Hohlraum Wall Material Research Articles

Self-consistent and precise measurement of time-dependent radiative albedo of gold based on specially symmetrical triple-cavity Hohlraum

A self-consistent and precise method to determine the time-dependent radiative albedo, i.e., the ratio of the reemission flux to the incident flux, for an indirect-drive inertial confinement fusion Hohlraum wall material is proposed. A specially designed symmetrical triple-cavity gold Hohlraum is used to create approximately constant and near-equilibrium uniform radiation with a peak temperature of 160 eV. The incident flux at the secondary cavity waist is obtained from flux balance analysis and from the shock velocity of a standard sample. The results agree well owing to the symmetrical radiation in the secondary cavity. A self-consistent and precise time-dependent radiative albedo is deduced from the reliable reemission flux and the incident flux, and the result from the shock velocity is found to have a smaller uncertainty than that from the multi-angle flux balance analysis, and also to agree well with the result of a simulation using the HYADES opacity.

Performance scaling with an applied magnetic field in indirect-drive inertial confinement fusion implosions

Magnetizing a cryogenic deuterium–tritium (DT)-layered inertial confinement fusion (ICF) implosion can improve performance by reducing thermal conduction and improving DT-alpha confinement in the hot spot. A room-temperature, magnetized indirect-drive ICF platform at the National Ignition Facility has been developed, using a high-Z, high-resistivity AuTa4 alloy as the hohlraum wall material. Experiments show a 2.5× increase in deuterium–deuterium (DD) neutron yield and a 0.8-keV increase in hot-spot temperature with the application of a 12-T B-field. For an initial 26-T B-field, we observed a 2.9× yield increase and a 1.1-keV temperature increase, with the inferred burn-averaged B-field in the compressed hot spot estimated to be 7.1 ± 1.8 kT using measured primary DD-n and secondary DT-n neutron yields.

Investigation of various methods for wall loss reduction in Inertial Confinement Fusion hohlraums

In this paper, different techniques are explored to reduce wall loss in Inertial Confinement Fusion (ICF) hohlraums. Along with the traditional high-Z element gold, uranium has also been used as the hohlraum wall material as it has higher opacity compared to Au at higher temperatures. Further, opacity is found to increase on adding other high-Z elements like gold, lead and tungsten to uranium in some specific proportions. For the densities and temperatures encountered in reemission zones, cocktails (U0.7Au0.3, U0.8Pb0.2 and U0.7W0.3) show the highest opacity enhancement. Radiation hydrodynamic (RHD) simulations are performed on the planar foils made up of above mentioned hohlraum wall materials by using tabular Equation of State (EOS) and opacity data. Significant reduction in wall loss occurs for the cocktails compared to pure Au on using a constant, time dependent temperature drive (obtained from constant 1 ns laser pulse) and a generic NIF ignition drive. Our work also quantitatively estimates the reduction in wall loss for both elements and cocktails on reducing their initial densities as a result of lower hydrodynamic losses. By combining the effects of both material mixing and density reduction, we have reported wall loss lowering as much as ∼ 24%. Wall loss scaling relations have also been obtained for high-Z elements and cocktails for constant and time dependent temperature drives. Resulting scaling relations are further used to obtain the temperature in empty hohlraum driven by constant power 1 ns laser pulse using hohlraum energy balance model. Hohlraum temperatures are found to increase by ∼ 9 eV for uranium and ∼ 11 eV for the cocktails compared to pure gold for 60 TW laser power. These results will prove to be useful in choosing the ICF hohlraum wall material.

Application of quantum-statistical methods to studies of thermodynamic and radiative processes in hot dense plasmas

Calculations of thermodynamic and radiative characteristics of hot dense plasmas within different quantum-statistical approaches, such as the use of the Hartree–Fock–Slater model and the ion model, are presented. Calculated equations of state of different substances are used to investigate findings from absolute and relative measurements of the compressibility of solid aluminum samples in strong shock waves. It is shown that our calculated Hugoniot adiabat of aluminum is in a good agreement with experimental data and other theoretical results from first principles. We also present a review of the most important applications of the quantum-statistical approach to the study of radiative properties of hot dense plasmas. It includes the optimization problem of hohlraum wall materials for laser inertial fusion, calculations of the radiative efficiency of complex materials for optically thin plasma in X-pinch, modeling of radiative and gas-dynamic processes in plasma for experiments, where both intense laser and heavy ion beams are used, and temperature diagnostics for X- and Z-pinch plasmas.

