Abstract
We study the onset of motion of a vortex lattice in response to a driving current that is turned on suddenly. For sufficiently high currents we find that the average vortex velocity (obtained from the longitudinal voltage drop) grows smoothly with time and obeys a stretched exponential with an exponent $\beta \sim 0.7$. The response is governed by two time scales: a delay time (t0) before the appearance of a measurable voltage drop, and a rise time ($\tau $) characterizing the evolution of voltage response. Both time scales increase with decreasing saturation voltage V0 : $\tau $ decreases exponentially, while t0 as a power law. When the saturation velocity is too low the response slows down significantly and shows a step structure indicating plastic response.