Chapter 1 Final

Chapter 1: Introduction

CHAPTER 1

INTRODUCTION

1.1 Dielectric barrier discharge (DBD) introductionThe DBD was discovered first by W. Siemens, in 1857 and proposed a novel type of electrical gas discharge that could generate ozone from atmospheric pressure oxygen [1]. According to his studies, it consists of two coaxial glass tubes, one within the other, separated by a small space. The dielectric barrier discharges (DBDs), also known as barrier discharges, are generated in the discharge configurations with at least one dielectric barrier between the two electrodes [2]. Andrews and Tait in 1860 [3] showed the results of the characteristics of the DBD and invented the name silent discharge for DBDs. In addition to the production of ozone (O3) from DBDs observed by Siemens [1], Hautefeuille and Chappuis in 1881[4] and Warburg and Leitha in 1909 [5] noticed the production of nitric-oxide (NO). After identifying NO, Warburg worked on increasing an understanding of the physics associated with the DBDs [6]. In the last century, the extensive use of dielectric barrier discharges (DBDs) started in Europe. The main application was focused on the production of ozone, which was used to decontaminate water [7, 8]. In the recent decades, the span of the applications included the excimer light sources for UV curing of photo-reactive polymers, plasma display panels, photo-deposition of large area on patterned thin metal or semiconductor films of high and low dielectric insulating layers, polymer etching and micro-structuring of polymer surfaces, etc. [9-14]. It is due to the most prominent characteristics of the DBDs, which states that the dielectric barrier discharges are non-equilibrium and non-thermal glow discharge plasmas and occurs at sub-atmospheric gas pressures, and as a consequence find large number of cold plasma applications. The DBD applications for the surface treatment have been extensively studied in which the DBD plasma reacts with the material surface and induces physical and chemical changes, such as, surface cleaning, functionalization (e.g., to increase wettability or adhesion), eching or deposition of a film, etc. To enhance the wettability of the surfaces, Z F Xiangqun Qiu and his team has worked and enhance the hydrophobicity of the glass surface which is sprayed with the silicon liquid before it is treated with the DBD for better insulation performance [15].Most recently the DBD technology opened possible emerging novel applications in biology and medical field [16, 17]. Also, the ultraviolet (UV) and vacuum ultraviolet (VUV) sources based on DBD emerged as a superior tool for many industrial applications [18]. The mercury based UV/VUV sources have excellent efficiency but also have one major drawback. The decomposition of mercury in air or water can cause serious health problems and also environmental pollution. Efforts are going on worldwide to find the alternatives for the mercury free light sources. Very recently, a new generation of lamps has been developed based on excimer radiations that are capable of producing efficient UV to deep UV light radiations [1922]. The DBD based excimers are the best alternative for the generation of UV/VUV radiation either with rare gases or with rare gas mixture of halogens [23-25]. It is well known that the excimers, which are unstable excited molecular complexes, are generated in a number of ways, e.g., by dielectric barrier discharges, high-energy electron beams, x-rays, synchrotron radiations, protons, heavy ions and microwave discharges [24,26].The DBD plasmas are used as effective sources of the excited dimmer which terminates to the unbound or weakly bound metastable states and leads to generate VUV/UV or visible light radiations effectively [24-25]. This is one of the most proficient ways to produce the necessary precursor for formation of excimers because DBDs have ability of producing high energetic electrons and excited dimmers at high pressures [27]. Furthermore, the DBD plasmas have potential adoptability for industrial applications because of their simplicity or the geometric freedom, high efficiency, low cost, etc. [28]. The UV/VUV radiation dynamics from DBD driven excilamp was experimentally studied for the first time by Lomaev et al. [23]. In earlier days the disadvantage of DBDs was that the UV radiation generation was characterized only by 10-15% electrical to optical convergence efficiency. However, Vollkommer and Hitzhschke [29] solved certain basic hindrances and improved the efficiency. The new configuration led to develop Xeradex source as shown in Fig. 1.1 with efficiency as maximum as 40% in converting electrical energy into ultraviolet radiation at wavelength 172 nm during the glow discharge.

