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Guo, Z. B.

Publications and source records attributed to Guo, Z. B..

10 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Curvature of Radial Electric Field Aggravates Edge Magnetohydrodynamics Mode in Toroidally Confined Plasmas

We show that the radial electric field (E r ) plays a dual role in edge magnetohydrodynamics (MHD) activity. While E r shear (first spatial derivative of E r ) dephases radial velocity and displacement, and so is stabilizing, a new finding here is that E r curvature (second spatial derivative of E r ) tends to synchronize the radial velocity and displacement, and so destabilizes MHD. As a highlighted result, we analytically demonstrate that E r curvature can destabilize an otherwise stable kink mode, and so form a joint vortexkink mode. The synergetic effects of E r shear and E r curvature in edge MHD extend the familiar E × B shearing paradigm. This theory thus explains the experimental findings that a deeper E × B well may aggravate edge MHD, and so trigger the formation of the edge harmonic oscillation. Here, a simple criterion linking E r structure and the edge MHD activity is derived.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Edge state selection by modulating E × B shearing profile in toroidally confined plasmas

We investigate how the E × B shearing profile impacts the energy transport induced by the MHD mode in the edge of H-mode plasmas. By enhancing the outer layer (ψ nor ≈ 0.9–1) shear, the growth rate spectrum γ(n) gets narrowed with the fastest growth rate having a lower toroidal mode number. In the nonlinear stage, the MHD fluctuations tend to reach a coherent, oscillating state, which is analogous to a quiescent (Q) H mode state with edge-harmonic-oscillations. By enhancing the inner layer (ψ nor ≈ 0.7–0.8) shear, γ(n) shifts to lower n, and its width does not change much. In the nonlinear stage, the edge fluctuations tend to stay in an incoherent, oscillating state, which is analogous to a turbulent QH-mode state. Here, we also explore the underlying physics of the results and show that they are closely related to the different cross-phase dynamics of the edge MHD fluctuations induced by the variation of the edge E × B shearing profile.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Bistable dynamics of turbulence spreading in a corrugated temperature profile

We present a new model of turbulence spreading in magnetically confined plasma. A basic question in turbulence spreading is how to sustain finite amplitude fluctuations in a stable subcritical region, where linear dissipation of the turbulence is strong? The answer to this question relies on a consistent treatment of mesoscale temperature profile corrugation and microscale turbulence. We argue that inhomogeneous mixing of the turbulence corrugates the mean temperature profile and that the temperature corrugation then induces subcritical bifurcation of the turbulence. Thus, the system will transition from a metastable “laminar” state to an absolutely stable, excited state. Incorporating spatial coupling of the locally excited turbulent regions, a front forms. This front connects the excited and laminar states and penetrates the linear stable region efficiently. Here, we argue that such bistable turbulence spreading can explain observations of hysteresis in the intensity of L-mode core turbulence.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Small scale coherent vortex generation in drift wave-zonal flow turbulence

We present a paradigm for the generation of small scale coherent vortex (SSCV) in drift wave-zonal flow (DW-ZF) turbulence. We demonstrate that phases of DWs can couple coherently, mediated by the ZF shearing. A SSCV is formed when the phases of the DWs are “attracted” to form a stable “phase cluster.” We show that the ZF shearing induces asymmetry between “attractive” and “repulsive” phase couplings, so that a net attractive phase coupling results. The turbulent DWs will (partially)synchronize into a stable SSCV at locations, where the attractive phase coupling induced by the ZF shearing exceeds the “detuning” effects by the DW dispersion and random phase scattering. We also discuss the “self-binding” effect of the newly formed SSCV.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Perpendicular magnetic anisotropy in Ta|Co{sub 40}Fe{sub 40}B{sub 20}|MgAl{sub 2}O{sub 4} structures and perpendicular CoFeB|MgAl{sub 2}O{sub 4}|CoFeB magnetic tunnel junction

Magnetic properties of Co{sub 40}Fe{sub 40}B{sub 20} (CoFeB) thin films sandwiched between Ta and MgAl{sub 2}O{sub 4} layers have been systematically studied. For as-grown state, Ta/CoFeB/MgAl{sub 2}O{sub 4} structures exhibit good perpendicular magnetic anisotropy (PMA) with interface anisotropy K{sub i} = 1.22 erg/cm{sup 2}, which further increases to 1.30 erg/cm{sup 2} after annealing, while MgAl{sub 2}O{sub 4}/CoFeB/Ta multilayer shows in-plane magnetic anisotropy and must be annealed in order to achieve PMA. For bottom CoFeB layer, the thickness window for PMA is from 0.6 to 1.0 nm, while that for top CoFeB layer is between 0.8 and 1.4 nm. Perpendicular magnetic tunnel junctions (p-MTJs) with a core structure of CoFeB/MgAl{sub 2}O{sub 4}/CoFeB have also been fabricated and tunneling magnetoresistance ratio of about 36% at room temperature and 63% at low temperature have been obtained. The intrinsic excitations in the p-MTJs have been identified by inelastic electron-tunneling spectroscopy.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Large negative magnetoresistance in reactive sputtered polycrystalline GdN{sub x} films

Polycrystalline ferromagnetic GdN{sub x} films were fabricated at different N{sub 2} flow rates (f{sub N2}) to modify N-vacancy concentration so as to study its influence on electrotransport. Metal-semiconductor transition appears at Curie temperature (T{sub C}) of {approx}40 K. Temperature-dependent magnetoresistance (MR) shows a peak at T{sub C}. The films at f{sub N2} = 5, 10, 15, and 20 sccm show MR of -38%, -42%, -46%, and -86% at 5 K and 50 kOe, respectively. Above 15 K, MR is from colossal MR and from both colossal and tunneling MR below 15 K. The enhanced MR at f{sub N2} = 20 sccm is attributed to large spin polarization of half-metallicity in GdN{sub x} with low N vacancies.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

MAGNETIC RECONNECTION, HELICITY DYNAMICS, AND HYPER-DIFFUSION

We examine the influence of noise and Alfven wave turbulence on magnetic reconnection in a reduced magnetohydrodynamics model. We focus on the dynamics of magnetic helicity density. Helicity conservation is then used to calculate the global reconnection rate in terms of the helicity density flux. Two specific scenarios are explored-noisy reconnection and Alfven wave turbulent reconnection. For noisy reconnection, the current sheet is assumed to sit in a noisy state, marginal to plasmoid formation instability. The scaling of the reconnection rate in the presence of noise is proportional to (S {sup 2}{sub 0}/V{sub A}L {sup 2}){sup 1/11}, where S {sup 2}{sub 0}/V{sub A}L {sup 2} is the relative amplitude of the noise. We obtain this prediction using a symmetry analysis of the helicity density flux. For Alfven wave turbulent reconnection, a mean field closure scheme is applied. A reconnection rate proportional to (< B-tilde {sup 2}>/ {sup 2}){sup 1/8} is obtained, where < B-tilde {sup 2}>/ {sup 2} and (B) are the relative energy of Alfven wave turbulence and the reconnecting field. The constraint on reconnection rate enforced mean-square magnetic potential conservation is reexamined. A critical magnetic Reynolds number R{sub m,c} is identified. For R{sub m} >> R{sub m,c}, the reconnection rate becomes independent of Spitzer resistivity and thus can be higher than the Sweet-Parker model prediction. Both cases exhibit a weak dependence of the reconnection rate on the amplitude of the turbulence. Therefore, even noise or weak turbulence can trigger fast reconnection if the system is marginally stable. The important distinction between turbulent reconnection and turbulent dissipation of magnetic energy is also discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