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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
DOE OSTI2014

Ion temperature gradient driven turbulence with strong trapped ion resonance

A theory to describe basic characterization of ion temperature gradient driven turbulence with strong trapped ion resonance is presented. The role of trapped ion granulations, clusters of trapped ions correlated by precession resonance, is the focus. Microscopically, the presence of trapped ion granulations leads to a sharp (logarithmic) divergence of two point phase space density correlation at small scales. Macroscopically, trapped ion granulations excite potential fluctuations that do not satisfy dispersion relation and so broaden frequency spectrum. The line width from emission due only to trapped ion granulations is calculated. The result shows that the line width depends on ion free energy and electron dissipation, which implies that non-adiabatic electrons are essential to recover non-trivial dynamics of trapped ion granulations. Relevant testable predictions are summarized.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Plasma size and power scaling of ion temperature gradient driven turbulence

The transport scaling with respect to plasma size and heating power is studied for ion temperature gradient driven turbulence using a fixed-flux full-f gyrokinetic Eulerian code. It is found that when heating power is scaled with plasma size, the ion heat diffusivity increases with plasma size in a local limit regime, where fixed-gradient δf simulations predict a gyro-Bohm scaling. In the local limit regime, the transport scaling is strongly affected by the stiffness of ion temperature profiles, which is related to the power degradation of confinement.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2024

Limit cycle oscillations in the zonal-flow-catalyzed interactions of ion-temperature-gradient turbulence

Limit-cycle oscillations are studied for ion temperature gradient turbulence, which, in the absence of large diamagnetic (mean) shear flows, saturates through energy transfer from unstable modes to large-scale stable modes via zonal-flow intermediary modes. Oscillations of zonal flow and turbulence levels are strongly constrained by the reactive, largely non-dissipative character of the zonal flows. Since existing predator–prey models for observed oscillations in experiments do not include energy transfer through zonal flows to stable modes, low-order fluid models with this physics are constructed and investigated. A simple three-wave truncation produces low-amplitude zonal flows that slowly oscillate around a zero mean, with turbulence oscillations between coupled wavenumbers that exceed linear frequencies by orders of magnitude. This inconsistency with experimental observations is caused by the weak non-linear drive of zonal flows in three-wave systems and the lack of multiple-wavenumber turbulent interactions. A more comprehensive model that preserves multiple wavenumber interactions within the context of conservative zonal-flow-mediated energy transfer to stable modes accurately reflects observed dynamics when the phase between stable and unstable modes is occasionally randomized.

Li, P. -Y. (ORCID:0000000295254171)↗
NASA NTRS1979

Temperature-gradient oven

Tubular oven operates on principle of lengthwise linear temperature gradient of homogeneous conductive rod in absence of radiative or convective heat loss. Oven can be applied to controlled heating or cooling of test specimens.

Jue, S.↗
DOE OSTI2014

Coupling of ion temperature gradient and trapped electron modes in the presence of impurities in tokamak plasmas

The coupling of ion temperature gradient (ITG or η{sub i}) mode and trapped electron mode (TEM) in the presence of impurity ions is numerically investigated in toroidal collisionless plasmas, using the gyrokinetic integral eigenmode equation. A framework for excitations of the ITG modes and TEMs with respect to their driving sources is formulated first, and then the roles of impurity ions played in are analyzed comprehensively. In particular, the characteristics of the ITG and TEM instabilities in the presence of impurity ions are emphasized for both strong and weak coupling (hybrid and coexistent) cases. It is found that the impurity ions with inwardly (outwardly) peaked density profiles have stabilizing (destabilizing) effects on the hybrid (namely the TE-ITG) modes in consistence with previous works. A new finding of this work is that the impurity ions have stabilizing effects on TEMs in small η{sub i} (η{sub i}≤1) regime regardless of peaking directions of their density profiles whereas the impurity ions with density gradient L{sub ez}=L{sub ne}/L{sub nz}>1 (L{sub ez}<1) destabilize (stabilize) the TEMs in large η{sub i} (η{sub i}≥1) regime. In addition, the dependences of the growth rate, real frequency, eigenmode structure, and wave spectrum on charge concentration, charge number, and mass of impurity ions are analyzed in detail. The necessity for taking impurity ion effects on the features of turbulence into account in future transport experimental data analyses is also discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Non-modal theory of the kinetic ion temperature gradient driven instability of plasma shear flows across the magnetic field

