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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 505 records · Page 28
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

A comprehensive numerical investigation on spray models for Direct-Injection Spark-Ignition engines

Gasoline direct-injection spark-ignition (DISI) engines generate a large portion of their unburned hydrocarbon (UHC) and soot emissions during the cold-start phase. A predictive computational fluid dynamics (CFD) modeling framework can be used to understand the physical processes that characterize fuel spray evolution and fuel-film formation at cold start conditions, which can help to reduce engine-out particulate emissions. This study systematically evaluated spray submodels and developed a set of simulation best practices for physical-numerical submodels with the goal of enabling accurate simulations of liquid spray behavior in a DISI engine. Three comprehensive experimental datasets containing free-spray projected liquid volume (PLV), liquid volume fraction (LVF), and near-field X-ray radiography data were used to validate the simulation results and evaluate the spray submodels. Systematic analysis delved into injected parcel distribution, droplet collision, spray breakup, and evaporation via a detailed assessment of the relevant spray submodels. Moreover, the effects of turbulence models and the initial turbulent flow properties on the liquid spray evolution were examined. Based on extensive calibration efforts, a set of simulation best practices for the free spray was developed and validated against the PLV/LVF data. Simulation results indicated that the uniform distribution for parcel initialization, coupled with appropriate droplet collision submodels, provides an improved spray morphology compared to the cluster distribution. The findings also underscored the importance of calibrating the Kelvin-Helmholtz Rayleigh-Taylor (KH-RT) breakup model constants and droplet heat transfer coefficient scaling factor to achieve favorable agreement regarding measured liquid penetration and spray widths. In conclusion, this study marks a substantial stride towards accurately predicting fuel film evolution and soot formation within DISI engine performance.

ECN Spray G↗
DOE OSTI2025

Overview of oxygen opacity experiments at the National Ignition Facility and investigation of potential systematic errors

Experiments to measure oxygen opacity at stellar interior conditions have been performed at the National Ignition Facility in a Discovery Science campaign. These experiments utilize the Opacity-on-NIF platform with a sample comprised of O, Mg, and Si. The spectral data from the Opacity Spectrometer cover the 1000–2000 eV photon energy range showing bound-free continuum absorption from O and line absorption from Mg and Si. DANTE and the Gated X-ray Detector are employed to measure the sample plasma’s temperature and density, respectively. Initial data show lower transmission than expected by theoretical models, raising questions of whether potential background or data uniformity concerns could produce systematic errors in the inferred transmission. Here, we investigate three concerns thought to be important for the oxygen opacity data, including instrumental scattered background, sample self-emission non-uniformity, and backlight continuum non-uniformity. Additionally, we show the effect of a recently developed method to account for 2nd order crystal reflection. The total effect of these concerns on one experiment is found to be small compared to the observed difference between the inferred transmission and a model calculation at the inferred temperature and density. Thus, we conclude that these potential sources of systematic error cannot account for the observed difference, increasing the likelihood of a real effect due to the high temperature and density conditions. However, because this is only a single experiment, we cannot make a firm conclusion. More experiments measuring the opacity and necessary calibrations are needed to assess the reproducibility and uncertainty of this result.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2025

3D Radio Frequency Mapping and Polarization Observations Show Lightning Flashes Were Ignited by Cosmic‐Ray Showers

Previous 2D radio frequency interferometric observations showed that lightning is commonly started with a positive fast discharge (+FD) before a normal negative leader continues from the origin of the +FD to the ensuing lightning flash. However, the inception and development of the +FD cannot be convincingly explained by existing discharge theories. With our new 3D broadband interferometric mapping and polarization system, we observed that the +FD was sometimes followed by an even faster and more extensive negative discharge (−FD) that propagated backward and overshoot the origin by a few hundred meters, as reported by an earlier 2D study. Surprisingly, the signal polarization, which measures the orientation of the discharge current, systematically slanted from the discharge propagation direction and rotated between the two opposite discharges, showing the +FD and −FD were driven by other storm-independent factors in addition to the storm electric field, or otherwise all would align in the same direction. Assuming a cosmic-ray shower (CRS) piercing through the cloud immediately before the +FD/−FD discharge, we found that their path is consistent with a pre-ionized path by the CRS, and the polarizations for the two opposite discharges are consistent with the respective deflected trajectories of high-energy positrons and electrons in the geomagnetic and an electric field. We further analyzed the more commonly observed +FD and showed that it is consistent with the CRS interpretation, suggesting these flashes in thunderstorms were ignited by cosmic-ray showers.

