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Title: Particle dynamics in asymmetry-induced transport

Abstract

The particle dynamics of asymmetry-induced transport are studied using a single-particle computer simulation. For the case of a helical asymmetry with axial and azimuthal wavenumbers (k,l) and with periodic boundary conditions, behaviors consistent with analytical theory are observed. For the typical experimental case of a standing wave asymmetry, the code reveals dynamical behaviors not included in the analytical theory of this transport. The resonances associated with the two constituent helical waves typically overlap and produce a region of stochastic motion. In addition, particles near the radius where the asymmetry frequency {omega} matches l times the ExB rotation frequency {omega}{sub R} can be trapped in the potential of the applied asymmetry and confined to one end of the device. Both behaviors are associated with large radial excursions and mainly affect particles with low velocities, i.e., v{sub z}<2{omega}{sub T}/k, where {omega}{sub T} is the trapping frequency. For the case of a helical asymmetry with specularly reflecting boundaries, large radial excursions are observed for all velocities near the radius, where {omega}=l{omega}{sub R}. Minor modifications to these results are observed when the code is run with realistic end potentials.

Authors:
 [1]
  1. Occidental College, Physics Department, Los Angeles, California 90041 (United States)
Publication Date:
OSTI Identifier:
20960090
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 14; Journal Issue: 1; Other Information: DOI: 10.1063/1.2424431; (c) 2007 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; ASYMMETRY; BOUNDARY CONDITIONS; COMPUTERIZED SIMULATION; ELECTROMAGNETIC FIELDS; EXCURSIONS; PARTICLES; PERIODICITY; PLASMA; PLASMA SIMULATION; RESONANCE; STANDING WAVES; STOCHASTIC PROCESSES; TRAPPING; TRAPS

Citation Formats

Eggleston, D. L. Particle dynamics in asymmetry-induced transport. United States: N. p., 2007. Web. doi:10.1063/1.2424431.
Eggleston, D. L. Particle dynamics in asymmetry-induced transport. United States. doi:10.1063/1.2424431.
Eggleston, D. L. Mon . "Particle dynamics in asymmetry-induced transport". United States. doi:10.1063/1.2424431.
@article{osti_20960090,
title = {Particle dynamics in asymmetry-induced transport},
author = {Eggleston, D. L.},
abstractNote = {The particle dynamics of asymmetry-induced transport are studied using a single-particle computer simulation. For the case of a helical asymmetry with axial and azimuthal wavenumbers (k,l) and with periodic boundary conditions, behaviors consistent with analytical theory are observed. For the typical experimental case of a standing wave asymmetry, the code reveals dynamical behaviors not included in the analytical theory of this transport. The resonances associated with the two constituent helical waves typically overlap and produce a region of stochastic motion. In addition, particles near the radius where the asymmetry frequency {omega} matches l times the ExB rotation frequency {omega}{sub R} can be trapped in the potential of the applied asymmetry and confined to one end of the device. Both behaviors are associated with large radial excursions and mainly affect particles with low velocities, i.e., v{sub z}<2{omega}{sub T}/k, where {omega}{sub T} is the trapping frequency. For the case of a helical asymmetry with specularly reflecting boundaries, large radial excursions are observed for all velocities near the radius, where {omega}=l{omega}{sub R}. Minor modifications to these results are observed when the code is run with realistic end potentials.},
doi = {10.1063/1.2424431},
journal = {Physics of Plasmas},
number = 1,
volume = 14,
place = {United States},
year = {Mon Jan 15 00:00:00 EST 2007},
month = {Mon Jan 15 00:00:00 EST 2007}
}
  • While it is easy to experimentally demonstrate that applied field asymmetries produce radial transport, convincing comparisons of experiment and theory have yet to be made. A key prediction of the theory is that the transport will be dominated by particles that move in resonance with the asymmetry. For the general case of a time-varying asymmetry, the resonance condition is {omega}-l{omega}{sub R}-kv=0, where v is the axial velocity, {omega}{sub R} is the ExB rotation frequency, and {omega}, l and k are the asymmetry frequency, azimuthal and axial wavenumbers, respectively. We present experiments on our low density trap in which {omega}, {omega}{submore » R}, and k are varied and the resulting radial particle flux is measured. The experiments show a resonance in the flux similar to that predicted by theory. The peak frequency of this resonance increases with {omega}{sub R} and k, but not in the way theory predicts. The peak magnitude of the measured transport is roughly forty times smaller than the theoretical prediction, and low-frequency asymmetries are especially ineffective at producing transport.« less
  • Weak axial variations in B(z) or {phi}(z) in 'axisymmetric' plasma traps cause a fraction of the particles to be trapped axially, with a velocity-space separatrix between trapped and passing populations. The trapped and passing particles experience different dynamics in response to a variety of {theta}-asymmetries in the E x B rotating plasma, so a discontinuity in the velocity-space distribution f(v) tends to form at the separatrix. Collisional scatterings thus cause large fluxes as they smooth the distribution in a boundary layer near the separatrix. In essence, this separatrix dissipation damps the AC or DC longitudinal currents induced by plasma wavesmore » or confinement field asymmetries. This trapped-particlemediated damping and 'neoclassical' particle transport often dominates in cylindrical pure electron plasmas, and may be important in other nominally symmetric open systems.« less
  • Cited by 9
  • Radial transport produced by static nonaxisymmetric fields is thought to limit the confinement of non-neutral plasmas and experiments with applied asymmetries have verified that such fields do produce transport. A theoretical model of such transport is presented which is appropriate for long, thin plasmas. The theory allows for asymmetries with nonzero frequency and includes the linear collective response to applied wall voltages. For the regime where the effective collision frequency is large, the asymmetry-induced radial particle flux is derived from the drift kinetic/Poisson equations including collisions. For low collision frequencies a heuristic derivation is given. In both regimes the resultingmore » transport is dominated by particles that move in resonance with the asymmetry. Possible applications of the theory to several experiments are discussed. {copyright} {ital 1999 American Institute of Physics.}« less
  • It has been suggested that magnetically trapped particles play a role in the asymmetry-induced radial transport observed in the Occidental non-neutral plasma trap. This magnetic trapping would occur due to a small increase ({beta}{identical_to}{delta}B/B{approx_equal}0.4%) in magnetic field at the center of our solenoid and would keep low velocity particles confined to the ends of the trap. To test this suggestion, three coils of additional windings have been added to the trap solenoid thus allowing adjustment of the axial field variation {delta}B. The effect of these adjustments on typical radial flux resonances is investigated. Making B as uniform as possible reducesmore » {beta} by a factor of 5.9, but this produces little change in the transport. Varying {beta} over the broader range from -8.5% to 9.5% gives variations of 20%-90% in the magnitude, peak frequency, and width of the flux resonances, but these variations do not match the predictions of a simple model of trapped particle transport based on isotropic particle distributions.« less