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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 1,585 records · Page 88
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Calculating the momentum enhancement factor for asteroid deflection studies

The possibility of kinetic-impact deflection of threatening near-Earth asteroids will be tested for the first time in the proposed AIDA (Asteroid Impact Deflection Assessment) mission, involving NASAs DART (Double Asteroid Redirection Test). The impact of the DART spacecraft onto the secondary of the binary asteroid 65803 Didymos at a speed of 5 to 7 km/s is expected to alter the mutual orbit by an observable amount. Furthermore, the velocity transferred to the secondary depends largely on the momentum enhancement factor, typically referred to as beta. Here, we use two hydrocodes developed at Los Alamos, RAGE and PAGOSA, to calculate an approximate value for beta in laboratory-scale benchmark experiments. Convergence studies comparing the two codes show the importance of mesh size in estimating this crucial parameter.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Neutron radiation hardness testing of 650V / 7.5 A GaN power HEMT

In this work, we have performed neutron radiation hardness testing for commercially available 650 V/7.5 A GaN power HEMT. The devices were tested at Los Alamos Neutron Science Center (LANSCE) inside Irradiation of Chips Electronics, ICE house-1 at a flux density of 10(6) n/cm(2) -s and energy level above 1 MeV, which is approximately 10(6) times higher than neutron radiation at airliner flight height (30,000 ft). During the experiments, the devices were stressed electrically, thermally and irradiated with neutron radiation. We have observed degradation of AlGaN layer due to accelerated neutron radiation (1.54 x 10(10) n/cm(2) above 1.5 MeV neutron energy) under electrical stress condition. We have performed parametric defect analysis through step-by-step application of neutron irradiation, thermal stress and electrical bias, and observed highest degradation of threshold voltage (0.14 V) for combined electrical stress and neutron irradiation.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Qualifying for the Green500: Experience with the newest generation of supercomputers at LANL

The High Performance Computing Division of Los Alamos National Laboratory recently brought four new supercomputing platforms on line: Trinity with separate partitions built around the Haswell and Knights Landing CPU architectures for capability computing and Grizzly, Fire, and Ice for capacity computing applications. The power monitoring infrastructure of these machines is significantly enhanced over previous supercomputing generations at LANL and all were qualified at the highest level of the Green500 benchmark. Here, this paper discusses supercomputing at LANL, the Green500 benchmark, and notes on our experience meeting the Green500's reporting requirements.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2023

Moon, Mars and Minds: Evaluating Parkinson’s disease mortality among U.S. radiation workers and veterans in the million person study of low-dose effects

Background: Radiation is one of the most important stressors related to missions in space beyond Earth’s orbit. Epidemiologic studies of exposed workers have reported elevated rates of Parkinson’s disease. The importance of cognitive dysfunction related to low-dose rate radiation in humans is not defined. A meta-analysis was conducted of six cohorts in the Million Person Study (MPS) of low-dose health effects to learn whether there is consistent evidence that Parkinson’s disease is associated with radiation dose to brain. Materials and methods: The MPS evaluates all causes of death among U.S. radiation workers and veterans, including Parkinson’s disease. Systematic and consistent methods are applied to study all categories of workers including medical radiation workers, industrial radiographers, nuclear power plant workers, atomic veterans, and Manhattan Projects workers at the Los Alamos National Laboratory and at Rocky Flats. Consistent methods for all cohorts are used to estimate organ-specific doses and to obtain vital status and cause of death. Results: The meta-analysis include 6 cohorts within the MPS, consisting of 517,608 workers and 17,219,001 person-years of observation. The mean dose to brain ranged from 6.9 to 47.6 mGy and the maximum dose from 0.76 to 2.7 Gy. Five of the 6 cohorts revealed positive associations with Parkinson’s disease. The overall summary estimate from the meta-analysis was statistically significant based on 1573 deaths due to Parkinson’s disease. The summary excess relative risk at 100 mGy was 0.17 (95% CI: 0.05; 0.29). Conclusions: Parkinson’s disease was positively associated with radiation in the MPS cohorts indicating the need for careful evaluation as to causality in other studies, delineation of possible mechanisms, and assessing possible implications for space travel as well as radiation protection guidance for terrestrial workers.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2020

The effect of initial conditions on mixing transition of the Richtmyer–Meshkov instability

