ALBERT

Notes: Hot electrons and sub- and supersonic flows of cold ions around a charged dust particle create steady state wake and Debye screening fields. These linear, electrostatic fields are studied in two-dimensional planar or cylindrical geometry. An asymptotic analysis in the limit of large (compared to Debye length) downstream coordinate z yields analytic wakefields that are in good agreement with numerical integrations of the linear, steady state response function.© 2000 American Institute of Physics.

Notes: 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 MλDe(1−M2)−1/2 for M〈1, where M is the Mach number and λDe 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 MλDe in both regimes. The amplitude of the wake peaks at M(approximate)1 in both the one- and two-dimensional simulations. © 2000 American Institute of Physics.

Notes: Hybrid simulations with kinetic ions and massless fluid electrons are used to investigate the linear and nonlinear behavior of the magnetized Rayleigh–Taylor instability in slab geometry with the plasma subject to a constant gravity. Three regimes are found, which are determined by the magnitude of the complex frequency ω=ωr+iγ. For |ω|(very-much-less-than)Ωi(Ωi= ion gyrofrequency), one finds the typical behavior of the usual fluid regime, namely the development of "mushroom-head'' spikes and bubbles in the density and a strongly convoluted boundary between the plasma and magnetic field, where the initial gradient is not relaxed much. A second regime, where |ω|∼0.1Ωi, is characterized by the importance of the Hall term. Linearly, the developing flute modes are more finger-like and tilted along the interface; nonlinearly, clump-like structures form, leading to a significant broadening of the interface. The third regime is characterized by unmagnetized ion behavior, with |ω|∼Ωi. Density clumps, rather than flutes, form in the linear stage, while nonlinearly, longer-wavelength modes that resemble those in fluid regime dominate. Finite Larmor radius stabilization of short-wavelength modes is observed in each regime. © 1996 American Institute of Physics.

Notes: Hybrid simulations with kinetic ions and massless fluid electrons are used to investigate the linear and nonlinear behavior of the Rayleigh-Taylor instability in a collisionless, magnetized plasma in slab geometry with the plasma subject to a time varying gravity. In particular, cases where the sign of gravity is reversed for some time interval are compared with the corresponding case with constant gravity. Consistent with simple theory, the effect of the gravity reversal is to stop the growth of the instability. And when the gravity is restored to its initial direction, the instability resumes at a rate that is commensurate with its earlier value. Several ways to estimate the rate of growth of the thickness of the mixing layer when g(vector) is not constant are suggested and compared with the simulations. © 1997 American Institute of Physics.

Notes: Ion interpenetration, stagnation, and energization processes are studied in colliding laser-produced plasma configurations relevant to Trident [R. G. Watt, Rev. Sci. Instrum. 64, 1770 (1993)] experiments using four different numerical methods: one-dimensional Monte Carlo and Lagrangian multifluid codes, and one- and two-dimensional hybrid (particle ions, fluid electrons) and single-fluid Lagrangian codes. Results from the four methodologies are compared for plasmas generated with gold and deuterated polyethylene (CD2) targets. Overall, the various codes give similar results concerning the initial expansion of the plasmas and their collisional interaction, the degree of stagnation, stagnation time, and amount of ion thermalization for gold targets, while multispecies techniques indicate a much softer stagnation for CD2 plasmas than the single-fluid model. Variations in the results of the calculations due to somewhat different initializations and parameters, as well as to different physics in the codes, are discussed. © 1996 American Institute of Physics.

Notes: Structuring that results from plasma streaming at sub-Alfvénic speeds across an external magnetic field is considered. Previously, it has been proposed the lower hybrid drift instability enhanced by the deceleration of the plasma by the field produces the flute modes observed on the surface of expanding laser produced plasmas and the AMPTE magnetotail releases [Eos (Trans) 63, 843 (1982)]. An appropriate dispersion equation to describe the properties of the unstable waves has been derived and particle simulations carried out to show the growth and evolution of the instability. The salient features of this earlier work are reviewed here, and then additions and refinements to the theory and simulations are described. In particular, the scaling of the wave properties with the ratio of the ion gyroradius to the magnetic confinement radius is discussed and the nonlinear evolution of the instability is investigated more thoroughly. The consequences of these results, both for the laser experiments and for AMPTE, are also considered. To this end, a comparison of the linear and nonlinear properties of the waves observed in the simulations with those seen in the experiments is carried out. While there is considerable discrepancy between the observed and predicted wavelengths of the modes, the effects considered here are in the direction of reducing the disagreement.

