Inertial Confinement Fusion Capsule Research Articles

Thickness dependence of microstructure and properties in Be2C coatings as a promising ablation material

Beryllium carbide (Be2C) thin films have proven to be promising ablation materials, but the properties of Be2C coatings of the greater thickness required for inertial confinement fusion capsules are still unknown. In this work, Be2C coatings of various thicknesses (0.3–32.9 µm) are prepared by DC reactive magnetron sputtering. The influence of thickness on crystal properties, microstructure, and optical properties is investigated. The results indicate that the crystallinity of polycrystalline Be2C films improves with increasing thickness, while the grain size (∼5 nm) and texture properties (without a preferred orientation) have only a weak dependence on thickness. A uniform featureless microstructure and smooth surface (root mean square roughness ∼8 nm) are observed even in thick (32.9 µm) films, despite the presence of defects induced by contaminants. High densities (2.19–2.31 g/cm3) and high deposition rates (∼270 nm/h) are realized, with the latter corresponding to the upper limit for the fabrication of Be2C coatings by magnetron sputtering. The transmittance of the films in the near-infrared region remains at a high level (>80%) and has only a weak dependence on thickness, while the transmittance in the visible region decreases with increasing thickness. In addition, the optical bandgap is estimated to be about 1.9 eV and decreases with increasing thickness owing to the presence of defects.

Measurement of laser differential confocal geometrical parameters for ICF capsule

A method based on the laser differential confocal principle is proposed for measurement of the uniformity of the inner and outer radius and shell thickness for an inertial confinement fusion (ICF) capsule. Firstly, this method uses the laser differential confocal measurement system (LDCS) driven by a precision air-bearing slide to scan and measure radially the outer radius, R, inner radius, r, and shell thickness, T, accurately. Secondly, a precision air-bearing rotation system is used to drive the capsule to rotate an angle, θ, in sequence, and the LDCS is used to measure R, r and T at the corresponding angle. Finally, the uniformity of the ICF capsule’s R, r and T can be calculated by the values of R, r and T measured at the position of each rotation angle. This method provides an approach for achieving high-precision, non-destructive, comprehensive, and rapid measurement of the uniformity of the inner and outer radius and shell thickness of an ICF capsule. Preliminary experiments indicate that measurement precision, using the proposed method for the uniformity of the outer radius, shell thickness and inner radius of the capsule, can reach 7.02 × 10−5, 5.87 × 10−4 and 6.52 × 10−5, respectively.

The Crystal Backlighter Imager: A spherically bent crystal imager for radiography on the National Ignition Facility.

The Crystal Backlighter Imager (CBI) is a quasi-monochromatic, near-normal incidence, spherically bent crystal imager developed for the National Ignition Facility (NIF), which will allow inertial confinement fusion capsule implosions to be radiographed close to stagnation. This is not possible using the standard pinhole-based area-backlighter configuration, as the self-emission from the capsule hotspot overwhelms the backlighter signal in the final stages of the implosion. The CBI mitigates the broadband self-emission from the capsule hot spot by using the extremely narrow bandwidth inherent to near-normal-incidence Bragg diffraction. Implementing a backlighter system based on near-normal reflection in the NIF chamber presents unique challenges, requiring the CBI to adopt novel engineering and operational strategies. The CBI currently operates with an 11.6 keV backlighter, making it the highest energy radiography diagnostic based on spherically bent crystals to date. For a given velocity, Doppler shift is proportional to the emitted photon energy. At 11.6 keV, the ablation velocity of the backlighter plasma results in a Doppler shift that is significant compared to the bandwidth of the instrument and the width of the atomic line, requiring that the shift be measured to high accuracy and the optics aligned accordingly to compensate. Experiments will be presented that used the CBI itself to measure the backlighter Doppler shift to an accuracy of better than 1 eV. These experiments also measured the spatial resolution of CBI radiographs at 7.0 μm, close to theoretical predictions. Finally, results will be presented from an experiment in which the CBI radiographed a capsule implosion driven by a 1 MJ NIF laser pulse, demonstrating a significant (>100) improvement in the backlighter to self-emission ratio compared to the pinhole-based area-backlighter configuration.

