Implosion Cores Research Articles

Sensitivity of capsule implosion symmetry due to laser beam imbalance in a scale 0.2 hot hohlraum at Omega

Results are shown from recent experiments at the Omega laser facility, using 40 Omega beams driving the hohlraum with 3 cones from each side and up to 19.5 kJ of laser energy. Beam phasing is achieved by decreasing the energy separately in each of the three cones, by 3 kJ, for a total drive energy of 16.5 kJ. This results in a more asymmetric drive, which will vary the shape of the imploded symmetry capsule core from round to oblate or prolate in a systematic and controlled manner. These results show the sensitivity of capsule implosion symmetry for implosions in "high temperature" (275 eV) hohlraums at Omega. Dante measurements confirmed the predicted peak drive temperatures of 275 eV. Implosion core time dependent x-ray images were obtained from framing camera data which show the expected change in symmetry due to beam imbalance and which also agree well with post processed hydro code calculations.

Argon K-shell and bound-free emission from OMEGA direct-drive implosion cores

We discuss calculations of synthetic spectra for the interpretation and analysis of K-shell and bound-free emission from argon-doped deuterium-filled OMEGA direct-drive implosion cores. The spectra are computed using a model that considers collisional-radiative atomic kinetics, continuum-lowering, detailed Stark-broadened line shapes, line overlapping, and radiation transport effects. The photon energy range covers the moderately optically thick n = 3 → n = 1 and n = 4 → n = 1 line transitions in He- and H-like Ar, their associated satellite lines in Li- and He-like Ar, and several radiative recombination edges. At the high-densities characteristic of implosion cores, the radiative recombination edges substantially shift to lower energies thus overlapping with several line transitions. We discuss the application of the spectra to spectroscopic analysis of doped implosion cores.

Applied plasma spectroscopy: Laser-fusion experiments

High-energy-density plasmas created in laser-fusion experiments are diagnosed with X-ray spectroscopy. Hans Griem, considered the father of modern plasma spectroscopy, provided an excellent foundation for this research. He studied the effect of plasma particles, in particular the fast-moving free electrons, on the Stark-broadening of spectral line shapes in plasmas [H. Griem, Phys. Rev. 125 (1962) 177]. Over the last three decades, X-ray spectroscopy has been used to record the remarkable progress made in inertial confinement fusion research. Four areas of X-ray spectroscopy for laser-fusion experiments are highlighted in this paper: K α emission spectroscopy to diagnose target preheat by suprathermal electrons, Stark-broadened K-shell emissions of mid- Z elements to diagnose compressed densities and temperatures of implosion cores, K- and L-shell absorption spectroscopy to diagnose the relatively cold imploding shell (the “piston”) that does not emit X rays, and multispectral monochromatic imaging of implosions to diagnose core temperature and density profiles. The seminal research leading to the original X-ray spectroscopy experiments in these areas will be discussed and compared to current state-of-the-art measurements.

Inference of ICF implosion core mix using experimental dataand theoretical mix modeling

The mixing between fuel and shell materials in Inertial Confinement Fusion (ICF) implosion cores is a current topic of interest. The goal of this work was to design direct drive ICF experiments which have varying levels of mix, and subsequently to extract information on mixing directly from the experimental data using spectroscopic techniques. The experimental design was accomplished using hydrodynamic simulations in conjunction with Haan's saturation model, which was used to predict the mix levels of candidate experimental configurations. These theoretical predictions were then compared to the mixing information which was extracted from the experimental data, and it was found that Haan's mix model predicted trends in the width of the mix layer as a function of initial shell thickness. These results contribute to an assessment of the range of validity and predictive capability of the Haan saturation model, as well as increasing confidence in the methods used to extract mixing information from experimental data.

Interactive tools designed to study mix in inertial confinement fusion implosions

Graphical user interface tools have been built in IDL to study mix in inertial confinement fusion (ICF) implosion cores. FLAME, a code which investigates yield degradation due to mix, was designed to post-process 1D hydrodynamic simulation output by implementing a variety of mix models. Three of these mix models are based on the physics of the fall-line. In addition, mixing data from other sources can be incorporated into the yield degradation analysis. Two independent tools called HAME and YAME were developed to calculate the spatial extent of the mix region according to the Haan saturation model and Youngs' phenomenological model, respectively. FLAME facilitates a direct comparison to experimental data. The FLAME, HAME, and YAME interfaces are user-friendly, flexible, and platform-independent.

Comparison of genetic-algorithm and emissivity-ratio analyses of image data from OMEGA implosion cores

Detailed analysis of x-ray narrow-band images from argon-doped deuterium-filled inertial confinement fusion implosion experiments yields information about the temperature spatial structure in the core at the collapse of the implosion. We discuss the analysis of direct-drive implosion experiments at OMEGA, in which multiple narrow-band images were recorded with a multimonochromatic x-ray imaging instrument. The temperature spatial structure is investigated by using the sensitivity of the Ly beta/He beta line emissivity ratio to the temperature. Three analysis methods that consider the argon He beta and Ly beta image data are discussed and the results compared. The methods are based on a ratio of image intensities, ratio of Abel-inverted emissivities, and a search and reconstruction technique driven by a Pareto genetic algorithm.