Fig. 1.1. Osram Xeradex 20 excimer light source emitting 172 nm of wavelength [30]

These new types of mercury free light sources are highly applicable for the medical applications. Galina Matafonova and Valeriy Batoev published a review article on recent advances on excilamps in 2012 [31] and described detail applications of excilamps in microbial inactivation by direct UV treatment. There is existence of many applications related to the use of DBD in medicines and biology [32-37]. Fridman et al. [34] demonstrated that without any visible or microscopic damage to the skin tissues the blood-clot formation in seconds is possible by the direct treatment using floating electrode DBD air plasma. This discharge configuration used in the treatment of living tissues and is shown in Fig. 1.2.

Fig. 1.2. Treatment of living tissue without damage [34]The radiation properties of the DBDs (308 nm UV light) pay important role for skin treatment. Nevertheless, this UV radiation was used in the skin treatment from the excimer laser for a particular decease psoriasis. The recent literature reveals that XeCl excilamp is much cheaper and simple to use than the excimer lasers for this particular decease [38].

(a)(b)

Fig. 1.3. UVB treatment at wavelength 308 nm (a) Before, (b) After [38]

The effect of XeBr*, KrCl*, and KrClKrBr* excilamps on five microbiological cultures have also been compared [39,40]. Matafonova et al. in 2008 have observed that rapid inactivation of bacteria happens while KrCl* excilamp with 222 nm wavelength [41]. They further showed that the Escherichia coli (E-coli) and Staphylococcus aureus (S-aureus) bacteria are the most sensitive at this wavelength and 100% deactivation of these bacteria occurred in couple of tens of seconds. Moreover, the XeBr* excilamp (289nm) provided the higher inactivation efficiency for E. coli and Pseudomonas aeruginosa [42] as compared to the mercury-lamp. Fig. 1.4 illustrates the flow chart for associated physical and chemical components of DBD along with most prominent applications. The discharge physics and plasma chemistry play important role in deciding the specific application that still requires many researches.

Fig. 1.4. Flow chart involving DBD applications

This dissertation involves the augmentation of DBD discharge physics understanding and to leverage it for the development of an efficient DBD based VUV/UV excimer light source with practical applications. However, the performance of DBD plasma depends on the treatment time for a definite applied voltage in a specific electrode geometry and discharge arrangement. A homogeneous discharge is advantageous than a filamentary discharge in most DBD plasma applications. The homogeneous or diffused discharge offers uniform radiations in indirect treatment and also delivery of active species uniformly in direct treatment and have better control over physical and chemical changes. There is the number of ways by which the discharge parameters of a homogeneous discharge can be controlled [4346]. To have better understanding, the classification of DBDs along with discharge mechanism understanding is most important.

1.2. Definition and classification of DBDs Dielectric barrier discharge is defined as a non-equilibrium and non-thermal gas discharge plasma in which at least one of the electrodes is covered by dielectric material. The plasma discharge consists of a large number of microdischarges of short duration that is distributed randomly in the gas gap of the discharging electrodes [1]. The electrons have high mean energy but lower heavy particle temperatures like neutrals and ions. In the DBDs several chemically active species, such as, electrons, radicals, metastables and ions with low gas heating are produced [47]. The DBD plasma exhibits a very specific characteristics that it is operated at higher working pressure in glow discharge mode that is, in general, not possible. It can basically depart from the equilibrium state that is non-equilibrium condition of the plasma due to the reduced electric field in the plasma by changing the external parameters like operating gas pressure, gas gap, power mode, etc. [27]. The DBD generally operates in filamentary mode due to the existence of microdischarges and can be transformed in to homogeneous mode depending on the control of plasma operating parameters [27] (see Fig. 1.5.). In most cases, DBD results in a filament mode of operation with formation of microdischarges and consequent filaments are visible to the human eye (see Fig. 1.5(a)). The diffused discharge is shown in Fig. 1.5(b) and is more applicable for most practical applications.