The temporal evolution of the kinetic ion temperature gradient driven instability and of the related anomalous transport of the ion thermal energy of plasma shear flow across the magnetic field is investigated analytically. This instability develops in a steady plasma due to the inverse ion Landau damping and has the growth rate of the order of the frequency when the ion temperature is equal to or above the electron temperature. The investigation is performed employing the non-modal methodology of the shearing modes which are the waves that have a static spatial structure in the frame of the background flow. The solution of the governing linear integral equation for the perturbed potential displays that the instability experiences the non-modal temporal evolution in the shearing flow during which the unstable perturbation becomes very different from a canonical modal form. It transforms into the non-modal structure with vanishing frequency and growth rate with time. The obtained solution of the nonlinear integral equation, which accounts for the random scattering of the angle of the ion gyro-motion due to the interaction of ions with ensemble of shearing waves, reveals similar but accelerated process of the transformations of the perturbations into the zero frequency structures. It was obtained that in the shear flow the anomalous ion thermal conductivity decays with time. It is a strictly non-modal effect, which originates from the temporal evolution of the shearing modes turbulence.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1982

The effect of small temperature gradients on flow in a continuous flow electrophoresis chamber

Continuous flow electrophoresis employs an electric field to separate biological cells suspended in a flowing liquid buffer solution. Good separations based on differences in electrophoretic mobility are obtained only when a unidirectional flow is maintained. The desired flow has a parabolic structure in the narrow dimension of the chamber and is uniform acros the width, except near the edges where the no-slip condition prevails. However, because of buoyancy, very small laterall or axial temperature gradients deform the flow significantly. The results of experiments conducted with a specially instrumented chamber show the origin and structure of the buoyancy-driven perturbations. It is found that very small temperature gradients can disturb the flow significantly, as was predicted by earlier theoretical work.

Rhodes, P. H.↗
DOE OSTI2015

On the nonlinear stability of a quasi-two-dimensional drift kinetic model for ion temperature gradient turbulence

We study a quasi-two-dimensional electrostatic drift kinetic system as a model for near-marginal ion temperature gradient driven turbulence. A proof is given for the nonlinear stability of this system under conditions of linear stability. This proof is achieved using a transformation that diagonalizes the linear dynamics and also commutes with nonlinear E × B advection. For the case when linear instability is present, a corollary is found that forbids nonlinear energy transfer between appropriately defined sets of stable and unstable modes. It is speculated that this may explain the preservation of linear eigenmodes in nonlinear gyrokinetic simulations. Based on this property, a dimensionally reduced (∞×∞→1) system is derived that may be useful for understanding dynamics around the critical gradient of Dimits.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Ion scale nonlinear interaction triggered by disparate scale electron temperature gradient mode

We have observed that the disparate scale nonlinear interactions between the high-frequency (∼0.4 MHz) electron temperature gradient (ETG) mode and the ion-scale low-frequency fluctuations (∼kHz) were enhanced when the amplitude of the ETG mode exceeded a certain threshold. The dynamics of nonlinear coupling between the ETG mode and the drift wave (DW) mode has already been reported [C. Moon, T. Kaneko, and R. Hatakeyama, Phys. Rev. Lett. (2013)]. Here, we have newly observed that another low-frequency fluctuation with f ≃ 3.6 kHz, i.e., the flute mode, was enhanced, corresponding to the saturation of the DW mode growth. Specifically, the bicoherence between the flute mode and the DW mode reaches a significant level when the ∇T{sub e}/T{sub e} strength exceeded 0.54 cm{sup −1}. Thus, it is shown that the ETG mode energy was transferred to the DW mode, and then the energy was ultimately transferred to the flute mode, which was triggered by the disparate scale nonlinear interactions between the ETG and ion-scale low-frequency modes.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

Effect of triangularity on ion-temperature-gradient-driven turbulence

The linear and nonlinear properties of ion-temperature-gradient-driven turbulence with adiabatic electrons are modeled for axisymmetric configurations for a broad range of triangularities δ, both negative and positive. Peak linear growth rates decrease with negative δ but increase and shift toward a finite radial wavenumber kx with positive δ. The growth-rate spectrum broadens as a function of kx with negative δ and significantly narrows with positive δ. Here, the effect of triangularity on linear instability properties can be explained through its impact on magnetic polarization and curvature. Nonlinear heat flux is weakly dependent on triangularity for |δ|≤0.5, decreasing significantly with extreme δ, regardless of sign. Zonal modes play an important role in nonlinear saturation in the configurations studied, and artificially suppressing zonal modes increased nonlinear heat flux by a factor of about four for negative δ, increasing with positive δ by almost a factor of 20. Proxies for zonal-flow damping and drive suggest that zonal flows are enhanced with increasing positive δ.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