58 GEOSCIENCES↗
DOE OSTI2024

Measurement of early time outer laser beam reflection inside a cylindrical hohlraum at the National Ignition Facility

In a National Ignition Facility (NIF) cylindrical hohlraum, any specular reflection (“glint”) off the wall from the outer cone (incident angle of 50° and 44° relative to the surface of the wall) laser beam will irradiate the capsule poles. If the glint power is sufficiently large during the picket (early time) of the laser pulse, it may seed high-mode perturbations on the capsule surface that can grow during the implosion. To quantify the glint power on the capsule during the picket by the outer beams, we performed dedicated experiments on NIF using a flat witness foil as a surrogate for the capsule in a half-hohlraum target. We found that the measured glint power is approximately comparable to simulations using a nominal electron conduction flux limiter of f = 0.15, and over an order of magnitude lower than that predicted using f = 0.03 in the wall. Based on our current understanding, we conclude that the glint from the outer beams plays an insignificant role in capsule drive asymmetry.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2025

Using real-time nuclear activation detectors for measuring neutron yields from D(D, T)n reactions on the national ignition facility (NIF)

The National Ignition Facility (NIF) has 48 Real-Time Nuclear Activation Detectors distributed around the target chamber capable of measuring deuterium-triton reaction neutron yields with high precision. Here, in this work, we extend this functionality to deuterium–deuterium (DD) reaction neutrons using a nuclear reaction that occurs in the detector’s scintillator material. The corresponding decay of the activated material has a very short half-life of 5 s, which necessitates rapid data collection immediately following an experiment. In this regime, dead time can be very high (>50%) adding significant uncertainty to the measurement. To combat this, we have developed a dead time model that can self-consistently describe the measured data. Initial results show reasonable agreement (within 20%) with DD neutron yields from neutron time-of-flight spectrometers.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2024

X-ray imaging and electron temperature evolution in laser-driven magnetic reconnection experiments at the national ignition facility

We present results from x-ray imaging of high-aspect-ratio magnetic reconnection experiments driven at the National Ignition Facility. Two parallel, self-magnetized, elongated laser-driven plumes are produced by tiling 40 laser beams. A magnetic reconnection layer is formed by the collision of the plumes. A gated x-ray framing pinhole camera with micro-channel plate detector produces multiple images through various filters of the formation and evolution of both the plumes and current sheet. As the diagnostic integrates plasma self-emission along the line of sight, two-dimensional electron temperature maps ⟨Te⟩Y are constructed by taking the ratio of intensity of these images obtained with different filters. The plumes have a characteristic temperature ⟨Te⟩Y=240 ± 20 eV at 2 ns after the initial laser irradiation and exhibit a slow cooling up to 4 ns. The reconnection layer forms at 3 ns with a temperature ⟨Te⟩Y=280 ± 50 eV as the result of the collision of the plumes. The error bars of the plumes and current sheet temperatures separate at 4 ns, showing the heating of the current sheet from colder inflows. Using a semi-analytical model, we survey various heating mechanisms in the current sheet. We find that reconnection energy conversion would dominate at low density (ne≲7×1018 cm−3) and electron-ion collisional drag at high-density (≳1019 cm−3).

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2024

Electrical design of the flexible imaging diffraction diagnostic for laser experiments (FIDDLE) at the National Ignition Facility (NIF)—Requirements, design, and performance

The Flexible Imaging Diffraction Diagnostic for Laser Experiments (FIDDLE) is a newly developed diagnostic for imaging time resolved diffraction in experiments at the National Ignition Facility (NIF). It builds on the successes of its predecessor, the Gated Diffraction Development Diagnostic (G3D). The FIDDLE was designed to support eight Daedalus version 2 sensors (six more hCMOS sensors than any other hCMOS-based diagnostic in NIF to date) and an integrated streak camera. We will review the electrical requirements, design, and performance of the electrical subsystems that were created to support this large number of cameras in the FIDDLE. The analysis of the data that the FIDDLE is intended to collect relies heavily on the accurate and well-understood timing of each sensor. We report camera-to-camera timing jitter of less than 100 ps rms and sensor integration times of 2.2 ns FWHM in 2-2 timing mode. Additionally, diffraction experiments on the NIF produce electric fields (EMI) on the order of 1 kV/m, which have been observed to negatively impact the performance of some electrical components of the FIDDLE. We report on the results of testing hCMOS camera electronics in a similar EMI environment generated in an offline lab. In addition, we also summarize the use of a novel approach to using a vector network analyzer as an EMI leak detector to understand and reduce the negative impacts of EMI on the FIDDLE.

