The influence of tube diameter and gas gap on breakdown characteristic of capillary discharge

In this experiment the DC breakdown phenomenon of needle-needle in a quartz tube is studied in air at atmospheric pressure. The breakdown voltage of needle-needle electrodes in a narrow tube is higher than that without tube, unlike that the typical surface flashover with dielectric gets lower breakdown voltage. Besides, the smaller the tube diameter (D) is and the larger the gas gap (d) is, the more the breakdown voltage increases. A special experimental device is used to exactly observe the position of the capillary discharge channel in the microspace with two vertical views. The photographs show that on the condition of narrower tube and larger gas gap, the surface discharge propagates along the dielectric surface and it proceeds to a spark following the tube axis at a higher applied voltage for accumulated charge and the leakage current through tube wall. The characteristic of capillary discharge is mainly influenced by the value of D and d. The experiment result indicates the critical size of D and d is approximately 2.5 mm as the same order of 2 mm that is the transverse estimated size of the electron avalanche of the streamer in air at atmospheric pressure.