High Energy Density Experiments Research Articles

Fabrication of Pixelated GaAs Detectors for Ultrafast Hard X-Ray Imaging

Sandia National Laboratories is developing GaAs photodiode arrays compatible with the Ultra-Fast X-ray Imager (UXI) platform for sub-nanosecond hard x-ray diagnostics. These 25 µm pitch pixelated detector arrays are particularly useful in the diagnostics of high energy density (HED) experiments to capture spatial variations as they evolve. Existing UXI sensors consist of a backside illuminated Si detector array bonded to a read-out integrated circuit (ROIC), which offers peak sensitivity to soft (<10keV) x-rays [1]. Substituting the Si detector with materials with higher atomic numbers Z, such as GaAs and CdTe, the sensitivity of the imager can be improved for higher x-ray energies [2]. The integration of these higher Z materials at useful pixel sizes and appropriate thicknesses (10s of µm) is challenging due to less mature processing and integration techniques. Here, we report on the fabrication of small pixel arrays using epitaxially-grown GaAs p-i-n diodes, integrated onto Si fanout packages with sub-nanosecond response times.We have previously reported on discrete fast, hard x-ray GaAs detectors [3]. This paper focuses on transferring this prior work on single-element detectors to fine-pitch pixelated imaging arrays. Fabrication of the backside-illuminated detector array begins with the separate fabrication of the Si readout and GaAs detector chips, continues with In bump flip-chip bonding, and is then completed by removing the GaAs substrate and making backside contact to bias the detectors. A cross-sectional schematic of the fabricated device is given in Fig. 1.The Si fanouts were fabricated in the Sandia MESA SiFab facility. Each fanout chip has a series of 1x1 mm2 bonding sites with 25 µm pitch pixels arrays ranging in size from 3x3 to 36x36. Growth and fabrication of the GaAs detectors was carried out in the Sandia MESA MicroFab facility. Molecular beam epitaxy is used to grow a series of wafers with GaAs p-i-n diode structures on commercial n-type GaAs wafers. The epitaxial structure consists of, from the substrate up, 200 nm of undoped GaAs, 400 nm of undoped Al0.4Ga0.6As to serve as an etch stop, 1 µm of n-type GaAs, 4 to 40 µm of undoped GaAs, and 400 nm of p-type GaAs.The GaAs devices are fabricated using standard III-V semiconductor processing techniques. The bond-side (p-contact) metal contact is made with a Ti/Pt/Au metal stack. Next, the pixels are electrically isolated by wet etching through the p-GaAs and into the undoped GaAs region. A silicon nitride passivation is then deposited by PECVD and patterned to reduce dark current [3]. Finally, underbump metal and In bumps are deposited on both the Si fanout and GaAs detector chips. After In deposition, the chips are singulated and flip-chip bonded. After bonding, the parts are underfilled with a low viscosity epoxy to provide additional mechanical stability. The left chip in Fig. 2(a) shows a bonded pair after flip-chip bonding.After bonding, the GaAs substrate is removed to allow backside metallization and illumination. The bulk of the substrate is first removed by mechanical lapping to thin the detector chip from approximately 630 µm to 70 µm. Next, a selective wet etch (5:1 citric acid:H2O2) is used to uniformly etch to the stop layer. Finally, a timed wet etch is used to remove the etch stop layer and expose the backside n-contact. These thinning steps are shown in Figs. 2(a) and (b). Backside metal contacts are formed using a low-temperature process to prevent melting the In bumps [4]. After fabrication, devices are packaged onto a custom PCB designed to allow for flexible readout as shown in Fig. 2(d).Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government.SAND2020-5273AREFERENCES[1] L. Claus, et al., “Design and characterization of an improved, 2 ns, multi-frame imager for the Ultra-Fast X-ray Imager (UXI) program at Sandia National Laboratories,” Proc. SPIE, 10390, 2017.[2] Q. Looker, et al., “Synchrotron characterization of high-Z, current-mode x-ray detectors,” Rev. Sci. Instrum. 91, 023509, 2020.[3] Q. Looker, et al., “GaAs x-ray detectors with sub-nanosecond temporal response,” Rev. Sci. Instrum. 90, 113505, 2019.[4] M. G. Wood, et al., “Formation of Ohmic contacts to n-GaAs at temperatures compatible with indium flip-chip bonding,” Proc. Electrochem. Soc., 881, 2019. Figure 1

Constraining physical models at gigabar pressures.

