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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 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
DOE OSTI2010

Synergy of Edge Shear Layer and Micro Turbulence in the Dynamics of the Density Limit of H-mode Discharges

Theoretical model for the physical mechanism of Greenwald density limit is presented. The importance of the role of shear layer associated with Zonal Flows (ZFs) and edge turbulence driven by high-m drift resistive ballooning mode (high-m DRBM) for Greenwald scaling is established. We have studied the multiscale interaction of zonal flows (ZFs) in the background of high-m DRBM turbulence. When the collision damping weak and model predicts unrealistically low value of saturated primary turbulence, we have studied the tertiary instability of ZFs, which take the place of collisional damping of ZFs. A self-consistent, simplified low-dimensional model of these interactions is constructed and derived the density scaling closed to Greenwald limit, which is based on experimental observations i.e., n{sub G{alpha}} I {sub P} /a{sup 2}(where n{sub G}, the plasma density and I{sub P}, the current).

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2010

Flute waves at the ion Larmor radius scales

The theory of the magnetic Rayleigh-Taylor instability (RTI) is discussed. Modified linear kinetic theory allows us to investigate RTI and flute waves with arbitrary perpendicular spatial scales compared to the ion Larmor radius. It is shown that in the linear limit a Fourier transform of these equations yields the dispersion relation which in the so-called Pade approximation corresponds to results of the kinetic theory. This analysis represents an extension of the previous study of the magnetic RTI obtained in the large wave scale approximation. It is shown that incorporation of the effects associated with wave scales of the order of the ion Larmor radius leads to a broader wave number range of the magnetic RTI.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Saturn's magnetosphere: An example of dynamic planetary systems

Planetary magnetospheres are prime examples of interacting plasma regimes at different scales. There is the principal interaction with the solar wind that seems to be the main driver of the dynamics at Mercury and Earth. But these inner planet magnetospheres are relatively simple when compared to those of the outer planets which are primarily driven by planetary rotation and include internal plasma sources from various moons and rings, in addition to those from the planetary ionospheres and the solar wind. Io's volcanic source at Jupiter is a prime example, but now Enceladus at Saturn has joined the fray, while Titan is a surprisingly minor player despite its thick nitrogen atmosphere and its continued bombardment by energetic particles. Mass loading of plasma leads to interchange instability in the inner magnetospheres at both Jupiter and Saturn, while ionospheric slippage, among other processes, seems to contribute to a variable rotation period in the spin-aligned dipole field of Saturn, manifested in auroral kilometric radiation (SKR), components of the magnetic field itself, and the plasma periodicities measured at several energies. Through use of the ENA (energetic neutral atom) technique, it is now possible to observe bulk motions of the plasma and their connection to planetary auroral processes. Such imaging at Saturn by Cassini has revealed the location of a region of post-midnight acceleration events that seem to corotate with the planet and coincide with auroral brightening and SKR. Periodic injections of plasma have been identified and repeat at the Kronian rotation period of 10.8 hours. A semi-permanent but asymmetric ring current has also been imaged, located between the orbits of the satellites Rhea ({approx}9 RS) and Titan ({approx}20 R{sub S}), with a maximum at {approx}10{+-} 1R{sub S} and dominated by the hot (>3 keV) plasma component.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

Competitions between Rayleigh-Taylor instability and Kelvin-Helmholtz instability with continuous density and velocity profiles

