[show abstract][hide abstract] ABSTRACT: We have measured time-integrated and time-gated electron temperature (Te) and density (Ne) spatial profiles from indirect-drive implosions. In our experiments, we used a multiple-pinhole two-dimensional imaging spectrometer to obtain multispectral X-ray images of the imploded core. Quantitative comparisons between quasi-monochromatic images in different energy bands allowed Te and Ne spatial profiles to be determined using two independent and validated techniques: a multi-objective search and reconstruction analysis, and an analytical analysis. We then compared the results to a simple one-dimensional (1D) mix-free hydrodynamics simulation in order to evaluate the ability of such a model to predict our experiments. Our data show spatial Te profiles that are qualitatively consistent with the predictions of our 1D simulations, but we observe central cores that are 10–25% cooler and emit X-rays as late as 200ps after peak compression. We infer time-gated spatial Ne profiles that are consistent with our 1D simulations near the times of peak compression, but we find significant disagreement between time-integrated data and 1D simulation predictions at large radii. Careful analysis of the time-gated and time-integrated Te and Ne spatial profiles, together with streaked X-ray emission spectra from core and shell dopants, suggests mixing of shell material into the core is an important process that our 1D hydrodynamics simulations fail to capture, and comparison between image data and a simple analytical model suggests that ∼2–5μm of the initial inner shell thickness mixes into the core during the time period of significant X-ray emission. This mix width is consistent with the predictions of a growth-factor analysis that treats instability growth seeded by capsule surface roughness, and points to the need to consider time-dependent mixing effects when interpreting Te and Ne spatial profiles derived from multispectral X-ray image data, particularly at large radii where mixing effects will be most significant.
High Energy Density Physics 04/2008; 4:1. · 1.60 Impact Factor
[show abstract][hide abstract] ABSTRACT: We discuss a method for the simultaneous and self-consistent fitting of a set of intensity or emissivity spatial profiles from several narrow-band x-ray pinhole images from argon-doped inertial confinement fusion implosion cores, and the space-integrated line spectrum. A Pareto genetic algorithm (PGA) combines the search and optimization capabilities of a single-objective genetic algorithm with the Pareto domination technique of multiobjective optimization. Further, the PGA search is followed up by a fine-tuning step based on a nonlinear least-squares-minimization procedure. The result is a robust search and reconstruction method that finds the optimal core spatial structure subject to multiple constraints. This method is independent of geometry inversions and could take advantage of not only optically thin but also optically thick image data. Results are shown for two combinations of three-objectives based on gated argon He β and Ly β image data and the line spectrum.
The Review of scientific instruments 11/2006; · 1.52 Impact Factor
[show abstract][hide abstract] ABSTRACT: The spectroscopic analysis of x-ray narrow-band images and space-integrated x-ray line spectra from argon-doped deuterium-filled inertial confinement fusion implosion experiments yields information about the spatial profiles in the compressed core. We discuss the analysis of direct-drive implosion experiments at OMEGA, in which images were obtained with a multimonochromatic imaging instrument. The analysis method considers data based on the argon Heβ and Lyβ spectral features and their associated Li- and He-like satellites. The temperature gradient structure is investigated by using the sensitivity of the Lyβ/Heβ emissivity ratio to the temperature.
The Review of scientific instruments 10/2006; 77(10):10E320-10E320-3. · 1.52 Impact Factor
[show abstract][hide abstract] ABSTRACT: We report on the experiments aimed at obtaining core temperature and density maps in direct drive implosions at the Omega laser facility using multimonochromatic x-ray imagers. These instruments use an array of pinholes and a flat multilayer mirror to provide unique multispectral images distributed over a wide spectral range. Using argon as a dopant in the direct-drive filled plastic shells produces emission images in the Ar He-β and Ly-β spectral regions. These images allow the retrieval of temperature and density maps of the plasma. We deployed three identical multimonochromatic x-ray imagers in a quasiorthogonal line-of-sight configuration to allow tomographic reconstruction of the structure of the imploding core.
