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  • 11
    Electronic Resource
    Electronic Resource
    Nonlinear mean field electrodynamics of turbulent dynamos (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1853-1857 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Mean field electrodynamics of turbulent dynamos taking into account Lorentz forces of the generated magnetic fields is derived and studied. Small-scale magnetic fields that are much stronger than the mean field B0 modify the fluid turbulence in such a way as to suppress the α effect: α=α0(1+R)−1, where α0 is the kinematic value, and the reduction factor is proportional to the magnetic Reynolds number Rm, R=RmB20/(4πρv2), and v is the characteristic turbulent velocity. In two dimensions the analog is turbulent magnetic diffusivity suppression. Suppression becomes noticeable at very low values of the mean magnetic field, B20∼ρv2/Rm. The modification of turbulence by the small-scale magnetic fields is discussed. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871981
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  • 12
    Electronic Resource
    Electronic Resource
    Role of neutrals in the phase transition model (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 4106-4114 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The phase transition model for the transition from the low-confinement regime (L mode) to high-confinement regime (H mode) is extended to incorporate the effect of the neutral particles on the transition threshold. For usual edge plasma parameters, the increase of effective poloidal viscosity through charge exchange damping and reduction of the effective fluxes by both ionization and charge exchange increases the threshold power required for the transition. For plasmas with the effective energy flux smaller than the convective energy flux, the transition may be triggered by a neutral influx. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871543
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  • 13
    Electronic Resource
    Electronic Resource
    The dynamics of marginality and self-organized criticality as a paradigm for turbulent transport (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1858-1866 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A general paradigm, based on the concept of self-organized criticality (SOC), for turbulent transport in magnetically confined plasmas, has been recently suggested as an explanation for some of the apparent discrepancies between most theoretical models of turbulent transport and experimental observations of the transport in magnetically confined plasmas. This model describes the dynamics of the transport without relying on the underlying local fluctuation mechanisms. Computations based on a cellular automata realization of such a model have found that noise-driven SOC systems can maintain average profiles that are linearly stable (submarginal) and yet are able to sustain active transport dynamics. It is also found that the dominant scales in the transport dynamics in the absence of sheared flow are system scales rather than the underlying local fluctuation scales. The addition of sheared flow into the dynamics leads to a large reduction of the system-scale transport events and a commensurate increase in the fluctuation-scale transport events needed to maintain the constant flux. The dynamics of these models and the potential ramifications for transport studies are discussed. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871681
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  • 14
    Electronic Resource
    Electronic Resource
    The effect of an external torque on low to high confinement transitions (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 3044-3048 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A number of experiments have attempted, with varying degrees of success, to influence the low to high confinement mode (L–H) transition through the application of an external flow shear forcing mechanism. In order to theoretically describe this type of experiment, an external torque is included in the phase transition model of Diamond et al. [Phys. Rev. Lett. 72, 2965 (1994)] for the L–H bifurcation. These equations exhibit a bifurcation between an L-mode fixed point and an H-mode fixed point with variation of a critical parameter, corresponding to the edge gradient, across a threshold value. With the addition of the external torque [from biased limiter, probe, or radio frequency (rf) wave], the character of the transition changes in a manner analogous to the addition of an external magnetic field to a ferromagnetic material. In addition to the change in the transition dynamics, there is a marked decrease in the threshold level. In this simple model, the decrease in threshold level is proportional to the applied torque to the two-thirds power, in approximate functional agreement with that seen by some experiments. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871201
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  • 15
    Electronic Resource
    Electronic Resource
    Fluid models for kinetic effects on coherent nonlinear Alfvén waves. I. Fundamental theory (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 863-873 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Collisionless regime kinetic models for coherent nonlinear Alfvén wave dynamics are studied using fluid moment equations with an approximate closure anzatz. Resonant particle effects are modeled by incorporating an additional term representing dissipation akin to parallel heat conduction. Unlike collisional dissipation, parallel heat conduction is presented by an integral operator. The modified derivative nonlinear Schrödinger equation thus has a spatially nonlocal nonlinear term describing the long-time evolution of the envelope of parallel-propagating Alfvén waves, as well. Coefficients in the nonlinear terms are free of the (1−β)−1 singularity usually encountered in previous analyses, and have a very simple form that clarifies the physical processes governing the large-amplitude Alfvénic nonlinear dynamics. The nonlinearity appears via coupling of an Alfvénic mode to a kinetic ion-acoustic mode. Damping of the nonlinear Alfvén wave appears via strong Landau damping of the ion-acoustic wave when the electron-to-ion temperature ratio is close to unity. For a (slightly) obliquely propagating wave, there are finite Larmor radius corrections in the dynamical equation. This effect depends on the angle of wave propagation relative to B0 and vanishes for the limit of strictly parallel propagation. Explicit magnetic perturbation envelope equations amenable to further analysis and numerical solution are obtained. Implications of these models for collisionless shock dynamics are discussed. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871790
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  • 16
    Electronic Resource
