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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 1,009 records · Page 56
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Switching efficiency improvement in spin torque majority gates

Spin torque majority gate (STMG) is one of the promising options for beyond complementary metal-oxide-semiconductor logic. Improvement of its performance—switching speed vs. required current—is critical for its competitiveness. In this paper, (a) we identify an optimized layout of the gate comprised of thin magnetic wires with in-plane magnetization; (b) we optimize geometries of perpendicular magnetization spin torque majority gates. Micromagnetic simulations demonstrate an improvement in switching current for in-plane magnetization (with less than 1 ns switching time) from 6 mA in the original scheme to 1.5 mA in the present one. Additionally, failures of switching caused by vortex formation are eliminated and desired output magnetization is achieved. Various geometries of STMG with perpendicular magnetization are explored. The scheme with a straight cross proves to be the most advantageous. It is predicted to operate with the switching current of 50 μA and less than 4 ns switching time.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Comparison of 1D and 2D particle-in-cell simulations for DC magnetron sputtering discharges

One-dimensional and two-dimensional particle-in-cell (PIC)/Monte Carlo collision simulations are performed for a balanced direct current magnetron sputtering (DCMS) discharge. The plasma parameters obtained by both simulations above the target racetrack share similar features and magnitudes under similar discharge intensities, i.e., similar local discharge current densities. The commonly observed high-energy tail of electron energy distribution in DCMS discharges is reproduced in both simulations. Additionally, the accuracy and applicability of using onedimensional simulations to qualitatively investigate the characteristics of balanced DCMS discharge are confirmed. In terms of capturing the key physical mechanisms in DCMS, one-dimensional PIC simulation is an efficient method, which could largely alleviate the computational expense and preserve the physical fidelity of modeling results.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2012

ASYMMETRIC MAGNETIC RECONNECTION IN SOLAR FLARE AND CORONAL MASS EJECTION CURRENT SHEETS

We present two-dimensional resistive magnetohydrodynamic simulations of line-tied asymmetric magnetic reconnection in the context of solar flare and coronal mass ejection current sheets. The reconnection process is made asymmetric along the inflow direction by allowing the initial upstream magnetic field strengths and densities to differ, and along the outflow direction by placing the initial perturbation near a conducting wall boundary that represents the photosphere. When the upstream magnetic fields are asymmetric, the post-flare loop structure is distorted into a characteristic skewed candle flame shape. The simulations can thus be used to provide constraints on the reconnection asymmetry in post-flare loops. More hard X-ray emission is expected to occur at the footpoint on the weak magnetic field side because energetic particles are more likely to escape the magnetic mirror there than at the strong magnetic field footpoint. The footpoint on the weak magnetic field side is predicted to move more quickly because of the requirement in two dimensions that equal amounts of flux must be reconnected from each upstream region. The X-line drifts away from the conducting wall in all simulations with asymmetric outflow and into the strong magnetic field region during most of the simulations with asymmetric inflow. There is net plasma flow across the X-line for both the inflow and outflow directions. The reconnection exhaust directed away from the obstructing wall is significantly faster than the exhaust directed toward it. The asymmetric inflow condition allows net vorticity in the rising outflow plasmoid which would appear as rolling motions about the flux rope axis.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Adjoint-based deviational Monte Carlo methods for phonon transport calculations

In the field of linear transport, adjoint formulations exploit linearity to derive powerful reciprocity relations between a variety of quantities of interest. We develop an adjoint formulation of the linearized Boltzmann transport equation for phonon transport. We use this formulation for accelerating deviational Monte Carlo simulations of complex, multiscale problems. Benefits include significant computational savings via direct variance reduction, or by enabling formulations which allow more efficient use of computational resources, such as formulations which provide high resolution in a particular phase-space dimension (e.g., spectral). We show that the proposed adjoint-based methods are particularly well suited to problems involving a wide range of length scales (e.g., nanometers to hundreds of microns) and lead to computational methods that can calculate quantities of interest with a cost that is independent of the system characteristic length scale, thus removing the traditional stiffness of kinetic descriptions. Applications to problems of current interest, such as simulation of transient thermoreflectance experiments or spectrally resolved calculation of the effective thermal conductivity of nanostructured materials, are presented and discussed in detail.

