Neutron backscatter edge: A measure of the hydrodynamic properties of the dense DT fuel at stagnation in ICF experiments

The kinematic lower bound for the single scattering of neutrons produced in deuterium-tritium (DT) fusion reactions produces a backscatter edge in the measured neutron spectrum. The energy spectrum of backscattered neutrons is dependent on the scattering ion velocity distribution. As the neutrons preferentially scatter in the densest regions of the capsule, the neutron backscatter edge presents a unique measurement of the hydrodynamic conditions in the dense DT fuel. It is shown that the spectral shape of the edge is determined by the scattering rate weighted fluid velocity and temperature of the dense DT fuel layer during neutron production. In order to fit the neutron spectrum, a model for the various backgrounds around the backscatter edge is developed and tested on synthetic data produced from hydrodynamic simulations of OMEGA implosions. It is determined that the analysis could be utilized on current inertial confinement fusion experiments in order to measure the dense fuel properties

Synthetic nuclear diagnostics for inferring plasma properties of inertial confinement fusion implosions

A suite of synthetic nuclear diagnostics has been developed to post-process radiation hydrodynamics simulations performed with the code Chimera. These provide experimental observables based on simulated capsule properties and are used to assess alternative experimental and data analysis techniques. These diagnostics include neutron spectroscopy, primary and scattered neutron imaging, neutron activation, γ-ray time histories and carbon γ-ray imaging. Novel features of the neutron spectrum have been analysed to infer plasma parameters. The nT and nD backscatter edges have been shown to provide a shell velocity measurement. Areal density asymmetries created by low mode perturbations have been inferred from the slope of the downscatter spectrum down to 10 MeV. Neutron activation diagnostics showed significant aliasing of high mode areal density asymmetries when observing a capsule implosion with 3D multimode perturbations applied. Carbon γ-ray imaging could be used to image the ablator at a high convergence ratio. Time histories of both the fusion and carbon γ signals showed a greater time difference between peak intensities for the perturbed case when compared to a symmetric simulation

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

Impact of stalk on directly driven inertial confinement fusion implosions

Low-mode asymmetries have emerged as one of the primary challenges to achieving high-performing inertial confinement fusion (ICF) implosions. In direct-drive ICF, an important potential seed of such asymmetries is the capsule stalk mount, the impact of which has remained a contentious question. In this paper, we describe the results from an experiment on the OMEGA laser with intentional offsets at varying angles to the capsule stalk mount, which clearly demonstrates the impact of the stalk mount on implosion dynamics. The angle between stalk and offset is found to significantly impact observables. Specifically, a larger directional flow is observed in neutron spectrum measurements when the offset is toward rather than away from the stalk, while an offset at 42° to the stalk gives minimal directional flow but still generates a large flow field in the implosion. No significant directional flow is seen due to stalk only. Time-integrated x-ray images support these flow observations. A trend is also seen in implosion yield, with lower yield obtained for offsets with a smaller angle than with a larger angle toward the stalk. Radiation hydrodynamic simulations using 2D DRACO and 2D/3D Chimera not including the stalk mount and using 2D xRAGE including the stalk mount are brought to bear on the data. The yield trend, the minimal directional flow with stalk only, and the larger flow enhancement observed with the offset toward the stalk are all reproduced in the xRAGE simulations. The results strongly indicate that the stalk impact must be considered and mitigated to achieve high-performing implosions

Effect of strongly magnetized electrons and Ions on heat flow and symmetry of inertial fusion implosions

This Letter presents the first observation on how a strong, 500 kG, externally applied B field increases the mode-two asymmetry in shock-heated inertial fusion implosions. Using a direct-drive implosion with polar illumination and imposed field, we observed that magnetization produces a significant increase in the implosion oblateness (a 2.5 × larger P 2 amplitude in x-ray self-emission images) compared with reference experiments with identical drive but with no field applied. The implosions produce strongly magnetized electrons ( ω e τ e ≫ 1 ) and ions ( ω i τ i > 1 ) that, as shown using simulations, restrict the cross field heat flow necessary for lateral distribution of the laser and shock heating from the implosion pole to the waist, causing the enhanced mode-two shape

Self-Generated magnetic fields in the stagnation phase of indirect-drive implosions on the national ignition facility

Three-dimensional extended-magnetohydrodynamic simulations of the stagnation phase of inertial confinement fusion implosion experiments at the National Ignition Facility are presented, showing self-generated magnetic fields over 10^4 T. Angular high mode-number perturbations develop large magnetic fields, but are localized to the cold, dense hot-spot surface, which is hard to magnetize. When low-mode perturbations are also present, the magnetic fields are injected into the hot core, reaching significant magnetizations, with peak local thermal conductivity reductions greater than 90%. However, Righi-Leduc heat transport effectively cools the hot spot and lowers the neutron spectra-inferred ion temperatures compared to the unmagnetized case. The Nernst effect qualitatively changes the results by demagnetizing the hot-spot core, while increasing magnetizations at the edge and near regions of large heat loss

