Session ZI01: Invited: ICF and HEDP Diagnostics

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The hot, dense stagnation plasma generated by most imploding fusion targets contains a mixture of fuel and pusher materials embedded in complicated temperature and density gradients. Measuring these gradients and the amount of pusher ``mix'' is important for assessing the quality of the implosion as well as enabling accurate comparisons to simulation tools. Yet these types of measurements are challenging and likely require new diagnostics. Here we present the design of a new x-ray imager based on reflective-crystal optics that were designed to measure the electron-temperature gradients inside the stagnation plasma generated by a magneto-inertial fusion target known as MagLIF, which is presently being developed on Sandia's Z-machine [Gomez, M. R., et al. (2019) \underline {IEEE }47]. By using spherically-bent crystals we discovered that we could exploit the narrow bandpass provided by the crystal reflection in order to isolate spectral-line emission and hence a chosen ionic species (e.g., He-like Cobalt). We first developed a two-crystal imaging technique that allowed us to visualize, in two-dimensions, the mixing between different emission sources such as the hydrogen fuel and tracer elements coated onto the pusher. By extending this technique to a three-crystal imaging system, we have now measured spectral-line ratios and hence the variation in electron temperature over the stagnation plasma. These measurements are further supported by high-resolution spectroscopy data, which give additional insight into the plasma conditions and composition. Overall, the development of these imaging and spectroscopic techniques represents an important advancement in diagnostic capabilities for Z, and we hope to further advance these capabilities by implementing time-resolved detectors in the near future.   

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White Dwarfs (WDs) are the final evolutionary state of nearly all stars in the sky, including our Sun. Fundamental WD parameters such as surface temperature (T$_{\mathrm{eff}})$ and gravity (log $g)$ can constrain many aspects stellar evolution. The origin of helium atmosphere WDs (DBs) is unknown and thus highlights deficiencies in current stellar evolution models. Several DB evolutionary channels have been proposed, but a lack of accurate DB log $g$ measurements makes discriminating between different models difficult. DB log $g$ values are obtained by fitting model atmospheres to observed spectra. The derived log $g$ strongly depends on our understanding of line broadening in DB atmospheres. Results presented in Bergeron et al. (2011) and Kepler et al. (2015) show an unexpected DB log $g$ increase at T$_{\mathrm{eff\thinspace \thinspace }}$\textless 16,000 K, indicating that the modeled line widths at those temperatures are severely underestimated. An incomplete understanding of neutral broadening has been identified as the leading hypothesis for this behavior. We investigated this phenomenon by performing first-of-their-kind at-parameter neutral broadening experiments at Sandia National Laboratories' Z-machine, the most energetic pulsed x-ray source on earth. Our line width measurements of He I at 5875 \textunderscore , the strongest optical transition in DB spectra, are at least a factor of 1.5 wider than that of any previous experiment or Stark broadening prediction for this feature. The varying neutral helium fraction in these experiments provides evidence that the extra broadening is most likely caused by neutrals in the plasma. Derrider et al. (1975), the neutral broadening theory used by the WD community, underpredicts the neutral broadening contribution by at least an order of magnitude. This experimental evidence suggests that the DB log $g$ upturn could result from an incomplete neutral broadening theory and could thereby also constrain DB evolution.   

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Spectral line broadening calculations are important for atomic behavior in high-energy-density plasmas and in wide-ranging applications that include stellar opacities and plasma diagnostics. Spectral lines are shifted and broadened due to the random perturbations of the atomic states by the surrounding plasma ions and electrons, caused by quantum mechanical many-body Coulomb interactions. Thus, line-shape theory is complex, multi-disciplinary, and employs many approximations. Laboratory measurements and astronomical observations sometimes question the validity of those approximations. We explore the validity of these approximations, thereby refining line-shape calculations. In this talk, we present our recent re-scrutiny of line-shape formalism, aiming to resolve decades-old \textit{isolated-line} problem [Ralchenko 2003] where measured Li-like line widths are significantly broader than calculated. We found that the commonly used line-widths formula neglects a potentially important contribution from \textit{electron-capture} (radiation-less recombination), which is the inverse process of autoionization. Including this effect removes most of the theory-experiment discrepancies in Li-like ions [Gomez (2020)]. This effect, which has been broadly neglected, is important for certain lines. We will discuss when this effect becomes important and how this may impact systems other than Li-like ions. We will further also examine possible future directions in the field of line broadening.   

