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Copper-doped beryllium spheres are an attractive ablator for inertial confinement fusion experiments. Beryllium spheres are made by sputtering beryllium onto spherical plastic mandrels which must t...

Achieving a high conversion efficiency into relativistic electrons is central to short-pulse laser application and fundamentally relies on creating interaction regions with intensities ${\gg}10^{18}$~W/cm$^2$. Small focal length optics are typically employed to achieve this goal; however, this solution is impractical for large kJ-class systems that are constrained by facility geometry, debris concerns, and component costs. We fielded target-mounted compound parabolic concentrators to overcome these limitations and achieved nearly an order of magnitude increase to the conversion efficiency and more than tripled electron temperature compared to flat targets. Particle-in-cell simulations demonstrate that plasma confinement within the cone and formation of turbulent laser fields that develop from cone wall reflections are responsible for the improved laser-to-target coupling. {These passive target components can be used to improve the coupling efficiency for all high-intensity short-pulse laser applications, particularly at large facilities with long focal length optics., 6 pages, 5 figures

Picosecond-scale laser–matter interactions using compound parabolic concentrators have demonstrated strongly relativistic ponderomotive effects with ∼ 10 × increase in x-ray source brightness, positron production and multi-MeV proton acceleration versus flat targets, using a marginally relativistic intensity laser.

The point design of beryllium capsules includes three Cu-doped layers in a 160-μm-thick beryllium shell to achieve the desired X-ray absorption profile. The beryllium capsules were deposited on glo...

The NIF point design uses a five-layer capsule to modify the X-ray absorption in order to achieve optimized shock timing. A stepped copper dopant design defines the layer structure. The production of the capsule involves pyrolysis to remove the inner plastic mandrel. Copper atoms diffuse radially and azimuthally throughout the capsule during pyrolysis. This diffusion significantly diminishes the capsule performance during implosion. Thermal and coated oxide barrier layers employed between layers mitigate the diffusion of copper during the mandrel removal process. The copper atoms do not diffuse through this barrier during pyrolysis. A capsule fabrication method that produces a capsule with a thin oxide layer will be discussed.

A new method (a variant of tumble finishing) for polishing and achieving local planarization on precision spherical, plastic capsules is described. Such capsules have niche applications, such as ablators used in high-peak-power laser targets for fusion energy research. The as-manufactured ablators contain many shallow domes (many 100's of nm high and a few 10's of μm wide) on the outer surface which are undesirable due to contributions to instabilities during implosion. These capsules were polished (i.e., tumble finished) by rotating a cylindrical vial containing the capsule, many borosilicate glass or zirconia media, and an aqueous-based colloidal silica polishing slurry. During tumble finishing, the relative media/capsule motions cause multiple, random sliding spherical–spherical Hertzian contacts, resulting in material removal, and possibly plastic deformation, on the capsule. As a result, the domes were observed to locally planarize (i.e., converge to lower heights). Utilizing the correct kinematics (i.e., the characteristics of the media/capsule motions), as controlled by the vial rotation rate and the fill fraction of media and slurry, the high velocity downward circumferential media motions were avoided, preventing fracturing of the fragile capsules. Also, the resulting post-polished surface roughness on the capsule was found to scale with the initial media surface roughness. Hence, pre-polishing the media greatly reduced the roughness of the media and thus the roughness of the polished capsule. A material removal model is described based on the Preston model and spherical–spherical Hertzian contacts which shows reasonable agreement with measured average removal rates of 35 ± 15 nm/day and which serves as a valuable tool to scale the polishing behavior with changes in process variables. Narrow domes were observed to planarize more rapidly than wider domes. A local planarization convergence model is also described, based on the concept of workpiece–lap mismatch where the local pressure, and hence removal, varies with the gap at the interface contact. The calculated rate and shape evolution of various size isolated domes compares well with the experimental data.

Thick (>150 μm) beryllium coatings are studied as an ablator material of interest for fusion fuel capsules for the National Ignition Facility. DC magnetron sputtering is used because of the relative controllability of the processing temperature and energy of the deposits. However, coatings produced by DC magnetron sputtering leak the fuel gas D2. By using ion-assisted DC magnetron, sputtered coatings can be made that are leak-tight. Transmission electron microscopy (TEM) studies revealed microstructural changes that lead to leak-tight coating. Ultrasmall angle x-ray spectroscopy is used to characterize the void distribution and volume along the spherical surface along with a combination of focused ion beam, scanning electron microscope, and TEM. An in situ multibeam optical stress sensor was used to measure the stress behavior of thick beryllium coatings on flat substrates as the material was being deposited.

