James R. Langenbrunner

Anomalous yield reduction in direct-drive deuterium/tritium implosions due to H3e additiona)

Glass capsules were imploded in direct drive on the OMEGA laser [Boehly et al., Opt. Commun. 133, 495 (1997)] to look for anomalous degradation in deuterium/tritium (DT) yield and changes in reaction history with H3e addition. Such anomalies have previously been reported for D/H3e plasmas but had not yet been investigated for DT/H3e. Anomalies such as these provide fertile ground for furthering our physics understanding of inertial confinement fusion implosions and capsule performance. Anomalous degradation in the compression component of yield was observed, consistent with the “factor of 2” degradation previously reported by Massachusetts Institute of Technology (MIT) at a 50% H3e atom fraction in D2 using plastic capsules [Rygg, Phys. Plasmas 13, 052702 (2006)]. However, clean calculations (i.e., no fuel-shell mixing) predict the shock component of yield quite well, contrary to the result reported by MIT but consistent with Los Alamos National Laboratory results in D2/H3e [Wilson et al., J. Phys.: Conf....

D-T gamma-to-neutron branching ratio determined from inertial confinement fusion plasmasa)

A new deuterium-tritium (D-T) fusion gamma-to-neutron branching ratio [3H(d,γ)5He/3H(d,n)4He] value of (4.2 ± 2.0) × 10−5 was recently reported by this group [Y. Kim et al. Phys. Rev. C (submitted)]. This measurement, conducted at the OMEGA laser facility located at the University of Rochester, was made for the first time using inertial confinement fusion (ICF) plasmas. Neutron-induced backgrounds are significantly reduced in these experiments as compared to traditional beam-target accelerator-based experiments due to the short pulse nature of ICF implosions and the use of gas Cherenkov γ-ray detectors with fast temporal responses and inherent energy thresholds. It is expected that this ICF-based measurement will help resolve the large and long-standing inconsistencies in previously reported accelerator-based values, which vary by a factor of approximately 30. The reported value at ICF conditions was determined by averaging the results of two methods: (1) a direct measurement of ICF D-T γ-ray and neutron ...

Measurement of areal density in the ablators of inertial-confinement-fusion capsules via detection of ablator (n, n′γ) gamma-ray emission

We report the first gamma-ray-based measurements of the areal density of ablators in inertial-confinement-fusion capsule implosions. The measurements, made at the OMEGA laser [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)], used observations of gamma rays arising from inelastic scattering of 14.1-MeV deuterium-tritium (DT) neutrons on 12C nuclei in the compressed plastic ablators. The emission of 12C(n,n′γ) gamma rays from the capsules is detected using the Gamma Reaction History instrument [H. W. Herrmann et al., J. Phys.: Conf. Ser. 244, 032047 (2010)] operating at OMEGA. From the ratio of a capsules 12C(n,n′γ) emission to the emission from the same processes in an in situ reference graphite “puck” of known mass and geometry [N. M. Hoffman et al., in IFSA 2011 proceedings (submitted)], we determine the time-averaged areal density of 12C in the capsules compressed ablator. Measured values of total ablator areal density for thirteen imploded capsules, in the range 23 ± 10 to 58 ± 14 mg/cm2, are comp...

An Uncertainty Inventory Demonstration—A Primary Step in Uncertainty Quantification

Tools, methods, and theories for assessing and quantifying uncertainties vary by application. Uncertainty quantification tasks have unique desiderata and circumstances. To realistically assess uncertainty requires the engineer/scientist to specify mathematical models, the physical phenomena of interest, and the theory or framework for assessments. For example, Probabilistic Risk Assessment (PRA) specifically identifies uncertainties using probability theory, and therefore, PRA’s lack formal procedures for quantifying uncertainties that are not probabilistic. The Phenomena Identification and Ranking Technique (PIRT) proceeds by ranking phenomena using scoring criteria that results in linguistic descriptors, such as importance ranked with words, “High/Medium/Low.” The use of words allows PIRT to be flexible, but the analysis may then be difficult to combine with other uncertainty theories. We propose that a necessary step for the development of a procedure or protocol for uncertainty quantification (UQ) is the application of an Uncertainty Inventory. An Uncertainty Inventory should be considered and performed in the earliest stages of UQ.

Inference Uncertainty Quantification Instead of Full-scale Testing

We give simple examples illustrating the concept and importance of inference uncertainty, which can be defined as the difference between what is measured (the observable quantity) and what is desired (the unobserved quantity). Quantification of uncertainty arising from inference has an important role to play in lieu of full-scale testing, because system-level uncertainties may not be observable by observing separate effects tests. Yet, little attention has been paid to this type of uncertainty, which is prevalent in numerous scientific and engineering applications. We propose that inference uncertainty can be mathematically characterized using different theories of uncertainty, including probability theory. A metric for the quantification of margins and uncertainties relating to factor of safety is discussed, and an example of information integration is illustrated. (Manuscript approved for unlimited, public release, LA-UR-08-1669, Unclassified.)

