Hohlraum Wall Research Articles

Analysis of the National Ignition Facility ignition hohlraum energetics experiments

A series of 40 experiments on the National Ignition Facility (NIF) [E. I. Moses et al., Phys. Plasmas 16, 041006 (2009)] to study energy balance and implosion symmetry in reduced- and full-scale ignition hohlraums was shot at energies up to 1.3 MJ. This paper reports the findings of the analysis of the ensemble of experimental data obtained that has produced an improved model for simulating ignition hohlraums. Last year the first observation in a NIF hohlraum of energy transfer between cones of beams as a function of wavelength shift between those cones was reported [P. Michel et al., Phys. Plasmas 17, 056305 (2010)]. Detailed analysis of hohlraum wall emission as measured through the laser entrance hole (LEH) has allowed the amount of energy transferred versus wavelength shift to be quantified. The change in outer beam brightness is found to be quantitatively consistent with LASNEX [G. B. Zimmerman and W. L. Kruer, Comments Plasma Phys. Controlled Fusion 2, 51 (1975)] simulations using the predicted energy transfer when possible saturation of the plasma wave mediating the transfer is included. The effect of the predicted energy transfer on implosion symmetry is also found to be in good agreement with gated x-ray framing camera images. Hohlraum energy balance, as measured by x-ray power escaping the LEH, is quantitatively consistent with revised estimates of backscatter and incident laser energy combined with a more rigorous non-local-thermodynamic-equilibrium atomic physics model with greater emissivity than the simpler average-atom model used in the original design of NIF targets.

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.

Images of the laser entrance hole from the static x-ray imager at NIF

The static x-ray imager at the National Ignition Facility is a pinhole camera using a CCD detector to obtain images of Hohlraum wall x-ray drive illumination patterns seen through the laser entrance hole (LEH). Carefully chosen filters, combined with the CCD response, allow recording images in the x-ray range of 3-5 keV with 60 μm spatial resolution. The routines used to obtain the apparent size of the backlit LEH and the location and intensity of beam spots are discussed and compared to predictions. A new soft x-ray channel centered at 870 eV (near the x-ray peak of a 300 eV temperature ignition Hohlraum) is discussed.

Measurements of preheat and shock melting in Be ablators during the first few nanoseconds of a National Ignition Facility ignition drive using the Omega laser

A scaled Hohlraum platform was used to experimentally measure preheat in ablator materials during the first few nanoseconds of a radiation drive proposed for ignition experiments at the National Ignition Facility [J. A. Paisner et al., Laser Focus World 30, 75 (1994)]. The platform design approximates the radiation environment of the pole of the capsule by matching both the laser spot intensity and illuminated Hohlraum wall fraction in scaled halfraums driven by the OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)]. Back surface motion measured via VISAR reflecting from the rear surface of the sample was used to measure sample motion prior to shock breakout. The experiments show that the first ∼20 μm of a Be ablator will be melted by radiation preheat, with subsequent material melted by the initial shock, in agreement with simulations. The experiments also show no evidence of anomalous heating of buried high-Z doped layers in the ablator.

NIF-scale re-emission sphere measurements of early-time Tr=100eV hohlraum symmetry (invited)

The indirect-drive National Ignition Campaign (NIC) proposes to set the first 2 ns of hohlraum radiation symmetry by observing the instantaneous soft x-ray re-emission pattern from a high-Z sphere in place of the ignition capsule. To assess this technique under NIC conditions, we used the Omega Laser Facility to image the re-emission of Bi coated spheres with 200 ps temporal, 50-100 microm spatial, and 30% spectral resolution. The sphere is driven by 70% NIC-scale vacuum Au hohlraums heated to Tr=100 eV using two cones per side laser beam illumination. The experiments have demonstrated the required accuracies of <3%P(2)/P(0) and <3%P(4)/P(0) Legendre mode flux asymmetry at both 900 and 1200 eV re-emission photon energies. The re-emission patterns at 900 and 1200 eV are also consistent with each other and their relative dependence on radiation temperature. We measured the P(2)/P(0) and P(4)/P(0) dependence to laser cone power ratio. View factor calculations are in agreement with the experimentally measured radiation flux and re-emit images when assuming 55% inner beam and 100 % outer beam coupling into x rays at the hohlraum wall.

