  We investigate the production of axion quanta during the early universe
evolution of an axion-like field rolling down a wiggly potential. We compute
the growth of quantum fluctuations and their back-reaction on the homogeneous
zero-mode. We evaluate the transfer of kinetic energy from the zero mode to the
quantum fluctuations and the conditions to decelerate the axion zero-mode as a
function of the Hubble rate, the slope of the potential, the size of the
barriers and the initial field velocity. We discuss how these effects impact
the relaxion mechanism.
