Session NO05: HED: Diagnostic and Computational Techniques

Live

The Center for Laboratory Astrophysics at the University of Michigan is unique among universities in that we have been fabricating targets for high-energy-density physics experiments for well over the past decade. We utilize the process of machined bodies of various materials and tightly toleranced mating components that serve as constraints, enabling our group to build precise, repeatable targets. We favor traditional machining, utilizing 3D printing when it suits, taking advantage of the very best part of both of these methods of creating precision parts for our targets. In addition to building targets for our group, we serve the greater HEDP community by building targets and providing shotday support for LaserNetUS users and National Laboratory PIs as well. Here we present several campaigns shot at OMEGA and several other facilities around the world.   

Live

Talbot-Lau X-ray Deflectometry (TXD) diagnostics for OMEGA EP can measure electron density gradients to characterize HED plasmas through phase-contrast. To optimize TXD, a suite of backlighter targets were studied on MTW considering source size and spectral quality within the 8 keV interferometer contrast curve. The EP-TXD diagnostic mapped the ablation front of a foil irradiated by 150 J, 1 ns laser pulse (\textgreater 2.4 x 10$^{\mathrm{17}}$ W/cm$^{\mathrm{2}})$. For the first time, a Moir\'{e} fringe pattern was recorded at 5 ns detecting refraction angles \textless 150 \textmu radians, equivalent to line-integrated electron density gradients of \textless 1.8 x 10$^{\mathrm{25}}$ cm$^{\mathrm{-3}}$. Additional Moir\'{e} images can help benchmark MHD codes and simulations by mapping the evolution of the ablation front near critical density. Further developments to the TXD platform will include monochromatic backlighting to improve fringe contrast and spatial resolution so that foils can be irradiated at higher intensities.   

Live

Two-dimensional x-ray radiography is used to measure hydrodynamic perturbation evolution in a laser-driven foil with a rippled rear surface. The ablatively driven system was generated on the OMEGA laser with up to few-nanosecond-duration laser pulses at focused intensities above 10$^{\mathrm{14}}$ W/cm$^{\mathrm{2}}$. The modulated rear-surface dynamics were imaged following the passage of a strong, unsupported shock wave. Radiographs were obtained using a 4.75-keV Ti or 6.70-keV Fe He-like resonance line area backlighting source coupled to a Fresnel phase zone plate imager and an SI-800 x-ray charge-coupled device. Static resolution grid tests confirm the achievement of sub-2-$\mu $m spatial resolution. The hydrodynamic evolution of planar targets with initial rear surface perturbations of varying wavelength, with and without mid-Z dopants or a high-Z rear-surface coating, are studied and compared with synthetic x-ray radiographs generated from numerical simulations using the computer codes DRACO and Spect3D.   

Live

The experimental measurement of concentration-driven diffusion between two species in warm dense matter (WDM) is important for modeling the structure of Jovian planets and for simulating instability growth in inertial confinement fusion (ICF) experiments. We are developing x-ray Fresnel diffractive-refractive radiography (FDR) for use at the NIF and the OMEGA laser facility, which combines ultra-small source size (1 $\mu $m slits) with a thin cylindrical sample (d \textless 10 $\mu $m) that will be isochorically heated to \textasciitilde 10 eV. Measurements are sensitive to Fresnel diffraction signatures, and can resolve density gradient changes with sub -- 1 -- $\mu $m resolution. We will discuss results from the first OMEGA experiments and experimental design plans for NIF experiments.   

Live

The temperature of dynamically compressed materials is the largest uncertainty in modern equation of state modeling, and developing new tools to measure temperature is important to complement data from existing diffraction and equation-of-state platforms. In experiments performed at the National Ignition Facility (NIF), x-ray absorption fine structure (XAFS) has been measured and used to constrain temperature and density in Cu at 450 GPa. These fine-structure modulations in the x-ray absorption are caused by photoelectron scattering off nearby atoms, and are sensitive to both local atomic spacing and thermal disorder. A new high-resolution crystal spectrometer (HiRAXS) with 3-eV resolution between 8.9 - 9.8 keV was used to measure Cu XAFS signals. Cu XAFS sensitivity to temperature, density, and crystal structure at different temperatures along an isochore will be discussed.   

