Implosion Asymmetry Research Articles

Enhanced sensitivity to target offset when using cross-beam energy transfer mitigation techniques in direct-drive inertial confinement fusion implosions

In direct-drive inertial confinement fusion, target offset from the target chamber center (or center of beam convergence) may lead to significant implosion asymmetry and fusion yield degradation. In addition, cross-beam energy transfer (CBET) has been shown to be a significant source of laser energy scattering and leads to a reduction in implosion velocity and yield. To improve energy coupling and implosion performance, several techniques for CBET mitigation have been proposed. Recent simulations, however, have shown that CBET also substantially mitigates the effect of target offset on implosion asymmetry and yield [Anderson et al., Phys. Plasmas 27, 112713 (2020)]. Furthermore, the inclusion of CBET models in radiation-hydrodynamics codes was shown to greatly improve agreement between simulations and experiments involving substantial target offset distances. This paper explores the intensity dependence of this CBET–offset effect. In addition, it is shown that enhanced sensitivity to target offset can be expected when CBET-mitigation techniques are used in direct-drive implosions. This is shown through simulations of two such CBET-mitigation techniques on the OMEGA laser: (1) decreased beam-to-target radius, and (2) beam-to-beam frequency detuning. For the typical target offset distances (<15 μm) observed in experiments on OMEGA, however, overall yield is still anticipated to be substantially higher when CBET-mitigation techniques are employed.

Basic principles and optical system design of 17.48keV high-throughput modified Wolter x-ray microscope.

High-precision x-ray imaging diagnostics of hotspot at the stagnation stage are essential for regulating implosion asymmetry and retrieving physical implosion parameters. With regard to 10-20keV energy band imaging, existing diagnostic instruments such as Kirkpatrick-Baez microscopes and pinhole cameras are insufficient in terms of spatial resolution and collection efficiency. The situation is even worse when high-speed, time-resolved imaging diagnostics are performed by coupling framing cameras or line-of-sight imagers. This article presents the basic principles and optical system design of a 17.48keV modified Wolter x-ray microscope, to resolve the problems encountered in high-energy imaging diagnostics. The proposed optical configuration offers a better spatial resolution, greater depth of field, and preliminary compliance with the requirements of high precision optical processing techniques. The spatial resolution is better than 1 µm in a field range ±150 µm, and is better than 3 µm in a total field of view ∼408 µm in diameter. The geometric solid angle is calculated as 3.0 × 10-5 sr and is estimated to be 1.2 × 10-6 sr, considering the reflectivity of the double mirrors. The proposed microscope is expected to effectively improve spatial resolution and signal-to-noise ratio for high-energy imaging diagnostics.

Double cylinder target design for study of hydrodynamic instabilities in multi-shell ICF

Cylindrical implosions are used to study hydrodynamic instability growth for inertial confinement fusion (ICF) applications, as the cylindrical geometry allows for easier diagnostic access while retaining convergence effects. In this work, we use the established cylindrical implosion platform [Palaniyappan et al., Phys. Plasmas 27, 042708 (2020)] to inform the double shell ICF campaign [Montgomery et al., Phys. Plasmas 25, 092706 (2018)]. We present a design for a double cylindrical target as an analogue to the double shell ICF capsule in order to study hydrodynamic instability growth on the high-Z inner shell. Our design work is done with two-dimensional (2D) Eulerian radiation-hydrodynamics simulations, considering the axial uniformity of the implosion and feasibility of measuring the instability growth of pre-seeded single mode sinusoidal perturbations. We discuss in depth the design for a target to be directly driven at the OMEGA laser facility [Boehly et al., Opt. Commun. 133, 495 (1997)]. We evaluate the design for axial implosion symmetry and visibility of instability growth using synthetic radiographs constructed from the simulations, as the instability growth on the inner cylinder is experimentally measured using x-ray radiography of the implosion. We find that the seeded perturbation growth on the inner cylinder should be visible in an experiment, even with axial implosion asymmetry and preheat. We compare our 2D simulations with linear theory predictions for perturbation growth and show that a cylinder with lower azimuthal mode number (mode-20) perturbations compares more favorably with linear theory, while a cylinder with higher azimuthal mode number (mode-40) perturbations at the same starting amplitude saturates and is over-predicted by linear theory.

