Shockwaves from pulsed streamer discharge imaged at ultra-high speeds

Pulsed electrical discharges into water have become a common way to create a plasma discharge, which is useful in a wide array of applications, including electrical insulation, environmental remediation, nanoscience, plasma medicine and agriculture. These discharges release a chain of spherical shockwaves along a filament that travels through the water. Understanding the fundamental physics behind these shocks is important to increasing their future applications.

Wen et al. study the individual shock fronts created by pulsed positive streamer discharge in water. Using a high-speed camera system that included four intensified charge coupled device (CCD) imagers and a light splitter, the authors were able to take four successive images of a discharge with 10 nanosecond exposures at intervals of 30 nanoseconds. This allowed the individual shock fronts to be captured and the step length between successive shock waves measured.

The results found the average shock separation was around 160 micrometers. The analysis additionally showed varying the water conductivity and applied voltage had no noticeable effect on the shock separation. The authors reported this could be explained by the bubble theory of electrical discharge in water. In this explanation, the electrical breakdown of a gaseous segment at the head of the streamer triggers a shock. A subsequent shock is released only after another gaseous segment forms and reaches a critical breaking point. This process produces streamer filaments that can travel 20 to 30 kilometers per second in water.

Source: “Shock wave release behavior of a pulsed positive streamer discharge in water,” by Xiaoqiong Wen and Xiaodong Xue, AIP Advances (2019). The article can be accessed at https://doi.org/10.1063/1.5108547 .