Sputter coating of beryllium on spherical mandrels has been used at Lawrence Livermore National Laboratory and at General Atomics to produce graded, copper doped beryllium shells. While these coatings have consistent microstructure and acceptable void content, different coaters produced different results with respect to argon implantation. Each individual system met the requirements for argon implantation, but the deviation from one system to another and from run to run exceeded the variability requirements as specified by the National Ignition Facility target design requirements. We redesigned the fixturing within one system to improve reproducibility. Then, we reconfigured the coaters so that the vertical and lateral alignments of the shells under the gun varied

We use X-ray phase contrast imaging to characterize the inner surface roughness of deuterium-tritium (D-T) ice layers in capsules for future ignition experiments. It is therefore important to quantify how well the X-ray data correlate with the actual ice roughness. We benchmarked the accuracy of our system using surrogates with fabricated roughness characterized with high precision standard techniques. Cylindrical surrogates with azimuthally uniform sinusoidal perturbations with 100-μm period and 1-μm amplitude demonstrated 0.02-μm accuracy limited by the resolution of the imager and the source size ofour phase contrast system. Spherical surrogates with random roughness close to that required for the D-T ice for a successful ignition experiment were used to correlate the actual surface roughness to that obtained from the X-ray measurements. We compare first the average power spectra of individual measurements. The accuracy mode number limits of the X-ray phase contrast system benchmarked against surface characterization performed by atomic force microscopy are 60 and 90 for surrogates smoother and rougher than the required roughness for the ice. These agreement mode number limits are about 100 when comparing matching individual measurements. We will discuss the implications for interpreting D-T ice roughness data derived from phase contrast X-ray imaging.

Sputtered beryllium and copper-doped beryllium coatings as thick as 170μm have been deposited on spherical substrates to produce hollow shells that are required as targets for inertial fusion experiments. Be coatings by magnetron sputtering achieved ∼95±2% bulk density consistently up to 170μm thick. Coatings on the spherical substrates exhibit the typical columnar structure throughout the entire thickness. Transmission electron microscopy indicates the presence of submicron, nearly spherical voids mainly aggregated along the columnar structure and grain boundaries, as well as some smaller intragranular elongated voids. Holes have been drilled in beryllium shells produced in this manner to allow filling with deuterium (the fusion fuel). Gas retention of these shells has been examined using mass spectrometry. It appears that a fraction of the pores in the coatings are interconnected, which leads to leakage of the hollow shells. Boron-doped Be layers near the eutectic phase, at a concentration of ∼11at.%, h...

Various morphologies have been observed in sputter-deposited Be ablator capsules, including nodular growth, cone growth and twisted grain growth. By devising an agitation method that includes both bouncing and rolling the spherical mandrels during deposition, and by reducing the coating rate, consistent columnar grain structure has now been obtained up to 170 mm. Low mode deformation of the shells is observed on thin CH mandrels, but is suppressed if stiffer mandrels are used. Ablator density measured by weighing and x-ray radiography is 93%–95% of bulk density of Be. Transmission electron microscopy shows 100.200 nm size voids in the film and striations inside the grains. Be shells produced with rolling agitation have met most of the NIF specifications. Some of the few remaining issues will be discussed.

Ablative targets for the National Ignition Campaign (NIC) have been fabricated by sputter coating spherical mandrels made of glow discharge polymer (GDP) with graded copper doped beryllium (Be) layers. The inner mandrel must be completely removed to meet specific ignition design requirements. The process of removing the mandrel requires elevated temperature in the presence of oxygen. However, elevating the temperature in air also oxidizes the Be and can cause blistering on the inner surface of the Be shell. This paper will discuss a refined technique, which removes the GDP mandrel without compromising the integrity of the inner Be surface. The oxygen gradient that develops during the mandrel removal and the impact of its presence will also be discussed.

The choice of the mandrel material has a significant bearing on the properties of the sputter-coated beryllium shell needed for NIF targets. Here, we present our work on screening four mandrel materials, their impact on the Be shell and issues related to their subsequent removal. Beryllium shells sputter deposited on hollow glow discharge polymer or GDP spheres met most of the target specifications. However, they had greater opacity due to partial oxidation of the Be during the GDP burnout step. Poly (α-methyl styrene), silicon and nickel beads were explored as alternative mandrels but were plagued with problems such as cracking of the Be shell or incomplete removal. The most promising approach was a two-step coating process mediated by a thin 6μm Be mandrel that is made using GDP.

