pact pulsed power and switching systems. An insulating fluid such as transformer oil is a critical component in With a fast coaxial setup using a needle/plane geometry high voltage and pulsed power systems. The desire for and a high sensitivity electrometer, conduction mecha-compact pulsed power systems drives research to develop nisms in transformer oil at varying temperature and hy-a model that is capable of describing the basic processes drostatic pressure are quantified. There are 3 stages in the of liquid breakdown, similar to the promotion of the conduction process prior to breakdown for highly non-model for gaseous breakdown almost a century ago. A uniform field geometries. Stage I is characterized by a descriptive model of conduction currents leading to diresistive current at low fields. Stage II consists of a rapid electric breakdown will aid the development of more effirise in the injection current associated with increasing cient switches and improved insulating systems. field due to a "tunneling" mechanism through the This paper discusses experiments on the basic conducmetal/dielectric interface. The transition from the resistive tion mechanisms associated with liquid dielectrics for to tunneling stage occurs when the applied field reduces non-uniform and uniform applied fields. Past studies the barrier at the metal/insulator interface to a point where have done similar tests but at much smaller gaps or only tunneling of charge carriers through the barrier begins. for a single geometry [1,2]. The variation of electrode This transition point is polarity dependent. In stage III, at material tests the conduction current as a result of varying high fields the current reaches space charge saturation at metal/liquid interfaces. Also, changes to the pressure clarelectron mobilities >100 cm2/V*s prior to breakdown. ify these mechanisms. The data leads to the explanation of The processes of final breakdown show distinct polarity the conduction current and how it evolves with applied dependence. Data for the negative needle exhibits strong voltage. Also, experiments on the investigation of the pressure dependence of the breakdown voltage, which is basic breakdown mechanisms as a function of reduced reduced by 5000 if the hydrostatic pressure is lowered hydrostatic pressure are performed. Optical diagnostics from atmospheric pressure to hundreds of mtorr. Such a combined with fast electrical measurements strongly supstrong pressure dependence, at reduced hydrostatic pres-port a bubble or vapor model for cathode initiated breaksure, indicates breakdown is gaseous in nature. This is down. Data presented reveals that cathode initiated breaksupported by images of bubble/low density regions form-down is strongly pressure dependent, while anode initiing at the current injection point. Positive needle dis-ated breakdown is less dependent upon the pressure. charges show a reduction of only about 10% in breakdown voltage for the reduced pressure case. A weak pres-II. EXPERIMENTAL SETUP sure dependence indicates the break...
