Ambipolar diffusion
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ambipolar diffusion
(¦am·bē′pōl·ər dif′yü·zhən)
(physics) The diffusion in a plasma of charged particles, such as electrons or ions, as a result of the almost exact local charge neutrality required.
Wiley Book of Astronomy:
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ambipolar diffusion
An important process in the initial stage of star formation whereby clumps of a molecular cloud uncouple from the interstellar magnetic field, which would otherwise resist the further gravitational collapse of the clumps (see stellar evolution). Magnetic fields thread all interstellar space and act upon ions (mostly protons) and electrons. These charged particles, in turn, collide with neutral atoms and thereby, in most cases, couple them to the field. However, in a molecular cloud, where the fractional ionization is very low (one part per million or less), neutral particles only rarely encounter charged particles, and so are not well coupled. Thus, the interstellar magnetic field can drag the ions through the neutral gas without acting as a significant brake on the cloud's collapse.
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Ambipolar diffusion
Ambipolar diffusion is diffusion of positive and negative particles in a plasma at the same rate due to their interaction via the electric field. It is closely related to the concept of quasineutrality.
In most plasmas, the forces acting on the ions are different from those acting on the electrons, so naively one would expect one species to be transported faster than the other, whether by diffusion or convection or some other process. If such differential transport has a divergence, then it will result in a change of the charge density, which will in return create an electric field that will alter the transport of one or both species in such a way that they become equal.
The simplest example is a plasma localized in an unmagnetized vacuum. (See Inertial confinement fusion.) Both electrons and ions will stream outward with their respective thermal velocity. If the ions are relatively cold, their thermal velocity will be small. The thermal velocity of the electrons will be fast due to their high temperature and low mass: 
. As the electrons leave the initial volume, they will leave behind a positive charge density of ions, which will result in an outwardly-directed electric field. This field will act on the electrons to slow them down and on the ions to speed them up. The net result is that both ions and electrons stream outward at the speed of sound, 
, which is much smaller than the electron thermal velocity, but usually much larger than the ion thermal velocity.
In astrophysics, "ambipolar diffusion" refers specifically to the decoupling of neutral particles from plasma in the initial stage of star formation. The neutral particles in this case are mostly hydrogen molecules in a cloud that would undergo gravitational collapse if it were not collisionally coupled to the plasma. The plasma is composed of ions (mostly protons) and electrons, which are tied to the interstellar magnetic field and therefore resist collapse. In a molecular cloud where the fractional ionization is very low (one part per million or less), neutral particles only rarely encounter charged particles, and so are not hindered in their collapse into a star.
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