PERFORMANCE ANALYSIS OF A DIELECTRIC … · Geometri-geometri elektrod seperti jumlah elektrod...

PERFORMANCE ANALYSIS OF A DIELECTRIC BARRIER DISCHARGE (DBD) PLASMA ACTUATOR

LAI KOON CHUN

A thesis submitted in fulfillment of requirements for the degree of Master of Engineering

Faculty of Engineering UNIVERSITI MALAYSIA SARA W AK

2009

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ABSTRACT

\ Dielectric Barrier Discharge (DBD) is a form of gas discharge by inserting

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dielectric layers into the discharge regimes) (t is a non-thermal discharge under

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atmospheric pressure and can generate low-temperature plasma in the air) ([he plasma

actuator which can generate the surface discharge has been designed and fabricate~l [he

principles related to plasma formation process, electrical performances, and the

unavoidable material degradation phenomenon were investigated) I

The plasma characteristics were analysed by images captured with a high-speed

thermal infrared (lR) camera, by studying the operation cycles and the self-organizing

of the microdischarges. The plasma on voltage, which measured the minimum required

voltages for generating surface discharges and thus electrohydrodynamic (EHD) airflow,

was varied with the dielectric thickness. The operation cycles for plasma formation and

deformation process were dissimilar. The plasma which was in non-thermal-equilibrium

stage was investigated. A suggestion was thus made in order to maximize the velocity

of the induced EHD flow for aerodynamic applications.

The electrical performances in terms of electrical limit and discharge power

were studied. Scaling laws for discharge power were elucidated. The relationship

between the maximum electric field and the total discharge-free surface area on the

plasma actuators was investigated. Electrode geometries, e.g. total electrodes of an

actuator were found to have little effect on the generated discharge power. This

discharge power, however, was mainly affected by the driving frequency and voltage as

well as the dielectric geometries.

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The degradation of the dielectric was observed after the plasma operation.

Effects of the operation period, magnitude of the driving voltage and electrode

geometry on the degradation severity were discussed. Overall, the driving voltage to the

plasma actuators was found to be the dominant factor and an improved actuator design

was suggested to reduce the undesired damages on the plasma actuator surfaces.

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ABSTRAK

Dielectric Barrier Discharge (DBD) merupakan satu pelepasan cas dalam

bentuk gas dengan memasukkan lapisan dielektrik ke dalam rejim-rejim nyahcas. Ia

adalah satu pelepasan bukan terma di bawah tekanan atmosfera dan dapat menghasilkan

plasma yang bersuhu rendah di udara. Plasma aktuator yang dapat menghasilkan

pelepasan cas pada permukaan telah direkabentuk dan siap dibuat. Pelbagai prinsip yang

berkait dengan proses pembentukan plasma, prestasi-prestasi elektrik dan fenomena

degradasi bahan yang tidak dapat dielakkan telah diselidik.

Ciri-ciri plasma telah diperinci dengan gambar-gambar yang diambil daripada

satu kamera terma inframerah (IR) yang berpecutan tinggi, dengan menyiasat kitaran

operasi dan proses penganjuran diri oleh microdischarges. Plasma on voltage, yang

mengukur voltan minimum diperlukan bagi menghasilkan pelepasan cas pada

permukaan dan electrohydrodynamic (ERD) ali ran udara, adalah bervariasi dengan

ketebalan lapisan dielektrik. Kitaran-kitaran operasi bagi proses formasi dan deformasi

adalah berbeza. Plasma yang berada di peringkat bukan terma keseimbangan telah

disiasat. Satu cadangan telah dibuat sedemikian demi memaksimakan halaju hasil aliran

udara ERD itu dalam aplikasi-aplikasi aerodinamik.

Prestasi-prestasi elektrik bagi soal had elektrik dan kuasa pelepasan cas telah

dipelajari. Penskalaan hukum-hukum untuk kuasa pelepasan cas telah dijelaskan.

Rubungan antara medan elektrik maksimum dan jumlah luas permukaan bebas dari

pengeluaran cas pada plasma aktuator telah disiasat. Geometri-geometri elektrod seperti

jumlah elektrod dalam satu aktuator, didapati tidak mempunyai kesan besar pada hasilan

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kuasa. Kuasa pelepasan cas tersebut adalah dipengaruhi terutama oleh frekuensi dan

voltan serta geometri-geometri dielektrik.

