Ian Lee Tregillis

Abstract Computational physicists are commonly faced with the task of resolving discrepancies between the predictions of a complex, integrated multi-physics numerical simulation and corresponding experimental datasets. Such efforts commonly require a slow iterative procedure. However, a different approach is available in cases where the multi-physics system of interest admits closed-form analytic solutions. In this situation, the ambiguity is conveniently broken into separate consideration of theory-simulation comparisons (issues of verification) and theory-data comparisons (issues of validation). We demonstrate this methodology via application to the specific example of a fluid-instability based ejecta source model (“RMI+SSVD”) under development at Los Alamos National Laboratory and implemented in FLAG, a Los Alamos continuum mechanics code. The formalism is conducted in the forward sense (i.e., from source to measurement) and enables us to compute, purely analytically, piezoelectric ejecta mass measurements for a specific class of explosively driven metal coupon experiments. We incorporate published measurement uncertainties on relevant experimental parameters to estimate a time-dependent uncertainty on these analytic predictions. This motivates the introduction of a “compatibility score” metric, our primary tool for quantitative analysis of the RMI+SSVD model.

Capsules driven with polar drive [1, 2] on the National Ignition Facility [3] are being used [4] to study mix in convergent geometry. In preparation for experiments that will utilize deuterated plastic shells with a pure tritium fill, hydrogen-filled capsules with copper doped deuterated layers have been imploded on NIF to provide spectroscopic and nuclear measurements of capsule performance. Experiments have shown that the mix region, when composed of shell material doped with about 1% copper (by atom), reaches temperatures of about 2 keV, while undoped mixed regions reach about 3 keV. Based on the yield from these implosions, we estimate the thickness of CD that mixed into the gas as between about 0.25 and 0.43 mu\m of the inner capsule surface, corresponding to about 5 to 9 mu g of material. Using 5 atm of tritium as the fill gas should result in over 1013 DT neutrons being produced, which is sufficient for neutron imaging [5].

The Defect Induced Mix Experiment (DIME-II) will measure the implosion and mix characteristics of CH capsules filled with 5 atmospheres of DT by incorporating mid-Z dopant layers of Ge and Ga. This polar direct drive (PDD) experiment also will demonstrate the filling of a CH capsule at target chamber center using a fill tube. Diagnostics for these experiments include areal x-ray backlighting to obtain early time images of the implosion trajectory and a multiple-monochromatic imager (MMI) to collect spectrally-resolved images of the capsule dopant line emission near bangtime. The inclusion of two (or more) thin dopant layers at separate depths within the capsule shell facilitates spatial correlation of mix between the layers and the hot gas core on a single shot. The dopant layers are typically 2 mu m thick and contain dopant concentrations of 1.5%. Three dimensional Hydra simulations have been performed to assess the effects of PDD asymmetry on capsule performance.

Inertial Confinement Fusion experiments at the National Ignition Facility (NIF) are designed to understand and test the basic principles of self-sustaining fusion reactions by laser driven compression of deuterium-tritium (DT) filled cryogenic plastic (CH) capsules. The experimental campaign is ongoing to tune the implosions and characterize the burning plasma conditions. Nuclear diagnostics play an important role in measuring the characteristics of these burning plasmas, providing feedback to improve the implosion dynamics. The Neutron Imaging (NI) diagnostic provides information on the distribution of the central fusion reaction region and the surrounding DT fuel by collecting images at two different energy bands for primary (13-15 MeV) and downscattered (10-12 MeV) neutrons. From these distributions, the final shape and size of the compressed capsule can be estimated and the symmetry of the compression can be inferred. The first downscattered neutron images from imploding ICF capsules are shown in this paper.

We have fielded a neutron imaging system at the National Ignition Facility to collect images of fusion neutrons produced in the implosion of inertial confinement fusion experiments and scattered neutrons from (n, n') reactions of the source neutrons in the surrounding dense material. A description of the neutron imaging system is presented, including the pinhole array aperture, the line-of-sight collimation, the scintillator-based detection system and the alignment systems and methods. Discussion of the alignment and resolution of the system is presented. We also discuss future improvements to the system hardware.

A summary of data and results from the first neutron images produced by the National Ignition Facility (NIF), Lawrence Livermore National Laboratory, Livermore, CA, USA are presented. An overview of the neutron imaging technique is presented, as well as a synopsis of data and measurements made to date. Data from directly driven, DT filled microballoons, as well as indirectly driven, cryogenically layered ignition experiments are presented. The data show that the primary cores from directly driven implosions are approximately twice as large, 64 +/- 3 mu m, as indirectly driven cores, 25 +/- 4 and 29 +/- 4 mu m and more asymmetric, P2/P0 = 47% vs. -14% and 7%. Further, comparison with the size and shape of X-ray image data on the same implosions show good agreement, indicating X-ray emission is dominated by the hot regions of the implosion.

Summary form only given. The neutron imaging diagnostic has recently been commissioned at the National Ignition Facility. We will present the diagnostic performance characteristics, which have been measured with the collection of these first neutron images. The goal for this diagnostic is to collect two pinhole images at two different times. The long flight path results in a chromatic separation of the neutrons, the first image will be of the 14 MeV neutrons and the second image of the 10-12 MeV neutrons. The combination of these two images will provide data on the size and shape of the compressed capsule as well as a measure of the quantity and spatial distribution of the cold fuel surrounding this core. The imager uses an array of 20 pinholes and three mini-penumbra machined in 20 cm of layered gold and tungsten, with an apex at 32.5 cm from the source to produce images in a scintillator array at 2800 cm. This geometry provides a magnification factor of 85 at the scintillator. The scintillator is a coherent array of scintillating fibers, which is viewed from the two ends by two fast-gated image collection systems. The first neutron images, collected in February, 2011, have provided the first measure of system performance at NIF. These results will be presented along with an interpretation of future system performance.

Neutron imaging is currently being developed as a primary diagnostic for inertial fusion studies at the National Ignition Facility (NIF). It is an attractive diagnostic for measuring asymmetries in the burn region and will be able to operate at neutron fluences found during ignition scale implosions. The most straightforward technique for imaging of the spatial distribution of deuterium-tritium (DT) fusion neutrons utilizes a simple pinhole aperture, which blocks all neutrons outside of the solid angle defined by the pinhole and results in a blurred image at the detector. We are currently investigating source image reconstruction techniques from detector images. Source reconstructions from Monte Carlo neutron transport (MCNP) calculations are shown to emulate hydrodynamic simulations with imposed Legendre asymmetries to high accuracy.

The effect of small localized perturbations, such as fill tubes and mounting tents, on the NIF ignition capsule and the effect of hemi-joints on high gain double shell capsules are an important issue in achieving ignition on NIF. Our codes have difficulty modeling small features and their effect on mix. Because of issues of symmetry, shock timing, high-z shells, mix etc. trying to understand the effect of localized perturbations ("defect") on these high gain NIF capsules will be difficult. To begin the study of defects on DT burn, a direct-drive exploding pusher is used. Experimental results are presented for capsules with and without defects. The unperturbed capsules give reproducible yields while the perturbed capsules show significant drops in yield. Both AMR and Lasnex codes over predict the unperturbed capsule yield