D. R. Farley

X-ray backlighting for the National Ignition Facility (invited)

X-ray backlighting is a powerful tool for diagnosing a large variety of high-energy-density phenomena. Traditional area backlighting techniques used at Nova and Omega cannot be extended efficiently to NIF-scale. New, more efficient backlighting sources and techniques are required and have begun to show promising results. These include a backlit-pinhole point projection technique, pinhole and slit arrays, distributed polychromatic sources, and picket fence backlighters. In parallel, there have been developments in improving the data SNR and hence quality by switching from film to CCD-based recording media and by removing the fixed-pattern noise of MCP-based cameras.

Implosion dynamics measurements at the National Ignition Facility

Measurements have been made of the in-flight dynamics of imploding capsules indirectly driven by laser energies of 1–1.7 MJ at the National Ignition Facility [Miller et al., Nucl. Fusion 44, 228 (2004)]. These experiments were part of the National Ignition Campaign [Landen et al., Phys. Plasmas 18, 051002 (2011)] to iteratively optimize the inputs required to achieve thermonuclear ignition in the laboratory. Using gated or streaked hard x-ray radiography, a suite of ablator performance parameters, including the time-resolved radius, velocity, mass, and thickness, have been determined throughout the acceleration history of surrogate gas-filled implosions. These measurements have been used to establish a dynamically consistent model of the ablative drive history and shell compressibility throughout the implosion trajectory. First results showed that the peak velocity of the original 1.3-MJ Ge-doped polymer (CH) point design using Au hohlraums reached only 75% of the required ignition velocity. Several capsu...

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...

Hot-spot mix in ignition-scale implosions on the NIF

Ignition of an inertial confinement fusion (ICF) target depends on the formation of a central hot spot with sufficient temperature and areal density. Radiative and conductive losses from the hot spot can be enhanced by hydrodynamic instabilities. The concentric spherical layers of current National Ignition Facility (NIF) ignition targets consist of a plastic ablator surrounding a thin shell of cryogenic thermonuclear fuel (i.e., hydrogen isotopes), with fuel vapor filling the interior volume [S. W. Haan et al., Phys. Plasmas 18, 051001 (2011)]. The Rev. 5 ablator is doped with Ge to minimize preheat of the ablator closest to the DT ice caused by Au M-band emission from the hohlraum x-ray drive [D. S. Clark et al., Phys. Plasmas 17, 052703 (2010)]. Richtmyer–Meshkov and Rayleigh–Taylor hydrodynamic instabilities seeded by high-mode (50<l<200) ablator-surface perturbations can cause Ge-doped ablator to mix into the interior of the shell at the end of the acceleration phase [B. A. Hammel et al., Phys. Plasma...

Progress in the indirect-drive National Ignition Campaign

We have carried out precision optimization of inertial confinement fusion ignition scale implosions. We have achieved hohlraum temperatures in excess of the 300 eV ignition goal with hot-spot symmetry and shock timing near ignition specs. Using slower rise pulses to peak power and extended pulses resulted in lower hot-spot adiabat and higher main fuel areal density at about 80% of the ignition goal. Yields are within a factor of 5–6 of that required to initiate alpha dominated burn. It is likely we will require thicker shells (+15–20%) to reach ignition velocity without mixing of ablator material into the hot spot.

Testing astrophysical radiation hydrodynamics codes with hypervelocity jet experiments on the nova laser

Recent shock-tube experiments using the Nova laser facility have demonstrated that strong shocks and highly supersonic flows similar to those encountered in astrophysical jets can be studied in detail through carefully controlled experiments. We propose the use of high-power lasers such as Nova, Omega, Gekko, and the National Ignition Facility (NIF) to perform experiments on radiation hydrodynamic problems such as jets involving the multidimensional dynamics of strong shocks. High-power lasers are the only experimental facilities that can reach the very high Mach number regime. The experiments will serve as diagnostics of astrophysically interesting gasdynamic problems and could also form the basis of test problems for numerical algorithms for astrophysical radiation hydrodynamic codes. The potential for experimentally achieving a strongly radiative jet seems very good.

Progress toward ignition at the National Ignition Facility

Progress toward ignition at the National Ignition Facility (NIF) has been focused on furthering the understanding of implosion performance. Implosion performance depends on the capsule fuel shape, on higher mode asymmetries that may cause hydrodynamic instabilities to quench ignition, on time-dependent asymmetries introduced by the hohlraum target, and on ablator performance. Significant findings in each of these four areas is reported. These investigations have led to improved in-flight capsule shape, a demonstration that a capsule robust to mix can generate high levels of neutrons (7.7 × 10 14 ), hohlraum modifications that should ultimately provide improved beam propagation and better laser coupling, and fielding of capsules with high-density carbon (HDC) ablators. A capsule just fielded with a HDC ablator and filled with DT gas generated a preliminary record level of neutrons at 1.6 × 10 15 , or 5kJ of energy. Future plans include further improvements to fuel shape and hohlraum performance, fielding robust capsules at higher laser power and energy, and tuning the HDC capsule. A capsule with a nanocrystalline diamond (HDC) ablator on a DT ice layer will be fielded at NIF later this year.

High Mach number mix instability experiments of an unstable density interface using a single-mode, nonlinear initial perturbation

The growth of an unstable density interface from a nonlinear, single-mode, two-dimensional initial perturbation (10 μm amplitude; 23 μm wavelength) has been studied experimentally using a miniature shock tube attached to a gold hohlraum irradiated by the Nova laser [J. T. Hunt and D. R. Speck, Opt. Eng. 28, 461 (1989)]. The initial perturbation was machined into a brominated plastic ablator (1.22 g/cm3) adjacent to a low density carbon foam (0.10 g/cm3). Upon laser illumination of the hohlraum and x-ray ablation of the plastic, a strong shock wave (Mach∼30) propagated across the perturbed density interface causing the onset of the Richtmyer–Meshkov (RM) instability. The interface subsequently experienced a relatively weak Rayleigh–Taylor (RT) unstable deceleration. The nonlinear growth of the mixing layer was obtained from time-resolved radiography of the x-ray transmission through the shock tube, and the decompression-corrected results were compared to published incompressible models, including a Lagrang...

Single-Mode, Nonlinear Mix Experiments at High Mach Number using Nova

Nonlinear growth of an unstable density interface from single-mode initial perturbations at high Mach number was studied using the Nova laser. A variety of initially nonlinear perturbations were used, having amplitude-to-wavelength ratios of a0/λ = 0.11, 0.22, and 0.43. The measured mix width data from these experiments were nondimensionalized and exhibit self-similar growth.

Diagnosing implosions at the national ignition facility with X-ray spectroscopy

X-ray spectroscopy is used at the National Ignition Facility (NIF) to diagnose plasma conditions in the hot spot and the compressed shell of ignition-scale inertial confinement fusion (ICF) implosions. Ignition of an ICF target depends on the formation of a central hot spot with sufficient temperature and areal density. The concentric spherical layers of current NIF ignition targets consist of a plastic ablator surrounding a thin shell of cryogenic thermonuclear fuel (i.e., hydrogen isotopes), with fuel vapor filling the interior volume. A fraction of the ablator has a Ge dopant to minimize preheat of the ablator closest to the DT ice caused by Au M-band emission from the hohlraum x-ray drive. This paper concentrates on three spectral features of the implosion: Ge Heα emission, Ge Kα emission, and the Ge K edge. Hydrodynamic instabilities seeded by highmode (50 < l < 200) ablator-surface perturbations on ignition-scale targets can cause mixing of Ge-doped ablator into the interior of the shell at the end ...