Session BO10: ICF: X-Ray Diagnostics

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Magnetized target fusion devices involve fast compression to high density and are compact. These properties make diagnosis difficult compared to conventional magnetic confinement devices such as tokamaks because of the tight space, the high power density, and the fast time scale. These issues put a premium on fast imaging diagnostics having a substantial standoff from the plasma. X-ray emission is an important indicator of plasma shape, evolution, and hot spots so an X-ray imaging camera is desirable. X-rays cannot be focused easily (if at all), so it is necessary to use an imaging system that avoids mirrors and refractive optics. A pinhole camera would be one possible solution, but a single aperture reduces sensitivity. To address these considerations, we are developing a 1D coded aperture imaging system with $\sim$30 ns resolution. This system, based on an earlier visible light proof-of-principle, will use a linear pixelated scintillator coupled via an optical fiber array to four 32-channel photomultiplier modules. The system will first be tested on the Caltech MHD jet to image a localized X-ray burst. It is then planned to diagnose MTF devices such as FuZE and MIFTI.   

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Laser-plasma x-ray sources have garnered interest from various communities due to their ability to generate high photon-energies from a small source size. The passive imaging of high-energy x-rays and neutrons is also a useful diagnostic in laser-driven fusion as well as laboratory astrophysics experiments which aim to study small samples of transient electron-positron plasmas. Here we demonstrate a coded aperture with scatter and partial attenuation included, which we have dubbed a ‘CASPA’. We compare CASPAs to the more common method of pinhole imaging, confirming the well-known throughput increase of coded apertures, and show that the decoding algorithm relaxes the need for a thick substrate. We simulate a 511 keV x-ray source through ray-tracing and Geant4 simulations to show how partial attenuation of the source by the CASPA allows for a superior signal to noise ratio with respect to a standard pinhole system. In addition, we demonstrate successful imaging of high-energy emission at higher resolution than previously attainable. Finally, we note the potential applications in fusion neutron imaging, and outline how this technique could be applied to measurements of implosion asymmetry.   

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X-ray spectroscopy of direct- and indirect-drive implosions is a powerful diagnostic of the core plasma conditions achieved in experiments. We discuss the temperature and density dependence of the x-ray line emission of L-shell krypton ions afforded through the characteristic sensitivities of the atomic level populations and charged state distributions and detailed Stark-broadened spectral line shapes due to plasma electric microfields, respectively. We have found that krypton-tracer atomic concentrations in the range from 0.02{\%} to 0.04{\%} of the main fill gas produce krypton L-shell n$=$4-2 line emission with values of optical depth that are less than 1 and intensity comparable to previous observations of argon K-shell spectra. In particular, modeling calculations suggest that L-shell emission of Be- and Li-like krypton ions can be used to diagnose electron temperatures in the 1.5keV to 3keV range in dense implosion cores. Furthermore, since the photon energy ranges of krypton L-shell and argon K-shell emissions are comparable, streaked and imaging spectrometers employed for argon spectroscopy$^{\mathrm{1}}$ can be used for krypton L-shell as well. $^{\mathrm{1}}$D. T. Cliche and R. C. Mancini, Applied Optics \textbf{58}, 4753 (2019).   

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X-ray tracer spectroscopy has proven to be a powerful diagnostic tool in inertial confinement fusion (ICF) plasmas using electron temperature dependent line ratios and electron density sensitive Stark widths. However, mid- to high-Z spectral dopants may reduce neutron yield due to radiation cooling and are incompatible with DT cryogenic ICF targets. These issues can be overcome by avoiding the use of spectral tracers and performing a spectroscopic analysis of the continuum x-ray emission from the ionized fuel. Imaging the x-ray continuum at different photon energies enables an image ratio analysis to extract a two-dimensional electron temperature map of the implosion core plasma.$^{\mathrm{1}}$ The OMEGA multi-monochromatic x-ray imager (MMI) instrument records arrays of spectrally resolved images that can be employed for this purpose. We discuss the continuum x-ray emission from the implosion core recorded with MMI in warm-shell OMEGA implosions, the data processing and extraction of continuum narrowband images, and the comparison between experiment and simulation core electron temperature maps. This work is supported by a contract from LLE. $^{\mathrm{1}}$J. A. Koch, S. W. Haan and R. C. Mancini, JQSRT 88, 433 (2004)   