Live

Direct and accurate measurements of thermodynamic and transport properties are essential for understanding the behavior of extreme states of matter. While X-ray diffraction measurements at large laser facilities or Free Electron Lasers, such as the LCLS, have allowed \textit{in situ} measurement of structure and density, however, the direct measurement of bulk temperature remains a challenge. Here, we present the development of a platform using inelastic X-ray scattering in a Johann geometry to measure temperature by the use of the principle of detailed balance. A proof-of-principle experiment was conducted at the HED beamline at the European XFEL on resistively heated single crystal diamond at 500 K. This technique was then combined with a cryogenic jet of argon compressed with a short pulse laser at the MEC endstation at LCLS, allowing the direct measurement of the temperature of laser compressed matter.   

Live

Physics phenomena not formerly accessible in high-energy-density plasmas have been revealed by measured x-ray spectra with high resolving power (E/dE $\sim 10^4$) at the extreme-contrast ALEPH 400 nm laser using precision layered targets. Fine structure x-ray emission from laser-heated H-, He-, and Li-like Ti has been recorded by a trio of x-ray crystal spectrometers. The set of targets, with thin (0.1$-$1 $\mu$m) Ti tracer layers sandwiched inside Al foils to provide spatial resolution, were heated by high-intensity ($10^{21}$ W/cm$^2$) light with high temporal contrast ($10^{12}$ at 25 ps). The Ti He$\beta$ lineshape appears strongly double-peaked, with peak separation compared to Stark lineshape code MERL to constrain the ion temperature and electron density. Furthermore, the x-ray emission of Ti Ly$\alpha$ and He$\alpha$ resonance lines reveal large Doppler shifts due to ion ablation and target expansion; these shifts were viewed from two sides so that directional velocity distribution profiles can be inferred. These temperature, density, and velocity profiles provide information on heat transport of these expanding plasmas and thus establish a benchmark for hydrodynamic models of the plasma evolution.   

Live

In dynamic compression experiments, materials experience dramatic changes in their physical and chemical properties, manifesting in the material's optical emission. For experiments involving high density, temperature is not a well-constrained parameter, requiring sophisticated models to infer thermodynamic properties. To this end, our work developed an optical spectroscopy diagnostic for the OMEGA EP Laser System at the Laboratory for Laser Energetics. This diagnostic measures the time- and wavelength-resolved thermal emission of laser-compressed materials. We present the design of the system, its performance, and experiments on decaying shocks in transparent crystals. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE\textunderscore NA0001944, the University of Rochester, and the New York State Energy Research and Development Authority. The support of DOE does not constitute an endorsement by DOE of the views expressed in this abstract.   

Live

Inertial Confinement Fusion (ICF) experiments implode deuterium-tritium fuel at velocities approaching 400 km/s to densities of 1g/cc and temperatures of 5 keV. Any implosion asymmetry results in reduced compression, yield, and motion of the fusing hot-spot. Models of the hot-spot plasma show that the shift in the mean of the neutron energy distribution away from the rest frame DT birth energy (14.028 MeV) is a result of hot-spot motion and the distribution of thermal ions undergoing fusion in the Gamov peak. On the National Ignition Facility (NIF) hot-spot velocity is measured using Fused-Silica Cherenkov detectors deployed on five lines of sight which is necessary to test the model, and derive the ion temperature of the fusing plasma. Ion temperatures derived from the width of the neutron spectrum are affected by flows and can deviate from the thermal temperature. Electron temperature measurements are often complicated by x-ray opacity and may not reflect the ion distribution in the Gamov peak. High-precision neutron time-of-flight measurements of the hot-spot velocity represent a new tool in diagnosing in implosion drive asymmetry and in understanding the ion distribution in the hot-spot. Measurements from DT-layered implosions on NIF are presented and the inference of thermal ion temperature compared with simulation.   

Live

We present time-resolved interferometry to simultaneously measure plasma densities and electron collision times for strong field laser-matter interactions. First, an intense femtosecond pump pulse generates plasma in a solid and second, a weak 800-nm femtosecond probe traverses the pump-induced plasma and is sent to an interferometer with controlled time delay between pump and probe. By analyzing the interferograms using Fourie methods, we can extract plasma densities and electron collision times in plasma simultaneously with micrometer spatial and femtosecond temporal resolutions. Using the technique, we study the plasma dynamics when a wavelength-varied ($\lambda =$1.2-2.3 $\mu $m) pump pulse undergoes laser filamentation in solid materials [1]. [1] G. Nagar et al., submitted.   

