R. Cook

Point design targets, specifications, and requirements for the 2010 ignition campaign on the National Ignition Facility

Point design targets have been specified for the initial ignition campaign on the National Ignition Facility [G. H. Miller, E. I. Moses, and C. R. Wuest, Opt. Eng. 443, 2841 (2004)]. The targets contain D-T fusion fuel in an ablator of either CH with Ge doping, or Be with Cu. These shells are imploded in a U or Au hohlraum with a peak radiation temperature set between 270 and 300 eV. Considerations determining the point design include laser-plasma interactions, hydrodynamic instabilities, laser operations, and target fabrication. Simulations were used to evaluate choices, and to define requirements and specifications. Simulation techniques and their experimental validation are summarized. Simulations were used to estimate the sensitivity of target performance to uncertainties and variations in experimental conditions. A formalism is described that evaluates margin for ignition, summarized in a parameter the Ignition Threshold Factor (ITF). Uncertainty and shot-to-shot variability in ITF are evaluated, and...

Review of indirect-drive ignition design options for the National Ignition Facility

Several inertial confinement fusion (ICF) capsule designs have been proposed as possible candidates for achieving ignition by indirect drive on the National Ignition Facility (NIF) laser [Paisner et al., Laser Focus World 30, 75 (1994)]. This article reviews these designs, their predicted performance using one-, two-, and three-dimensional numerical simulations, and their fabricability. Recent design work at a peak x-ray drive temperature of 250 eV with either 900 or 1300 kJ total laser energy confirms earlier capsule performance estimates [Lindl, Phys. Plasmas 2, 3933 (1995)] that were based on hydrodynamic stability arguments. These simulations at 250 eV and others at the nominal 300 eV drive show that capsules having either copper doped beryllium (Be+Cu) or polyimide (C22H10N2O4) ablators have favorable implosion stability and material fabrication properties. Prototypes of capsules using these ablator materials are being constructed using several techniques: brazing together machined hemishells (Be+Cu)...

Time-resolved x-ray imaging of high-power laser-irradiated underdense silica aerogels and agar foams

This article presents the results of experiments in which a high‐power laser was used to irradiate low density (4–9 mg/cm3) silica aerogel and agar foam targets. The laser–solid interaction and energy transport through the material were monitored with time‐resolved imaging diagnostics, and the data show the production and propagation of an x‐ray emission front in the plasma. The emission‐front trajectory data are found to be in significant disagreement with detailed simulations, which predict a much more rapid heating of the cold material, and the data suggest that this discrepancy is not explainable by target inhomogeneities. Evidence suggests that energy transport into the cold material may be dominated by thermal conduction; however, no completely satisfactory explanation for the discrepancies is identified, and further experimental and theoretical research is necessary in order to resolve this important problem in laser–plasma interaction physics.

Sputter-deposited Be ablators for NIF target capsules

We have performed a series of preliminary experiments to determine whether sputter deposition of doped Be is a practical route to producing NIF target capsules with Be ablators. Films ranging in thickness from 7 to {approximately} 120 {micro}m have been deposited on spherical polymer mandrels using a bounce pan to ensure uniform coating. With no voltage bias applied to the pan, relatively porous coatings were formed that were highly permeable to hydrogen. The surface finish of these films ranged from {approximately}250 nm rms for 13-{micro}m-thick films to a minimum of {approximately}75 nm rms for an 80-{micro}m-thick film. Application of a voltage bias was found to significantly modify the film morphology. At a bias of 120 V, 7-{micro}m-thick films with a dense, fine-grained microstructure were produced. These capsules had a reflective surface with a 50 nm rms roughness. Finally, to demonstrate the ability to produce a graded dopant profile, a coating was produced in which the concentration of added Cu was varied from 2.5 atom % at the beginning to zero after 40 {micro}m of deposition.

X-RAY SPECTROSCOPIC DIAGNOSTICS OF MIX IN HIGH GROWTH FACTOR SPHERICAL IMPLOSIONS

Abstract Rayleigh-Taylor (RT) instability of the pusher-fuel interface occurring upon acceleration and deceleration of the pusher is of major concern for current and future ICF experiments. One common diagnostic technique for measuring pusher-fuel mix in spherical implosion experiments involves placing spectroscopic dopants both in the capsule fuel region and the innermost region of the capsule wall adjacent to the fuel. As the degree of pusher-fuel mix is increased the pusher dopant x-ray emission increases relative to that of the fuel dopant. Spherical implosion experiments of this type using Ar and Ti dopants in the fuel and pusher, respectively, are being carried out on Nova. We first show that the Ti He-α Ar He-β line ratio shows promise as a mix diagnostic for high growth factor targets. We then discuss some of the important physical processes underlying Ar and Ti spectral line formation in these targets and discuss how these processes affect the calculation of simulated spectra. The importance of radiative transfer as well as high-density plasma phenomena such as continuum lowering and Stark broadening is demonstrated. The simulated spectra are also observed to be sensitive to assumptions regarding the treatment of electron thermal conduction in the mix region. Spectral postprocessing of 2-D hydrodynamic simulations using detailed line transfer methods has been carried out and implies that simple escape factor treatments must be tested carefully before they can be relied upon. Preliminary comparisons of experimental data with simulation are presented. It is shown that the computed spectra is sensitive to the laser energy and pusher temperature. These comparisons to data also imply that the inclusion of convective effects in computing the electron temperature profile through the mix region is necessary in order to satisfactorily model experimental spectra.

