Understanding the Formation of Stable and Unstable Cathode Spots in Plasma-Liquid Interactions

Interactions of plasmas with liquid cathode electrodes have gained significant attention in recent years due to their potential applications in various fields, including environmental remediation, biomedicine, and materials synthesis. Cathode spots, localized regions of intense plasma discharge near the cathode surface, play a crucial role in these interactions. Herein, we have investigated the formation of stable and unstable cathode spots on the surface of liquid cathode. Plasma is generated by flowing helium (He) through a tungsten tube enclosed inside a tee tubing & observations of cathode spot is made through the variation of liquid conductivity, temperature, gap distance, applied voltage & He flow rate. At a gap distance of 5 mm and He flow rate of 0.8 liters per minute, the unstable cathode spot transforms to stable one as the conductivity and temperature reach critical values of ~65.4µS and ~92.3°F respectively. Increasing the gap distance or the addition of electronegative gas inhibits the formation of the stable cathode spot. Further investigations showed that at increasing values of applied voltage, the stable shaped pattern transforms to a cone shaped pattern. Increment of He flow induces liquid instability & results in the unstable shape of the cathode spot. The characterization of the cathode spot using optical emission spectroscopy and time-resolved imaging will be presented.

This work was supported by the NSF EPSCoR RII-Track-1 Cooperative Agreement OIA-2148653 and DOE DE-SC0021391. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.