Frequency Sweeping of Phase Space Structures

Holes and clumps, spontaneously formed in the nonlinear development of a single mode driven by a weakly destabilizing kinetic distribution function in a dissipative medium, were expected to persist for an appropriate collisional time scale. However, Fokker-Planck calculations for the nonlinear system abruptly break down in the course of the calculation for some initial conditions. We nd that this is because an adiabatic description of phase space structures at zero collisionality does not necessarily lead to continual adiabatic frequency sweeping. Criteria are found for a class of initial distribution functions that determine whether adiabatic frequency sweeping continues inde nitely, or suddenly terminates. Passing particle contribution is found to be unimportant in the dynamics of the phase space structures in the framework of adiabatic description. An unresolved bifurcation problem is also observed under certain conditions. Typeset using REVTEX 1