Interpolating individual line-of-sight neutron spectrometer measurements onto the "sky" at the National Ignition Facility (NIF).

Nuclear diagnostics provide measurements of inertial confinement fusion implosions used as metrics of performance for the shot. The interpretation of these measurements for shots with low mode asymmetries requires a way of combining the data to produce a "sky map" where the individual line-of-sight values are used to interpolate to other positions in the sky. These interpolations can provide information regarding the orientation of the low mode asymmetries. We describe the interpolation method, associated uncertainties, and correlations between different metrics, e.g., Tion, down scatter ratio, and hot-spot velocity direction. This work is also related to recently reported studies [H. G. Rinderknecht et al., Phys. Rev. Lett. 124, 145002 (2020) and K. M. Woo et al., Phys. Plasmas 27, 062702 (2020)] of low mode asymmetries. We report an analysis that makes use of a newly commissioned line of sight, a scheme for incorporating multiple neutron spectrum measurement types, and recent work on the sources of implosion asymmetry to provide a more complete picture of implosion performance.

Effect of cross-beam energy transfer on target-offset asymmetry in direct-drive inertial confinement fusion implosions

The unintentional mispositioning of inertial confinement fusion (ICF) capsules from the center of laser beam convergence has long been shown in simulations to generate large ℓ=1 asymmetry and significantly degrade implosion symmetry and fusion yields. Experimental yields on the OMEGA laser system, however, have shown much less sensitivity to this initial target offset. This paper presents simulations of offset ICF implosions improved by including a physics model of cross-beam energy transfer (CBET), a mechanism of laser energy scattering from one beam to another. Room-temperature OMEGA implosion experiments with prescribed target offsets are simulated with and without CBET, illustrating that CBET mitigates the ℓ=1 implosion asymmetry from the target offset. Comparison of simulations to multiple complementary experimental observables indicates that the addition of CBET physics in offset simulations is necessary to match experimental results.

Study of the asymmetry of hot-spot self-emission imaging of inertial confinement fusion implosion driven by high-power laser facilities

Implosion asymmetry is a crucial problem quenching ignition in the field of inertial confinement fusion. A forward-calculation method based on 1D and 2D hydrodynamic simulations has been developed to generate and study the x-ray images of hot-spot self-emission, indicating asymmetry integrated over the entire drive pulse. It is shown that the x-ray imaging photon energy should be higher to avoid the influence of the remaining shell. The contour level (percentage of the maximum emission intensity) and spatial resolution should be as low as possible, optimally less than 20% and 3 µm, for characterization of higher-mode signatures such as P8–P12 by x-ray self-emission images. On the contrary, signatures of lower-mode such as P2 remain clear at all contour levels and spatial resolutions. These key results can help determine the optimal diagnostics, laser, and target parameters for implosion experiments. Recent typical hot-spot asymmetry measurements and applications on the Shenguang 100 kJ class laser facility are also reported.

Azimuthal Drive Asymmetry in Inertial Confinement Fusion Implosions on the National Ignition Facility.

Data from nuclear diagnostics present correlated signatures of azimuthal implosion asymmetry in recent indirect-drive inertial confinement fusion (ICF) implosion campaigns performed at the National Ignition Facility (NIF). The mean hot-spot velocity, inferred from the Doppler shift of 14MeV neutrons produced by deuterium-tritium (DT) fusion, is systematically directed toward one azimuthal half of the NIF target chamber, centered on ϕ≈70°. Areal density (ρR) asymmetry of the converged DT fuel, inferred from nuclear activation diagnostics, presents a minimum ρR in the same direction as the hot-spot velocity and with ΔρR amplitude correlated with velocity magnitude. These two correlated observations, which are seen in all recent campaigns with cryogenic layers of DT fuel, are a known signature of asymmetry in the fuel convergence, implying a systematic azimuthal drive asymmetry across a wide range of shot and target configurations. The direction of the implied radiation asymmetry is observed to cluster toward the hohlraum diagnostic windows. This low-mode asymmetry degrades hot-spot conditions at peak convergence and limits implosion performance and yield.

