Publications

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The MAGnetized Liner Inertial Fusion (MagLIF) scheme has achieved thermonuclear fusion yields on the Z Facility by imploding a cylindrical liner filled with D₂ fuel that is preheated with a multi-kJ laser and pre-magnetized with an axial Bz=10 T magnetic field. Preheating (T_e = 100-200 eV) and pre-magnetizing (10-30 T) the fuel serves to reduce the implosion velocity required to achieve multi-keV fusion-relevant temperatures at stagnation with a modest radial convergence. The challenge of fuel preheat in MagLIF is to deposit multiple-kJ of energy into the underdense (n_e/n_c<0.1) fuel over ~10 mm target length efficiently and without introducing contaminants. Once the fuel is heated the applied axial magnetic field (ω_ceτ_e ~ 10) needs to suppress electron thermal conduction sufficiently to prevent unacceptable heat losses to the liner walls. In this LDRD we investigated laser energy deposition at two facilities: The OMEGA-EP laser at the Laboratory for Laser Energetics and the Z-beamlet laser at Sandia National Labs utilizing the PECOS chamber. Multiple experiments were carried out investigating laser transmission through LEH foils, laser heating of underdense gasses and the effects of magnetization on laser preheat. The studies find that magneto-hydrodynamic simulations are able to reproduce energy deposition at MagLIF-like conditions but that at the intensities currently used to preheat MagLIF significant laser plasma instabilities (LPI) occur which partly explain the inability of codes to reproduce previous MagLIF preheat studies. The experiments find that reducing the intensity and smoothing the beam dramatically reduces the amount of stimulated Brillouin backscatter and produces deposition profiles more similar to those produced in simulations. The experiments have provided a large and varied dataset that can be compared to simulations. As part of the LDRD new experimental capabilities have also been developed that will be used to design future MagLIF integrated experiments and investigate fuel magnetization.

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This powerpoint presentation goes over the Fe opacity measurement platform, including how the experiment works, what can be gathered from the measurements, what can be gathered from the simulations, and the limitations of the experiment.

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Recently, frequency-resolved iron opacity measurements at electron temperatures of 170-200 eV and electron densities of (0.7-4.0)×1022cm-3 revealed a 30-400% disagreement with the calculated opacities [J. E. Bailey et al., Nature (London) 517, 56 (2015)NATUAS0028-083610.1038/nature14048]. The discrepancies have a high impact on astrophysics, atomic physics, and high-energy density physics, and it is important to verify our understanding of the experimental platform with simulations. Reliable simulations are challenging because the temporal and spatial evolution of the source radiation and of the sample plasma are both complex and incompletely diagnosed. In this article, we describe simulations that reproduce the measured temperature and density in recent iron opacity experiments performed at the Sandia National Laboratories Z facility. The time-dependent spectral irradiance at the sample is estimated using the measured time- and space-dependent source radiation distribution, in situ source-to-sample distance measurements, and a three-dimensional (3D) view-factor code. The inferred spectral irradiance is used to drive 1D sample radiation hydrodynamics simulations. The images recorded by slit-imaged space-resolved spectrometers are modeled by solving radiation transport of the source radiation through the sample. We find that the same drive radiation time history successfully reproduces the measured plasma conditions for eight different opacity experiments. These results provide a quantitative physical explanation for the observed dependence of both temperature and density on the sample configuration. Simulated spectral images for the experiments without the FeMg sample show quantitative agreement with the measured spectral images. The agreement in spectral profile, spatial profile, and brightness provides further confidence in our understanding of the backlight-radiation time history and image formation. These simulations bridge the static-uniform picture of the data interpretation and the dynamic-gradient reality of the experiments, and they will allow us to quantitatively assess the impact of effects neglected in the data interpretation.

