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The effect of current prepulse on the initiation of Al wire arrays, ablation dynamics and x-ray production was investigated on the 1-MA ZEBRA accelerator (University of Nevada, Reno). It is shown that increasing the number of wires lowers the temperature of the wire cores at the time of breakdown. Al arrays with cold wire cores demonstrate long and inhomogeneous ablation, and a less intense, wider x-ray pulse. Shortening the current prepulse by a flashover switch causes an increased wire-core temperature, symmetrization and synchronization of the wires’ ablation, and improvement of the amplitude and shape of the x-ray pulse. Application of a vacuum flashover switch can be important for shortening the current prepulse on the upcoming 28-MA ZR-accelerator at Sandia National Laboratories to optimize the x-ray production and shot-to-shot reproducibility from wire-array Z pinches. [ABSTRACT FROM AUTHOR]