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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 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Temperature Gradients on the Cell Wall in the Critical Viscosity Experiment

Because of the diverging susceptibility delta rho/delta Tau near the liquid-vapor critical point, temperature gradients must be kept small to maintain adequate sample homogeneity. In our Science Requirements Document we paid particular attention to radial density gradients caused by equilibration of the xenon sample. Axial density gradients were addressed through the requirement that the cell's copper wall have a gradient less than 22 microK/m. This report re-examines the cell wall's temperature distribution in more detail by estimating all known significant contributions to temperature differences on the cell's wall.

Berg, Robert F.↗
DOE OSTI2021

Effects of zonal flows on ion temperature gradient instability in the scrape-off layer of a field-reversed configuration

Gyrokinetic simulations of long wavelength ion temperature gradient (ITG) turbulence in the scrape-off layer (SOL) of a field-reversed configuration (FRC) find that zonal flows are nonlinearly generated and are the dominant mechanism for the nonlinear saturation of the ITG instability. After the ITG saturation, zonal flows remain undamped and gradually suppress the turbulent transport to a very low level. In the simulations with collisions, collisional damping gradually reduces zonal flow amplitude to a lower level, which allows finite ITG turbulence intensity and ion heat transport in the SOL. The steady state turbulence intensity and ion heat transport are found to be proportional to the collision frequency. Furthermore, this favorable scaling suggests that minimizing collisions (e.g. increasing temperature, reducing impurity content, etc) and preserving toroidal symmetry could improve plasma confinement in the FRC.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2025

Electron temperature gradient instability and transport analysis in NSTX and NSTX-U plasmas

Extensive linear and nonlinear simulations to study electron temperature gradient (ETG) stability and thermal transport in National Spherical Torus Experiment (NSTX) and NSTX-U plasmas were performed using the fully electromagnetic gyrokinetic code CGYRO. Linear simulations were performed to determine ETG thresholds in different discharges, showing that ETG modes in spherical tokamaks can present different scalings compared to conventional aspect-ratio tokamaks. Nonlinear gyrokinetic simulations were conducted for selected cases to calculate electron thermal transport and compare to experimental values. Results are also compared with those of ETG modes in the multi-mode model and the Trapped-Gyro-Landau-Fluid reduced model codes, to better understand their applicability in spherical tokamaks.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2026

Global gyro-kinetic ion temperature gradient and trapped electron mode turbulence modelling in X -point geometry in negative and positive triangularity

Comparative modelling of ion temperature gradient (ITG)/trapped electron mode (TEM) turbulence in negative (NT) and positive (PT) triangularity plasma shapes was done using the nonlinear global full-f gyrokinetic particle code JOREK-GK in the realistic X-point tokamak geometry including the Scrape Off Layer (SOL) for TCV and DIII-D parameters. A comparison of JOREK-GK code with the gyrokinetic codes GS2 and GENE was done using NT/PT triangularity TCV L-modes parameters showing good agreement between codes in linear growth rates and clear beneficial effect of NT as compared to PT. Global non-linear modelling of the ITG/TEM saturated turbulence for realistic DIII-D NT pulses was done and compared with numerically constructed PT equilibrium with the same plasma profiles. Existence of longer correlation length of density fluctuations in PT compared to NT was demonstrated. Stronger and more sheared zonal flows are generated via Reynolds stress in NT compared to PT. These factors are stabilizing for TEM/ITG turbulence in NT and lead to smaller heat fluxes and heat conductivities in NT compared to PT configuration. Weak dependence of plasma confinement on collisionality and plasma rotation was found in modelling of DIII-D NT shots similar to the experiment. The confinement scaling with normalized ion gyro-radius ρ* was estimated both for NT and PT. Bohm-like scaling was obtained in both configurations, however with better confinement for NT compared to PT which could be favourable factor for reactor size machines.

ITG↗
DOE OSTI2023

Global gyrokinetic simulations of the impact of magnetic island on ion temperature gradient driven turbulence

