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At least 1,099 records · Page 61
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Experiment-modeling studies comparing energy dissipation in the DIII-D SAS and SAS-VW divertors

Recent DIII-D experiments on Small Angle Slot (SAS) divertors have confirmed that a combination of divertor closure and target shaping can enhance cooling across the divertor target and increase energy dissipation, but with significant dependence on B T (toroidal magnetic field) direction. In these novel divertors, the roles of closure, target shaping, drifts, and scale lengths are all interconnected in optimizing dissipation, with the separatrix electron density n eSEP being the key parameter associated with the level of dissipation/detachment. After modifying the original flat-targeted graphite SAS to include a V shape with a tungsten coating on the outer side of the divertor (SAS-VW), matched series of discharges were run to compare to detailed SOLPS-ITER modeling. Experimentally, when run as designed with the outer strike point at the slot vertex, SAS-VW requires nearly identical n eSEP for detachment as the original SAS, with little difference in dissipation for the new geometry. This is in contrast to (1) earlier modeling predictions that a small change of the SAS geometry to a V shape should enhance dissipation at the same n eSEP for magnetic configurations having better H-mode access (ion B × ∇B drift directed into the divertor), and (2) despite the achievement of significantly higher (2-7x) neutral pressures and compression in the SAS-VW slot. Comparisons of experimental density scans to the most recent SOLPS-ITER modeling with ExB drifts show reasonable agreement for dissipation/detachment onset when using separatrix density as the independent parameter. In order to help understand the discrepancy in modeled vs actual performance for the new configuration, additional measurements varying gas injection location and impurity injection were undertaken. In-slot D 2 gas fueling is more effective (5–22 %) in promoting detachment, in accord with modeling. In-slot impurity injection (N 2 or Ne) can yield 30 % lower core Z eff and 15 % less confinement degradation after detachment compared to main chamber puffing, as well as relatively lower tungsten leakage from the divertor. Modeling can also reproduce the improved detachment seen as the strike point moves inboard of the slot vertex. While we can explain the effects of the most important parameters causing energy dissipation in these slot divertors, it remains that many aspects of their behavior cannot be accurately modeled using state-of-art codes such as SOLPS-ITER. This is of concern for future model-driven designs utilizing similar V-shaped geometries.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Simultaneous inference of the compressibility and inelastic response of tantalum under extreme loading

We study the deformation of tantalum under extreme loading conditions. Experimental velocity data are drawn from both ramp loading experiments on Sandia’s Z-machine and gas gun compression experiments. The drive conditions enable the study of materials under pressures greater than 100 GPa. We provide a detailed forward model of the experiments including a model of the magnetic drive for the Z-machine. Utilizing these experiments, we simultaneously infer several different types of physically motivated parameters describing equation of state, plasticity, and anelasticity via the computational device of Bayesian model calibration. Characteristics of the resulting calculated posterior distributions illustrate relationships among the parameters of interest via the degree of cross correlation. The calibrated velocity traces display good agreement with the experiments up to experimental uncertainty as well as improvement over previous calibrations. Examining the Z-shots and gun-shots together and separately reveals a trade-off between accuracy and transferability across different experimental conditions. Implications for model calibration, limitations from model form, and suggestions for improvements are discussed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Prediction of strain, inter-layer interaction and critical current in CORC ® wires under axial strain by T-A modeling

