146 research outputs found
Collective modes of two-dimensional classical Coulomb fluids
Molecular dynamics simulations have been performed to investigate in detail
collective modes spectra of two-dimensional Coulomb fluids in a wide range of
coupling. The obtained dispersion relations are compared with theoretical
approaches based on quasi-crystalline approximation (QCA), also known as the
quasi-localized charge approximation (QLCA) in the plasma-related context. An
overall satisfactory agreement between theory and simulations is documented for
the longitudinal mode at moderate coupling and in the long-wavelength domain at
strong coupling. For the transverse mode, satisfactory agreement in the
long-wavelength domain is only reached at very strong coupling, when the cutoff
wave-number below which shear waves cannot propagate becomes small. The
dependence of the cutoff wave-number for shear waves on the coupling parameter
is obtained.Comment: 10 pages, 6 figure
Three dimensional complex plasma structures in a combined radio frequency and direct current discharge
We report on the first detailed analysis of large three dimensional (3D)
complex plasma structures in experiments performed in pure rf and combined
rf+dc discharge modes. Inductively coupled plasma (ICP) is generated by an rf
coil wrapped around the vertically positioned cylindrical glass tube at a
pressure of 0.3 mbar. In addition, dc plasma can be generated by applying
voltage to the electrodes at the ends of the tube far from the rf coil. The
injected monodisperse particles are levitated in the plasma below the coil. A
scanning laser sheet and a high resolution camera are used to determine the 3D
positions of about particles. The observed bowl-shaped particle clouds
reveal coexistence of various structures, including well-distinguished
solid-like, less ordered liquid-like, and pronounced string-like phases. New
criteria to identify string-like structures are proposed.Comment: 6 pages, 7 figure
Negative ions in liquid helium
The structure of negative ions in liquid4He is analyzed. The possibility of
cluster or bubble formation around impurity ions of both signs is discussed.
It is shown that in superfluidhelium, bubbles form around negative alkaline
earth metal ions and clusters form around halogen ions. The nature of "fast"
and "exotic" negative ions is also discussed. It is assumed that "fast" ions
are negative ions of helium excimer molecules localized inside bubbles.
"Exotic" ions are stable negative impurity ions, which are always present in
small amounts in gas discharge plasmas. Bubbles or clusters with radii smaller
the radius of electron bubbles develop around these ions
Structural properties of dense hard sphere packings
The structural properties of dense random packings of identical hard spheres
(HS) are investigated. The bond order parameter method is used to obtain
detailed information on the local structural properties of the system for
different packing fractions , in the range between and
. A new order parameter, based on the cumulative properties of
spheres distribution over the rotational invariant , is proposed to
characterize crystallization of randomly packed HS systems. It is shown that an
increase in the packing fraction of the crystallized HS system first results in
the transformation of the individual crystalline clusters into the global
three-dimensional crystalline structure, which, upon further densification,
transforms into alternating planar layers formed by different lattice types.Comment: 4 pages, 5 figure
Effect of ionization/recombination processes on the electrical interactions between positively charged particles in highly collisional plasmas
The effect of ionization and recombination processes on the electrical
interactions between a pair of small charged particles in highly collisional
plasmas is discussed. In particular, it is shown that these processes suppress
the long-range attraction between positively charged particles. The condition
corresponding to the vanishing of attraction is derived. The role of the effect
for conditions of existing experiments is estimated
Electron Clusters in Inert Gases
The paper addresses counterintuitive behavior of electrons injected into
dense cryogenic media with negative scattering length . Instead of
expected polaronic effect (formation of density enhancement clusters) which
should substantially reduce the electron mobility, an opposite picture is
observed: with increasing (the trend taking place for inert gases with
the growth of atomic number) and the medium density, the electrons remain
practically free. An explanation of this behaviour is provided based on
consistent accounting for the non-linearity of electron interaction with the
gaseous medium in the gas atom number density
Freezing and melting of 3D complex plasma structures under microgravity conditions driven by neutral gas pressure manipulation
Freezing and melting of large three-dimensional complex plasmas under
microgravity conditions is investigated. The neutral gas pressure is used as a
control parameter to trigger the phase changes: Complex plasma freezes (melts)
by decreasing (increasing) the pressure. Evolution of complex plasma structural
properties upon pressure variation is studied. Theoretical estimates allow us
to identify main factors responsible for the observed behavior.Comment: Phys. Rev. Lett. (in press); 4 pages, 4 figure
Particle flows in a dc discharge in laboratory and microgravity conditions
We describe a series of experiments on dust particles flows in a positive
column of a horizontal dc discharge operating in laboratory and microgravity
conditions. The main observation is that the particle flow velocities in
laboratory experiments are systematically higher than in microgravity
experiments, for otherwise identical discharge conditions. The paper provides
an explanation for this interesting and unexpected observation. The explanation
is based on a physical model, which properly takes into account main
plasma-particle interaction mechanisms relevant to the described experimental
study. Comparison of experimentally measured particle velocities and those
calculated using the proposed model demonstrates reasonable agreement, both in
laboratory and microgravity conditions, in the entire range of discharge
parameters investigated.Comment: 9 pages, 7 figure
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