Graded copper-doped Be shells have been fabricated by sputter coating on spherical mandrels. While such coatings have consistent microstructure and acceptable void content and size, we have found that they suffer from sufficient interconnected porosity leading to relatively rapid gas leakage. In this paper, we present an extensive study of D 2 leakage out of Be shells made by sputter coating. The leakage appears to follow molecular flow dynamics as determined by examining the temperature dependence of the flow. Furthermore, the time dependence of the leakage suggests that the flow channels are nanometerish in diameter, propagating through the thickness of the coating, possibly brought about by residual stress in the coatings. We have investigated the D 2 leakage time constant as a function of a large number of coating parameters, including the effect of introducing boron-doped layers. Addition of thin 0.25 μm amorphous boron-doped layers near the inside surface has been most effective in producing shells with long time constants (greater than 7 days to immeasurable) with yield of greater than 50%. There is still substantial scatter in the data, even within a given coating batch, suggesting a possible stochastic cracking process driven by residual stress in the coating.

In ablator shell fabrication, trace elements and impurities are introduced in the deposition and the pyrolysis process, which must be controlled below a critical level. However, it is the opacity, not the individual elements, which matters in an Inertial Confinement Fusion (ICF) implosion. Radiography measures the opacity, allowing the accurate determination of the total impurity effect in a lump sum. Furthermore, by using the sputter target trace element information, we can determine the radial profile of oxygen to ±0.4 at. %. Oxygen is very difficult to measure by any other method, but is critically important for beryllium process development such as mandrel removal. To ensure measurement accuracy, we use a local standard to remove fluctuation in film developing and a step wedge to calibrate the film model.

This procedure describes a method for the fabrication of large-area and ultrathin free-standing polymer films. Typically, ultrathin films are prepared using either sacrificial layers, which may damage the film or affect its mechanical properties, or they are made on freshly cleaved mica, a substrate that is difficult to scale. Further, the size of ultrathin film is typically limited to a few square millimeters. In this method, we modify a surface with a polyelectrolyte that alters the strength of adhesion between polymer and deposition substrate. The polyelectrolyte can be shown to remain on the wafer using spectroscopy, and a treated wafer can be used to produce multiple films, indicating that at best minimal amounts of the polyelectrolyte are added to the film. The process has thus far been shown to be limited in scalability only by the size of the coating equipment, and is expected to be readily scalable to industrial processes. In this study, the protocol for making the solutions, preparing the deposition surface, and producing the films is described.

When used with 1.06-mm beryllium (Be) capsules on the National Ignition Facility, gold hohlraums with the inner diameter of 5.75 mm and helium gas fill density of 1.6 mg/cm3 exhibit significant drive degradation due to laser energy backscatter (of order 14%–17%) and “missing” X-ray drive energy (about 32% during the main pulse). Also, hard to simulate cross-beam energy transfer (CBET) must be used to control the implosion symmetry. Larger, 6.72-mm hohlraums with fill densities ≤0.6 mg/cm3 generally offer improved drive efficiency, reduced hot-electron preheat, and better control of the implosion symmetry without CBET. Recently, we carried out an exploratory campaign to evaluate performance of 1.06-mm Be capsules in such hohlraums and determine optimal hohlraum parameters. Specifically, we performed a hohlraum fill-density scan with a three-shock, 9.5-ns laser pulse and found that an appropriate axial laser repointing and azimuthal outer-quad splitting resulted in significantly improved hohlraum energetics...

Free-standing polymer thin films are typically fabricated using a sacrificial underlayer (between the film and its deposition substrate) or overlayer (on top of the film to assist peeling) in order to facilitate removal of the thin film from its deposition substrate. We show the direct delamination of extraordinarily thin (as thin as 8 nm) films of poly(vinyl formal) (PVF), polystyrene, and poly(methyl methacrylate). Large (up to 13 cm diameter) films of PVF could be captured on wire supports to produce free-standing films. By modifying the substrate to lower the interfacial energy resisting film-substrate separation, the conditions for spontaneous delamination are satisfied even for very thin films. The substrate modification is based on the electrostatic adsorption of a cationic polyelectrolyte. Eliminating the use of sacrificial materials and instead relying on naturally self-limiting adsorption makes this method suitable for large areas. We have observed delamination of films with aspect ratios (ratio of lateral dimension between supports to thickness) of 10(7) and have captured dry, free-standing films with aspect ratios10(6). Films with an aspect ratio of 10(5) can bear loads up to 10(6) times the mass of the film itself. The presence of the adsorbed layer can be observed using X-ray photoelectron spectroscopy, and this layer is persistent through multiple uses. In the system studied, elimination of sacrificial materials leads to an enhancement in the failure strength of the free-standing thin film. The robustness, persistence, and the self-optimizing nature distinguish this method from various fabrication methods utilizing sacrificial materials and make it a potentially scalable process for the fabrication of ultrathin free-standing or transferrable films for filtration, MEMS, or tissue engineering applications.