Superconducting conductors on round core (CORC ® ) cables and wires can meet the needs of large high-field magnets, such as particle accelerators and compact nuclear fusion machines, due to their simple cabling process, high current-carrying capacity and reliable operation under high mechanical stresses. Many high-field magnets require CORC ® cables to carry a current of thousands of amperes in a background magnetic field exceeding 20 T. As a result, the large electromagnetic forces will deform the cable in the axial direction due to hoop stress and in the transverse direction by compressive stress. Therefore, it is essential to determine the irreversible deformation limit of the CORC ® cable under axial tensile load and optimize the cabling parameters to potentially extend this limit. Analytical and numerical methods are developed to assess the performance degradation of CORC ® wires under axial tensile load. The strain level, interlayer contact pressure and friction and their impact on the critical current are calculated by combining the mechanical response and the T-A method. Analyzing the results shows that the winding angle of the tape and the Poisson’s ratio of the inner core are key factors affecting the irreversible tensile strain limit of CORC ® wires. The smaller the winding angle and the higher the Poisson’s ratio of the inner core, the higher the irreversible tensile strain limit. For multi-layer CORC ® wires, the initial contact pressure caused by the cabling process must also be considered. The inter-layer interaction is coupled with the tape strain of each layer. The results of this research can serve as a basis for optimizing and designing CORC ® wires with extended irreversible strain limits.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Overview of new MAST physics in anticipation of first results from MAST Upgrade

We report that the mega amp spherical tokamak (MAST) was a low aspect ratio device (R/a = 0.85/0.65 ~ 1.3) with similar poloidal cross-section to other medium-size tokamaks. The physics programme concentrates on addressing key physics issues for the operation of ITER, design of DEMO and future spherical tokamaks by utilising high resolution diagnostic measurements closely coupled with theory and modelling to significantly advance our understanding. An empirical scaling of the energy confinement time that favours higher power, lower collisionality devices is consistent with gyrokinetic modelling of electron scale turbulence. Measurements of ion scale turbulence with beam emission spectroscopy and gyrokinetic modelling in up-down symmetric plasmas find that the symmetry of the turbulence is broken by flow shear. Near the non-linear stability threshold, flow shear tilts the density fluctuation correlation function and skews the fluctuation amplitude distribution. Results from fast particle physics studies include the observation that sawteeth are found to redistribute passing and trapped fast particles injected from neutral beam injectors in equal measure, suggesting that resonances between the m = 1 perturbation and the fast ion orbits may be playing a dominant role in the fast ion transport. Measured D–D fusion products from a neutron camera and a charged fusion product detector are 40% lower than predictions from TRANSP/NUBEAM, highlighting possible deficiencies in the guiding centre approximation. Modelling of fast ion losses in the presence of resonant magnetic perturbations (RMPs) can reproduce trends observed in experiments when the plasma response and charge-exchange losses are accounted for. Measurements with a neutral particle analyser during merging-compression start-up indicate the acceleration of ions and electrons. Transport at the plasma edge has been improved through reciprocating probe measurements that have characterised a geodesic acoustic mode at the edge of an ohmic L-mode plasma and particle-in-cell modelling has improved the interpretation of plasma potential estimates from ball-pen probes. The application of RMPs leads to a reduction in particle confinement in L-mode and H-mode and an increase in the core ionization source. The ejection of secondary filaments following type-I ELMs correlates with interactions with surfaces near the X-point. Simulations of the interaction between pairs of filaments in the scrape-off layer suggest this results in modest changes to their velocity, and in most cases can be treated as moving independently. A stochastic model of scrape-off layer profile formation based on the superposition of non-interacting filaments is in good agreement with measured time-average profiles. Transport in the divertor has been improved through fast camera imaging, indicating the presence of a quiescent region devoid of filament near the X-point, extending from the separatrix to $ψ_n$ ~ 1.02. Simulations of turbulent transport in the divertor show that the angle between the divertor leg on the curvature vector strongly influences transport into the private flux region via the interchange mechanism. Coherence imaging measurements show counter-streaming flows of impurities due to gas puffing increasing the pressure on field lines where the gas is ionised. MAST Upgrade is based on the original MAST device, with substantially improved capabilities to operate with a Super-X divertor to test extended divertor leg concepts. SOLPS-ITER modelling predicts the detachment threshold will be reduced by more than a factor of 2, in terms of upstream density, in the Super-X compared with a conventional configuration and that the radiation front movement is passively stabilised before it reaches the X-point. 1D fluid modelling reveals the key role of momentum and power loss mechanisms in governing detachment onset and evolution. Analytic modelling indicates that long legs placed at large major radius, or equivalently low at the target compared with the X-point are more amenable to external control. With MAST Upgrade experiments expected in 2019, a thorough characterisation of the sources of the intrinsic error field has been carried out and a mitigation strategy developed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2024

