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At least 793 records · Page 44
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

Magnetic island evolution in hot ion plasmas

Effects of finite ion temperature on magnetic island evolution are studied by means of numerical simulations of a reduced set of two-fluid equations which include ion as well as electron diamagnetism in slab geometry. The polarization current is found to be almost an order of magnitude larger in hot than in cold ion plasmas, due to the strong shear of ion velocity around the separatrix of the magnetic islands. As a function of the island width, the propagation speed decreases from the electron drift velocity (for islands thinner than the Larmor radius) to values close to the guiding-center velocity (for islands of order 10 times the Larmor radius). In the latter regime, the polarization current is destabilizing (i.e., it drives magnetic island growth). This is in contrast to cold ion plasmas, where the polarization current is generally found to have a healing effect on freely propagating magnetic island.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Kink modes and surface currents associated with vertical displacement events

The fast termination phase of a vertical displacement event (VDE) in a tokamak is modeled as a sequence of shrinking equilibria, where the core current profile remains constant so that the safety-factor at the axis, q{sub axis}, remains fixed and the q{sub edge} systematically decreases. At some point, the n = 1 kink mode is destabilized. Kink modes distort the magnetic field lines outside the plasma, and surface currents are required to nullify the normal component of the B-field at the plasma boundary and maintain equilibrium at finite pressure. If the plasma touches a conductor, the current can be transferred to the conductor, and may be measurable by the halo current monitors. This report describes a practical method to model the plasma as it evolves during a VDE, and determine the surface currents, needed to maintain equilibrium. The main results are that the onset conditions for the disruption are that the growth-rate of the n = 1 kink exceeds half the Alfven time and the associated surface current needed to maintain equilibrium exceeds one half of the core plasma current. This occurs when q{sub edge} drops below a low integer, usually 2. Application to NSTX provides favorable comparison with non-axisymmetric halo-current measurements. The model is also applied to ITER and shows that the 2/1 mode is projected to be the most likely cause of the final disruption.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Asymmetric radiative damping of low shear toroidal Alfven eigenmodes

Radiative damping of toroidicity-induced Alfven eigenmodes (TAEs) in tokamaks, caused by coupling to the kinetic Alfven wave (KAW), is investigated analytically in the limit of low magnetic shear. A significant asymmetry is found between the radiative damping of the odd TAE, whose frequency lies above the central TAE gap frequency {omega}{sub 0}, and that of the even TAE, with frequency {omega}<{omega}{sub 0}. For the even TAE, which consists of a symmetric combination of neighboring poloidal harmonics (and therefore has ballooning-type mode structure), the coupling results in two non-overlapping, outgoing fluxes of KAWs that propagate radially away from each other and the TAE localization region. In contrast, the odd TAE consists of an antisymmetric combination of neighboring poloidal harmonics, resulting in anti-ballooning mode structure. For this mode, the KAWs initially propagate towards each other and form an interference pattern in the TAE localization region, resulting in a negligibly small escaping flux and a correspondingly low radiative damping rate. As a result of the up/down asymmetry in radiative damping with respect to the mode frequency, the odd TAE may be destabilized by fusion born alpha particles more easily than the usual, even TAE.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Modification of {Delta}{sup Prime} by magnetic feedback and kinetic effects

Two possible ways of modifying the linear tearing mode index, by active magnetic feedback and by drift kinetic effects of deeply trapped particles, are analytically investigated. Magnetic feedback schemes, studied in this work, are found generally stabilizing for {Delta}{sup Prime }. The drift kinetic effects from both thermal particles and hot ions tend to reduce the power of the large solution from the outer region. This generally leads to a destabilization of {Delta} Prime for the toroidal analytic equilibria considered here.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Hybrid Alfven resonant mode generation in the magnetosphere-ionosphere coupling system

