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  • 1
    Electronic Resource
    Electronic Resource
    Dispersion of ideal particles in a two-dimensional model of electrostatic turbulence (1999)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 6 (1999), S. 4575-4585 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The dispersion of ideal test particles in electrostatic drift-wave turbulence is investigated numerically. A self-consistent model with an internal instability drive is used to obtain the turbulent two-dimensional (2D) flow-field. It is shown that nonlinear couplings lead to the formation of coherent vortical structures in the flow. The dispersion of the particles is found to be anisotropic, with the weakest dispersion in the direction of the density gradient. By distinguishing between particles trapped in structures and free particles, it is demonstrated that the trapping and subsequent displacement of particles by nonlinear vortex structures enhances the particle diffusion in the direction of the background density gradient. Conditional diffusion coefficients are obtained showing that particles trapped by the vortex structures are convected by the structures. The time a particle on the average stays trapped in the structure is closely related to the lifetime of the vortical structures. The relation between the diffusion coefficient obtained from the test particle dispersion and an effective diffusion coefficient obtained from the cross-field turbulent flux is discussed. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873745
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  • 2
    Electronic Resource
    Electronic Resource
    Turbulent transport in low-β plasmas (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1530-1544 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Low-frequency electrostatic fluctuations are studied experimentally in a low-β plasma, with particular attention to their importance for the anomalous plasma transport across magnetic field lines. The presence of large coherent structures in a turbulent background at the edge of the plasma column is demonstrated by a statistical analysis. The importance of these structures for the turbulent transport is investigated. The study is extended by a multichannel conditional analysis to illustrate detailed properties and parameter dependences of the turbulent transport. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872008
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  • 3
    Electronic Resource
    Electronic Resource
    Nonlinear evolution of whistler wave modulational instability (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 3302-3319 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The nonlinear evolution of the modulational instability of whistler waves coupled to fast magnetosonic waves (FMS) and to slow magnetosonic waves (SMS) is investigated. Results from direct numerical solutions in two spatial dimensions agree with simplified results from a set of ordinary differential equations obtained from a Hamiltonian formulation of the governing equations. The long-time evolution of the modulational instability for the FMS-coupling shows a quasi-recurrent behavior with a slow spreading of the energy to higher and higher mode numbers. For the SMS-coupling, no recurrent behavior is found and the energy is gradually leaking to higher mode numbers while the spatial evolution of the modulation tends to develop small scale "spikes.'' © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871451
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  • 4
    Electronic Resource
    Electronic Resource
    Vortices associated with toroidal ion-temperature-gradient-driven fluctuations (1992)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 4 (1992), S. 3336-3343 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The three nonlinear hydrodynamic equations for potential, parallel ion velocity and ion pressure used in simulations of the toroidal ion-temperature-gradient-driven fluctuations and transport in a shear magnetic field are analyzed for coherent vortex structures. Two types of vortex structures are found: one type for weak shear that is a generalization of the usual modon vortex construction and the second type of solution for strong magnetic shear where the convective nonlinearity in the parallel velocity field generates a cubic trapping nonlinearity in the vorticity equation. These vortex structures show the possibility of explaining the saturated states observed in the numerical simulations as self-organized nonlinear states in contrast to wave turbulence.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860388
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  • 5
    Electronic Resource
    Electronic Resource
    Generation of tripolar vortical structures on the beta plane (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 1674-1678 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A new feature of the long-time evolution of a strong vortex with initially monotonic potential vorticity is found by direct numerical solution of the quasigeostrophic equivalent barotropic equation. Two satellites, which emerge after splitting of an annulus, appear at the vortex periphery. Rotation and oscillation of the tripolar structure may lead to increased mixing near the boundary of the vortex core. The translation of strong monopoles is found to be well described, even for times longer than the linear Rossby wave period, by a recent approximate theory for the evolution of an azimuthal perturbation with mode number l=1.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.858844