Boron carbide films with reduced nodular defect density deposited by full-face erosion radio-frequency magnetron sputtering

Boron carbide coatings deposited by sputtering with conventional planar magnetrons exhibit nodular growth defects. Here, we demonstrate that the density of nodular defects in B 4 C films can be greatly reduced by using a full-face-erosion magnetron source that achieves a more uniform target erosion by sweeping the confining magnetic field over the target surface. Here, we systematically study properties of B 4 C films deposited on substrates with different tilt angles and demonstrate deposition of ultrathick films with compressive residual stress below 200MPa.

36 MATERIALS SCIENCE↗
NASA NTRS1966

Bench vise adapter grips tubing securely and safely

Plastic self-compressing adapter with grooves, attached to the jaws of a bench vise, secures thin-wall tubing vertically or horizontally during cutting and flaring operations without marring or damaging it. Magnets incorporated in both sections of the adapter prevent detachment from the jaws when the vise is opened.

Howland, B. T.↗
NASA NTRS1995

Large Amplitude IMF Fluctuations in Corotating Interaction Regions: Ulysses at Midlatitudes

Corotating Interaction Regions (CIRs), formed by high-speed corotating streams interacting with slow speed streams, have been examined from -20 deg to -36 deg heliolatitudes. The high-speed streams emanate from a polar coronal hole that Ulysses eventually becomes fully embedded in as it travels towards the south pole. We find that the trailing portion of the CIR, from the interface surface (IF) to the reverse shock (RS), contains both large amplitude transverse fluctuations and magnitude fluctuations. Similar fluctuations have been previously noted to exist within CIRs detected in the ecliptic plane, but their existence has not been explained. The normalized magnetic field component variances within this portion of the CIR and in the trailing high-speed stream are approximately the same, indicating that the fluctuations in the CIR are compressed Alfven waves. Mirror mode structures with lower intensities are also observed in the trailing portion of the CIR, presumably generated from a local instability driven by free energy associated with compression of the high-speed solar wind plasma. The mixture of these two modes (compressed Alfven waves and mirror modes) plus other modes generated by three wave processes (wave-shock interactions) lead to a lower Alfvenicity within the trailing portion of the CfR than in the high-speed stream proper. The results presented in this paper suggest a mechanism for generation of large amplitude B(sub z) fluctuations within CIRS. Such phenomena have been noted to be responsible for the generation of moderate geomagnetic storms during the declining phase of the solar cycle.

Tsurutani, Bruce T.↗
NASA NTRS2003

Divergence Free High Order Filter Methods for the Compressible MHD Equations

The generalization of a class of low-dissipative high order filter finite difference methods for long time wave propagation of shock/turbulence/combustion compressible viscous gas dynamic flows to compressible MHD equations for structured curvilinear grids has been achieved. The new scheme is shown to provide a natural and efficient way for the minimization of the divergence of the magnetic field numerical error. Standard diver- gence cleaning is not required by the present filter approach. For certain MHD test cases, divergence free preservation of the magnetic fields has been achieved.

Yea, H. C.↗
NASA NTRS2003

High Order Filter Methods for the Non-ideal Compressible MHD Equations

The generalization of a class of low-dissipative high order filter finite difference methods for long time wave propagation of shock/turbulence/combustion compressible viscous gas dynamic flows to compressible MHD equations for structured curvilinear grids has been achieved. The new scheme is shown to provide a natural and efficient way for the minimization of the divergence of the magnetic field numerical error. Standard divergence cleaning is not required by the present filter approach. For certain non-ideal MHD test cases, divergence free preservation of the magnetic fields has been achieved.

