Double-shell Target Research Articles

Bayesian batch optimization for molybdenum versus tungsten inertial confinement fusion double shell target design

AbstractAccess to reliable, clean energy sources is a major concern for national security. Much research is focused on the “grand challenge” of producing energy via controlled fusion reactions in a laboratory setting. For fusion experiments, specifically inertial confinement fusion (ICF), to produce sufficient energy, the fusion reactions in the ICF fuel need to become self‐sustaining and burn deuterium‐tritium (DT) fuel efficiently. The recent record‐breaking NIF ignition shot was able to achieve this goal as well as produce more energy than used to drive the experiment. This achievement brings self‐sustaining fusion‐based power systems closer than ever before, capable of providing humans with access to secure, renewable energy. In order to further progress toward the actualization of such power systems, more ICF experiments need to be conducted at large laser facilities such as the United States's National Ignition Facility (NIF) or France's Laser Mega‐Joule. The high cost per shot and limited number of shots that are possible per year make it prohibitive to perform large numbers of experiments. As such, experimental design relies heavily on complex predictive physics simulations for high‐fidelity “preshot” analysis. These multidimensional, multi‐physics, high‐fidelity simulations have to account for a variety of input parameters as well as modeling the extreme conditions (pressures and densities) present at ignition. Such simulations (especially in 3D) can become computationally prohibitive to turn around for each ICF experiment. In this work, we explore using Bayesian optimization with Gaussian processes (GPs) to find optimal designs for ICF double shell targets, while keeping computational costs to manageable levels. These double shell targets have an inner shell that grades from beryllium on the outer surface to the higher Z material molybdenum, as opposed to the nominally used tungsten, on the inside in order to trade off between the high performance associated with high density inner shells and capsule stability. We describe our results for “capsule‐only” xRAGE simulations to study the physics between different capsule designs, inner shell materials, and potential for future experiments.

Outer shell symmetry for double shell capsules with aluminum ablators

Double shell targets are a promising potential avenue to obtain robust neutron yield at current laser facilities. Similar to single shell designs, double shells require the symmetric implosion of an ablator in order to uniformly compress and heat a fuel volume, with the goal of achieving thermonuclear burn. Significant differences between double and single shells include the usage of an aluminum ablator as well as a reverse ramp laser pulse. In addition, double shells require a different convergence than single shells for fuel ignition. Numerical implosion studies at various energies with comparisons to experimental outcomes are required to gain confidence that simulations can capture the ablator shape from subscale to full scale. The current work builds on previous implosion experiments conducted at 1-MJ laser energy to confirm achieved ablator symmetry at 1.25 and 1.5 MJ. Average ablator P2 and P4 shapes measured in these experiments are within 5% of the simulated shape, which merits the platforms for further experimental studies.

Demonstration of low-mode shape control in indirect-drive double shell implosions at the NIF

Double shell inertial confinement fusion is a concept for achieving robust thermonuclear burn that uses dense metal shells to compress deuterium-tritium (DT) fuel to fusion conditions. Double shell implosions are typically indirectly driven and involve a target that consists of a low-Z ablator, a foam layer, and a high-Z pusher surrounding the DT fuel. The goal of the campaign is to achieve a volumetric burn as radiation losses from the DT fuel are trapped by the opaque high-Z shell. The overall performance of double shell implosions relies on the efficient collisional transfer of kinetic energy between layers. The efficiency of this transfer (and therefore the overall performance of a given implosion) is degraded by the presence of low-mode asymmetries. P2 asymmetries are often observed in spatially resolved 2D radiographs of nominal double shell implosions. This work discusses three such experiments: one with an oblate P2 asymmetry, one with a prolate P2 asymmetry, and one with an approximate spherical symmetry. After performing a shape analysis of the oblate and prolate implosions to quantify asymmetries, these experimental results were compared with the results of hydrodynamic simulations for the two experiments. Differences between the experiment and simulation were then used to design an approximately spherical implosion by altering the incident laser cone fraction. Radiographs from the experiment that implemented the modified cone fraction show evidence of an implosion that is approximately spherical until bang time. This design is intended to serve as a point design for future studies that will seek to optimize various aspects of the double shell target.

