Characteristics of vapor coverage formation on an RF-driven metal electrode to discharge a plasma in saline solution

Abstract

The generation condition of sub-mm sized non-thermal plasmas (named aqua-plasmas) inside electrically conductive liquids (saline) is investigated with various electrical conductivities of the electrolyte. The breakdown condition of an RF-driven (380 kHz and less than 50 W) aqua-plasma discharge on a metal electrode is investigated using both experimental and numerical simulation methods. Since breakdown occurs in the vapor covering the powered electrode surface, the boiling process and the influence of fluidic dynamics on the vapor coverage oscillation are considered in detail. The vapor coverage is formed on the metal electrode surface via three phases: individual bubble generation, merging of bubbles and maintaining the bubbles during the aqua-plasma discharge inside the full vapor coverage. The surface temperature is evaluated from a numerical simulation based on the proposed one-dimensional aqua-plasma equivalent circuit electro-fluid power balance, and the bubble coverage can be sustained due to boiling of the film on the heated electrode by a current flow of electrolyte ions, especially Na+ in this case. The periodic discharge of the aqua-plasma inside the shrunken vapor coverage is investigated by a model that considers the electrolyte viscosity and surface charge model. The vapor coverage periodically shrinks under the heat balance between the gain, mainly plasma heating and heat flux from the heated tip, and the loss to the saline, so it is a feature that the aqua-plasma is discharged during the period of shrinkage. Moreover, the tip, heated up to meet the temperature condition of film boiling ∼300 °C, reduces the breakdown field due to the reduced work function of the electrode material.