Notes: The theory of cross-field ion streaming instabilities is applied to the parameter regime of the end cells of the upgraded Tandem Mirror Experiment with neutral beam injection. The cross-field ion drift as well as the electron thermal anisotropy Te⊥〉Te(parallel) provides the free-energy that drives various instabilities. Three instabilities, a nearly perpendicular propagating modified two-stream instability, an obliquely propagating ion–ion streaming instability, and an obliquely propagating electromagnetic lower-hybrid instability, have been identified. The first two waves are electrostatic and have the largest growth rate. For the actual operation conditions of the tandem mirror machines, it is found that the ion–ion streaming instability has the largest growth rate. When more energetic neutral beams become available, the two-stream instability may play the dominant role in the stability of these devices.

Notes: A comprehensive theoretical treatment of the linear stability of a sub-Alfvénic plasma expansion is developed. The analysis is similar to those performed for the lower-hybrid-drift instability and the drift cyclotron instability. In addition to the diamagnetic drift (Vdi) that drives these instabilities, the gravitational drift (Vg) caused by the deceleration of the plasma shell, and the Pedersen drift (VP) caused by ion–neutral collisions and neutral gas flow, are included. The emphasis of the paper is on the instability driven by the gravitational drift. The theory is fully kinetic and includes finite-beta effects (i.e., electromagnetic coupling and electron ∇B drift-wave resonances), collisional effects (electron–ion, electron–neutral, and ion–neutral collisions), and neutral gas flow, effects that have not been considered to date. The analysis is carried out in a slab geometry although the applications are to spherical expansions. The main conclusions are as follows. In the strong drift limit (Vg〉vi and Vdi∼vi, where vi is the ion thermal velocity) it is found that (1) finite-beta effects are stabilizing and reduce the wavelength of the maximum growth rate; (2) ion–neutral collisions are stabilizing and do not affect the wavelength of the maximum growth rate; (3) electron–neutral collisions are stabilizing and increase the wavelength of the maximum growth rate; (4) the gravitational drift driven mode maximizes the growth rate at longer wavelengths than the diamagnetic drift driven mode; (5) the Pedersen drift effectively reduces the gravitational drift, and is therefore a stabilizing influence; and (6) the instability splits into two modes for Te(very-much-greater-than)Ti in finite-beta plasmas: the lower-hybrid-drift instability at high frequencies and short wavelengths, and a gravitational mode at lower frequencies and longer wavelengths.In the weak drift regime (Vg〈vi and Vdi〈vi) it is found that (1) finite-beta effects are stabilizing and increase the wavelength of the maximum growth rate; (2) ion–neutral collisions are destabilizing and decrease the wavelength of the maximum growth rate; and (3) electron–ion and electron–neutral collisions are stabilizing, and increase the wavelength of the maximum growth rate. When the growth rate becomes less than the ion cyclotron instability (γ〈Ωi), the growth rate as a function of wave number "breaks up'' into a discrete set of modes which is associated with the coupling of the drift waves to ion cyclotron waves. These results are applied to the AMPTE magnetotail release [J. Geophys. Res. 92, 5777 (1987)], the Naval Research Laboratory laser experiment [Phys. Rev. Lett. 59, 2299 (1987)], and the upcoming CRRES GTO releases [D. Reasoner (private communication, 1989)].

Notes: We quantify a model which incorporates observed features of contaminant particle growth in plasma processing reactors. According to the model, large "predator'' particles grow by adsorbing smaller, typically neutral, "prey'' protoparticles. The latter are supplied by an assumed constant mass injection of contaminant material. Scaling laws and quantitative predictions compare favorably with published experimental results. © 1996 American Institute of Physics.