Anisotropic Diffusion in Electron Swarms

We show that the distribution functions derived by Parker (1963) in his analysis of the TownsendHuxley experiment can be used to calculate DL/D, the ratio of longitudinal to isotropic diffusion coefficients for electron swarms in electrostatic fields in gases. In the case of a constant collision frequency interaction our results agree with previous calculations, whilst for a constant mean free path we find DL/D = 0�58. This result is some 16% higher than previously published values but provides better agreement with experiment for electrons in helium.

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Cation mobilities in liquid and supercritical C1–C4 hydrocarbons

Canadian Journal of Chemistry ◽

10.1139/v80-236 ◽

1980 ◽

Vol 58 (14) ◽

pp. 1490-1494 ◽

Cited By ~ 13

Author(s):

Norman Gee ◽

Gordon R. Freeman

Keyword(s):

Free Path ◽

Ion Mobility ◽

Diffusion Coefficients ◽

Critical Region ◽

Mean Free Path ◽

Liquid Viscosity ◽

Boiling Points ◽

Rough Approximation ◽

The Relationship ◽

Lower Liquid

The relationship between ion mobility and liquid viscosity is commonly expressed as μ [Formula: see text] η−m. In hydrocarbons the value of m tends to be near 1.0 at η > 5 mP, m > 1.0 at ~5 < η < 1 mP, and m < 1.0 at η < 0.5 mP. Thus there is a maximum in a plot of μη against η−1 and Walden's rule (m = 1.0) is only a rough approximation. The decrease of μη as the critical region is approached is accompanied by an increase in the ratio of diffusion coefficients Dmolec/Dion. Ion mobilities in the liquids well below their normal boiling points are chiefly controlled by the fluidity. At higher temperatures and concomitant lower liquid densities and viscosities μη first increases, due to an increasing ion mean free path, then decreases as the critical region is approached, due to the increasing liquid compressibility and consequent electrostriction about the ion.

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New low-frequency waves and negative mass instability in dusty plasmas

Journal of Plasma Physics ◽

10.1017/s002237781000036x ◽

2010 ◽

Vol 76 (6) ◽

pp. 929-937

Author(s):

D. P. RESENDES ◽

R. BINGHAM ◽

S. MOTA ◽

V. N. TSYTOVICH

Keyword(s):

Electron Temperature ◽

Free Path ◽

Collision Frequency ◽

Low Frequency ◽

Mean Free Path ◽

Sound Waves ◽

Ion Temperature ◽

Plasma Particle ◽

Charging Mode ◽

Dust Charging

AbstractLow-frequency dusty plasma waves with frequencies much smaller than the frequency of charging collisions of plasma particles with dust particles are considered taking into account elastic and charging collisions of plasma particles with dust and neutrals. The usual dust sound waves with an upper frequency equal to the dust plasma frequency are found to be present only for wavelengths much smaller than the plasma particle effective mean free path due to the effective collision frequency. The effectice collision frequency is found to be inversely proportional to the square root of the product of the charging frequency and the frequency of particle momentum losses, involving processes due to elastic plasma particle–dust collisions and collisions with neutrals. It is shown that when the wavelength of the wave is much larger than the mean free path for effective collisions, the properties of the waves are different from those considered previously. A negative mass instability is found in this domain of frequencies when the effective mean free path of ions is larger than the effective mean free path of electrons. In the absence of neutrals, this appears to be possible only if the temperature of ions exceeds the electron temperature. This can occur in laboratory experiments and space plasmas but not in plasma-etching experiments. In the absence of instability, a new dust oscillation, a dust charging mode, is found, whose frequency is almost constant over a certain range of wave numbers. It is inversely proportional to the dust mass and charging frequency of the dust. A new dust electron sound wave is found for frequencies less than the frequency of the dust charging mode. The velocity of the dust electron sound wave is determined by the electron temperature but not the ion temperature, as for the usual dust sound waves, with the electron temperature substantially exceeding the ion temperature.

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Departure from MHD prescriptions in shock formation over a guiding magnetic field

Laser and Particle Beams ◽

10.1017/s0263034617000519 ◽

2017 ◽

Vol 35 (3) ◽

pp. 513-519 ◽

Cited By ~ 1

Author(s):

A. Bret ◽

A. Pe'er ◽

L. Sironi ◽

M.E. Dieckmann ◽

R. Narayan

Keyword(s):

Magnetic Field ◽

Free Path ◽

Mean Free Path ◽

Distribution Functions ◽

Shock Formation ◽

Physical Analysis ◽

Particle In Cell ◽

The Mean ◽

Collective Plasma ◽

Aligned Magnetic Field

AbstractIn plasmas where the mean-free-path is much larger than the size of the system, shock waves can arise with a front much shorter than the mean-free-path. These so-called “collisionless shocks” are mediated by collective plasma interactions. Studies conducted so far on these shocks found that although binary collisions are absent, the distribution functions are thermalized downstream by scattering on the fields, so that magnetohydrodynamics prescriptions may apply. Here we show a clear departure from this pattern in the case of Weibel shocks forming over a flow-aligned magnetic field. A micro-physical analysis of the particle motion in the Weibel filaments shows how they become unable to trap the flow in the presence of too strong a field, inhibiting the mechanism of shock formation. Particle-in-cell simulations confirm these results.

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