We have developed an empirical electrical-breakdown relation that can be used to design large-area water-insulated pulsed-power systems. Such systems often form an integral part of multiterawatt pulsedpower accelerators, and may be incorporated in future petawatt-class machines. We find that complete dielectric failure is likely to occur in water between a significantly field-enhanced anode and a lessenhanced cathode when E p 0:3300:026 eff 0:135 0:009. In this expression E p V p =d is the peak

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... value in time of the spatially averaged electric field between the anode and cathode (in MV=cm), V p is the peak voltage across the electrodes, d is the distance between the anode and cathode, and eff is the temporal width (in s) of the voltage pulse at 63% of peak. This relation is based on 25 measurements for which 1 V p 4:10 MV, 1:25 d 22 cm, and 0:011 eff 0:6 s. The normalized standard deviation of the differences between these measurements and the associated predictions of the relation is 12%.