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.

Controlling Stimulated Brillouin Backscatter with Beam Smoothing in Weakly Damped Systems

We derive an analytical estimate of the effect of temporal smoothing of laser beams on stimulated Brillouin scattering (SBS) in a regime relevant to indirect drive ignition. We predict a strong reduction of SBS in the gold plasma expanding from the Hohlraum wall with temporal smoothing. This is a new regime far above threshold where the time to reach convective saturation allows for an effective contrast reduction of the beam intensity driving the instability. This result agrees with three dimensional simulations. Polarization smoothing is shown to double the effective bandwidth.

Developing depleted uranium and gold cocktail hohlraums for the National Ignition Facility

Fusion ignition experiments are planned to begin at the National Ignition Facility (NIF) [J. A. Paisner, E. M. Campbell, and W. J. Hogan, Fusion Technol. 26, 755 (1994)] using the indirect drive configuration [J. D. Lindl, P. Amendt, R. L. Berger, S. G. Glendinning, S. H. Glenzer, S. W. Haan, R. L, Kauffman, O. L. Landen, and L. J. Suter, Phys. Plasmas 11, 339 (2004)]. Although the x-ray drive in this configuration is highly symmetric, energy is lost in the conversion process due to x-ray penetration into the hohlraum wall. To mitigate this loss, depleted uranium is incorporated into the traditional gold hohlraum to increase the efficiency of the laser to x-ray energy conversion by making the wall more opaque to the x rays [H. Nishumura, T. Endo, H. Shiraga, U. Kato, and S. Nakai, Appl. Phys. Lett. 62, 1344 (1993)]. Multilayered depleted uranium (DU) and gold hohlraums are deposited by sputtering by alternately rotating a hohlraum mold in front of separate DU and Au sources to build up multilayers to the desired wall thickness. This mold is removed to leave a freestanding hohlraum half; two halves are used to assemble the complete NIF hohlraum to the design specifications. In practice, exposed DU oxidizes in air and other chemicals necessary to hohlraum production, so research has focused on developing a fabrication process that protects the U from damaging environments. This paper reports on the most current depleted uranium and gold cocktail hohlraum fabrication techniques, including characterization by Auger electron spectroscopy, which is used to verify sample composition and the amount of oxygen uptake over time.

Hard x-ray imaging for measuring laser absorption spatial profiles on the National Ignition Facility

Hard x-ray (“thin wall”) imaging will be employed on the National Ignition Facility (NIF) to spatially locate laser beam energy deposition regions on the hohlraum walls in indirect drive Inertial Confinement Fusion (ICF) experiments, relevant for ICF symmetry tuning. Based on time resolved imaging of the hard x ray emission of the laser spots, this method will be used to infer hohlraum wall motion due to x ray and laser ablation and any beam refraction caused by plasma density gradients. In optimizing this measurement, issues that have to be addressed are hard x-ray visibility during the entire ignition laser pulse with intensities ranging from 1013to1015W∕cm2, as well as simultaneous visibility of the inner and the outer laser drive cones. In this work we will compare the hard x-ray emission calculated by LASNEX and analytical modeling with thin wall imaging data recorded previously on Omega and during the first hohlraum experiments on NIF. Based on these calculations and comparisons the thin wall imaging will be optimized for ICF/NIF experiments.