42 ENGINEERING↗
DOE OSTI2014

Transport Studies and Modeling in PEM Fuel Cells

This project’s aim was to develop fuel cell components (i.e. membranes, gas-diffusion media (GDM), bipolar plates and flow fields) that possess specific properties (i.e. water transport and conductivity). A computational fluid dynamics model was developed to elucidate the effect of certain parameters on these specific properties. Ultimately, the model will be used to determine sensitivity of fuel cell performance to component properties to determine limiting components and to guide research. We have successfully reached our objectives and achieved most of the milestones of this project. We have designed and synthesized a variety of hydrocarbon block polymer membranes with lower equivalent weight, structure, chemistry, phase separation and process conditions. These membranes provide a broad selection with optimized water transport properties. We have also designed and constructed a variety of devices that are capable of accurately measuring the water transport properties (water uptake, water diffusivity and electro-osmatic drag) of these membranes. These transport properties are correlated to the membranes’ structures derived from X-ray and microscopy techniques to determine the structure-property relationship. We successfully integrated hydrocarbon membrane MEAs with a current distribution board (CBD) to study the impact of hydrocarbon membrane on water transport in fuel cells. We have designed and fabricated various GDM with varying substrate, diffusivity and micro-porous layers (MPL) and characterized their pore structure, tortuosity and hydrophobicity. We have derived a universal chart (MacMullin number as function of wet proofing and porosity) that can be used to characterize various GDM. The abovementioned GDMs have been evaluated in operating fuel cells; their performance is correlated to various pore structure, tortuosity and hydrophobicity of the GDM. Unfortunately, determining a universal relationship between the MacMullin number and these properties was not achieved. We have simulated fuel cell performance, current distribution and water distribution at various values of the water uptake, membrane diffusivity, and electro-osmotic drag coefficient (EODC) and compared modeling results with segmented-cell data for both serpentine and parallel flow-fields. We have developed iterations of fuel cell flow fields to achieve specific water transport and thermal management targets. This work demonstrated the importance of membrane diffusivity on fuel cell performance, the necessity of a high membrane diffusion coefficient, and the desirability of a low EODC at low levels of relative humidity.

08 HYDROGEN↗
DOE OSTI2018

Effect of Grain Boundaries on the Performance of Thin-Film-Based Polycrystalline Silicon Solar Cells: A Numerical Modeling

Solar cells/photovoltaic, a renewable energy source, is appraised to be the most effective alternative to the conventional electrical energy generator. A cost-effective alternative of crystalline wafer-based solar cell is thin-film polycrystalline-based solar cell. This paper reports the numerical analysis of dependency of the solar cell parameters (i.e., efficiency, fill factor, open-circuit voltage and short-circuit current density) on grain size for thin-film-based polycrystalline silicon (Si) solar cells. A minority carrier lifetime model is proposed to do a correlation between the grains, grain boundaries and lifetime for thin-film-based polycrystalline Si solar cells in MATLAB environment. As observed, the increment in the grain size diameter results in increase in minority carrier lifetime in polycrystalline Si thin film. A non-equivalent series resistance double-diode model is used to find the dark as well as light (AM1.5) current–voltage (I-V) characteristics for thin-film-based polycrystalline Si solar cells. To optimize the effectiveness of the proposed model, a successive approximation method is used and the corresponding fitting parameters are obtained. The model is validated with the experimentally obtained results reported elsewhere. The experimentally reported solar cell parameters can be found using the proposed model described here.