Feedback unstable Alfven waves involving global field-line oscillations and the ionospheric Alfven resonator (IAR) were comprehensively studied to clarify their properties of frequency dispersion, growth rate, and eigenfunctions. It is discovered that a new mode called here the hybrid Alfven resonant (HAR) mode can be destabilized in the magnetosphere-ionosphere coupling system with a realistic Alfven velocity profile. The HAR mode found in a high frequency range over 0.3 Hz is caused by coupling of IAR modes with strong dispersion and magnetospheric cavity resonances. The harmonic relation of HAR eigenfrequencies is characterized by a constant frequency shift from those of IAR modes. The three modes are robustly found even if effects of two-fluid process and ionospheric collision are taken into account and thus are anticipated to be detected by magnetic field observations in a frequency range of 0.3-1 Hz in auroral and polar-cap regions.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2012

Kinetics of chemical ordering in a Ag-Pt nanoalloy particle via first-principles simulations

The energetics and kinetic energy barriers of vacancy/atom exchange in a 37-atom truncated octahedron Ag-Pt binary cluster in the Ag-rich range of compositions are investigated via a first-principles atomistic approach. The energy of the local minima obtained considering various distributions of a single vacancy and a few Pt atoms within the cluster and the energy barriers connecting them are evaluated using accurate density-functional calculations. The effects of the simultaneous presence of a vacancy and Pt atoms are found to be simply additive when their distances are larger than first-neighbors, whereas when they can be stabilizing at low Pt content due to the release of strain by the Pt/vacancy interaction or destabilizing close to a perfect Pt(core)/Ag(shell) arrangement. It is found that alloying with Pt appreciably increases the barriers for homotops transformations, thus rationalizing the issues encountered at the experimental level in producing Ag-Pt equilibrated nanoparticles and bulk phase diagram.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Observations of pressure gradient driven m = 1 internal kink mode in EAST tokamak

Pressure gradient driven m = 1 internal kink mode destabilization that follows an L-H transition is observed in the operational region of the EAST tokamak, which manifests in periodic oscillations in soft x-ray (SXR) and Mirnov coil signals. Using tomography with the high resolution soft x-ray detection array, we find that the rotation direction of the 1/1 kink mode is in the ion diamagnetic drift direction in poloidal cross-section. A large displacement of the hot core is attributable to the shift of the 1/1 internal kink mode. In contrast to stationary oscillations with fixed frequency, various frequency chirping behavior is observed with this 1/1 kink mode. Furthermore, we also occasionally observe that a 2/1 neoclassical tearing mode (NTM) is triggered by a 1/1 internal kink mode via mode coupling in a high-performance plasma. The spatial structure of a 2/2 mode, which is the harmonic mode of the 1/1 kink mode, is also presented in this paper. Large amounts of medium-Z impurities accumulate in the central plasma region where the 1/1 kink mode instability bursts. Finally, we also find that the frequency beating associated with a 1/1 kink mode is a consequence of plasma rotation. Based on all of these observations, we propose that the plasma pressure gradient, the driving force in kink modes, is plausibly the product of an intense concentration of impurities, which are related to plasma rotation.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Effect of neutral collision and radiative heat-loss function on self-gravitational instability of viscous thermally conducting partially-ionized plasma

The problem of thermal instability and gravitational instability is investigated for a partially ionized self-gravitating plasma which has connection in astrophysical condensations. We use normal mode analysis method in this problem. The general dispersion relation is derived using linearized perturbation equations of the problem. Effects of collisions with neutrals, radiative heat-loss function, viscosity, thermal conductivity and magnetic field strength, on the instability of the system are discussed. The conditions of instability are derived for a temperature-dependent and density-dependent heat-loss function with thermal conductivity. Numerical calculations have been performed to discuss the effect of various physical parameters on the growth rate of the gravitational instability. The temperature-dependent heat-loss function, thermal conductivity, viscosity, magnetic field and neutral collision have stabilizing effect, while density-dependent heat-loss function has a destabilizing effect on the growth rate of the gravitational instability. With the help of Routh-Hurwitz's criterion, the stability of the system is discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Roles of poloidal rotation in the q = 1 high-order harmonic tearing modes in a tokamak plasma

