Ambipolar Diffusion

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.

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Ambipolar Diffusion 

or bipolar diffusion, the simultaneous transport, in an ionized medium, of charged particles of both signs (for example, electrons and positive ions), leading to a decrease of their concentration in the medium. Ambipolar diffusion can be seen very clearly in gas and liquid plasmas, which are ionized mediums where the densities of charge of both signs are equal to each other. If in such a medium the concentrations of charge carriers n⁺ and n~ change from one point to another (for example, decrease from level 1 to level 2, as shown in Figure 1), then the charge carriers (ions or electrons) diffuse from 1 to 2. The charge carriers which diffuse faster (for example, the positive ones) will at first move in great numbers from 1 to 2. As a result, level 2 will have an excess of that charge (in this case, positive) and level 1 will have an excess of the opposite charge. Therefore, an electric field will form between levels 1 and 2 (in this case, directed from 2 to 1); this will slow the movement of the more mobile carriers and speed the movement of the slower carriers. An equilibrium state will be reached when the intensity of field E is such that the transference velocities of charge carriers of both signs are equal to each other. This is what characterizes ambipolar diffusion. The flux of charge carriers v of either sign is determined by the combined effect of diffusion and the electrical field. The velocity of ambipolar diffusion is governed by a law similar to that governing the velocity of ordinary diffusion, but with a different coefficient.

Figure 1

Ambipolar diffusion takes place in liquids (electrolytes) when there is a difference in electrolyte concentrations. For example, when an acid is poured into water, the cations H⁺, which are more mobile than the acid anions, overtake the latter and produce a positive potential in regions of lower concentration. The resultant difference in potential between regions of different concentration can be measured and serves as a measure of the concentrational electromotive force which appears as a result of ambipolar diffusion.

Ambipolar diffusion can also take place in semiconductors which have free carriers of charges of both signs. Ambipolar diffusion plays an important role in gas discharge devices (phanotrons, thyratrons, and others).