Underwater pulsed spark discharge influenced by the relative position between the top of a pin electrode and an insulating tube

Needle-to-plane geometry has been widely investigated and used in underwater pulsed discharges. The position relationship between the needle tip and insulation layer significantly affects the discharge patterns. We carried out experiments on underwater pulsed discharge with the needle tip protruding from, recessing into, and flushing with the insulating tube. The results are as follows. First, underwater pulsed discharge has a strong randomness under the experimental conditions. Different discharge patterns appeared under the same experimental environment. Second, recession into the insulator surface led to a higher probability of occurrence but a lower strength of spark discharge than protrusion. Third, between the needle tip protruding from and recessing into the insulation material, the average speed of propagation of underwater pulsed spark discharge decreased by an order of magnitude. The study shows that the optimum length of needle tip protruding from the insulation layer is 1 mm to obtain a strong underwater pulsed spark discharge.