We investigate the late-time Richtmyer–Meshkov instability (RMI) growth of sinuous perturbations on an air/sulphur hexafluoride interface (Atwood number, A ~ 0.67 ) subjected to a Mach 1.2 planar shock wave at Los Alamos National Laboratory's vertical shock tube facility. Interface perturbations are established using a novel membraneless technique where cross-flowing air and SF 6 separated by an oscillating splitter plate create a perturbed density interface. The interface formed has multi-modal features and residual small perturbations, however, a dominant mode is still noticeable. The late-time perturbation growths scale with $ka_0$ initial conditions (where $k$ is the wavenumber and $a_0$ is the initial amplitude of the dominant mode) as measured at the pre-shock interface. Past nonlinear models based on potential-flow theory, heuristic/interpolation approaches, Padé approximants and numerical simulations are evaluated against present experimental results. Accounting for an explicit $ka_0$ dependence in Sadot et al.'s model, we propose an empirical rational function that captures the asymptotic behaviour of perturbation growth for a broad range of initial conditions ( 0.30 $≤ ka_0 ≤$ 0.86 ). The onset of mixing transition and its initial condition dependence are investigated with respect to the minimum state criterion ($Re =$ 1.6 $\times$ 10 5 ) for unsteady flows by Zhou. Earlier mixing transitions for higher ka0 initial conditions are noted from local and global Reynolds number estimates which are corroborated by the existence of an inertial sub-range and formation of mixing regions indicating the physical significance of the minimum state criterion in RMI flows. The transition is accompanied by the increasing teapot-like appearance of joint probability density functions of $p–q$ (invariants of the reduced velocity gradient tensor), establishing the technique as a useful tool for turbulence detection in two-dimensional diagnostics.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Laboratory plasma physics experiments using merging supersonic plasma jets

We describe a laboratory plasma physics experiment at Los Alamos National Laboratory that uses two merging supersonic plasma jets formed and launched by pulsed-power-driven railguns. The jets can be formed using any atomic species or mixture available in a compressed-gas bottle and have the following nominal initial parameters at the railgun nozzle exit: n e ≈ n i ~ 10¹⁶ cm⁻³, T e ≈ T i ≈ 1.4 eV, V jet ≈ 30–100 km/s, mean charge $\bar{Z}$ ≈ 1, sonic Mach number M s ≡ V jet /C s > 10, jet diameter = 5 cm, and jet length ≈ 20 cm. Experiments to date have focused on the study of merging-jet dynamics and the shocks that form as a result of the interaction, in both collisional and collisionless regimes with respect to the inter-jet classical ion mean free path, and with and without an applied magnetic field. However, many other studies are also possible, as discussed in this paper.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

Spatially resolved texture and microstructure evolution of additively manufactured and gas gun deformed 304L stainless steel investigated by neutron diffraction and electron backscatter diffraction

In this study, we report the characterization of a 304L stainless steel cylindrical projectile produced by additive manufacturing. The projectile was compressively deformed using a Taylor Anvil Gas Gun, leading to a huge strain gradient along the axis of the deformed cylinder. Spatially resolved neutron diffraction measurements on the HIgh Pressure Preferred Orientation time-of-flight diffractometer (HIPPO) and Spectrometer for Materials Research at Temperature and Stress diffractometer (SMARTS) beamlines at the Los Alamos Neutron Science CEnter (LANSCE) with Rietveld and single-peak analysis were used to quantitatively evaluate the volume fractions of the α, γ, and ε phases as well as residual strain and texture. The texture of the γ phase is consistent with uniaxial compression, while the α texture can be explained by the Kurdjumov–Sachs relationship from the γ texture after deformation. This indicates that the material first deformed in the γ phase and subsequently transformed at larger strains. The ε phase was only found in volumes close to the undeformed material with a texture connected to the γ texture by the Shoji–Nishiyama orientation relationship. This allows us to conclude that the ε phase occurs as an intermediate phase at lower strain, and is superseded by the α phase when strain increases further. Furthe, we found a proportionality between the root-mean-squared microstrain of the γ phase, dominated by the dislocation density, with the α volume fraction, consistent with strain-induced martensite α formation. In conclusion, knowledge of the sample volume with the ε phase from the neutron diffraction analysis allowed us to identify the ε phase by electron back scatter diffraction analysis, complementing the neutron diffraction analysis with characterization on the grain level.