Radiation and electron thermal conduction damping of acoustic perturbations in igniting deuterium–tritium gas

We derive a dispersion relation for the damping of acoustic waves in equi-molar deuterium–tritium (DT) gas due to radiation coupling and electron thermal conduction and discuss its significance for inertial confinement fusion (ICF) targets with high-Z shells surrounding a central DT fuel region. As the shell implodes around DT fuel in such a target, shocks and waves are transmitted through the DT gas. If the shell is perturbed due to drive non-uniformity or manufacturing imperfection, these shocks and waves may be perturbed as well, and can potentially re-perturb the shell. This can complicate calculation of shell stability and implosion asymmetry and in general make the target less robust against implosion non-uniformity. Damping of perturbations in DT gas can alleviate these complications. Also, damping of low-order modes, which is primarily due to radiation coupling, can drive the DT gas to an isobaric and isothermal ‘equilibrium’ configuration during ignition. We find that for the range of common ignition temperatures in targets with high-Z shells, $2.5\lesssim T_{ig}\lesssim 3.5$ keV, damping of low-order modes is significant for areal densities ( $\unicode[STIX]{x1D70C}r$ ) in the broad range of $0.6\lesssim \unicode[STIX]{x1D70C}r\lesssim 1.8~\text{g}~\text{cm}^{-2}$ . This suggests it is advantageous to design these targets to achieve areal densities at ignition within this range. Furthermore, we derive a simple constraint between areal density and temperature, $\unicode[STIX]{x1D70C}r=0.34T_{o}$ where $T_{o}$ is in keV, such that DT gas undergoing equilibrium ignition is optimally robust against non-uniformity.

Interpreting the electron temperature inferred from x-ray continuum emission for direct-drive inertial confinement fusion implosions on OMEGA

In this paper, we present a theoretical framework for interpreting the hot-spot electron temperature (Te) inferred from hard (10- to 20-keV) x-ray continuum emission for inertial confinement fusion implosions on OMEGA. We first show that the inferred Te represents the emission-weighted, harmonic mean of the hot-spot Te distribution, both spatially and temporally. A scheme is then provided for selecting a photon energy of which the emission weighting approximates neutron weighting. Simulations are then used to quantify the predicted relationship between the inferred Te, neutron-weighted Ti, and implosion performance on OMEGA. In an ensemble of 1-D simulations, it was observed that hot-spot thermal nonequilibrium precluded a sufficiently unique mapping between the inferred Te and neutron-weighted Ti. The inferred Te and hard x-ray yield's sensitivity to implosion asymmetry was studied using a 3-D simulation case study with low-harmonic-mode perturbations (i.e., laser beam power imbalance, target offset, and beam port geometry departures from spherical symmetry) and laser imprint (lmax = 200).

Development of a polar-view Kirkpatrick-Baez X-ray microscope for implosion asymmetry studies.

The development of a polar-view Kirkpatrick-Baez microscope, fielded in the upper polar zone of the Shenguang-III laser fusion facility, is presented. With this microscope, the resolving power of polar-direction X-ray imaging diagnostics is improved, to the 3 ~5 μm scale. The microscope is designed for implosion asymmetry studies, with response energy points at 1.2 keV, 3.5 keV, and 8 keV. A biperiodic multilayer scheme is adopted to accommodate multiple implosion stages. We present the overall optical system design, target aiming scheme, characteristic composite imaging diagnostic experiments and initial results. The inertial-driven quasi-one-dimensional spherical implosions were observed from orthogonal directions with a convergence ratio of ~14.4. Fine features of the stagnating hot spot core are also resolved.

Effects of asymmetry and hot-spot shape on ignition capsules

Asymmetric implosion of inertial confinement fusion capsules is known, both experimentally and computationally, to reduce thermonuclear performance. This work shows that low-mode asymmetries degrade performance as a result of a decrease in the hydrodynamic disassembly time of the hot-spot core, which scales with the minimum dimension of the hot spot. The asymmetric shape of a hot spot results in decreased temperatures and areal densities and allows more alpha particles to escape, relative to an ideal spherical implosion, thus reducing alpha-energy deposition in the hot spot. Here, we extend previous ignition theory to include the hot-spot shape and quantify the effects of implosion asymmetry on both the ignition criterion and the capsule performance. The ignition criterion becomes more stringent with increasing deformation of the hot spot. The new theoretical results are validated by comparison with existing experimental data obtained at the National Ignition Facility. The shape effects on thermonuclear performance are relatively more noticeable for capsules having self-heating and high yields. The degradation of thermonuclear burn can be as high as 45% for shots with a yield lower than 2×10^{15} and less than 30% for shots with a higher yield.