High-energy-density (HED) experiments in convergent geometry are able to test physical models at pressures beyond hundreds of millions of atmospheres. The measurements from these experiments are generally highly integrated and require unique analysis techniques to procure quantitative information. This work describes a methodology to constrain the physics in convergent HED experiments by adapting the methods common to many other fields of physics. As an example, a mechanical model of an imploding shell is constrained by data from a thin-shelled direct-drive exploding-pusher experiment on the OMEGA laser system using Bayesian inference, resulting in the reconstruction of the shell dynamics and energy transfer during the implosion. The model is tested by analyzing synthetic data from a one-dimensional hydrodynamics code and is sampled using a Markov chain Monte Carlo to generate the posterior distributions of the model parameters. The goal of this work is to demonstrate a general methodology that can be used to draw conclusions from a wide variety of HED experiments.

Review of the first charged-particle transport coefficient comparison workshop

We present the results of the first Charged-Particle Transport Coefficient Code Comparison Workshop, which was held in Albuquerque, NM October 4–6, 2016. In this first workshop, scientists from eight institutions and four countries gathered to compare calculations of transport coefficients including thermal and electrical conduction, electron–ion coupling, inter-ion diffusion, ion viscosity, and charged particle stopping powers. Here, we give general background on Coulomb coupling and computational expense, review where some transport coefficients appear in hydrodynamic equations, and present the submitted data. Large variations are found when either the relevant Coulomb coupling parameter is large or computational expense causes difficulties. Understanding the general accuracy and uncertainty associated with such transport coefficients is important for quantifying errors in hydrodynamic simulations of inertial confinement fusion and high-energy density experiments.

Model for photothermal ionization and molecular recombination during pulsed ablation of polyethylene

The surface erosion of polyethylene is studied under conditions characteristic of extremely high-rate heating expected in laser-driven and high energy density experiments. A coupled model for photoionization and volumetric photothermal degradation of polyethylene is developed that includes finite rate chemistry and molecular recombination of radicals. First, the model is used to explore the evolution of radicals and hydrocarbons during isochoric heating, for which it is found that polyethylene is not in chemical equilibrium for rates &amp;gt;108K/s. Then, the model is used to explore the cooperativity between photoionization and photothermal ionization during one-dimensional ablation from a pulsed heating source, accounting for coupled energy deposition, thermokinetics, thermochemistry, hydrodynamics, mechanics, and thermal conduction in the ALEGRA multi-physics code. The ablation depth per energy pulse is found to agree with an analytical model for instantaneous energy deposition and absent molecular recombination or photoionization. Otherwise, molecular recombination of radicals reduces the depth of ablation per pulse, owing to thermal stabilization on forming branched molecules. Radical formation through photoionization can be compensated for by recombination, but it accelerates ablation at high enough photoionization rates. Finally, the influence of molecular recombination on the ablation depth is found to decrease for long pulses, owing to decreased recombination reaction rates at lower vapor densities near the ablated surface. These and other findings highlight competition between the processes of scission, recombination, and photoionization during pulsed ablation of polyethylene.

Measuring the oscillator strength of intercombination lines of helium-like V ions in a laser-produced-plasma

We present results of measurements of the oscillator strength of intercombination lines of He-like Vanadium ions in high energy density (HED) laser-produced-plasmas and compare them with the simulations from commonly used codes and data from the NIST database. Whilst not yet sufficiently accurate to constrain different trusted atomic-physics models for the particular system studied, our results are in agreement with the available data within experimental error bars, yet differ from cruder approximations of the oscillator strength used in certain atomic-kinetics packages, suggesting that this general method could be further extended to be used as a measurement of the oscillator strength of additional atomic transitions under the extreme conditions that are achieved in HED experiments.

Material Characterization of Hierarchical Tunable Pore Size Polymer Foams Used in the MARBLE Mix Morphology Experiment

One of the great challenges of inertial confinement fusion and high energy density experiments is understanding the effects of mix on thermonuclear burn. The MARBLE campaign, conceived at Los Alamos National Laboratory, aims to gather new insights into this issue by utilizing unique target capsules containing polymer foams of variable pore sizes, tunable over an order of magnitude. Such capsules allow the degree of initial heterogeneity to be controlled experimentally for the first time. Here, we describe the various characterization efforts used to gain understanding of the chemical structure and behavior of the foam. Previous experiments were not sensitive to foam physical properties, and the MARBLE platform has aided in the development of techniques to measure foam properties such as deuterium content, density variation, hydrogen adsorption, and pore size and volume distribution.

Hybrid target design for imprint mitigation in direct-drive inertial confinement fusion.

