Charged particles interact according to the Coulomb force law, according to which the interaction strength diminishes slowly with distance. Charged particles, moving in a plasma, constantly interact with other charged particles via this long-range force. For this reason the path of a particle is not a broken one (with discontinuous derivatives) as is the case with neutral particles, but a smooth curve (Fig. 43). However, a detailed analysis shows that thermal motion and transport processes can still be described in terms of a collision theory by introducing the concept of Coulomb collisions. This means that the actual curved path of the particles can be replaced by a series of broken lines (the dashed line in Fig. 43). Each time the direction of a particle changes by approximately 90° as continuous interactions we say that the particle has experienced a Coulomb collision; this corresponds to a change in direction in the dashed line. In order to describe thermal motion in a plasma quantitatively by a collision theory, we must determine an effective cross section for Coulomb collisions. We will call this the Coulomb cross section. It consists of two factors; these take account of the near and remote interactions.
KeywordsImpact Parameter Thermal Motion Effective Cross Section Coulomb Collision Collision Theory
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