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Ambipolar diffusion

ambipolar diffusion, ambipolar diffusion coefficient
Ambipolar diffusion is diffusion of positive and negative species with opposite electrical charge due to their interaction via an electric field In the case of ionic crystals, the fluxes of the diffusing species are coupled, while in a plasma the various species diffuse at the same rate

Diffusion in plasmas

In plasma physics, ambipolar diffusion 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 results in a change of the charge density The latter will in turn create an electric field that can 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: v e ≈ k B T e / m e \approx T_/m_}}} 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, c s ≈ k B T e / m i \approx T_/m_}}} , 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, for example 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 they 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 entirely hindered in their collapse note that now is dynamical collapse, not free fall into a star


  1. ^ Kizilyalli, M; Corish, J; Metselaar, R 1999 "Definitions of terms for diffusion in the solid state IUPAC Recommendations 1999" PDF Pure Appl Chem 71 7: 1307–1325 doi:101351/pac199971071307 

Further reading

  • Mathematical analysis of ambipolar diffusion - dead link

ambipolar diffusion, ambipolar diffusion coefficient, ambipolar diffusion plasma, ambipolar diffusion star formation, video lecture on ambipolar diffusion youtube

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