Space Charge Fields in the Development of a Townsend Discharge in Nitrogen

As the CUlTent through a spark rises into the milliampere range, ionization due to fields produced by space charge plays an increasingly dominant role. Infor� mation on these fields can be obtained from analysis of the light intensity emitted at different points along the axis of the discharge. In an earlier publication (Doran 1968) photomultiplier records of the light output obtained at various times during the growth of a Townsend discharge in nitrogen (pd = 600 torr cm) were analysed by an approximate method. In the present paper a more exact relation between the light emitted and the variation in local electric field is derived, enabling the previous results to be re.analysed. This has provided a quantitative picture, spatially and temporally resolved, of the development of these fields, which are associated with luminous fronts observed to propagate back and forth across the discharge gap. The magnitude of the field variations is in general about 10-20% of the applied electric field.