Plasma Jet

In its simplest design, it consists of a cylindrical shaped cathode, an outer glass/dielectric shield wrapped by the anode. By feeding any carrier gas or air between the cathode and shield, and by applying DC/RF power, an arc between the electrodes may be ignited and sustained. Higher voltages (kV) are required for gas breakdown at atmospheric pressure as required by the Paschen law. At higher pressure the collision rate is high and effective energy exchange occurs between the electrons and neutral molecules, so that thermal equilibrium achieved, and electron temperature lowers to T_e = T_n. In this case, the electron density usually ranges from 10¹⁶ –10¹⁹ cm^-3.

Figure 1 Schematic of plasma jet setup consisting of ring electrodes around 2 mm diameter quartz tube. The imaging region is 1.5 mm from tube exit. Inset photo shows gas/droplet stream in liquid-filled cuvette. (DOI: https://doi.org/10.1063/1.4922034)

Example of a low temperature plasma jet device driven by low voltage Ac and used for wound healing is discussed here (figure 2). It comprises of two copper electrodes in a coaxial layout. The diameter of inner solid positive electrode is 8 mm and inner diameter of outer hollow electrode is 25 mm which is biased negatively. The two electrodes are attached at base with a dielectric. There is 8.5 mm gap between the electrodes and the middle cavity forms the gas channel. Plasma is generated by the ionization of the air between the two electrodes. The active species are propelled outside of nozzle by the airflow. This system was tested for clinical and bio medical applications and have been successfully demonstrated its efficacy towards the in-depth wound healing.

Figure 2 (A) Schematic illustration of the device, (B) The prototype, (C) Plasma jet operated with surrounded air (DOI: https://doi.org/10.1111/iwj.13652)