Anharmonicity in resonant perturbations and its effect on stochasticity extension

We analyse the effect of the separatrix shape on stochasticity onset and on diffusion in the two-wave system, and in particular the role of the separatrix angle at the hyperbolic point. Introducing anharmonicity in the expression of the perturbation, we can adjust this angle, and eventually have it go to zero. We show that, in this latter case, stochasticity appears for a substantially smaller amplitude of the perturbation and that, for a given amplitude, the diffusion coefficient is strongly enhanced. Applications to specific physical problems are discussed.