**3.2 Atmospheric pressure plasma jet**

One of the most versatile techniques for the generation of CAP plasma for biomedical application is the nonthermal atmospheric pressure plasma jets. Because of their practical capability to produce plasmas that are not spatially bound or

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

confined by electrodes, they can be used for direct treatment on any object irrespective of their shape and size. As a result, they can deliver the plasma generated essential short lifetime active radicals and charged particles to the sample to be treated. Many plasma jets with different configuration have been reported in the literature to generate CAP plasma. There exist various classifications schemes for CAP plasma jets. Some authors classify CAP plasma jets according to the power sources' excitation frequency to generate the plasma. This frequency range can vary from DC to GHz. Accordingly, they are named DC plasma jet, pulsed-dc plasma jet, KHz operated plasma jet, RF operated plasma jet and Microwave driven plasma jet [16, 23–25]. Some authors also use the names 'plasma flame', 'plasma plume', 'plasma gun', 'plasma stream', 'plasma pencil' etc., for plasma jets [24]. The plasma jets are operated with inert gases such as helium, argon etc. or a mixture of inert gas and a few percent of reactive gases of interest. The earliest known CAP plasma jet was developed by Koinuma et al. in 1992. It was powered by an RF source [26].

A comprehensive collection of various types of nonthermal plasma jet arrangements has been discussed in detail by Lu et al. [27]. They are classified into four different groups, namely dielectric-free electrode (DFE) jets, dielectric barrier discharge (DBD) jets, DBD-like jets and single electrode (SE) jets. The DFE jets consists of an inner powered electrode and an outer grounded electrode, as shown in **Figure 5(a)**. There is no dielectric material between the two electrodes. The gas temperature of this jet is relatively high, and cooling water is needed for continuous operation and to keep the temperature low. There is always a risk of arcing in this jet when standard operating conditions are not met. The DFE jet is not suitable for direct biomedical applications. However, it is effective for surface sterilization. It is operated by an RF power source.

In DBD jets, a dielectric layer is present between the two electrodes, and the plasma is not in contact with any electrode. The power consumed by this plasma jet is very less (of the order of few watts). Due to the presence of dielectric, these plasma jets are relatively safe as there is no risk of arcing and is ideal for biomedical applications. The DBD jets can be powered by a KHz ac source or by a pulsed-dc source. **Figure 5(b)** shows the typical electrode configurations of DBD jets.

In DBD-like plasma jets, the discharge is more or less DBD-like when plasma is not in contact with any object. There is no dielectric material between the live electrode and the object to be treated. In this type of plasma jets, easily more power can be delivered to the plasma, and the plasma can be very reactive. As long as arcing can be avoided, this type of jets has their own advantages. For biomedical applications, this kind of devices should be handled carefully because of the risk of arcing. These plasma jets can be powered by KHz ac, RF or pulsed dc sources. The typical configuration of a DBD-like plasma jet is shown in **Figure 5(c)**.

The SE jets are similar to DBD-like jets, except there is no electrode outside the dielectric tube. These jets can be operated by dc, KHz ac, RF or pulsed dc power sources. These kind of jets are not suitable for biomedical application due to the risk of arcing. **Figure 5(d)** shows the basic electrode configuration of a SE plasma jet.

Although the plasma produced by CAP plasma jets looks homogeneous to the naked eye, it is actually discrete in nature when observed by using fast imaging. The plasma volume consists of some "bullet "-like structure, with a propagation speed of more than ten kms−1. This discrete nature of plasma jet was first reported by Teschke et al*.* using an RF-driven plasma jet [28] and by Lu et al*.* using a pulsed dc plasma jet [29].

### **3.3 Corona discharge**

A corona discharge is a well-known non-equilibrium discharge that occurs around a pin or thin wire electrode where the electric field is higher near the

**Figure 6.** *Typical electrode configurations of corona discharge. (a) Point to plane and (b) wire to cylinder configuration.*

electrode edge but decreases quickly with increase of distance [18, 30]. Due to this highly non-uniform electric field, the gas breakdown occurs near the pointed electrode. The electric field strength is high enough to form a conductive region but insufficient to cause electrical breakdown to nearby objects. This type of non-uniform electric field can be created using an asymmetric electrode pair arrangement such as point to plane or wire to cylindrical electrodes, as shown in **Figure 6**.

The corona discharge can be classified into two types depending on the polarity of the HV electrode. The physics of these positive and negative corona discharge is considerably different. This happens due to the vast difference in the mass of electrons and ions. In the positive corona, the electrons are attracted to the HV electrode, and the positive ions are repelled. The secondary electrons are created by photoionization in the gas near the electrode. The electrons are then attracted towards the electrode, which begins the process of further electron avalanche through inelastic collision with neutral gas molecules.

On the other hand, in the negative corona, the electrons move away from the HV electrode. In this case, the secondary electrons are primarily generated by the photoelectric effect from the electrode surface itself. The process is similar to the Townsend breakdown. The electron avalanche then multiplies through impact ionization. As we go away from the electrode, positive ion accumulation occurs, and the electric field becomes weak. As a result, ionization diminishes there.

A corona discharge can be driven by direct-current (DC), alternating-current (AC), or pulsed voltage. It has widespread applications in various fields, such as ozone synthesis, material processing, water purification, electrophotography, copier machine, bacterial inactivation, wound healing and medical surface preparation etc. This type of plasma provides substantial flexibility in treating various products and materials used in the medical industry, for example, syringe barrels, pill bottles, catheter tubing, IV tubes and surgical gowns etc.