A target design for mitigating the Rayleigh-Taylor instability is proposed for use in high energy density and direct-drive inertial confinement fusion experiments. In this scheme, a thin gold membrane is offset from the main target by several-hundred microns. A strong picket on the drive beams is incident upon this membrane to produce x rays which generate the initial shock through the target. The main drive follows shortly thereafter, passing through the ablated shell and directly driving the main target. The efficacy of this scheme is demonstrated through experiments performed at the OMEGA EP facility, showing a reduction of the Rayleigh-Taylor instability growth which scales exponentially with frequency, suppressing development by at least a factor of 5 for all wavelengths below 100 μm. This results in a delay in the time of target perforation by ∼40%.

Shock compression response of diamond single crystals at multimegabar stresses

Shock compressed diamond response at multimegabar stresses---of fundamental interest to high pressure science and relevant for high energy density experiments related to inertial confinement fusion---is often assumed to be hydrodynamic. To examine this assumption, plate impact experiments were conducted to measure wave profiles in diamond single crystals shocked to $\ensuremath{\sim}900\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ elastic impact stress (EIS). For the [110] and [111] orientations, two-wave structures (elastic-inelastic response) were observed to \ensuremath{\sim}900 GPa EIS; in contrast, single (overdriven) waves were observed at 480 GPa EIS and above for the [100] orientation. The elastic wave velocities for the [110] and [111] orientations were significantly larger than those for the [100] orientation. Strong orientation dependence was also observed for the elastic wave amplitudes; for [110] and [111] diamond, the amplitudes increased significantly with increasing elastic impact stress. The observed two-wave structures and the strong orientation dependence (elastic wave speeds and amplitudes) demonstrate unequivocally that the shock response of diamond single crystals is not hydrodynamic at stresses below the melt transition. As such, appropriate elastic-inelastic material descriptions are needed to accurately model the high stress response of diamond and other strong solids.

Correlations between conduction electrons in dense plasmas.

Most treatments of electron-electron correlations in dense plasmas either ignore them entirely (random phase approximation) or neglect the role of ions (jellium approximation). In this work, we go beyond both these approximations to derive a formula for the electron-electron static structure factor which properly accounts for the contributions of both ionic structure and quantum-mechanical dynamic response in the electrons. The result can be viewed as a natural extension of the quantum Ornstein-Zernike theory of ionic and electronic correlations, and it is suitable for dense plasmas in which the ions are classical and the conduction electrons are quantum-mechanical. The corresponding electron-electron pair distribution functions are compared with the results of path integral Monte Carlo simulations, showing good agreement whenever no strong electron resonance states are present. We construct approximate potentials of mean force which describe the effective screened interaction between electrons. Significant deviations from Debye-Hückel screening are present at temperatures and densities relevant to high-energy density experiments involving warm and hot dense plasmas. The presence of correlations between conduction electrons is likely to influence the electron-electron contribution to the electrical and thermal conductivity. It is expected that excitation processes involving the conduction electrons (e.g., free-free absorption) will also be affected.

Three dimensional ion-acoustic rogons in quantized anisotropic magnetoplasmas with trapped/untrapped electrons.

Three-dimensional (3D) modulational instability (MI) and ion-acoustic (IA) envelopes are studied in a quantized degenerate magnetoplasma, whose constituents are the trapped/untrapped electrons and anisotropic positive ions. By using quantum hydrodynamic equations and the multiscale reductive perturbation technique, a 3D nonlinear Schrödinger equation is derived to account for electron quantization and ion pressure anisotrophy effects. The potential excitations are shown stable (unstable) against the perturbations for Kc<0(Kc>0), where Kc is a critical parameter that accounts for the longitudinal (transverse) dispersion(s) and nonlinearity effects. Numerically, the nonlinear evolution of IA wavepackets into a 3D MI may be revealed in the ranges of low and high frequencies 0<ω≤0.05 and 0.75≤ω≤1.1. The quantizing magnetic field reduces (enhances) the group speed (wave frequency) of IA excitations, concentrating the wave energy to favor the modulational instability. Finite electronic temperature (viz.,Te≤10keV) enhances the untrapped electrons and significantly widens the instability domain Kc>0. The ionic pressure anisotropy increases the wave frequency (ω), piles up the harmonics under Kc>0, and give rise to modulational instability. The quantized magnetic field and anisotropic pressure reduce the amplitude and spatial extension of the IA rogons. This study is important for understanding the 3D MI and unstable excitations in degenerate plasmas, relevant to white dwarfs, neutron stars, and high-energy density experiments, where strong magnetic field quantizes the dynamics of trapped/untrapped electrons.