Electron temperature gradient driven transport model for tokamak plasmas

A new model for electron temperature gradient (ETG) modes is developed as a component of the multi-mode anomalous transport module [Rafiq et al., Phys Plasmas 20, 032506 (2013)] to predict a time-dependent electron temperature profile in conventional and low aspect ratio tokamaks. This model is based on two-fluid equations that govern the dynamics of low-frequency short- and long-wavelength electromagnetic toroidal ETG driven drift modes. A low collisionality NSTX discharge is used to scan the plasma parameter dependence on the ETG real frequency, growth rate, and electron thermal diffusivity. Electron thermal transport is discovered in the deep core region where modes are more electromagnetic in nature. Several previously reported gyrokinetic trends are reproduced, including the dependencies of density gradients, magnetic shear, β and gradient of β (β'), collisionality, safety factor, and toroidicity, where β is the ratio of the plasma pressure to the magnetic pressure. Furthermore, the electron heat diffusivity associated with the ETG mode is discovered to be on a scale consistent with the experimental diffusivity determined by power balance analysis.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

Electron Temperature Gradient Driven Transport Model for Tokamak Plasmas

A new model for electron temperature gradient (ETG) modes is developed as a component of the Multi-Mode anomalous transport module [T. Rafiq \textit{et al.,} Phys Plasmas \textbf{20}, 032506 (2013)] to predict a time dependent electron temperature profile in conventional and low aspect ratio tokamaks. This model is based on two-fluid equations that govern the dynamics of low-frequency short- and long-wavelength electromagnetic toroidal ETG driven drift modes. A low collisionality NSTX discharge is used to scan the plasma parameter dependence on the ETG real frequency, growth rate, and electron thermal diffusivity. Electron thermal transport is discovered in the deep core region where modes are more electromagnetic in nature. Several previously reported gyrokinetic trends are reproduced, including the dependencies of density gradients, magnetic shear, $\beta$ and gradient of $\beta$ $(\betap)$, collisionality, safety factor, and toroidicity, where $\beta$ is the ratio of plasma pressure to the magnetic pressure. The electron heat diffusivity associated with the ETG mode is discovered to be on a scale consistent with the experimental diffusivity determined by power balance analysis.

ETG↗
DOE OSTI2015

Electron temperature gradient mode instability and stationary vortices with elliptic and circular boundary conditions in non-Maxwellian plasmas

Linear and nonlinear dynamics of electron temperature gradient mode along with parallel electron dynamics is investigated by considering hydrodynamic electrons and non-Maxwellian ions. It is noticed that the growth rate of η{sub e}-mode driven linear instability decreases by increasing the value of spectral index and increases by reducing the ion/electron temperature ratio along the magnetic field lines. The eigen mode dispersion relation is also found in the ballooning mode limit. Stationary solutions in the form of dipolar vortices are obtained for both circular and elliptic boundary conditions. It is shown that the dynamics of both circular and elliptic vortices changes with the inclusion of inhomogeneity and non-Maxwellian effects.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

THE EFFECT OF VERTICAL TEMPERATURE GRADIENT ON THE PROPAGATION OF THREE-DIMENSIONAL WAVES IN A PROTOPLANETARY DISK

Excitation and propagation of waves in a thermally stratified disk with an arbitrary vertical temperature profile are studied. Previous analytical studies of three-dimensional waves had been focused on either isothermal or polytropic vertical disk structures. However, at the location in a protoplanetary disk where the dominant heating source is stellar irradiation, the temperature gradient may become positive in the vertical direction. We extend the analysis to study the effects of the vertical temperature structure on the waves that are excited at the Lindblad resonances. For a hotter disk atmosphere, the f-mode contributes less to the torque and remains confined near the midplane as it propagates away from the resonances. On the other hand, the excitation of the g-modes is stronger. As they propagate, they channel to the top of disk atmosphere and their group velocities decrease. The differences compared to previous studies may have implications in understanding the wave dynamics in a realistic disk structure.

79 ASTRONOMY AND ASTROPHYSICS↗