Mode transition of aqua-plasma generated by micro metal tip surrounded dielectric material in electrolyte

The aqua-plasma, non-thermal plasmas in liquids, can be discharged by forming the vapor coverage by Joule heating on the metal electrode. The electric field is decided by thickness of vapor coverage with applied voltage. It implies that the thickness of vapor coverage is the key parameter to the control the characteristics of aqua-plasma. The aquaplasma electrode consists with discharge tip, dielectric, and counter electrode. The tip covering vapor attaches on the dielectric with high surface tension coefficient. In this work, the effects of dielectric temperature on the mode transition of aqua-plasma are studied. The aqua-plasmas are discharged by applying ac driven voltage on the saline immersed discharge tip, size of 400 μm. The temperature of dielectric is controlled in range of 330 K - 500 K by varying the thickness from 90 μm to 780 μm. Ground electrode is placed under 30 mm of the tip. The V-I signals are monitored by using the voltage and current probes. The aqua-plasma shows three different modes with the thickness of dielectric; Townsend mode, glow-like mode, and spark mode. In case of 0.09 mm, dielectric temperature is relatively low, 330 K, due to thermal conduction from heated electrode to electrolyte. The spark discharge simultaneously occurs with the formation of vapor, 150 μm, with electric field of 5.2 MV/m. The vapor coverage is collapsed by high dose of plasma charge and re-built. In the 0.78 mm, dielectric temperature reaches to 500 K by low thermal conduction. Thickness of vapor coverage increases to 220 μm. The electric field is reduced to 3.4 MV/m and vapor coverage is maintained, Townsend mode streamer discharge takes place. The vapor coverage is oscillating since streamer heats up the vapor surface. In the intermediate dielectric thickness, 0.35 mm, glow-like discharge occurs. Also the rotation temperatures of aquaplasma produced ·OH* (T_{rot_·OH*}) are estimated by using the Boltzmann plot method on the OES. The two different T_{rot_·OH*} are observed with the ·OH* production location. The ·OH* at the vapor surface is lower than the ·OH* in plasma due to electrolyte quenching. The difference between the two T_{rot_·OH*} increases from 50 K to 400 K with vapor thickness, due to the separation of ·OH* generation location. Thus the structure of dielectric is should be considered in electrode design to generate the aqua plasma with the suitable characteristics for the application.