In this research, competitions between Rayleigh-Taylor instability (RTI) and Kelvin-Helmholtz instability (KHI) in two-dimensional incompressible fluids within a linear growth regime are investigated analytically. Normalized linear growth rate formulas for both the RTI, suitable for arbitrary density ratio with continuous density profile, and the KHI, suitable for arbitrary density ratio with continuous density and velocity profiles, are obtained. The linear growth rates of pure RTI ({gamma}{sub RT}), pure KHI ({gamma}{sub KH}), and combined RTI and KHI ({gamma}{sub total}) are investigated, respectively. In the pure RTI, it is found that the effect of the finite thickness of the density transition layer (L{sub {rho}}) reduces the linear growth of the RTI (stabilizes the RTI). In the pure KHI, it is found that conversely, the effect of the finite thickness of the density transition layer increases the linear growth of the KHI (destabilizes the KHI). It is found that the effect of the finite thickness of the density transition layer decreases the ''effective'' or ''local'' Atwood number (A) for both the RTI and the KHI. However, based on the properties of {gamma}{sub RT}{proportional_to}{radical}(A) and {gamma}{sub KH}{proportional_to}{radical}(1-A{sup 2}), the effect of the finite thickness of the density transition layer therefore has a completely opposite role on the RTI and the KHI noted above. In addition, it is found that the effect of the finite thickness of the velocity shear layer (L{sub u}) stabilizes the KHI, and for the most cases, the combined effects of the finite thickness of the density transition layer and the velocity shear layer (L{sub {rho}=}L{sub u}) also stabilize the KHI. Regarding the combined RTI and KHI, it is found that there is a competition between the RTI and the KHI because of the completely opposite effect of the finite thickness of the density transition layer on these two kinds of instability. It is found that the competitions between the RTI and the KHI depend, respectively, on the Froude number, the density ratio of the light fluid to the heavy one, and the finite thicknesses of the density transition layer and the velocity shear layer. Furthermore, for the fixed Froude number, the linear growth rate ratio of the RTI to the KHI decreases with both the density ratio and the finite thickness of the density transition layer, but increases with the finite thickness of the velocity shear layer and the combined finite thicknesses of the density transition layer and the velocity shear layer (L{sub {rho}=}L{sub u}). In summary, our analytical results show that the effect of the finite thickness of the density transition layer stabilizes the RTI and the overall combined effects of the finite thickness of the density transition layer and the velocity shear layer (L{sub {rho}=}L{sub u}) also stabilize the KHI. Thus, it should be included in applications where the transition layer effect plays an important role, such as the formation of large-scale structures (jets) in high energy density physics and astrophysics and turbulent mixing.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

The magnetic Rayleigh-Taylor instability and flute waves at the ion Larmor radius scales

The theory of flute waves (with arbitrary spatial scales compared to the ion Larmor radius) driven by the Rayleigh-Taylor instability (RTI) is developed. Both the kinetic and hydrodynamic models are considered. In this way we have extended the previous analysis of RTI carried out in the long wavelength limit. It is found that complete finite ion Larmor radius stabilization is absent when the ion diamagnetic velocity attains the ion gravitation drift velocity. The hydrodynamic approach allowed us to deduce a new set of nonlinear equations for flute waves with arbitrary spatial scales. It is shown that the previously deduced equations are inadequate when the wavelength becomes of the order of the ion Larmor radius. In the linear limit a Fourier transform of these equations yields the dispersion relation which in the so-called Pade approximation corresponds to the results of the fully kinetic treatment. The development of such a theory gives us enough grounds for an adequate description of the RTI stabilization by the finite ion Larmor radius effect.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Coupling of Kelvin-Helmholtz instability and buoyancy instability in a thermally laminar plasma

Thermal convective instability is investigated in a thermally stratified plasma in the presence of shear flow, which is known to give rise to the Kelvin-Helmholtz (KH) instability. We examine how the KH instability and magnetothermal instability (MTI) affect each other. Based on the sharp boundary model, the KH instability coupled with the MTI is studied. We present the growth rate and instability criteria. The shear flow is shown to significantly alter the critical condition for the occurrence of thermal convective instability.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Scaling of the power exhaust channel in Alcator C-Mod

Parametric dependences of the heat flux footprint on the outer divertor target plate are explored in EDA H-mode and ohmic L-mode plasmas over a wide range of parameters with attached plasma conditions. Heat flux profile shapes are found to be independent of toroidal field strength, independent of power flow along magnetic field lines and insensitive to x-point topology (single-null versus double-null). The magnitudes and widths closely follow that of the ''upstream'' pressure profile, which are correlated to plasma thermal energy content and plasma current. Heat flux decay lengths near the strike-point in H- and L-mode plasmas scale approximately with the inverse of plasma current, with a diminished dependence at high collisionality in L-mode. Consistent with previous studies, pressure gradients in the boundary scale with plasma current squared, holding the magnetohydrodynamic ballooning parameter approximately invariant at fixed collisionality--strong evidence that critical-gradient transport physics plays a key role in setting the power exhaust channel.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Formation of high-{beta} plasma and stable confinement of toroidal electron plasma in Ring Trap 1