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A general approach for estimating hot-spot mix is being pursued for direct-drive layered implosions on OMEGA. Previously, ablator mix was estimated at the National Ignition Facility by comparing the ratio of measured x-ray over neutron yield to an initially no-mix, ice-block model estimate [T. Ma \textit{et al.}, Phys. Rev. Lett. \textbf{111}, 085004 (2013)]. However, tests with the 1-D LILAC code show that mix can be overestimated because temperature nonequilibrium in OMEGA-scale hot spots causes a breakdown of the ice-block model assumption of equal neutron and x-ray emission volumes. Crucially, this overestimation can be of the order considered important. We explore using a spatially varying hot-spot profile model [R. Betti \textit{et al.}, Phys. Plasmas \textbf{8}, 5257 (2001)] that allows for differing emission volumes to ameliorate mix discrepancies associated with the ice-block assumption. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

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In recent cryogenic ICF implosions at Omega, multiple x-ray histories were measured with a 4-channel x-ray temporal diagnostic using scintillators coupled to an optical streak camera. The relative signal amplitudes of the different channels are modeled by an time-dependent exponential bremsstrahlung emission spectrum, from which time-resolved electron temperature, Te(t), is inferred. This provides valuable diagnostic information, as Te is unaffected by residual flows and other non-thermal effects. Measurement of Te(t) will be used to investigate effects related to time-resolved hot spot energy balance including high-mode and low-mode asymmetries, residual ion-kinetic energy, radiative losses, and ion-electron equilibration in Omega cryogenic implosions. The current prototype diagnostic uses the existing neutron temporal diagnostic infrastructure which allows measurement of Te(t) with 40 ps time resolution and 15{\%} uncertainty at peak emission. A proposed dedicated diagnostic would achieve 5{\%} uncertainty with 20 ps time resolution.   

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The electron temperature (Te) of the hot spot within the core of imploded inertial confinement fusion capsules is an effective indicator of implosion performance. An x-ray streak camera at the National Ignition Facility uses thick Titanium filters to sample x-rays from specific energy regions. The time resolved signal from each filter is used in a forward fit algorithm with a hot spot emission model to generate Te as a function of time. This is a complex problem because the instrument impulse response through each filter is distinct. In addition, the algorithm must distinguish emission from the hot spot from that from ablator material. Herein, the strategies in solving this problem is discussed. Preliminary Te results and their correlation with other performance metrics are presented. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-ABS- 811772   

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Double shell capsules provide a complementary and alternative path to the single shell inertial confinement fusion (ICF) approach. Generically, a double shell capsule consists of an outer shell, a medium between the shells and a high-Z inner shell filled with DT fuel. Double shell targets rely on effectively transferring the kinetic energy of the outer shell to the inner shell to compress the DT fuel. We need to understand the shape of the inner shell surface pushing against the DT, however, current designs use a W or Au inner shell, requiring MeV x-rays to radiograph the inner shell. Surrogate inner shell materials such as Cr allow one to study the same physics and can be radiographed with much lower-energy x-rays (10's of keV). We have developed a plan to study the evolution and shape of the inner shell starting with surrogate materials and utilizing the Advanced Radiographic Capability (ARC) on the National Ignition Facility (NIF). We will discuss our experimental requirements and our plans to utilize ARC to radiograph the inner shell during the implosion.   