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Electron temperature is a fundamental parameter used to characterize plasmas of all types. Yet, in the case of HED plasmas, there are very few temperature diagnostics, and many have limited utility. Here we present a technique using x-ray fluorescence (XRF) spectroscopy, which enables temperature measurements in HED experiments where other temperature diagnostics are not currently available. XRF interrogates the plasma with an external energy source (typically x-rays or electrons) to fluoresce specific ions in a plasma to determine the ion distribution, and in turn the plasma temperature. By optimizing the fluorescing element and emission lines for the expected plasma conditions, excellent temperature sensitivity can be achieved over a broad range of plasma conditions. XRF measurements have several advantages over other diagnostics. For example, XRF can probe a specific spatial region of the plasma either by localizing the fluoreser element to the region of interest, by exposing only a limited region of the plasma to the probe source, or a combination of both. XRF is also sensitive to much lower temperatures than traditional x-ray emission spectroscopy for a given photon energy, owing to the fact that the fluorescer ions are at a lower charge state. In addition, the signal is much brighter than similar scattering measurements due to the fact that XRF uses the photoelectric absorption cross section, which is several orders of magnitude larger than scattering cross sections for photon energies of typical x-ray backlighters in HED experiments. We present experimental results using XRF spectroscopy to infer temperatures in shocked foams at the Trident laser facility [1] and ongoing experiments at the Omega laser facility to improve atomic models required to interpret XRF data to extract plasma temperatures. This talk will explore how to optimize XRF measurements for various plasma conditions and how the technique could be applied to HED hydrodynamics experiments where there is a significant need for reliable temperature measurements. [1] M. J. MacDonald et al. J. Appl. Phys. 120, 125901 (2016).   

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The interface between the capsule ablator and fuel ice layer is susceptible to hydrodynamic instabilities. The subsequent mixing of hot ablator material into the ice reduces fuel compression at stagnation and is a candidate for reduced capsule performance. The ability to diagnose ice-ablator mix is critical to understanding and improving stability at this interface. Combining the Crystal Backlighter Imager with the Single Line of Sight camera on the NIF provides multiple quasi-monochromatic 7.2keV radiographs of layered capsule implosions per experiment, with high spatial and temporal resolution. The narrow bandwidth of this diagnostic platform allows radiography of the inner edge of the capsule limb close to stagnation without capsule self-emission contaminating the data, providing a wealth of information that can be used to assess the stability of the ice-ablator interface. An important factor which affects the stability of the ice-ablator interface is preheating of the ablator adjacent to the fuel by energetic components of the x-ray drive, destabilizing the interface. Adding a high-Z dopant layer to the ablator mitigates this effect. Results will be presented from a campaign in which this radiographic technique was used to measure the effect of tungsten dopant concentration on the ice-ablator interface for a series of 3-shock High Density Carbon implosions. These measurements reveal that while the addition of dopant provides an overall stabilizing effect, there are significant differences between the equatorial and polar regions of the capsule. It is hypothesized that this is due to the anisotropy of the energetic components of the x-ray drive, suggesting that this must be considered when addressing preheat. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.   

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Precise knowledge of ionization is required to accurately model compressibility, heat capacity, and the equation of state of materials at extreme conditions. This requires a good understanding of ionization potential depression in dense plasmas, which is an area of vigorous scientific debate, motivated by experiments that have found higher ionization than predicted by widely used theoretical models.[1,2] NIF enables near-degenerate plasmas to be probed at unprecedented densities and pressures. We have developed an experimental platform for x-ray Thomson scattering (XRTS) at NIF to characterize the plasma conditions in ICF capsule implosions near stagnation.[3,4] The electron density and temperature can be inferred from the inelastic Doppler-broadened Compton scattering signal. The ratio of elastic to inelastic scattering is a measure of ionization level.\newline I will present XRTS results from Be, carbon, and CH capsule implosions that reached mass densities up to 50 g/cm$^3$, accessing near- degenerate plasmas where the electron density exceeds 1e+25 cm$^{-3}$, the Fermi energy approaches 200 eV, and the pressure exceeds 1 Gbar. These experiments find higher ionization than predicted by widely-used ionization models, and we see evidence for pressure ionization of the Be K-shell. At mass densities greater than 20 g/cm$^3$, the results show an additional reduction of elastic scattering associated with the delocalization of bound K-shell electrons, which eventually leads to a fully ionized plasma at sufficiently high densities, and affects the compressibility of matter, which can be measured by radiography.[5] \newline [1] S. Vinko et al., Nature {\bf 482}, 59 (2012).\newline [2] D. Kraus, T. D\"oppner et al., Phys. Rev. E {\bf 94}, 011202(R) (2016).\newline [3] T. D\"oppner et al., J. Phys. Conf. Ser. {\bf 500}, 192019 (2014).\newline [4] D. Kraus, T. D\"oppner et al., J. Phys. Conf. Ser. {\bf 717}, 012067 (2016). \newline [5] T. D\"oppner, Phys. Rev. Lett. {\bf 121}, 025001 (2018).\newline