Roles of poloidal rotation in stabilizing the m/n=1/1 kink-tearing mode and exciting its high-order harmonic tearing modes are numerically investigated by using a reduced magnetohydrodynamic model. It is found that the high-order harmonic tearing modes, such as m/n=2/2, m/n=3/3, or even much higher-m harmonics, can be destabilized so significantly by rotation shear as to be more unstable than or comparable to the m/n=1/1 mode. Moreover, the short wave-length Kelvin- Helmholtz (KH) instabilities can be excited in the large rotation shear regime. The scaling power laws of the linear growth rate for each harmonic mode in different rotation shear regimes are verified by the previous relevant theoretical results based on the non-constant-{psi} and constant-{psi} behavior categories in tearing modes. During the nonlinear evolution, the m/n=2/2 mode dominated phase first appears and then is followed by the m/n=1/1 mode dominated nonlinear phase instead. Afterward, some smaller sub-islands due to the high-order harmonics are produced in the large irregular m=1 crescent-shaped island, and then a coalescence process of turbulent island chains occurs before the decay phase.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Electrostatic 'bounce' instability in a magnetotail configuration

To understand the possible destabilization of two-dimensional current sheets, a kinetic model is proposed to describe the resonant interaction between electrostatic modes and trapped particles that bounce within the sheet. This work follows the initial investigation by Tur et al.[Phys. Plasmas 17, 102905 (2010)] that is revised and extended. Using a quasi-parabolic equilibrium state, the linearized gyro-kinetic Vlasov equation is solved for electrostatic fluctuations with period of the order of the electron bounce period. Using an appropriated Fourier expansion of the particle motion along the magnetic field, the complete time integration of the non-local perturbed distribution functions is performed. The dispersion relation for electrostatic modes is then obtained through the quasineutrality condition. It is found that strongly unstable electrostatic modes may develop provided that the current sheet is moderately stretched and, more important, that the proportion of passing particle remains small (less than typically 10%). This strong but finely tuned instability may offer opportunities to explain features of magnetospheric substorms.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Fast wave stabilization/destabilization of drift waves in a plasma

Four wave-nonlinear coupling of a large amplitude whistler with low frequency drift wave and whistler wave sidebands is examined. The pump and whistler sidebands exert a low frequency ponderomotive force on electrons introducing a frequency shift in the drift wave. For whistler pump propagating along the ambient magnetic field B{sub s}z-caret with wave number k(vector sign){sub 0}, drift waves of wave number k(vector sign)=k(vector sign){sub Up-Tack }+k{sub ||}z-caret see an upward frequency shift when k{sub Up-Tack }{sup 2}/k{sub 0}{sup 2}>4k{sub ||}/k{sub 0} and are stabilized once the whistler power exceeds a threshold value. The drift waves of low transverse wavelength tend to be destabilized by the nonlinear coupling. Oblique propagating whistler pump with transverse wave vector parallel to k(vector sign){sub Up-Tack} is also effective but with reduced effectiveness.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Vibrational, elastic, and structural properties of cubic silicon carbide under pressure up to 75 GPa: Implication for a primary pressure scale

We present results of concomitant measurements of synchrotron x-ray diffraction (XRD), Brillouin, and Raman spectroscopy on the single crystal samples of cubic silicon carbide (3C-SiC) under quasi-hydrostatic pressures up to 65 GPa, as well as x-ray diffraction and Raman spectroscopy up to 75 GPa. We determined the equation of state of 3C-SiC and pressure dependencies of the zone-center phonon, elastic tensor, and mode Gruneisen parameters. Cubic SiC lattice was found to be stable up to 75 GPa, but there is a tendency for destabilization above 40 GPa, based on softening of a transverse sound velocity. By applying the concomitant density and elasticity measurements, we determined the pressure on the SiC sample without referring to any other pressure scale thus establishing a new primary pressure scale with a 2%–4% precision up to 65 GPa. We proposed corrections to the existing ruby and neon pressure scales, and also calibrated cubic SiC as a pressure marker for the x-ray diffraction and Raman experiments.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Five-field simulations of peeling-ballooning modes using BOUT++ code