Yield reduction via the Knudsen layer effect in a mixture of fuel and pusher material

Hydrodynamic mix in inertial confinement fusion capsules produces chunk mix between deuterium–tritium fuel and inert pusher materials. The characteristic size of the intermingled fuel and pusher materials can be rapidly reduced by fluid mix, especially when the streamlines of the flow field are chaotic. This leads to an increasing Knudsen number for the fuel segments and a decreasing size of the pusher remnant with respect to the fuel ion mean-free-path. The former exacerbates Knudsen layer yield degradation due to fast ion losses, while the latter tends to alleviate Knudsen layer yield reduction due to other kinetic effects such as diffusive tunneling.

Laser differential confocal measurement of the outer surface profile of a laser inertial confinement fusion capsule

A new laser differential confocal measurement method with a high axial resolution and insensitivity to surface inclination is proposed for high-precision measurement of the outer surface profile of a laser inertial confinement fusion (ICF) capsule. This method uses an eccentricity automatic adjustment system to rapidly adjust the capsule’s centre to be aligned with the centreline of the rotary shaft. The capsule is then rotated by a high-precision rotation system, and the laser differential confocal measurement system (LDCS) radially measures the geometrical coordinates of each sampling point position in order to obtain the outer surface profile data of the ICF capsule. The proposed method achieves high-precision, non-contact, automatic, and rapid measurement of the outer surface profile of an ICF capsule. Theoretical analyses and preliminary experiments indicate that for the proposed method the measurement repeatability for the outer surface profile of an ICF capsule can reach 20 nm.

Null interferometric microscope for ICF-capsule surface-defect detection.

Isolated defects on the surface of the inertial confinement fusion (ICF) capsule reduce the probability of ignition. Here, to the best of our knowledge, we present the first null interferometric microscope (NIM) for direct and large-field surface defects detection on ICF capsules. The planar reference mirror in conventional interferometric microscopes is replaced by a spherical reference mirror to achieve null interference in the full field of view. Further, via the use of a short-coherence light source system, parasitic fringes are avoided. The feasibility of the NIM is verified via experiments on a 0.7mm diameter capsule. A 1mm diameter ICF capsule is also tested by the NIM to prove that the NIM has the ability to measure capsules with different diameters.

Implementing time resolved electron temperature capability at the NIF using a streak camera.

A new capability at the National Ignition Facility (NIF) has been implemented to measure the temperature of x-ray emitting sources. Although it is designed primarily for Inertial Confinement Fusion (ICF), it can be used for any hot emitting source that is well modeled. The electron temperature (Te) of the hot spot within the core of imploded ICF capsules is an effective indicator of implosion performance. Currently, there are spatially and temporally integrated Te inferences using image plates. A temporally resolved measurement of Te will help elucidate the mechanisms for hot spot heating and cooling such as conduction to fuel, alpha-heating, mix, and radiative losses. To determine the temporally resolved Te of hot spots, specific filters are added to an existing x-ray streak camera "streaked polar instrumentation for diagnosing energetic radiation" to probe the emission spectrum during the x-ray burn history of implosions at the NIF. One of the difficulties in inferring the hot spot temperature is the attenuation of the emission due to opacity from the shell and fuel. Therefore, a series of increasingly thick titanium filters were implemented to isolate the emission in specific energy regions that are sensitive to temperatures above 3 keV while not significantly influenced by the shell/fuel attenuation. Additionally, a relatively thin zinc filter was used to measure the contribution of colder emission sources. Since the signal levels of the emission through the thicker filters are relatively poor, a dual slit (aperture) was designed to increase the detected signal at the higher end of the spectrum. Herein, the design of the filters and slit is described, an overview of the solving technique is provided, and the initial electron temperature results are reported.

Probing the seeding of hydrodynamic instabilities from nonuniformities in ablator materials using 2D velocimetry

Despite the extensive work done to characterize and improve the smoothness of ablator materials used in inertial confinement fusion (ICF), features indicative of seeded instability growth in these materials are still observed. A two-dimensional imaging velocimetry technique has been used on Omega to measure the velocity non-uniformities of shock fronts launched by indirect drive in the three ablator materials of current interest, glow-discharge polymer, beryllium, and high-density carbon ablators. Observed features are deviations from shock front planarity with amplitudes of a few tens of nanometers, local velocity variations of a few tens of m/s, and transverse spatial scales ranging from 5 to 200 μm. These data will help develop a full understanding of the effects of surface topography, dynamic material response, and internal heterogeneities on the stability of ICF capsules. For all three ablators, we have quantified perturbations at amplitudes that can dominate conventional surface roughness seeds to hydrodynamic instability.

