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Hydrodynamic instabilities are a major obstacle in the quest to achieve ignition as they cause preexisting capsule defects to grow and ultimately quench the fusion burn in experiments at the National Ignition Facility. Unstable growth at the ablation front has been dramatically reduced in implosions with "high-foot" drives as measured using x-ray radiography of modulations at the most dangerous wavelengths (Legendre mode numbers of 30-90). These growth reductions have helped to improve the performance of layered DT implosions reported by O. A. Hurricane et al. [Nature (London) 506, 343 (2014)], when compared to previous "low-foot" experiments, demonstrating the value of stabilizing ablation-front growth and providing directions for future ignition designs.

First time-integrated neutron images of a deuterium gas filled capsule were obtained using arrival time gating with the Neutron Imaging System at the National Ignition Facility. Images exist from DT (deuterium and tritium mixture) filled capsules in several energy bands but only at the Omega laser had DD (pure deuterium) filled capsules been imaged. A composite image was derived from an assembly of multiple penumbral neutron images using an iterative Maximum Likelihood reconstruction technique. This was compared with a simulated image from a radiation-hydrodynamic calculation. The observed image size, and shape agree, as do the primary DD, secondary DT neutron yields, and the burn duration. However, the observed cross-sectional profiles, although smaller in half width, extend outside the calculated, suggesting that deuterium has mixed outward into the carbon ablator. The observed X-ray image size (61 μm) is larger than the observed neutron image (51 μm). The calculations also reflect this. X-ray brightness includes carbon as well as deuterium emission. A bright spot, “meteor,” in the X-ray image is seen to move in time-gated images, but is not evident in the neutron image. It does not appear to degrade the neutron yield. [ABSTRACT FROM AUTHOR]

We report on the first layered deuterium-tritium (DT) capsule implosions indirectly driven by a "highfoot" laser pulse that were fielded in depleted uranium hohlraums at the National Ignition Facility. Recently, high-foot implosions have demonstrated improved resistance to ablation-front Rayleigh-Taylor instability induced mixing of ablator material into the DT hot spot [Hurricane et a l, Nature (London) 506, 343 (2014)]. Uranium hohlraums provide a higher albedo and thus an increased drive equivalent to an additional 25 TW laser power at the peak of the drive compared to standard gold hohlraums leading to higher implosion velocity. Additionally, we observe an improved hot-spot shape closer to round which indicates enhanced drive from the waist. In contrast to findings in the National Ignition Campaign, now all of our highest performing experiments have been done in uranium hohlraums and achieved total yields approaching 1016 neutrons where more than 50% of the yield was due to additional heating of alpha particles stopping in the DT fuel. [ABSTRACT FROM AUTHOR]

This Letter reports on a series of high-adiabat implosions of cryogenic layered deuterium-tritium (DT) capsules indirectly driven by a "high-foot" laser drive pulse at the National Ignition Facility. High-foot implosions have high ablation velocities and large density gradient scale lengths and are more resistant to ablation-front Rayleigh-Taylor instability induced mixing of ablator material into the DT hot spot. Indeed, the observed hot spot mix in these implosions was low and the measured neutron yields were typically 50% (or higher) of the yields predicted by simulation. On one high performing shot (N130812), 1.7 MJ of laser energy at a peak power of 350 TW was used to obtain a peak hohlraum radiation temperature of ~300 eV. The resulting experimental neutron yield was (2.4±0.05)×1015 DT, the fuel ρR was (0.86±0.063) g/cm², and the measured Tion was (4.2±0.16) keV, corresponding to 8 kJ of fusion yield, with ~1/3 of the yield caused by self-heating of the fuel by a particles emitted in the initial reactions. The generalized Lawson criteria, an ignition metric, was 0.43 and the neutron yield was ~70% of the value predicted by simulations that include α-particle self-heating. [ABSTRACT FROM AUTHOR]

The first measurements of multiple, high-pressure shock waves in cryogenic deuterium-tritium (DT) ice layered capsule implosions on the National Ignition Facility have been performed. The strength and relative timing of these shocks must be adjusted to very high precision in order to keep the DT fuel entropy low and compressibility high. All previous measurements of shock timing in inertial confinement fusion implosions [T. R. Boehly et al., Phys. Rev. Lett. 106 195005 (2011), H. F. Robey et al., Phys. Rev. Lett. 108 215004 (2012)] have been performed in surrogate targets, where the solid DT ice shell and central DT gas regions were replaced with a continuous liquid deuterium (D2) fill. This report presents the first experimental validation of the assumptions underlying this surrogate technique. [ABSTRACT FROM AUTHOR]