## **4. Diagnostics of CAP plasma**

Due to the small size and transient discharge behavior of CAP plasma, plasma diagnostics is very challenging. The use of invasive diagnostic techniques such as Langmuir probe is not suitable as they significantly disturb the plasma and, as a result, yield incorrect values. Therefore, various non-invasive optical diagnostic techniques are the choices of interest for determining the plasma characteristic of CAP plasma [31]. One such most widely used technique is Optical emission spectroscopy (OES).

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

It is a relatively simple and easy to implement method for determining various plasma properties. The light emitted by the plasma due to deexcitation contains various valuable information regarding the plasma. An optical emission spectrometer can capture this radiation, from which one can extract information on the different species present in the plasma. Also, using the emission spectrum, one can estimate various plasma parameters such as electron/excitation temperature, neutral or heavy particles gas temperature, electron density, concentration of different reactive excited species etc. **Figure 7** shows a typical emission spectrum obtained from an argon CAP plasma jet.

The emission spectrum of CAP plasma often contains emission from molecular species like 2 2 *N N OH* , , <sup>+</sup> etc. The highly energetic electrons of plasma can easily transfer their energy to the low lying molecular rotational and vibrational states. In atmospheric pressure condition, the rotational and translational degrees of freedom of gas molecules remain in equilibrium through collisions. Consequently, the rotational temperature gives the value of gas temperature. Generally, the *OH* rotational band around 306–309 nm, the second positive system of *N*2 and the first negative system of *N*<sup>2</sup> <sup>+</sup> are used to obtain the gas temperature. From the best fit between the experimental spectrum and a simulated theoretical spectrum of a particular molecular band, the rotational temperature or the gas temperature of the plasma can be determined. The simulated spectrum can be calculated using software like *Specair* [32] and *Lifbase* [33]. The Boltzmann distribution of the rotational levels is assumed to obtain the temperature.

Another important plasma parameter, the electron/excitation temperature, can be obtained using the Boltzmann plot technique [34]. In this technique, the spectral line intensity (*I*) and the excitation temperature (*T*exc) is related by the formula: ln / ( ) / *<sup>k</sup> k exc I g A E kT C* λ =− + , where λ is the wavelength of the line, *g*k is the upper state degeneracy, *E*k is the upper level energy and *A* is the transition probability. If the Boltzmann law is satisfied, the plot of ln / (*I gA* λ *<sup>k</sup>* ) vs. *Ek* becomes a straight line, and the inverse of the slope gives the excitation temperature. Typically, the electron temperature is found to be near 1 eV in CAP plasmas. In low-temperature plasmas, the low energy electron number is much higher than that of high energy electrons. So the bound electrons on the higher excited levels can be in collisional equilibrium with the free electrons because the energy difference between higher excited levels and the ionization energy is small. So they can satisfy Boltzmann law.

**Figure 7.** *Emission spectrum of an argon CAP plasma jet.*

The population of electrons on the lower excited levels usually do not satisfy the Boltzmann law because they are not in collisional equilibrium with the free electrons. The excitation temperature can also be measured using another well-established method called the line intensity ratio method [35].

The electron density can be measured from the study of spectral line broadening. Spectral lines are always affected by various broadening mechanisms, such as Stark, Van-der-Waals, instrumental, Doppler broadening etc. By extracting the Stark part from the total broadening, the electron density can be determined. The popular lines used for this measurement is the hydrogen Balmer lines. These lines can appear as an impurity from the moisture, or hydrogen can be added in a small amount to the discharge for diagnostic purposes. The Hβ 486.13 nm line is most widely used because of very strong Stark broadening and less self-absorption. It is also not much affected by broadening due to ion dynamics and temperature variations. Electron density as low as 5 × 1013 cm−3 can be measured using this method [31]. The Hα line at 656.3 nm can also be used for this purpose. However, the accuracy of the electron density value obtained from this line is relatively less. Other non-hydrogenic atomic lines can also be used to determine the electron density using this technique. For a detailed discussion on electron density measurement from the Stark broadening, an interested reader can go through the references [36, 37]. Typically, the electron density can vary between 1010 and 1014 cm−3 in CAP plasmas [31].

Apart from OES, there are many other techniques to study CAP plasmas. The active laser spectroscopy techniques such as laser induced fluorescence (LIF), two-photon absorption laser induced fluorescence (TALIF) can be used to obtain information on the ground state and long-lived, nonradiative excited atoms, molecules or radicals [2]. This technique has been used for many years for plasma diagnostics. Popular laser sources used in LIF are the Nd:YAG laser, dye lasers, excimer lasers, and ion lasers. Other well-known techniques such as Thompson scattering can give direct information on the electron density and temperature. Rayleigh and Raman scattering can provide information on the gas density and temperatures. The optical absorptions spectroscopy and cavity ring down spectroscopy (CRDS) can determine the absolute densities of certain plasma species. Other techniques known from chemical analysis such as UV and FTIR absorption spectroscopy, mass spectrometry, gas chromatography or electron paramagnetic resonance spectroscopy etc., are also used to identify and quantify ions and reactive species in the plasma and to track its transition from plasma phase to liquid phase [6, 18, 38].