The effect of island width on the multi-scale interactions between magnetic island (MI) and ion temperature gradient (ITG) turbulence has been investigated based on the global gyrokinetic approach. It is found that the coupling between the island and turbulence is enhanced when the MI width (w) becomes larger. A vortex flow that is highly sensitive to the width of the MI can be triggered, ultimately resulting in a potent E × B shear flow and a consequent reduction in turbulent transport. The shearing rate induced by the vortex flow is minimum at the O-point while it is maximum at both of the two reconnection points of the island, i.e. the X-points, regardless of the island width. There exists a nonmonotonic relationship between zonal flow (ZF) amplitude and island width, showing that the ZF is partially suppressed by medium-sized MIs whereas enhanced in the case of large island. A larger MI can tremendously damage the ITG mode structure, resulting in higher turbulent transport at the X-point whereas a lower one at the O-point, respectively. Such phenomenon will be less distinct at very small island widths below w/a ~ 8% ≈ 12ρ i (a is the minor radius and ρ i the ion gyroradius), where it shows that turbulence near the X-point is hardly affected although it is still suppressed inside the island. Furthermore, the influence of different island sizes on turbulence transport level is also discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1985

Variable-Temperature-Gradient Device for Solidification Research

Device for research in solidification and crystal growth allows crystallization of melt observed as occurs. Temperature gradient across melt specimen increased or decreased rapidly while solidification front proceeds at constant speed across sample. Device moves sample at same speed, thereby holding position of liquid/solid interface stationary within field of optical microscope. Device, variabletemperature-gradient microscope stage, used to study crystal growth at constant rate while thermal driving force is varied.

Kaukler, W. F.↗
DOE OSTI2026

Suppression of ion temperature gradient modes by Alfvén activity above a drive threshold in DIII-D

A recent study demonstrates that the suppression of ion temperature gradient (ITG) modes can occur during the nonlinear evolution of toroidicity-induced Alfvén eigenmodes (TAEs) in the DIII-D tokamak [Du et al., Phys. Rev. Lett. 135, 265101 (2025)]. In that work, ITG was suppressed by the formation of a narrow, TAE-induced shear flow layer, whose shearing rate exceeded the ITG decorrelation rate. Here, the shear flow arises from an imbalance between Reynolds and Maxwell stress forces, as the TAE departs from the conventional shear Alfvén wave polarization. This follow-up paper systematically identifies the plasma conditions required for robust ITG suppression through a series of comparative experiments. The results show that TAEs routinely suppress ITG turbulence in plasmas with higher local safety factor (q), elevated fast-ion beta, and larger populations of fast ions on passing orbits, i.e., conditions consistent with the substantial TAE drive. Database analysis further reveals the existence of a threshold in TAE drive for ITG suppression. That is, once the drive exceeds a critical value within a favorable q window, the system undergoes the nonlinear bifurcation process, characterized by a sharp increase in TAE saturated amplitudes at a nearly fixed fast ion drive, suppression of ITG turbulence, and the formation of an internal thermal transport barrier at the localized radii.

Du, X. D. [General Atomics, San Diego, CA (United ↗
NASA NTRS1979

Non-Maxwellian velocity distribution functions associated with steep temperature gradients in the solar transition region. Paper 1: Estimate of the electron velocity distribution functions

It was shown that, in the presence of the steep temperature gradients characteristic of EUV models of the solar transition region, the electron and proton velocity distribution functions are non-Maxwellian and are characterized by high energy tails. The magnitude of these tails are estimated for a model of the transition region and the heat flux is calculated at a maximum of 30 percent greater than predicted by collision-dominated theory.

Roussel-Dupre, R.↗
DOE OSTI2020

Apparatus, methods and system for temperature gradient aging with in-situ electrical monitoring

An apparatus for making in-situ electrical measurements of a sample while the sample is subjected to a temperature gradient between a first side of the sample and a second side of the sample. The apparatus comprises a first heater and a second heater that are positioned in contact with the first side of the sample and the second side of the sample, respectively. The first heater and the second heater comprise respective faces that each have electrical contacts embedded therein. Electrical measurements pertaining to the sample can be made by way of the electrical contacts embedded in the faces. A temperature at either side of the sample can be monitored by way of thermocouples positioned inside the heaters in proximity to the faces of the heaters.

Reyes, Karla Rosa↗
DOE OSTI2023

On the effect of negative triangularity on ion temperature gradient turbulence in tokamaks

Considering the same magnetic equilibrium and plasma conditions as in Duff et al., we perform linear and nonlinear simulations of electrostatic ion temperature gradient turbulence investigating the role of triangularity δ. Differently from what was previously reported, we find that triangularity increases the transport level regardless of its sign, but more strongly when δ is positive. Here, for the case analyzed, we identify the shear of triangularity as the critical parameter determining the transport level, indicating that even in the local limit negative triangularity can reduce the transport efficiently, suggesting that confinement improvement can also be expected for larger devices.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1987