The simulations of edge localized modes (ELMs) with a 5-field peeling-ballooning (P-B) model using BOUT++ code are reported in this paper. In order to study the particle and energy transport in the pedestal region, the pressure equation is separated into ion density and ion and electron temperature equations. Through the simulations, the length scale L{sub n} of the gradient of equilibrium density n{sub i0} is found to destabilize the P-B modes in ideal MHD model. With ion diamagnetic effects, the growth rate is inversely proportional to n{sub i0} at medium toroidal mode number n. For the nonlinear simulations, the gradient of n{sub i0} in the pedestal region can more than double the ELM size. This increasing effect can be suppressed by thermal diffusivities χ{sub ∥}, employing the flux limited expression. Thermal diffusivities are sufficient to suppress the perturbations at the top of pedestal region. These suppressing effects lead to smaller ELM size of P-B modes.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Stability of compressible reduced magnetohydrodynamic equilibria-Analogy with magnetorotational instability

Stability analyses for equilibria of the compressible reduced magnetohydrodynamics (CRMHD) model are carried out by means of the Energy-Casimir (EC) method. Stability results are compared with those obtained for ideal magnetohydrodynamics (MHD) from the classical {delta}W criterion. An identification of the terms in the second variation of the free energy functional for CRMHD with those of {delta}W is made: two destabilizing effects present for CRMHD turn out to correspond to the kink and interchange instabilities in usual MHD, while the stabilizing roles of field line bending and compressibility are also identified in the reduced model. Also, using the EC method, stability conditions in the presence of toroidal flow are obtained. A formal analogy between CRMHD and a reduced incompressible model for magnetized rotating disks, due to Julien and Knobloch [EAS Pub. Series, 21, 81 (2006)], is discovered. In light of this analogy, energy stability analysis shows that the condition for magnetorotational instability (MRI) for the latter model corresponds to the condition for interchange instability in CRMHD, with the Coriolis term and shear velocity playing the roles of the curvature term and pressure gradient, respectively. Using the EC method, stability conditions for the rotating disk model, for a large class of equilibria with possible non-uniform magnetic fields, are obtained. In particular, this shows it is possible for the MRI system to undergo, in addition to the MRI, another instability that is analogous to the kink instability. For vanishing magnetic field, the Rayleigh hydrodynamical stability condition is recovered.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Ideal ballooning modes in the tokamak scrape-off layer

A drift-reduced Braginskii fluid model is used to carry out a linear and non-linear study of ideal ballooning modes in the tokamak scrape-off layer. First, it is shown that the scrape-off layer finite connection length and boundary conditions modify the ideal stability limit with respect to the closed flux-surface result. Then, in a two-fluid description, it is found that magnetic induction effects can destabilize long wavelength resistive ballooning modes below marginal ideal stability. Non-linear simulations confirm a gradual transition from small scale quasi-electrostatic interchange turbulence to longer wavelength modes as the plasma beta is increased. The transition to global ideal ballooning modes occurs, roughly, at the linearly obtained stability threshold. The transport levels and the pressure gradient as a function of plasma beta obtained in non-linear simulations can be predicted using the non-linear flattening of the pressure profile from the linear modes as a turbulent saturation mechanism.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Comparison of kinetic and extended magnetohydrodynamics computational models for the linear ion temperature gradient instability in slab geometry

