Robert Tipton

A comparison of three-dimensional multimode hydrodynamic instability growth on various National Ignition Facility capsule designs with HYDRA simulations

Three similar cryogenic ignition capsule designs for the National Ignition Facility [J. Lindl, Phys. Plasmas 2, 3933 (1995)] are analyzed to determine surface roughness specifications required to mitigate the growth of hydrodynamic instabilities. These capsule utilize brominated plastic, polyimid and copper-doped beryllium ablator materials respectively. Direct three-dimensional numerical simulations with the HYDRA radiation hydrodynamic code [M. M. Marinak et al., Phys. Plasmas 3, 2070 (1996)] examine the growth of multimode perturbations seeded by roughness on the outer ablator and inner ice surfaces. The simulations, which showed weakly nonlinear behavior for optimized surfaces, were carried through ignition and burn. A three-dimensional multimode perturbation achieves somewhat larger amplitudes in the nonlinear regime than a corresponding two-dimensional simulation of the same rms amplitude. The beryllium and polyimid capsules exhibit enhanced tolerance of roughness on both the ice and ablator surfaces.

Indirect-Drive Noncryogenic Double-Shell Ignition Targets for the National Ignition Facility: Design and Analysis

The central goal of the National Ignition Facility (NIF) is demonstration of controlled thermonuclear ignition. The mainline ignition target is a low-Z, single-shell cryogenic capsule designed to have weakly nonlinear Rayleigh-Taylor growth of surface perturbations. Double-shell targets are an alternative design concept that avoids the complexity of cryogenic preparation but has greater physics uncertainties associated with performance-degrading mix. A typical double-shell design involves a high-Z inner capsule filled with DT gas and supported within a low-Z ablator shell. The largest source of uncertainty for this target is the degree of highly evolved nonlinear mix on the inner surface of the high-Z shell. High Atwood numbers and feed-through of strong outer surface perturbation growth to the inner surface promote high levels of instability. The main challenge of the double-shell target designs is controlling the resulting nonlinear mix to levels that allow ignition to occur. Design and analysis of a suite of indirect-drive NIF double-shell targets with hohlraum temperatures of 200 eV and 250 eV are presented. Analysis of these targets includes assessment of two-dimensional radiation asymmetry as well as nonlinear mix. Two-dimensional integrated hohlraum simulations indicate that the x-ray illumination can be adjusted to provide adequate symmetry control in hohlraums specially designed to have high laser-coupling efficiency [Suter et al., Phys. Plasmas 5, 2092 (2000)]. These simulations also reveal the need to diagnose and control localized 10-15 keV x-ray emission from the high-Z hohlraum wall because of strong absorption by the high-Z inner shell. Preliminary estimates of the degree of laser backscatter from an assortment of laser-plasma interactions suggest comparatively benign hohlraum conditions. Application of a variety of nonlinear mix models and phenomenological tools, including buoyancy-drag models, multimode simulations and fall-line optimization, indicates a possibility of achieving ignition, i.e., fusion yields greater than 1 MJ. Planned experiments on the Omega laser to test current understanding of high-energy radiation flux asymmetry and mix-induced yield degradation in double-shell targets are described.

K-L turbulence model for the self-similar growth of the Rayleigh-Taylor and Richtmyer-Meshkov instabilities

A turbulence model is developed to described the self-similar growth of the Rayleigh-Taylor (RT) and Richtmyer-Meshkov (RM) instabilities. The model describes the dominant eddies in the mixing zone with evolutionary equations for their characteristic dimension L and energy per unit mass K≡V2∕2. The equations are based on the successful buoyancy-drag models for RT and RM flows, but constructed only with local parameters so that it can be applied to multidimensional flows with multiple shells of materials. The model has several unknown coefficients that are determined by comparing analytical and numerical solutions with RT and RM experiments.

Hydrodynamic instability growth and mix experiments at the National Ignition Facilitya)

Hydrodynamic instability growth and its effects on implosion performance were studied at the National Ignition Facility [G. H. Miller, E. I. Moses, and C. R. Wuest, Opt. Eng. 443, 2841 (2004)]. Implosion performance and mix have been measured at peak compression using plastic shells filled with tritium gas and containing embedded localized carbon-deuterium diagnostic layers in various locations in the ablator. Neutron yield and ion temperature of the deuterium-tritium fusion reactions were used as a measure of shell-gas mix, while neutron yield of the tritium-tritium fusion reaction was used as a measure of implosion performance. The results have indicated that the low-mode hydrodynamic instabilities due to surface roughness were the primary culprits for yield degradation, with atomic ablator-gas mix playing a secondary role. In addition, spherical shells with pre-imposed 2D modulations were used to measure instability growth in the acceleration phase of the implosions. The capsules were imploded using ig...

