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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.

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At least 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Inference of the electron temperature in inertial confinement fusion implosions from the hard X-ray spectral continuum

Using the free-free continuum self-emission spectrum at photon energies above 15 keV is one of the most promising concepts for assessing the electron temperature in inertial confinement fusion (ICF) experiments. However, these photons are due to suprathermal electrons whose mean free path is much larger than the thermal one, making their distribution deviate from Maxwellian in a finite-size hotspot. Here, the first study of the free-free X-ray emission from an ICF implosion is conducted, accounting for the kinetic modifications to the electron distribution. These modifications are found to result in qualitatively new features in the hard X-ray spectral continuum. Finally, inference of the electron temperature as if the emitting electrons are Maxwellian is shown to give a lower value than the actual one.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Ensemble simulations of inertial confinement fusion implosions

The achievement of inertial confinement fusion ignition on the National Ignition Facility relies on the collection and interpretation of a limited (and expensive) set of experimental data. These data are therefore supplemented with state-of-the-art multi-dimensional radiation-hydrodynamic simulations to provide a better understanding of implosion dynamics and behavior. We present a relatively large number (~4000) of systematically perturbed 2D simulations to probe our understanding of low-mode fuel and ablator asymmetries seeded by asymmetric illumination. We find that Gaussian process surrogate models are able to predict both the total neutron yield and the degradation in performance due to asymmetries. Furthermore, the surrogates are then applied to simulations containing new sources of degradation to quantify the impact of the new source.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2020

3D xRAGE simulation of inertial confinement fusion implosion with imposed mode 2 laser drive asymmetry

Low-mode asymmetries represent an important obstacle to achieving high-gain inertial confinement fusion implosions. As a step in learning how to control such effects, an OMEGA experiment with imposed mode 2 laser drive asymmetries was done to study the expected signatures of this type of asymmetry [M. Gatu Johnson et al., PRE 2018]. In the present work, a 3D xRAGE simulation including the stalk mount has been brought to bear on the data from that experiment. Furthermore, comprehensive comparisons between simulated and measured observables are made. Good agreement between simulated and measured x-ray image-inferred shell trajectories, bang times and neutron emission widths are seen, showing that the hydrodynamics are well captured in the simulation. Asymmetries seen in simulated and measured time-resolved and time-integrated x-ray images and areal densities also compare well, showing impact of both stalk and mode 2. On the other hand, important differences in measured and simulated neutron emission histories, yield, and ion temperature (T ion ) asymmetries are seen, suggesting that the simulation is overestimating shock yield. Overall, the results clearly demonstrate the importance of considering all asymmetry sources when interpreting measured signatures of asymmetry.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

Probing atomic physics at ultrahigh pressure using laser-driven implosions

Abstract Spectroscopic measurements of dense plasmas at billions of atmospheres provide tests to our fundamental understanding of how matter behaves at extreme conditions. Developing reliable atomic physics models at these conditions, benchmarked by experimental data, is crucial to an improved understanding of radiation transport in both stars and inertial fusion targets. However, detailed spectroscopic measurements at these conditions are rare, and traditional collisional-radiative equilibrium models, based on isolated-atom calculations and ad hoc continuum lowering models, have proved questionable at and beyond solid density. Here we report time-integrated and time-resolved x-ray spectroscopy measurements at several billion atmospheres using laser-driven implosions of Cu-doped targets. We use the imploding shell and its hot core at stagnation to probe the spectral changes of Cu-doped witness layer. These measurements indicate the necessity and viability of modeling dense plasmas with self-consistent methods like density-functional theory, which impact the accuracy of radiation transport simulations used to describe stellar evolution and the design of inertial fusion targets.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2020

Generation of megatesla magnetic fields by intense-laser-driven microtube implosions

