4,937 research outputs found
Near-lunar proton velocity distribution explained by electrostatic acceleration
The observation of parallel ion velocity in the near-lunar wake approximately
equal to external solar wind velocity \emph{can} be explained within
uncertainties by an analytic electrostatic expansion model. The one-dimensional
model frequently used is inadequate because it does not account for the moon's
spherical shape. However, application of a more recent generalization to
three-dimensions of the solution along characteristics predicts higher
velocities, and is probably sufficient to account for the SARA observations on
the Chandrayaan-1 space-craft.Comment: 1 figure, 1 tabl
Ion collection by oblique surfaces of an object in a transversely-flowing strongly-magnetized plasma
The equations governing a collisionless obliquely-flowing plasma around an
ion-absorbing object in a strong magnetic field are shown to have an exact
analytic solution even for arbitrary (two-dimensional) object-shape, when
temperature is uniform, and diffusive transport can be ignored. The solution
has an extremely simple geometric embodiment. It shows that the ion collection
flux density to a convex body's surface depends only upon the orientation of
the surface, and provides the theoretical justification and calibration of
oblique `Mach-probes'. The exponential form of this exact solution helps
explain the approximate fit of this function to previous numerical solutions.Comment: Four pages, 2 figures. Submitted to Phys. Rev. Letter
Experimental Divertor Similarity Database Parameters
A set of experimentally-determined dimensionless parameters is proposed for
characterizing the regime of divertor operation. The objective is to be able to
compare as unambiguously as possible the operation of different divertors and
to understand what physical similarities and differences they represent.
Examples from Alcator C-Mod are given.Comment: Plain Tex (8 pages) plus 5 postscipt figure
Non-linear Plasma Wake Growth of Electron Holes
An object's wake in a plasma with small Debye length that drifts
\emph{across} the magnetic field is subject to electrostatic electron
instabilities. Such situations include, for example, the moon in the solar wind
wake and probes in magnetized laboratory plasmas. The instability drive
mechanism can equivalently be considered drift down the potential-energy
gradient or drift up the density-gradient. The gradients arise because the
plasma wake has a region of depressed density and electrostatic potential into
which ions are attracted along the field. The non-linear consequences of the
instability are analysed in this paper. At physical ratios of electron to ion
mass, neither linear nor quasilinear treatment can explain the observation of
large-amplitude perturbations that disrupt the ion streams well before they
become ion-ion unstable. We show here, however, that electron holes, once
formed, continue to grow, driven by the drift mechanism, and if they remain in
the wake may reach a maximum non-linearly stable size, beyond which their
uncontrolled growth disrupts the ions. The hole growth calculations provide a
quantitative prediction of hole profile and size evolution. Hole growth appears
to explain the observations of recent particle-in-cell simulations
Transverse instability magnetic field thresholds of electron phase-space holes
A detailed comparison is presented of analytical and particle-in-cell (PIC)
simulation investigation of the transverse instability, in two dimensions, of
initially one-dimensional electron phase-space hole equilibria. Good
quantitative agreement is found between the shift-mode analysis and the
simulations for the magnetic field () threshold at which the instability
becomes overstable (time-oscillatory) and for the real and imaginary parts of
the frequency. The simulation -threshold for full stabilization exceeds the
predictions of shift-mode analysis by 20 to 30\%, because the mode becomes
substantially narrower in spatial extent than a pure shift. This threshold
shift is qualitatively explained by the kinematic mechanism of instability.Comment: 6 figure
Oblate Electron Holes are not attributable to Anisotropic Shielding
Shielding mechanisms' influence on the ratio of perpendicular to parallel
scale lengths of multidimensional plasma electron hole equilibria are analyzed
theoretically and computationally. It is shown that the ``gyrokinetic'' model,
invoking perpendicular polarization, is based on a misunderstanding and cannot
explain the observational trend that greater transverse extent accompanies
lower magnetic field. Instead, the potential in the wings of the hole, outside
the region of trapped-electron depletion, has isotropic shielding giving
, with the shielding length equal to the
Debye length for holes much slower than the electron thermal speed. Particle in
cell simulations confirm the analysis
Comment on "Evolution Equations of Nonlinearly Permissible, Coherent Hole Structures Propagating Persistently in Collisionless Plasmas''
Recent critical remarks, published in "Annalen der Physik", about the present
author's analysis of electron and ion holes and their stability are addressed
and shown to be misunderstandings and misrepresentations
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