Density-functional-theory calculations of the optical properties of Al2O3: From solid-state to warm dense matter conditions

Aluminium oxide (Al2O3) in extreme conditions is present in many high energy density (HED) experiments, as well as in disintegrating extra-solar planets (exoplanets). Understanding the changes in the optical properties of Al2O3 in conditions of extreme temperatures is necessary for designing HED experiments, and the interpretation of astronomical observations. However, there are no calculations of the optical properties of HED Al2O3 in the literature. Here, we present density functional theory (DFT) calculations of the optical properties of Al2O3 for photon energies from 0 to 90 eV in conditions ranging from crystalline to warm dense matter conditions, at solid densities. Changes of the optical properties due to increasing temperature are explained in terms of the changing electronic structure and electron occupation. To benchmark the DFT approach adopted here, the calculations are compared to available experimental data of cold solid Al2O3 from other studies.

Improvement in Thomson scattering diagnostic precision via fitting the multiple-wavenumber spectra simultaneously.

Thomson scattering (TS) is a powerful diagnostics for understanding the plasma conditions in high energy density experiments. With the aid of Monte Carlo simulation and statistical analysis, we demonstrated unreported high precisions of ne, Te, Ti, etc., via fitting the multiple-wavenumber spectra of ion-acoustic featured TS simultaneously. For instance, utilizing this method in the current typical conditions on SG-180kJ laser facility, the precisions of ne, Te would be better than 8% and 0.5%, respectively. We presented the fitting precisions at different cases and the chi-square trends of the single- and dual-branch TS. This diagnostic technique is found to be applicable within a wide range of plasma parameters and wavenumbers, which is practical to prompt much more precise plasma diagnostics in experiments.

Investigation of high X-ray conversion efficiency Kr filled gas sources at the National Ignition Facility

We report on the performance of high x-ray fluence Kr K-shell sources that are being developed for high energy density experiments. These targets are 4.1 mm in diameter 4.4 mm tall hollow epoxy tubes having a 40 μm thick wall holding 1.5 atm of Kr gas. For these shots, the National Ignition Facility laser delivered a nominally constant total energy of ≈750 kJ of 351 nm (3ω) light at the three power levels [e.g., ≈120 (low), ≈145 (medium), and ≈210 TW (high)]. The Kr K-shell (Ephoton = 8–20 keV) x-ray radiant intensity and radiant energy (kJ/sr) of these sources were found to increase as a function of laser power but began to plateau at the highest laser power. The Kr K-shell radiant energy increased from ≈1 kJ/sr at ≈120 TW to ≈2 kJ/sr at ≈210 TW. Radiation hydrodynamics simulations predict radiant energies to be always higher than these measurements. The increase in K-shell emission is attributed to its strong dependence on the electron temperature. Electron temperature distributions were inferred from measured Heα and Lyα line emission through the use of a genetic algorithm and Scram modeling. The inferred temperatures from the experiment are 20% to 30% higher than those predicted from modeling.

Properties of laser-driven hard x-ray sources over a wide range of laser intensities

We present measurements of the hard x-ray emission from targets irradiated at relativistic laser intensities, with the objective of comprehensively characterizing source properties relevant to x-ray radiography backlighting in high energy density experiments. Thin gold foil and tungsten wire targets were irradiated at peak laser intensities varying between 1018 and 1021 W/cm2, with laser pulse energies &amp;gt;100 J. We have measured the absolute x-ray yield in the spectral range between 20 and 200 keV, angularly resolved over a large range of emission angles with respect to the incident laser. In addition, we have determined the x-ray source sizes for the two target types in the direction both along and across the target. The results are compared with the predictions of a simple model for the hot electron propagation, x-ray generation, collisional stopping, and expansion cooling. Based on this model, our measurements allow extraction of the laser to hot electron conversion over the wide range of intensities covered by our experiment.

Development and validation of the TROLL radiation-hydrodynamics code for 3D hohlraum calculations

Two-dimensionnal (2D) radiation hydrodynamics codes are the workhorses with which current inertial confinement fusion (ICF) and high energy density (HED) experiments are most often prepared and analyzed. Yet significant scientific and programmatic benefits can be gained from a 3D capability. The TROLL code at CEA DAM is meant to address this need. This article illustrates the phases of integrated verification and physical validation through which this project is currently progressing.

High-energy X-ray radiography of laser shock loaded metal dynamic fragmentation using high-intensity short-pulse laser.