(a)

(b)

Fig. 1.5. Image of discharge in parallel plate DBD (a) filamentary discharge (b) Diffused discharge The filamentary mode operation consists of thin plasma channels stochastically distributed in the gas gap between the working electrodes. These microdischarges are several tens of microns in diameter and last for several tens of nanoseconds. On the other hand homogeneous discharge fills the entire gap with practically uniform plasma. Filamentary DBDs are less used in surface and plasma radiation processing because of the non-uniform plasma interactions, so the real challenge was to get diffused discharge at atmospheric/sub-atmospheric pressure operation of DBDs. Diffused DBD under atmospheric pressure was first reported by Kanazawa et al. in 1988 [48]. They found that the discharge current waveform of the diffused DBD is one discharge pulse (about 1 s) per half-cycle of the applied voltage, while the filamentary DBD consists of many short-lived pulses (about 10 ns) per half-cycle [49,50]. It is reported that for the different gases and mixture of the gases, DBD is filamentary in nature at atmospheric pressure discharge operations and diffusive at sub-atmospheric pressure discharge operations [51]. The controlling of the mean energy of the electron with the variation of the external parameters such as geometry and type of dielectric used also play an important role in diffused discharge operation of the DBDs [27]. Accordingly, the operating pressure, the inter-electrode gap (geometry) and the type of the dielectric play important role to influence the spectral characteristics. Hence depending on the application, specific requirement for DBD have led to use the different configurations. The DBDs are mainly classified in three basic configurations: the volume discharge, (VD) surface discharge (SD) and coplanar discharge (CD) arrangements [52]. Fig. 1.6 illustrates the different configurations arrangement of DBDs. Fig. 1.6 (a-d) describe the volume discharge arrangement, Fig. 1.6(e) shows the surface discharge and Fig. 1.6(f) depicts the coplanar discharge which is the combination of surface discharge and the planer discharge arrangements. The VD arrangement of the DBD consists of two parallel electrodes in which either one or both electrodes are covered by the dielectric layer. The discharge is executed in a gas gap between two parallel plates or concentric cylindrical electrodes covered by the dielectrics. The discharge occurs in the form of mcirodischarges or filaments, which are distributed in the gas gap between the electrodes. The SD arrangement consists of number of surface electrodes on the dielectric layer and a counter electrode on its reverse side. It means one electrode is exposed to the air and the other is covered by the dielectric material. There is no clearly defined discharge gap in this case. The microdischarges are, in this case, rather individual discharge channels that occur in a thin layer on the dielectric surface and can be considered homogenous over a definite distance.(a)(b)

(c)

(d)

(e)(f)

Fig. 1.6. Schematic view of different DBD configurations

The plasma actuator is the one of best example for SD arrangement of DBDs [52]. The CD arrangement is characterized by pairs of long parallel electrodes with opposite polarity. The discharge occurs in the gas gap and on the surface of the dielectric. Typical gas spaces vary from several tens of micrometers to several centimeters. The CD arrangement is the combination of two configurations. It is characterized by the pairs of electrodes of opposite polarity situated within the bulk of the dielectric as shown in Fig. 1.6(f). The VD arrangement of DBD has been investigated by several researchers whereas the investigations of SDs are rare [54, 55]. The efficiency of SDs is lower than that of VDs and perhaps this is the reason. So, the volume discharge configuration in the DBDs is the appropriate choice to study the discharge behavior of the DBDs. Hence VD configuration of the DBD has been considered in the present research work. It is to be mentioned that the most influencing factors to the efficiency of the DBDs is dielectric material [56]. The choice of the dielectric material is based on the good insulating material properties so as to sustain the high voltage stresses and also have small loss angle. The most preferred dielectric materials are glass, quartz and alumina (ceramics) albeit the choice of dielectric material depends on the specific application. Shape of electrodes is also useful parameters in term of efficiency enhancement. The most used forms are planar or cylindrical but metal plate, metal grid, or narrow metal strips can also be used.

