Ambipolar diffusion


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: v_e approx sqrt{k_BT_e/m_e}. 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 = sqrt{k_BT_e/m_i}, 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.

For a mathematical analysis, see [http://www.timedomaincvd.com/CVD_Fundamentals/plasmas/ambipolar_diffusion.html Ambipolar Diffusion] .


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  • diffusion ambipolaire — dvipolė difuzija statusas T sritis fizika atitikmenys: angl. ambipolar diffusion; bipolar diffusion vok. ambipolare Diffusion, f rus. амбиполярная диффузия, f; биполярная диффузия, f; двуполярная диффузия, f pranc. diffusion ambipolaire, f …   Fizikos terminų žodynas

  • ambipolar — ˌ adjective Etymology: ambi + polar : relating to or consisting of both electrons and positive ions moving in opposite directions ambipolar diffusion …   Useful english dictionary

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  • ambipolare Diffusion — dvipolė difuzija statusas T sritis fizika atitikmenys: angl. ambipolar diffusion; bipolar diffusion vok. ambipolare Diffusion, f rus. амбиполярная диффузия, f; биполярная диффузия, f; двуполярная диффузия, f pranc. diffusion ambipolaire, f …   Fizikos terminų žodynas

  • bipolar diffusion — dvipolė difuzija statusas T sritis fizika atitikmenys: angl. ambipolar diffusion; bipolar diffusion vok. ambipolare Diffusion, f rus. амбиполярная диффузия, f; биполярная диффузия, f; двуполярная диффузия, f pranc. diffusion ambipolaire, f …   Fizikos terminų žodynas

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