The dynamic fragmentation of shock-loaded high-Z metal is of considerable importance for both basic and applied science. The areal density and mass-velocity distribution of dynamic fragmentation are crucial factors in understanding this issue. Experimental methods, such as pulsed X-ray radiography and proton radiography, have been utilized to obtain information on such factors; however, they are restricted to a complex device, and the spatial resolution is in the order of 100 μm. In this work, we present the high-quality radiography of the dynamic fragmentation of laser shock-loaded tin, with good two-dimensional (2D) spatial resolution. Dynamic fragmentation is generated via high-intensity ns-laser shock-loaded tin. A high-energy X-ray source in the 50-200 keV range is realized by the interaction of a high-intensity ps-pulse with an Au microwire target, attached to a low-Z substrate material. A high 2D resolution of 12 μm is achieved by point-projection radiography. The dynamic-fragmentation radiography is clear, and the signal-to-noise ratio is sufficiently high for a single-shot experiment. This unique technique has potential application in high-energy density experiments.

Sub-nanosecond single line-of-sight (SLOS) x-ray imagers (invited).

A new generation of fast-gated x-ray framing cameras have been developed that are capable of capturing multiple frames along a single line-of-sight with 30 ps temporal resolution. The instruments are constructed by integrating pulse-dilation electron imaging with burst mode hybrid-complimentary metal-oxide-semiconductor sensors. Two such instruments have been developed, characterized, and fielded at the National Ignition Facility and the OMEGA laser. These instruments are particularly suited for advanced x-ray imaging applications in Inertial Confinement Fusion and High energy density experiments. Here, we discuss the system architecture and the techniques required for tuning the instruments to achieve optimal performance. Characterization results are also presented along with planned future improvements to the design.

Characterizing the modulation transfer function for X-ray radiography in high energy density experiments.

The Modulation Transfer Function (MTF) is an established means for characterizing imaging performance of X-ray radiography systems. We report on experiments using high energy, laser-driven X-ray radiography systems that assess performance using MTF values measured with the knife-edge projection method. The broadband, hard X-ray systems under study use line-projection imaging produced by narrowing the laser-generated X-ray source with a slit. We find that good contrast resolution can be achieved (the MTF = 0.5 at 75 μm wavelength) and that this performance is reproduced on different laser facilities. We also find that the MTF is sensitive both to the thickness of the line-projection slit and to the backing material thickness under the knife-edge. Both these sensitivities are due to a common mechanism, namely induced changes in the spectrally-averaged spatial widths of the X-ray source. The same line-projection system is also used on experimental campaigns measuring Rayleigh-Taylor instability growth by dynamically imaging sinusoidal, high Z micro-targets with wavelengths of 100 μm or less. By applying the measured MTF values to correct the ripple target contrast measurements, we can predict ripple growth to approximately 10% accuracy.

An x-ray optic calibration facility for high energy density diagnostics.

A facility to calibrate x-ray imaging optics was built at Lawrence Livermore National Laboratory to support high energy density (HED) and inertial confinement fusion (ICF) diagnostics such as those at the National Ignition Facility and the Sandia Z-Machine. Calibration of the spectral reflectivity and resolution of these x-ray diagnostics enable absolute determination of the x-ray flux and wavelengths generated in the HED and ICF experiments. Measurement of the optic point spread function is used to determine spatial resolution of the optic. This facility was constructed to measure (1) the x-ray reflectivity to ±5% over a spectral range from 5 to 60 keV; (2) point spread functions with a resolution of 50 μm (currently) and 13 μm (future) in the image plane; and (3) optic distance relative to the x-ray source and detector to within ±100 μm in each dimension. This article describes the capabilities of the calibration facility, concept of operations, and initial data from selected x-ray optics.

X-ray backlighter requirements for refraction-based electron density diagnostics through Talbot-Lau deflectometry.

Talbot-Lau x-ray interferometers can map electron density gradients in High Energy Density (HED) samples. In the deflectometer configuration, it can provide refraction, attenuation, elemental composition, and scatter information from a single image. X-ray backlighters in Talbot-Lau deflectometry must meet specific requirements regarding source size and x-ray spectra, amongst others, to accurately diagnose a wide range of HED experiments. 8 keV sources produced in the high-power laser and pulsed power environment were evaluated as x-ray backlighters for Talbot-Lau x-ray deflectometry. In high-power laser experiments, K-shell emission was produced by irradiating copper targets (500 × 500 × 12.5 μm3 foils, 20 μm diameter wire, and >10 μm diameter spheres) with 30 J, 8-30 ps laser pulses and a 25 μm copper wire with a 60 J, 10 ps laser pulse. In the pulsed power environment, single (2 × 40 μm) and double (4 × 25 μm) copper x-pinches were driven at ∼1 kA/ns. Moiré fringe formation was demonstrated for all x-ray sources explored, and detector performance was evaluated for x-ray films, x-ray CCDs, and imaging plates in context of spatial resolution, x-ray emission, and fringe contrast.