**3. Methods of CAP plasma production**

Production of atmospheric pressure nonthermal plasma is quite challenging due to high electron-neutral collision frequency, and low applied electric field. Fortunately, several methods have been developed over the years to overcome these challenges. Different production methods have been reported to produce cold plasma in the open environment. These include Dielectric Barrier Discharge (DBD), Atmospheric Pressure Plasma Jet (APPJ), corona discharge etc. Several different working gases such as Helium, Argon, Nitrogen, Heliox (a mix of helium and oxygen), air etc., are used to produce CAP plasma. This section gives a brief overview of the commonly used CAP plasma generation techniques.

### **3.1 Dielectric barrier discharge (DBD)**

One of the most widely used techniques for generating CAP plasma is the dielectric barrier discharge (DBD) using alternating or pulsed electric field. As the name itself suggests, a dielectric cover is used at least at one of the two electrodes for producing the discharge. The function of the dielectric layer is to suppress the spark or arc transition by limiting the discharge current. DBDs are also called

### *Cold Atmospheric Pressure Plasma Technology for Biomedical Application DOI: http://dx.doi.org/10.5772/intechopen.98895*

"silent" discharges as it produces no sound during discharge. Typical electrode gap distance in a DBD varies from 0.1 mm to several centimeters. Different dielectric materials such as glass, quartz, ceramics and polymers etc., are used in DBDs. To avoid a spark or arc transition, sufficient breakdown strength of the dielectric layer is necessary for insulation of discharge current. But a thicker layer requires a higher voltage, so a compromise must be made here. The electrode arrangement is generally enclosed in a chamber to introduce various gas mixtures between the two electrodes [16]. High voltages sources with frequencies in the kHz range generally drive DBDs. There are many different configurations of DBD are available, but the concept behind them all is the same. These include planar, parallel plates separated by a dielectric or a cylinder, or coaxial plates with a dielectric tube between them. Some basic DBD electrode configurations are shown in **Figure 4**.

More recently, Fridman et al. developed a floating electrode DBD (FE-DBD) [17]*.* It is similar to the original DBD and consists of two electrodes: an insulated high voltage electrode and an active electrode. The difference between FE-DBD and DBD is that the second electrode is active, meaning it is not grounded. The second electrode can be human skin, a sample, or any other target. Here, the powered electrode needs to be close to the surface of the second electrode to create the discharge.

The discharge in a DBD at atmospheric pressure is generally non-uniform filamentary type which can result in non-uniform treatment of the sample. The dynamic distribution of these filaments determines the appearance of the discharge. Although DBDs usually produce filamentary plasmas, under certain conditions, homogeneous diffuse plasma can also be created. Several groups have reported successful production of diffuse homogeneous atmospheric pressure glow DBD plasmas [18–21]. The mechanism of generating a glow DBD is to initiate a Townsend breakdown instead of a streamer breakdown [22]. To form an avalanche under a lower electric field and avoid growing a large number of positive space charges, sufficient initial seed electrons should exist in the gap before breakdown.

**Figure 4.** *Schematics of DBD with different electrode configurations.*

**Figure 5.**

*Typical electrode configurations of (a) DFE jet, (b) DBD jet, (c) DBD-like jet and (d) SE jet. H.V. = high voltage.*

In DBDs, residual species from the previous half period of the applied voltage provide the seed electrons or enhance the initial field for the next discharge cycle. This is the so-called memory effect [19].