Observation of hydrodynamic phenomena of plasma interaction in hohlraums

In indirect-drive inertial confinement fusion (ICF), laser beams are injected into a high-Z hohlraum and the laser energy is converted into intense X-ray radiation, which ablates a capsule located in the center of the hohlraum, and thus making it implode. To achieve high implosion efficiency, it is required that the hohlraum inner wall plasma movement, which will block further laser injection through the laser entrance hole (LEH), be suppressed. Evolution of hohlraum radiation nonuniformity caused by the plasma movement will result in implosion asymmetry which will prevent the ignition from happening. Therefore it is very important to study the hydrodynamic movement of high-Z plasma in ICF experiment.<br/>In ICF hohlraum, various plasmas of laser spots, corona, radiation ablation and jets move in different ways driven by laser ablation and X-ray radiation ablation, which is hard to observe and study. An X-ray dual spectral band time-resolved imaging method is developed to clearly observe the motion of various plasmas in hohlraum. Based on the time-resolved X-ray framing camera, using the typical gold plasma emission spectrum, the gold microstrip MCP response spectrum, and the 1.5 μm Al or 3 μm Ti filter transmittance spectrum, the two narrow-band X-ray peaks at 0.8 keV and 2.5 keV are highlighted. The 0.8 keV X-ray shows the Planck spectrum of gold plasma, and 2.5 keV X-ray indicates the M-band of gold plasma.<br/>In the vacuum hohlraum, jets are observed clearly, which are verified to be 4 times the sound speed experimentally. The generation mechanism of gold plasma jets in the ICF hohlraum is mainly due to collision rather than magnetic field, because it is estimated that thermal pressure is much bigger than magnetic pressure. In the gas-filled hohlraum, low-Z C<sub>5</sub>H<sub>12</sub> gas can effectively eliminate high-Z gold jets and suppress the high-Z gold coronal plasma movement. The interface between the low-Z and high-Z substance is observed clearly, and gold plasma is accumulated obviously in the later period at the interface. Moreover, spike and filamentous structure occur at the interface between the two substances, which is probably caused by the hydrodynamic instability. The 0.8 keV rather than 2.5 keV X-ray is observed around inner wall, which originates from the low-temperature plasma driven by radiation ablation and is predicted by simulation code. Furthermore, the pressure balance between the two substances and the density steepness at the interface are also analyzed.

Influence of Au M-band flux asymmetry on implosion symmetry

AbstractIn indirect-drive inertial confinement fusion, the radiation symmetry must be controlled for the achievement of hotspot ignition. The radiation symmetry is of great importance. In this paper, we investigate the drive asymmetry of the M-band (2–5 keV) radiation emitted from an Au holhraum wall by using the three-dimensional view-factor code IRAD3D. Analysis of the M-band flux drive at the Shenguang-III laser facility shows that it is asymmetric and that the asymmetry varies with time. For a given cross section over the pole, the initial M-band flux asymmetries are P2 = 11.59, P4 = 1.41, and P6 = −0.64%. When the asymmetries are artificially added to a symmetric radiation drive, the position of the deuterium-tritium (DT) ice/gas interface is asymmetric for a National Ignition Facility capsule in 1D simulation. This means that M-band flux asymmetry can lead to implosion asymmetry even if the total radiation is symmetric. Pure CH and Si-doped CH capsules are considered. The results show that a mid-Z dopant can partly reduce the asymmetry. However, the asymmetry is still very large. Thus, it is necessary to study the M-band flux asymmetry and its influence on the implosion symmetry.