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We report experimental results and simulations showing efficient laser energy coupling into plasmas at conditions relevant to the magnetized liner inertial fusion (MagLIF) concept. In MagLIF, to limit convergence and increase the hydrodynamic stability of the implosion, the fuel must be efficiently preheated. To determine the efficiency and physics of preheating by a laser, an Ar plasma with ne/ncrit∼0.04 is irradiated by a multi-ns, multi-kJ, 0.35-μm, phase-plate-smoothed laser at spot-averaged intensities ranging from 1.0×1014 to 2.5×1014W/cm2 and pulse widths from 2 to 10 ns. Time-resolved x-ray images of the laser-heated plasma are compared to two-dimensional radiation-hydrodynamic simulations that show agreement with the propagating emission front, a comparison that constrains laser energy deposition to the plasma. The experiments show that long-pulse, modest-intensity (I=1.5×1014W/cm2) beams can efficiently couple energy (∼82% of the incident energy) to MagLIF-relevant long-length (9.5 mm) underdense plasmas. The demonstrated heating efficiency is significantly higher than is thought to have been achieved in early integrated MagLIF experiments [A. B. Sefkow, Phys. Plasmas 21, 072711 (2014)10.1063/1.4890298].

We present a platform on the OMEGA EP Laser Facility that creates and diagnoses the conditions present during the preheat stage of the MAGnetized Liner Inertial Fusion (MagLIF) concept. Experiments were conducted using 9 kJ of 3ω (355 nm) light to heat an underdense deuterium gas (electron density: 2.5×1020 cm-3=0.025 of critical density) magnetized with a 10 T axial field. Results show that the deuterium plasma reached a peak electron temperature of 670 ± 140 eV, diagnosed using streaked spectroscopy of an argon dopant. The results demonstrate that plasmas relevant to the preheat stage of MagLIF can be produced at multiple laser facilities, thereby enabling more rapid progress in understanding magnetized preheat. Results are compared with magneto-radiation-hydrodynamics simulations, and plans for future experiments are described.

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Temperature and density asymmetry diagnosis is critical to advance inertial confinement fusion (ICF) science. A multimonochromatic x-ray imager, MMI, records the spectral signature from an ICF implosion core with time resolution, 2D spatial resolution and spectral resolution. While narrow-band images and 2D space-resolved spectra from the MMI data constrain the temperature and the density spatial structure of the core, the accuracy of the images and the spectra highly depends on the quality of the MMI data and the processing tools. Here, we synthetically investigate the criterion for reliable MMI diagnostics and its effects on the accuracy of the reconstructed images. The pinhole array tilt determines the object spatial sampling efficiency and the minimum reconstruction width, $\textit{w}$. When the spectral width associated with $\textit{w}$ is significantly narrower than the spectral linewidth, the line images reconstructed from the MMI data become reliable. The MMI setup has to be optimized for every application to meet this criterion for reliable ICF diagnostics.

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We spectroscopically measure multiple hydrogen Balmer line profiles from laboratory plasmas to investigate the theoretical line profiles used in white dwarf (WD) atmosphere models. X-ray radiation produced at the Z Pulsed Power Facility at Sandia National Laboratories initiates plasma formation in a hydrogen-filled gas cell, replicating WD photospheric conditions. We also present time-resolved measurements of Hβ and fit this line using different theoretical line profiles to diagnose electron density, n_e, and n = 2 level population, n₂. Aided by synthetic tests, we characterize the validity of our diagnostic method for this experimental platform. During a single experiment, we infer a continuous range of electron densities increasing from n_e ~ 4 to ~30 × 10¹⁶ cm^-3 throughout a 120-ns evolution of our plasma. Also, we observe n₂ to be initially elevated with respect to local thermodynamic equilibrium (LTE); it then equilibrates within ~55 ns to become consistent with LTE. This also supports our electron-temperature determination of T_e ~ 1.3 eV (~15,000 K) after this time. At n_e≲ 10¹⁷ cm^-3, we find that computer-simulation-based line-profile calculations provide better fits (lower reduced χ²) than the line profiles currently used in the WD astronomy community. The inferred conditions, however, are in good quantitative agreement. Lastly, this work establishes an experimental foundation for the future investigation of relative shapes and strengths between different hydrogen Balmer lines.

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