Ambipolar diffusion
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Ambipolar diffusion is diffusion of positive and negative particles with opposite electrical charge (such as electrons and positive ions) due to their interaction via an electric field.
Ambipolar diffusion is diffusion of positive and negative particles with opposite electrical charge (such as electrons and positive ions) due to their interaction via an electric field. 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, resulting 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 \approx \sqrt{k_BT_e/m_i} , which is much smaller than the electron thermal velocity, but usually much larger than the ion thermal velocity.
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