P2 asymmetry of Au's M-band flux and its smoothing effect due to high-Z ablator dopants

X-ray drive asymmetry is one of the main seeds of low-mode implosion asymmetry that blocks further improvement of the nuclear performance of “high-foot” experiments on the National Ignition Facility [Miller et al., Nucl. Fusion 44, S228 (2004)]. More particularly, the P2 asymmetry of Au's M-band flux can also severely influence the implosion performance of ignition capsules [Li et al., Phys. Plasmas 23, 072705 (2016)]. Here we study the smoothing effect of mid- and/or high-Z dopants in ablator on Au's M-band flux asymmetries, by modeling and comparing the implosion processes of a Ge-doped ignition capsule and a Si-doped one driven by X-ray sources with P2 M-band flux asymmetry. As the results, (1) mid- or high-Z dopants absorb hard X-rays (M-band flux) and re-emit isotropically, which helps to smooth the asymmetric M-band flux arriving at the ablation front, therefore reducing the P2 asymmetries of the imploding shell and hot spot; (2) the smoothing effect of Ge-dopant is more remarkable than Si-dopant because its opacity in Au's M-band is higher than the latter's; and (3) placing the doped layer at a larger radius in ablator is more efficient. Applying this effect may not be a main measure to reduce the low-mode implosion asymmetry, but might be of significance in some critical situations such as inertial confinement fusion (ICF) experiments very near the performance cliffs of asymmetric X-ray drives.

A hybrid-drive nonisobaric-ignition scheme for inertial confinement fusion

A new hybrid-drive (HD) nonisobaric ignition scheme of inertial confinement fusion (ICF) is proposed, in which a HD pressure to drive implosion dynamics increases via increasing density rather than temperature in the conventional indirect drive (ID) and direct drive (DD) approaches. In this HD (combination of ID and DD) scheme, an assembled target of a spherical hohlraum and a layered deuterium-tritium capsule inside is used. The ID lasers first drive the shock to perform a spherical symmetry implosion and produce a large-scale corona plasma. Then, the DD lasers, whose critical surface in ID corona plasma is far from the radiation ablation front, drive a supersonic electron thermal wave, which slows down to a high-pressure electron compression wave, like a snowplow, piling up the corona plasma into high density and forming a HD pressurized plateau with a large width. The HD pressure is several times the conventional ID and DD ablation pressure and launches an enhanced precursor shock and a continuous compression wave, which give rise to the HD capsule implosion dynamics in a large implosion velocity. The hydrodynamic instabilities at imploding capsule interfaces are suppressed, and the continuous HD compression wave provides main pdV work large enough to hotspot, resulting in the HD nonisobaric ignition. The ignition condition and target design based on this scheme are given theoretically and by numerical simulations. It shows that the novel scheme can significantly suppress implosion asymmetry and hydrodynamic instabilities of current isobaric hotspot ignition design, and a high-gain ICF is promising.

Three-dimensional simulations of National Ignition Facility implosions: Insight into experimental observablesa)

We simulate in 3D both the hydrodynamics and, simultaneously, the X-ray and neutron diagnostic signatures of National Ignition Facility (NIF) implosions. We apply asymmetric radiation drive to study the impact of low mode asymmetry on diagnostic observables. We examine X-ray and neutron images as well as neutron spectra for these perturbed implosions. The X-ray images show hot spot evolution on small length scales and short time scales, reflecting the incomplete stagnation seen in the simulation. The neutron images show surprising differences from the X-ray images. The neutron spectra provide additional measures of implosion asymmetry. Flow in the hot spot alters the neutron spectral peak, namely, the peak location and width. The changes in the width lead to a variation in the apparent temperature with viewing angle that signals underlying hot spot asymmetry. We compare our new expectations based on the simulated data with NIF data. We find that some recent cryogenic layered experiments show appreciable temperature anisotropy indicating residual flow in the hot spot. We also find some trends in the data that do not reflect our simulation and theoretical understanding.