The doubled CO2 climate - Impact of the sea surface temperature gradient

The Goddard Institute for Space Studies (GISS) GCM of Hansen et al. (1983) was run, with 4 deg x 5 deg resolution, with doubled CO2 and two sets of sea surface temperature gradient distributions. One set was derived from the equilibrium doubled CO2 run of the 8 deg x 10 deg GISS GCM, with minimal high latitude amplification. The other set resembled closely the GFDL model results, with greater amplification. Both experiments had the same global mean surface air temperature change. The two experiments were often found to produce substantially different climate characteristics. With reduced high latitude amplification (set one), and thus, more equatorial warming, there was a greater increase in specific humidity and the greenhouse capacity of the atmosphere, resulting in a warmer atmosphere in general. Features such as the low-latitude precipitation, Hadley cell intensity, jet stream magnitude, and atmospheric energy transports all increased in comparison with the control run. In contrast, these features all decreased in the experiment with greater high latitude amplification (set two).

Rind, David↗
NASA NTRS1989

Acoustic response of a rectangular waveguide with a strong transverse temperature gradient

An acoustic wave equation was developed for a perfect gas with spatially-variable temperature. The strong-gradient wave equation was used to analyze the response of a rectangular wave guide containing a thermally-stratified gas. It was assumed that the temperature gradient is constant, representing one-dimensional heat transfer with a constant coefficient of conductivity. The analysis of the waveguide shows that the resonant frequencies of the waveguide are shifted away from the values that would be expected from the average temperature of the waveguide. For small gradients, the frequency shift is proportional to the square of the gradient. The factor of proportionality is a quadratic function of the natural frequency of the waveguide with uniform temperature. An experiment is designed to verify the essential features of the strong-gradient theory.

Zorumski, William E.↗
NASA NTRS1990

Influence of temperature gradients on the measurement accuracy of IR imaging systems

Implementing a new experimental aerodynamic technique, a long electrically heated wire was placed across flows with general velocity distributions (wakes, jets). By measuring the temperature distribution along the wire with an IR camera, the flow behavior was identified. Furthermore, it is suggested that by using Nusselt number correlations, the velocity distributions can be deduced. Comparing the results with predictions based on heat transfer correlations, it was found that the IR camera cannot accurately track high gradient temperature fields. A correction procedure is outlined to account for the errors found in the measured results.

Gartenberg, Ehud↗
DOE OSTI2020

Linear gyrokinetic simulations of reversed shear Alfvén eigenmodes and ion temperature gradient modes in DIII-D tokamak

Abstract Global linear gyrokinetic simulations using realistic DIII-D tokamak geometry and plasma profiles find co-existence of unstable reversed shear Alfvén eigenmodes (RSAE) with low toroidal mode number n and electromagnetic ion temperature gradient (ITG) instabilities with higher toroidal mode number n . For intermediate n = [10, 12], RSAE and ITG co-exist and overlap weakly in the radial domain with similar growth rates but different real frequencies. Both RSAE and ITG growth rates decrease less than 5% when compressible magnetic perturbations are neglected in the simulations. The ITG growth rates increase less than 7% when fast ions are not included in the simulations. Finally, the effects of trapped electrons on the RSAE are negligible.

Physics↗
DOE OSTI2021

Separate-Effects Tests for Studying Temperature-Gradient-Driven Cracking in UO 2 Pellets

We report a variety of normal operation and accident scenarios can generate thermal stresses large enough to cause cracking in light-water reactor (LWR) fuel pellets. Cracking of fuel pellets can lead to reduced heat removal, higher centerline temperatures, and localized stress in cladding, all of which impact fuel performance. It is important to experimentally characterize the thermal and mechanical behavior in the pellet before and after cracking to improve cracking models in fuel performance codes. However, in-reactor observation and measurement of cracking is very challenging due to the harsh environment and logistics. Recently, an experimental pellet cracking test stand was developed for separate effects testing of normal operations and accident temperature conditions, using thermal imaging to capture the pellet surface temperature for evaluation of thermal stresses and optical imaging to capture the evolution of cracking in real time. Experiments were performed using depleted uranium dioxide (UO 2 ) pellets, which are useful for collecting data valuable for development and validation of cracking models. A combination of induction and resistance heating was used to create an average temperature gradient of 236°C/cm and 193°C/cm before and after cracking respectively. Characterization of the pellets were done before as well as after cracking. The cracking patterns are somewhat different than those expected in a typical reactor because of the differences in thermal conditions and pellet microstructure. However, if the actual conditions of these experiments are reproduced in computational models, these out-of-pile tests on UO 2 pellets provide relevant data for modeling purposes.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