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Delettrez, J.

Publications and source records attributed to Delettrez, J..

15 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Inferences of hot electron preheat and its spatial distribution in OMEGA direct drive implosions

Hot electrons generated from laser plasma instabilities degrade performance of direct drive implosions by preheating the deuterium and tritium (DT) fuel resulting in early decompression and lower areal densities at stagnation. A technique to quantify the hot electron preheat of the dense DT fuel and connect it to the degradation in areal density is described in detail. Hot electrons are measured primarily from the hard x-rays they emit as they slow down in the target. The DT preheat is inferred from a comparison of the hard x-ray signals between a DT-layered implosion and its mass equivalent ablator only implosion. The preheat energy spatial distribution within the imploding shell is inferred from experiments using high Z payloads of varying thicknesses. It is found that the electrons deposit their energy uniformly throughout the shell material. For typical direct-drive OMEGA implosions driven with an overlapped intensity of ∼9·1014 W/cm2, approximately ∼0.02%–0.03% of the laser energy is converted into preheat of the stagnated fuel which corresponds to areal density degradations of 10%–20%. The degradations in areal density explain some of the observed discrepancies between the simulated and measured areal densities.

Christopherson, A. R. (ORCID:000000033578635X)↗
DOE OSTI2021

2H(p, gamma) 3He cross section measurement using high-energy-density plasmas

An absolute cross section for the radiative capture reaction 2H(p, γ ) 3He has been measured at the OMEGA laser facility using inertially confined plasmas. These high-temperature plasmas are created by imploding a fuel containing capsule using laser ablation, and are advantageous in that they better mimic astrophysical systems. We measure an S factor for this reaction of 0.429 ± 0.026stat ± 0.072sys eV b at Ec.m. = 16.35 ± 0.40 keV, which is higher than the adopted evaluations. This reaction is important as a source of nuclear energy in protostars and brown dwarfs. It is also a critical reaction during big-bang nucleosynthesis, and an accurate cross section can be used as a constraint on cosmology.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2020

2 H(p,γ) 3 He cross section measurement using high-energy-density plasmas

An absolute cross section for the radiative capture reaction 2 H(p,γ) 3 He has been measured at the OMEGA laser facility using inertially confined plasmas. These high-temperature plasmas are created by imploding a fuel-containing capsule using laser ablation, and are advantageous in that they better mimic astrophysical systems. We measure an S factor for this reaction of 0.429 ± 0.026 stat ± 0.072 sys eV b at E c.m . = 16.35 ± 0.40 keV, which is higher than the adopted evaluations. Furthermore, this reaction is important as a source of nuclear energy in protostars and brown dwarfs. It is also a critical reaction during big-bang nucleosynthesis, and an accurate cross section can be used as a constraint on cosmology.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Impact of imposed mode 2 laser drive asymmetry on inertial confinement fusion implosions

Low-mode asymmetries have emerged as one of the primary challenges to achieving high-performing inertial confinement fusion implosions. These asymmetries seed flows in the implosions, which will manifest as modifications to the measured ion temperature (Tion) as inferred from the broadening of primary neutron spectra. The effects are important to understand both to learn to control and mitigate low-mode asymmetries, and to experimentally more closely capture thermal Tion used as input in implosion performance metric calculations. In this paper, results from and simulations of a set of experiments with a seeded mode 2 in the laser drive are described. The goal of this intentionally asymmetrically driven experiment was to test our capability to predict and measure the signatures of flows seeded by the low-mode asymmetry. The results from these experiments (first discussed in M Gatu Johnson et al, Phys. Rev. E 98, 051201(R) (2018)) demonstrate the importance of interplay of flows seeded by various asymmetry seeds. In particular, measured Tion and self-emission x-ray asymmetries are expected to be well captured by interplay between flow seeded by the imposed mode 2 and the capsule stalk mount. Measurements of areal density asymmetry also indicate the importance of the stalk mount as an asymmetry seed in these implosions. The simulations brought to bear on the problem (1D LILAC, 2D xRAGE, 3D ASTER and 3D Chimera) show how thermal Tion is expected to be significantly lower than Tion as inferred from the broadening of measured neutron spectra. They also show that the electron temperature is not expected to be the same as Tion for these implosions.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Impact of imposed mode 2 laser drive asymmetry on inertial confinement fusion implosions