36 MATERIALS SCIENCE↗
DOE OSTI2013

THE FORCE-FREE MAGNETOSPHERE OF A ROTATING BLACK HOLE

We revisit the Blandford-Znajek process and solve the fundamental equation that governs the structure of the steady-state force-free magnetosphere around a Kerr black hole. The solution depends on the distributions of the magnetic field angular velocity {omega} and the poloidal electric current I. These are not arbitrary. They are determined self-consistently by requiring that magnetic field lines cross smoothly the two singular surfaces of the problem: the inner ''light surface'' located inside the ergosphere and the outer ''light surface'' which is the generalization of the pulsar light cylinder. We find the solution for the simplest possible magnetic field configuration, the split monopole, through a numerical iterative relaxation method analogous to the one that yields the structure of the steady-state axisymmetric force-free pulsar magnetosphere. We obtain the rate of electromagnetic extraction of energy and confirm the results of Blandford and Znajek and of previous time-dependent simulations. Furthermore, we discuss the physical applicability of magnetic field configurations that do not cross both ''light surfaces''.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2025

Qutrit and qubit circuits for three-flavor collective neutrino oscillations

We explore the utility of qutrits and qubits for simulating the flavor dynamics of dense neutrino systems. The evolution of such systems impacts some important astrophysical processes, such as core-collapse supernovae and the nucleosynthesis of heavy nuclei. Many-body simulations require classical resources beyond current computing capabilities for physically relevant system sizes. Quantum computers are therefore a promising candidate to efficiently simulate the many-body dynamics of collective neutrino oscillations. Previous quantum simulation efforts have primarily focused on properties of the two-flavor approximation due to their direct mapping to qubits. Furthermore, we present new quantum circuits for simulating three-flavor neutrino systems on qutrit- and qubit-based platforms, and demonstrate their feasibility by simulating systems of two, four, and eight neutrinos on IBM and Quantinuum quantum computers.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2019

Synergy of two lower hybrid waves with different frequencies on EAST

Lower hybrid current drive (LHCD) is one of the major approaches maintaining long pulse discharges on EAST. There are two lower hybrid (LH) systems on EAST launching waves at 4.6 GHz and 2.45 GHz into the plasma, respectively, and the input power of the 4.6 GHz wave is typically dominant. Here, the synergy of the two waves and the modification of the power deposition of the 4.6 GHz wave by coupling the 2.45 GHz wave are investigated theoretically. According to the phase space analysis, the wave propagation domains of the two LH waves always overlap substantially with each other for typical LHCD experimental parameters on EAST, indicating that the coupling between them might be strong. Ray-tracing/Fokker-Planck simulations for a LH current drive experiment on EAST show that the change of the power deposition profile due to the coupling could be understood by the rise and fall of the tails of the parallel electron distribution functions on different flux surfaces. If either the injected wave power at 2.45 GHz is comparable with that at 4.6 GHz or the incident N ∥ of the 2.45 GHz wave reaches a high value (e.g., $5.7/\sqrt{T_{e,0}}$), the power deposition profile of the 4.6 GHz wave can be modified greatly due to the coupling with the 2.45 GHz wave. With the parameters compatible with the ability of the LH wave system on EAST, injection of the 2.45 GHz wave can still modify the power deposition profile of the 4.6 GHz wave significantly by combining those two approaches. Finally, it is shown how a 4.6 GHz wave which cannot be Landau damped when injected alone is absorbed in low density plasmas with a magnetic shear reversal configuration in the presence of a 2.45 GHz wave.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2023

Roadmap for IES Modeling and Simulation Activities: Update 2023

Since inception of the DOE-NE Integrated Energy Systems (IES) program, several program and modeling visions and roadmaps have been published. In 2020, a last comprehensive roadmap for the DOE-NE Integrated Energy Systems program was published. In 2017, a separate modeling and simulation capability development plan was published that was extensively updated in 2022. The current report incrementally updates the 2022 modeling and simulation capability roadmap. This is not a new report, but individual sections have been updated to reflect recent accomplishments and new directions of the program. The role of modeling and simulation within IES is to support the demonstration of new coupled integrated technologies along every step of the technology maturation from the strategic analysis of preferred system architecture to preliminary design, to laboratory testing, up to full commercial testing and integration. To achieve this role, modeling and simulation must be able to assess the technical performance and the economic viability of potential IES. Also, modeling and simulation must provide support for experimental evaluations (i.e., support component design and real-time operations of experimental demonstration systems). Finally, modeling and simulation can help scale-up experimental systems to the final commercial systems. This report details the current state of the modeling and simulation efforts within IES for the above-mentioned areas, provides a gap analysis, and proposes next steps for a time horizon over the next years.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2012

