B. J. Kozioziemski

The experimental plan for cryogenic layered target implosions on the National Ignition Facility—The inertial confinement approach to fusion

Ignition requires precisely controlled, high convergence implosions to assemble a dense shell of deuterium-tritium (DT) fuel with ρR>∼1 g/cm2 surrounding a 10 keV hot spot with ρR ∼ 0.3 g/cm2. A working definition of ignition has been a yield of ∼1 MJ. At this yield the α-particle energy deposited in the fuel would have been ∼200 kJ, which is already ∼10 × more than the kinetic energy of a typical implosion. The National Ignition Campaign includes low yield implosions with dudded fuel layers to study and optimize the hydrodynamic assembly of the fuel in a diagnostics rich environment. The fuel is a mixture of tritium-hydrogen-deuterium (THD) with a density equivalent to DT. The fraction of D can be adjusted to control the neutron yield. Yields of ∼1014−15 14 MeV (primary) neutrons are adequate to diagnose the hot spot as well as the dense fuel properties via down scattering of the primary neutrons. X-ray imaging diagnostics can function in this low yield environment providing additional information about ...

The high-foot implosion campaign on the National Ignition Facilitya)

The “High-Foot” platform manipulates the laser pulse-shape coming from the National Ignition Facility laser to create an indirect drive 3-shock implosion that is significantly more robust against instability growth involving the ablator and also modestly reduces implosion convergence ratio. This strategy gives up on theoretical high-gain in an inertial confinement fusion implosion in order to obtain better control of the implosion and bring experimental performance in-line with calculated performance, yet keeps the absolute capsule performance relatively high. In this paper, we will cover the various experimental and theoretical motivations for the high-foot drive as well as cover the experimental results that have come out of the high-foot experimental campaign. At the time of this writing, the high-foot implosion has demonstrated record total deuterium-tritium yields (9.3×1015) with low levels of inferred mix, excellent agreement with implosion simulations, fuel energy gains exceeding unity, and evidenc...

A high-resolution integrated model of the National Ignition Campaign cryogenic layered experiments

A detailed simulation-based model of the June 2011 National Ignition Campaign cryogenic DT experiments is presented. The model is based on integrated hohlraum-capsule simulations that utilize the best available models for the hohlraum wall, ablator, and DT equations of state and opacities. The calculated radiation drive was adjusted by changing the input laser power to match the experimentally measured shock speeds, shock merger times, peak implosion velocity, and bangtime. The crossbeam energy transfer model was tuned to match the measured time-dependent symmetry. Mid-mode mix was included by directly modeling the ablator and ice surface perturbations up to mode 60. Simulated experimental values were extracted from the simulation and compared against the experiment. Although by design the model is able to reproduce the 1D in-flight implosion parameters and low-mode asymmetries, it is not able to accurately predict the measured and inferred stagnation properties and levels of mix. In particular, the measu...

Diamond spheres for inertial confinement fusion

The National Ignition Facility (NIF) will allow scientists to prove the feasibility of inertial confinement fusion (ICF). The success of ICF experiments at NIF will critically depend on the availability of robust targets. Guided by computer simulations, we generated a new target design that takes advantage of the extreme atomic density of synthetic diamond, and developed a process that allows us to produce large quantities of these ultrahigh precision diamond targets via a low-cost batch process. Computer simulations were used to assess the performance and the robustness of these diamond targets. The results demonstrate that diamond has the potential to outperform other target materials in terms of energy efficiency and implosion stability, thus making successful ignition more likely.

Quantitative characterization of inertial confinement fusion capsules using phase contrast enhanced x-ray imaging

Current designs for inertial confinement fusion capsules for the National Ignition Facility consist of a solid deuterium–tritium (D–T) fuel layer inside of a copper doped beryllium, Be(Cu), shell. Phase contrast enhanced x-ray imaging is shown to render the D–T layer visible inside the Be(Cu) shell. Phase contrast imaging is experimentally demonstrated for several surrogate capsules and validates computational models. Polyimide and low density divinyl benzene foam shells were imaged at the Advanced Photon Source synchrotron. The surrogates demonstrate that phase contrast enhanced imaging provides a method to characterize surfaces when absorption imaging cannot be used. Our computational models demonstrate that a rough surface can be accurately characterized using phase contrast enhanced x-ray images.

