  We present switching current measurements in niobium-aluminum oxide-niobium
underdamped Josephson rings. Underdamped Josephson rings display hysteretic
current-voltage curves which can be characterized by their switching current,
the value of applied current at which the junctions switch to the energy-gap
voltage. The value of the switching current is strongly affected by temperature
and by the presence of fluxons in the system. We observe a very small voltage
(0.2 nV) across the ring prior to switching, indicating a low-velocity fluxon
diffusion state before the jump to a full running state. In analogy with
previous work on single junctions, we analyze the switching current data with
the process of thermal activation over a dissipation barrier, where the system
switches from a low velocity state to a high velocity state. We find that our
data agrees qualitatively with this description, further supporting the
observation of fluxon diffusion.
