Resistive diffusion in magnetized ICF implosions: Reduced magnetic stabilization of the Richtmyer–Meshkov instability
Not Available
Walsh, C. A. (ORCID:0000000266393543)↗
SEARCH · Search NASA
Results for “IMPLOSIONS”
Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
Three-dimensional reconstruction of implosion stagnation in laser direct drive on OMEGA
Multidimensional effects on hot-spot formation must be considered to better understand the current limits on the performance of direct-drive inertial confinement fusion experiments on OMEGA with cryogenically layered solid deuterium–tritium targets. A comprehensive reconstruction effort has been established at the Laboratory for Laser Energetics to infer hot-spot and shell conditions at stagnation from a large collection of x-ray, neutron, and particle detectors along multiple lines of sight. Several time-gated and time-integrated x-ray imagers are being used to record the shape of the hot-spot plasma. A 3D hot-spot x-ray emission tomography technique has been developed to infer low-mode drive asymmetries from the hot-spot shape. A suite of neutron diagnostics is used to provide measurements of hot-spot flow velocity, ion temperature, and areal density. Here, the information obtained from the x-ray and neutron detectors will be combined into a coherent model of the shape of the hot spot and shell assembly.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Measurements of the ablation-front trajectory and low-mode nonuniformity in direct-drive implosions using x-ray self-emission shadowgraphy
Self-emission x-ray shadowgraphy provides a method to measure the ablation-front trajectory and low-mode nonuniformity of a target imploded by directly illuminating a fusion capsule with laser beams. The technique uses time-resolved images of soft x-rays (> 1 keV) emitted from the coronal plasma of the target imaged onto an x-ray framing camera to determine the position of the ablation front. Methods used to accurately measure the ablation-front radius ( ${\it\delta}R=\pm 1.15~{\rm\mu}\text{m}$ ), image-to-image timing ( ${\it\delta}({\rm\Delta}t)=\pm 2.5$ ps) and absolute timing ( ${\it\delta}t=\pm 10$ ps) are presented. Angular averaging of the images provides an average radius measurement of ${\it\delta}(R_{\text{av}})=\pm 0.15~{\rm\mu}\text{m}$ and an error in velocity of ${\it\delta}V/V=\pm 3\%$ . This technique was applied on the Omega Laser Facility and the National Ignition Facility.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Demonstration of hot-spot fuel gain exceeding unity in direct-drive inertial confinement fusion implosions
Irradiating a small capsule containing deuterium and tritium fuel directly with intense laser light causes it to implode, which creates a plasma hot enough to initiate fusion reactions between the fuel nuclei. Here we report on such laser direct-drive experiments and observe that the fusion reactions produce more energy than the amount of energy in the central so-called hot-spot plasma. This condition is identified as having a hot-spot fuel gain greater than unity. A hot-spot fuel gain of around four was previously accomplished at the National Ignition Facility in indirect-drive inertial confinement fusion experiments where the capsule is irradiated by X-rays. In that case, up to 1.9 MJ of laser energy was used, but in contrast, our experiments on the OMEGA laser system require as little as 28 kJ. As the hot-spot fuel gain is predicted to grow with laser energy and target size, our work establishes the direct-drive approach to inertial fusion as a promising path towards burning and ignited plasmas in the laboratory. Additionally, we report a record (direct-drive) fusion yield of 0.9 kJ on OMEGA, which we achieved with thin-ice deuterium–tritium liner targets.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Nanowire implosion under laser amplified spontaneous emission pedestal irradiation
Nanowire array targets exhibit high optical absorption when interacting with short, intense laser pulses. This leads to an increased yield in the production of accelerated particles for a variety of applications. However, these interactions are sensitive to the laser prepulse and could be significantly affected. Here, we show that an array of aligned nanowires is imploded when irradiated by an Amplified Spontaneous Emission pedestal of a 1 PW laser with an intensity on the order of 10 11 W cm –2 . Using radiation hydrodynamics simulations, we demonstrate that the electron density profile is radially compressed at the tip by the rocket-like propulsion of the ablated plasma. The mass density compression increases up to 2.9x when a more dense nanowire array is used. This is due to the ablation pressure from the neighboring nanowires. These findings offer valuable information for selecting an appropriate target design for experiments aimed at enhancing production of accelerated particles.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Physics consistent machine learning framework for inverse modeling with applications to ICF capsule implosions
In high energy density physics (HEDP) and inertial confinement fusion (ICF), predictive modeling is complicated by uncertainty in parameters that characterize various aspects of the modeled system, such as those characterizing material properties, equation of state (EOS), opacities, and initial conditions. Typically, however, these parameters are not directly observable. What is observed instead is a time sequence of radiographic projections using X-rays. In this work, we define a set of sparse hydrodynamic features derived from the outgoing shock profile and outer material edge, which can be obtained from radiographic measurements, to directly infer such parameters. Our machine learning (ML)-based methodology involves a pipeline of two architectures, a radiograph-to-features network (R2FNet) and a features-to-parameters network (F2PNet), that are trained independently and later combined to approximate a posterior distribution for the parameters from radiographs. We show that the machine learning architectures are able to accurately infer initial conditions and EOS parameters, and that the estimated parameters can be used in a hydrodynamics code to obtain density fields, shocks, and material interfaces that satisfy thermodynamic and hydrodynamic consistency. Finally, we demonstrate that features resulting from an unknown EOS model can be successfully mapped onto parameters of a chosen analytical EOS model, implying that network predictions are learning physics, with a degree of invariance to the underlying choice of EOS model. To the best of our knowledge, our framework is the first demonstration of recovering both thermodynamic and hydrodynamic consistent density fields from noisy radiographs.