Diagnosis of indirectly driven double shell targets with point-projection hard x-ray radiography

We present an application of short-pulse laser-generated hard x rays for the diagnosis of indirectly driven double shell targets. Cone-inserted double shell targets were imploded through an indirect drive approach on the upgraded SG-II laser facility. Then, based on the point-projection hard x-ray radiography technique, time-resolved radiography of the double shell targets, including that of their near-peak compression, were obtained. The backlighter source was created by the interactions of a high-intensity short pulsed laser with a metal microwire target. Images of the target near peak compression were obtained with an Au microwire. In addition, radiation hydrodynamic simulations were performed, and the target evolution obtained agrees well with the experimental results. Using the radiographic images, areal densities of the targets were evaluated.

Quantification of Thin Walls and Capsules Using X-Ray Computed Tomography

X-ray computed tomography (CT) is widely used in material science to understand the inner morphology of a specimen. Often, it is used to qualitatively understand the distribution of salient features such as cracks, voids, or particles. There are many challenges in using X-ray CT in a quantitative manner. These include a coarser resolution for comparable fields of view when compared to other imaging techniques (i.e., electron or optical microscopy), imaging artifacts (i.e., beam hardening and phase contrast), and the plethora of imaging and processing parameters that are chosen by the instrument/software user that can significantly affect the resultant measures. These limitations must be considered and quantified to acquire accurate and precise measurements. X-ray CT is powerful in that it can measure, in three dimensions, salient features that are subsurface and cannot be imaged with other direct line-of-sight imaging techniques. In this work, we discuss the use of X-ray CT to measure the thickness variations of thin walls of opacity capsules as well as the measurement of double-shell targets to understand the concentricity of the capsules within each other. Morphological measurements needed for target characterization require very high accuracy and precision. This paper will describe the application for the first time of a variety of measurements and will explore their robustness and pros and cons to identify areas of research to improve their accuracy and precision.

Machine learning for detection of 3D features using sparse x-ray tomographic reconstruction.

In many inertial confinement fusion (ICF) experiments, the neutron yield and other parameters cannot be completely accounted for with one and two dimensional models. This discrepancy suggests that there are three dimensional effects that may be significant. Sources of these effects include defects in the shells and defects in shell interfaces, the fill tube of the capsule, and the joint feature in double shell targets. Due to their ability to penetrate materials, x rays are used to capture the internal structure of objects. Methods such as computational tomography use x-ray radiographs from hundreds of projections, in order to reconstruct a three dimensional model of the object. In experimental environments, such as the National Ignition Facility and Omega-60, the availability of these views is scarce, and in many cases only consists of a single line of sight. Mathematical reconstruction of a 3D object from sparse views is an ill-posed inverse problem. These types of problems are typically solved by utilizing prior information. Neural networks have been used for the task of 3D reconstruction as they are capable of encoding and leveraging this prior information. We utilize half a dozen, different convolutional neural networks to produce different 3D representations of ICF implosions from the experimental data. Deep supervision is utilized to train a neural network to produce high-resolution reconstructions. These representations are used to track 3D features of the capsules, such as the ablator, inner shell, and the joint between shell hemispheres. Machine learning, supplemented by different priors, is a promising method for 3D reconstructions in ICF and x-ray radiography, in general.

Mechanisms of shape transfer and preheating in indirect-drive double shell collisions

Implosions of Hohlraum-driven double shell targets as an alternative inertial confinement fusion concept are underway at the National Ignition Facility. The double shell system relies on a series of energy transfer processes starting from thermal x-ray absorption by the outer shell, followed by collisional transfer of kinetic energy to a heavy metal inner shell, and finally, conversion to the internal energy of the deuterium-tritium fuel. During each of these energy transfer stages, low-mode asymmetries can act to reduce the ideal transfer efficiency degrading double shell performance. Mechanisms, such as hard x-ray preheat from the Hohlraum, not only decrease the efficiency of kinetic energy transfer but may also be a source of low-mode asymmetry. In this article, we evaluate the shape transfer processes through the time of shell collision using two-dimensional integrated Hohlraum and capsule computations. We find that the dominant mode of the shape transfer is well described using a “radial impulse” model from the shape of the foam pressure reservoir. To evaluate the importance of preheat on inner shell shape, we also report on first measurements of Au L-shell preheat asymmetry in a double shell with a tungsten pusher. These measurements showed a 65% higher preheat velocity at the pole of the capsule relative to the equator. We also found that the experiments provided rigorous constraints by which to test the Hohlraum model settings that impact the amount and symmetry of Au L-shell preheat via the plasma conditions inside the outer cone Au bubble.