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  • 6
    Electronic Resource
    Electronic Resource
    Formation and temporal evolution of the Lamb-dipole (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 9 (1997), S. 982-991 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The formation and dynamics of dipolar vortex structures in two-dimensional flows are studied. Localized initial structures possessing a finite linear momentum are found to develop into dipoles by direct numerical solutions of the two-dimensional Navier-Stokes equations. The detailed structure of the evolving dipoles depend on the initial condition. However, the gross properties of their evolution are only weakly dependent on the detailed structure and can be well-described by the so-called Lamb-dipole solution. The viscous decay of the Lamb-dipole, leading to an expansion and a decreasing velocity, is well described by an adiabatic theory. During the expansion the dipole is found to trap fluid as it evolves. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.869193
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  • 7
    Electronic Resource
    Electronic Resource
    Interaction of plasma vortices with resonant particles (1990)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 2 (1990), S. 2035-2041 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Kinetic effects associated with the electron motion along magnetic field lines in low-beta plasmas are studied. Using the gyrokinetic description of electrons, a kinetic analog of the reduced magnetohydrodynamic equations is derived, and it is shown that in the strongly nonlinear regime they possess localized solutions in the form of dipolar vortices, which can efficiently interact with resonant electrons. In the adiabatic limit, evolution equations are derived for the vortex parameters, describing exchange of the energy, enstrophy, and of the Poynting vector between the vortex and resonant particles. The evolution equations indicate the possibility of excitation of plasma vortices by electron beams.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859424
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  • 8
    Electronic Resource
    Electronic Resource
    Vortex merging and spectral cascade in two-dimensional flows (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 8 (1996), S. 2263-2265 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The merging of two identical vortices is studied numerically using a spectral code. It is noted that the enstrophy cascade is most active on the distorted vortex boundaries, with a Kolmogorov-like spectrum E(k)≈k−α, α≤4, developed at high wave numbers. The inverse energy cascade is completed when the vortices merge into one of larger size. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.869027
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  • 9
    Electronic Resource
    Electronic Resource
    Dynamics of a nonlinear dipole vortex (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 7 (1995), S. 2220-2229 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A localized stationary dipole solution to the Euler equations with a relationship between the vorticity and streamfunction given as ω=−ψ+ψ3 is presented. By numerical integration of the Euler equations this dipole is shown to be unstable. However, the initially unstable dipole reorganizes itself into a new nonlinear dipole, which is found to be stable. This new structure has a functional relationship given as ω=αψ+βψ3−γψ5. Such dipoles are stable to head-on collisions and they are capable of creating tripolar structures when colliding off axis. The effects of increasing Newtonian viscosity on the nonlinear dipole is studied revealing that even though the nonlinearity is weakening, the dipole does not relax towards a Lamb dipole. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.868470
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  • 10
    Electronic Resource
    Electronic Resource
    Dynamical aspects of electrostatic double layers (1988)
    Springer
    Astrophysics and space science 144 (1988), S. 43-71 
    Details
    ISSN: 1572-946X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Electrostatic double layers have been proposed as an acceleration mechanism in solar flares and other astrophysical objects. They have been extensively studied in the laboratory and by means of computer simulations. The theory of steady-state double layers implies several existence criteria, in particular the Bohm criteria, restricting the conditions under which double layers may form. In the present paper several already published theoretical models of different types of double layers are discussed. It is shown that the existence conditions often imply current-driven instabilities in the ambient plasma, at least for strong double layers, and it is argued that such conditions must be used with care when applied to real plasmas. Laboratory double layers, and by implication those arising in astrophysical plasmas often produce instabilities in the surrounding plasma and are generally time-dependent structures. Naturally occuring double layers should, therefore, be far more common than the restrictions deduced from idealised time-independent models would imply. In particular it is necessary to understand more fully the time-dependent behaviour of double layers. In the present paper the dynamics of weak double layers is discussed. Also a model for a moving strong double layer, where an associated potential minimum plays a significant role, is presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00793172
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