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A laser-shadow technique of high time resolution was applied to study the erosion of high-current Cu vacuum arcs in situ. Cathodic processes lead to emission of high-velocity droplets shortly before and after current-zero. Increasing movements of the anodic melt produce large droplets several milliseconds after the arc. The many particles generated are responsible for the slow decay of vapor measured by laser-induced fluorescence (LIF). Densities greater than 1012 cm-3 were obtained near current-zero for the diffuse mode. Because of the optical thickness of the vapor to resonance radiation, radiative transfer had to be considered.