The first indirect drive implosion experiments using Beryllium (Be) capsules at the National Ignition Facility confirm the superior ablation properties and elucidate possible Be-ablator issues such as hohlraum filling by ablator material. Since the 1990s, Be has been the preferred Inertial Confinement Fusion (ICF) ablator because of its higher mass ablation rate compared to that of carbon-based ablators. This enables ICF target designs with higher implosion velocities at lower radiation temperatures and improved hydrodynamic stability through greater ablative stabilization. Recent experiments to demonstrate the viability of Be ablator target designs measured the backscattered laser energy, capsule implosion velocity, core implosion shape from self-emission, and in-flight capsule shape from backlit imaging. The laser backscatter is similar to that from comparable plastic (CH) targets under the same hohlraum conditions. Implosion velocity measurements from backlit streaked radiography show that laser energy coupling to the hohlraum wall is comparable to plastic ablators. The measured implosion shape indicates no significant reduction of laser energy from the inner laser cone beams reaching the hohlraum wall as compared with plastic and high-density carbon ablators. These results indicate that the high mass ablation rate for beryllium capsules does not significantly alter hohlraum energetics. In addition, these data, together with data for low fill-density hohlraum performance, indicate that laser power multipliers, required to reconcile simulations with experimental observations, are likely due to our limited understanding of the hohlraum rather than the capsule physics since similar multipliers are needed for both Be and CH capsules as seen in experiments.

We demonstrate the hohlraum radiation temperature and symmetry required for ignition-scale inertial confinement fusion capsule implosions. Cryogenic gas-filled hohlraums with 2.2 mm-diameter capsules are heated with unprecedented laser energies of 1.2 MJ delivered by 192 ultraviolet laser beams on the National Ignition Facility. Laser backscatter measurements show that these hohlraums absorb 87% to 91% of the incident laser power resulting in peak radiation temperatures of T(RAD)=300 eV and a symmetric implosion to a 100 μm diameter hot core.

Thick (>150 μm) beryllium coatings are studied as an ablator material of interest for fusion fuel capsules for the National Ignition Facility (NIF). As an added complication, the coatings are deposited on mm-scale spherical substrates, as opposed to flats. DC magnetron sputtering is used because of the relative controllability of the processing temperature and energy of the deposits. We used ultra small angle x-ray spectroscopy (USAXS) to characterize the void fraction and distribution along the spherical surface. We investigated the void structure using a combination focused ion beam (FIB) and scanning electron microscope (SEM), along with transmission electron microscopy (TEM). Our results show a few volume percent of voids and a typical void diameter of less than two hundred nanometers. Understanding how the stresses in the deposited material develop with thickness is important so that we can minimize film cracking and delamination. To that end, an in-situ multiple optical beam stress sensor (MOSS) was used to measure the stress behavior of thick Beryllium coatings on flat substrates as the material was being deposited. We will show how the film stress saturates with thickness and changes with pressure.

We demonstrate controlled doping of a glow discharge polymer by implantation with 500 keV Xe ions at room temperature. The Xe retention exhibits a threshold behavior, with a threshold dose of ∼2 × 1014 cm−2. Doping is accompanied by irradiation-induced changes in the polymer composition, including gradual H loss and a more complex non-monotonic behavior of the O concentration. The matrix composition saturates at C0.77H0.22O0.01 for Xe doses above ∼5 × 1014 cm−2 and up to the maximum dose studied (5 × 1015 cm−2). The retention mechanism is attributed to the modification of the polymer from a chain-like to clustered ring structure. The dopant profile and the elemental composition of the implanted polymer exhibit good stability upon thermal annealing up to 305 °C.

The National Ignition Facility point design uses a five-layer capsule to modify the X-ray absorption in order to achieve optimized shock timing. A stepwise copper dopant design defines the layer st...