304L stainless steel↗
DOE OSTI2018

Fifth User Workshop on high-power lasers at the Linac Coherent Light Source

The fifth international user workshop focusing on high-power lasers at the Linac Coherent Light Source (LCLS) was held in Menlo Park, CA, USA on September 27-28, 2017. The workshop was co-organized by SLAC National Accelerator Laboratory (SLAC) and Los Alamos National Laboratory, and it had approximately 100 scientists in attendance. For the first time, this year’s workshop was combined with the larger LCLS/SSRL User Meeting. Participants discussed the recent experiments that have been performed using high power lasers at the LCLS-Matter in Extreme Conditions (MEC) endstation. Ten journal articles have been published during the previous year from research at the MEC instrument, and research performed at MEC has also contributed to several PhD theses. This workshop provided an opportunity for the scientific community to learn about recent accomplishments, as well as to discuss future facility upgrades. SLAC has already begun construction for a major upgrade of the X-ray free electron laser LCLS to LCLS-II, which made the community feedback particularly timely at this workshop. LCLS will be closed during 2019 for the upgrade and will re-commission new and enhanced capabilities in early 2020.

47 OTHER INSTRUMENTATION↗
DOE OSTI2012

Controlling Axial p-n Heterojunction Abruptness Through Catalyst Alloying in Vapor-Liquid-Solid Grown Semiconductor Nanowires

The p-n junction can be regarded as the most important electronic structure that is responsible for the ubiquity of semiconductor microelectronics today. Efforts to continually scale down the size of electronic components is guiding research to explore the use of nanomaterials synthesized from a bottom-up approach - group-IV semiconductor nanowires being one such material. However, Au-catalyzed synthesis of Si/Si1-x-Gex semiconductor nanowire heterojunctions using the commonly-used vapor-liquid-solid (VLS) growth technique results in diffuse heterojunction interfaces [1], leading to doubts of producing compositionally-sharp p-n junctions using this approach. However, we have recently reported the ability to increase Ge-Si nanowire heterojunction abruptness by VLS synthesis from a Au(1-x)Ga(x) catalyst alloy as shown by EDX analysis in an SEM [2]. In this work, we have extended the use of a AuGa catalyst alloy to produce more compositionally abrupt p-n junction interfaces compared to using pure Au as directly measured by atom probe tomography. As shown in Figure 1(a-b), individual Ge-Si heterostructured nanowires were grown vertically atop Ge(111) microposts. Direct growth on the microposts provides a facile approach to nanowire analysis which circumvents the need to use FIB-based sample preparation techniques. Both nanowires grown from pure Au and a AuGa catalyst alloy were analyzed. The corresponding 3D APT reconstruction of an individual heterostructured nanowire is shown in Figure 1(c) with the corresponding materials labeled. A 1-dimensional composition profile along the analysis direction in Figure 1(d) confirms an increase in heterojunction abruptness for nanowires grown from AuGa (~10nm) compared to nanowires grown from pure Au (~65nm). Analysis of the P distribution within the Si region (Figure 1(e)) indicates that P reaches a constant distribution over approximately 10nm when incorporated through the AuGa catalyst, whereas it continually increases over 100’s of nanometers when incorporated through pure Au. The apparent lower overall P concentration within the nanowire grown from the AuGa alloy suggests that the solubility of P in the alloy is lower compared to pure Au. The ability to controllably increase nanowire p-n junction abruptness is important for nanowire applications as solar cells and tunneling field effect transistors where an increase in device performance is expected from shaper p-n junction interfaces. [1] T.E. Clark et al., Nano Lett. 8 (2008) 1246. [2] D.E. Perea et al., Nano Lett. 11 (2011) 3117. [3] The research was supported through the user program at both the Center for Integrated Nanotechnologies at Los Alamos National Laboratory and the Environmental Molecular Sciences Laboratory at Pacific Northwest National Laboratory.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2020

Insight into the Chemistry of PETN Under Shock Compression Through Ultrafast Broadband Mid-Infrared Absorption Spectroscopy