A microtube implosion driven by ultraintense laser pulses is used to produce ultrahigh magnetic fields. Due to the laser-produced hot electrons with energies of mega-electron volts, cold ions in the inner wall surface implode towards the central axis. By pre-seeding uniform magnetic fields on the kilotesla order, the Lorenz force induces the Larmor gyromotion of the imploding ions and electrons. Due to the resultant collective motion of relativistic charged particles around the central axis, strong spin current densities of ~ peta-ampere/cm 2 are produced with a few tens of nm size, generating megatesla-order magnetic fields. The underlying physics and important scaling are revealed by particle simulations and a simple analytical model. The concept holds promise to open new frontiers in many branches of fundamental physics and applications in terms of ultrahigh magnetic fields.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Near-vacuum hohlraums for driving fusion implosions with high density carbon ablatorsa)

Recent experiments at the National Ignition Facility [M. J. Edwards et al., Phys. Plasmas 20, 070501 (2013)] have explored driving high-density carbon ablators with near-vacuum hohlraums, which use a minimal amount of helium gas fill. These hohlraums show improved efficiency relative to conventional gas-filled hohlraums in terms of minimal backscatter, minimal generation of suprathermal electrons, and increased hohlraum-capsule coupling. Given these advantages, near-vacuum hohlraums are a promising choice for pursuing high neutron yield implosions. Long pulse symmetry control, though, remains a challenge, as the hohlraum volume fills with material. Here, two mitigation methodologies have been explored, dynamic beam phasing and increased case-to-capsule ratio (larger hohlraum size relative to capsule). Unexpectedly, experiments have demonstrated that the inner laser beam propagation is better than predicted by nominal simulations, and an enhanced beam propagation model is required to match measured hot spot symmetry. Ongoing work is focused on developing a physical model which captures this enhanced propagation and on utilizing the enhanced propagation to drive longer laser pulses than originally predicted in order to reach alpha-heating dominated neutron yields.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Ion-kinetic simulations of D- 3 He gas-filled inertial confinement fusion target implosions with moderate to large Knudsen number

Experiments designed to investigate the transition to non-collisional behavior in D 3 He-gas inertial confinement fusion target implosions display increasingly large discrepancies with respect to simulations by standard hydrodynamics codes as the expected ion mean-free-paths λ c increase with respect to the target radius R (i.e., when the Knudsen number N K = λ c /R grows). To take properly into account large N K 's, multi-ion-species Vlasov-Fokker-Planck computations of the inner gas in the capsules have been performed, for two different values of N K , one moderate and one large. The results, including nuclear yield, reactivity-weighted ion temperatures, nuclear emissivities, and surface brightness, have been compared with the experimental data and with the results of hydrodynamical simulations, some of which include an ad hocmodeling of kinetic effects. The experimental results are quite accurately rendered by the kinetic calculations in the smaller-N K case, much better than by the hydrodynamical calculations. The kinetic effects at play in this case are thus correctly understood. However, in the higher-N K case, the agreement is much worse. Furthermore, the remaining discrepancies are shown to arise from kinetic phenomena (e.g., inter-species diffusion) occurring at the gas-pusher interface, which should be investigated in the future work.

42 ENGINEERING↗
DOE OSTI2017

Improving ICF implosion performance with alternative capsule supports

The thin membrane that holds the capsule in-place in the hohlraum is recognized as one of the most significant contributors to reduced performance in indirect drive inertial confinement fusion (ICF) experiments on the National Ignition Facility. This membrane, known as the “tent,” seeds a perturbation that is amplified by Rayleigh-Taylor and can rupture the capsule. A less damaging capsule support mechanism is under development. Possible alternatives include the micron-scale rods spanning the hohlraum width and supporting either the capsule or stiffening the fill-tube, a larger fill-tube to both fill and support the capsule, or a low-density foam layer that protects the capsule from the tent impact. Experiments are testing these support features to measure their imprint on the capsule. These experiments are revealing unexpected aspects about perturbation development in indirect drive ICF, such as the importance of shadows coming from bright spots in the hohlraum. Two dimensional and 3D models are used to explain these features and assess the impact on implosion performance. Experiments and modeling suggest that the fill-tube supported by a perpendicular rod can mount the capsule without any additional perturbation beyond that of the fill tube.