Numerical performance assessment of double-shell targets for Z-pinch dynamic hohlraum

A Z-pinch dynamic hohlraum can create the high-temperature radiation field required by indirect-drive inertial confinement fusion. A dynamic hohlraum with peak radiation temperature over 300 eV can be obtained with a >50 MA Z-pinch driver according to the scaling law of dynamic hohlraum radiation temperature vs drive current. Based on a uniform 300 eV radiation temperature profile with a width of 10 ns, three double-shell capsules with radii of 2, 2.5, and 3 mm are proposed, and the corresponding fusion yields from a one-dimensional calculation are 28.8, 56.1, and 101.6 MJ. The implosion dynamics of the 2.5 mm-radius capsule is investigated in detail. At ignition, the areal density of the fuel is about 0.53 g/cm2, the fuel pressure is about 80 Gbar, and the central ion temperature is about 4.5 keV, according to the one-dimensional simulation. A two-dimensional simulation indicates that the double-shell capsule can implode nearly spherically when driven by the radiation field of a Z-pinch dynamic hohlraum. The sensitivities of the fusion performance to the radiation temperature profiles and to deviations in the capsule parameter are investigated through one-dimensional simulation, and it is found that the capsule fusion yields are rather stable in a quite large parameter space. A one-dimensional simulation of a capsule embedded in 50 mg/cm3 CH foam indicates that the capsule performance does not change greatly in the mimicked environment of a Z-pinch dynamic hohlraum. The double-shell capsules designed here are also applicable to laser indirect-drive inertial fusion, if a laser facility can produce a uniform 300 eV radiation field and sustain it for about 10 ns.

Single and double shell ignition targets for the national ignition facility at 527 nm

Converting and using the National Ignition Facility (NIF) to deliver 527 nm light instead of its current 351 nm would allow the laser to deliver more energy and power to ignition targets. We update previous 527 nm target design work to reflect more contemporary target designs using high-density carbon capsules and low density helium gas filled Hohlraums. We extend single shell capsule designs based on current experimental results to higher energy and power and also explore double shell capsules, both driven by green light. These studies were completed using detailed pulse shapes found for targets that converged with acceptable 2D implosion symmetries and then used the Lava Lamp II code to confirm their feasibility at NIF. A 1.2× dimensional scaleup of one tuned NIF target at the limit of its current 351 nm capabilities and shot 170827 uses 3.3 MJ, at the limit of the current NIF's 527 nm capability. With the less-structured pulse of a double shell target, 3.7 MJ could be delivered by the laser. Our LPI calculations do not preclude operation at 527 nm, particularly for low fill Hohlraums, and suggest that the stimulated Raman backscatter may be no worse than the small quantities seen in 170827; stimulated forward Raman scattering may be present. If Stimulated Brillouin Scattering is too great, the much greater laser bandwidth available at 527 nm could be used to decrease backscatter. These larger targets with higher energy and power may offer a better chance of achieving ignition with only modest changes to the NIF laser.

Constraining computational modeling of indirect drive double shell capsule implosions using experiments

Double shell capsule implosions are an alternative approach to achieving alpha heating on the National Ignition Facility. Current machining techniques construct the outer shell as two hemispheres that are glued together, and the deuterium and tritium (DT) liquid inside the inner shell will be injected by a fill tube. These features introduce asymmetries and jetting that may disrupt the confinement of the DT fuel if not carefully controlled. Simulations indicate that in order to achieve high yields in the laboratory, these features as well as susceptibility to the Rayleigh–Taylor instability (RTI) must be mitigated. Due to uncertainties in computational models and the expense of using the best physics models at adequate resolution in three dimensions, our computational modeling must be constrained by experiments. We report on the results of recent hydrogrowth radiography and dual-axis keyhole experiments with double shell targets that have been used to evaluate our modeling of the outer shell joint as well as the impacts of high-energy x-ray preheat that strongly impacts RTI growth. Our simulations show good agreement with the experimental data and inform several important modeling choices.

