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  • 1
    Electronic Resource
    Electronic Resource
    Experimental study of lower-hybrid drift turbulence in a reconnecting current sheet (2002)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 9 (2002), S. 3272-3288 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The role of turbulence in the process of magnetic reconnection has been the subject of a great deal of study and debate in the theoretical literature. At issue in this debate is whether turbulence is essential for fast magnetic reconnection to occur in collisionless current sheets. Some theories claim it is necessary in order to provide anomalous resistivity, while others present a laminar fast reconnection mechanism based on the Hall term in the generalized Ohm's law. In this work, a thorough study of electrostatic potential fluctuations in the current sheet of the magnetic reconnection experiment (MRX) [Yamada et al., Phys. Plasmas 4, 1936 (1997)] was performed in order to ascertain the importance of turbulence in a laboratory reconnection experiment. Using amplified floating Langmuir probes, broadband fluctuations in the lower hybrid frequency range (fLH∼5–15 MHz) were measured which arise with the formation of the current sheet in MRX. The frequency spectrum, spatial amplitude profile, and spatial correlation characteristics of the measured turbulence were examined carefully, finding consistency with theories of the lower-hybrid drift instability (LHDI). The LHDI and its role in magnetic reconnection has been studied theoretically for decades, but this work represents the first detection and detailed study of the LHDI in a laboratory current sheet. The observation of the LHDI in MRX has provided the unique opportunity to uncover the role of this instability in collisionless reconnection. It was found that: (1) the LHDI fluctuations are confined to the low-beta edge of current sheets in MRX; (2) the LHDI amplitude does not correlate well in time or space with the reconnection electric field, which is directly related to the rate of reconnection; and (3) significant LHDI amplitude persists in high-collisionality current sheets where the reconnection rate is classical. These findings suggest that the measured LHDI fluctuations do not play an essential role in determining the reconnection rate in MRX. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1494433
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  • 2
    Electronic Resource
    Electronic Resource
    Two-dimensional numerical simulation of the resistive reconnection layer (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 4018-4030 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In this paper a two-dimensional, time dependent, numerical simulation of a reconnection current layer in incompressible resistive magnetohydrodynamics with uniform resistivity in the limit of very large Lundquist numbers is presented. Realistic boundary conditions derived consistently from the outside magnetic field are used, and the effect of the back pressure from flow into the separatrix region is also taken into account. It is found that within a few Alfvén times the system evolves from an arbitrary initial state to a steady state consistent with the Sweet–Parker model, even if the initial state is Petschek-like. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1308081
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  • 3
    Electronic Resource
    Electronic Resource
    Experimental study of ion heating and acceleration during magnetic reconnection : 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. 1916-1928 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Ion heating and acceleration has been studied in the well-characterized reconnection layer of the Magnetic Reconnection Experiment [M. Yamada et al., Phys. Plasmas 4, 1936 (1997)]. Ion temperature in the layer rises substantially during null-helicity reconnection in which reconnecting field lines are anti-parallel. The plasma outflow is sub-Alfvénic due to a downstream back pressure. An ion energy balance calculation based on the data and including classical viscous heating indicates that ions are heated largely via nonclassical mechanisms. The Ti rise is much smaller during co-helicity reconnection in which field lines reconnect obliquely. This is consistent with a slower reconnection rate and a smaller resistivity enhancement over the Spitzer value. These observations show that nonclassical dissipation mechanisms can play an important role both in heating the ions and in facilitating the reconnection process. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1356737
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  • 4
    Electronic Resource
    Electronic Resource
    Propagation of ion beams through a tenuous magnetized plasma (1986)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 29 (1986), S. 1675-1681 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: When an ion beam is propagated through a plasma, the question of charge neutralization is critical to its propagation. This paper considers such a problem where the plasma is magnetized with the magnetic field perpendicular to the beam. The plasma-number density and beam-number density are assumed comparable. This problem is reduced to a two-dimensional model and solved. It is shown that it should be possible to attain charge neutralization if the beam density is properly varied along itself.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.865632
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  • 5
    Electronic Resource
    Electronic Resource
    Forced magnetic reconnection (1985)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 28 (1985), S. 2412-2418 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: By studying a simple model problem, the time evolution of magnetic field islands which are induced by perturbing the boundary surrounding an incompressible plasma with a resonant surface inside is examined. The reconnection and island formation process for sufficiently small boundary perturbations occurs on the tearing mode time scale defined by Furth, Killeen, and Rosenbluth [Phys. Fluids 6, 459 (1963)]. For larger perturbations the time scale is that defined by Rutherford [Phys. Fluids 16, 1903 (1973)]. The resulting asymptotic equilibrium is such that surface currents in the resonant region vanish. A detailed analytical picture of this reconnection process is presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.865247
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  • 6
