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CR-39 detectors are used routinely in inertial confinement fusion (ICF) experiments as a part of nuclear diagnostics. CR-39 is filtered to stop fast ablator ions which have been accelerated from an ICF implosion due to electric fields caused by laser-plasma interactions. In some experiments, the filtering is insufficient to block these ions and the fusion-product signal tracks are lost in the large background of accelerated ion tracks. A technique for recovering signal in these scenarios has been developed, tested, and implemented successfully. The technique involves removing material from the surface of the CR-39 to a depth beyond the endpoint of the ablator ion tracks. The technique preserves signal magnitude (yield) as well as structure in radiograph images. The technique is effective when signal particle range is at least 10 μm deeper than the necessary bulk material removal. [ABSTRACT FROM AUTHOR]

We describe a radiochemical measurement of the ratio of isotope concentrations produced in a gold hohlraum surrounding an Inertial Confinement Fusion capsule at the National Ignition Facility (NIF). We relate the ratio of the concentrations of (n,γ) and (n,2n) products in the gold hohlraum matrix to the down-scatter of neutrons in the compressed fuel and, consequently, to the fuel's areal density. The observed ratio of the concentrations of 198m+gAu and 196gAu is a performance signature of ablator areal density and the fuel assembly confinement time. We identify the measurement of nuclear cross sections of astrophysical importance as a potential application of the neutrons generated at the NIF. [ABSTRACT FROM AUTHOR]

A study was conducted with a line-imaging velocity interferometer on sandwich targets at the Shen Guang-III prototype laser facility in China, with the goal of eliminating the preheat effect. A sandwich target structure was used to reduce the x-ray preheat limitation (radiation temperature ∼170 eV) in a radiative drive shock experiment. With a thick ablator, the preheat effect appeared before the shock arrived at the window. After adding a shield layer of high-Z material on the ablator, x-rays which penetrated the ablator were so weak that the blank-out effect could not be measured. This experiment indicates that the sandwich target may provide a valuable technique in experiments such as equation of state and shock timing for inertial confinement fusion studies. [ABSTRACT FROM AUTHOR]

The velocity and remaining ablator mass of an imploding capsule are critical metrics for assessing the progress toward ignition of an inertially confined fusion experiment. These and other ablator rocket parameters have been measured using a single streaked x-ray radiograph. A regularization technique has been used to determine the ablator density profile ρ(r) at each time step; moments of ρ(r) then provide the areal density, average radius, and mass of the unablated, or remaining, ablator material, with the velocity determined from the time derivative of the average radius. The technique has been implemented on experiments at the OMEGA laser facility. [ABSTRACT FROM AUTHOR]

A methodology is presented for determining the compressive response of a porous material’s surface layer. Microscopic strain measurements are combined with an analytical model that describes the strain in the surface layer, at a given compressive stress, as a nonlinear function of surface depth. The model yields the effective thickness of the surface layer, explains quantitatively an increase in apparent material stiffness with increasing sample thickness, and yields accurate stress-strain curves (prior to yielding) for samples of arbitrary thickness. The methodology is applied successfully to silicone impregnated reusable ceramic ablator, a thermal protection system material developed and used by NASA for planetary missions, including the Mars rovers. [ABSTRACT FROM AUTHOR]

A new radiochemical method for determining deuterium-tritium (DT) fuel and plastic ablator (CH) areal densities (ρR) in high-convergence, cryogenic inertial confinement fusion implosions at the National Ignition Facility is described. It is based on measuring the 198Au/196Au activation ratio using the collected post-shot debris of the Au hohlraum. The Au ratio combined with the independently measured neutron down scatter ratio uniquely determines the areal densities ρR(DT) and ρR(CH) during burn in the context of a simple 1-dimensional capsule model. The results show larger than expected ρR(CH) values, hinting at the presence of cold fuel-ablator mix. [ABSTRACT FROM AUTHOR]

Fusion neutrons streaming from a burning inertial confinement fusion capsule generate gamma rays via inelastic nuclear scattering in the ablator of the capsule. The intensity of gamma-ray emission is proportional to the product of the ablator areal density (ρR) and the yield of fusion neutrons, so by detecting the gamma rays we can infer the ablator areal density, provided we also have a measurement of the capsule's total neutron yield. In plastic-shell capsules, for example, 12C nuclei emit gamma rays at 4.44 MeV after excitation by 14.1 MeV neutrons from D+T fusion. These gamma rays can be measured by a new gamma-ray detector under development. Analysis of predicted signals is in progress, with results to date indicating that the method promises to be useful for diagnosing imploded capsules. [ABSTRACT FROM AUTHOR]

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 1 MJ 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. [ABSTRACT FROM AUTHOR]

At Los Alamos, we have been working on two-dimensional capsule-Hohlraum implosion calculations for the NIF that utilize a laser drive pulse that peaks at ∼300 eV and uses about 1 MJ of laser energy. We use a 0.3 at. % uniformly Cu doped beryllium ablator capsule that has an inner ice radius of 753 μm, inner ablator radius of 825 μm, and an outer ablator radius of 1000 μm. Our Hohlraum has a diameter of 5.02 mm and a length of 8.56 mm. We use a Hohlraum gas fill density of 1.3 mg/cm3 and obtain ignition with yields up to 9.8 MJ, but we also have some failures due to bad shock timing or different pointing positions of the inner and outer laser cones. We describe the salient features of our implosions and show postprocessed diagnostic signatures from x-ray and neutron images, along with reaction history plots to show how we can diagnose “failures” and how to correct for them based on these diagnostics. [ABSTRACT FROM AUTHOR]

If x-radiation penetrates the ablator of an ignition capsule too deeply or too little, ignition can fail. Typically an ablator areal density of ∼0.2 g/cm2 remains at ignition time. 20% more or less flux in the final pulse of radiation driving the capsule can change that areal density by ∼0.1 g/cm2 and halve the capsule yield. We propose a technique to measure ablator areal density in a surrogate capsule that would complement Cu activation [D. C. Wilson et al., Phys. Plasmas 5, 1953 (1998)]. Replace the DT ice layer by deuterium and 3He gas. Diagnose the number and spectra of D–3He protons escaping the surrogate capsule with wedge range filters. For a 0.02 g/cm3 gas fill and the optimal radiation drive, 1.2e+11 protons escape with energies above 3 MeV and a peak at 5 MeV. If ignition would fail due to ablator burn through, 1.7e+11 protons would escape but the peak moves to 11 MeV. If ignition would fail from too much ablator remaining, 3.2e+9 protons escape the surrogate and the peak moves near 3 MeV. The D+D neutron yield remains roughly constant over this range. Wedge range filters would be able to diagnose these proton yields and peaks above ∼4 MeV with a resolution of ∼0.3 MeV. [ABSTRACT FROM AUTHOR]

Radiochemical analysis of post-ignition debris inside the National Ignition Facility (NIF) target chamber can help determine various diagnostic parameters associated with the implosion efficiency of the fusion capsule. This technique is limited by the ability to distinguish ablator material from other debris and by the collection efficiency of the capsule debris after implosion. Prior to designing an on-line collection system, the chemical nature and distribution of the debris inside the chamber must be determined. The focus of our current work has been on evaluating capture of activated Au hohlraum debris on passive foils (5 cm diameter, 50 cm from target center) post-shot. Preliminary data suggest that debris distribution is locally heterogeneous along the equatorial and polar line-of-sights. [ABSTRACT FROM AUTHOR]