The Review of scientific instruments 09/2006; 77(10):10E303. · 1.52 Impact Factor
[show abstract][hide abstract] ABSTRACT: We have developed a software package for reconstruction of quasimonochromatic images from a multiple monochromatic x-ray imager for inertial confinement fusion implosions. The instrument consists of a pinhole array, a multilayer Bragg mirror, and an image detector. The pinhole array projects hundreds of images onto the detector after reflection off the multilayer Bragg mirror, which introduces spectral dispersion along the reflection axis. The quasimonochromatic images of line emissions and continuum emissions can be used for measurement of temperature and density maps of implosion plasmas. In this article, we describe a computer-aided processing technique for systematic reconstruction of quasimonochromatic images from raw data. This technique provides flexible spectral bandwidth selection and allows systematic subtraction of continuum emission from line emission images.
The Review of scientific instruments 09/2006; · 1.52 Impact Factor
[show abstract][hide abstract] ABSTRACT: We present recent results using multi-monochromatic X-ray imaging of direct drive implosions at the OMEGA laser facility. An array of pinholes coupled to a flat multilayer mirror provides multi-spectral images distributed over a wide spectral range. Using Argon as a dopant in the DD-filled plastic shells produces emission images in the Ar He-beta and Ly-beta spectral regions. When used in conjunction with gated imaging detectors the instrument provides images with spatial resolution of similar to 10 mu m and temporal resolution of similar to 50ps. A special algorithm has been developed to reconstruct narrow-band images, which will allow the retrieval of temperature, and density maps of the core as it evolves through peak compression.
[show abstract][hide abstract] ABSTRACT: We present results from simulations performed to investigate the effects of dopant radiative cooling in inertial confinement fusion indirect-drive capsule implosion experiments. Using a one-dimensional radiation-hydrodynamics code that includes inline collisional-radiative modeling, we compute in detail the non-local thermodynamic equilibrium atomic kinetics and spectral characteristics for Ar-doped DD fuel. Specifically, we present results from a series of calculations in which the concentration of the Ar is varied, and examine the sensitivity of the fuel conditions (e.g., electron temperature) and neutron yield to the Ar dopant concentration. Simulation results are compared with data obtained in OMEGA indirect-drive experiments in which monochromatic imaging and spectral measurements of Ar Hebeta and Lybeta line emission were recorded. The incident radiation drive on the capsule is computed with a three-dimensional view factor code using the laser beam pointings and powers from the OMEGA experiments. We also examine the sensitivity of the calculated compressed core electron temperatures and neutron yields to the radiation drive on the capsule and to the radiation and atomic modeling in the simulations.
[show abstract][hide abstract] ABSTRACT: We discuss a three-objective search and reconstruction method for the extraction of spatial structure information from argon-doped implosion cores based on the simultaneous analysis of He beta and Ly beta X-ray narrow-band images and space-integrated line spectrum data. The search in parameter space is driven by a multi-objective Pareto genetic algorithm. Two alternative implementations of the three objectives are considered. Two- and three-objective analysis of the same set of time-integrated data produce similar temperature profiles but different density profiles. (C) 2005 Elsevier Ltd. All rights reserved.
Journal of Quantitative Spectroscopy and Radiative Transfer 01/2006; 99(1-3):649-657. · 2.38 Impact Factor
[show abstract][hide abstract] ABSTRACT: Summary form only given. Recent spectroscopic analysis, based on image intensity distributions and line spectra of a series of indirect-drive inertial confinement fusion implosions at the University of Rochester's OMEGA laser, has demonstrated a discrepancy between density gradients inferred from the data analysis and those calculated in "clean" hydrodynamic simulations. The cause of this discrepancy is currently under investigation. One likely cause is the presence of mixing between the shell material and the fuel, which is not included in the "clean" hydrodynamic simulations. The application of a mix model to the hydrodynamic output will help to define the extent of mixing in the problem, and also assess how important mixing is in the spectroscopic data analysis
[show abstract][hide abstract] ABSTRACT: We describe a multiple monochromatic x-ray imager designed for implosion experiments. This instrument uses an array of pinholes in front of a flat multilayered Bragg mirror to provide many individual quasimonochromatic x-ray pinhole images spread over a wide spectral range. We discuss design constraints and optimizations, and we discuss the specific details of the instrument we have used to obtain temperature and density maps of implosion plasmas. (c) 2005 American Institute of Physics.
