Cold Atmospheric Plasma (CAP)

Abstract

Typical cold plasma system configurations: (a) plasma jet, (b) dielectric barrier discharges (DBD), (c) microwave discharge, and (d) tip-plate discharges. There are many categories to classify CAP sources, including plasma discharge mode, interaction characteristics with objects, and excitation frequency. CAP discharge modes can include any combination of plasma discharge such as glow discharge, dielectric barrier discharge (DBD), spark discharge, microwave discharge, and arc discharge. Discharges that operate at lower energy densities have applications for medicine, agriculture, foods, environment, materials, surface engineering, etc. (Bárdos and Baránková, 2010; Brandenburg, 2017; Fridman et al., 2005; Kim et al., 2015e; Laroussi et al., 2017; Lee et al., 2011; Nguyen et al., 2018; Ostrikov et al., 2013; Puač et al., 2018; Sakudo et al., 2020; Sanbhal et al., 2018; Setsuhara, 2016). Higher-energy discharges are generally employed for blood coagulation and incising tissues (Guild III et al., 2017; Nold et al., 2018). Classification according to electrical configuration and plasma/object interaction characteristics provides the general distinction, with various kinds of CAP developed for each. As shown in Figure 2.1, plasma jet, DBD, microwave discharge, and tip-plate discharge are common sources for cold plasma generating at atmospheric pressure. Plasma jets (Figure 2.1a) include different kinds of configurations that enable gas discharge in an open (non-sealed) electrode arrangement and project the discharge plasma reactive species (Nishime et al., 2017). These are often referred to as atmospheric pressure plasma jets (APPJs). DBDs (Figure 2.1b) are generated between two electrodes separated with the dielectric layers that reduce stray currents and spark formation (Moreau et al., 2008). Microwave- driven discharge plasma (Figure 2.1c) is generated by a magnetron and and employs a wave guide to transmit energy to the gas electrons. (Tolouie et al., 2018). Collisions that are primarily elastic occur between electrons and heavy plasma species and their interactions with the applied electric field result in heating of the electrons and slight heating of heavy species. Through these interactions, the electrons gain enough energy to produce inelastic exciting and ionizing collisions. Thus, the gas becomes partially ionized and becomes a plasma that supports electromagnetic wave propagation (Kabouzi et al., 2002). Applications of microwave plasma have driven the development of portable power modules: the integration of a microwave power amplifier chip with a power module to build a portable power source (Park et al., 2010). Tip-plate discharges (Figure 2.1d) use one sharp electrode (thin wires, edges, or tapred points) at atmospheric pressure where electric fields are high enough to accelerate electrons to energies sufficient to ionize the molecules and atoms of surrounding gas, leading to corona, spark, or arc discharges depending on operating conditions (Phan et al., 2017a). DBDs and APPJs are widely used to generate CAP, as the dielectric material between electrodes mediates the discharge to prevent the transition to a purely arc discharge and limit the discharge current through the electrodes. These plasma sources can be operated with a wide range of gases. For example, noble gases such as helium (He) and argon (Ar) are widely used to assist in the generation of CAP and typically exhibit a stable discharge with low gas temperatures. Other plasma sources convert just the ambient air (or surrounding gas) in close vicinity to the object surface into plasma (Kogelschatz, 2003). When the jet is exposed to air or air is mixed with assisting gases (sometimes called “feed” gases), gas-phase chemistry dominated by oxygen and nitrogen reactive species (ROS and RNS) is formed. These CAP reactive species are sometimes collectively identified as reactive oxygen and nitrogen species (RONS). ROS and RNS fluxes and composition are important factors for CAP applications and depend on many factors of the CAP device and operating conditions, including the power density, electrode spacing, electrode surface area, and gas composition and humidity.