Streamer-to-spark transitions in deionized water: unsymmetrical structure and two-stage model

This letter focuses on the streamer-to-spark transitions generated by microsecond pulsed discharges (±20 kV of amplitude, 150 μs of pulse width) in deionized water to provide a quantitative insight into the underwater breakdown process. The discharge channel during underwater electrical breakdown is generally considered as a homogenous blackbody radiator by the arc model. However, we report an unsymmetrical structure of negative breakdown channel in water which shows great discrepancy from the frequently observed uniform structure. An improved two-stage analytical model is proposed to quantitatively estimate the parameter variations of the discharge channels, e.g. radius, temperature, pressure. The calculated results show that the discrepancy in parameters of different parts of negative discharge channel is tremendous. Specifically, during negative electrical breakdown, the region of discharge channel near ground electrode owns greatest values in characteristic parameters (e.g. T _max = 22 300 K, P _max = 0.145 GPa and v _max = 219 m s⁻¹), which are far above the counterparts of main body of channel. By introducing field ionization and impact ionization mechanisms during the propagation of the streamer, we explain the rapid expansion of positive discharge channel and the regions of negative channel near the needle tip as well as the ground electrode. Besides, the deposited homocharges at the gas-liquid interface weaken the ionizations in main body of negative breakdown channel, and hence causing the formation of the unsymmetrical structure.