97 MATHEMATICS AND COMPUTING↗
Laser irradiance scaling in polar direct drive implosions on the National Ignition Facility
We report polar-direct-drive experiments conducted at the National Ignition Facility [E. I. Moses, Fusion Sci. Technol. 54, 361 (2008)] performed at laser irradiance between 1 and 2×10 15 W/cm 2 exhibit increased hard x-ray emission, decreased neutron yield, and reduced areal density as the irradiance is increased. Experimental x-ray images at the higher irradiances show x-ray emission at the equator, as well as degraded symmetry, that is not predicted in hydrodynamic simulations using flux-limited energy transport, but that appear when non-local electron transport together with a model to account for cross beam energy transfer (CBET) is utilized. The reduction in laser power for equatorial beams required in the simulations to reproduce the effects of CBET on the observed symmetry also reproduces the yield degradation consistent with experimental data.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Shock formation in Ne, Ar, Kr, and Xe on deuterium gas puff implosions
1- and 2-D simulations of 1-cm radius, gas-puff liners of Ne, Ar, Kr, and Xe imploding onto a deuterium target are conducted using the discharge parameters for the Zebra (1 MA, 130 ns) driver using the resistive MHD code MACH2. This is an implementation of the Staged Z-pinch concept, in which the target is driven to high-energy-density first by shock compression launched by a diffused azimuthal magnetic field (J×B force), and then by the adiabatic compression as the liner converges on axis. During the run-in phase, the initial shock heating preheats the deuterium plasma, with a subsequent stable, adiabatic compression heating the target to high energy density. Shock compression of the target coincides with the development of a J×B force at the target/liner interface. Stronger B-field transport and earlier shock compression increases with higher-Z liners, which results in an earlier shock arrival on axis. As a result, delayed shock formation in lower-Z liners yields a relative increase in shock heating, however, the 2-D simulations show an increased target isolation from magneto-Rayleigh-Taylor instability penetration, suggesting that an optimal balance between these two effects is reached in an Ar or Kr liner, rather than with Xe.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Effects of a Xe dopant on an Ar gas-puff implosion on Z
Two Ar gas-puff experiments, each using loads consisting of two annuli and a center jet, were recently performed on Sandia National Laboratories' Z machine. These shots had the same load except that one of them (Z2603) employed a Xe dopant of 0.8% by number in the jet, which was not present in the otherwise identical Z2605. The extensive diagnostics deployed in these experiments reveal that the presence of this small fraction of Xe had a significant effect on the emitted K-shell radiation. Use of the Xe dramatically reduced the Ar K-shell yield from 373 ± 9% to 129 ± 9% kJ. However, the total yield increased, from 1.02 ± 17% to 1.14 ± 17% MJ. Also, the K-shell power pulse for Z2603 exhibited two nearly equal peaks separated by ∼5 ns, but that of Z2605 consisted of a more conventional single peak. Analysis of time- and space resolved K-shell spectra using a detailed atomic model for Ar ions indicates that a much greater fraction of the load mass was heated to the K-shell in the non-Xe shot Z2605. Previous average-ion atomic calculations of Xe cooling rates [Post et al., At. Data Nucl. Data Tables 20, 397 (1977)] show that the radiative cooling of the small 0.8% Xe fraction is nonetheless significant compared to that of the Ar and is therefore likely responsible for the differences in the shots.
Apruzese, J. P.↗
Use of 41Ar production to measure ablator areal density in NIF beryllium implosions
For the first time, 41Ar produced by the (n,ϒ) reaction from 40Ar in the beryllium shell of a DT filled Inertial Confinement Fusion capsule has been measured. Ar is co-deposited with beryllium in the sputter deposition of the capsule shell. Combined with a measurement of the neutron yield, the radioactive 41Ar then quantifies the areal density of beryllium during the DT neutron production. The measured 1.15 ± 0.17 × 10+8 atoms of 41Ar are 2.5 times that from the best post-shot calculation, suggesting that the Ar and Be areal densities are correspondingly higher than those calculated. Possible explanations are that (1) the beryllium shell is compressed more than calculated, (2) beryllium has mixed into the cold DT ice, or more likely (3) less beryllium is ablated than calculated. Since only one DT filled beryllium capsule has been fielded at NIF, these results can be confirmed and expanded in the future.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Direct measurement of the inertial confinement time in a magnetically driven implosion
Not Available
Knapp, P. F.↗
On krypton-doped capsule implosion experiments at the National Ignition Facility
Not Available
Chen, Hui↗
Using multiple x-ray emission images of inertially confined implosions to identify spatial variations and estimate confinement volumes (invited)
We describe two methods to analyze multiple x-ray images of a small, self-emitting object, and we apply these methods to the stagnating hotspots in inertial confinement fusion experiments. The first method, the common integrated profile, can be employed to assess and quantify spatial variations in opacity. It is both a simple assessment of consistency and a sophisticated measurement of variations in a region that is otherwise difficult to observe. Second, we present a method to estimate volumes of highly asymmetric objects using multiple images of x-ray emission. The method is based on image intensities and does not require any explicit assumption of symmetry
47 OTHER INSTRUMENTATION↗
Ar and Kr on deuterium gas-puff staged Z-pinch implosions on a 1-MA driver: Experiment and simulation
Not Available
Rahman, H. U.↗