Weber, C. R.↗
DOE OSTI2017

Monochromatic backlighting of direct-drive cryogenic DT implosions on OMEGA

Backlighting is a powerful technique to observe the flow of cold and dense material in highenergy- density-plasma experiments. High-performance, direct-drive cryogenic deuterium-tritium (DT) implosions are a challenging backlighting configuration because of the low opacity of the DT shell, the high shell velocity, the small size of the stagnating shell, and the very bright selfemission of the hot core.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Observation and modeling of interspecies ion separation in inertial confinement fusion implosions via imaging x-ray spectroscopy

In this article, we report the first direct experimental evidence of interspecies ion separation in direct-drive inertial confinement fusion experiments performed at the OMEGA laser facility via spectrally, temporally, and spatially resolved imaging x-ray-spectroscopy data [S. C. Hsu et al., Europhys. Lett. 115, 65001 (2016)]. These experiments were designed based on the expectation that interspecies ion thermo-diffusion would be the strongest for species with a large mass and charge difference. The targets were spherical plastic shells filled with D2 and a trace amount of Ar (0.1% or 1% by atom). Ar K-shell spectral features were observed primarily between the time of first-shock convergence and slightly before the neutron bang time, using a time- and space-integrated spectrometer, a streaked crystal spectrometer, and two gated multi-monochromatic x-ray imagers fielded along quasi-orthogonal lines of sight. Detailed spectroscopic analyses of spatially resolved Ar K-shell lines reveal the deviation from the initial 1% Ar gas fill and show both Ar-concentration enhancement and depletion at different times and radial positions of the implosion. The experimental results are interpreted using radiation-hydrodynamic simulations that include recently implemented, first-principles models of interspecies ion diffusion. Lastly, the experimentally inferred Ar-atom fraction profiles agree reasonably with calculated profiles associated with the incoming and rebounding first shock.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

The effects of convergence ratio on the implosion behavior of DT layered inertial confinement fusion capsules

The wetted foam capsule design for inertial confinement fusion capsules, which includes a foam layer wetted with deuterium-tritium liquid, enables layered capsule implosions with a wide range of hot-spot convergence ratios (CR) on the National Ignition Facility. In this paper, we present a full-scale wetted foam capsule design that demonstrates high gain in one-dimensional simulations. In these simulations, increasing the convergence ratio leads to an improved capsule yield due to higher hot-spot temperatures and increased fuel areal density. High-resolution two-dimensional simulations of this design are presented with detailed and well resolved models for the capsule fill tube, support tent, surface roughness, and predicted asymmetries in the x-ray drive. Our modeling of these asymmetries is validated by comparisons with available experimental data. In 2D simulations of the full-scale wetted foam capsule design, jetting caused by the fill tube is prevented by the expansion of the tungsten-doped shell layer due to preheat. While the impacts of surface roughness and predicted asymmetries in the x-ray drive are enhanced by convergence effects, likely underpredicted in 2D at high CR, simulations predict that the capsule is robust to these features. Nevertheless, the design is highly susceptible to the effects of the capsule support tent, which negates all of the one-dimensional benefits of increasing the convergence ratio. Indeed, when the support tent is included in simulations, the yield decreases as the convergence ratio is increased for CR > 20. Finally and nevertheless, the results suggest that the full-scale wetted foam design has the potential to outperform ice layer capsules given currently achievable levels of asymmetries when fielded at low convergence ratios (CR < 20).