1.3. DBD mechanism and diagnostics

A basic VD configuration of DBD as shown in Fig. 1.6(c) has been considered to understand the discharge phenomena of the DBDs. At initial stage many microdischarges occurs in the discharge region. In fact, when a dielectric barrier layer is placed in front of the electrodes [57] and high voltage is applied between the two electrodes, it limits the discharge current but streamers are formed. To understand it more clearly, when the applied high voltage kept remains constant, the charges from plasma accumulates on the dielectric surface which are basically wall charges (Fig. 1.7) and these reduce the effective electric field inside the discharge gap leading to quench the discharge in short time [58]. To illustrate it more clearly, the induced field due to accumulated charges localizes the net field and discharge is extinguished when the net field is lower than the breakdown field. The charge accumulation on the surface of the dielectric barrier reduces the electric field at the location of microdischarges which results in the current termination within nanoseconds. These short duration current discharges are clearly known as the microdischarges and lead to lower the heat dissipation and the DBD plasma remains firmly non-thermal and non-equilibrium.

Fig. 1.7 Schematic view of the DBD discharge: the charge density on the dielectric barriers generates a field that opposes the applied field (s= Surface charge density, Es=Electric field generated due to charge deposition at the dielectric layers). In order to sustain the sub-atmospheric/ atmospheric pressure glow discharge in the used VD configuration of the DBD, the applied voltage needs to transform over time, which is essential condition for such discharges [59]. The applied frequency, pulse width, pulse rise and pulse shape are the parameters, which controls the discharge dynamics [27]. The applied pulses basically controls the charges accumulated on the insulator surfaces and also accordingly removed. This is called memory effect which is the most dominant feature of the barrier discharges [60]. In fact, when we apply the external pulse/sinusoidal voltage to the DBD and the external voltage increases continuously, the microdischarges are initiated randomly at new locations because of the presence of residual charges on the dielectric surfaces. These residual charges reduce the electric field at the positions where microdischarges have already appeared. When the voltage reversal occurs, new microdischarges are generated at the previous microdischarges locations and due to the residual charges on the dielectric surface it will take less external voltage for the breakdown in the subsequent half cycle. If the applied voltage is adequately enough, the accumulated charges are sufficient to initiate the discharge at next half cycle of the applied voltage. The basic phenomenon is first the accumulated charge store energy before the discharge initiation and then release the energy in subsequent discharge. The more stored energy of these charges results the discharge power increasing. For the low applied voltage discharges, the discharge is always in Townsend phase and current density is less than 1mA cm-2 [61]. In this type of discharge, the influence of accumulated charge can be neglected. In general, the charge accumulation on the dielectric layer increases very fast with the increase in applied voltage and accordingly the memory effect increases. This increased memory effect further leads to form more microdischarges. Naude and his co-workers have shown the transition of Townsend-like to filamentary discharge clearly [62]. They further showed that the bunch of filamentary streamers or microdischarges can be transformed to the diffuse discharge, which depends on the driving frequency, operating pressure, and detailed chemistry coupled through the surrounding gases. The applied power is also most influencing parameter and it has been found that for increased applied power the patterned discharge becomes homogeneous gradually [44,63-65]. Somekawa and his group (2005) explained the theoretical analysis of transition between filamentary to diffuse mode and also explained the concept of self erasing effect for the succeeding discharge cycle [66]. For efficient electrical to radiative energy conversion in the plasma, the homogeneity in the plasma play an important role [67-69] though there are aforementioned large number of influencing parameters, which need to be understood before arriving to a final conclusion