Formation of high-{beta} electron cyclotron resonance heating plasma and stable confinement of pure electron plasma have been realized in the Ring Trap 1 device, a magnetospheric configuration generated by a levitated dipole field magnet. The effects of coil levitation resulted in drastic improvements of the confinement properties, and the maximum local {beta} value has exceeded 70%. Hot electrons are major component of electron populations, and its particle confinement time is 0.5 s. Plasma has a peaked density profile in strong field region [H. Saitoh et al., 23rd IAEA Fusion Energy Conference EXC/9-4Rb (2010)]. In pure electron plasma experiment, inward particle diffusion is realized, and electrons are stably trapped for more than 300 s. When the plasma is in turbulent state during beam injection, plasma flow has a shear, which activates the diocotron (Kelvin-Helmholtz) instability. The canonical angular momentum of the particle is not conserved in this phase, realizing the radial diffusion of charged particles across closed magnetic surfaces. [Z. Yoshida et al., Phys Rev. Lett. 104, 235004 (2010); H. Saitoh et al., Phys. Plasmas 17, 112111 (2010).].

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Structure of an exploding laser-produced plasma

Currents and instabilities associated with an expanding dense plasma embedded in a magnetized background plasma are investigated by direct volumetric probe measurements of the magnetic field and floating potential. A diamagnetic cavity is formed and found to collapse rapidly compared to the expected magnetic diffusion time. The three-dimensional current density within the expanding plasma includes currents along the background magnetic field, in addition to the diamagnetic current. Correlation measurements reveal that flutelike structures at the plasma surface translate with the expanding plasma across the magnetic field and extend into the current system that sustains the diamagnetic cavity, possibly contributing to its collapse.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Gyrokinetic analysis of tearing instabilities in a collisionless plasma

Using a gyrokinetic description, an analytic investigation of tearing instabilities is carried out for a collisionless tokamak plasma, with particular emphasis on delineating the effects associated with Landau and {nabla}B resonances. The linear characteristics of {Delta}{sup '}-driven tearing modes are studied by including short wavelength variations across the confining magnetic field and long wavelength variations along the field. For the case when electrons are adiabatic and ions are fluidlike, the dispersion relation is solved analytically for mode widths lying between electron and ion excursion lengths. It is shown that electron Landau damping effect can significantly influence the tearing mode growth rate by making it proportional to ({Delta}{sup '}){sup 1/2} in contrast to earlier kinetic results, which show a linear dependence on {Delta}{sup '}. The growth rate can further slow down when compressional mode coupling effects are taken into account. Likewise, analytic conditions for the growth of the gyrokinetic tearing mode in the presence of electron {nabla}B resonance effect are obtained for both the {Delta}{sup '} driven global mode as well as the large {Delta}{sup '} branch of this instability and expressions for the real frequency and growth rate of the modes are given. Our analytic results, besides providing physical insights into the influence of these 'resonance' effects, can also serve as useful benchmark signatures to look for in large scale numerical gyrokinetic simulations.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Stabilization of the resistive wall mode instability by trapped energetic particles

A theoretical model for investigating the effect of the trapped energetic particles (EPs) on the resistive wall mode (RWM) instability is proposed. The results demonstrate that the trapped EPs have a dramatic stabilizing effect on the RWM because of resonant interaction between the mode and the magnetic precession drift motion of the trapped EPs. The results also show that the effect of the trapped EPs depends on the wall position. In addition, the stabilizing effect becomes stronger when the plasma rotation is taken into account. For sufficiently fast plasma rotation, the trapped EPs can lead to the complete stabilization of the RWM. Furthermore, the trapped EPs can induce a finite real frequency of the RWM in the absence of plasma rotation.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

