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  • 1
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    Fast Collisionless Magnetic Reconnection: Fluid Regime and Kinetic Effects (2001)
    Institute of Physics
    Details
    Publication Date: 2001-01-01
    Print ISSN: 0031-8949
    Electronic ISSN: 1402-4896
    Topics: Physics
    Published by Institute of Physics
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    PAPER CURRENT
  • 2
    Electronic Resource
    Electronic Resource
    Nonlinear modeling of kinetic plasma instabilities : The 40th annual meeting of the division of plasma physics of the american physical society (1999)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 6 (1999), S. 1822-1829 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Many kinetic plasma instabilities, in quite different physical systems, share a genuinely similar mathematical structure near isolated phase-space islands. For this reason, dynamical features such as faster-than-exponential growth of the instability, as well as nonlinear frequency sweeping, are found to be universal. Numerical δf methods, which follow the evolution of the (nonlinear) perturbed distribution function along single-particle orbits, have been applied to analytic models, which include a continuous particle source, resonant particle collisions, and wave damping. The result is a series of codes that can reliably model the nonlinear evolution of kinetic instabilities, including some specific to tokamak plasmas, over experimentally relevant time scales. New results include (i) nonlinear simulations of two-species, one-degree-of-freedom plasmas; (ii) simulations of fishbone bursts in tokamak plasmas; (iii) nonlinear modeling of beam-driven toroidal Alfvén eigenmode activity in tokamaks. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873440
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  • 3
    Electronic Resource
    Electronic Resource
    On the theory of internal kink oscillations (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 2326-2333 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In this paper a time evolution equation for internal kink oscillations is derived. It is valid for both stable and unstable plasma regimes, and incorporates the response of an energetic particle population. A linear analysis reveals a parallel between (i) the time evolution of the spatial derivative of the internal kink radial displacement and (ii) the time evolution of the perturbed particle distribution function in the field of an electrostatic wave (Landau problem). It is shown that diamagnetic drift effects make the asymptotic decay of internal kink perturbations in a stable plasma algebraic rather than exponential. However, under certain conditions the stable root of the dispersion relation can dominate the response of the on-axis displacement for a significant period of time. The form of the evolution equation naturally allows one to include a nonlinear, fully toroidal treatment of energetic particles into the theory of internal kink oscillations. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872906
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  • 4
    Electronic Resource
    Electronic Resource
    Fast nonlinear magnetic reconnection (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 4104-4117 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The nonlinear evolution of magnetic reconnection in collisionless and weakly collisional regimes is analyzed on the basis of a two-dimensional incompressible fluid model. The initial equilibria are unstable to tearing modes. In the limit where the stability parameter Δ′ is relatively large, the mode structure is characterized by global convective cells. It is found that the system exhibits a quasiexplosive time behavior in the early nonlinear stage, where the fluid displacement is larger than the inertial skin depth but smaller than the typical size of the convective cells. The reconnection time is an order of magnitude shorter than the Sweet–Parker time for values of the inertial skin depth, of the ion Larmor radius, and of the magnetic Reynolds number typical of the core of magnetic fusion experiments. The reconnection process is accompanied by the formation of a current density sublayer narrower than the skin depth. In the strict dissipationless limit, this sublayer shrinks indefinitely in time. Physical mechanisms limiting this tendency to a singular current density profile are also discussed. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871033
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  • 5
    Electronic Resource
    Electronic Resource
    Solution of the drift-kinetic equation for global plasma modes and finite particle orbit widths (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 470-480 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The response of a collisionless plasma to global electromagnetic perturbations of an axisymmetric toroidal equilibrium is derived. By adopting a variational formulation for guiding center motion, the perturbed distribution function is expressed in terms of the linearized guiding center Lagrangian. Finite orbit widths are retained. In particular, the high particle energy limit where mirror-trapped banana orbits are distorted into "potato-shaped'' orbits is considered. In this limit, the time scales associated with the drift and bounce motions of a mirror-trapped orbit become comparable, yielding important consequences on plasma stability. Quadratic forms are constructed in the context of kinetic-magnetohydrodynamic (MHD) models of plasmas composed of a thermal component obeying fluid-like equations and a high-energy component described in terms of the collisionless drift-kinetic equation. Relevant applications include improved modeling of energetic ion effects on toroidicity-induced Alfvén gap modes and internal kinks.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870792
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  • 6
