Simultaneous timehyphen;resolved spectroscopic and current measurements have been made during the early stages of current growth leading to breakdown between planehyphen;parallel electrodes of dissimilar material subjected to stephyphen;function impulsive voltages. Measurements of the breakdown delay time following the application of the impulse in the range 1ndash;200 mgr;s for an electrode separation of 0.1 cm have been correlated with the optical resonance radiation, characteristic of each electrode material, emitted from the gap during the early stages of breakdown. Data have been obtained from three spatial regions, near the cathode, midgap, and near the anode. The results show that breakdown delay times le;20 mgr;s are characterized by the initial emission of cathode vapor radiation, whereas for times ge;20 mgr;s anode vapor radiation is emitted prior to cathode vapor radiation. It is concluded that a transition in the impulse breakdown mechanism occurs from one involving initiation in anode vapor to one in which initiation takes place in cathode vapor, as the magnitude of the applied voltage is increased above the dc breakdown value. For an electrode separation of 0.1 cm, this transition corresponds to a breakdown delay time sim;20 mgr;s.
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