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.

Increasing stagnation pressure and thermonuclear performance of inertial confinement fusion capsules by the introduction of a high-Z dopant

Inertial confinement fusion requires the inertia of the imploding mass to provide the necessary confinement such that the core reaches adequate high density, temperature, and pressure. Experiments utilize low-Z capsules filled with hydrogenic fuel, which are subject to multiple instabilities at the interfaces during the implosion. To improve the stability of the fuel:capsule interface and narrow the imploding shell profile, capsules are doped with a small atomic percentage of a high-Z material. A series of recent indirect-drive experiments executed at the National Ignition Facility with tungsten-doped high density carbon capsules has demonstrated that the presence of this dopant serves to increase the in-flight aspect ratio of the shell and increase the compression and neutron yield performance of both gas-filled and deuterium-tritium cryogenically layered targets. These experiments definitively demonstrate that benefits accrued by the introduction of a high-Z dopant into the capsule can outweigh the detrimentally reduced stability of the ablation front, avoiding shell breakup or significant radiative cooling of the hot spot. Future experiments will utilize these types of capsules to further increase nuclear performance.

Ignition and pusher adiabat

In the last five years, large amounts of high quality data on inertial confinement fusion (ICF) experiments were produced at the National Ignition Facility (NIF). From this data we have significantly advanced our scientific understanding of the physics of thermonuclear (TN) ignition and identified critical issues that must be addressed to achieve a burning hotspot, such as implosion energetics, pusher adiabat, tamping effects, and confinement time. In this paper we present a review of recently developed TN ignition and implosion scaling theory (Cheng et al 2013 Phys. Rev. E 88 041101; Cheng et al 2014 Phys. Plasmas 21 10270) that characterizes the thermodynamic properties of the hotspot and the ignition criteria for ICF. We compare our theoretical predictions with NIF data and find good agreement between theory and experiments. We demonstrate the fundamental effects of the pusher adiabat on the energy partition between the cold shell and the hot deuterium–tritium (DT) gas, and thus on the integrated performance of ICF capsules. Theoretical analysis of NIF experiments (Cheng et al 2015 Phys. Plasmas 22 082704; Melvin et al 2015 Phys. Plasmas 22 022708; Cheng et al 2016 Phys. Plasmas 23 120702) and physical explanations of the discrepancies between theory, data, and simulations are presented. It is shown that the true experimental adiabat of the cold DT fuel can be inferred from neutron image data of a capsule implosion. We show that the ablator mix and preheat in the cold fuel can be estimated from the experimentally inferred hotspot mix. Finally, possible paths forward to reach higher yields at NIF implied by the theory are discussed.

Rapid measurement and compensation method of eccentricity in automatic profile measurement of the ICF capsule.

In this paper, we propose a new measurement and compensation method for the eccentricity of the inertial confinement fusion (ICF) capsule, which combines computer vision and the laser differential confocal method to align the capsule in rotation measurement. This technique measures the eccentricity of the capsule by obtaining the sub-pixel profile with a moment-based algorithm, then performs the preliminary alignment by the two-dimensional adjustment. Next, we use the laser differential confocal sensor to measure the height data of the equatorial surface of the capsule by turning it around, then obtain and compensate the remaining eccentricity ultimately. This method is a non-contact, automatic, rapid, high-precision measurement and compensation technique of eccentricity for the capsule. Theoretical analyses and preliminary experiments indicate that the maximum measurement range of eccentricity of this proposed method is 1.8mm for the capsule with a diameter of 1mm, and it could eliminate the eccentricity to less than 0.5μm in 30s.

Mitigation of X-ray shadow seeding of hydrodynamic instabilities on inertial confinement fusion capsules using a reduced diameter fuel fill-tube

We report a reduced X-ray shadow imprint of hydrodynamic instabilities on the high-density carbon ablator surface of inertial confinement fusion (ICF) capsules using a reduced diameter fuel fill tube on the National Ignition Facility (NIF). The perturbation seed mass from hydrodynamic instabilities was reduced by approximately an order of magnitude by reducing both the diameter and wall thickness of the fill tube by ∼2×, consistent with analytical estimates. This work demonstrates a successful mitigation strategy for engineered features for ICF implosions on the NIF.