Direct-drive cryogenic-target implosion experiments on SGIII prototype laser facility

Directly driven cryogenic target implosion experiments are performed on the SGIII prototype laser facility. X-ray pinhole images reveal frozen condensation on the sealing film. The influence of the condensation on the delivery of laser energy to the capsule surface is then quantified experimentally. It is found that, with a carefully chosen pre-pulse duration, the influence can be reduced, and the neutron yield is increased by an order of magnitude. Subsequently, the cryogenic layered capsule and cryogenic gas-filled capsule are imploded using 6.5-kJ laser energy. The implosion performance is characterized by the neutron yield, the 2D self-emission images of the in-flight shell, and the primary proton spectrum. The neutron yield is 2 × 107 for the gas-filled capsule and 2.8 × 107 for the layered capsule. The 2D self-emission images of the in-flight shell exhibit significant implosion asymmetry. The energy downshift of the proton spectrum is used to infer the areal density. For the gas-filled capsule, the spectrum is downshifted by 0.1 MeV, yielding an areal density of 1–3 mg/cm2. For the layered capsule, the spectrum is downshifted by 0.5 MeV, yielding an areal density of 4–6 mg/cm2. Improving the implosion symmetry would help to further increase the areal density.

Designing symmetric polar direct drive implosions on the Omega laser facility

Achieving symmetric capsule implosions with Polar Direct Drive [S. Skupsky et al., Phys. Plasmas 11, 2763 (2004); R. S. Craxton et al., Phys. Plasmas 12, 056304 (2005); F. J. Marshall et al., J. Phys. IV France 133, 153–157 (2006)] has been explored during recent Defect Induced Mix Experiment campaign on the Omega facility at the Laboratory for Laser Energetics. To minimize the implosion asymmetry due to laser drive, optimized laser cone powers, as well as improved beam pointings, were designed using 3D radiation-hydrodynamics code HYDRA [M. M. Marinak et al., Phys. Plasmas 3, 2070 (1996)]. Experimental back-lit radiographic and self-emission images revealed improved polar symmetry and increased neutron yield which were in good agreement with 2D HYDRA simulations. In particular, by reducing the energy in Omega's 21.4° polar rings by 16.75%, while increasing the energy in the 58.9° equatorial rings by 8.25% in such a way as to keep the overall energy to the target at 16 kJ, the second Legendre mode (P2) was reduced by a factor of 2, to less than 4% at bang time. At the same time the neutron yield increased by 62%. The polar symmetry was also improved relative to nominal DIME settings by a more radical repointing of OMEGA's 42.0° and 58.9° degree beams, to compensate for oblique incidence and reduced absorption at the equator, resulting in virtually no P2 around bang time and 33% more yield.

Variations of implosion asymmetry with hohlraum length and time in indirect-drive inertial confinement fusion

In order to achieve the high density compression in laser indirect-drive inertial confinement fusion, the implosion symmetry and hohlraum radiation uniformity are strictly required. To study the variations of implosion asymmetry with hohlraum length and time, three kinds of hohlraum lengths are adopted in experiment. X-ray emission from capsule fuel is measured by an X-ray framing camera. Based on measured capsule compression process and ellipticity variation, it is preliminarily judged that the medium hohlraum of 1700 μm long is the closest to implosion symmetry demand of Shenguang Ⅲ prototype laser facility. Time-resolved implosion asymmetry is derived from a simplified analytic model, in which used is the time-resolved hohlraum radiation nonuniformity derived from a view-factor code. The derived results of the time-resolved implosion asymmetry are basically in agreement with experimental results. The physical mechanism for how hohlraum radiation nonuniformity evolution induces the variations of implosion asymmetry with hohlraum length and time is analyzed.

Direct drive implosion experiments on SGIII prototype laser facility: Assessing energy coupling efficiency and implosion symmetry

Direct drive implosion experiments were conducted on SGIII prototype laser facility. From the time resolved x-ray images, the bright ring and the central bright spot are observed. The radial velocity of the convergent bright ring indicates the shell velocity, and the times when the central bright spot is first seen and becomes most intensive indicate the times of shock convergence and later stagnation, respectively. Radiation hydrodynamic simulations were carried out by changing laser energy deposition factors. When the simulated results are brought close to the measured ones, it is found that the energy coupling efficiency is around 70%. The implosion symmetry is indicated by the core x-ray emission pattern which is pancake when viewing from the equator, and splits into several bright spots when viewing close to the pole. A simple model is developed to understand this asymmetry. It is speculated that the observed implosion asymmetry can be attributed to the laser arrangement which is originally designed for indirect drive experiments. Further improvements of energy coupling efficiency and implosion symmetry in future experiments can be achieved by optimizing target design and laser arrangement.