Search NASA⌕ Search

Engineering topics

Kagan, Grigory

Publications and source records attributed to Kagan, Grigory.

18 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Observations of Multiple Nuclear Reaction Histories and Fuel-Ion Species Dynamics in Shock-Driven Inertial Confinement Fusion Implosions

Fuel-ion species dynamics in DT 3 He-gas-filled Inertial Confinement Fusion implosion is quantitatively assessed for the first time using simultaneously measured D 3 He and DT reaction histories. Here, these reaction histories are measured with the Particle X-ray Temporal Diagnostic, which captures the relative timing between different nuclear burns with unprecedented precision (~10 ps). The observed 50±10 ps earlier D 3 He reaction history timing (relative to DT) cannot be explained by average-ion hydrodynamic simulations, and is attributed to fuel-ion species separation between the D, T, and 3 He ions during shock convergence and rebound. At the onset of the shock burn, inferred 3 He/T fuel ratio in the burn region using the measured reaction histories is much higher as compared to the initial gas-fill ratio. As T and 3 He have the same mass but different charge, these results indicate that the charge-to-mass ratio plays an important role in driving fuel-ion species separation during strong shock propagation.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2018

Summaries of FY 2018 LANL experimental campaigns at the OMEGA and OMEGA-EP Laser Facilities

In FY 2018, Los Alamos National Laboratory carried out 21 shot days on the OMEGA and OMEGA-EP laser facilities in the areas of High Energy Density (HED) Science and Inertial Confinement Fusion (ICF). Our focus areas were on radiation flow, hydrodynamic turbulent mix and burn, and coupled Kelvin-Helmholtz (KH)/Richtmyer-Meshkov (RM) instability growth including in convergent geometry. We also had several focused shot days on transport properties in the kinetic regime. We continue to develop advanced diagnostics such as Neutron Imaging , and performed tests of a new imaging concept to be installed on NIF. We executed experiments in support of our double-shell campaign, and we began initial experiments using the new Revolver platform. Below is a summary of our campaigns, their motivation, and main results from this year.

47 OTHER INSTRUMENTATION↗
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 OSTI2018

Self-similar solutions for multi-species plasma mixing by gradient driven transport

We report that multi-species transport of plasma ions across an initial interface between DT and CH is shown to exhibit self-similar species density profiles under 1D isobaric conditions. Results using transport theory from recent studies and using a Maxwell–Stephan multi-species approximation are found to be in good agreement for the self-similar mix profiles of the four ions under isothermal and isobaric conditions. The individual ion species mass flux and molar flux profile results through the mixing layer are examined using transport theory. The sum over species mass flux is confirmed to be zero as required, and the sum over species molar flux is related to a local velocity divergence needed to maintain pressure equilibrium during the transport process. The light ion species mass fluxes are dominated by the diagonal coefficients of the diffusion transport matrix, while for the heaviest ion species (C in this case), the ion flux with only the diagonal term is reduced by about a factor two from that using the full diffusion matrix, implying the heavy species moves more by frictional collisions with the lighter species than by its own gradient force. Temperature gradient forces were examined by comparing profile results with and without imposing constant temperature gradients chosen to be of realistic magnitude for ICF experimental conditions at a fuel-capsule interface (10 μm scale length or greater). The temperature gradients clearly modify the relative concentrations of the ions, for example near the fuel center, however the mixing across the fuel-capsule interface appears to be minimally influenced by the temperature gradient forces within the expected compression and burn time. Finally, discussion considers the application of the self-similar profiles to specific conditions in ICF.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Influence of coupling on thermal forces and dynamic friction in plasmas with multiple ion species

The recently proposed effective potential theory is used to investigate the influence of coupling on inter-ion-species diffusion and momentum exchange in multi-component plasmas. Thermo-diffusion and the thermal force are found to diminish rapidly as strong coupling onsets. We found that for the same coupling parameters, the dynamic friction coefficient there tends to be unity. Our results provide an impetus for addressing the role of coupling on diffusive processes in inertial confinement fusion experiments.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Systematic Fuel Cavity Asymmetries in Directly Driven Inertial Confinement Fusion Implosions

Here, we present narrow-band self-emission x-ray images from a titanium tracer layer placed at the fuel-shell interface in 60-laser-beam implosion experiments at the OMEGA facility. The images are acquired during deceleration with inferred convergences of ~9-14. Novel here is that a systematically observed asymmetry of the emission is linked, using full sphere 3D implosion modeling, to performance-limiting low mode asymmetry of the drive.

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

Study of the ion kinetic effects in ICF run-away burn using a quasi-1D hybrid model

Here, the loss of fuel ions in the Gamow peak and other kinetic effects related to the α particles during ignition, run-away burn, and disassembly stages of an inertial confinement fusion D-T capsule are investigated with a quasi-1D hybrid volume ignition model that includes kinetic ions, fluid electrons, Planckian radiation photons, and a metallic pusher. The fuel ion loss due to the Knudsen effect at the fuel-pusher interface is accounted for by a local-loss model by with an albedo model for ions returning from the pusher wall. The tail refilling and relaxation of the fuel ion distribution are captured with a nonlinear Fokker-Planck solver. Alpha heating of the fuel ions is modeled kinetically while simple models for finite alpha range and electron heating are used. This dynamical model is benchmarked with a 3 T hydrodynamic burn model employing similar assumptions. For an energetic pusher (~40 kJ) that compresses the fuel to an areal density of ~1.07g/cm 2 at ignition, the simulation shows that the Knudsen effect can substantially limit ion temperature rise in runaway burn. While the final yield decreases modestly from kinetic effects of the α particles, large reduction of the fuel reactivity during ignition and runaway burn may require a higher Knudsen loss rate compared to the rise time of the temperatures above ~25 keV when the broad D-T Gamow peak merges into the bulk Maxwellian distribution.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Observation of interspecies ion separation in inertial-confinement-fusion implosions