The Review of scientific instruments 01/2005; 76:4. · 1.52 Impact Factor
[show abstract][hide abstract] ABSTRACT: We have developed a multiple monochromatic x-ray imaging diagnostic that uses an array of pinholes coupled to a multilayer Bragg mirror, and we have used this diagnostic to obtain unique multispectral imaging data of inertial-confinement fusion implosion plasmas. Argon dopants in the fuel allow emission images to be obtained in the Ar He-beta and Ly-beta spectral bands, and these images provide information on core temperature and density profiles. We have analyzed these images to obtain quasi-three-dimensional maps of electron temperature and scaled electron density within the core for several cases of drive symmetry, and we find quantitative and qualitative disagreement with one-dimensional hydrodynamics simulations. We also observed a two-lobed structure evolving for increasingly prolatc-asymmetric drive.
[show abstract][hide abstract] ABSTRACT: We have developed a multiple monochromatic x-ray imaging diagnostic using an array of pinholes coupled to a multilayer Bragg mirror, and we have used this diagnostic to obtain unique multispectral imaging data of inertial-confinement fusion implosion plasmas. Argon dopants in the fuel allow emission images to be obtained in the Ar He-b and Ly-b spectral regions, and these images provide data on core temperature and density profiles. We have analyzed these data to obtain quasi-three-dimensional maps of electron temperature and scaled electron density within the core for several cases of drive symmetry, and we observed a two-lobed structure evolving for increasingly prolate-asymmetric drive. This structure is invisible in broad-band x-ray images. Future work will concentrate on hydrodynamics simulations for comparison with the data.
[show abstract][hide abstract] ABSTRACT: We report on an experimental and modeling study of the spatial structure in indirectly driven implosion cores. To this end, Ar-doped D-2-filled plastic shells were irradiated with the OMEGA laser system. We focus on processing and analysis of data recorded with a new X-ray imager (MMI-2) that records numerous narrow-band images that include both line and continuum emission. The temperature and density gradients are determined by using a multi-objective analysis of X-ray narrow-band images and X-ray line spectra. This analysis self-consistently determines the temperature and density gradients that yield the best fits to the spatially-resolved emissivity and space-integrated spectral line shapes. (C) 2003 Elsevier Science Ltd. All rights reserved.
Journal of Quantitative Spectroscopy and Radiative Transfer 01/2003; 81(1-4):487-497. · 2.38 Impact Factor
[show abstract][hide abstract] ABSTRACT: A Multi-Monochromatic X-ray Imager (MMI-3) was fielded in OMEGA indirect-drive implosion experiments to record time-resolved narrow-band X-ray core images. MMI-3 uses an array of pinholes and slits that permit the simultaneous recording of images and space-integrated line spectra. Data was obtained with argon-doped deuterium-filled plastic capsules. To process MMI-3 images, a graphical user interface tool was designed in IDL to conveniently and objectively process the data. The results of the data processing can be used to extract time-dependent information about the space structure of the core. This next step in the x-ray spectroscopy of implosion cores results in an improved description of core plasma conditions.
[show abstract][hide abstract] ABSTRACT: We have developed a multiple monochromatic x-ray imaging diagnostic using an array of pinholes coupled to a multilayer Bragg mirror, and we have used this diagnostic to obtain unique multispectral imaging data of indirect-drive inertial-confinement fusion implosion plasmas. Titanium dopants allow core-backlit images to be obtained in the spectral regions of the Ti K-alpha and K-edge absorption bands, and these images provide data on areal density and ionization gradients in the shell. Argon dopants in the fuel also allow emission images to be obtained in the Ar He-beta and Ly-beta spectral regions, and these images provide data on core temperature gradients. We discuss this data and the information it provides, and we discuss future plans to use a similar diagnostic to provide core temperature and density gradient data in cryogenic deuterium implosions.
[show abstract][hide abstract] ABSTRACT: K-shell emission spectroscopy is commonly used to diagnose core temperature and density of Ar-doped ICF implosions at OMEGA and Z. To investigate details of spectra formation, we perform simulations of Ar-doped indirect laser-driven implosions and generate synthetic spectra and plasma core images observable at the collapse of the implosion. VisRad is a user-friendly view factor code used to simulate the radiation environment in three-dimensional objects. It predicts temperature and radiative flux distributions throughout target components in high-power laser and z-pinch laboratory plasma experiments. Those time-dependent hohlraum radiation temperatures and fluxes are used to initialize target implosion simulation with a 1D rad-hydro code BUCKY. Plasma core temperature and density distributions from the hydro code are then used to compute Ar emission spectra and core images. To this end we utilize multi-dimensional collisional-radiative, spectral analysis code SPECT3D. We will discuss details of the calculations and compare our results against experimental data.