INDIRECTLY DRIVEN, HIGH GROWTH RAYLEIGH-TAYLOR IMPLOSIONS ON NOVA

Abstract Indirectly-driven implosions for which the predicted Rayleigh-Taylor (RT) instability growth rates of pre-imposed capsule surface perturbations are substantially increased by mid-Z-doping of the ablators have been fielded on the Nova laser. The multiple effects on implosion performance of the additional x-ray opacity provided by the ablator dopant is discussed. For best surface finish capsules, the addition of increasing ablator dopant levels is shown to improve the neutron yield. However, as capsule surface roughness is increased, so that RT instability growth increases, this trend is reversed, leading to decreasing yields with increased dopant content. The RT-induced mixing between shell and fuel is further investigated by diagnosing the x-ray emission levels and time histories from Ti and Ar dopants in capsules with predetermined surface roughness. The x-ray line ratios show the expected decrease in fuel temperature with increasing surface roughness. The spectral content, intensity and duration of the Ti spectra, however, suggest 2- or 3-D rather than just 1-D effects are important so that higher than 1-D models of the mix region may be needed.

Diagnosis of pusher‐fuel mix in spherical implosions using x‐ray spectroscopy (invited)

Of primary concern in next generation inertial confinement fusion (ICF) implosion experiments is Rayleigh–Taylor (RT) instability of the pusher‐fuel interface occurring upon acceleration and deceleration of the pusher. This results in mixing of hot fuel with cold pusher material. One method of diagnosing mix in this case is to place spectroscopic dopants both in the capsule fuel region and the innermost region of the capsule wall adjacent to the fuel. As the degree of pusher/fuel mix is increased (typically through placement of controlled perturbations on the outer surface of the capsule) the pusher dopant x‐ray emission increases relative to that of the fuel dopant. Experiments of this type using indirectly driven implosions have been carried out on Nova. In this paper we describe some of the important physics issues underlying spectral line formation in these targets and discuss how they are manifested in the modeling and interpretation of experimental data. The importance of radiative transfer as well ...

Effects of variable x‐ray preheat shielding in indirectly driven implosions

The performance of indirectly driven fusion capsules has been improved by mid Z doping of the plastic capsule ablator. The doping increases x‐ray preheat shielding leading to a more isentropic compression, higher convergence, and higher neutron yield. A 4× increase in neutron yield is both calculated and observed as the Ge doping level is increased from 0% to 3% by atomic fraction. A predicted 40% decrease in x‐ray image core size with increasing Ge content is confirmed.

Experimental Study of Fill-Tube Hydrodynamical Effects on Implosions

Summary form only given. Planned cryogenic ignition experiments at the National Ignition Facility (NIF) are expected to use a fill tube to introduce liquid DT into the capsule prior to solid layer formation. During the implosion, this fill tube is expected to form a hydrodynamic jet at deceleration that could quench ignition if this jet is sufficiently large and penetrates the core early enough. We have begun the first indirect-drive experiments to explore the hydrodynamic effects of fill tubes on implosion performance. In these experiments, we have concentrated on developing diagnostic techniques by replacing the fill tube with a bump on the outer shell surface. This bump is hydrodynamically similar to a fill tube but much easier to fabricate and simulate. We present experimental data and calculations for various bump diameters and heights obtained from two techniques: emission imaging of Ti-doped shell material that has been swept into the core at times near peak compression, and backlit imaging of the bump at early times

Diagnosis of pusher‐fuel mixing for high growth‐factor implosions (abstract)a)

The degradation in implosion performance of surface‐pitted, indirectly driven Br‐doped capsules which have been intentionally designed to be susceptible to large hydrodynamic instability growth has been systematically and reproducibly measured by a wide array of x‐ray and neutron diagnostics. Primary and secondary neutron yields, and x‐ray line ratios and durations from Ar and Ti fuel and pusher dopants are presented, showing clear sensitivity to increasing surface roughness and by inference, increasing pusher‐fuel mix. The sensitivity of core x‐ray emissivity and neutron bang‐times to micron variations in capsule ablator thickness is also demonstrated. The experimental trends are compared to expectations based on two different models for the effects of instability growth at the pusher‐fuel interface (atomic mixing versus Rayleigh–Taylor bubble‐and‐spike growth).