| Hydrodynamic processes following the electrical breakdown in polar liquids, mainly water and propylene carbonate, are studied with electrical pulse-probe systems and optical diagnostics including shadowgraphy and Schlieren techniques. The dielectric strength of water in a repetitively pulsed power system is found to be 750 kV/cm and 650 kV/cm for pulses of 200 nanosecond and 8 microsecond duration, respectively. For energies less than 1 J, the discharge plasma decays in approximately one microsecond. The mechanical effect created by the expanding plasma is a shock wave which has an initial pressure of 1 GPa and decays within approximately 5 microseconds. The observed shock wave pattern is determined by the electrode geometry. The percentage of energy converted to shock energy is decreased as total energy rises, for example, 20% for total energy of 15 J and 8% for 50 J. The process following shock wave emission is the formation of a vapor bubble caused by the Joule heating of the liquid. It takes approximately 250 microseconds for the vapor bubble to reach its maximum size and a millisecond to decay. The presence of the vapor bubble determines the recovery of the dielectric strength after breakdown as shown with the electrical pulse-probe measurements. The vapor bubble formation scales with the energy input. The duration of vapor bubble is increased by a factor of ten when the energy is increased from 1 J to 50 J. Besides water, propylene carbonate is also studied with regard to its dielectric strength, breakdown, and postbreakdown development. Propylene carbonate has shown a breakdown strength of 760 kV/cm. Electrical breakdown in propylene carbonate at 1 J has also shown similar temporal development as water for plasma decay, shock wave emission, and vapor bubble expansion. However, due to the formation of polymers during the discharge, it takes several seconds to restore its dielectric strength. Both water and propylene carbonate are self-healing polar liquids, allowing repetitive applications. The repetition rate can be increased to values exceeding 1 kHz by applying laminar flow. |
