41 research outputs found
Space-charge sheath with ions accelerated into the plasma
International audienceThe conventional model of near-cathode space-charge sheath with ions entering the sheath from the quasi-neutral plasma may be not applicable to discharges burning in cathode vapor, e.g., vacuum arcs, where ionization of emitted atoms may occur inside the sheath with some of the produced ions returning to the cathode and others moving into the plasma. In this connection, a simple model is considered of a sheath formed by electrons and positive ions injected into the sheath with a very low velocity and moving from the sheath into the plasma. It is shown that such sheath is possible provided that the sheath voltage is equal to or exceeds approximately 1.256kT e /e. This limitation is due to the space charge in the sheath and is in this sense analogous to the limitation of ion current in a vacuum diode expressed by the Child-Langmuir law. The ions leave a sheath and enter the plasma with a velocity equal to or exceeding approximately 1.585u B
Nanopowder management and control of plasma parameters in electronegative SiH4 plasmas
Management of nanosize powder particles via control of plasma parameters in a low-pressure SiH4
discharge for silicon microfabrication technologies is considered. The spatial profiles of electron and
positive/negative ion number densities, electron temperature, and charge of the fine particles are
obtained using a self-consistent fluid model of the electronegative plasmas in the parallel plate
reactor geometry. The model accounts for variable powder size and number density, powder-charge
distribution, local plasma nonuniformity, as well as UV photodetachment of electrons from the
nanoparticles. The relations between the equilibrium discharge state and powder properties and the
input power and neutral gas pressure are studied. Methods for controlling the electron temperature
and SiH3- anion (here assumed to be the powder precursor) density, and hence the powder growth
process, are proposed. It is shown that by controlling the neutral gas pressure, input power, and
powder size and density, plasma density profiles with high levels of uniformity can be achieved.
Management of powder charge distribution is also possible through control of the external
parameters
L'épizootie asiatique de la grippe aviaire et la menace de pandémie humaine
LILLE2-BU Santé-Recherche (593502101) / SudocSudocFranceF