Degradasi pada lapisan dielektrik dapat diperhatikan setelah operasi plasma.

Kesan-kesan bagi tempoh operasi, kekuatan voltan dan geometri elektrod pada

fenomena degradasi telah dibincangkan. Secara keseluruhan, voltan yang dibekalkan

kepada plasma aktuator merupakan faktor yang dominan dan satu reka bentuk aktuator

yang lebih baik telah dicadang demi mengurangkan kerosakan yang tidak diingini pada

permukaan-permukaan plasma aktuator.

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TABLE OF CONTENTS

TITLE

ACKNOWLEDGEMENT

ABSTRACT 11

LIST OF TABLES Xlll

ABSTRAK IV

TABLE OF CONTENTS VI

LIST OF FIGURES x

ABBREVIA TIONS XIV

NOMENCLATURES xv

CHAPTER 1: INTRODUCTION AND LITERATURE REVIEW

1.1 Research Background

1.2 Problem Statements 3

1.3 Objectives of the Thesis 5

1.4 Outline of the Thesis 5

1.5 Literature Review 6

1.5.1 Fundamental Theory of Gas Discharges 6

1.5.2 Type of Gas Discharge 7

1.5.2.1 Low Pressure Gas Discharge 7

1.5.2.2 High Pressure Gas Discharge 11

1.5.3 Plasma Actuator 12

1.5.4 DBD Plasma Actuator and Its Typical Designs 13

1.5.4.1 Parallel Plate Actuator 14

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1.5.4.2 Coplanar plate Actuator 15

1.5.5 Glow Discharge Regime in DBD 16

1.5.6 Basic Principles ofDBD Plasma Actuator and EHD Force 18

1.5.7 Ignition Voltage 22

1.5.8 Non-Equilibrium Plasma Thermology 23

1.5.9 Potential Performance Influencing Factors 24

1.5.9.1 Voltage and Frequency Applied 25

1.5.9.2 Dielectric Thickness and Permittivity 26

1.5.9.3 Dielectric Material and Heating 27

1.5.9.4 Electrode Gap 28

1.5.10 Variation Laws 30

1.6 Summary 30

CHAPTER 2: METHODOLOGY

2.1 Introduction 34

2.2 Experiment Setup 34

2.2.1 High Voltage Power Amplifier 35

2.2.2 Digital Oscilloscope 35

2.2.3 Infrared (IR) Camera 36

2.2.4 Environment 36

2.3 DBD Plasma Actuator Designs 37

2.3.1 Designing Rules and Parameters 41

2.3.1.1 Electrode Gap 41

2.3.1.2 Electrode Width 44

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2.3.1.3 Electrodes Amount 46

2.3.1.4 Dielectric Thickness 47

2.3.1.5 Dielectric Materials 48

2.3.2 Available Free Surface Area 49

2.3.3 Fabrication ofDBD Plasma Actuator 50

2.4 Discharge Power Measurement Techniques 51

2.5 Summary 54

CHAPTER 3: RESULTS AND DISCUSSIONS

3.1 Plasma Formation 56

3.1.1 Glow Discharge Emission 56

3.1.2 Determining Plasma On Voltage (POV) 57

3.1.3 Thermal Imaging Analysis 59

3.1.3.1 Formation and Deformation Cycles 61

3.1.3.2 Thermophoresis of Non-Equilibrium Plasma 63

3.1.4 Discharge Formation and Expansion Phases 67

3.1.5 Summary 69

3.2 Electrical Performances 70

3.2.1 Electrical Limit, VMAX 72

3.2.1.1 Applied Voltage, V and Operating Frequency, f 72

3.2.1.2 Electrode Gap, <I> 73

3.2.1.3 Total Electrodes, n and Electrode Width, a 75

3.2.1.4 Dielectric Thickness, t and Constant, G,. 78

3.2.2 Discharge Power, P Dependence on Actuator Geometry 80

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3.2.2.1 Applied Voltage, V and Operating Frequency, f

3.2.2.2 The Effects of Electrode Geometry

3.2.2.3 The Effects ofDielectric Geometry

3.2.3 Summary

3.3 Material Degradation

3.3.1 Degradation

3.3.2 Parametric Study

3.3.2.1 Effect of Operation Time

3.3.2.2 Effect of Voltage Magnitude

3.3.2.3 Effect of Electrode Geometry

3.3.3 Summary

CHAPTER 4: CONCLUSIONS AND FURTHER WORK

4.1 Introduction

4.2 Conclusions

4.3 Further Work

References

Appendices

Appendix A

Appendix B

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LIST OF FIGURES

Figure 1.1: Schematic representation of two dielectric barrier discharges: (a) 2 Volume DBD and (b) Surface DBD