Analysis of time-resolved argon line spectra from OMEGA direct-drive implosions

We discuss the observation and data analysis of argon K-shell line spectra from argon-doped deuterium-filled OMEGA direct-drive implosion cores based on data recorded with two streaked crystal spectrometers. The targets were 870 microm in diameter, 27 microm wall thickness plastic shells filled with 20 atm of deuterium gas, and a tracer amount of argon for diagnostic purposes. The argon K-shell line spectrum is primarily emitted at the collapse of the implosion and its analysis provides a spectroscopic diagnostic of the core implosion conditions. The observed spectra includes the He alpha, Ly alpha, He beta, He gamma, Ly beta, and Ly gamma line emissions and their associated He- and Li-like satellites thus covering a broad photon energy range from 3100 to 4200 eV with a spectral resolution power of approximately 500. The data analysis relies on detailed atomic and spectral models that take into account nonequilibrium collisional-radiative atomic kinetics, Stark-broadened line shapes, and radiation transport calculations.

Four-objective analysis including an optically thick line to extract electron temperature and density profiles in ICF implosion cores

We discuss a search and reconstruction method for the simultaneous and self-consistent analysis of a set of image intensity spatial profiles from several narrow-band x-ray pinhole images from argon-doped inertial confinement fusion implosion cores, and the space-integrated line spectrum. The atomic and radiation physics model is embedded into an inversion algorithm, which consists of a Pareto genetic algorithm followed by a Levenberg-Marquardt non-linear least-squares-minimization step, in order to extract the core spatial structure. To find out the best combination of experimental objectives for the analysis, two different combinations of objectives, including optically thick argon Lyα line, were tested and their results were compared with the results from a previous 3-objective analysis. © 2007 American Institute of Physics.

Initial experiments on the shock-ignition inertial confinement fusion concept

Shock ignition is a two-step inertial confinement fusion concept where a strong shock wave is launched at the end of the laser pulse to ignite the compressed core of a low-velocity implosion. Initial shock-ignition technique experiments were performed at the OMEGA Laser Facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] using 40-μm-thick, 0.9-mm-diam, warm surrogate plastic shells filled with deuterium gas. The experiments showed a significant improvement in the performance of low-adiabat, low-velocity implosions compared to conventional “hot-spot” implosions. High areal densities with average values exceeding ∼0.2g∕cm2 and peak areal densities above 0.3g∕cm2 were measured, which is in good agreement with one-dimensional hydrodynamical simulation predictions. Shock-ignition technique implosions with cryogenic deuterium and deuterium-tritium ice shells produced areal densities close to the 1D prediction and achieved up to 12% of the predicted 1D fusion yield.

Spectroscopic determination of temperature and density spatial profiles and mix in indirect-drive implosion cores

In the field of inertial confinement fusion (ICF), work has been consistently progressing in the past decade toward a more fundamental understanding of the plasma conditions in ICF implosion cores. The research presented here represents a substantial evolution in the ability to diagnose plasma temperatures and densities, along with characteristics of mixing between fuel and shell materials. Mixing is a vital property to study and quantify, since it can significantly affect implosion quality. We employ a number of new spectroscopic techniques that allow us to probe these important quantities. The first technique developed is an emissivity analysis, which uses the emissivity ratio of the optically thin Lybeta and Hebeta lines to spectroscopically extract temperature profiles, followed by the solution of emissivity equations to infer density profiles. The second technique, an intensity analysis, models the radiation transport through the implosion core. The nature of the intensity analysis allows us to use an optically thick line, the Lyalpha, to extract information on mixing near the core edge. With this work, it is now possible to extract directly from experimental data not only detailed temperature and density maps of the core, but also spatial mixing profiles.

Development of two mix model postprocessors for the investigation of shell mix in indirect drive implosion cores

The presence of shell mix in inertial confinement fusion implosion cores is an important characteristic. Mixing in this experimental regime is primarily due to hydrodynamic instabilities, such as Rayleigh-Taylor and Richtmyer-Meshkov, which can affect implosion dynamics. Two independent theoretical mix models, Youngs’ model and the Haan saturation model, were used to estimate the level of Rayleigh-Taylor mixing in a series of indirect drive experiments. The models were used to predict the radial width of the region containing mixed fuel and shell materials. The results for Rayleigh-Taylor mixing provided by Youngs’ model are considered to be a lower bound for the mix width, while those generated by Haan’s model incorporate more experimental characteristics and consequently have larger mix widths. These results are compared with an independent experimental analysis, which infers a larger mix width based on all instabilities and effects captured in the experimental data.