National Ignition Facility target design and fabrication

The current capsule target design for the first ignition experiments at the NIF Facility beginning in 2009 will be a copper-doped beryllium capsule, roughly 2 mm in diameter with 160-{micro}m walls. The capsule will have a 75-{micro}m layer of solid DT on the inside surface, and the capsule will driven with x-rays generated from a gold/uranium cocktail hohlraum. The design specifications are extremely rigorous, particularly with respect to interfaces, which must be very smooth to inhibit Rayleigh-Taylor instability growth. This paper outlines the current design, and focuses on the challenges and advances in capsule fabrication and characterization; hohlraum fabrication, and D-T layering and characterization.

Symmetry control of an indirectly driven high-density-carbon implosion at high convergence and high velocity

We report on the most recent and successful effort at controlling the trajectory and symmetry of a high density carbon implosion at the National Ignition Facility. We use a low gasfill (0.3 mg/cc He) bare depleted uranium hohlraum with around 1 MJ of laser energy to drive a 3-shock-ignition relevant implosion. We assess drive performance and we demonstrate symmetry control at convergence 1, 3–5, 12, and 27 to better than ±5 μm using a succession of experimental platforms. The symmetry control was maintained at a peak fuel velocity of 380 km/s. Overall, implosion symmetry measurements are consistent with the pole-equator symmetry of the X-ray drive on the capsule being better than 5% in the foot of the drive (when shocks are launched) and better than 1% during peak drive (main acceleration phase). This level of residual asymmetry should have little impact on implosion performance.

Cryogenic tritium-hydrogen-deuterium and deuterium-tritium layer implosions with high density carbon ablators in near-vacuum hohlraums

High Density Carbon (or diamond) is a promising ablator material for use in near-vacuum hohlraums, as its high density allows for ignition designs with laser pulse durations of <10 ns. A series of Inertial Confinement Fusion (ICF) experiments in 2013 on the National Ignition Facility [Moses et al., Phys. Plasmas 16, 041006 (2009)] culminated in a deuterium-tritium (DT) layered implosion driven by a 6.8 ns, 2-shock laser pulse. This paper describes these experiments and comparisons with ICF design code simulations. Backlit radiography of a tritium-hydrogen-deuterium (THD) layered capsule demonstrated an ablator implosion velocity of 385 km/s with a slightly oblate hot spot shape. Other diagnostics suggested an asymmetric compressed fuel layer. A streak camera-based hot spot self-emission diagnostic (SPIDER) showed a double-peaked history of the capsule self-emission. Simulations suggest that this is a signature of low quality hot spot formation. Changes to the laser pulse and pointing for a subsequent DT i...

Solid deuterium-tritium surface roughness in a beryllium inertial confinement fusion shell

Solid deuterium-tritium (D-T) fuel layers for inertial confinement fusion experiments were formed inside of a 2 mm diameter beryllium shell and were characterized using phase-contrast enhanced x-ray imaging. The solid D-T surface roughness is found to be 0.4 {micro}m for modes 7-128 at 1.5 K below the melting temperature. The layer roughness is found to increase with decreasing temperature, in agreement with previous visible light characterization studies. However, phase-contrast enhanced x-ray imaging provides a more robust surface roughness measurement than visible light methods. The new x-ray imaging results demonstrate clearly that the surface roughness decreases with time for solid D-T layers held at 1.5 K below the melting temperature.

X-Ray Imaging Of Cryogenic Deuterium-Tritium Layers In A Beryllium Shell

Solid deuterium-tritium (D-T) fuel layers inside copper-doped beryllium shells are robust inertial confinement fusion fuel pellets. This paper describes the first characterization of such layers using phase-contrast x-ray imaging. Good agreement is found between calculation and experimental contrast at the layer interfaces. Uniform solid D-T layers and their response to thermal asymmetries were measured in the Be(Cu) shell. The solid D-T redistribution time constant was measured to be 28 min in the Be(Cu) shell.