Showing papers by "Siegfried Glenzer published in 2000"

Ionization processes and charge-state distribution in a highly ionized high- Z laser-produced plasma

Abstract: The charge-state distribution in a well-characterized highly ionized Au plasma was accurately determined using time-resolved x-ray spectroscopy. Simultaneous measurements of the electron temperature and density allow the first direct comparisons with nonlocal thermodynamic equilibrium model predictions for the charge-state distribution of a highly ionized high-Z plasma in a nonradiative environment. The importance of two-electron atomic processes is clearly demonstrated. (c) 2000 The American Physical Society.

Accurate determination of the charge state distribution in a well characterized highly ionized Au plasma

Abstract: The density, temperature and charge state distribution are accurately determined in a highly ionized non-LTE Au sample. Laser heated Au microdots buried in a thin Be foil, reach temperatures of 2 keV and ionize into the M-shell. During expansion, the tamped Au samples remain uniform and in near steady-state ionization equilibrium. The electron temperature is measured with time and space resolved Thomson scattering while the density is determined from time-gated X-ray imaging the expanded Au sample. The charge state distribution is obtained from analysis of emission measurements of Au 5f–3d transition arrays in the wavelength range 3.3–3.9 A, allowing the average charge to be determined to within ∼1% accuracy.

Accuracy of K -shell spectra modeling in high-density plasmas

Abstract: We present spectroscopic measurements of the helium-like and lithium-like argon emission supported by Thomson scattering diagnostics on gas bag targets These data provide critical tests of plasma spectroscopic K-shell models In particular, we have measured the line radiation in the wavelength region of the He-like $\mathrm{Ar}{1s}^{2}--1s3l$ transition $(\mathrm{He}\ensuremath{-}\ensuremath{\beta})$ that is of interest for density and temperature measurements of plasmas from gas-filled targets ${(n}_{e}l~{10}^{21} {\mathrm{cm}}^{\ensuremath{-}3}),$ laser ablation targets ${(n}_{e}l~{10}^{22} {\mathrm{cm}}^{\ensuremath{-}3}),$ and inertial confinement fusion capsule implosions ${(n}_{e}g~{10}^{24} {\mathrm{cm}}^{\ensuremath{-}3})$ The spectra show lithium-like dielectronic satellites on the red wing of the $\mathrm{He}\ensuremath{-}\ensuremath{\beta}$ line that are temperature sensitive and are known to influence the shape of the Stark-broadened line profiles observed from implosions To examine the kinetics modeling of this complex, ie, the $\mathrm{He}\ensuremath{-}\ensuremath{\beta}$ and its associated satellites, we have performed experiments in gas bag plasmas at densities of $(06--11)\ifmmode\times\else\texttimes\fi{}{10}^{21} {\mathrm{cm}}^{\ensuremath{-}3}$ where we independently determine the electron temperature with ultraviolet Thomson scattering The comparison of the satellite intensities with kinetics modeling shows good agreement for satellites whose upper states are populated by dielectronic capture, but shows discrepancies for inner-shell collisional excited transitions

Hohlraum energetics with smoothed laser beams

Abstract: Measurements of radiation temperatures from empty and gas-filled hohlraums heated at the Nova Laser Facility [E. M. Campbell et al., Laser Part. Beams 9, 209 (1991)] show efficient coupling of the laser power to the target when applying laser beam smoothing techniques. Scattering losses are reduced to the 3% level while the radiation temperatures increased by ∼15 eV for smoothed laser beams. The experimental findings and supporting calculations indicate that filamentation and gain for stimulated Raman and Brillouin scattering is suppressed in the hohlraum plasma for smoothed laser beams. The scaling of the radiation temperature is well described by integrated radiation hydrodynamic LASNEX modeling [G. B. Zimmerman and W. L. Kruer, Comments Plasma Phys. Controlled Fusion 2, 85 (1975)] following the Marshak scaling. Peak radiation temperatures are in excess of 230 eV in gas-filled hohlraums in agreement with the detailed LASNEX modeling.

Testing Astrophysical Radiation Hydrodynamics Codes with Hypervelocity Jet Experiments on the Nova Laser

Abstract: Recent shock-tube experiments using the Nova laser facility have demonstrated that strong shocks and highly supersonic flows similar to those encountered in astrophysical jets can be studied in detail through carefully controlled experiments. We propose the use of high-power lasers such as Nova, Omega, Gekko, and the National Ignition Facility (NIF) to perform experiments on radiation hydrodynamic problems such as jets involving the multidimensional dynamics of strong shocks. High-power lasers are the only experimental facilities that can reach the very high Mach number regime. The experiments will serve as diagnostics of astrophysically interesting gasdynamic problems and could also form the basis of test problems for numerical algorithms for astrophysical radiation hydrodynamic codes. The potential for experimentally achieving a strongly radiative jet seems very good.

Modeling of Thomson Scattering Spectra in High-Z, Laser-produced Plasmas

Abstract: Theoretical calculations of a Thomson scattering cross section and dynamical form factors are presented for high-Z, laser-produced, inhomogeneous plasmas. Relevance of these results to astrophysical plasmas is pointed out. Comparisons with recent experimental observations are discussed with emphasis on the effects of plasma inhomogeneity, ion-ion collisions, and non-Maxwellian distribution functions.

