  We study the thermal depinning of single fluxons in rings made of Josephson
junctions. Due to thermal fluctuations a fluxon can be excited from its energy
minima and move through the array, causing a voltage across each junction. We
find that for the initial depinning, the fluxon behaves as a single particle
and follows a Kramers-type escape law. However, under some conditions this
single particle description breaks down. At low values of the discreteness
parameter and low values of the damping, the depinning rate is larger than the
single particle result would suggest. In addition, for some values of the
parameters the fluxon can undergo low-voltage diffusion before switching to the
high-voltage whirling mode. This type of diffusion is similar to phase
diffusion in a single junction, but occurs without frequency-dependent damping.
We study the switching to the whirling state as well.