Fif,TUre 1.2: Typical SDRD plasma actuator configurations 3

Figure 1.3: Potential optimization parameters of a DBD plasma actuator 4

Figure 1.4: The voltage-current characteristic between parallel plate 8 electrodes in a low pressure environment

Figure 1.5: Parallel plate DBD actuator 14

Figure 1.6: Coplanar plate DBD actuator 15

Figure 1.7: Asymmetric coplanar plate DBD actuator 16

Figure 1.8: Various sub-regimes in glow discharge regime 16

Figure 1.9: Surface DBD with asymmetric electrode configuration 19

Figure 1.10: Spatial distribution of the ion density (grey levels) and electric 20 potential (contours) at three different times when the cathode is below the dielectric layer

Figure 1.11: Time integrated force per unit volume, F, parallel to the 21 dielectric surface for (a) cathode is below the dielectric (b) cathode is above the dielectric

Figure 1.12: Charge composition and surface discharge of the plasma actuator 23

Figure 1.13: AC applied voltage and discharge current versus time 25

Figure 1.14: Spatial distribution of the time averaged force parallel to the 27 dielectric surface)

Figure 1.15: Teflon actuator at various electrode gaps at 7kV 29

Figure 2.1: Schematic diagram of the experiment layout and connections 34

Figure 2.2: Plasma actuator design overview 37

Figure 2.3: Drawing dimensions for each layer 38

Figure 2.4: Drawing dimensions of each layers in stack 40

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Figure 2.5: Drawing dimensions of actuators 40

Figure 2.6: Dimensional parameters of plasma actuators 42

Figure 2.7: Eight different gapping designs 43

Figure 2.8: Four different electrode width designs 45

Figure 2.9: Five different designs varied by electrodes amount 46

Figure 2.10: Image of finished fabrication 51

Figure 2.11: Schematic diagram of the experiment layout 52

Figure 2.12: Typical Lissajous pattern 52

Figure 2.13: Lissajous pattern used in this research 53

Figure 3.1: Glow discharge emission on both surfaces of the plasma actuator 57

Figure 3.2: The formation of plasma inspected by IR camera 60

Figure 3.3: The deformation of plasma inspected by IR camera 60

Figure 3.4: The formation cycle inspection in lOs interval 62

Figure 3.5: The deformation cycle inspection in 20s interval 62

Figure 3.6: Temperature distribution at 7.5 kV at 0.5 kHz 63

Figure 3.7: Temperature distribution at 13.2 kV at 0.5 kHz 65

Figure 3.8: High-angle thermal image 66

Figure 3.9: A suggestion to maximize the induced EHD flow velocity of the 67 plasma actuator by operating it inside an opened box

Figure 3.10: Plasma propagation observed under a fully-ionized stage 68

Figure 3.11: Plasma propagation observed under a partially-ionized stage 68

Figure 3.12: The voltage waveform distorts when the electrical limit is 71 reached

Figure 3.13: Relationship between maximum withstand voltage and operating 73 frequency

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Figure 3.14: Relationship between maximum withstand voltage and electrode 74 gap

Figure 3.15: Relationship between maximum withstand voltage and total 76 electrodes

Figure 3.16: Relationship between maximum withstand voltage and electrode 77 width

Figure 3.17: Relationship between maximum withstand voltage and dielectric 78 thickness

Figure 3.18: Relationship between maximum withstand voltage and dielectric 79 constant

Figure 3.19: Power response of the supplied voltage 80

Figure 3.20: Power response of the operating frequency 81

Figure 3.21: Power response of the electrode gap 82

Figure 3.22: Power response of the total electrodes 83

Figure 3.23: Power response of the electrode width 83

Figure 3.24: Power response of the dielectric thickness 84

Figure 3.25: Power response of the dielectric constant 84

Figure 3.26: Final image (a) before (b) after operation for 2 hours 88

Figure 3.27: Electron micrograph of scratches at electrode tips caused by CD 89 process

Figure 3.28: Observations for different operation periods (a) 30 minutes (b) 2 90 hours (c) 4 hours, (d), (e), (f) were the corresponding close up view images