    Electronic Resource
    Electronic Resource
    Large gyroradius m=1 Alfvén modes and energetic particles (1991)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 1319-1325 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Nonmagnetohydrodynamic effects, caused by a minority population of energetic particles and/or by the large gyroradii of the main ion population, strongly modify the stability of a high temperature toroidal plasma against m=1 internal modes. It is shown that the enhanced stability due to the presence of the energetic particles is reduced in the large gyroradius regime, where the width of the transition layer of the mode is determined by the gyroradius of the main ions. This modification results from the presence of a discrete spectrum of resistively damped Alfvén-type modes which can be destabilized through the resonance with the energetic particles.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859826
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  • 7
    Electronic Resource
    Electronic Resource
    Stabilization of collisional drift-tearing modes at the breakdown of the constant-Ψ approximation (1991)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 1338-1345 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The linear growth rate of the collisional drift-tearing mode is found to be a nonmonotonic function of the stability parameter Δ'. It reaches a maximum for Δ'≈0.5Δ'1=mωA/ (square root of)ωˆ*e(ωˆ*e−ωdi) (approximately-greater-than) 0, where ωA, ωˆ*e, ωdi are the Alfvén, electron drift wave, and ion diamagnetic frequencies, and becomes negative for Δ' ≥ Δ'1, corresponding to the regime where the constant-Ψ approximation breaks down. A second mode, identified as the diamagnetic modification of the ε1/3η mode near the condition for ideal magnetohydrodynamic (MHD) marginal stability, becomes unstable for Δ'≥Δ'2 = (ωˆ*e/ωAεη)1/2 (approximately-greater-than) Δ'1, leaving a stable window in values of Δ'. Applications to the stability of modes with poloidal and toroidal mode numbers m = n = 1 are presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859828
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  • 8
    Electronic Resource
    Electronic Resource
    Internal kink modes in the ion-kinetic regime (1989)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 1 (1989), S. 364-374 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The m=1 kink mode is investigated in the high temperature regime where the width of the singular layer is determined by the mean ion gyroradius. This regime is reached in a number of present-day fusion experiments with strong auxiliary heating. A dispersion relation that contains the full kinetic response of the ions is derived and analyzed. The growth rates are larger than the corresponding ones obtained from fluid theory. Diamagnetic stabilization is weaker than in the fluid case. Ion temperature gradients are shown to be stabilizing at low values of the diamagnetic frequency and destabilizing at large values.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859149
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  • 9
    Electronic Resource
    Electronic Resource
    Global modes and high-energy particles in ignited plasmas (1990)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 2 (1990), S. 927-943 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Fusion-produced alpha particles may spontaneously enhance the stability of an ignited plasma against m=1 internal modes. Stable values of the poloidal beta of the thermal plasma component significantly in excess of the ideal magnetohydrodynamic threshold can be attained, provided the region where the magnetic helical parameter q is below unity is not too wide and the ignition temperature is not too high. A comprehensive analysis of the different instability regimes is presented, with special attention to so-called "fishbone oscillations'' and their influence on the ignition energy balance. The theoretical predictions compare favorably with sawtooth suppression experiments in the JET tokamak [Phys. Rev. Lett. 60, 2148 (1988)] with high-power, ion-cyclotron resonant frequency heating, where energetic ions in the MeV range are produced.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859240
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  • 10
    Electronic Resource
    Electronic Resource
    Macroscopic plasma oscillation bursts (fishbones) resulting from high-energy populations (1988)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 31 (1988), S. 1630-1648 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Oscillation bursts (fishbones) of magnetically confined plasmas are associated with the excitation of an m0=1 mode when the ideal magnetohydrodynamic (MHD) threshold for the instability of this mode is reached. Near this threshold and in the absence of an effective "viscous'' dissipative process, this mode remains marginally stable as a result of finite ion Larmor radius effects and has a real frequency of oscillation near the ion diamagnetic frequency. The release of the mode excitation energy related to the gradient of the plasma pressure, in the case of perpendicular neutral beam injection, is allowed by the resonant interaction of the mode with fast trapped ions that precess around the torus as a result of the curvature and the gradient of the confining magnetic field. This process plays the role of an effective viscosity. A consequence of the presented interpretation is that fishbone oscillations may also be excited in the case of parallel beam injection. In fact, for relatively low values of the beam transverse pressure, the basic mode frequency is related to the parameters of the target plasma and does not depend on the magnetic drift frequency of the beam particles that are injected nor on their velocity distribution.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.866702
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