Development of a randomized 3D cell model for Monte Carlo microdosimetry simulations

Purpose: The objective of the current work was to develop an algorithm for growing a macroscopic tumor volume from individual randomized quasi-realistic cells. The major physical and chemical components of the cell need to be modeled. It is intended to import the tumor volume into GEANT4 (and potentially other Monte Carlo packages) to simulate ionization events within the cell regions. Methods: A MATLAB Copyright-Sign code was developed to produce a tumor coordinate system consisting of individual ellipsoidal cells randomized in their spatial coordinates, sizes, and rotations. An eigenvalue method using a mathematical equation to represent individual cells was used to detect overlapping cells. GEANT4 code was then developed to import the coordinate system into GEANT4 and populate it with individual cells of varying sizes and composed of the membrane, cytoplasm, reticulum, nucleus, and nucleolus. Each region is composed of chemically realistic materials. Results: The in-house developed MATLAB Copyright-Sign code was able to grow semi-realistic cell distributions ({approx}2 Multiplication-Sign 10{sup 8} cells in 1 cm{sup 3}) in under 36 h. The cell distribution can be used in any number of Monte Carlo particle tracking toolkits including GEANT4, which has been demonstrated in this work. Conclusions: Using the cell distribution and GEANT4, the authors were able to simulate ionization events in the individual cell components resulting from 80 keV gamma radiation (the code is applicable to other particles and a wide range of energies). This virtual microdosimetry tool will allow for a more complete picture of cell damage to be developed.

61 RADIATION PROTECTION AND DOSIMETRY↗
DOE OSTI2019

Effect Laws of Different Factors on Levitation Characteristics of High-T{sub c} Superconducting Maglev System with Numerical Solutions

The interaction between the high-temperature superconducting (HTS) bulk and permanent magnet guideway (PMG) has been a topic among the existing HTS magnetic levitation systems. Up to now, the theories about the electromagnetic and force characteristics have been relatively consummated. Here, using the finite element model by COMSOL Multiphysics 5.3a, we investigated the impact of the intrinsic and external factors on the levitation characteristics of HTS maglev systems, the application laws are concluded. Factors including the critical current density and geometric dimensions of the bulk superconductor, as well as the moving velocity, are discussed. The effects of the multiple back and forth movements are also analyzed. By comparing calculation results of the distributions of the induced current and the levitation force of the bulk superconductor, we can clearly confirm the effect laws of various factors on the electromagnetic characteristics and levitation force. These observations support the physical parameters that are difficult to be tested by experiments. And they also have a significant contribution to the experimental condition selections and optimization of the high-temperature superconducting magnetic levitation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

The evolution of the ion diffusion region during collisionless magnetic reconnection in a force-free current sheet

Two-dimensional particle-in-cell simulation is performed to investigate magnetic reconnection in a force-free current sheet. The results show that the evolution of the ion diffusion region has two different phases. In the first phase, the electrons flow toward the X line along one pair of separatrices and away from the X line along the other pair of separatrices. Therefore, in the ion diffusion region, a distorted quadrupole structure of the out-of-plane magnetic field is formed, which is similar to that of a typical guide field reconnection in the Harris current sheet. In the second phase, the electrons move toward the X line along the separatrices and then flow away from the X line at the inner side of the separatrices. In the ion diffusion region, the out-of-plane magnetic field exhibits a characteristic quadrupole pattern with a good symmetry, which is similar to that of antiparallel reconnection in the Harris current sheet.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

Charge carrier motion and effect of fixed oxide charge in a microstructured silicon radiation detector

