THE PHYSICAL-MECHANISMS LEADING TO ELECTRICAL BREAKDOWN IN UNDERWATER ARC SOUND SOURCES

Underwater electrical arcs have long been used as an impulsive sound source, although efforts to improve efficiency and control acoustic output have been limited by inadequate understanding of the arc initiation process. An experimental study of electrical breakdown in seawater is presented and a model for arc initiation is proposed. It is shown that breakdown is not driven by the achievement of a critical electric field strength, but is an energy-dependent process. Provided the field is applied for a sufficiently long time, breakdown is demonstrated to occur at very low field strength. The breakdown time is shown to be linearly correlated with the time at which a threshold vaporization energy is dissipated in the fluid. This threshold energy is determined acoustically, by measuring the minimum electrical pulse energy required to observe steam bubble formation and collapse. These observations are used to support a model for breakdown in seawater based on vaporization as the initiating mechanism. The model is shown to be consistent with experiments up to field strengths of 50 kV/cm.