Positive-Point-to-Plane Discharge in Air at Atmospheric Pressure

Abstract

Investigations have been carried out on current-voltage characteristics of positive-point-to-plane discharge. All currents start abruptly and increase at first linearly and then more steeply with voltage, until finally breakdown into a spark occurs. The corona process in air is shown to be a discontinuous one, involving the alternate propagation and space charge quenching of positive streamers out from the point. Photoelectric ionization in the gas is shown to account for the rapid self-propagation of streamers. Attachment of electrons to form negative ions and subsequent detachment of these electrons in the high field near the point provides means of starting new streamers for self-maintaining corona process. Fields at onset are calculated by the use of confocal paraboloids as electrodes, and it is shown that the fields necessary for such corona process are available. Introduction of a radioactive source into the gap indicates that the sharp cut-off of the corona current as the voltage is decreased is due to lack of electrons for initiating new streamers. Offset voltage marks the point at which the self-perpetuating ionization process begins. The initial current rise is found to increase as the gap distance is decreased and this is shown to be caused by the change in field distribution and consequent further extension of streamers into the gap. Below a certain minimum gap distance, depending on the point geometry, the initial streamers propagate completely across the gap and this introduction of the highly efficient ionization at the cathode causes spark breakdown. The initial current-voltage slopes are expressed as a resistance which is found to increase linearly with the gap and to account for approximately 1 percent of the total voltage drop in the gap. Extrapolation of resistance to zero resistance gives the minimum gap at which stable corona will flow. Above the initial current rise, the current increases as the square of the difference between the applied and offset voltage. Evidence indicates that this is caused by a superposition upon the otherwise linear rise in current with voltage, of an increase proportional to the current itself. Visual observation near breakdown indicates that this increase proportional to the current is associated with the utilization by later streamers of the residual paths of their predecessors. This becomes more pronounced as the current and voltage increase; the effect of earlier streamers in this way increases the current in later streamers. Spark breakdown occurs when the amalgamated streamers succeed in crossing to the plate.