Low-mode asymmetries have emerged as one of the primary challenges to achieving high-performing inertial confinement fusion implosions. These asymmetries seed flows in the implosions, which will manifest as modifications to the measured ion temperature (T ion ) as inferred from the broadening of primary neutron spectra. The effects are important to understand (i) to learn to control and mitigate low-mode asymmetries and (ii) to experimentally more closely capture thermal T ion used as input in implosion performance metric calculations. Here, in this paper, results from and simulations of a set of experiments with a seeded mode 2 in the laser drive are described. The goal of this intentionally asymmetrically driven experiment was to test our capability to predict and measure the signatures of flows seeded by the low-mode asymmetry. The results from these experiments [first discussed in M. Gatu Johnson et al., Phys. Rev. E 98, 051201(R) (2018)] demonstrate the importance of interplay of flows seeded by various asymmetry seeds. In particular, measured T ion and self-emission x-ray asymmetries are expected to be well captured by interplay between flows seeded by the imposed mode 2 and the capsule stalk mount. Measurements of areal density asymmetry also indicate the importance of the stalk mount as an asymmetry seed in these implosions. The simulations brought to bear on the problem (1D LILAC, 2D xRAGE, 3D ASTER, and 3D Chimera) show how thermal T ion is expected to be significantly lower than T ion as inferred from the broadening of measured neutron spectra. They also show that the electron temperature is not expected to be the same as T ion for these implosions.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

Impact of asymmetries on fuel performance in Inertial Confinement Fusion

Low-mode asymmetries prevent effective compression, confinement and heating of the fuel in inertial confinement fusion (ICF) implosions and their control is essential to achieving ignition. Ion temperatures (Tion) in ICF experiments are inferred from the broadening of primary neutron spectra. Directional motion (flow) of the fuel at burn also impacts broadening and will lead to artificially inflated “Tion” values. Flow due to low-mode asymmetries is expected to give rise to line-of-sight variations in measured Tion. We report on intentionally asymmetrically driven experiments at the OMEGA laser facility designed to test the ability to accurately predict and measure line-of-sight differences in apparent Tion due to low-mode asymmetry-seeded flows. Contrasted to Chimera and xRAGE simulations, the measurements demonstrate how all asymmetry seeds have to be considered to fully capture the flow field in an implosion. In particular, flow induced by the stalk that holds the target is found to interfere with the seeded asymmetry. A substantial stalk-seeded asymmetry in the areal density of the implosion is also observed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

Impact of asymmetries on fuel performance in inertial confinement fusion

Low-mode asymmetries prevent effective compression, confinement, and heating of the fuel in inertial confinement fusion (ICF) implosions, and their control is essential to achieving ignition. Ion temperatures (T ion ) in ICF experiments are inferred from the broadening of primary neutron spectra. Directional motion (flow) of the fuel at burn also impacts broadening and will lead to artificially inflated “T ion ” values. Flow due to low-mode asymmetries is expected to give rise to line-of-sight variations in measured T ion . Here we report on intentionally asymmetrically driven experiments at the OMEGA laser facility designed to test the ability to accurately predict and measure line-of-sight differences in apparent T ion due to low-mode asymmetry-seeded flows. Contrasted to chimera and xrage simulations, the measurements demonstrate how all asymmetry seeds have to be considered to fully capture the flow field in an implosion. In particular, flow induced by the stalk that holds the target is found to interfere with the seeded asymmetry. A substantial stalk-seeded asymmetry in the areal density of the implosion is also observed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Transport and spatial energy deposition of relativistic electrons in copper-doped fast ignition plasmas

Fast electron transport and spatial energy deposition are investigated in integrated cone-guided Fast Ignition experiments by measuring fast electron induced copper K-shell emission using a copper tracer added to deuterated plastic shells with a geometrically reentrant gold cone. Experiments were carried out at the Laboratory for Laser Energetics on the OMEGA/OMEGA-EP Laser where the plastic shells were imploded using 54 of the 60 OMEGA60 beams (3ω, 20 kJ), while the high intensity OMEGA-EP (BL2) beam (1 ω, 10 ps, 500 J, I peak > 10 19 W/cm 2 ) was focused onto the inner cone tip. Here, a retrograde analysis using the hybrid-PIC electron transport code, ZUMA, is performed to examine the sensitivity of the copper Kα spatial profile on the laser-produced fast electrons, facilitating the optimization of new target point designs and laser configurations to improve the compressed core areal density by a factor of 4 and the fast electron energy coupling by a factor of 3.5.