In this study, thin films of pentaerythritol tetranitrate (PETN) were shock compressed using the laser driven shock apparatus at Los Alamos National Laboratory (LANL). Two spectroscopic probes were available to this apparatus: visible white light transient absorption spectroscopy (VIS) from 400 to 700 nm and mid-infrared transient absorption spectroscopy (MIR) from 1150 to 3800 cm –1 . Important PETN vibrational modes are the symmetric and antisymmetric NO 2 stretches at 1280 and 1650 cm –1 , respectively, as well as CH stretches at ~2900 cm –1 . Shock strength was varied from approximately 3 to 55 GPa to span from the chemically unreactive regime to the regime in which fast chemical reaction took place on the 250 ps time scale of the measurements. VIS and MIR results suggest irreversible chemistry was induced in PETN at pressures above 30 GPa. At lower shock pressures, the spectroscopy showed minimal changes attributable to pressure induced effects. Under the higher-pressure reactive conditions, the frequency region at the antisymmetric NO 2 stretch mode had a significantly increased absorption while the region around the symmetric NO 2 stretch did not. No observable increased absorption occurred in the higher frequency regions where CH-, NH-, and OH- bond absorptions would be observed. A broad absorption appeared on the shoulder at the red-edge of the CO 2 vibrational band around 2200 cm –1 . In addition to the experiments, reactive molecular dynamics were carried out under equivalent shock conditions to correlate the evolution of the infrared spectrum to molecular processes. The simulations show results consistent to experiments up to 30 GPa but suggest that NO and NO 2 related features provided the strongest contributions to the shocked infrared changes. Proposed mechanisms for shocked PETN chemistry are analyzed as consistent or inconsistent with the data presented here. Our experimental data suggests C≡O or N 2 O bond formation, nitrite formation, and absence of significant hydroxyl or amine concentrations in the initial chemistry steps in PETN shocked above 30 GPa.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Tribute to Lawrence R. Pratt

Lawrence Pratt’s career following completion of his Ph.D. at the University of Illinois Urbana Champaign has taken him from Harvard University, to the University of California, Berkeley, and Los Alamos National Laboratory. Most recently, he joined the faculty of the Department of Chemical and Biomolecular Engineering at Tulane University in 2008. Over his career Lawrence has been a leader in theoretical physical chemistry, making influential contributions to a number of areas including the theory of the hydrophobic effect, the development of transition path sampling, contributions to orbital free density functional theory, and the theory of liquids and solutions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Southwestern U.S. Biomass Burning Smoke Hygroscopicity: The Role of Plant Phenology, Chemical Composition, and Combustion Properties

Biomass burning emissions have substantially increased with continued warming and drying in the southwestern U.S., impacting air quality and atmospheric processes. To better quantify impacts of biomass burning aerosols, an extensive laboratory study of fresh smoke emissions was conducted at Los Alamos National Laboratory. Laboratory burn experiments with selected native and invasive southwestern U.S. fuels were used to elucidate the role of fuel type, chemical composition, and ignition method on the hygroscopicity of smoke. Here we focus on a custom controlled relative humidity (RH) nephelometry system using the direct measurement of aerosol light scattering with two nephelometers—one at dry conditions and one at a controlled high RH (RH ~ 85%). Aerosol hygroscopicity was highly variable with the enhancement in light scattering coefficient in the range of 1.02 < f (RH = 85%) < 2.1 and corresponding to the kappa parameter (κ neph ) ranging from ~0 to 0.18. Hygroscopicity is determined primarily by the fuel's inorganic ion content. For example, invasive halophytes with high inorganic salt content exhibit much greater water uptake than native coniferous species with low inorganic content. Combustion temperature and phase, flaming or smoldering, play a secondary role in the water uptake of smoke. High–temperature ignition methods create flaming conditions that enhance hygroscopicity while lower–temperature smoldering conditions diminish hygroscopicity. Our results construct an empirical relation between κ neph and the inorganic content of the fuel and smoke to predict water uptake.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2018

Oceanic Heat Delivery to the Antarctic Continental Shelf: Large-Scale, Low-Frequency Variability

