Publications (218) View all
-
Article: Inelastic X-Ray Scattering from Shocked Liquid Deuterium.
[show abstract] [hide abstract]
ABSTRACT: The Fermi-degenerate plasma conditions created in liquid deuterium by a laser-ablation-driven shock wave were probed with noncollective, spectrally resolved, inelastic x-ray Thomson scattering employing Cl Ly_{α} line emission at 2.96 keV. These first x-ray Thomson scattering measurements of the microscopic properties of shocked deuterium show an inferred spatially averaged electron temperature of 8±5 eV, an electron density of 2.2(±0.5)×10^{23} cm^{-3}, and an ionization of 0.8 (-0.25, +0.15). Two-dimensional hydrodynamic simulations using equation-of-state models suited for the extreme parameters occurring in inertial confinement fusion research and planetary interiors are consistent with the experimental results.Physical Review Letters 12/2012; 109(26):265003. · 7.37 Impact Factor -
Article: Stochastic Ion Heating from Many Overlapping Laser Beams in Fusion Plasmas.
[show abstract] [hide abstract]
ABSTRACT: In this Letter, we show through numerical simulations and analytical results that overlapping multiple (N) laser beams in plasmas can lead to strong stochastic ion heating from many (∝N^{2}) electrostatic perturbations driven by beat waves between pairs of laser beams. For conditions typical of inertial-confinement-fusion experiment conditions, hundreds of such beat waves are driven in mm^{3}-scale plasmas, leading to ion heating rates of several keV/ns. This mechanism saturates cross-beam energy transfer, with a reduction of linear gains by a factor ∼4-5 and can strongly modify the overall hydrodynamics evolution of such laser-plasma systems.Physical Review Letters 11/2012; 109(19):195004. · 7.37 Impact Factor -
Article: Hot-spot mix in ignition-scale implosions on the NIF
[show abstract] [hide abstract]
ABSTRACT: Ignition of an inertial confinement fusion (ICF) target depends on the formation of a central hot spot with sufficient temperature and areal density. Radiative and conductive losses from the hot spot can be enhanced by hydrodynamic instabilities. The concentric spherical layers of current National Ignition Facility (NIF) ignition targets consist of a plastic ablator surrounding a thin shell of cryogenic thermonuclear fuel (i.e., hydrogen isotopes), with fuel vapor filling the interior volume [S. W. Haan et al., Phys. Plasmas 18, 051001 (2011)]. The Rev. 5 ablator is doped with Ge to minimize preheat of the ablator closest to the DT ice caused by Au M-band emission from the hohlraum x-ray drive [D. S. Clark et al., Phys. Plasmas 17, 052703 (2010)]. Richtmyer-Meshkov and Rayleigh-Taylor hydrodynamic instabilities seeded by high-mode (50 < l < 200) ablator-surface perturbations can cause Ge-doped ablator to mix into the interior of the shell at the end of the acceleration phase [B. A. Hammel et al., Phys. Plasmas 18, 056310 (2011)]. As the shell decelerates, it compresses the fuel vapor, forming a hot spot. K-shell line emission from the ionized Ge that has penetrated into the hot spot provides an experimental signature of hot-spot mix. The Ge emission from tritium-hydrogen-deuterium (THD) and deuterium-tritium (DT) cryogenic targets and gas-filled plastic-shell capsules, which replace the THD layer with a mass-equivalent CH layer, was examined. The inferred amount of hot-spot-mix mass, estimated from the Ge K-shell line brightness using a detailed atomic physics code [J. J. MacFarlane et al., High Energy Density Phys. 3, 181 (2006)], is typically below the 75-ng allowance for hot-spot mix [S. W. Haan et al., Phys. Plasmas 18, 051001 (2011)]. Predictions of a simple mix model, based on linear growth of the measured surface-mass modulations, are consistent with the experimental results. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3694057]Physics of Plasmas 05/2012; 19(5). · 2.15 Impact Factor -
Article: Multistep redirection by cross-beam power transfer of ultrahigh-power lasers in a plasma
[show abstract] [hide abstract]
ABSTRACT: Laser redirection by cross-beam power transfer in a plasma is an important example of a nonlinear optics process which uses laser-plasma instabilities to one's advantage. We have demonstrated this in a hohlraum plasma at the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory. A four-wave mixing process causes laser power in multiple beams to change direction and add to the laser power of a selected beam. The process is controlled by setting the wavelength separation of the interacting laser beams. This technique provides a method to remotely re-point or combine high-powered laser beams without the need of local optical apparatus.Nature Physics 04/2012; 8(4):344-349. · 18.97 Impact Factor -
Article: Observation of inhibited electron-ion coupling in strongly heated graphite.
[show abstract] [hide abstract]
ABSTRACT: Creating non-equilibrium states of matter with highly unequal electron and lattice temperatures (T(ele)≠T(ion)) allows unsurpassed insight into the dynamic coupling between electrons and ions through time-resolved energy relaxation measurements. Recent studies on low-temperature laser-heated graphite suggest a complex energy exchange when compared to other materials. To avoid problems related to surface preparation, crystal quality and poor understanding of the energy deposition and transport mechanisms, we apply a different energy deposition mechanism, via laser-accelerated protons, to isochorically and non-radiatively heat macroscopic graphite samples up to temperatures close to the melting threshold. Using time-resolved x ray diffraction, we show clear evidence of a very small electron-ion energy transfer, yielding approximately three times longer relaxation times than previously reported. This is indicative of the existence of an energy transfer bottleneck in non-equilibrium warm dense matter.Scientific Reports 01/2012; 2:889.