Calculation of re-emission diagnostic in NIF ignition Hohlraum at 1MJ laser energy

Preliminary results of a two-dimensional design study are discussed for a SiO2 foam filled Hohlraum containing a Bi-coated re-emission capsule. The Hohlraum wall consists of a Au–U “cocktail” designed to maximize the amount of x-ray energy produced for the capsule to absorb, given the 1MJ laser energy into the Hohlraum. The foam fill acts to minimize wall expansion while maintaining symmetric drive on the capsule. Various foam densities and laser pointings for most efficient drive are considered. Sensitivities to drive asymmetries during the long “foot” portion of the laser drive are calculated and shown for the re-emission capsule. The foam fill was found to affect the re-emit symmetry much more than a H∕He gas fill. Compensating effects in beam balance or pointing are required to maintain best symmetry. The effect of the diagnostic on the Hohlraum environment with a Cu-doped Be ignition capsule is discussed. Effects of optimal filtering for maximum signal detectability are considered.

Chemical Compatibility of Silica Aerogel Processes with ICF Hohlraums

Inertial Confinement Fusion (ICF) energy hohlraums are composed of a high-Z material filled with foam. Because of the small pore size and transparency, silica aerogels are used in some ICF targets. The traditional synthesis of silica aerogels require sol-gel polymerization of silicon alkoxide followed by supercritical drying. Some constituents in sol-gel polymerization have been found to contribute to leaching of certain metals at the silica/metal interface. Since the hohlraums are composed of metals, possible chemical reactivity at the silica aerogel and metal hohlraum interface was investigated. The hohlraums studied are aluminum lined with either copper or copper/chromium. Upon initial inspection, the aerogel appeared transparent and uniform, however, closer inspection of the copper wall suggested possible leaching. Alternatively the quality of the aerogel in the copper-chromium hohlraum was very poor with the chromium layer of the hohlraum and some copper completely etched. Control experiments were used to determine the cause of the leaching. When copper is in the presence of sol-gel constituents, Cu2+ ion formed, thus leaching copper from the hohlraum walls. In the presence of chromium, Cr2O72- or CrO42- was identified in solution with the Cu2+, these anions are believed to form copper chromite under the aerogel synthesis procedures utilized.

Progress in Coating Multi-Layered Cocktail Hohlraums

The process of making multi-layered depleted uranium (DU) and gold “cocktail” hohlraums is being developed in a sputter-coater designed and assembled at General Atomics. These elements have been chosen to increase the hohlraum wall albedo, targeting the composition that results in the highest hohlraum efficiency. Rather than co-sputtering the cocktail constituents as was done previously, the approach taken in this work is to sputter alternating multiple thin layers of DU and gold. The intended outcome of creating a multi-layered structure is to encapsulate the DU in gold, thus reducing or perhaps preventing rapid oxidation of uranium, a known problem in the co-sputtered materials. Residual stress in coatings has been reduced to sufficiently low levels by optimizing deposition pressure allowing fabrication of free-standing cylinders and foils. Characterization of this type of sandwich material is difficult due to the fact that the cocktail region consists of buried interfaces, though promising results from Auger depth profiling show that the materials have sufficiently low oxygen content (≤4 at. %) as well as the targeted composition.

Theory and Numerical Modelling of the Effects of Ablator Wall and Hohlraum Transfer Gas Thermal Resistances on Deuterium-Tritium Redistribution Rates

The influence of capsule wall material and the transfer gas surrounding the capsule on the time required for beta-heating-induced redistribution of a 50-50 mole percent mixture of deuterium and tritium (DT) in a spherical capsule are investigated analytically and numerically. The derivation of an analytical solution for the redistribution time in a one-dimensional binary diffusion model, which includes the thermal resistance of the capsule, is first described. This result shows that the redistribution time for a high conductivity capsule wall is approximately doubled after 8 days of 3He formation. In contrast, with a low thermal conductivity capsule wall (e.g., polyimide), the redistribution time would increase by less than 10%The substantial effect of the capsule wall resistance suggested that the resistance to heat transfer from the capsule through the surrounding transfer gas to the hohlraum wall would also influence the redistribution process. This was investigated with a spherical model, which was based on accounting for energy transfer by diffusion with a conduction heat transfer approximation. This made it possible to solve for the continuous temperature distribution throughout the capsule and surrounding gas. As with the capsule the redistribution times depended on the relative values of the thermal resistances of the vapor in the capsule and the transfer gas. With increasing vapor thermal resistance (increased concentration of 3He) redistributions times for hydrocarbon capsules were less than the minimum one-dimensional value of 27 minutes. Further analytical and experimental investigation of the thermal interaction between the capsule and hohlraum is recommended.

