Exponential Fermi acceleration in general time-dependent billiards.

We show, that under very general conditions, a generic time-dependent billiard, for which a phase space of corresponding static (frozen) billiards is of the mixed type, exhibits the exponential Fermi acceleration in the adiabatic limit. The velocity dynamics in the adiabatic regime is represented as an integral over a path through the abstract space of invariant components of corresponding static billiards, where the paths are generated probabilistically in terms of transition-probability matrices. We study the statistical properties of possible paths and deduce the conditions for the exponential Fermi acceleration. The exponential Fermi acceleration and theoretical concepts presented in the paper are demonstrated numerically in four different time-dependent billiards.