on the discharge parameter dependency. Hence in the presented VD configuration of the DBDs, an effort has been made to understand the dielectric barrier discharge in filamentary and diffused mode of operation at different discharge operating conditions for optimum VUV/UV efficiency so as to develop a practical device for water purification application. To identify the discharge operating parameters, appropriate diagnostics of DBDs are needed. However, in very thin geometries of DBDs in-situ diagnostics are not possible [70] but passive diagnostics, such as, Electrical Analysis [71], Image Intensified Charge Couple Device (ICCD) imaging [72], Optical Emission Spectroscopy (OES) [73] and particle-in-cell (PIC) simulations [70] can provide appropriate information about the discharges, and such diagnostics studies are necessary for the characterization of plasma devices. As the discharge is triggered from the applied voltage waveform, a non negligible capacitive current component is added to the real current of the DBD plasma [74]. Moreover, due to the accumulated charges on the dielectric barrier, there is an induced voltage in the plasma which is quite different from the applied voltage. Hence a specific data processing through an electrical analysis can compute the whole measurements. This calculation requires an equivalent circuit model [71] analysis, which includes all capacitors under consideration. The capacitive and discharge current separation from the total current has not been clearly worked out so far and this can further help in diagnosing the electron plasma density through discharge resistivity method [75]. The ICCD camera imaging can investigate the space and time resolved characteristics of the plasma discharge within discharge region [76] and can easily determine the discharge structures of the gas gap so as to compare with the PIC simulations [60]. Based on such comparison one can obtained certain useful parameters and a comparison of the discharge appearance under sinusoidal and pulse excitation through ICCD showed that under unipolar pulse excitation more diffuse discharge is observed. In recent years, the optical emission spectroscopy (OES) has proven to be very versatile technique for passive plasma diagnostic especially in low and atmospheric pressure discharges [77]. In fact, there are many atomic and physical processes involved in the DBDs which mainly depends on the electron energy, electron plasma temperature and electron plasma density. Using OES technique one can easily determine the electron plasma densities (Ne) and electron plasma temperatures (Te) and many other parameters of interest [78]. Techniques based on OES are non-invasive and require only moderate spectroscopic equipment, easy to implement and measurements are usually fast. The OES technique combines measurements of certain line-intensities that can be compared with the collisional-radiative (CR) model analysis of a plasma discharge depending on the discharge equilibrium under consideration [79]. This analysis provides the relative intensities of spectral lines emitted from different upper levels which are proportional to the relative populations of the excited states and consequently depend on the plasma parameters. Such analyses have been rarely practiced in the DBDs and hence focused efforts are needed in this direction. To compliment the passive plasma diagnostics, computational modeling can successfully be applied for studying physical phenomena that cannot be effectively investigated experimentally [80]. Using simulations a small change in the operating parameters of the discharge system can be easily studied. Some preliminary efforts have been made using PIC simulations to interpret DBDs and a few codes have been developed [81]. The Oopic-Pro is an object oriented particle-in-cell 2-D simulation code. It solves for the fields on the grid and calculates the particle trajectories including self-consistently the effects of charged particles on the fields with respect to the space and time variations. This also treats collision and ionization processes of a background neutral gas with Monte Carlo collisions method and able to find the electrical as well as the plasma parameters throughout the gas gap which cannot be calculated through the experiments. To measure the plasma parameters through the experiments using OES the line-of-sight measurements of the DBD gives average output but using the PIC simulation a distributive and clear output from every point of the gas gap can be taken out.

