563 research outputs found
Dust particle charge in plasma with ion flow and electron depletion
The charge of micrometer-sized dust particles suspended in plasma above the
powered electrode of radio-frequency (RF) discharges is studied. Using a
self-consistent fluid model, the plasma profiles above the electrode are
calculated and the electron depletion towards the electrode, as well as the
increasing flow speed of ions towards the electrode, are considered in the
calculation of the dust particle floating potential. The results are compared
with those reported in literature and the importance of the spatial dust charge
variation is investigated
Charging of Aggregate Grains in Astrophysical Environments
The charging of dust grains in astrophysical environments has been
investigated with the assumption these grains are homogeneous spheres. However,
there is evidence which suggests many grains in astrophysical environments are
irregularly-shaped aggregates. Recent studies have shown that aggregates
acquire higher charge-to-mass ratios due to their complex structures, which in
turn may alter their subsequent dynamics and evolution. In this paper, the
charging of aggregates is examined including secondary electron emission and
photoemission in addition to primary plasma currents. The results show that the
equilibrium charge on aggregates can differ markedly from spherical grains with
the same mass, but that the charge can be estimated for a given environment
based on structural characteristics of the grain. The "small particle effect"
due to secondary electron emission is also important for determining the charge
of micron-sized aggregates consisting of nano-sized particles.Comment: 9 figures. arXiv admin note: substantial text overlap with
arXiv:1107.028
Probing the sheath electric field with a crystal lattice by using thermophoresis in dusty plasma
A two-dimensional dust crystal levitated in the sheath of a modified Gaseous
Electronics Conference (GEC) reference cell is manipulated by heating or
cooling the lower electrode. The dust charge is obtained by measuring global
characteristics of the levitated crystal obtained from top-view pictures. From
the force balance, the electric field in the sheath is reconstructed. From the
Bohm criterion, we conclude that the dust crystal is levitated mainly above and
just below the classical Bohm point
Glow and dust in plasma boundaries
The sheath region is probed in different complex plasma experiments using
dust particles in addition to measurement of the optical emission originating
from the plasma. The local maximum in optical emission coincides with the
breaking of quasi-neutrality at the sheath boundary as indicated by the
vertical force profile reconstructed from dust particle trajectories, as well
as by the local onset of dust density waves in high density dust clouds
suspended in a dielectric box
Modeling Agglomeration of Dust Particles in Plasma
The charge on an aggregate immersed in a plasma environment distributes
itself over the aggregate's surface; this can be approximated theoretically by
assuming a multipole distribution. The dipole-dipole (or higher order) charge
interactions between fractal aggregates lead to rotations of the grains as they
interact. Other properties of the dust grains also influence the agglomeration
process, such as the monomer shape (spherical or ellipsoidal) or the presence
of magnetic material. Finally, the plasma and grain properties also determine
the morphology of the resultant aggregates. Porous and fluffy aggregates are
more strongly coupled to the gas, leading to reduced collisional velocities,
and greater collisional cross sections. These factors in turn can determine the
growth rate of the aggregates and evolution of the dust cloud. This paper gives
an overview of the numerical and experimental methods used to study dust
agglomeration at CASPER and highlights some recent results
Determination of the levitation limits of dust particles within the sheath in complex plasma experiments
Experiments are performed in which dust particles are levitated at varying
heights above the powered electrode in a RF plasma discharge by changing the
discharge power. The trajectories of particles dropped from the top of the
discharge chamber are used to reconstruct the vertical electric force acting on
the particles. The resulting data, together with the results from a
selfconsistent fluid model, are used to determine the lower levitation limit
for dust particles in the discharge and the approximate height above the lower
electrode where quasineutrality is attained, locating the sheath edge. These
results are then compared with current sheath models. It is also shown that
particles levitated within a few electron Debye lengths of the sheath edge are
located outside the linearly increasing portion of the electric field
The 1981 United States survey of cardiac pacing practices
A survey of physicians implanting pacemakers was conducted to obtain a profile of permanent cardiac pacing practices in the United States during 1981. Questionnaires were mailed to 5,832 implanters with 765 responses (13%) received and 680 analyzed. It was estimated that there were approximately 5,600 physicians, 66% surgeons and 34% nonsurgeons, implanting pacemakers at 3,670 centers. About 118,000 new primary implants were performed, or 518 per million population. Only 17% of implantation procedures in 1981 were replacements compared with 31% in 1978. Roughly half the respondents worked in teams, most implanting from 46 to 55 pacemakers annually.The chief indications for permanent pacing were sick sinus syndrome (48%) and impairment of conduction in the atrioventricular node and His-Purkinje system (42%). Ninety-five percent of pacing leads were implanted transvenously. Seventy percent of the respondents had had experience with atrial and dual-chamber pacemakers, used largely to increase cardiac output. The use of ventricular demand (VVI) pacing decreased accordingly from 91% in 1975 to 84% in 1981. Although approximately 90% of primary pacemakers were programmable to some degree, almost half were not reprogrammed within the first 3 months after implantation and 30% were never reprogrammed. Most patients (85%) were followed up by transtelephonic electrocardiographic monitoring, 68% in conjunction with private office visits. The respondents estimated that dual-chamber pacing, accounting for 10% of implants in 1981, would increase to 37% by 1985.Early electrode malfunctions were less frequent when implantation was performed by high volume and solo implanters, and in public and community hospitals. It is concluded that periodic surveys of this type disclose important trends in the practice of cardiac pacing
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