Hugoniot equation of state of Si-doped glow discharge polymer and scaling to other plastic ablators

Pressure, density, and temperature were measured along the principal Hugoniot of the Si-doped Glow Discharge Polymer used in Inertial Confinement Fusion (ICF) capsules up to 5 Mbar, covering conditions beyond the first shock in a full-scale Inertial Confinement Fusion (ICF) capsule. The experiments were performed using the GEKKOXII laser at the Institute of Laser Engineering at Osaka University in Japan. Results are in good agreement with predictions obtained from ab initio Hugoniot calculations, but softer than the quotidian equation of state average atom model. Ab initio calculations show that dissociation of carbon bonds need to be taken into account in order to explain Hugoniot compressibility.

Visualizing deceleration-phase instabilities in inertial confinement fusion implosions using an “enhanced self-emission” technique at the National Ignition Facility

High-mode perturbations and low-mode asymmetries were measured in the deceleration phase of indirectly driven, deuterium gas filled inertial confinement fusion capsule implosions at convergence ratios of 10 to 15, using a new “enhanced emission” technique at the National Ignition Facility [E. M. Campbell et al., AIP Conf. Proc. 429, 3 (1998)]. In these experiments, a high spatial resolution Kirkpatrick-Baez microscope was used to image the x-ray emission from the inner surface of a high-density-carbon capsule's shell. The use of a high atomic number dopant in the shell enabled time-resolved observations of shell perturbations penetrating into the hot spot. This allowed the effects of the perturbations and asymmetries on degrading neutron yield to be directly measured. In particular, mix induced radiation losses of ∼400 J from the hot spot resulted in a neutron yield reduction of a factor of ∼2. In a subsequent experiment with a significantly increased level of short-mode initial perturbations, shown through the enhanced imaging technique to be highly organized radially, the neutron yield dropped an additional factor of ∼2.

Yield degradation in inertial-confinement-fusion implosions due to shock-driven kinetic fuel-species stratification and viscous heating

Anomalous thermonuclear yield degradation (i.e., that not describable by single-fluid radiation hydrodynamics) in Inertial Confinement Fusion (ICF) implosions is ubiquitously observed in both Omega and National Ignition experiments. Multiple experimental and theoretical studies have been carried out to investigate the origin of such a degradation. Relative concentration changes of fuel-ion species, as well as kinetically enhanced viscous heating, have been among possible explanations proposed for certain classes of ICF experiments. In this study, we investigate the role of such kinetic plasma effects in detail. To this end, we use the iFP code to perform multi-species ion Vlasov-Fokker-Planck simulations of ICF capsule implosions with the fuel comprising various hydrodynamically equivalent mixtures of deuterium (D) and helium-3 (3He), as in the original Rygg experiments [J. R. Rygg et al., Phys. Plasmas 13, 052702 (2006)]. We employ the same computational setup as in O. Larroche [Phys. Plasmas 19, 122706 (2012)], which was the first to simulate the experiments kinetically. However, unlike the Larroche study, and in partial agreement with experimental data, we find a systematic yield degradation in multi-species simulations versus averaged-ion simulations when the D-fuel fraction is decreased. This yield degradation originates in the fuel-ion species stratification induced by plasma shocks, which imprints the imploding system and results in the relocation of the D ions from the core of the capsule to its periphery, thereby reducing the yield relative to a non-separable averaged-ion case. By comparing yields from the averaged-ion kinetic simulations and from the hydrodynamic scaling, we also observe yield variations associated with ion kinetic effects other than fuel-ion stratification, such as ion viscous heating, which is typically neglected in hydrodynamic implosions' simulations. Since our kinetic simulations are driven by hydrodynamic boundary conditions at the fuel-ablator interface, they cannot capture the effects of ion viscosity on the capsule compression, or effects associated with the interface, which are expected to be important. Studies of such effects are left for future work.