Yee, H. C.↗
NASA NTRS2015

Scaling of Magnetic Reconnection in Relativistic Collisionless Pair Plasmas

Using fully kinetic simulations, we study the scaling of the inflow speed of collisionless magnetic reconnection in electron-positron plasmas from the non-relativistic to ultra-relativistic limit. In the anti-parallel configuration, the inflow speed increases with the upstream magnetization parameter sigma and approaches the speed of light when sigma is greater than O(100), leading to an enhanced reconnection rate. In all regimes, the divergence of the pressure tensor is the dominant term responsible for breaking the frozen-in condition at the x-line. The observed scaling agrees well with a simple model that accounts for the Lorentz contraction of the plasma passing through the diffusion region. The results demonstrate that the aspect ratio of the diffusion region, modified by the compression factor of proper density, remains approximately 0.1 in both the non-relativistic and relativistic limits.

plasma↗
DOE OSTI2023

Mapping the intrinsic photocurrent streamlines through micromagnetic heterostructure devices

Photocurrent in quantum materials is often collected at global contacts far away from the initial photoexcitation. This collection process is highly nonlocal. It involves an intricate spatial pattern of photocurrent flow (streamlines) away from its primary photoexcitation that depends sensitively on the configuration of current collecting contacts as well as the spatial nonuniformity and tensor structure of conductivity. Direct imaging to track photocurrent streamlines is challenging. Here, we demonstrate a microscopy method to image photocurrent streamlines through ultrathin heterostructure devices comprising platinum on yttrium iron garnet (YIG). We accomplish this by combining scanning photovoltage microscopy with a uniform rotating magnetic field. Here, local photocurrent is generated through a photo-Nernst type effect with its direction controlled by the external magnetic field. This enables the mapping of photocurrent streamlines in a variety of geometries that include conventional Hall bar-type devices, but also unconventional wing-shaped devices called electrofoils. In these, we find that photocurrent streamlines display contortion, compression, and expansion behavior depending on the shape and angle of attack of the electrofoil devices, much in the same way as tracers in a wind tunnel map the flow of air around an aerodynamic airfoil. This affords a powerful tool to visualize and characterize charge flow in optoelectronic devices.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Pressure-induced polymorphism and piezochromism in Mn 2 FeSbO 6

In the last decade, major efforts have been devoted to searching for polar magnets due to their vast potential applications in spintronic devices. However, the polar magnets are rare because of conflicting electronic configuration requirements of ferromagnetism and electric polarization. Double-perovskite oxides with a polar structure containing transition metal elements represent excellent candidates for the polar magnet design. In this work, the crystal structure evolution of Mn 2 FeSbO 6 (MFSO) was investigated at pressures reaching ~50 GPa by in situ synchrotron X-ray diffraction (XRD), Raman scattering, and ab initio calculation techniques. The XRD results reveal ilmenite- to perovskite-type phase transition at around 35 GPa. An additional intermediate phase, observed in the range of 31–36 GPa by Raman spectroscopy, but not the XRD technique, is proposed to represent the polar LiNbO 3 phase. It is argued that this phase emerged due to the heating effect of the Raman-excitation laser. The LiNbO 3 -type MFSO compounds, displaying an intrinsic dipole ordering, represent a promising candidate for multiferroic materials. The detected phase transitions were found to be reversible although a significant hysteresis was noticeable between compression and decompression runs. Moreover, a pressure-induced piezochromism, signifying a bandgap change, was discovered by the direct visual observations and corroborated by ab initio calculations. The present study benefits an efficient high-pressure synthesis of polar magnetic double-perovskite oxides in the future.