We perform linear stability studies of the ion temperature gradient (ITG) instability in unsheared slab geometry using kinetic and extended magnetohydrodynamics (MHD) models, in the regime k{sub ∥}/k{sub ⊥}≪1. The ITG is a parallel (to B) sound wave that may be destabilized by finite ion Larmor radius (FLR) effects in the presence of a gradient in the equilibrium ion temperature. The ITG is stable in both ideal and resistive MHD; for a given temperature scale length L{sub Ti0}, instability requires that either k{sub ⊥}ρ{sub i} or ρ{sub i}/L{sub Ti0} be sufficiently large. Kinetic models capture FLR effects to all orders in either parameter. In the extended MHD model, these effects are captured only to lowest order by means of the Braginskii ion gyro-viscous stress tensor and the ion diamagnetic heat flux. We present the linear electrostatic dispersion relations for the ITG for both kinetic Vlasov and extended MHD (two-fluid) models in the local approximation. In the low frequency fluid regime, these reduce to the same cubic equation for the complex eigenvalue ω=ω{sub r}+iγ. An explicit solution is derived for the growth rate and real frequency in this regime. These are found to depend on a single non-dimensional parameter. We also compute the eigenvalues and the eigenfunctions with the extended MHD code NIMROD, and a hybrid kinetic δf code that assumes six-dimensional Vlasov ions and isothermal fluid electrons, as functions of k{sub ⊥}ρ{sub i} and ρ{sub i}/L{sub Ti0} using a spatially dependent equilibrium. These solutions are compared with each other, and with the predictions of the local kinetic and fluid dispersion relations. Kinetic and fluid calculations agree well at and near the marginal stability point, but diverge as k{sub ⊥}ρ{sub i} or ρ{sub i}/L{sub Ti0} increases. There is good qualitative agreement between the models for the shape of the unstable global eigenfunction for L{sub Ti0}/ρ{sub i}=30 and 20. The results quantify how far fluid calculations can be extended accurately into the kinetic regime. We conclude that for the linear ITG problem in slab geometry with unsheared magnetic field when k{sub ∥}/k{sub ⊥}≪1, the extended MHD model may be a reliable physical model for this problem when ρ{sub i}/L{sub Ti0}<10{sup −2} and k{sub ⊥}ρ{sub i}<0.2.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

The plasmoid instability during asymmetric inflow magnetic reconnection

Theoretical studies of the plasmoid instability generally assume that the reconnecting magnetic fields are symmetric. We relax this assumption by performing two-dimensional resistive magnetohydrodynamic simulations of the plasmoid instability during asymmetric inflow magnetic reconnection. Magnetic asymmetry modifies the onset, scaling, and dynamics of this instability. Magnetic islands develop preferentially into the weak magnetic field upstream region. Outflow jets from individual X-points impact plasmoids obliquely rather than directly as in the symmetric case. Consequently, deposition of momentum by the outflow jets into the plasmoids is less efficient, the plasmoids develop net vorticity, and shear flow slows down secondary merging between islands. Secondary merging events have asymmetry along both the inflow and outflow directions. Downstream plasma is more turbulent in cases with magnetic asymmetry because islands are able to roll around each other after exiting the current sheet. As in the symmetric case, plasmoid formation facilitates faster reconnection for at least small and moderate magnetic asymmetries. However, when the upstream magnetic field strengths differ by a factor of 4, the reconnection rate plateaus at a lower value than expected from scaling the symmetric results. We perform a parameter study to investigate the onset of the plasmoid instability as a function of magnetic asymmetry and domain size. There exist domain sizes for which symmetric simulations are stable but asymmetric simulations are unstable, suggesting that moderate magnetic asymmetry is somewhat destabilizing. We discuss the implications for plasmoid and flux rope formation in solar eruptions, laboratory reconnection experiments, and space plasmas. The differences between symmetric and asymmetric simulations provide some hints regarding the nature of the three-dimensional plasmoid instability.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

An easy-to-implement filter for separating photo-excited signals from topography in scanning tunneling microscopy

In order to achieve elemental and chemical sensitivity in scanning tunneling microscopy (STM), synchrotron x-rays have been applied to excite core-level electrons during tunneling. The x-ray photo-excitations result in tip currents that are superimposed onto conventional tunneling currents. While carrying important physical information, the varying x-ray induced currents can destabilize the feedback loop causing it to be unable to maintain a constant tunneling current, sometimes even causing the tip to retract fully or crash. In this paper, we report on an easy-to-implement filter circuit that can separate the x-ray induced currents from conventional tunneling currents, thereby allowing simultaneous measurements of topography and chemical contrasts. The filter and the schematic presented here can also be applied to other variants of light-assisted STM such as laser STM.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