    Electronic Resource
    Electronic Resource
    On zero-frequency magnetic fluctuations in plasmas (1992)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 4 (1992), S. 2338-2345 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A plasma sustains fluctuations of electromagnetic fields and particle density even in thermal equilibrium and such fluctuations have a large zero-frequency peak. The level of fluctuations in the plasma for a given wavelength and frequency of electromagnetic fields is calculated through the fluctuation–dissipation theorem. The frequency spectrum shows that the energy contained in this peak is complementary to the energy "lost'' by the plasma cutoff effect. The level of the zero (or nearly zero) frequency magnetic fields is computed as 〈B2〉0/8π = (1/2π3)T(ωp/c)3, where T and ωp are the temperature and plasma frequency. The relation between the nonradiative and radiative fluctuations is elucidated. Both a simple collision model and a kinetic theoretic treatment are presented with essentially the same results. The size of the fluctuations is λ ∼ (c/ωp)(η/ω)1/2, where η and ω are the collision frequency and the (nearly zero) frequency of magnetic fields oscillations. Perhaps the most dramatic application of the present theory, however, is to the cosmological plasma of early epoch (say t = 10−2–100 sec after the Big Bang). Implications of these magnetic fields in the early Universe are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860204
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  • 7
    Electronic Resource
    Electronic Resource
    Considerations of ion-temperature-gradient-driven turbulence (1991)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 2767-2782 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The ion-temperature-gradient-driven instability is considered in this paper. Physical pictures are presented to clarify the nature of the instability. The saturation of a single eddy is modeled by a simple nonlinear equation. It is shown that eddies that are elongated in the direction of the temperature gradient are the most unstable and have the highest saturation amplitudes. In a sheared magnetic field, such elongated eddies twist with the field lines. This structure is shown to be an alternative to the usual Fourier mode picture in which the mode is localized around the surface where k(parallel) =0. These elongated twisting eddies, which are an integral part of the "ballooning mode'' structure, could survive in a torus. The elongated eddies are shown to be unstable to secondary instabilities that are driven by the large gradients in the long eddy. It is argued that the "mixing length'' is affected by this nonlinear process, and is unlikely to be a linear eigenmode width.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859913
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  • 8
    Electronic Resource
    Electronic Resource
    Linear stability of tearing modes (1986)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 29 (1986), S. 3230-3244 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: This paper examines the stability of tearing modes in a sheared slab when the width of the tearing layer is much smaller than the ion Larmor radius. The ion response is nonlocal, and the quasineutrality retains its full integral form. An expansion procedure is introduced to solve the quasineutrality equation in powers of the width of the tearing layer over the ion Larmor radius. The expansion procedure is applied to the collisionless and semicollisional tearing modes. It is found that first-order terms in the expansion are strongly stabilizing. The physics of the mode and of the stabilization are discussed. Tearing modes are observed in experiments even though the slab theory predicts stability. It is proposed that these modes grow from an equilibrium with islands at the rational surfaces. If the equilibrium islands are wider than the ion Larmor radius, the mode is unstable when Δ' is positive.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.865841
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  • 9
    Electronic Resource
    Electronic Resource
    An analytic solution of high-beta equilibrium in a large aspect ratio tokamak (1991)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 2066-2077 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An analytic solution of the high-beta (εβ¯p∼βq2/ε(very-much-greater-than)1) equilibrium of a large aspect ratio tokamak is presented. Two arbitrary flux functions, the pressure profile p(ψ) and the safety factor profile q(ψ), specify the equilibrium. The solution splits into two asymptotic regions: the core region where ψ is a function of the major radius alone and a narrow boundary layer region adjoining the conducting wall. The solutions in the two regions are asymptotically matched to each other. For monotonic pressure profiles, the Shafranov shift is equal to the minor radius. For beta much bigger than 1, the solution contains a region (in place of the magnetic axis) of zero magnetic field and constant pressure. At high beta the quantity βI, which is essentially proportional to the pressure over the total current squared, is largely independent of pressure. The important ramifications of limited βI for high-beta reactors are discussed. Generalizations to shaped cross sections and hollow pressure profiles are outlined. The problem of equilibrium reconstruction in the high-beta regime is also considered.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859991
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  • 10
    Electronic Resource
    Electronic Resource
    Analysis of anomalous resistivity during the conduction phase of the plasma erosion opening switch (1988)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 31 (1988), S. 1741-1747 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: During the conduction phase of the plasma erosion opening switch, the current channel width, in the body of the plasma away from the electrodes, has been observed to be many times wider than the collisionless skin depth. Anomalous collisions have been invoked to explain this discrepancy. Here the problem is analyzed using an electrostatic Vlasov approach, and an unstable ion acoustic mode is identified. The derived growth rate is fast enough and the nonlinear saturation level is high enough to explain the observed magnetic field penetration.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.866662
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