Here we report direct experimental evidence of interspecies ion separation in direct-drive, inertial-confinement-fusion experiments on the OMEGA laser facility. These experiments, which used plastic capsules with D 2 /Ar gas fill (1% Ar by atom), were designed specifically to reveal interspecies ion separation by exploiting the predicted, strong ion thermo-diffusion between ion species of large mass and charge difference. Via detailed analyses of imaging x-ray-spectroscopy data, we extract Ar-atom-fraction radial profiles at different times, and observe both enhancement and depletion compared to the initial 1%-Ar gas fill. The experimental results are interpreted with radiation-hydrodynamic simulations that include recently implemented, first-principles models of interspecies ion diffusion. Finally, the experimentally inferred Ar-atom-fraction profiles agree reasonably, but not exactly, with calculated profiles associated with the incoming and rebounding first shock.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Kinetic studies of ICF implosions

Here, kinetic effects on inertial confinement fusion have been investigated. In particular, inter-ion-species diffusion and suprathermal ion distribution have been analyzed. The former drives separation of the fuel constituents in the hot reacting core and governs mix at the shell/fuel interface. The latter underlie measurements obtained with nuclear diagnostics, including the fusion yield and inferred ion burn temperatures. Basic mechanisms behind and practical consequences from these effects are discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Plasma physics effects on thermonuclear burn rate in the presence of hydrodynamic mix

Hydrodynamic mix can significantly degrade thermonuclear burn rate in an inertial confinement fusion (ICF) target. Successful mitigation requires a detailed understanding of the physical mechanisms by which mix affects burn. Here we summarize the roles of three distinct plasma physics effects on burn rate. The first is the well-known effect of enhanced thermal energy loss from the hot spot and the mitigating role of self-generated or externally-applied magnetic field. The second is the fuel ion separation via inter-species ion diffusion driven by the powerful thermodynamic forces exacerbated by mix during the implosion process. The third is the fusion reactivity modification by fast ion transport in a mix-dominated ICF target, where hot plasma is intermingled with cold fuel.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Self-similar structure and experimental signatures of suprathermal ion distribution in inertial confinement fusion implosions

The distribution function of suprathermal ions is found to be self-similar under conditions relevant to inertial confinement fusion hot spots. By utilizing this feature, interference between the hydrodynamic instabilities and kinetic effects is for the first time assessed quantitatively to find that the instabilities substantially aggravate the fusion reactivity reduction. Thus, the ion tail depletion is also shown to lower the experimentally inferred ion temperature, a novel kinetic effect that may explain the discrepancy between the exploding pusher experiments and rad-hydro simulations and contribute to the observation that temperature inferred from DD reaction products is lower than from DT at the National Ignition Facility.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Combined x-ray scattering, radiography, and velocity interferometry/streaked optical pyrometry measurements of warm dense carbon using a novel technique of shock-and-release

This work focused on a new application of the shock-and-release technique for equation of state (EOS) measurements. Warm dense matter states at near normal solid density and at temperatures close to 10 eV in diamond and graphite samples were created using a deep release from a laser-driven shock at the OMEGA laser facility. Independent temperature, density, and pressure measurements that do not depend on any theoretical models or simulations were obtained using imaging x-ray Thomson scattering, radiography, velocity interferometry, and streaked optical pyrometry. The experimental results were reproduced by the 2-D FLASH radiation hydrodynamics simulations finding a good agreement. The final EOS measurement was then compared with widely used SESAME EOS models as well as quantum molecular dynamics simulation results for carbon, which were very consistent with the experimental data.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Thermodynamic evaluation of mass diffusion in ionic mixtures

The thermodynamic technique of Landau and Lifshitz originally developed for inter-species diffusion in a binary neutral gas mixture is extended to a quasi-neutral plasma with two ion species. It is shown that, while baro- and electro-diffusion coefficients depend on the choice of the thermodynamic system, prediction for the total diffusive mass flux is invariant.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Electro-diffusion in a plasma with two ion species

Electric field is a thermodynamic force that can drive collisional inter-ion-species transport in a multicomponent plasma. In an inertial confinement fusion capsule, such transport causes fuel ion separation even with a target initially prepared to have equal number densities for the two fuel ion species. Unlike the baro-diffusion driven by ion pressure gradient and the thermo-diffusion driven by ion and electron temperature gradients, electro-diffusion has a critical dependence on the charge-to-mass ratio of the ion species. Specifically, it is shown here that electro-diffusion vanishes if the ion species have the same charge-to-mass ratio. An explicit expression for the electro-diffusion ratio is obtained and used to investigate the relative importance of electro- and baro-diffusion mechanisms. In particular, it is found that electro-diffusion reinforces baro-diffusion in the deuterium and tritium mix, but tends to cancel it in the deuterium and helium-3 mix.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Bounce-free spherical hydrodynamic implosion

In a bounce-free spherical hydrodynamic implosion, the post-stagnation hot core plasma does not expand against the imploding flow. Such an implosion scheme has the advantage of improving the dwell time of the burning fuel, resulting in a higher fusion burn-up fraction. The existence of bounce-free spherical implosions is demonstrated by explicitly constructing a family of self-similar solutions to the spherically symmetric ideal hydrodynamic equations. When applied to a specific example of plasma liner driven magneto-inertial fusion, the bounce-free solution is found to produce at least a factor of four improvement in dwell time and fusion energy gain.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