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.

Experimental measurement of Au M-band flux in indirectly driven double-shell implosions

Indirectly driven double-shell implosions are being investigated as a possible noncryogenic path to ignition on the National Ignition Facility [J. A. Paisner, J. D. Boyes, S. A. Kumpan, W. H. Lowdermilk, and M. S. Sorem, Laser Focus World 30, 75 (1994)]. In recent double-shell experiments, the inner shell trajectory was shown to exhibit a strong sensitivity to the temporal history of the M-band (2–5keV) radiation emitted from the Au hohlraum wall. A large time-dependent discrepancy was observed between measurement and simulation of the x-ray flux in this range. In order to better characterize the radiation environment seen in these implosions, an experimental campaign was conducted on the Omega laser. A number of diagnostics were used to measure both the temporal and spectral nature of the M-band flux. Results were obtained from an absolutely calibrated 12-channel filtered x-ray diode array (Dante) as well as two streaked crystal spectrometers and an absolutely calibrated time-integrated spectrometer (Henway). X-ray backlighting was also used to directly measure the effect of M-band radiation on the trajectory of the inner shell. The data from all diagnostics are shown to be in excellent agreement and provide a consistent picture of the M-band flux. These results are being used to constrain and improve the simulation of hohlraum-generated M-band radiation that will be necessary for the design of future double-shell implosions employing higher-Z inner shells.

Target Physics Scaling for Z-Pinch Inertial Fusion Energy

The Z-pinch fusion energy power plant concept is based upon an X-ray driven inertial confinement fusion (ICF) capsule having a hypothetical yield of 3 GJ with an overall target gain in the range of 50-100. In the present paper, a combination of analytic arguments, results of radiation-hydrodynamic computational simulations, and empirical scalings from Z-pinch hohlraum experiments are used to demonstrate that the absorption of approximately 6 MJ of X-ray energy by the capsule and 26 MJ by the hohlraum walls of an ICF target (~ 32 MJ total X-ray input) will be adequate to provide a 3 GJ yield. As a result, it appears that the Ref. 1 assumption of a 3 GJ thermonuclear yield with an overall target gain approaching 100 is conceptually feasible.

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.

Quantified reduction of wall material influx during Hohlraum experiments

Heating the gold walls of a Hohlraum with intense laser beams produces a rapidly expanding gold plasma. Eventually, the wall material will converge on the axis of the Hohlraum with a density sufficient to be opaque to any standard radiography source. The gold expansion makes radiography of the back wall through the laser entrance hole of a Hohlraum driven from one side difficult. This experiment demonstrates a reduction of Au influx when the Hohlraum walls are coated with 0.44 μm of parylene-N. The reduction is quantified, using an x-ray framing camera, in a cylindrical Hohlraum driven by 6.8 kJ of laser light.

Late-time simulation of National Ignition Facility hohlraums

The late-time (t ≥ 80 ns) behaviour of hohlraums designed for the National Ignition Facility (NIF) is simulated using the multiphysics radiation hydrodynamics codes LASNEX and HYDRA. The spatial distribution of x-radiation outside the hohlraum is shown as a function of time. The energy spectrum of the x-ray emission is presented for various hohlraum viewing angles. We have made refinements to the grid motion algorithms in the arbitrary Lagrangian Eulerian (ALE) hydrodynamics code HYDRA to obtain the first late-time simulations of a hohlraum that can be used to give the spatial distribution of the vaporized hohlraum wall. The importance of late-time simulations in determining the lifetimes of debris shields on NIF is discussed.