  When a chaotic, ergodic Hamiltonian system with $N$ degrees of freedom is
subject to sufficiently rapid periodic driving, its energy evolves diffusively.
We derive a Fokker-Planck equation that governs the evolution of the system's
probability distribution in energy space, and we provide explicit expressions
for the energy drift and diffusion rates. Our analysis suggests that the system
generically relaxes to a long-lived "prethermal" state characterized by minimal
energy absorption, eventually followed by more rapid heating. When $N\gg 1$,
the system ultimately absorbs energy indefinitely from the drive, or at least
until an infinite temperature state is reached.
