A model of local metastable states due to the pinning induces plastic
deformations allows to describe the nonlinear I-V curves in sliding density
waves -DW. With increasing the DW velocity v, the metastable states of
decreasing lifetimes ~1/v are accessed. The characteristic second threshold
field is reached when configurations of shortest life time are accessed by the
fast moving DW. Thus the DW works as a kind of a ``linear accelerator'' testing
virtual states.Comment: To be published in Proceedings of ECRYS-99, J. de Physique, Coll.,
December 1999, http://ipnweb.in2p3.fr/~lptms/membres/brazov