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Breakdown voltage and prebreakdown current between parallel plane copper electrodes in a vacuum were measured in the gap range from 0.002 to 0.2 mm using an impulse voltage of 550Ã5000 Â¿s. Breakdown field strength, effective field enhancement factor, and field emission current just prior to breakdown were ascertained as a function of gap spacing. With a decrease in the gap spacing from 0.2 mm, the field emission current just prior to breakdown increased sharply and at about 0.04 mm, it was 10 or 20 times larger than that when the gapspacing is 0.2 mm. However, in the gap range from 0.04 mm to 0.002 mm, the field emission current was maintained at about a constant value. The gap spacing dependence of the field emission current could be explained by the introduction of two ideas, which are a space charge effect by emitted electrons, and a change in the breakdown mechanism.