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In inertial confinement fusion, high-Z material mixed into the fuel degrades implosions, dulling ignition. Mix is often understood to be primarily fluid instabilities (Rayleigh Taylor, Richtmyer-Meshkov) and has been benchmarked by separated-reactant experiments. New higher spatial resolution separated-reactant target (150 nm deuterated layer instead of a 2 $\mu $m layer) experiments coupled with nuclear diagnostics - neutron time-of-flight and time-resolved gamma-ray diagnostics - reveal a more complex shell-to-fuel mix landscape. Recent implosion experiments at OMEGA Laser Facility reveal that the dominant mix mechanism is diffusion even for a moderate temperature (6 keV) and convergence (10), which were traditionally understood with a hydrodynamic mix width. Supporting Fokker-Planck simulations capture the species specific ion movement independent of fluid instability growth. The thinner reactant layer helps inform the transition to the canonical hydrodynamic mix. Understanding the interplay between diffusion and standard mix mechanisms gives insight into the regimes kinetic effects plays an important role for inertial confinement fusion evolution.   

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The dHIRES (DIM based high resolution) x-ray spectrometer measures Kr He$\alpha $ and He$\beta $ spectra from NIF compressed capsules with 12-eV spectral and 30-ps temporal resolution. Comparison of the measured Kr He$\beta $ spectra with theoretical line shapes provides a measure of the time history of the electron density, n$_{\mathrm{e}}$. Electron temperatures, T$_{\mathrm{e}}$, are inferred by comparison of ratios of Li-like to He-like Kr line intensities to calculations by SCRAM. Spatial profile effects are calculated by averaging assumed optically thin SCRAM spectra over spherical shells defined by n$_{\mathrm{e}}$(r) and T$_{\mathrm{e}}$(r) from LASNEX. Comparisons of measured spectra with SCRAM and CRETIN-TOTAL simulations will be shown.   

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When interpreting trends in MagLIF experiments, we wish to know the relationship between stagnation conditions, performance, and morphology. Typically, stagnation conditions are inferred through a combination of imaging, nTOF, and neutron and x-ray yield measurements. Unfortunately, the observed morphology can strongly bias our inference of stagnation conditions which depends on an understanding of the fuel volume. The volume is constrained by 2D x-ray emission images, which tend to overestimate the neutron producing volume. We show here how the incorporation of K-shell line ratios of Fe ions that are mixed into the fuel into a Bayesian analysis of stagnation that self-consistently includes imaging as well as x-ray and neutron yields, can dramatically improve our understanding of stagnation conditions. The reason for this improvement is that K-shell line ratios are relatively insensitive to the fuel structure, providing an independent constraint on important quantities such as pressure. This improved accuracy leads to a higher fidelity understanding of the observed performance trends.   

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Thin crystals bent into some desirable shape for monochromatic x-ray imaging of plasmas are rarely the same in the images they give, even when they are made by the same people the same way. Here the crystal of interest is quartz cut along the (10\=11) crystal plane. Highly detailed measurements have shows that nominally identical samples of thin wafers that could be bent, but held flat for ease of measurement still reflect x-rays differently from each other, and much less uniformly than thick crystals. When bent, such thin crystals should differ from each other in their x-ray reflection as well and be consistent with a performance difference between monochromatic imagers. This paper is to discuss any progress we may have made in obtaining better thin quartz crystals, and to demonstrate their x-ray reflection with any measurements we may have made in the interim, on flat or bent versions of these crystals.   

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H. G. Rinderknecht, R. Epstein, D. Thorn, T. E. Weber\newline X-ray diagnostics for the electron temperature $T_e$ in ICF implosions are currently being developed. We demonstrate for the first time that the X-ray diagnostics also allow constraining the Te spatial profile in a realistic hot-spot. A series of Omega implosions have been performed with both soft (3-6 keV) and hard (20-30 keV) X-ray diagnostics employed simultaneously. The apparent temperatures inferred from the respective data have been found to differ by a factor of about 2. This reflects the fact that the higher energy photons are produced closer to the center of the hot-spot, thus enabling us to quantify the difference between peak and peripheral temperatures in the fusion fuel.