Charged particle propagation in strong disordered magnetic fields

The use of quasilinear theory in calculating particle trajectories in strong random fields is beset by a basic difficulty concerning the breakdown of the basic assumption of the theory. This assumption is that the random force acting on the particle becomes self-incoherent before it has changed the particle's trajectory in phase space, ie., its position or velocity, by a significant amount. The various time scales involved in this problem are defined and compared to determine in which regions the basic assumptions of quasilinear theory are violated. It is shown how nonlinear theories in general and the Partially Averaged Field theory (Jones et al. 1973) can partly alleviate these difficulties.