Collisions in a Plasma of Finite-Size Particles

Collisions in one-, two-, and three-dimensional plasmas of finite-size particles, clouds, are studied from two different points of view. The first approach starts from the dynamics of binary collisions and proceeds to obtain the interaction force between two clouds including the effect of static shielding which is then used to obtain the scattering cross section. The calculation shows that cloud cross sections are reduced by orders of magnitude compared with those of point particles as the cloud radius is made comparable to the Debye length. The second approach makes use of the kinetic equations of Balescu-Lenard and Landau from which the Fokker-Planck coefficients for a test cloud in an equilibrium plasma are calculated. It is shown that the Fokker-Planck coefficients for clouds also become considerably smaller than those for point particles when the cloud radius becomes comparable to the Debye length; in addition, the velocity dependence of the coefficients for thick clouds are different from those for point particles. As an application, it is shown that the high collision frequencies encountered in computer plasma simulation (ND too small by normal plasma criteria) can be greatly reduced by use of clouds of radius comparable to the Debye length.