A magnetospheric energy principle extended to include neutral atmosphere

The problem of ideal magnetohydrodynamic stability of plasmas in a magnetosphere-atmosphere system, in which the unperturbed magnetic field is assumed to be perpendicular to the plasma-atmosphere interface (ionospheric surface), is investigated by means of an extended magnetospheric energy principle. The derivation of the principle and conditions under which it applies to a real terrestrial magnetosphere is given. In the principle, the atmosphere is considered to be a very heavy and compressible gas with finite pressure. A thin ionospheric layer is taken into account as boundary conditions, but energetics within it are neglected. The solid-earth surface is assumed to be a perfectly conducting wall for perturbations. For a perturbation that satisfies either rigid or horizontally free boundary conditions at the plasma-atmosphere interface, the self-adjointness of the force operator is satisfied and an extended magnetospheric energy principle can be developed on the basis of the extended energy principle for fusion plasmas. These two boundary conditions are shown to be realized in the magnetosphere when the ionospheric conductivity is either very large or very small. Whereas in fusion plasmas the perturbed magnetic energy in the vacuum makes a stabilizing contribution to the potential energy, in the magnetosphere the perturbed magnetic energy in the atmosphere makes no such stabilizing contribution. This is due to the difference of the assumed field configurations of the magnetospheric and fusion plasmas. The ionospheric surface makes a destabilizing negative contribution to the potential energy owing to a horizontal plasma displacement on the spherical ionospheric surface. The method is applied to magnetospheric ballooning and interchange instabilities. The existence of a new type of magnetospheric interchange instability is shown and its structure in the magnetosphere-atmosphere system is clarified. Possible consequences of the instabilities and their relevance to magnetospheric physics are discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Effect of thermoelectric current splitting on the magnetic topology in DIII-D

An extended model is proposed to describe the magnetic topology during appearance of edge localized modes (ELMs). It is applied to an ELMing H-mode in a lower single null discharge at DIII-D [J. L. Luxon, Nucl. Fusion 42, 614 (2002)]. The process of flux tube formation is discussed based on a previously proposed two-step model. Large currents, as found in measurements in DIII-D, are assumed running through newly formed large flux tubes. Two different realizations of the current distribution within the tubes are compared, namely a single filament in each tube and a scenario where the current in each tube is split into subfilaments. The latter scenario is shown to be the more realistic distribution because it leads to much better agreement with infrared camera observations. It is demonstrated that stripe patterns in the divertor heat flux produced by an ELM in the DIII-D tokamak can be reproduced numerically by taking into account the magnetic perturbation caused by the thermoelectric current subfilaments.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Effect of nonaxisymmetric magnetic perturbations on divertor heat and particle flux profiles in National Spherical Torus Experiment

Small, nonaxisymmetric magnetic perturbations generated by external coils have been found to break the axisymmetry of heat and particle flux deposition pattern in the divertor area in the National Spherical Torus Experiment (NSTX). This breaking by the applied 3-D field causes strike point splitting that is represented as local peaks and valleys in the divertor profiles. In case of n = 3 fields application, the broken toroidal symmetry of the divertor profile shows 120 deg. of spatial periodicity while data for n = 1 fields provide a fully nonaxisymmetric heat and particle deposition. Field line tracing showed good agreement with the measured heat and particle flux profiles. Higher toroidal mode number (n = 3) of the applied perturbation produced more and finer striations in the divertor profiles than in the lower mode number (n = 1) case. Following the previous result of the intrinsic strike point splitting by the n = 3 error fields [Nucl. Fusion 50, 045010 (2010); J. Nucl. Mater. (2011), doi:10.1016/j.jnucmat.2011.01.115], result of the connection length profile from field line tracing identifies intrinsic error field as a possible source of the intrinsic splitting. In determining the strike point splitting pattern, q{sub 95} is found to play an important role; higher q{sub 95} produces finer striations and induces higher fraction of heat flux to flow through the split strike point channels. Higher pedestal electron collisionality also made the striations in the D{sub {alpha}} profile more pronounced in the given range of collisionality variation. The radial location of local peaks in the profiles during the triggered edge localized modes (ELMs) by the applied n = 3 fields stays similar before and after the application. This shows that the heat flux from the triggered ELMs follows the mode number of the applied perturbation. The external magnetic perturbation can reattach detached divertor plasma, but this can be overcome by detaching the plasma with additional divertor gas puffing.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Dust Accelerators And Their Applications In High-Temperature Plasmas

