[en] The structure of the wake potential downstream of a stationary dust grain in a flowing plasma is studied on ion time scales using particle-in-cell simulation methods. The scaling of the wake is investigated as a function of Mach number and other parameters as well as the dimensionality of the system. The results are compared and discussed in relation to various theoretical expressions for the wake. Consistent with theory, in one dimension the wake wavelength scales as Mλ_De(1-M²)^-1/2 for M<1, where M is the Mach number and λ_De is the electron Debye length, while no wake forms for M>1. In two dimensions, a wake is formed for both M<1 and M>1, while the wake wavelength scales as Mλ_De in both regimes. The amplitude of the wake peaks at M≅1 in both the one- and two-dimensional simulations. (c) 2000 American Institute of Physics

[en] A new type of density soliton, which we call open-quotes dipole density soliton,close-quote close-quote is discovered in data from the Freja satellite. Like the dip or hump density solitons that were recently discovered in the Freja data [D.-J. Wu, G.-L. Huang, and D.-Y. Wang, Phys. Plasmas 3, 2879 (1996)], the dipole density solitons are also associated with strong electric spikes (∼ a few 100 mV/m) and have a spatial scale length of a few 100 m. This indicates that the three types of density solitons (dip, hump, and dipole) probably have the same physical nature. In this paper, a two-dimensional solitary kinetic Alfvacute en wave (SKAW) model with a dipole vortex structure is proposed to account for the three kinds of density solitons (dip, hump, and dipole), in which the differences in their appearances can naturally be attributed to differences in the positions and directions at which the satellite crosses dipole vortex structures. Some features of this two-dimensional SKAW model are discussed, and the results are compared to the one-dimensional cases. copyright 1997 American Institute of Physics

[en] Core plasma rotation is observed to change from counter direction to co-current direction during the transition from low (L) to high (H) confinement mode, in Alcator C-Mod plasmas that are heated purely Ohmically and, hence, have no momentum input. The changes of the toroidal velocities, deduced independently from impurity Doppler measurements and from magnetic perturbations associated with sawteeth, agree. The magnitude of the change is consistent with the previously documented scaling for rotation in ion cyclotron rf-heated H modes. The rotation in this Ohmic experiment is obviously not an rf effect but demonstrates unequivocally a transport effect accelerating the plasma. (c) 2000 The American Physical Society

[en] The problem of mode locking due to forced magnetic reconnection in rotating cylindrical plasmas is revisited. Forced reconnection is characterized by very large values of the parameter Δ^', which makes the constant-ψ approximation generally inapplicable in the linear and early nonlinear regimes. The nonlinear dynamics of rotating non-constant-ψ islands is distinguished by the persistence of current sheets spanning Y points. Mode locking due to a suddenly imposed error field is discussed. Temporal dynamics and locking thresholds that differ significantly from the predictions of the constant-ψ theory [R. Fitzpatrick and T. C. Hender, Phys. Fluids B 3, 644 (1991)] are obtained. The predictions of the present theory are compared with experimental tokamak observations. copyright 1997 American Institute of Physics

No abstract available

[en] A theoretical framework is developed for calculating the nonlinear rf forces that can drive sheared poloidal flow in a tokamak plasma. It is shown that the rf-induced flow drive can be calculated without first obtaining an explicit result for the nonlinear distribution function. Instead, for modes satisfying the eikonal approximation, the flow drive can be expressed entirely in terms of moments of the linearized plasma responses. The method is applied to obtain explicit results for poloidal force generation for sheared flow drive applications in a hot plasma slab that supports rf waves of arbitrary polarization. The theory is fully electromagnetic and retains k_{(perpendicularsign)}ρ_i∼1 (Bessel function) effects for the ion dynamics without approximation. An illustrative application to the ion Bernstein wave is presented. (c) 2000 American Institute of Physics

[en] Ohm close-quote s law in ideal magnetohydrodynamics (MHD) leads to an induction equation which can be interpreted in terms of magnetic flux being transported by the plasma flow. It is shown that this frozen-in condition is the non-relativistic limit of a corresponding relativistic condition for the electromagnetic field tensor. Several invariants for this type of transport are analyzed. The relativistic formulation also includes a broader class of transporting flows, which may differ from the plasma flow. A classification and interpretation of these transporting flows is given and it is shown that the corresponding evolutions of the electromagnetic field also includes cases of non-ideal MHD evolution. Thus it is possible to find invariants in non-ideal MHD similar to the magnetic flux for ideal plasma flows. copyright 1997 American Institute of Physics

[en] Debye screening potential and wake potential for a moving dust grain in a collisionless plasma with ion flow is studied. When a relative velocity of the dust grain exceeds the ion acoustic velocity, the oscillatory wake potential is formed in a circular cone behind the particle and produces potential minima in a periodic manner. The ion acoustic collective effects on dust particles contribute to the formation of the periodic structure. The characteristic spacing between the potential minima are several times of Debye wavelength in height and in radius. Such a periodic structure may be relevant to the formation of Coulomb quasilattices (plasma crystals) observed in the dusty plasma laboratory experiments. copyright 1997 American Institute of Physics

[en] The effects of finite plasma pressure and pressure anisotropy, toroidal rotation and gravity on the equilibrium, flow, and stability of plasma in dipolar magnetic configurations are considered. Dipolar equilibria are of interest for magnetic confinement experiments in the laboratory and understanding the physics of magnetospheric and astrophysical plasmas. It is demonstrated that realistic solutions of the appropriate ideal magnetohydrodynamics (MHD) equations can be found in a separable form which drastically simplifies the equations and even allows us to analytically obtain some limiting forms of the nonlinear solutions. The MHD stability of these equilibria is explicitly evaluated in some cases. (c) 2000 American Institute of Physics

[en] Differential geometry based upon the Cartan calculus of differential forms is applied to investigate invariant properties of equations that describe the motion of continuous media. The main feature of this approach is that physical quantities are treated as geometrical objects. The geometrical notion of invariance is introduced in terms of Lie derivatives and a general procedure for the construction of local and integral fluid invariants is presented. The solutions of the equations for invariant fields can be written in terms of Lagrange variables. A generalization of the Hamiltonian formalism for finite-dimensional systems to continuous media is proposed. Analogously to finite-dimensional systems, Hamiltonian fluids are introduced as systems that annihilate an exact two-form. It is shown that Euler and ideal, charged fluids satisfy this local definition of a Hamiltonian structure. A new class of scalar invariants of Hamiltonian fluids is constructed that generalizes the invariants that are related with gauge transformations and with symmetries (Noether). copyright 1997 American Institute of Physics