Development of spectroscopic tools for the determination of temperature and density spatial profiles in implosion cores

Two innovative spectroscopic techniques have been developed to characterize the temperature and density spatial profiles of Inertial Confinement Fusion (ICF) implosion cores. The analyses involve the emissivity ratio of Ly β/He β to extract temperature, followed by the solution of either emissivity or intensity equations based on the Ly β or He β line to infer density. In particular, the idea of solving intensity equations permits the consideration of the effects of opacity. Information on turbulent mixing of the shell material into the core can be extracted by including the optically-thick Ly α line in the intensity analysis. We focus on developing and validating these spectroscopic tools using synthetic data. Hydrodynamic simulations matching the experimental conditions are used to test the codes and to demonstrate the validity of the physical principles in our spectroscopic techniques.

Multiobjective method for fitting pinhole image intensity profiles of implosion cores driven by a Pareto genetic algorithm

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 Heβ and Lyβ image data and the line spectrum.

Spatial structure analysis of direct-drive implosion cores at OMEGA using x-ray narrow-band core images

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.

Dynamic hohlraum radiation hydrodynamics

Z-pinch dynamic hohlraums are a promising indirect-drive inertial confinement fusion approach. Comparison of multiple experimental methods with integrated Z-pinch∕hohlraum∕capsule computer simulations builds understanding of the hohlraum interior conditions. Time-resolved x-ray images determine the motion of the radiating shock that heats the hohlraum as it propagates toward the hohlraum axis. The images also measure the radius of radiation-driven capsules as they implode. Dynamic hohlraum LASNEX [G. Zimmerman and W. Kruer, Comments Plasma Phys. Control. Fusion 2, 85 (1975)] simulations are found to overpredict the shock velocity by ∼20–40%, but simulated capsule implosion trajectories agree reasonably well with the data. Measurements of the capsule implosion core conditions using time- and space-resolved Ar tracer x-ray spectroscopy and the fusion neutron yield provide additional tests of the integrated hohlraum-implosion system understanding. The neutron yield in the highest performing CH capsule implosions is ∼20–30% of the yield calculated with unperturbed 2D LASNEX simulations. The emissivity-averaged electron temperature and density peak at approximately 900eV and 4×1023cm−3, respectively. Synthetic spectra produced by postprocessing 1D LASNEX capsule implosion simulations possess spectral features from H-like and He-like Ar that are similar in duration to the measured spectra. However, the simulation emissivity-averaged density peaks at a value that is four times lower than the experiment, while the temperature is approximately 1.6 times higher. The agreement with the capsule trajectory measurements and the ability to design capsule implosions that routinely produce implosion cores hot and dense enough to emit fusion neutrons and Ar spectra are evidence for a respectable degree of dynamic hohlraum understanding. The hohlraum shock velocity and implosion core discrepancies imply that calculations of the hohlraum radiation driving capsule implosions require further refinement.

Line broadening analysis of implosion core conditions at Z using argon K-shell spectroscopy

We report on spectral line broadening analysis of Ar K-shell lines from argon-doped implosion cores driven by a dynamic hohlraum z-pinch. The observed Ar spectra include emissions from the resonance series in H- and He-like Ar ions, i.e., Ly α , Ly β and Ly γ , and He α , He β , He γ and He δ lines, respectively. The analysis accounts for opacity and Stark broadening to determine electron density, N e , and areal-density, N Δ R , values for the ground state populations of H- and He-like Ar ions. Furthermore, these results are combined with the ratio of H- and He-like ground state populations to extract the electron temperature, T e .

Multi-objective spectroscopic analysis of core gradients: Extension from two to three objectives

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 β and Ly β 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.

Analysis of the spatial structure of inertial confinement fusion implosion cores at OMEGA

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.

Processing of multi-monochromatic x-ray images from indirect drive implosions at OMEGA

We report here on the processing of multi-monochromatic x-ray images recorded with the MMI instrument in a series of stable and low-convergence indirect-drive implosion experiments in which Ar-doped D2-filled plastic shells were imploded with the OMEGA laser system. MMI records numerous narrow-band x-ray images over a broad photon energy range. From these images, specific line- and continuum-based subimages can be extracted. A procedure for processing data from the array of images recorded by MMI was developed and implemented into a convenient interactive data language code. Data from narrow-band x-ray images are critical for diagnosing the spatial structure of ICF implosion cores.

Spectroscopic determination of dynamic plasma gradients in implosion cores.

The time-dependent gradient structure of a laser-compressed, high-energy-density plasma has been determined using a method based on the simultaneous analysis of time-resolved x-ray monochromatic images and x-ray line spectra from Ar-doped D2 implosion cores. The analysis self-consistently determines the temperature and density gradients that yield the best fits to the spatial-emissivity profiles and spectral line shapes. This measurement is important for understanding the spectra formation and plasma dynamics associated with the implosion process. In addition, since the results are independent of hydrodynamic simulations, they are also important for comparison with fluid-dynamic models.