Recent progress of hohlraum physics experiments in indirect-driven ICF in China

In recent years, hohlraum experiments have been performed extensively on Shenguang series laser facilities in the context of laser indirect-drive inertial confinement fusion. Multiple aspects about the hohlraumenergetics, drive symmetry and plasma condition are studied by a variety of methods resolving different photon ranges and multiple viewing areas. To improve the experimental uncertainty, several diagnostics are optimized and calibrated, also the power balance and pointing accuracy of laser beams are evaluated and improved. These works lead a rapid progress on hohlraum experimental capabilities and a series of successful experimental campaigns. In order to further optimize the hohlraum performance, other hohlraum geometry (the spherical hohlram with six LEHs and the cylindrical hohlraum with six LEHs) and hohlraum wall material (depleted Uranium and foam Au) are explored as well. Hohlraum experiments and modeling on Shenguang series laser facilities demonstrated quantitative understanding of the laser conversion, X-ray ablation and plasma motion in different regions.

The I-Raum: A new shaped hohlraum for improved inner beam propagation in indirectly-driven ICF implosions on the National Ignition Facility

Recent work in indirectly-driven inertial confinement fusion implosions on the National Ignition Facility has indicated that late-time propagation of the inner cones of laser beams (23° and 30°) is impeded by the growth of a “bubble” of hohlraum wall material (Au or depleted uranium), which is initiated by and is located at the location where the higher-intensity outer beams (44° and 50°) hit the hohlraum wall. The absorption of the inner cone beams by this “bubble” reduces the laser energy reaching the hohlraum equator at late time driving an oblate or pancaked implosion, which limits implosion performance. In this article, we present the design of a new shaped hohlraum designed specifically to reduce the impact of this bubble by adding a recessed pocket at the location where the outer cones hit the hohlraum wall. This recessed pocket displaces the bubble radially outward, reducing the inward penetration of the bubble at all times throughout the implosion and increasing the time for inner beam propagation by approximately 1 ns. This increased laser propagation time allows one to drive a larger capsule, which absorbs more energy and is predicted to improve implosion performance. The new design is based on a recent National Ignition Facility shot, N170601, which produced a record neutron yield. The expansion rate and absorption of laser energy by the bubble is quantified for both cylindrical and shaped hohlraums, and the predicted performance is compared.

Efficient X-ray emission from laser-irradiated low-density lead target: a substitute for gold in hohlraum design

Numerical experiments are carried out to calculate continuum emissivity and opacity of plasmas produced from laser-irradiated Au and Pb targets as hohlraum wall materials. Targets are considered to be solid or porous with different initial densities. Simulation results show a good agreement compared with the measured data. The results show that under identical conditions, X-ray emission is higher for Au plasma; however, by decreasing initial densities, X-ray yield enhancement is greater for Pb plasma. By using a Pb target with initial density of about 1.14 g cm−3 instead of solid Au target, the same X-ray yield even more can be obtained. Calculations also show that in the conditions of solid density targets, Pb plasma offers a little lower opacity in soft X-ray region. Decreasing initial density of Pb causes its opacity to increase and get closer to the opacity of solid Au which in turn reduces energy losses in hohlraum wall.

Study on optimal inertial-confinement-fusion hohlraum wall radial density and wall loss

Reducing hohlraum wall loss is one of the most important ways to improve hohlraum coupling efficiency in laser indirect drive inertial-confinement fusion. It is appeared that a high Z metallic foam as hohlraum wall material will reduce wall loss. By adjusting initial hohlraum wall density distribution along radial axes to ρ0(r)=kr, the numerical simulation results show that it can indeed bring best savings of ∼40% general wall loss. We conclude that absorbed energy mainly decreases by restraining rarefactions, and a proper slope k can optimize internal energy loss of low density and increased kinetic loss by subsonic. Also saved energy ratio reduces with source temperature decreasing. This approach would cut the reactor driver that needs quite substantially if experiments demonstrate it.

