The effect of initiatory electrons on breakdown characteristics in highly nonuniform gap

Previous work has shown that the positive impulse breakdown voltage of SF₆, N₂ and SF₆/N₂ mixture in a point-plane gap is affected by the presence of space charge. With negative DC bias voltage on a corona needle arrangement external to the test gap, the impulse breakdown strength decreases significantly. This phenomenon has little to do with the electronegative properties of gases, since the same result is found in both electron-attaching and nonattaching gases. The reason was supposed to be the abundance of initiatory electrons resulting from detachment of plentiful negative ions injected. The aim of the present work is to confirm the above explanation. Experiments were carried out in air, SF₆ and an SF₆/N₂ mixture both with and without irradiation. The results show that with either artificial irradiation or a negative DC bias voltage, the impulse breakdown strength decreases. The present experimental results confirmed to a large extend that the breakdown strength of a nonuniform field gap is greatly affected by the availability of initiatory electrons within the gap volume