Live

Plasma flows encountered in high-energy density experiments display features that differ from those of equilibrium systems. Non-equilibrium approaches such as kinetic theory (KT) capture many, if not all, of these phenomena. However, KT requires closure information, which can be computed from microscale simulations and communicated to KT. We present a concurrent heterogeneous multiscale approach that couples molecular dynamics (MD) with KT in the limit of near-equilibrium flows. To reduce the cost of gathering information from MD, we use active learning to train neural nets on MD data obtained by randomly sampling a small subset of the parameter space. We apply this method to a plasma interfacial mixing problem relevant to warm dense matter, showing considerable computational gains when compared with the full kinetic-MD approach. We find that our approach enables the probing of Coulomb coupling physics across a broad range of temperatures and densities that are inaccessible with current theoretical models.   

Live

We have developed COAX [1] to provide spatial temperature profiles of a radiation wave through low density foams as a more detailed constraint for simulations than breakout measurements provide [2]. The COAX platform uses a halfraum to drive radiation down a titanium-laden silicon oxide foam at OMEGA-60 [1,3]. Point-projection X-ray absorption spectroscopy, for temperature, is paired with radiography to capture the developing shock as radiation flow becomes subsonic. Spectroscopic analysis of COAX data determines temperature by comparison to synthetic spectra post-processed from radiation-hydrodynamics simulations [2], with \underline {}$+$22.4$\mu $m spatial resolution and temperature uncertainty of \underline {}$+$ 4-8 eV. \begin{enumerate} \item H. M. Johns, C. L. Fryer, S. R. Wood, et al, submitted to HEDP \item C. Fryer, A. Hungerford, J. Kline, et al, HEDP, \textbf{35}, 2020, 100738 \item D. Capelli, C. A. Charsley-Groffman, R. B. Randolph, High Power Laser Sci. \textbf{5}, (2017) \end{enumerate}   

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Assessing plasma state through temperature and density measurements is a standard problem in HED science, yet inferring temperature from spectroscopic diagnostics often relies on complex simulations coarsely fitted to experimental data. These forward analyses fold in assumptions that are difficult to disentangle when puzzling inconsistencies arise. To date, inverse analysis of HED spectra has been limited to the line ratio method, which can only analyze a pair of spectral lines. We demonstrate a generalization of Boltzmann plots to the HED regime to provide an alternative inverse analysis method for obtaining temperature from spectral measurements. Boltzmann plots analyze multiple lines simultaneously to statistically constrain temperature, and provide additional points of comparison for checking assumptions in simulations. We demonstrate this technique on x-ray absorption spectra of titanium measured on the COAX platform on Omega-60. LA-UR-20-24580.   

Live

Proton imaging is a powerful tool for probing electromagnetic fields in high-energy-density plasmas, providing a path-integrated map of the field topology. However, inferring the underlying field structure from proton images of the filamentary magnetic fields produced by the Weibel instability in counterstreaming plasmas has been challenging. In this talk we show that, in general, proton image features directly correspond to the size of the individual magnetic structures and not to the spacing between them (unless they are correlated). For the Weibel instability, the size of the magnetic filaments can be accurately determined via Fourier analysis of the proton images. This work has been published in Phys. Plasmas 26, 102303 (2019); doi: 10.1063/1.5100728   

Live

Radiation transport and its interaction with matter is important for a wide range of problems.~ As simulations increase in fidelity and interfaces are more finely resolved, certain approximations become less valid and transport effects may arise, requiring improved methods for coupling radiation to mater. Transport effects can be provoked across interfaces, especially where the density or composition dramatically change.~ Here we present simulation results for two experimental campaigns that probe radiation flow across boundaries, COAX and Radishock. COAX develops an in-situ temperature probe using a Ti-laden aerogel. ~Radishock uses this probe examine a material interface that is constructed using a counter-propagating shock and dynamically evolves over time.