42 ENGINEERING↗
DOE OSTI2016

Visualizing fast electron energy transport into laser-compressed high-density fast-ignition targets

Understanding of fast electron generation inside the cone and their subsequent transport into hot dense plasmas is crucial to the success of the cone guided fast ignition (FI) scheme of inertial confinement fusion. In our prior work, we developed a new platform that measured, for the first time, the spatial map of fast electron transport in the vicinity of the core of an imploded shell in joint OMEGA and OMEGA EP experiments. In this University of California San Diego-led NLUF project we are using the same platform to investigate several methods of improving coupling efficiency into an imploded CH/CD shell attached to a re-entrant gold cone targets. The shell has an outer diameter of 870 µm consisting of a 15 µm thick outer CH ablator and a 23 µm thick inner CD layer. A Cu dopant (at ~ 1% atomic number density of CD) is added to the CD layer of the shell, which allows characterization of fast electrons transport via their induced Cu K-shell fluorescence radiation.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Ion Thermal Decoupling and Species Separation in Shock-Driven Implosions

Anomalous reduction of the fusion yields by 50% and anomalous scaling of the burn-averaged ion temperatures with the ion-species fraction has been observed for the first time in DHe 3 -filled shock-driven inertial confinement fusion implosions. Two ion kinetic mechanisms are used to explain the anomalous observations: thermal decoupling of the D and He 3 populations and diffusive species separation. The observed insensitivity of ion temperature to a varying deuterium fraction is shown to be a signature of ion thermal decoupling in shock-heated plasmas. The burn-averaged deuterium fraction calculated from the experimental data demonstrates a reduction in the average core deuterium density, as predicted by simulations that use a diffusion model. Accounting for each of these effects in simulations reproduces the observed yield trends.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Compressed shell conditions extracted from spectroscopic analysis of Ti K-shell absorption spectra with evaluation of line self-emission

Ti-doped tracer layers embedded in the shell at varying distances from the fuel-shell interface serve as a spectroscopic diagnostic for direct-drive experiments conducted at OMEGA. Detailed modeling of Ti K-shell absorption spectra produced in the tracer layer considers n = 1–2 transitions in F- through Li-like Ti ions in the 4400–4800 eV range, both including and excluding line self-emission. Testing the model on synthetic spectra generated from 1-D LILAC hydrodynamic simulations reveals that the model including self-emission best reproduces the simulation, while the model excluding self-emission overestimates electron temperature T{sub e} and density N{sub e} to a higher degree for layers closer to the core. The prediction of the simulation that the magnitude of T{sub e} and duration of Ti absorption will be strongly tied to the distance of the layer from the core is consistent with the idea that regions of the shell close to the core are more significantly heated by thermal transport out of the hot dense core, but more distant regions are less affected by it. The simulation predicts more time variation in the observed T{sub e}, N{sub e} conditions in the compressed shell than is observed in the experiment, analysis of which reveals conditions remain in the range T{sub e} = 400–600 eV and N{sub e} = 3.0–10.0 × 10{sup 24} cm{sup −3} for all but the most distant Ti-doped layer, with error bars ∼5% T{sub e} value and ∼10% N{sub e} on average. The T{sub e}, N{sub e} conditions of the simulation lead to a greater degree of ionization for zones close to the core than occurs experimentally, and less ionization for zones far from the core.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

First Observations of Nonhydrodynamic Mix at the Fuel-Shell Interface in Shock-Driven Inertial Confinement Implosions

A strong nonhydrodynamic mechanism generating atomic fuel-shell mix has been observed in strongly shocked inertial confinement fusion implosions of thin deuterated-plastic shells filled with 3 He gas. These implosions were found to produce D 3 He-proton shock yields comparable to implosions of identical shells filled with a hydroequivalent 50:50 D 3 He gas mixture. Standard hydrodynamic mixing cannot explain this observation, as hydrodynamic modeling including mix predicts a yield an order of magnitude lower than was observed. Instead, these results can be attributed to ion diffusive mix at the fuel-shell interface.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Instability-driven electromagnetic fields in coronal plasmas

Filamentary electromagnetic fields previously observed in the coronae of laser-driven spherical targets [F. H. Séguin et al., Phys. Plasma. 19, 012701 (2012)] have been further investigated in laser-irradiated plastic foils. Face-on proton-radiography provides an axial view of these filaments and shows coherent cellular structure regardless of initial foil-surface conditions. The observed cellular fields are shown to have an approximately constant scale size of ∼210 μm throughout the plasma evolution. A discussion of possible field-generation mechanisms is provided and it is demonstrated that the likely source of the cellular field structure is the magnetothermal instability. Using predicted temperature and density profiles, the fastest growing modes of this instability were found to be slowly varying in time and consistent with the observed cellular size.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