443 research outputs found
Cosmic ray secondary nuclei and the structure of the galaxy
The consequencies of diffusive acceleration of cosmic rays in supernova shocks propagation through an inhomogeneous interstellar medium are explored. The acceleration takes place in the hot, tenuous, intercloud gas, while nuclear collisions, leading to the production of cosmic ray secondaries, predominantly occur in those regions where the supernova shocks collide with interstellar clouds. A simple model is used to calculate the interaction of a (cosmic ray + gas) shock with a cloud, and thus determine the gross topology. Extending this to the whole system, using mean cloud sizes and space densities, allows us to calculate the secondary/primary cosmic ray abundance ratios for light and heavy nuclei
Anisotropic shear melting and recrystallization of a two-dimensional complex (dusty) plasma
A two-dimensional plasma crystal was melted by suddenly applying localized
shear stress. A stripe of particles in the crystal was pushed by the radiation
pressure force of a laser beam. We found that the response of the plasma
crystal to stress and the eventual shear melting depended strongly on the
crystal's angular orientation relative to the laser beam. Shear stress and
strain rate were measured, from which the spatially resolved shear viscosity
was calculated. The latter was shown to have minima in the regions with high
velocity shear, thus demonstrating shear thinning. Shear-induced reordering was
observed in the steady-state flow, where particles formed strings aligned in
the flow direction.Comment: 7 pages, 8 figures, submitted to Physical Review
Binary crystals in two-dimensional two-component Yukawa mixtures
The zero-temperature phase diagram of binary mixtures of particles
interacting via a screened Coulomb pair potential is calculated as a function
of composition and charge ratio. The potential energy obtained by a Lekner
summation is minimized among a variety of candidate two-dimensional crystals. A
wealth of different stable crystal structures is identified including
structures [ particles correspond to large
(small) charge.] Their elementary cells consist of triangular, square or
rhombic lattices of the particles with a basis comprising various
structures of and particles. For small charge asymmetry there are no
intermediate crystals besides the pure and triangular crystals.Comment: RevTeX 4 - 17 pages - 6 main figure
Onset of cavity deformation upon subsonic motion of a projectile in a fluid complex plasma
We study deformation of a cavity around a large projectile moving with
subsonic velocity in the cloud of small dust particles. To solve this problem,
we employ the Navier--Stokes equation for a compressible fluid with due regard
for friction between dust particles and atoms of neutral gas. The solutions
shows that due to friction, the pressure of dust cloud at the boundary of the
cavity behind the projectile can become negative, which entails formation of a
microscopic void free from dust particles -- the cavity deformation.
Corresponding threshold velocity is calculated, which is found to decrease with
increasing cavity size. Measurement of such velocity makes it possible to
estimate the static pressure inside the dust cloud.Comment: 7 pages, 3 figure
Microstructure of a liquid complex (dusty) plasma under shear
The microstructure of a strongly coupled liquid undergoing a shear flow was
studied experimentally. The liquid was a shear melted two-dimensional plasma
crystal, i.e., a single-layer suspension of micrometer-size particles in a rf
discharge plasma. Trajectories of particles were measured using video
microscopy. The resulting microstructure was anisotropic, with compressional
and extensional axes at around to the flow direction.
Corresponding ellipticity of the pair correlation function or
static structure factor gives the (normalized) shear rate of the
flow.Comment: 5 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
Observation of particle pairing in a two-dimensional plasma crystal
The observation is presented of naturally occurring pairing of particles and
their cooperative drift in a two-dimensional plasma crystal. A single layer of
plastic microspheres was suspended in the plasma sheath of a capacitively
coupled rf discharge in argon at a low pressure of 1 Pa. The particle dynamics
were studied by combining the top-view and side-view imaging of the suspension.
Cross analysis of the particle trajectories allowed us to identify naturally
occurring metastable pairs of particles. The lifetime of pairs was long enough
for their reliable identification.Comment: 5 pages, 4 figure
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