Theory of cumulative small-angle collisions in plasmas

A succession of small-angle binary collisions can be grouped into a unique binary collision with a large scattering angle. The latter is called a cumulative collision. This makes it possible to treat the cumulative collision like a collision between neutral molecules. A significant feature of the cumulative collision is that the probability density function for a deflection angle depends on the time spent by a charged particle while engaged in the cumulative collision. Here a simple analytic expression for the function is proposed which is easy to use together with the Monte Carlo method. The validity of the present theory is ascertained by calculating various relaxation phenomena in plasmas. The theory is best suited to particle simulation of plasmas.