The Effect of Oxygen on the Delay Time of Flashover by a Laser-Produced Plasma

The delay time of discharge triggered by a laser-produced plasma is investigated in a mixture of oxygen and nitrogen. When oxygen exists in the mixture, the delay time of discharge has two styles of “late discharge” and “early discharge”. The delay time of “late discharge” agrees with the transit time of oxygen negative ions between electrodes. The delay time of “early discharge” approaches the transit time of electrons between electrodes as the applied voltage between the electrodes increases. There exists a threshold voltage at which the delay time leaps from “late discharge” to “early discharge”. Calculation of the electron detachment cross section reveals that the leap of the delay time is caused by changing of negative particle triggering discharge from oxygen negative ions to electrons due to an increase in electron detachment as the applied voltage becomes higher.