A theoretical model for radiation heat conduction between deuterium–tritium fuel and a high-Z pusher in volume ignition in inertial confinement fusion

Volume ignition, in which fuel is usually surrounded by pushers made from high-Z materials, is an approach to inertial confinement fusion. High-Z pushers have high albedos, which means that they can re-radiate most of the radiation coming from the hot fuel. Hence, it is beneficial to realize ignition at lower temperatures. We develop a theoretical model comprising a set of coupled equations to model the radiation heat conduction between the hot fuel and the high-Z pusher. We then investigate the dependence of the albedo of the high-Z pusher on the state of the hot fuel. The results of our model show good consistency with numerical simulations. Furthermore, the physics of the radiation heat conduction between the hot fuel and the high-Z pusher in volume ignition is elucidated, which is important for follow-up studies of the critical values for volume ignition and for obtaining a physical picture of fusion burning. The model can also provide some theoretical basis for the design of double-shell targets and energy targets.

Microstructure and morphology evolution of high-Z/low-Z gradient coatings on PAMS microspheres

Gradient coatings exhibit excellent implosion performance in double-shell targets for Inertial Confinement Fusion (ICF). Here, we present the controllable preparation of high-Z/low-Z (Z is the atomic number) gradient alloy coatings which can be used for ICF double-shell targets. Tungsten/carbon (W/C), tungsten/silicon (W/Si) and tungsten/aluminum (W/Al) gradient coatings were successfully deposited on silicon substrates and poly-alpha-methyl-styrene (PAMS) spheres by direct-current magnetron sputtering. The morphology, component and surface roughness of gradient coatings were characterized. Although there are significant differences in morphology and surface roughness compared with planar gradient coatings, the spherical gradient coatings still have a dense microstructure and smooth surface. The root-mean-square (RMS) roughness of our gradient coatings experience an improvement compared with the previous films which used in ICF experiments. The gradient coatings deposited by co-sputtering of high-Z and low-Z elements in this work provide competitive candidate materials for ICF double-shell targetsee.

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 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.

Experimental investigation of Z-pinch radiation source for indirect drive inertial confinement fusion

Z-pinch dynamic hohlraums (ZPDHs) could potentially be used to drive inertial confinement fusion targets. Double- or multishell capsules using the technique of volume ignition could exploit the advantages of ZPDHs while tolerating their radiation asymmetry, which would be unacceptable for a central ignition target. In this paper, we review research on Z-pinch implosions and ZPDHs for indirect drive targets at the Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics. The characteristics of double-shell targets and the associated technical requirements are analyzed through a one-dimensional computer code developed from MULTI-IFE. Some key issues regarding the establishment of suitable sources for dynamic hohlraums are introduced, such as soft X-ray power optimization, novel methods for plasma profile modulation, and the use of thin-shell liner implosions to inhibit the generation of prior-stagnated plasma. Finally, shock propagation and radiation characteristics in a ZPDH are presented and discussed, together with some plans for future work.

Radiography of direct drive double shell targets with hard x-rays generated by a short pulse laser

Direct drive implosion experiments of a cone inserted double shell target were carried out at the SG-II Upgrade laser facility. Time-resolved radiographic images of the targets were obtained with hard x-rays generated by irradiating a short-pulse laser on a metal microwire embedded in a low-Z substrate. Since the SG-II Upgrade laser facility is mainly designed for indirect drive, it will lead to high direct illumination nonuniformity of the target surface. 2D nonuniformities of the inner shell were clearly shown by these images. Simple analytical estimations and radiation hydrodynamic simulations were also carried out, and calculations were consistent with the experimental results. Besides, areal density of the target was evaluated using the radiography images. Furthermore, it was found that the nanosecond laser had effects on hard x-ray generation, and the possibilities for improving the experimental results are discussed, which will be helpful to better understand the hydrodynamics of double shell or even triple shell target implosions.

Enhanced energy coupling for indirectly driven inertial confinement fusion

Recent experiments in the study of inertial confinement fusion (ICF) at the National Ignition Facility (NIF) in the United States have reached the so-called alpha-heating regime1–3, in which the self-heating by fusion products becomes dominant, with neutron yields now exceeding 1 × 1016 (ref. 4) However, there are still challenges on the path towards ignition, such as minimization of the drive asymmetry, suppression of laser-plasma instabilities, and mitigation of fabrication features5. In addition, in the current cylindrical-hohlraum indirect drive schemes for ICF, a strong limitation is the inefficient (≤10%) absorption of the laser-produced hohlraum X-rays by the capsule as set by relative capsule-to-hohlraum surface areas. Here we report an experiment demonstrating ~30% energy coupling to an aluminium capsule in a rugby-shaped6, gold hohlraum. This high coupling efficiency can substantially increase the tolerance to residual imperfections and improve the prospects for ignition, both in mainline single-shell hot-spot designs and potential double-shell targets. High coupling efficiency between laser-induced hohlraum X-rays and targets is essential for reaching long-sought regimes for viable inertial confinement fusion. Experiments with a rugby hohlraum shape and an improved capsule now allow demonstration of more than 30%.

