## 2 Plasma
### Plasma Sources  
There are several methods for generating CAPP from various gases such as (i) dielectric barrier discharge, (ii) plasma jet, (iii) corona discharge, (iv) gliding arc discharge, etc. (Figure 1) [10,25]. Commonly used working gas includes air, oxygen, nitrogen, helium, argon and their mixtures [14,18,21,26].  
Dielectric barrier discharge (DBD) is generated by applying a high voltage (\(\sim\)kV) electric DC or AC current at high frequency (\(\sim\)kHz) across an adjustable gap (ranging from tens of microns to several cm) between two electrodes separated by an insulating dielectric barrier. The geometry of DBDs usually consists of two parallel plates in planar or cylindrical  
Figure 1: Schematic drawing of diverse cold atmospheric pressure plasma devices.  
arrangements. DBDs use a dielectric material (e.g., quartz, glass, ceramics, enamel, silicon rubber, teflon, mica, plastic) to cover at least one of the electrodes [27]. The nonconducting coating eliminates the transition of the discharge to an electrical arc [10; 28; 29; 30].  
The atmospheric pressure plasma jet (APPJ) is a type of cold plasma discharge that produces a high velocity stream of highly reactive chemical species with weak emitted light [31; 32]. APPJs typically consist of two concentric cylindrical electrodes, where the inner electrode is connected to a RF or MW power source at high frequency, causing ionization of the working gas (mainly noble gases such as helium or argon). The gas exits through a nozzle, which gives a \"jet-like\" appearance [20]. Based on the configuration and used materials, APPJs can be divided into single electrode jets, dielectric-free electrode jets, DBD jets, etc. Miniature plasma jets are known as plasma pens, plasma torches or plasma needles [28; 30; 33]. The main component of plasma needles is an electrode with a sharpened tip inside of a tube. The feed gas (most frequently helium) is flowing through a tube and is mixed with air at the needle tip where a micro discharge is created [34]. The diameter of the generated plasma glow is a few millimeters [35].  
Corona discharge is generated by the application of high voltage between two or more sharp electrodes. The coronizing electrode is usually realized as a needle or a thin wire. The ionization process creates a crown around this active electrode. Coronas are very weak discharges, having very low electron and ion densities [36; 37].  
Gliding arc discharge plasma reactors are known as hot plasma sources, however, under specific conditions they may also produce cold plasma. The gliding arc plasma can combine the advantages of both thermal and nonthermal plasmas (nonthermal plasma conditions at higher power). The discharge is formed by a high voltage at the spot where the distance between diverging electrodes is the shortest (\(\sim\) in the range of millimeters). Electrodes are placed in a fast gas flow and the discharge increases its volume and length in the flow direction [38; 39; 40].  
The plasma sources can be applied directly (the target is in direct contact with the active plasma region) or indirectly (plasma afterglow or storable plasma-activated medium is used which contains various reactive species) to the object [41].