The perennial presence of dust in high-temperature plasma and fusion devices has been firmly established. Dust inventory must be controlled, in particular in the next-generation steady-state fusion machines like ITER, as it can pose significant safety hazards and potentially interfere with fusion energy production. Although much effort has been devoted to getting rid of the dust nuisance, there are instances where a controlled use of dust can be beneficial. We have recognized a number of dust-accelerators applications in magnetic fusion, including in plasma diagnostics, in studying dust-plasma interactions, and more recently in edge localized mode (ELM)'s pacing. With the applications in mind, we will compare various acceleration methods, including electrostatic, gas-drag, and plasma-drag acceleration. We will also describe laboratory experiments and results on dust acceleration.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Pedestal density fluctuation dynamics during the inter-ELM cycle in DIII-D

Detailed 2D measurements of long-wavelength density fluctuations in the pedestal region with beam emission spectroscopy during the period between edge localized modes (ELMs) indicate two distinct bands of fluctuations propagating in opposite poloidal directions in the plasma frame: one lower frequency band (50-150 kHz) advects in the ion-diamagnetic drift direction (ion mode) and a higher frequency band (200-400 kHz) advects in the electron diamagnetic drift direction (electron mode). The ion mode amplitude is modulated with the ELM cycle: it increases rapidly after an ELM and then saturates, similar to the evolution of the pedestal electron pressure and density gradients. The electron mode, in contrast, has no significant time evolution between ELMs. The decorrelation time of the ion mode is <5 {mu}s[{tau}{sub c}(c{sub s}/c{sub s}aa){<=}1], the radial correlation length is of order 10 {rho}{sub i} and has poloidal wave-number k{sub {theta}{rho}i{approx}}0.1, and the mode advects at near the ion diamagnetic velocity in the plasma frame. These spatiotemporal dynamics are qualitatively similar to features predicted for kinetic ballooning modes.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

A new matching method for linear stability analysis of MHD modes close to the ideal MHD marginal stability

A new matching method has been invented for linear stability analysis of magnetohydrodynamics (MHD) modes for plasmas marginally stable against ideal MHD. An inner region with a finite width is utilized as in our previous study [M. Furukawa, S. Tokuda, and L.-J. Zheng, Phys. Plasmas 17, 052502 (2010)]. An ordering scheme for the outer region has been newly developed, thereby it succeeds to include effects of small plasma inertia and resistivity perturbatively in the outer region. The corresponding boundary condition requires direct, not asymptotic, matching of the outer and inner solutions, which assumes nothing special for the behavior of parallel electric field across the matching points. The union of the ordering scheme and the boundary condition enables us to apply our matching method even for plasmas marginally stable against ideal MHD. Because our matching method is not asymptotic, it is easy to implement numerically. The stability analysis of resistive MHD modes, such as internal kink and tearing modes, is satisfactory.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Impact of a shear flow on double tearing nonlinear dynamics

The dynamics of global reconnection in the presence of a poloidal shear flow located in between magnetic islands is investigated. Different linear and nonlinear regimes are identified depending on the resistivity, the equilibrium velocity amplitude, and the distance between the low-order resonant surfaces. It is found that nonlinearly, the shear flow can significantly delay DTM generation and global reconnection. It is shown that this delay is linked to a symmetry breaking imposed by the shear flow and the generation of mean poloidal flows in the resistive layers. It is also found that turbulence can be generated by Kelvin-Helmholtz instability in between the resonance layers and enhance magnetic reconnection processes.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