47 OTHER INSTRUMENTATION↗
DOE OSTI2024

Testing the optical components for the National Ignition Facility time-resolved soft x-ray opacity spectrometer (OpSpecTR)

Opacity measurements are being carried out at the Z-facility at Sandia National Laboratories and at the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory. The current soft x-ray Opacity Spectrometer (OpSpec) used on the NIF uses two elliptically bent crystals in time-integrated mode on either an image plate or a film. Plans are under way to expand these opacity measurements into a mode of time-resolved detection, called OpSpecTR. Previously, considerations for the available hCMOS detector size and photometrics led to a crystal geometry redesign and the use of a grazing angle x-ray mirror. The mirror acts as a low-pass x-ray energy filter, reducing the contribution of higher energy x rays. The first tests of the mirror and the crystal for OpSpecTR are presented here. The size of the mirror reflection and the reflectivity is tested using a Manson x-ray source. The mirror coupled with the new elliptical crystal shape demonstrates OpSpecTR’s spectral coverage. Finally, the results from the x-ray optics performance testing are shown along with the intended design.

47 OTHER INSTRUMENTATION↗
DOE OSTI2024

Ex situ calibration of the scattered-light time-history diagnostic on the National Ignition Facility

The scattered-light time-history diagnostic (SLTD) suite measures time-resolved scattered light in three wavelength bands: stimulated Brillouin scattering (350–352 nm), stimulated Raman scattering (430–760 nm), and plasma emission at half the laser frequency (695–735 nm), at 15 locations around the National Ignition Facility (NIF) target chamber. The SLTD, along with the full-aperture backscatter station (FABS), collects scattered light from direct- and indirect-drive inertial confinement fusion experiments. The SLTD calibration was revisited after a discrepancy between FABS and SLTD measurements was observed on NIF polar direct-drive experiments. An integrated calibration of the SLTD was performed for the first time, and individual components were also calibrated for the wavelengths of 351, 527, and 532 nm. The optical transmission of the instrument was measured to be (1.12 ± 0.04) × 10 –7 and (1.96 ± 0.11) × 10 –7 for the wavelengths of 351 and 532 nm, respectively. The revised calibration at 351 nm brings the SLTD measured scattered energy in agreement with the FABS measured scattered energy after additionally accounting for the degradation of an optical element in FABS. Furthermore, this decreased the inferred absorption by 7% for a representative experiment. However, discrepancies remain between FABS and SLTD measurements in the SRS band (532 nm).

47 OTHER INSTRUMENTATION↗
DOE OSTI2024

Characterization of a CMOS camera based film digitization platform for gated x-ray imaging diagnostics at the National Ignition Facility

Hardened gated x-ray detectors use photographic film as the data recording medium due to its low sensitivity to the high-yield neutron environments at the National Ignition Facility (NIF). The photographic film is digitized with a Photometric Data Systems (PDS) microdensitometer, which measures the film’s optical density. The PDS scanner is able to measure a dynamic range of 0–5 OD; however, raster scanning the film is time consuming and maintenance of the instrument is challenging due to legacy technology. Since film usage at NIF is expected to continue in the foreseeable future, a digitization platform that is faster and more maintainable would benefit the NIF’s current and future operations. Here, this work presents the characterization of the digital transitions (DT) atom, a CMOS camera-based digitization platform that records film data in a single image capture very quickly and has widely available user support. The preliminary results suggest that the DT atom is able to reconstruct exposures accurately enough to be a competitive alternative to the PDS Scanner.

47 OTHER INSTRUMENTATION↗
DOE OSTI2024

Development of a compact magnetic spectrometer for use at the OMEGA Laser Facility and the National Ignition Facility

Measurement of proton spectra is an important diagnostic for a variety of high energy density physics experiments. Current diagnostics are either not designed to capture the spectrum of low-energy protons or are unsuitable for high debris experiments. To bridge the gap, a new CR-39 based compact magnetic spectrometer (MagSpec) has been developed to measure proton spectra in the 1–20 MeV energy range, with a particular focus on the low-energy (1–6 MeV) spectrum, for use in experiments at the OMEGA Laser Facility and the National Ignition Facility (NIF). In the MagSpec diagnostic, protons of different energies are dispersed as they pass through a magnetic field before impinging on a differentially filtered CR-39 surface, resulting in a spatial distribution of CR-39 tracks that corresponds to the energy spectrum. In this paper, we discuss details of the design and implementation of MagSpec on the NIF and OMEGA.

47 OTHER INSTRUMENTATION↗