  Fermi acceleration is the process of energy transfer from massive objects in
slow motion to light objects that move fast. The model for such process is a
time-dependent Hamiltonian system. As the parameters of the system change with
time, the energy is no longer conserved, which makes the acceleration possible.
One of the main problems is how to generate a sustained and robust energy
growth. We show that the non-ergodicity of any chaotic Hamiltonian system must
universally lead to the exponential growth of energy at a slow periodic
variation of parameters. We build a model for this process in terms of a
Geometric Brownian Motion with a positive drift, and relate it to the entropy
increase.
