration of the prediction that this would occur [M. D. Rosen and J. H. Hammer, Phys. Rev. E 72, 056403 (2005)10.1103/PhysRevE.72.056403]. For high density walls, the radiation front propagates subsonically, and part of the absorbed energy is wasted by the flow kinetic energy. For the lower wall density, the front velocity is supersonic and can devote almost all of the absorbed energy to heating the wall.

Submitted for the DPP08 Meeting of The American Physical Society Design of Asymmetrically driven hohlraum experiments on OMEGA STUART MCALPIN, MARK STEVENSON, KELLY VAUGHAN, JOHN FOSTER, MARK TAYLOR, AWE, AWE TEAM — A campaign of experiments is planned on the OMEGA laser to asymmetrically drive an imploding capsule within a hohlraum. This will act as a stringent test of the modeling of both the conditions inside the hohlraum and the evolution of complex hydrodynamic systems. These experiments will be modelled using a two step approach. A pure Lagrangian code linked to an Eulerian code is used to capture the late-time hydrodynamics and a single-step ALE (Arbitrary Lagrangian Eulerian) code is used as a fully integrated test. A number of techniques have been identified which potentially offer significant control of both the spatial and temporal asymmetry of the drive on the capsule. These will be tested systematically in two stages. The drive as a function of position and time will be measured by following the ablation front in aerogel spheres. The effect of the drive on the late time hydrodynamics will be tested using the implosion and potential jet formation in GDP coated glass capsules. In both cases the evolution of the configuration will be determined using titanium area backlighting at 4.7 keV combined with a gated x-ray imaging system. Richard Stevenson AWE Date submitted: 17 Jul 2008 Electronic form version 1.4

The National Ignition Facility (NIF), currently under construction at Lawrence Livermore National Laboratory (LLNL) comprises four clusters of 48 laser beams in a symmetric geometry. Each cluster is itself capable of producing potentially useful conditions for hohlraum interaction experiments. The three dimensional nature of the laser illumination geometry in each cluster presents a challenge when modelled with a 2D hydrocode. Recent experiments have been fielded on the OMEGA laser facility to investigate the validity of using such codes in modelling the plasma filling of targets in similar geometries to a NIF cluster. The extent to which plasma filling within hohlraum type targets is a significant issue can potentially be addressed by the observation of laser plasma instabilities, such as Raman backscatter. This can be a significant energy loss mechanism for targets that have electron densities up to quarter critical. Linear Raman gain theory has been applied to the output from AWE's 2D lagrangian radiation hydrodynamics code, NYM, to produce qualitative and quantitative comparisons with these experiments. This provides a basis for judging the fidelity of the approach.