**2. Fundamentals of nonthermal plasma**

The term nonthermal plasma refers to a plasma that is not in thermodynamic equilibrium, meaning that the temperature of electrons, ions and neutrals are not equal. In this type of plasma, the electrons remain at a very high temperature (up to a few eV, 1 eV ≈ 11,600 K), whereas the temperature of heavy particles is quite low. Because of this reason, they are also termed as non-equilibrium or cold plasma. The high energetic electrons provide the unique reaction chemistry of the cold plasma by facilitating excitation, ionization and chemical dissociation of atoms and molecules at a very low gas temperature. The cold plasma generated at atmospheric pressure produces a myriad of reactive and charged species, including electrons, ions, free radicals, neutral or excited atoms, UV photons etc. These exciting properties of cold plasma have led to their extensive use in various technological fields such as material processing, environmental remediation, nanomaterial synthesis, textile industry, food processing and biomedical applications etc. [13].

Plasmas can be generated by supplying electrical energy to a gas in the form of an electric field. When the applied electric field between the two electrodes is high enough to initiate a breakdown, plasma is formed. Electrons can rapidly gain energy from the applied electric field because of their tiny mass and high mobility. Then they transmit the energy to the neutral atoms and molecules through collisions, providing energy for ionization, excitation, dissociation and other chemical processes. Two types of collisions occur in plasmas [14]:

**Elastic collisions:** These type of collisions raise the kinetic energy of the neutral species but do not change their internal energies. They increase the temperature of the heavy particles.

**Inelastic collisions:** These type of collisions between electrons and heavy particles are excitative or ionizing. They modify the electronic structure of the neutral species. When the electronic energy is high enough, it can create excited species or ions. Most of the excited species of plasma have a very short lifetime. They come down to the ground state by emitting a photon. The metastable species are also excited states, but they can decay only by energy transfer through collisions as there are no allowed transitions. Hence, they have a longer lifetime. These collisions do not raise the temperature of heavy particles.

### **2.1 Paschen's law**

The voltage necessary to start a discharge in a gas between two electrodes is given by Paschen's law. It is named after Freidrich Paschen, who discovered it

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

empirically in 1889. The breakdown voltage depends on the electrode spacing *d* and the pressure *p* and is given by the formula [15]:

$$V\_b = \frac{B\left(p.d\right)}{\ln\left[A\left(p.d\right)\right] - \ln\left[\ln\left(1 + \frac{1}{\mathcal{I}\_\*}\right)\right]}$$

Here, *A* and *B* are constants determined experimentally, and *se* γ is the secondary electron emission coefficient of the cathode. **Figure 2** shows the dependence of breakdown voltage of various gases on the product of electrode spacing and pressure. It is seen that for a constant electrode spacing, the voltage required to ionize a gas is high towards higher pressure, which implies that a narrow gap is necessary to have a reasonable breakdown voltage at atmospheric pressure.

### **2.2 Current-Voltage characteristics**

The plasma behavior inside a discharge is determined by the values of current and voltage between the electrodes. A typical figure that almost every plasma physics textbook discusses is the current–voltage characteristic of a low-pressure (~ 1 mTorr) DC discharge, which describes different gas discharge regimes as shown in **Figure 3**. Arc discharged is characterized by a very high current and a low voltage between the anode and the cathode. Glow discharge occurs at a low current (typically in mA range) and a high voltage. The corona discharge is characterized by a very low current (few μA) and a very high voltage. For low-temperature atmospheric pressure applications, arc discharge is not acceptable as it produces a very high gas temperature. Therefore, a special setup is necessary to create a cold plasma and keep the plasma current low so that discharge remains in glow and corona regime.

**Figure 2.**

*Breakdown voltage in various gases as a function of the product of pressure, P and gap distance, d for plane parallel electrode.*

### **Figure 3.**

*Current–voltage characteristics of a low pressure DC discharge showing transitions from Townsend to glow and arc discharge.*

The necessary condition for a plasma to be suitable for biomedical application is that the plasma has to be produced at atmospheric pressure and the gas temperature has to be near room temperature to avoid thermal damage of biomaterials (tissue etc.) at the contact zone. For this purpose, the plasma needs to be near glow mode. However, glow discharge is generally produced at low pressure. At higher pressure, glow discharge is unstable, and a glow to arc transition can always occur. Therefore, a special electrode arrangement is required to maintain the discharge near glow and corona regime at atmospheric pressure. One general method of producing CAP plasma is to place a dielectric barrier between the two electrodes, and the resulting plasma is known as dielectric barrier discharge (DBD). The role of the dielectric is to limit the discharge current and thus keeps the plasma temperature low. The different types of CAP plasma generation methods are discussed in the next section.
