Effect of Intense Illumination on Time Lag in Static Spark Breakdown

The time lag of spark breakdown in static uniform fields and with intense illumination of the cathode has been studied in air, helium and carbon dioxide. An electro-optical shutter was used to observe the time lag, while an auxiliary spark gap supplied the intense cathode illumination. In air, the time lag was about 10^-7 second for overvoltages of a few percent and increased very rapidly with decreasing overvoltage. Increasing the overvoltage to above 30 or 40 percent reduced the time lag to a more or less constant value of 2 or 3×{}10^-8 second. This, in part, is shown to be due to the nature of the initiatory spark. The position of the midgap streamer observed in previous experiments has been found to depend on both the overvoltage and intensity of illumination. An explanation of these observations is advanced on the basis of space charge effects. The results in carbon dioxide were quite similar to those in air. In helium a much higher overvoltage was found necessary to produce a given time lag than in air. This is explained by considerations of the relative rates of gain of energy by electrons in the two gases.