Observations of a vacuum spark under different driver conditions of the applied voltage

Comparative observations of the Vacuum Spark discharge are presented under differing electrical drive conditions but identical geometrical conditions. A 1.5 Ω 120 nsec, coaxial line is used to provide maximum currents to 90 kA. The line may be operated in the usual switched mode to give a rectangular voltage and current wave form, or in the Hybrid mode, in which case the voltage builds slowly until breakdown occurs. In both modes of operation the value of dI/dt is about 1×1012 A/sec. A Nd:YAG is focused onto either electrode to initiate the discharge. With the laser focused onto the anode a low‐density plasma accompanied by intense electron beams is formed. With the laser focused on the cathode a much higher plasma density is observed in the gap, with the formation of a dense tight pinch close to the cathode, in which hot spots form at peak current. The Hybrid mode is favored for its reproducible behavior.