The dynamics of a vacuum-arc operated rail-gun plasma were numerically simulated for the undercritically damped regime and confirmed in experiments. Analytical solutions of arc current, its velocity, and position also showed good agreement with the results. The spatial development of the arc is shown here to follow a linear behavior, even though the propulsion velocity followed an exponential-step function due to the sinusoidal nature of the arc current. Peak arc current∼100 kA,15 &mgr;s period was propelled to∼106 cm/sas indicated in time-of-flight measurements and simulations. The voltage signal induced on aB-dot probe by the dynamic sinusoidal arc was simulated, which compared well with the measured probe signal. A Gaussian current distribution inside the measured arc length∼1–2 cm,was established by unfolding theB-dot probe signal. A peak magnetic field of the order 1–5 kG was also interpreted inside the arc. The emerging plasma contained ion current over tens of amperes in the pulse duration encompassing the arc envelope. ©1997 American Institute of Physics.