36 MATERIALS SCIENCE↗
NASA NTRS1978

The interplanetary modulation and transport of Jovian electrons

Based on simultaneous measurements by Pioneer 11 of the 3-6 MeV Jovian electron flux, interplanetary magnetic field magnitude, and solar wind speed, the interplanetary transport of energetic particles is studied. It is found that corotating interaction regions (CIR's) greatly inhibit electron transport across the average field direction. Cross-field transport is also influenced by the degree of compression of the solar wind since CIR's are areas of compressed solar wind plasma. The propagation of Jovian electrons is studied by a model that includes the effects of CIR's. The model tests whether or not the three-dimensional convection-diffusion theory adequately describes the cross-field transport of electrons. The model is also valid for Jovian electron observations from earth-orbiting satellites. The model may be further applied to 1 AU from the sun where it is found that the cross-field diffusion of electrons explains why Jovian electrons are detected at the earth even during periods when the interplanetary magnetic field does not connect the earth directly to Jupiter.

Conlon, T. F.↗
NASA NTRS1982

Magnetically suspended Stirling cryogenic space refrigerator Status report

At the 1979 Cryogenic Engineering Conference, attention was given to conceptual designs of spaceborne cryogenic refrigeration systems which can provide long-term, unattended operation. Since that time, efforts have continued to translate one of those concepts into an engineering model. The present investigation is concerned with a refrigerator which was designed to generate 5 W of cooling power at a temperature of 65 K. The compression heat of the refrigerator is dissipated at a temperature of 300 K, and the output of the system is to be maintained reliably for a period of five years or longer. The refrigerator design is based on the Stirling cycle, which has an ideal efficiency equal to that of the Carnot cycle. Attention is given to some background information concerning a cryogenic refrigerator, the design of the refrigerator components, and the development status. The magnetic bearings and the linear motors have been tested at the component level.

Daniels, A.↗
DOE OSTI2025

Energy Shifts and Broadening of Excitonic Resonances in Electrostatically Doped Semiconductors

Tuning the density of resident electrons or holes in semiconductors through electrostatic doping provides crucial insight into the composition of the many excitonic complexes that are routinely observed as absorption or photoluminescence resonances in optical studies. Moreover, we can change the way these resonances shift and broaden in energy by controlling the quantum numbers (e.g., spin and valley) of the resident carriers with applied magnetic fields and doping levels, as well as by selecting the quantum numbers of the photoexcited or recombining electron-hole (𝑒−ℎ) pair through optical polarization. Here, we discuss the roles of distinguishability and optimality of excitonic complexes, showing them to be key ingredients that determine the energy shifts and broadening of optical resonances in charge-tunable semiconductors. A distinguishable 𝑒−ℎ pair means that the electron and hole undergoing photoexcitation or recombination have quantum numbers that are not shared by any of the resident carriers. An optimal excitonic complex refers to a complex whose particles come with all available quantum numbers of the resident carriers. Based on the carrier density, magnetic field, and light polarization, all optical resonances may be classified as either distinct or indistinct depending on the distinguishability of the 𝑒−ℎ pair, and the underlying excitonic complex can be classified as either optimal or suboptimal. The universality of these classifications, inherited from the fundamental Pauli exclusion principle, allows us to understand how optical resonances shift in energy and whether they should broaden as doping is increased. This understanding is supported by conclusive evidence that the broadening and decay of optical resonances cannot be simply attributed to enhanced screening when resident carriers are added to a semiconductor. Finally, applying the classification scheme in either monolayer or moiré heterobilayer semiconductor systems, we relate the energy shift and amplitude of the neutral-exciton resonance to the compressibility of the resident carrier gas.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Tuning the band topology of GdSb by epitaxial strain

Rare-earth monopnictide (RE-V) semimetal crystals subjected to hydrostatic pressure have shown interesting trends in magnetoresistance, magnetic ordering, and superconductivity, with theory predicting pressure-induced band inversion. Yet, thus far, there have been no direct experimental reports of interchanged band order in RE-Vs due to strain. This work studies the evolution of band topology in biaxially strained GdSb(001) epitaxial films using angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT). As biaxial strain is tuned from tensile to compressive strain, the gap between the hole and the electron bands dispersed along [001] decreases. The conduction and valence band shifts seen in DFT and ARPES measurements are explained by a tight-binding model that accounts for the orbital symmetry of each band. Finally, we discuss the effect of biaxial strain on carrier compensation and magnetic ordering temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2021