Theoretical and experimental studies of material radiative properties and their applications to laser and heavy ion inertial fusion

AbstractTheoretical and experimental studies of radiative properties of substances heated by pulsed current devises or lasers and used as X-ray sources have been carried out depending on plasma conditions, and specific spectra of X-ray absorption and radiation for different materials have been calculated. Important features of the theoretical model, known as the ion model of plasma, are discussed. This model can be applied for calculations of the radiative properties of complex materials over a wide range of plasma parameters. For purposes of indirect-driven inertial fusion based on the hohlraum concept, an optimization method is used for the selection of an effective complex hohlraum wall material, which provides high radiation efficiency at laser interaction with the wall. The radiation efficiency of the resulting material is compared with the efficiency of other composite materials that have previously been evaluated theoretically. A similar theoretical study is performed for the optically thin X-pinch plasma produced by exploding wires. Theoretical estimations of radiative efficiency are compared with experimental data that have been obtained from measurements of X-pinch radiation energy yield using two exploding wire materials, NiCr and Alloy 188. It is shown that the theoretical results agree well with the experimental data. A symmetric multilayer X-pinch, where W and Mo wires are used, is as well considered. The theoretical explanation of experimental phenomena is discussed based on the W and Mo radiative spectra. The ion model was as well applied for interpretation of experimental results on opacities of CHO-plasma obtained via indirect heating of low density polymer layers by means of soft X-rays. The new diagnostics method based on the deformation of the of the Carbon absorption K-edge when foam layer is heated to plasma is discussed. The spectral coefficients for X-ray absorption in CHO-plasma are calculated in the photon energy region around the Carbon K-edge for different plasma temperatures and mean foam density. In this case, the Carbon K-edge position on the energy scale can be used for plasma temperature diagnostic.

X-ray conversion efficiency of high-Z hohlraum wall materials for indirect drive ignition

The conversion efficiency of 351nm laser light to soft x rays (0.1–5keV) was measured for Au, U, and high Z mixture “cocktails” used as hohlraum wall materials in indirect drive fusion experiments. For the spherical targets in a direct drive geometry, flattop laser pulses and laser smoothing with phase plates are employed to achieve constant and uniform laser intensities of 1014 and 1015W∕cm2 over the target surface that are relevant for the future ignition experiments at the National Ignition Facility [G. H. Miller, E. I. Moses, and C. R. Wuest, Nucl. Fusion 44, 228 (2004)]. The absolute time and spectrally resolved radiation flux is measured with a multichannel soft x-ray power diagnostic. The conversion efficiency is then calculated by dividing the measured x-ray power by the incident laser power from which the measured laser backscattering losses are subtracted. After ∼0.5ns, the time resolved x-ray conversion efficiency reaches a slowly increasing plateau of 95% at 1014W∕cm2 laser intensity and of 80% at 1015W∕cm2. The M-band flux (2–5keV) is negligible at 1014W∕cm2 reaching ∼1% of the total x-ray flux for all target materials. In contrast, the M-band flux is significant and depends on the target material at 1015W∕cm2 laser intensity, reaching values between 10% of the total flux for U and 27% for Au. LASNEX simulations [G. B. Zimmerman and W. L. Kruer, Comm. Plasma Phys. Contr. Fusion 2, 51 (1975)] show good agreement in conversion efficiency and radiated spectra with data when using XSN atomic physics model and a flux limiter of 0.15, but they underestimate the generated M-band flux.

Analytic expressions for optimal inertial-confinement-fusion hohlraum wall density and wall loss

Solutions to the radiation diffusion equation predict the absorbed energy ("wall loss") within an inertial confinement fusion (ICF) hohlraum. Comparing supersonic versus subsonic solutions suggests that a high metallic foam as hohlraum wall material will reduce hydrodynamic losses, and hence, net absorbed energy by . We derive an analytic expression for the optimal density (for any given drive temperature and pulse-length) that will achieve this reduction factor and which agrees well with numerical simulations. This approach can increase the coupling efficiency of indirectly driven ICF capsules.