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Simultaneous diagnosis of radial profiles and mix in NIF ignition-scale implosions via X-ray spectroscopy

In a National Ignition Facility implosion, hydrodynamic instabilities may cause the cold material from the imploding shell to be injected into the hot-spot (hot-spot mix), enhancing the radiative and conductive losses, which in turn may lead to a quenching of the ignition process. The bound-bound features of the spectrum emitted by high-Z ablator dopants that get mixed into the hot-spot have been previously used to infer the total amount of mixed mass; however, the typical errorbars are larger than the maximum tolerable mix. We present in this paper an improved 2D model for mix spectroscopy which can be used to retrieve information on both the amount of mixed mass and the full imploded plasma profile. By performing radiation transfer and simultaneously fitting all of the features exhibited by the spectra, we are able to constrain self-consistently the effect of the opacity of the external layers of the target on the emission, thus improving the accuracy of the inferred mixed mass. The model's predictive capabilities are first validated by fitting simulated spectra arising from fully characterized hydrodynamic simulations, and then, the model is applied to previously published experimental results, providing values of mix mass in agreement with previous estimates. Finally, we show that the new self consistent procedure leads to better constrained estimates of mix and also provides insight into the sensitivity of the hot-spot spectroscopy to the spatial properties of the imploded capsule, such as the in-flight aspect ratio of the cold fuel surrounding the hotspot.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

The control of hot-electron preheat in shock-ignition implosions

In the shock-ignition scheme for inertial confinement fusion, hot electrons resulting from laser–plasma instabilities can play a major role during the late stage of the implosion. This article presents the results of an experiment performed on OMEGA in the so-called “40 + 20 configuration.” Using a recent calibration of the time-resolved hard x-ray diagnostic, the hot electrons’ temperature and total energy were measured. One-dimensional radiation–hydrodynamic simulations have been performed that include hot electrons and are in agreement with the measured neutron-rate–averaged areal density. For an early spike launch, both experiment and simulations show the detrimental effect of hot electrons on areal density and neutron yield. Lastly, for a later spike launch, this effect is minimized because of a higher compression of the target.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

Design and testing of a magnetically driven implosion peak current diagnostic

We report that a critical component of the magnetically driven implosion experiments at Sandia National Laboratories is the delivery of high-current, 10s of MA, from the Z pulsed power facility to a target. In order to assess the performance of the experiment, it is necessary to measure the current delivered to the target. Recent Magnetized Liner Inertial Fusion (MagLIF) experiments have included velocimetry diagnostics, such as PDV (Photonic Doppler Velocimetry) or Velocity Interferometer System for Any Reflector, in the final power feed section in order to infer the load current as a function of time. However, due to the nonlinear volumetrically distributed magnetic force within a velocimetry flyer, a complete time-dependent load current unfold is typically a time-intensive process and the uncertainties in the unfold can be difficult to assess. In this paper, we discuss how a PDV diagnostic can be simplified to obtain a peak current by sufficiently increasing the thickness of the flyer. This effectively keeps the magnetic force localized to the flyer surface, resulting in fast and highly accurate measurements of the peak load current. Additionally, we show the results of experimental peak load current measurements from the PDV diagnostic in recent MagLIF experiments.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

Visualizing deceleration-phase instabilities in inertial confinement fusion implosions using an “enhanced self-emission” technique at the National Ignition Facility

High-mode perturbations and low-mode asymmetries were measured in the deceleration phase of indirectly-driven, deuterium gas filled inertial confinement fusion (ICF) capsule implosions at convergence ratios of 10 to 15, using a new “enhanced emission” technique at the National Ignition Facility (NIF) [E. M. Campbell, R. Cauble, and B. A. Remington, AIP Conf. Proc. 429, 3 (1998)]. In these experiments, a high spatial resolution Kirkpatrick-Baez microscope was used to image the x-ray emission from the inner surface of a high-density-carbon (HDC) capsule’s shell. Use of a high atomic number dopant in the shell enabled time-resolved observations of shell perturbations penetrating into the hot spot. This allowed the effects of the perturbations and asymmetries on degrading neutron yield to be directly measured. In particular, mix induced radiation losses of ~400 J from the hot spot resulted in a neutron yield reduction of a factor of ~2. In a subsequent experiment with a significantly increased level of short-mode initial perturbations, shown through the enhanced imaging technique to be highly organized radially, the neutron yield dropped an additional factor of ~2.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Progress on observations of interspecies ion separation in inertial-confinement-fusion implosions via imaging x-ray spectroscopy