The effect of areal density asymmetries on scattered neutron spectra in ICF implosions

Scattered neutron spectroscopy is a diagnostic technique commonly used to measure areal density in inertial confinement fusion experiments. Deleterious areal density asymmetries modify the shape of the scattered neutron spectrum. In this work, a novel analysis is developed, which can be used to fit the shape change. This will allow experimental scattered neutron spectroscopy to directly infer the amplitude and mode of the areal density asymmetries, with little sensitivity to confounding factors that affect other diagnostics for areal density. The model is tested on spectra produced by a neutron transport calculation with both isotropic and anisotropic primary fusion neutron sources. Multiple lines of sight are required to infer the areal density distribution over the whole sphere—we investigate the error propagation and optimal detector arrangement associated with the inference of mode 1 asymmetries

Burn regimes in the hydrodynamic scaling of perturbed inertial confinement fusion hotspots

We present simulations of ignition and burn based on the Highfoot and high-density carbon indirect drive designs of the National Ignition Facility for three regimes of alpha-heating—self-heating, robust ignition and propagating burn—exploring hotspot power balance, perturbations and hydrodynamic scaling. A Monte-Carlo particle-in-cell charged particle transport package for the radiation-magnetohydrodynamics code Chimera was developed for this purpose, using a linked-list type data structure. The hotspot power balance between alpha-heating, electron thermal conduction and radiation was investigated in 1D for the three burn regimes. Stronger alpha-heating levels alter the hydrodynamics: sharper temperature and density gradients at hotspot edge; and increased hotspot pressures which further compress the shell, increase hotspot size and induce faster re-expansion. The impact of perturbations on this power balance is explored in 3D using a single Rayleigh–Taylor spike. Heat flow into the perturbation from thermal conduction and alpha-heating increases by factors of , due to sharper temperature gradients and increased proximity of the cold, dense material to the main fusion regions respectively. The radiative contribution remains largely unaffected in magnitude. Hydrodynamic scaling with capsule size and laser energy of different perturbation scenarios (a short-wavelength multi-mode and a long-wavelength radiation asymmetry) is explored in 3D, demonstrating the differing hydrodynamic evolution of the three alpha-heating regimes. The multi-mode yield increases faster with scale factor due to more synchronous compression producing higher temperatures and densities, and therefore stronger bootstrapping of alpha-heating. The perturbed implosions exhibit differences in hydrodynamic evolution due to alpha-heating in addition to the 1D effects, including: reduced perturbation growth due to ablation from both fire-polishing and stronger thermal conduction; and faster re-expansion into regions of weak confinement, which can result in loss of confinement

Perturbation modifications by pre-magnetisation of inertial confinement fusion implosions

Pre-magnetisation of inertial confinement fusion implosions on the National Ignition Facility has the potential to raise current high-performing targets into the ignition regime [Perkins et al. "The potential of imposed magnetic fields for enhancing ignition probability and fusion energy yield in indirect-drive inertial confinement fusion," Phys. Plasmas 24, 062708 (2017)]. A key concern with this method is that the application of a magnetic field inherently increases asymmetry. This paper uses 3-D extended-magnetohydrodynamics Gorgon simulations to investigate how thermal conduction suppression, the Lorentz force, and α-particle magnetisation affect three hot-spot perturbation scenarios: a cold fuel spike, a time-dependent radiation drive asymmetry, and a multi-mode perturbation. For moderate magnetisations (B0 = 5 T), the single spike penetrates deeper into the hot-spot, as thermal ablative stabilisation is reduced. However, at higher magnetisations (B0 = 50 T), magnetic tension acts to stabilise the spike. While magnetisation of α-particle orbits increases the peak hot-spot temperature, no impact on the perturbation penetration depth is observed. The P4-dominated radiation drive asymmetry demonstrates the anisotropic nature of the thermal ablative stabilisation modifications, with perturbations perpendicular to the magnetic field penetrating deeper and perturbations parallel to the field being preferentially stabilised by increased heat-flows. Moderate magnetisations also increase the prevalence of high modes, while magnetic tension reduces vorticity at the hot-spot edge for larger magnetisations. For a simulated high-foot experiment, the yield doubles through the application of a 50 T magnetic field-an amplification which is expected to be larger for higher-performing configurations

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 (i) to learn to control and mitigate low-mode asymmetries and (ii) 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 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 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