Feasibility of Target Material Recycling as Waste Management Alternative

The issue of waste management has been studied simultaneously along with the development of the ARIES heavy-ion-driven inertial fusion energy (IFE) concept. Options for waste management include disposal in repositories, recycling, or clearance from regulatory control, following a reasonable cooling period. This paper concerns the feasibility of recycling the heavy-ion-beam targets, in particular the hohlraum wall materials that include, for example, Au/Gd, Au, W, Pb, Hg, Ta, Pb/Ta/Cs, Hg/W/Cs, Pb/Hf, Hf, solid Kr, and solid Xe. The choice between target material disposal and recycling depends on the amount of waste generated relative to the nuclear island, the strategy to solve the recycling problem, and the impact of the additional cost and complexity of the recycling process on the overall machine. A detailed flow diagram for the elements of the recycling process was developed to analyze two extreme activation cases: (a) one-shot use and then disposal in a repository and (b) recycling continuously during plant life without removal of transmutation products. Metrics for comparing the two scenarios included waste level, dose to recycling equipment, additional cost, and design complexity. Comparing the two approaches indicated a preference for the one-shot scenario as it generates 1 m3/yr of extremely low-level waste (Class A) and offers attractive design and economics features. Recycling reduces the target waste stream by a factor of 10 or more but introduces additional issues. It may produce high-level waste, requires remote handling, adds radioactive storage facilities, and increases the cost and complexity of the plant. The inventory analysis indicated that the heavy-ion-beam (HIB) target materials represent a very small waste stream compared to that of the nuclear island (<1% of the total waste). This means recycling is not a “must” requirement for IFE-HIB power plants unless the target materials have cost and/or resource problems (e.g., Au and Gd). In this case, the additional cost and complexity of the recycling process should be factored into the economics of IFE power plants.

Laser hohlraum coupling efficiency on the Shenguang II facility

Recently, hohlraum experiments were performed at the Shenguang-II (SG-II) laser facility [Lin et al., Chin. J. Lasers B10, Suppl. IV6 (2001)]. The measured maximum radiation temperature was 170 eV for the standard hohlraum and 150 eV for a 1.5-scaled one. This paper discusses the radiation temperature and laser hohlraum coupling efficiency in terms of a theoretical model [Phys. Plasmas 8, 1659 (2001)] and numerical simulation. A 2D laser–hohlraum coupling code, LARED-H [Chin. J. Comput. Phys. 19, 57 (2002)], gives a satisfactory coincidence with the measured time-resolved radiation temperature. Upon fitting the time-resolved curve, the theoretical model obtains the hohlraum coupling efficiency and, furthermore, the parameter n+s for the hohlraum wall material (Au) can be determined simultaneously, where n, s are the power exponents of temperature for the radiation Rosseland mean-free path and specific internal energy, respectively.

Target activation and radiological response of ARIES-IFE dry wall chamber

The choice of target coating and hohlraum wall materials is among the most critical decisions to be made for inertial fusion energy (IFE) designs. Gold and gold/gadolinium have long been considered to be the coating and hohlraum wall materials of choice for direct drive (DD) and indirect drive (ID) targets, respectively, offering high target performance and low beam energy losses. More recently, a variety of other materials have been considered, including W, Pb, Pt, Pd, and Ag for the DD target coating and Au, W, Pb, Hg, Ta, Cs, and Hf for the hohlraum wall of the ID target. The choice of the coating/hohlraum material is a tradeoff between the target design elements such as safety, target physics, economics, etc. We identified the key safety issues and have investigated the neutron-induced irradiation effects of the candidate coating/hohlraum materials using the radiation chamber conditions of the ARIES-IFE dry wall concept. The safety requirements have specific impacts in terms of the coating/hohlraum materials choice.

Progress in target physics and design for heavy ion fusion

Two-dimensional, integrated calculations of a close-coupled version of the distributed radiator, heavy ion target predict gain 130 from 3.3 MJ of beam energy. To achieve these results, the case-to-capsule ratio was decreased by about 25% from the previous heavy ion targets [M. Tabak and D. Callahan-Miller, Phys. Plasmas 5, 1895 (1998)]. These targets are robust to changes in the ion stopping model because changes in the ion stopping model can be accommodated by changes to the target. The capsule is also insensitive to changes in the deuterium–tritium (DT) gas fill in the center of the capsule over the range that is of interest for target fabrication and target injection. Single-mode Rayleigh–Taylor growth rates for this capsule are smaller than those for at least one National Ignition Facility (NIF) [J. A. Paisner et al., Laser Focus World 30, 75 (1994)] design. As a result, stability issues for the heavy ion capsule can be settled on NIF. The close-coupled target also opens up the possibility of a high gain engineering test facility from a 1.5–2 MJ driver; calculations predict that gain 90 is achievable from 1.75 MJ of beam energy. Finally, the choice of hohlraum wall material, which must satisfy constraints from target physics, environment and safety, chamber design, and target fabrication, is discussed.