Detailed characterization of laser plasmas for benchmarking of radiation-hydrodynamic modeling

Abstract: We have applied Thomson scattering for accurate measurements of densities, temperatures, and ionization levels in dense laser-produced gold plasmas. These experiments give a unique data set that has allowed us to make critical comparisons with two-dimensional radiation-hydrodynamic simulations showing that dielectronic recombination is an important process in cooling laser plasmas. Consistent with this result, additional X-ray spectroscopic measurements of the emission from the gold plasma also indicate a rapidly recombining plasma. These findings are important for the development of kinetics models relevant to high-Z laser-irradiated plasmas and to test our calculational capability for future high-density plasma experiments.

Towards an experimental benchmark for aluminum X-ray spectra

Abstract: To test the validity of kinetics for laser-produced plasmas, one would like to measure the X-ray spectrum emitted from a plasma volume whose characteristics are determined by diagnostics that do not rely on interpreting the X-ray spectrum itself. An experimental test bed has been developed at the Janus laser at Lawrence Livermore National Laboratory to simultaneously characterize the electron temperature, the electron density and the X-ray emission from laser-irradiated aluminum dot targets. Thomson scattering, interferometry and pinhole imaging are implemented to achieve this. Further, the X-ray spectrum from 1–2 keV is spatially integrated and is obtained using a flat crystal (PET) and an X-ray streak camera for time resolution. Spectra have been calculated using the HULLAC and FLY atomic physics codes which use the measured density and temperature as input, and DCA which calculates X-ray spectra from the 2-D expanding plasma calculated by a hydrodynamics code. Comparisons of the models with the data will be discussed and future directions will be indicated.

Experimental Investigation of Short Scalelength Density Fluctuations in Laser-Produced Plasmas

Abstract: The technique of near forward laser scattering is used to infer characteristics of intrinsic and controlled density fluctuations in laser-produced plasmas. Intrinsic fluctuations are studied in long-scale length plasmas where we find that the fluctuations exhibit scale sizes related to the intensity variation scales in the plasma-forming and interaction beams. Stimulated Brillouin forward scattering and filamentation appear to be the primary mechanism through which these fluctuations originate. The beam spray resulting from these fluctuations is important to understand since it can affect symmetry in an inertial confinement fusion (ICF) experiment. Controlled fluctuations are studied in foam and exploding foil targets. Forward scattered light from foam targets shows evidence that the initial target inhomogeneities remain after the target is laser heated. Forward scattered light from an exploding foil plasma shows that a regular intensity pattern can be used to produce a spatially correlated density fluctuation pattern. These results provide data which are being used to benchmark numerical models of beam spray.

Thomson Scattering in Inertial Confinement Fusion Research

Abstract: Thomson scattering [1] using short-wavelength probe lasers [2] is the standard diagnostic to characterize high-density inertial confinement fusion (ICF) plasmas [3]. Its unique capability to measure plasma parameters such as temperatures, densities, plasma flow, and ionization stages with high accuracy together with possible measurements of plasma wave fluctuations is now widely recognized in the area of laser-produced plasma research. Initially, many basic Thomson scattering experiments with applications to ICF have been performed [4] on the Nova laser facility at the Lawrence Livermore National Laboratory that produced hot mm-scale plasmas with laser energies of up to 30 kJ. More recently, we have also begun to perform Thomson scattering experiments at a second large facility namely the Omega laser facility at the University of Rochester [5].

Experimental study of laser beam transmission and power accounting in a large scale length laser plasma

Abstract: It is shown that the measured laser power transmission through a large scale length, high temperature plasma (which emulates an indirect drive ignition-scale plasma) is in approximate agreement with the simulated transmission provided the simulations account for the power loss due to scattering from laser-plasma instabilities. Detailed accounting of the incident, transmitted, scattered, and absorbed powers is used to infer the likely location in the target where most of the scattering occurs along the incident beam trajectory. This location is near the incident laser side of the target at peak electron temperatures for a range of laser intensities. As a result, the backscattered light measurements at peak electron temperature do not require significant adjustment to account for attenuation of the backscattered light as it propagates out through the plasma.

Analysis of discrepancies between quantal and semiclassical calculations of electron impact broadening in plasmas

Abstract: In this work we analyse the serious discrepancies found between quantal calculations and non-perturbative semiclassical calculations of electron impact broadening in the case of B2+2s–2p. We use the Coulomb Bethe (CB) approximation to calculate both electron impact broadening and excitation of 2s–2p for comparison with the other methods. We find good agreement between CB and the non-perturbative semiclassical method for the line width contributions of individual partial waves, except for low L, where strong collision effects enter. We also find good agreement between CB and the R-matrix and Coulomb Born methods for the excitation cross section partial wave contributions, again except for low L. There is disagreement for the high partial wave cross sections between the non-perturbative semiclassical method and all of the quantum methods; this is resolved by applying a symmetrized method, for which we demonstrate excellent agreement with CB. The area in which the semiclassical, Coulomb–Born and R-matrix methods disagree has been reduced to the first three partial waves, and the disagreement must be due to the treatment of strong collisions.

On the accuracy of x-ray spectra modeling of inertial confinement fusion plasmas

Abstract: X-ray spectroscopic measurements of the helium-like and lithium-like argon emission from gas bag targets are presented. These data are supported by Thomson scattering diagnostics providing critical tests of plasma spectroscopic K-shell models. The x-ray spectra show lithium-like dielectronic satellites on the red wing of the He-β line that are temperature sensitive and are known to influence the shape of the Stark-broadened line profiles observed from implosions. We find that the satellite intensities compare well to kinetics modeling that uses the independently measured electron temperature for satellites whose upper states are populated by dielectronic capture, but discrepancies are observed for inner-shell collisionally excited transitions.