LA-R43S6-L1 Postshot Report [Slides]

The Ranchero LA-R43S6-L1 test (formerly referred to as LA-43S-L1. The R specifically designates it is a Ranchero device and the 6 indicates a 6” high explosive charge diameter in anticipation of using larger diameters in the future.) was conducted on 3/8/17 with good success. The experiment was the first test of a “Swooped” Ranchero flux compression generator (FCG), and the load for the test was an aluminum imploding liner. In addition to the swoop, the stator was anodized to provide an insulating layer instead of one or more layers of polyethylene as had been the case for all earlier Ranchero tests. Using the Firing point TA-39-88 capacitor bank, 3.5 MA was delivered to the FCG for the initial magnetic field, and 36.4 MA were delivered to the load having an initial inductance of 2.5 nH. Implosion speeds over 1.1 cm/us were recorded, which exceeds previous LANL solid liner experimental results. The test was diagnosed with 18 PDV channels, 12 of which recorded liner implosion data, and the other 6 of which tracked the FCG armature during flux compression. These armature expansion data recorded during flux compression are the first of their kind, and it was previously unknown, due to unknown effects of the SF 6 in the flux compression volume, whether or not the expansion could be recorded using the PDV technique. Post shot 2D MHD calculations employed a complete external circuit model, and an improved flux diffusion model not available prior to the test. Results from these simulations show very good agreement with the experimental result. Four PDV channels recorded the liner implosion viewed radially outward from the center of the liner. All four of these probes recorded implosion velocities greater that 1 cm/µs, with the cylindrical center displaced by ~2 mm from actual center. Four PDV channels looked at +10° angles from the CMU wall, and four looked at -10° angles. A glide plane interaction is shown moving toward the center of the liner in MHD calculations, and PDV probes provide confirmation. The high explosive (HE) in the LA-R43S6-L1 test was PBX 9501 which had to be glued together in many pieces, and the test was preceded by a camera test to verify that the PBX 9501 could be assembled with acceptable tolerance in glue joints to prevent severing the armature during expansion. The camera test verified an acceptable armature performance, and results are given in complete detail in a post shot report LA-UR-19-20124, and summarized here. This report provides complete detail of the considerable body of data obtained on the test and in the post shot calculations. It is prepared in Power Point for ease of preparation and future review. Shot documentation available in the LANL on-line library are included as references, and non-referenceable documents will be cited and stored in an LA- R43S6-L1 post shot folder stored on an M-6 shared drive. Fabrication drawings are also maintained in an M-6 shared drive and paper files are available from M-6 personnel. This report will be maintained as a PowerPoint document on the shared drive, as well, since included movies will play and graphs from Excel files retain information in the PowerPoint versions. PDF versions are required for clearance and are not to large to send by e-mail.

42 ENGINEERING↗
NASA NTRS1976

Heating of the solar chromosphere and corona. I - Generalized inhomogeneous wave equation for magnetoacoustic motions

The generalized inhomogeneous wave equation that governs magnetoacoustic, vortical, and thermal motions in compressible fluids and that is applicable to the problem of heating of the solar chromosphere and corona is obtained. The effects of kinematic and bulk viscosity, heat conduction, Joule dissipation, and magnetic diffusivity are included. Under the usual assumptions, the generalized wave equation reduces to the well-known equations of Lighthill, Kulsrud, Phillips, and others. The major problems encountered in applying Lighthill's (1952) mechanism to sound generation in turbulent media are reviewed for both the subsonic and supersonic cases.

Anand, S. P. S.↗