1.4. Gaps in existing research

Demands for the improved efficiency, cost effectiveness and environmental benign for DBD based technologies led for renewed interest of the researchers in this emerging area of plasma science. In the DBD research, lots of efforts are underway for efficiency improvement of the VUV/UV excimer sources [23]. Many studies consisting of electrical discharge analysis and spectroscopic analysis have been performed [60,75], but still the internal discharge parameters need to be predicted to describe the internal discharge structure of the DBDs, which are not recognized till now for smaller geometries. An effort is needed to find out the possible internal electrical parameters, such as, discharge current (Idis), displacement current through the gap (Idg), gas gap voltage (Vg), dielectric voltage (Vd), memory voltage (Vmo), supplied power (Psup) and consumed power (Pdis) and some of the basic plasma parameters, such as, electron plasma density and temperature with the variation of external operating conditions. The electrical parameters on one hand can provide the selectivity of the power sources for optimum use while the basic plasma parameters can help in understanding the optimum use of the plasma radiations during the efficiency analysis of the DBD based VUV/UV light sources. The single shot images of the DBD in VUV and visible region were also of some interest [82] which showed the filamentary and diffused structures in the gap but the validation of these discharge structures is also required. In terms of discharge structure, many researchers are trying to get the discharge appearance during the process. However, it is necessary to record the short microdischarge plasmas in the thin geometries of the DBDs using high speed camera, such as, ICCD in order to get the space and time resolved discharge analysis of very short lived DBD filaments. This can help to conclude whether the DBD discharge is filamentary or diffused in nature. Simultaneously, the simulation also deserves a high degree of credibility when plasma discharges are transient [83]. So, to compare with the experimental results, a 2-dmensional time dynamic code like Oopic-Pro can be used [84]. This can help in understanding the discharge patterns and also can help in the measurements of the basic plasma parameters to compare with the experimental results. There are some reports of the single microdischarge simulations [85] but only measurements for a single microdischarge cannot provide the requisite information. In fact, it just reduces the complexity during the simulation but a single microdischarge cannot completely describe the array of the filaments which usually occurs in the DBD based excimer sources. The large area simulation can allow getting the information on the elementary processes in entire DBD discharge and can give complete description of the plasma during the discharge. Therefore, there is a requirement for self-consistent simulation by considering all the parameters simultaneously including, discharge geometry, gas type and pressure, type of dielectric, gas kinematics, surface chemistry, atomic processes, etc. To do this a packaged code is a better option and hence the Oopic-Pro code has been used. The theoretical calculations and simulation model results compared with the experimental results can provide better insight to describe the discharges in thin DBD geometries. The significant information obtained through the simulations, electrical and spectroscopic diagnostics can further help to make an efficient VUV/UV excimer source for their immediate use to society.

1.5. Research Objectives

The main objective of this research is to design and develop couple of DBD discharge sources of VD configurations to study and characterize them in terms of their electrical and optical parameters by having alternating and unipolar pulse discharge operations. Also based on the obtained optimum plasma parameters from simple configurations to develop an efficient VUV/UV DBD plasma discharge source for societal application. The following are the key objective of this research work;

1. Design and development of different dielectric barrier discharge sources of volume discharge configurations working at sub-atmospheric pressures.2. Electrical characterization of these DBD sources for alternating and unipolar pulsed voltage excitation mechanisms so as the power source is optimally used. 3. Optical emission spectroscopic characterization of these DBD sources for the measurements of plasma parameters to help in understanding the optimum use of the plasma radiations during the efficiency analysis of these DBD based VUV/UV light sources. 4. Carrying out the particle-in-cell simulation of the designed and developed DBD sources using electromagnetic simulation software OOPIC-Pro to understand the discharge patterns and also to help in the measurement of the basic plasma parameters to compare with the experimental results.5. Development of an optimized DBD VUV/UV excimer source for the water treatment application.

The essentials for the experiments and simulations including set-up, results, significance, and discussion along with an extensive summary are included in the subsequent chapters of this dissertation.

