In the quest for reaching ignition of deuterium–tritium (DT) fuel capsule implosions, experiments on the National Ignition Facility (NIF) have shown lower final fuel areal densities than simulated. Possible explanations for reduced compression are higher preheat due to energetic electrons or x rays generated through the laser interaction that deposit in the capsule shell (which can then increase the ablator–DT-ice density jump and induce mixing at that interface) or reverberating shocks. Our goal was to develop x-ray refraction enhanced radiography to infer the in-flight density profiles in layered fuel capsule implosions. The first experiments validated our setup by recording a streaked x-ray fringe pattern from an undriven high-density carbon capsule consistent with raytracing calculations at the required resolution of approximately 6 μm and 25 ps. Streaked refraction-enhanced radiography was then applied to in-flight, cryogenically layered, high-density carbon capsule implosions using a hydrogen–tritium fuel mix. The first refraction-enhanced radiography (RER) of an in-flight capsule revealed strong features associated with the ablation front and ice–ablator interface that are not visible in standard absorption in-flight radiographs.
Inertial confinement fusion (ICF) experiments on the NIF (Miller et al. 2004) use NIF's 192 laser beams (with up to 2 MJ energy) to heat the interior wall of a high-Z hohlraum (Lindl et al. 2004). A capsule placed inside of the hohlraum is filled with a DT fuel layer that is ablatively compressed by the soft x-ray flux. The fuel compressions are below that expected from simulations (Meezan et al. 2015). A likely reason for lower fuel compression is reduced density jump at the ablator–DT-ice interface due to hohlraum radiation preheat, which leads to Rayleigh Taylor instability growth and mixing at the interface.
The in-flight capsule ablator trajectory and thickness in NIF ICF implosions is usually characterized in convergent ablator (ConA) experiments (Hicks et al. 2012). ConA's use x-ray radiography created by area backlighter sources imaged by pinholes or slits. The ConA measurement is sensitive to capsule opacity (i.e., mainly to the high-density, low-temperature region of the ablator), ranging from the ice–ablator surface to the ablation front. By contrast, point or slit projection RER, has the potential to reveal in-flight density gradients at the ice–ablator interface and ablation front that are not visible or extractable by absorption ConA radiography. This is achieved by recording x-ray refraction features that appear in the presence of transverse electron-density gradients in the capsule in addition to x-ray absorption (Koch et al. 2009 and 2013, Ping et al. 2011).
In the radiography setup depicted in the figure below, an x-ray source (s) is positioned at a specific distance (p) away from a radial refractive index jump between two media of a certain radius (R), so the x rays cast a shadow for a specific distance (q). The incident radiation is both absorbed and refracted at the interface transverse to the x rays, toward the lower-density medium. Refraction can enhance the radiograph by bright and dark peaks (i.e., "fringes"), where detected flux can appear brighter or weaker than the transmitted source in the absence of refraction.
We conducted the first experimental demonstration of time-resolved RER on layered ICF indirect-drive implosions. We compared an RER data lineout to ray-tracing predictions from a pre-shot simulated density profile. Once we allowed for a temporal offset between data and simulations (since the measured capsule trajectory is slightly slower than in simulations), we observed good agreement between the measured and simulated RER ray trace. At this specific point early in the trajectory before acceleration, the simulated density was highest in the ablator due to shock compression. Furthermore, both the data and the simulated RER ray trace displayed fringes from the first shock front traversing the HT ice, the ice–ablator interface, the doped–undoped ablator interface, and finally from the ablation front, which was the most prominent feature due to a combination of highest density differential and long-gradient scale length.
Our research demonstrating the application of x-ray RER to low-Z dense hydrodynamic phenomena supports the NNSA and Lawrence Livermore National Laboratory goals of strengthening the science, technology, and engineering competencies in matters related to stockpile stewardship, as well as developing better models and experiments of radiation hydrodynamics. It also supports the Laboratory's objectives to create more opportunities for innovation and enhance the vitality of HED experimental science and maximize the utility of the NIF for exploring HED physics under more extreme conditions.
The RER technique will be made available to the wider ICF and HED science community. Examples of other possible uses include checking for unexpected shocks in opaque material and inferring preheat from the ratio of shock compression to shock velocity. For slower phenomena, RER could be extended to full two-dimensional imaging and use the full potential of single-line-of-sight (Theobald et al. 2018) gated x-ray detectors now available at the NIF. This would enable tracking the shape evolution of interfaces and shock fronts in-flight due to drive asymmetries and imposed perturbations.
We demonstrated the first use of x-ray in-flight RER measurements of layered ICF implosions with high-magnification slit projection streaked radiography. This required improving NIF radiographic capabilities to 5-μm, 20-ps resolution. The high magnification required careful target metrology. The recorded radiographs we obtained during this project contain a wealth of data not visible to traditional absorption radiography and are sensitive to greater than 1-μm density scale lengths and density differentials down to 0.5 g/cc.
Our results were sufficiently promising that we garnered more NIF shots for fiscal year 2019, which we will use to conduct experiments in mitigating hohlraum-related x-ray background, guided by one-dimensional code simulations. Our results have also led to proposals for developing a related technique, a time-resolved version of grid-image refractometry (Benattar and Godart 1984) that could extend sensitivity to shallower density jumps.
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——— . 2013. "Refraction-Enhanced Backlit Imaging of Axially Symmetric Inertial Confinement Fusion Plasmas." Applied Optics 52 (15): 3538–3556. doi: 10.1364/ao.52.003538.
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——— . 2018. "First Refraction Enhanced Radiography Experiments for Probing Inflight Density Profiles of ICF Capsule Implosions." 60th Annual Meeting of the APS Division of Plasma Physics, Portland, OR, 2018. LLNL-ABS-753326.
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