According to the article, onshore penetration of oceanic water across the Antarctic continental slope (ACS) plays a major role in global sea level rise by delivering heat to the Antarctic marginal seas, thus contributing to the basal melting of ice shelves. Here the time-mean (Φ mean ) and eddy (Φ eddy ) components of the heat transport (Φ) across the 1,000-m isobath along the entire ACS are investigated using a 0.1° global coupled ocean/sea ice simulation based on the Los Alamos Parallel Ocean Program (POP) and sea ice (Community Ice CodE) models. In this work, comparison with in situ hydrography shows that the model successfully represents the basic water mass structure, with a warm bias in the Circumpolar Deep Water layer. Segments of on-shelf Φ, with lengths of O(100–1,000 km), are found along the ACS. The circumpolar integral of the annually averaged Φ is O(20 TW), with Φ eddy always on-shelf, while Φ mean fluctuates between on-shelf and off-shelf. Stirring along isoneutral surfaces is often the dominant process by which eddies transport heat across the ACS, but advection of heat by both mean flow-topography interactions and eddies can also be significant depending on the along- and across-slope location. The seasonal and interannual variability of the circumpolarly integrated Φ mean is controlled by convergence of Ekman transport within the ACS. Prominent warming features at the bottom of the continental shelf (consistent with observed temperature trends) are found both during high-Southern Annular Mode and high-Niño 3.4 periods, suggesting that climate modes can modulate the heat transfer from the Southern Ocean to the ACS across the entire Antarctic margin.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2021

Geospace Plume and Its Impact on Dayside Magnetopause Reconnection Rate

The role a geospace plume in influencing the efficiency of magnetopause reconnection is an open question with two contrasting theories being debated. A local-control theory suggests that a plume decreases both local and global reconnection rates, whereas a global-control theory argues that the global reconnection rate is controlled by the solar wind rather than local physics. Observationally, limited numbers of point measurements from spacecraft cannot reveal whether a local change affects the global reconnection. A distributed observatory is hence needed to assess the validity of the two theories. We use THEMIS and Los Alamos National Laboratory spacecraft to identify the occurrence of a geospace plume and its contact with the magnetopause. Global evolution and morphology of the plume is traced using GPS measurements. SuperDARN is then used to monitor the distribution and the strength of dayside reconnection. Two storm-time geospace plume events are examined and show that as the plume contacts the magnetopause, the efficiency of reconnection decreases at the contact longitude. The amount of local decrease is 81% and 68% for the two events, and both values are consistent with the mass loading effect of the plume if the plume's atomic mass is ~4 amu. Reconnection in the surrounding is enhanced, and when the solar wind driving is stable, little variation is seen in the cross polar cap potential. This study illuminates a pathway to resolve the role of cold dense plasma on solar wind-magnetosphere coupling, and the observations suggest that plumes redistribute magnetopause reconnection activity without changing the global strength substantially.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

PreMevE Update: Forecasting Ultra-Relativistic Electrons Inside Earth's Outer Radiation Belt

Energetic electrons inside Earth's Van Allen belts pose a major radiation threat to spaceborne electronics that often play vital roles in modern society. Ultra-relativistic electrons with energies greater than or equal to two megaelectron-volt (MeV) are of particular interest, and thus forecasting these ≥2 MeV electrons has a significant meaning to all space sectors. Here, we update the latest development of the predictive model for MeV electrons in the outer radiation belt. The new version, called PREdictive MEV Electron (PreMevE)-2E, forecasts ultra-relativistic electron flux distributions across the outer belt, with no need for in situ measurements of the trapped MeV electron population except at the geosynchronous orbit (GEO). Model inputs include precipitating electrons observed in low-Earthorbits by NOAA satellites, upstream solar wind speeds and densities from solar wind monitors, as well as ultra-relativistic electrons measured by one Los Alamos GEO satellite. We evaluated 32 supervised machine learning models that fall into four different classes of linear and neural network architectures, and successfully tested ensemble forecasting by using groups of top-performing models. All models are individually trained, validated, and tested by in situ electron data from NASA's Van Allen Probes mission. It is shown that the final ensemble model outperforms individual models at most L-shells, and this PreMevE-2E model can provide 25-h (~1-day) and 50-h (~2-day) forecasts with high mean performance efficiency and correlation values. Our results also suggest that this new model is dominated by nonlinear components at L-shells <~4 for ultra-relativistic electrons, different from the dominance of linear components for 1 MeV electrons as previously discovered.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Three‐Dimensional Broadband Interferometric Mapping and Polarization (BIMAP‐3D) Observations of Lightning Discharge Processes

