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By means of numerical simulations, we demonstrate that ac field can supportstably moving collective nonlinear excitations in the form of dislocations(topological solitons, or kinks) in the Frenkel-Kontorova (FK) lattice withweak friction, which was qualitatively predicted by Bonilla and Malomed [Phys.Rev. B{\bf 43}, 11539 (1991)]. Direct generation of the moving dislocationsturns out to be virtually impossible; however, they can be generated initiallyin the lattice subject to an auxiliary spatial modulation of the on-sitepotential strength. Gradually relaxing the modulation, we are able to get thestable moving dislocations in the uniform FK lattice with the periodic boundaryconditions, provided that the driving frequency is close to the gap frequencyof the linear excitations in the uniform lattice. The excitations have a largeand noninteger index of commensurability with the lattice (suggesting that itsactual value is irrational). The simulations reveal two different types of themoving dislocations: broad ones, that extend, roughly, to half the full lengthof the periodic lattice (in that sense, they cannot be called solitons), andlocalized soliton-like dislocations, that can be found in an excited state,demonstrating strong persistent internal vibrations. The minimum (threshold)amplitude of the driving force necessary to support the traveling excitation isfound as a function of the friction coefficient. Its extrapolation suggeststhat the threshold does not vanish at the zero friction, which may be explainedby radiation losses. The moving dislocation can be observed experimentally inan array of coupled small Josephson junctions in the form of an {\it inverseJosephson effect}, i.e., a dc-voltage response to the uniformly applied ac biascurrent.Comment: Plain Latex, 13 pages + 9 PostScript figures. to appear on Journal of  Physics: condensed matte

The alternating-current-driven motion of dislocations in a weakly damped Frenkel - Kontorova lattice