Development of the CD Symcap platform to study gas-shell mix in implosions at the National Ignition Facility

Surrogate implosions play an important role at the National Ignition Facility (NIF) for isolating aspects of the complex physical processes associated with fully integrated ignition experiments. The newly developed CD Symcap platform has been designed to study gas-shell mix in indirectly driven, pure T2-gas filled CH-shell implosions equipped with 4 μm thick CD layers. This configuration provides a direct nuclear signature of mix as the DT yield (above a characterized D contamination background) is produced by D from the CD layer in the shell, mixing into the T-gas core. The CD layer can be placed at different locations within the CH shell to probe the depth and extent of mix. CD layers placed flush with the gas-shell interface and recessed up to 8 μm have shown that most of the mix occurs at the inner-shell surface. In addition, time-gated x-ray images of the hotspot show large brightly radiating objects traversing through the hotspot around bang-time, which are likely chunks of CH/CD plastic. This platf...

Measured dependence of nuclear burn region size on implosion parameters in inertial confinement fusion experiments

Radial profiles of nuclear burn in directly driven, inertial-confinement-fusion implosions have been systematically studied for the first time using a proton emission imaging system sensitive to energetic 14.7MeV protons from the fusion of deuterium (D) and 3-helium (He3) at the OMEGA laser facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)]. Experimental parameters that were varied include capsule size, shell composition and thickness, gas fill pressure, and laser energy. Clear relationships have been identified between changes in a number of these parameters and changes in the size of the burn region, which we characterize here by the median “burn radius” Rburn containing half of the total DHe3 reactions. Different laser and capsule parameters resulted in burn radii varying from 20to80μm. For example, reducing the DHe3 fill pressure from 18to3.6atm in capsules with 20μm thick CH shells resulted in Rburn changing from 31to25μm; this reduction is attributed to increased fuel-shell mix for the mor...

Symmetry tuning of a near one-dimensional 2-shock platform for code validation at the National Ignition Facility

We introduce a new quasi 1-D implosion experimental platform at the National Ignition Facility designed to validate physics models as well as to study various Inertial Confinement Fusion aspects such as implosion symmetry, convergence, hydrodynamic instabilities, and shock timing. The platform has been developed to maintain shell sphericity throughout the compression phase and produce a round hot core at stagnation. This platform utilizes a 2-shock 1 MJ pulse with 340 TW peak power in a near-vacuum Au Hohlraum and a CH ablator capsule uniformly doped with 1% Si. We have performed several inflight radiography, symmetry capsule, and shock timing experiments in order to tune the symmetry of the capsule to near round throughout several epochs of the implosion. Adjusting the relative powers of the inner and outer cones of beams has allowed us to control the drive at the poles and equator of the capsule, thus providing the mechanism to achieve a spherical capsule convergence. Details and results of the tuning e...

Simulations of indirectly driven gas-filled capsules at the National Ignition Facility

Gas-filled capsules imploded with indirect drive on the National Ignition Facility have been employed as symmetry surrogates for cryogenic-layered ignition capsules and to explore interfacial mix. Plastic capsules containing deuterated layers and filled with tritium gas provide a direct measure of mix of ablator into the gas fuel. Other plastic capsules have employed DT or D3He gas fill. We present the results of two-dimensional simulations of gas-filled capsule implosions with known degradation sources represented as in modeling of inertial confinement fusion ignition designs; these are time-dependent drive asymmetry, the capsule support tent, roughness at material interfaces, and prescribed gas-ablator interface mix. Unlike the case of cryogenic-layered implosions, many observables of gas-filled implosions are in reasonable agreement with predictions of these simulations. Yields of TT and DT neutrons as well as other x-ray and nuclear diagnostics are matched for CD-layered implosions. Yields of DT-fille...

Development of an inertial confinement fusion platform to study charged-particle-producing nuclear reactions relevant to nuclear astrophysics

This paper describes the development of a platform to study astrophysically relevant nuclear reactions using inertial-confinement fusion implosions on the OMEGA and National Ignition Facility laser facilities, with a particular focus on optimizing the implosions to study charged-particle-producing reactions. Primary requirements on the platform are high yield, for high statistics in the fusion product measurements, combined with low areal density, to allow the charged fusion products to escape. This is optimally achieved with direct-drive exploding pusher implosions using thin-glass-shell capsules. Mitigation strategies to eliminate a possible target sheath potential which would accelerate the emitted ions are discussed. The potential impact of kinetic effects on the implosions is also considered. The platform is initially employed to study the complementary T(t,2n)α, T(3He,np)α and 3He(3He,2p)α reactions. Proof-of-principle results from the first experiments demonstrating the ability to accurately measur...

Hydrodynamic growth and mix experiments at National Ignition Facility

Hydrodynamic growth and its effects on implosion performance and mix were studied at the National Ignition Facility (NIF). Spherical shells with pre-imposed 2D modulations were used to measure Rayleigh-Taylor (RT) instability growth in the acceleration phase of implosions using in-flight x-ray radiography. In addition, implosion performance and mix have been studied at peak compression using plastic shells filled with tritium gas and imbedding localized CD diagnostic layer in various locations in the ablator. Neutron yield and ion temperature of the DT fusion reactions were used as a measure of shell-gas mix, while neutron yield of the TT fusion reaction was used as a measure of implosion performance. The results have indicated that the low-mode hydrodynamic instabilities due to surface roughness were the primary culprits to yield degradation, with atomic ablator-gas mix playing a secondary role.