Abstract Following on our earlier single‐station, 2‐dimensional (2D) broadband interferometric mapping and polarization (BIMAP) observations of lightning discharges, we recently deployed two BIMAP stations at Los Alamos National Laboratory to map the lightning sources and their polarization in full 3‐dimensional (3D) space (BIMAP‐3D). The two stations are separated by 11.5‐km and each station consists of four antenna sets (instead of three for the original BIMAP) that form a Y ‐shaped array for improved interferometric performance. In this paper, we report the BIMAP‐3D system design, a generalized and analytical 2D interferometry technique for noncoplanar antenna array, a two‐stage 3D mapping technique based on geometric triangulation and baseline‐based differential time of arrival, and a technique to reconstruct the polarization orientation in 3D space by combining the 2D polarization results from the two‐station observations. Along with description of the techniques, we demonstrate and discuss the initial lightning results, including 3D maps for a hybrid intracloud and cloud‐to‐ground flash and for a normal intracloud flash, development of abnormal K‐change leaders, and polarization signatures for a K‐change leader. We find that with the two‐stage 3D mapping techniques, the sources can be located to meters accuracy for a favorable event that occurs between the two stations. We also find the polarization vectors for the example K leader are mostly orthogonal to the leader channel after the full 3D polarization analysis. The main purpose of this paper is to report the BIMAP‐3D techniques and capabilities. Detailed analysis of more specific discharge processes will be reported in later studies.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2022

New Lightning-Derived Vertical Total Electron Content Data Provide Unique Global Ionospheric Measurements

A newly released, novel ionospheric data set of global gridded vertical total electron content (VTEC) is introduced in this paper. This VTEC data set, provided by Los Alamos National Laboratory (LANL), is derived from very high frequency (VHF; defined as 30–300 MHz) broadband radio-frequency (RF) measurements of lightning made by U.S. Department of Defense sensing systems on board Global Positioning System (GPS) satellites. This paper presents the new data set (LANL VTEC), discusses the errors inherent in VHF TEC estimation due to ionospheric dispersion, and compares the LANL VTEC to two community standard VTEC gridded products: Jet Propulsion Laboratory's Global Ionospheric Model (JPL GIM) and the CEDAR community's Open Madrigal VTEC gridded measurements of L-band GNSS (global navigation satellite systems) TEC. We find that the LANL VTEC data have an offset of 3 TECU from CEDAR Madrigal GNSS VTEC and a full-width-half-maximum (FWHM) of 6 TECU. In comparison, the offset between LANL VTEC and the JPL GIM model is –3 TECU, but with a FWHM of 5 TECU. We also compare to Jason-3 VTEC measurements over the ocean, finding an offset of less than 0.5 TECU and an FWHM of <5 TECU. Because this technique uses a completely different methodology to determine TEC, the sources of errors are distinct from the typical ground-based GNSS L-band (GHz) TEC measurements. Also, because it is derived from RF lightning signals, this data set provides measurements in regions that are not well covered by ground-based GPS measurements, such as over oceans and over central Africa.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Insights Into Lightning K‐Leader Initiation and Development From Three Dimensional Broadband Interferometric Observations

Abstract We report detailed observations of K‐leaders and the activity between them with the three‐dimensional Broadband Interferometric Mapping and Polarization system (BIMAP‐3D) at Los Alamos National Laboratory. It is found that K‐leaders have a general propagation trend of initial acceleration and then gradual deceleration, and the corresponding very high frequency (VHF) radiation power is exponentially correlated with the leader speed. Based on the 3D development and simultaneous electric field change measurement, some simple K‐leaders can be modeled with time‐evolving point charges at the propagating leader tip and at the stationary origin. We found that the charge magnitude increases during the initial acceleration stage and stays relatively constant for the rest of the development. K‐leaders are observed to interact with other branches; the branches affect the leader's propagation speed, and may affect the charge transfer. After the occurrence of a K‐leader, VHF emissions are quenched for several milliseconds. VHF sources then reappear in an impulsive and scattered manner as “twinkling,” and these sources are found not uniquely on the so‐called needles, but also on the main channel. These twinkling sources start near the apparent positive leader tip, and extend back toward the direction of the flash origin at about 10 5 m/s, while the apparent positive tip continues to extend forward at about 10 4 m/s. The twinkling extending toward the direction of flash origin appears to initiate the following K‐leader, although it may be interrupted by a K‐leader along a different branch, or simply die out without more K‐leader activity.

54 ENVIRONMENTAL SCIENCES↗