ambipolar diffusion paxš-e ubâqotbi Fr.: diffusion ambipolaire A physical process which allows a → molecular cloud to decouple from → interstellar magnetic field in order to undergo → gravitational collapse. A cloud of pure molecular gas would form stars very fast through collapse since neutral matter does not respond to the magnetic field. However, the magnetic field holds up a collapse because the ions present in the cloud collide with the neutrals and tie them to the field. The collapse can then only proceed if the magnetic field can be separated from the gas. In denser molecular cores the ionization degree decreases substantially and therefore neutrals and ions decouple. |
diffusion paxš (#) Fr.: diffusion 1) Movement of a gas or liquid as a result of the random thermal motion of its atoms or
molecules. L. diffusionem, from stem of diffundere "scatter, pour out," from dif- "apart, in every direction," → dis-, + fundere "to melt, cast, pour out," from PIE *gheud-, from root *gheu- "to pour." Paxš, verbal noun and stem of paxšidan→ diffuse. |
diffusion coefficient hamgar-e paxš Fr.: coefficient de diffusion A factor of proportionality involved in the → diffusion equation. It may be defined as the amount of the quantity diffusing across a unit area through a unit concentration gradient in unit time. → magnetic diffusivity. → diffusion; → coefficient. |
diffusion equation hamugeš-e paxš Fr.: équation de diffusion An equation that expresses the time rate of change of a quantity in terms of the product of the diffusion coefficient and the → Laplacian operating on the quantity. For example the diffusion equation for temperature is: ∂T/∂t = D∇^{2}T. |
diffusion region nâhiye-ye paxš Fr.: région de diffusion A narrow boundary layer above the solar → photosphere, between two magnetic field lines, where the plasma becomes demagnetized or unfrozen. The presence of a localized magnetic region is necessary for → magnetic reconnection. |
gaseous diffusion paxš-e gâzi Fr.: diffusion gazeuse An → isotope separation process using the different diffusion speeds of → atoms or → molecules for separation. This process is used to divide → uranium hexafluoride (UF_{6}) into two separate streams of U-235 and U-238. Before processing by gaseous diffusion, uranium is first converted from → uranium oxide (U_{3}O_{8}) to UF_{6}. The UF_{6} is heated and converted from a solid to a gas. The gas is then forced through a series of compressors and converters that contain porous barriers. Because uranium-235 has a slightly lighter isotopic mass than uranium-238, UF_{6} molecules made with uranium-235 diffuse through the barriers at a slightly higher rate than the molecules containing uranium-238. At the end of the process, there are two UF_{6} streams, with one stream having a higher concentration of uranium-235 than the other (EVS, a Division of Argonne National Laboratory). |
magnetic diffusion paxš-e meqnâtisi Fr.: diffusion magnétique The process whereby the magnetic field tends to diffuse across the plasma and to smooth out any local inhomogeneities under the influence of a finite resistance in the plasma. For a stationary plasma the → induction equation becomes a pure → diffusion equation: ∂B/∂t = D_{m}∇^{2}B, where D_{m} = (μ_{0}σ_{0})^{-1} is the → magnetic diffusivity. |
radiative diffusion paxš-e tâbeši Fr.: diffusion radiative A process of → radiative transfer in which photons are repeatedly absorbed and re-emitted by matter particles. |