Gradient coating for NIF double shell targets

As opposed to a single shell, the double shell target design provides an alternative implosion platform for high energy density (HED) experiments at the National Ignition Facility (NIF) and related laboratories. The inner shell of this target scheme incorporates a density graded layer to suppress inhomogeneity induced Rayleigh-Taylor instabilities during the implosion. Here we report our efforts to fabricate density graded layers for use as the inner shell of a double shell target using magnetron sputtering as well as the characterization of these layers. Cold welding (spherical targets sticking to either each other or the deposition pan) was observed for many of metals surveyed and seems to be correlated with material ductility. A W-Be gradient layer was successfully fabricated as an inner shell. This material combination is advantageous due to the constituent's large inherent density difference. Microstructural changes are revealed with varying composition including a previously unknown amorphous phase.

Progress Toward Fabrication of Machined Metal Shells for the First Double-Shell Implosions at the National Ignition Facility

The double-shell platform fielded at the National Ignition Facility requires developments in new machining techniques and robotic assembly stations to meet the experimental specifications. Current double-shell target designs use a dense high-Z inner shell, a foam cushion, and a low-Z outer shell. The design requires that the inner shell be gas filled using a fill tube. This tube impacts the entire machining and assembly design. Other intermediate physics designs have to be fielded to answer physics questions and advance the technology to be able to fabricate the full point design in the near future. One of these intermediate designs is a mid-Z imaging design. The methods of designing, fabricating, and characterizing each of the major components of an imaging double shell are discussed with an emphasis on the fabrication of the machined outer metal shell.

Progress in Developing Novel Double-Shell Metal Targets Via Magnetron Sputtering

Double-shell inertial confinement fusion targets represent a unique platform for achieving ignition. They consist of a low-Z outer ablator, a high-Z inner pusher layer, and a low-density foam layer sandwiched in between. There is the possibility that double-shell targets may achieve ignition at lower ion temperatures due to the containment of radiation and conduction losses as well as requiring smaller convergence ratios. We have explored using magnetron sputtering to make the inner high-Z pusher layers and have demonstrated a W-Cr bilayer inner-shell design. An Al-Be mixture was explored as one of the outer ablator materials. This material takes advantage of Al X-ray M-band absorption to reduce preheating and still retain Be high-ablation speeds. Typical commercial Al-Be materials suffer from phase separation. However, by using magnetron sputtering we have been able to demonstrate homogeneous Al-Be ablator coatings. The sputtered material forms with nanosized grains and has demonstrated excellent machinability. As a second type of shell explored, pushered single shells can exploit large density gradients to stabilize Rayleigh-Taylor instabilities during compression. Sharp gradients will have higher ignition yields and larger grading lengths will be more stable. We were able to demonstrate pushered single shells made from W-Be gradient layers with various grading slopes and provide simulated results showing that the grading profiles can be influenced by the coating rates of two components.

Developing one-dimensional implosions for inertial confinement fusion science

Experiments on the National Ignition Facility show that multi-dimensional effects currently dominate the implosion performance. Low mode implosion symmetry and hydrodynamic instabilities seeded by capsule mounting features appear to be two key limiting factors for implosion performance. One reason these factors have a large impact on the performance of inertial confinement fusion implosions is the high convergence required to achieve high fusion gains. To tackle these problems, a predictable implosion platform is needed meaning experiments must trade-off high gain for performance. LANL has adopted three main approaches to develop a one-dimensional (1D) implosion platform where 1D means measured yield over the 1D clean calculation. A high adiabat, low convergence platform is being developed using beryllium capsules enabling larger case-to-capsule ratios to improve symmetry. The second approach is liquid fuel layers using wetted foam targets. With liquid fuel layers, the implosion convergence can be controlled via the initial vapor pressure set by the target fielding temperature. The last method is double shell targets. For double shells, the smaller inner shell houses the DT fuel and the convergence of this cavity is relatively small compared to hot spot ignition. However, double shell targets have a different set of trade-off versus advantages. Details for each of these approaches are described.