  Directed motion and depinning of topological solitons in a strongly discrete
damped and biharmonically ac-driven array of Josephson junctions is studied.
The mechanism of the depinning transition is investigated in detail. We show
that the depinning process takes place through chaotization of an initially
standing fluxon periodic orbit. Detailed investigation of the Floquet
multipliers of these orbits shows that depending on the depinning parameters
(either the driving amplitude or the phase shift between harmonics) the
chaotization process can take place either along the period-doubling scenario
or due to the type-I intermittency.
