Onset of stochasticity and the diffusion approximation in drift waves

The Hamiltonian structure of the E*B drift equations is exploited to describe the onset of stochasticity for test particles in drift waves. In contrast to a longitudinal plasma wave, a drift wave acts over the entire single particle phase space. This feature precludes a simple use of the techniques presently available for predicting the onset of chaos in Hamiltonian systems. For two drift waves a generalized Chirikov (1979) overlap criterion is derived. The present work gives conditions on the drift wave spectrum for global stochasticity and the validity of the diffusion approximation.