    Electronic Resource
    Effects of nonlinear electron dynamics in a fluid model of collisionless trapped-electron mode turbulence (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 1877-1881 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The role of the electron nonlinearity on saturation levels and particle transport in collisionless trapped electron mode turbulence is examined numerically using a two-dimensional fluid model. It is found that the removal of the electron nonlinearity results in an order of magnitude increase in fluctuation and particle transport levels. In addition, the qualitative behavior of the saturated state changes distinctly, including a decrease in RMS wave number in both cross-field directions, deviations from isotropy, changes in transport scalings, and changes in the spectral flow of energy. These results indicate that the electron nonlinearity is responsible for an efficient transfer of internal energy to small dissipation scales, thus resulting in lower fluctuation levels than predicted by ion mode coupling alone. The electron nonlinearity also decreases the phase angle between the density and potential leading to a decrease in the driving source. These results underscore the importance of the cross-correlation on saturation and demonstrate that numerical or theoretical use of "iδ'' models to describe weakly collisional or collisionless trapped-electron mode turbulence is not appropriate.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870643
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  • 17
    Electronic Resource
    Electronic Resource
    Dynamics of low to high ("L'' to "H'') confinement bifurcation: Poloidal flow and ion pressure gradient evolution (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 4014-4021 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A self-consistent model of the low to high ("L'' to "H'') transition is derived from coupled nonlinear envelope equations for the fluctuation level and radial electric field shear, Er', as determined by ion pressure gradient, ∇Pi, and poloidal flow. These equations extend the phase transition model of the L to H bifurcation by including ∇Pi effects. In this model, the transition occurs when the turbulence drive is large enough to overcome the damping of the total E×B flow. Near the critical power for transition, poloidal flow shear dominates Er', but at high power, ∇Pi gives the main contribution. The inclusion of ∇Pi also introduces a quenched fluctuation state that is accessible at high power and may be the experimentally observed H-mode state for P(very-much-greater-than)Pcrit. In this state, the radial electric field is determined only by ∇Pi because no fluctuation energy is available to produce a turbulent Reynolds stress. © 1994 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870871
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  • 18
    Electronic Resource
    Electronic Resource
    Theory of electric-field curvature effects on long-wavelength drift wave turbulence (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 1142-1153 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A simple model of long-wavelength drift waves is used to study the way in which a radial electric-field profile influences the growth and saturation of turbulence. For a fixed external field, the effect of curvature (∂2Er/∂r2) dominates that of shear (∂Er/∂r). In the linear regime, both affect the average k(parallel) at which ion damping occurs: shear by shifting the eigenmode off the resonant surface and curvature by changing the eigenmode width. Curvature damps more efficiently and also shifts the real frequency of the drift wave, changing the instability drive. In the nonlinear regime, radial trapping at large fluctuation levels limits the ability of an external electric-field profile to affect the spatial structure. Changes in damping are now less effective than the feedback between frequency shift and drive. The importance of the frequency shift caused by electric-field curvature in the presence of finite-amplitude fluctuations has been demonstrated by numerical calculations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870765
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  • 19
    Electronic Resource
    Electronic Resource
    Bifurcation and scaling of drift wave turbulence intensity with collisional zonal flow damping (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 3996-4009 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Interacting drift wave–zonal flow turbulence is examined at the spectral level of description using an extended "predator–prey" model. Analytic solutions that describe both the linear scaling of transport with ion–ion collisionality as well as the saturation regime are obtained for a simple model of drift wave turbulence. A theory of self-regulation in this system is presented. The possibility of bifurcation to a state with higher turbulence level and transport is demonstrated. This bifurcation is associated with the appearance of a condensate solution at the largest scales. The possible relevance of this phenomenon to the bursting events of turbulence and transport recently observed in gyrokinetic simulations of ITG instability is discussed. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1394760
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  • 20
    Electronic Resource
    Electronic Resource
    Modern theory of Fermi acceleration: A new challenge to plasma physics : The 42nd annual meeting of the division of plasma physics of the American Physical Society and the 10th international congress on plasma physics (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 2401-2406 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: One of the main features of astrophysical shocks is their ability to accelerate particles to extremely high energies. The leading acceleration mechanism, the diffusive shock acceleration, is reviewed. It is demonstrated that its efficiency critically depends on the injection of thermal plasma into acceleration which takes place at the subshock of the collisionless shock structure that, in turn, can be significantly smoothed by energetic particles. Furthermore, their inhomogeneous distribution provides free energy for magnetohydrodynamic (MHD) turbulence regulating the subshock strength and injection rate. Moreover, the MHD turbulence confines particles to the shock front controlling their maximum energy and bootstrapping acceleration. Therefore, the study of the MHD turbulence in a compressive plasma flow near a shock is a key to the understanding of the entire process. The calculation of the injection rate became part of the collisionless shock theory. It is argued that the further progress in diffusive shock acceleration theory is impossible without a significant advance in these two areas of plasma physics. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1345507
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