We report on the analyses of x-ray-imaging spectroscopy data from experiments to study interspecies ion separation in direct-drive inertial-confinement-fusion experiments on the Omega laser facility. This is a continuation of recent, related research [S. C. Hsu et al., Euro Phys. Lett. 115, 65001 (2016); T. R. Joshi et al., Phys. Plasmas 24, 056305 (2017)]. The targets were argon (Ar)-doped, deuterium (D2)-filled spherical plastic shells of varying D2-Ar relative and total gas pressures. We used a time- and space-integrated spectrometer, streaked crystal spectrometer, and up to three time-gated multi-monochromatic x-ray imagers (MMIs) fielded along different lines of sight to record x-ray spectral features obtained from the implosions. The MMI data were recorded between first-shock convergence and slightly before the neutron bang time. We confirm the presence of interspecies ion separation as reported in our recent work. Extensions to the previous work include (a) the inclusion of shell mix in the data analysis, which slightly changes the amount of inferred species separation, (b) observation of species separation closer to the neutron bang time, and (c) fielding of the particle x-ray temporal diagnostic (PXTD) [H. Sio et al., Rev. Sci. Instrum. 87, 11D701 (2016)] to infer the relative timing between the neutron bang time and peak x-ray emission. Experimentally inferred species separation is compared with radiation-hydrodynamic simulations that include a multi-ion-species transport model.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Implosion performance of subscale beryllium capsules on the NIF

Many inertial fusion designs use capsules made of beryllium, as its high mass ablation rate is advantageous. here, we present the first systematic experimental study of indirectly driven beryllium capsules with a cryogenic deuterium-tritium fuel layer. “Subscale” capsules, 80% of the nominal National Ignition Facility point design radius, show optimal performance with the remaining mass of ~6–7%. A buoyancy-drag mix model explains the implosion performance, suggesting that fuel-ablator mix is the dominant degradation mechanism. Increasing the capsule scale is predicted to reduce the impact of fuel-ablator mix and achieve high performance.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Computational study of instability and fill tube mitigation strategies for double shell implosions

Double shell capsules are an attractive alternative scheme for achieving robust alpha-heating at the National Ignition Facility due to their low convergence ratio and low predicted temperature for achieving volume ignition. Nevertheless, simulations suggest that double shell targets are more susceptible to the fill tube, used to fill the inner shell with liquid DT, than typical single-shell ignition capsule designs, due to the higher density gradient between the shell and the fill tube hole, a lower outer shell velocity, which prevents the implosion from catching up to the initial fill tube jet, and the absence of a rebounding shock through the foam to slow this jet. Double shells are also highly susceptible to the Rayleigh-Taylor instability at both interfaces with the high density inner shell. Combined, these effects are predicted by radiation-hydrodynamics simulations to reduce fuel confinement and temperature, resulting in reduced performance by a factor of ≈20–45, depending on design details, compared to idealized one-dimensional (1D) simulations. We discuss herein a mitigation strategy for both the interfacial instabilities and the fill tube that is predicted by simulations to decrease the yield degradation to a factor of ≈4. The mitigation strategy involves a modification of the capsule geometry as well as the use of a multishock pulse shape. The multishock pulse is required for the fill tube mitigation strategy and has the added benefit of stabilizing perturbations at the foam/pusher interface without decreasing 1D yield. In order to experimentally verify these predictions, we discuss the potential use of a hydrogrowth radiography platform that could be applied to test the proposed mitigation strategies.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