Breakdown of large vacuum gaps under lightning impulse stress

The breakdown of centimeter gaps in high vacuum is investigated using lightning impulse voltage 1.2/50 mu s. Prebreakdown currents were measured as a function of gap spacing for different electrode materials. The prebreakdown current delay time is in the range of 10-60 mu s. Two different breakdown mechanisms characterized by the breakdown delay time were observed. Short-delay breakdown is caused by explosive electron emission; long-delay breakdown is apparently initiated by an anode response and microparticle-induced effects. The range of prebreakdown current peak values depends significantly on electrode material and increase in the order aluminum, stainless steel, and copper. The electric strength increases in the order copper, stainless steel, and aluminum, and indicates a relationship between crest value of prebreakdown current and electric strength. The prebreakdown current delay time depends statistically on gap spacing and prebreakdown current crest value. The influence of AC breakdown and low pressure argon conditioning on electric strength was measured as a function of gap spacing.