Signal formation in a microstructured semiconductor neutron detector is more complex than in planar diode geometry. Three-dimensional microstructures are laterally smaller than the ionization cloud length, and the electric fields may be weak enough to exhibit plasma time effects. This work is the first detailed treatment of charge carrier motion in these complex semiconductor devices to replicate the time profile and signal magnitude. Simulations were performed using COMSOL Multiphysics to investigate various parameters that affect the propagation of the charge cloud. It was observed that the size of the simulated three-dimensional structure had an impact on the induced current pulse, indicating the importance of simulation geometry optimization to accurately simulate charge cloud expansion. COMSOL Multiphysics was used to replicate accurate charge creation profiles using energy deposition information imported from radiation transport codes. A detailed simulation methodology is presented to benchmark preamplifier event pulses along with complexities in modeling the charge carrier motion along the etched microstructured trenches with Si–SiO 2 boundary conditions, including fixed oxide charge and interface trapping.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

A new radiation stripline ICRF antenna design for EAST Tokamak

A new type of toroidal long Radiation Stripline Antenna (RSA) is presented, which can effectively improve antenna radiation, leading in reduction of max voltage on transmission line and decrease of the sensitivity to ELM's of the ICRF system at some frequencies. Based on the new concept, a 4-straps RSA is proposed for EAST device. Using 3-D computing simulator code (HFSS), RF current distribution, S-parameters and electromagnetic field distribution on and near the RSA ICRF antenna are analyzed and compared with present ICRF antenna on EAST.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

Physical properties and analytical models of band-to-band tunneling in low-bandgap semiconductors

Low-bandgap semiconductors, such as InAs and InSb, are widely considered to be ideal for use in tunnel field-effect transistors to ensure sufficient on-current boosting at low voltages. This work elucidates the physical and mathematical considerations of applying conventional band-to-band tunneling models in low-bandgap semiconductors, and presents a new analytical alternative for practical use. The high-bandgap tunneling generates most at maximum field region with shortest tunnel path, whereas the low-bandgap generations occur dispersedly because of narrow tunnel barrier. The local electrical field associated with tunneling-electron numbers dominates in low-bandgap materials. This work proposes decoupled electric-field terms in the pre-exponential factor and exponential function of generation-rate expressions. Without fitting, the analytical results and approximated forms exhibit great agreements with the sophisticated forms both in high- and low-bandgap semiconductors. Neither nonlocal nor local field is appropriate to be used in numerical simulations for predicting the tunneling generations in a variety of low- and high-bandgap semiconductors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Physics design of a 28 GHz electron heating system for the National Spherical Torus experiment upgrade

A megawatt-level, 28 GHz electron heating system is being designed to support non-inductive (NI) plasma current (I{sub p}) start-up and local heating and current drive (CD) in H-mode discharges in the National Spherical Torus Experiment Upgrade (NSTX-U). The development of fully NI I{sub p} start-up and ramp-up is an important goal of the NSTXU research program. 28 GHz electron cyclotron (EC) heating is predicted to rapidly increase the central electron temperature (T{sub e}(0)) of low density NI plasmas generated by Coaxial Helicity Injection (CHI). The increased T{sub e}(0) will significantly reduce the I{sub p} decay rate of CHI plasmas, allowing the coupling of fast wave heating and neutral beam injection. Also 28 GHz electron Bernstein wave (EBW) heating and CD can be used during the I{sub p} flat top in NSTX-U discharges when the plasma is overdense. Ray tracing and Fokker-Planck numerical simulation codes have been used to model EC and EBW heating and CD in NSTX-U. This paper presents a pre-conceptual design for the 28 GHz heating system and some of the results from the numerical simulations.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Basic factors for acquiring, correcting, and interpreting probe current-voltage characteristic in moderate-collisional plasma for determining electron energy distribution

Methods for inferring the electron distribution function (EDF) from a probe current–voltage trace are reviewed for moderate-collisional plasma. The explicit accounting of particle collisions, modeling sheath depletion, and electron refilling processes are rarely performed when analyzing probe measurements, yet they are critical for regime validity, EDF-profile accuracy, and space potential quantification precision. Furthermore, the advantages, limitations, and utilization of large-area wall probes in moderate-collisional plasma, capable of highly resolved indications of energetic electrons in plasma, are outlined.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