Statistical time lags have been measured in a discharge tube containing SF6at a pressure of 1 Torr, with the aim of determining the source of the electrons which initiate breakdownYlsquo;Te,hf cx pressure SF6. Measurements with and without careful shielding of the discharge tube give similar values of mean statistical time lag showing that the initiating electrons are not produced by external radiation. Measurements, in which the time between the 500 successive breakdowns used for a single measurement of mean statistical time lag is increased over the range 1ndash;103s, provide values of mean statistical time lag which increase over the range 10minus;5ndash;10minus;2s: This reveals that the breakdown process is controlled by the time since the discharge produced by a previous breakdown. Data from further such measurements, made for different applied tube voltages, are shown to fit well with the Fowlerndash;Nordheim theory of field emission. The conclusion drawn is that the initiating electrons are produced at the cathode surface by field emission, and that the state of the surface at any time depends primarily on the intensity of the previous discharge and the length of time since it took place. During the time between successive breakdowns, a surface layer on the cathode surface is continually developing and reducing the rate of field emission from the cathode. Although the applied electric fields are well below that required for field emission, microscopic fields exist at protrusions and craters on the cathode surface which are high enough to provide field emission: This is supported by experiments on electron emission made with the discharge tube evacuated below 10minus;6Torr.
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