1.6. Scope of the thesis:

The scope of proposed research work is well focused to develop the DBD based VUV/UV light sources and a primarily put forward an understanding to improve the efficiency of the DBD based excimer light sources for some immediate VUV/UV excimer application. The introduction to the DBDs and the broad literature review is presented in this chapter that covers a detailed discussion to the different types of DBD discharges and the physics behind the discharges under different geometries of DBDs. This chapter also gives a brief review of the useful applications related to the research work undertaken. The detailed description of equipments which are used for the experiments have been described in chapter 2. This includes, description of the different designs of the DBD geometries including parallel plate geometry, capillary source and a single barrier source. The experimental setup, details about the electrical measurement equipments, brief discussion about the used optical diagnostic tools and some details about the diagnostics tool limitations are also considered in this chapter. The discharge excitation results of three different geometries for sinusoidal and pulse excitation are presented in chapter 3. This chapter emphasizes the electrical characterization of developed sources to derive important electrical parameters using an equivalent electrical circuit model. The information derived for the electrical parameters includes, the discharge current (Idis), displacement current through the gap (Idg), gas gap voltage (Vg), dielectric voltage (Vd), memory voltage (Vmo), supplied power (Psup) and consumed power (Pdis), which are measured for different operating experimental conditions and in different gas pressures. This exercise has led to understand the temporal evolution processes of all the internal electrical quantities and quantitative estimations of the essential electrical parameters. A parametric study of the DBD is presented and dependency of the different operating parameters has been discussed which gives the insight of the discharge characteristics in both excitation cases. The two excitation methods are evaluated on one basic VD configuration (parallel plate configuration) and results showed that the pulse excitation is much efficient than the sinusoidal excitation and are explained based on the memory charges. Moreover the estimated values of the electrical parameters, such as, discharge current (Idis) and gas gap voltage (Vg) are used to measure the internal dynamic resistance of the gas gap which in turn is responsible for the measurement of the electron plasma density using discharge resistivity method. The estimated electron plasma density for the parallel plate DBD and capillary DBD is found to be in the range of 1010 -1011 cm-3 whereas in case of the single barrier DBD it is ~1012 cm-3. Chapter 4 contains detail about the DBD characterization using optical emission spectroscopy (OES) to measure the basic plasma parameters, such as, the electron plasma density (Ne) and electron plasma temperature (Te). The OES is a non-perturbing passive method for measuring the plasma parameters and has been best exploited in the thin DBD geometries. For electron plasma density and temperature estimations, the CR-model based line-ratio technique has been used and theoretically predicted ratios are compared with ratios of the spectral lines measured experimentally. The electron plasma density ~1010-1012 cm-3 and electron plasma temperature ~5-6 eV in the parallel plate and capillary DBD sources are reported, which has been verified using a 2-D PIC simulation code also. The kinetic simulations of the DBD sources are carried out for a symmetrical 2-D geometries of the DBDs using OOPIC-Pro code. The simulations are carried out for both geometries i.e., parallel plate and capillary DBD sources. The peak electron plasma densities are obtained from the simulations code at different radial distances of the discharge geometries for different simulations times and operating pressures. Since the discharges are filamentary in nature, at the initial operation of the simulations, the statistical analysis allowed us to measure the electron plasma density and temperature in the simulations to compare with the spectroscopic average measurements, which are found to be in close agreement. Chapter 5 demonstrates the optimization of the single barrier DBD source filled with xenon and chlorine for efficient use in the water purification application. The generation of excimer radiation from the mixtures of the rare-gas Xe2 together with halide gas Cl2 and air admixture has been investigated. The source has been optimized for maximum radiations of this UV-B (308 nm) light which occurs at ~ 2% Cl2 admixture in the xenon and at 25 kHz operating frequency. Furthermore, to make the source more cost effective in the admixture of Xe2 and Cl2, the air has been introduced. The application of this optimized single barrier DBD source using unipolar excitation is also discussed in this chapter. The optimized source has been pinched-off at the condition of maximum radiation efficiency and has been used for water purification study in particular for E-coli bacteria deactivation. The complete procedure to stop the DNA growth of the bacteria using direct plasma exposure to the bacteria has also been discussed in this chapter. The conclusion with overall discussion is presented in Chapter 6, which has been arrived from the characterization of the DBDs for measuring electrical and plasma parameters and optimization of the DBD for societal applications. The future scope of the thesis is also presented in this concluding chapter.

2

+sEdddEgEdg =1-sEsdddgdd