Evolving desiderata for validating engineered-physics systems without full-scale testing

Theory and principles of engineered-physics designs do not change over time, but the actual engineered product does evolve. Engineered components are prescient to the physics and change with time. Parts are never produced exactly as designed, assembled as designed, or remain unperturbed over time. For this reason, validation of performance may be regarded as evolving over time. Desired use of products evolves with time. These pragmatic realities require flexibility, understanding, and robustness-to-ignorance. Validation without full-scale testing involves engineering, small-scale experiments, physics theory and full-scale computer-simulation validation. We have previously published an approach to validation without full-scale testing using information integration, small-scale tests, theory and full-scale simulations [Langenbrunner et al. 2008]. This approach adds value, but also adds complexity and uncertainty due to inference. We illustrate a validation example that manages evolving desiderata without full-scale testing.

Validation of sea ice models using an uncertainty-based distance metric for multiple model variables: NEW METRIC FOR SEA ICE MODEL VALIDATION

We implement a variance-based distance metric (Dn) to objectively assess skill of sea ice models when multiple output variables or uncertainties in both model predictions and observations need to be considered. The metric compares observations and model data pairs on common spatial and temporal grids improving upon highly aggregated metrics (e.g., total sea ice extent or volume) by capturing the spatial character of model skill. The Dn metric is a gamma-distributed statistic that is more general than the χ2 statistic commonly used to assess model fit, which requires the assumption that the model is unbiased and can only incorporate observational error in the analysis. The Dn statistic does not assume that the model is unbiased, and allows the incorporation of multiple observational data sets for the same variable and simultaneously for different variables, along with different types of variances that can characterize uncertainties in both observations and the model. This approach represents a step to establish a systematic framework for probabilistic validation of sea ice models. The methodology is also useful for model tuning by using the Dn metric as a cost function and incorporating model parametric uncertainty as part of a scheme to optimize model functionality. We apply this approach to evaluate different configurations of the standalone Los Alamos sea ice model (CICE) encompassing the parametric uncertainty in the model, and to find new sets of model configurations that produce better agreement than previous configurations between model and observational estimates of sea ice concentration and thickness.

Knowledge Integration for Uncertainty Management

Knowledge integration is based upon gathering and aggregating all available data, information, and knowledge from theory, experience, computation and similar applications. Such a ”waste nothing” approach becomes important when the underlying theory is difficult to model, when observational data are sparse or difficult to measure, or when uncertainties are large. An inference approach is prescribed, providing common ground for many kinds of uncertainties arising from the sources of data, information and knowledge. These sources are integrated using a modified Saaty’s Analytic Hierarchy Process (AHP). A fusion physics application illustrates how to manage the uncertainties in the inference-based integration approach. Zadeh membership functions and possibility distributions contribute to this management.

Measurement of D-T branching ratio based on cross-calibration to D-3He

Summary form only given. Interest in the gamma-ray producing branch of D-T fusion has increased as the National Ignition Facility (NIF) is moving toward ICF D-T ignition. Measurements of the branching ratio for T(d,γ)5He relative to T(d,α)n are necessary if the gamma-rays are to be fully exploited as a fusion diagnostic. In the present experiment, gamma rays from T(d,γ)5He and D(3He,γ)5Li were measured using a Gas Cherenkov Detector (GCD) at Omega laser facility. The GCD is a high-energy gamma ray detector utilizing gamma-to-electron Compton scattering and electron-to-light Cherenkov principle. The high-bandwidth of the GCD allows for the detection of D-T fusion gamma rays before the 14.1 MeV neutrons arrive at the GCD, mitigating the neutron background issues that have plagued beam-target based measurements. The primary goal of the experiment was to cross-calibrate the D-T gamma/neutron branching ratio against the better known D-3He gamma/proton branching ratio. Accurate measurements of the neutron and proton fusion product yields are key components for this work. Initial experimental results show that D-T branching ratio is about 2.6 times less than the D-3He branching ratio, which is inconsistent with the assumption of charge independence of D-3He and D-T reactions. The refining of the exact branching ratio will be discussed at the conference.

A reduced model for the ICF gamma-ray reaction history diagnostic

An analytic model for the gamma reaction history (GRH) diagnostic to be fielded on the National Ignition Facility is described. The application of the GRH diagnostic for the measurement of capsule rho-R during burn using 4.4 MeV carbon gamma rays is demonstrated by simulation.