Detection of Electrode Vapor Between Plane Parallel Copper Electrodes Prior to Current Amplification and Breakdown in Vacuum

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Abstract
Simultaneous, time‐resolved measurements of current growth and resonance line absorption have been made for the time intervals 0.2 and 0.8 μsec, respectively, preceding breakdown between plane‐parallel copper electrodes in vacuum. These measurements have been carried out for an electrode separation of 0.1 cm and residual pressures in the low 10−9‐Torr range. The experimental data show that neutral copper vapor is present in the interelectrode volume before the current increases sufficiently to produce breakdown. It is found that the vapor is generated during times of the order of some microseconds prior to breakdown, and is highly localized to the region of the subsequent spark channel. Further, the vapor density decreases from cathode to anode along the path of the subsequent spark channel. The results are consistent with a model for vacuum breakdown proposed recently in which the transient production of vapor immediately prior to breakdown occurs by the evaporation of an anode macroparticle during its transit to the cathode; amplification of the prebreakdown current in this vapor then leads to breakdown. From the data obtained in the present experiment, it appears that breakdown occurs primarily while the macroparticle is located between mid‐gap and the cathode.