Metal Alloy ICF Capsules Created by Electrodeposition

Electrochemical deposition is an attractive alternative to physical vapor deposition and micromachining to produce metal capsules for inertial confinement fusion (ICF). Electrochemical deposition (also referred to as electrodeposition or plating) is expected to produce full-density metal capsules without seams or inclusions of unwanted atomic constituents, the current shortcomings of micromachine and physical vapor deposition, respectively.Here, we discuss new cathode designs that allow for the rapid electrodeposition of gold and copper alloys on spherical mandrels by making transient contact with the constantly moving spheres. Electrodeposition of pure gold, copper, platinum, and alloys of gold-copper and gold-silver are demonstrated, with nonporous coatings of >40 µm achieved in only a few hours of plating. The surface roughness of the spheres after electrodeposition is comparable to the starting mandrel, and the coatings appear to be fully dense with no inclusions.A detailed understanding of the electrodeposition conditions that result in different alloy compositions and plating rates will allow for the electrodeposition of graded alloys on spheres in the near future. This report on the electrodeposition of metals on spherical mandrels is an important first step toward the fabrication of graded-density metal capsules for ICF experiments at the National Ignition Facility.

Positron radiography of ignition-relevant ICF capsules

Laser-generated positrons are evaluated as a probe source to radiograph in-flight ignition-relevant inertial confinement fusion capsules. Current ultraintense laser facilities are capable of producing 2 × 1012 relativistic positrons in a narrow energy bandwidth and short time duration. Monte Carlo simulations suggest that the unique characteristics of such positrons allow for the reconstruction of both capsule shell radius and areal density between 0.002 and 2 g/cm2. The energy-downshifted positron spectrum and angular scattering of the source particles are sufficient to constrain the conditions of the capsule between preshot and stagnation. We evaluate the effects of magnetic fields near the capsule surface using analytic estimates where it is shown that this diagnostic can tolerate line integrated field strengths of 100 T mm.

Laser differential confocal inner-surface profile measurement method for an ICF capsule

To achieve the high-precision measurement of the inner surface profile of a laser inertial confinement fusion (ICF) capsule, a new laser differential confocal ICF target measurement method with high axial resolution and an anti-surface reflectivity capability is proposed for the inner surface profile of an ICF capsule. First, this method uses the laser differential confocal measurement system to radially measure the inner and outer surfaces of an ICF capsule located in the center of the rotary shaft, respectively, and it uses the measured location parameters of the inner and outer surfaces of the ICF capsule and the refractive index of the ICF capsule to obtain the geometrical parameters of the inner surface trigger point by ray tracing. Secondly, it rotates the capsule using a high-precision rotation system, and uses the laser differential confocal measuring system to scan and measure the inner surface profile of the equatorial section of the capsule. Then, it rotates the capsule to the other equatorial section using the auxiliary rotary system, and uses the laser differential confocal measuring system to measure the inner surface profile of other equatorial section of the capsule. Finally, all of the inner surface profile data obtained on each equatorial section are reconstructed to obtain the three-dimensional (3D) profile information of the capsule’s inner surface. For the first time, this proposed method achieves the high-precision nondestructive measurement of the inner surface profile of an ICF capsule. Theoretical analyses and preliminary experiments indicate that the repetitive measurement obtained using the proposed method for the inner surface profile of the capsule can reach 15 nm.

The effects of convergence ratio on the implosion behavior of DT layered inertial confinement fusion capsules

The wetted foam capsule design for inertial confinement fusion capsules, which includes a foam layer wetted with deuterium-tritium liquid, enables layered capsule implosions with a wide range of hot-spot convergence ratios (CR) on the National Ignition Facility. We present a full-scale wetted foam capsule design that demonstrates high gain in one-dimensional simulations. In these simulations, increasing the convergence ratio leads to an improved capsule yield due to higher hot-spot temperatures and increased fuel areal density. High-resolution two-dimensional simulations of this design are presented with detailed and well resolved models for the capsule fill tube, support tent, surface roughness, and predicted asymmetries in the x-ray drive. Our modeling of these asymmetries is validated by comparisons with available experimental data. In 2D simulations of the full-scale wetted foam capsule design, jetting caused by the fill tube is prevented by the expansion of the tungsten-doped shell layer due to preheat. While the impacts of surface roughness and predicted asymmetries in the x-ray drive are enhanced by convergence effects, likely underpredicted in 2D at high CR, simulations predict that the capsule is robust to these features. Nevertheless, the design is highly susceptible to the effects of the capsule support tent, which negates all of the one-dimensional benefits of increasing the convergence ratio. Indeed, when the support tent is included in simulations, the yield decreases as the convergence ratio is increased for CR > 20. Nevertheless, the results suggest that the full-scale wetted foam design has the potential to outperform ice layer capsules given currently achievable levels of asymmetries when fielded at low convergence ratios (CR < 20).