Initial stages of pulsed discharge in saline solutions in a vicinity of threshold voltages

Data are presented on an investigation of pulsed discharges in a solution of 30 g of NaCl per liter of distilled water. The interelectrode gap consists of the active pin electrode and the return plane electrode. The voltage pulse of positive polarity forms by means of an electric circuit where a capacitor bank of 40 mu F with an initial voltage V (0) of up to 1500 V is discharged via the gap. In a range of initial voltages V (0) = (550 - 1100) V vapor cavities in a form of the microbubbles are generated near the active electrode and at the electrode surface. The microbubbles are able to merge with each other thus forming macrobubbles and vapor layers, which shield the active electrode from current passage. The process of vapor cavity generation and the effect of shielding are discussed in detail. The discharge at the vapor layer appears at the critical voltage V (0) = 1100 V. The principal prerequisite for discharge arising is that the electrode has to be completely shielded by the vapor layer and the voltage at the layer at the instant of discharge initiation has to be at least at a level of 500 V. However, when the voltage at the gap exceeds the critical value, the condition of complete shielding is not obligatory.