*4.3. Sources of Corona Discharge*

Plasma sources realizing a corona discharge operate in an electric field with high intensity, at high air pressure. Ionization processes during discharge generation occur only near the corona electrode. The diagram of the device is shown in Figure 4.

Operating principle. Using a corona discharge for cleaning the surface is possible using non-uniform fields that occur at electrodes with large surface curvature. Such an independent discharge is typical for electrodes in point, thin wires. Near such cathodes, a corona-shaped glow appears, and the field strength has higher values. The flow of a corona discharge is observed if an electron arises during the random ionization of a molecule (neutral). It accelerates in the electric field, acquiring energy enough to ionize another molecule. As a result of the process, a new negative electron and a positively charged ion are formed. Thus, a stream of charged particles is formed that then passes into an avalanche. The characteristics of a corona discharge depend on the type of corona (negative or positive) and the pulse applied to the corona electrode [160,161]. The scheme of the device that implements the corona discharge is shown in Figure 6.

**Figure 6.** Corona devices used for surface cleaning.

Corona discharge device configurations use needle electrodes (thin wire) or planar electrodes. In this case, the energy is concentrated on the needle electrode, and the flat one is grounded. The addition of oxygen is necessary for the decomposition of organic pollutants. Voltage during processing is applied to the upper electrode. The dielectric in the system is located between two electrodes and air gaps. The bottom electrode is grounded. The gradually increasing voltage ionizes the air between the two electrodes and creates a corona discharge.

The main source advantage is in the ability to initiate a chemical reaction on the surface to be cleaned. Flaws are significant weakening of the plasma effect during processing, oxidation of the substrate surface due to ozone release, and the need for sophisticated ozone neutralization equipment.

### *4.4. High Frequency and Super High Frequency Discharges*

For excitation in sources of High Frequency (HF) and Super High Frequency (SHF) (microwave) discharges, two electrodes must be in direct contact with the plasma. The direct current glow discharge is ignited and maintained, which is necessary to implement the cleaning process. However, such conditions can lead to disruption of the discharge due to the formation of a non-conductive film on the electrodes. The diagram of the device is shown in Figure 7 [162,163].

**Figure 7.** Plasma source for surface cleaning.

Atmospheric glow discharge is generated using a high-frequency discharge and microwave (microwave) at high pressure. Plasma is generated in He, N2, air, and He with the addition of 1–3% molecular gas such as O2, N2, H2, or CF4, providing sufficient concentration of the plasma stream to clean the surface material effectively.

The atmospheric pressure plasma source is a device with an internal electrode and a grounded external electrode. The plasma gas flows between the electrodes at high speed and exits the nozzle. The plasma jet propagates from the anode (the anode is grounded). Plasma source operating modes: voltage of several hundred volts, power~1 kW. Surface treatment can be realized in HF and microwave modes. Plasma is generated in the resonant chamber and enters the processing zone through the opening-nozzle in the chamber [129,157]. The parameters of the plasma realizing the microwave discharge are close to the parameters of the arc and glow discharge of atmospheric pressure.

Device advantages are in the possibility of modifying the surface to be cleaned; the ability to control chemically active substances to improve the quality of cleaning.

Flaws are in limited areas of impact of HF and SHF discharge; there is significant heating of the substrate in the process of exposure to the source, and possible overheating of the surface. A plasma source based on atmospheric plasma based on a dielectric barrier discharge is most suitable for use as a source of plasma surface cleaning.