Figure 3.29: Observation for panel operated with (a) 9 kV for 2 hours (b) 9 92 kV for 4 hours (c) 12 kV for 2 hours, (d), (e), (f) were the corresponding close up VIew Images

Figure 3.30: Different gap designs (a) 2mm (b) 4mm (c) 5mm, (d), (e), (f) 93 were the corresponding close up view images

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LIST OF TABLES

Table 1.1: Characteristics ofvarious dielectric materials 28

Table 1.2: Summary of review on development of DBD 32

Table 1.3: Summary ofreview on electrical performance affecting parameters 33

Table 2.1: Detailed configurations of eight gapping designs 44

Table 2.2: Detailed parameters for four different width designs 45

Table 2.3: Detailed parameters for five different designs varied by electrodes 47 amount

Table 2.4: Detailed parameters for two different PCB materials 48

Table 3.1: Plasma actuators and the required POV s at 0.5 kHz 58

Table 3.2: Various electrode gapping designs and the available free surface 75 areas

Table 3.3: Electrodes amount and the available free surface areas 77

Table 3.4: Various electrode width designs and the available free surface 78 areas

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ABBREVIATIONS

Notation Description

AC Alternative current CCD Charge coupled device CD Corona discharge

DBD Dielectric barrier discharge DC Direct current

EHD Electrohydrodynamic EM Electromagnetic IR Infrared

OSP Organic solderability preservatives PCB Printed circuit board POV Plasma on voltage RF Radio-frequency

SDBD Surface dielectric barrier discharge SEM Scanning electron microscope

VDBD Volume dielectric barrier discharge

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NOMENCLATURES

Notation Description Unit C Capacitance F (Farad) E Dielectric strength kV/mm F Time integrated force per unit volume Ns/m3

f Frequency applied kHz .Ii) Threshold frequency kHz I Current A

LA Left margin of top actuator mm LB Left margin of bottom actuator mm m Constant ­n Total of electrodes ­p Pressure Torr P Time-average discharge power W (Watt) PD Dielectric dissipation power W (Watt) Q Charge C (Columns) r Ratio ­R Resistance n

RA Right margin of top actuator mm RB Right margin of bottom actuator mm

Sene Enclosed area of Lissajous Pattern J (Joule)/F t Dielectric thickness mm T Time cycle I Period s

tan (5 Dielectric loss tangent ­v Flow velocity mls V Supplied voltage (peak-to-peak) kV Vc Voltage across capacitor V

VMAX Maximum withstand voltage kV Wp Width of actuator plates mm

Greek a Electrode width mm

Electrode spacing mmf3 (5 Dielectric loss angle -Eo Free space permittivity F/m

Er Dielectric constant I Relative permittivity -Es Dielectric permittivity F/m

11: Pi ­Left gap I Electrode gap mm~ Right gap mm\I'

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CHAPTER!

INTRODUCTION AND LITERATURE REVIEW

1.1 RESEARCH BACKGROUND

For decades, plasma science is a field of growing interest. Plasma, which defines

the ionized state of gases, has become increasingly important for various industrial

applications. It represents the source of highest continuously maintainable temperature

(Chang et ai., 1995). Plasma can be formed by the voltage differences between the

electrodes, through a succession of microdischarges that are randomly distributed in

time and space (Moreau, 2007). The electric field will then direct the charged particles

in the plasma to transfer momentum to the surrounding neutral non-ionized air. Most of

these momentum transfers occur as a result of particle collisions and ionic wind which

is in general utilized for aerodynamic applications can be induced consequently. In this

study, one of the gas discharges, the dielectric barrier discharge, is studied on its

fundamental governing aspects with particular emphasis on the generated

e1ectrohydrodynamic (EHD) forces.

Dielectric Barrier Discharge (DBD), also known as silent discharge is a form of

gas discharge by inserting dielectric barriers or layers into the discharge regimes. It is a

non-thermal discharge under atmospheric pressure and can generate low-temperature

plasma in the air. The plasma is typically obtained between two parallel electrodes

separated by a gap of some millimetres and excited by high alternating current (AC)

voltage with a frequency of up to several kilohertz. The dielectric barrier interjected

between both electrodes is designed to stabilize the discharge. DBD has been widely

used for industrial applications (i.e. surface treatment, thin-film coating) and

1

aerodynamic flow control. Two discharge phenomena of DBD can be distinguished,

namely volume DBD and surface DBD (Bogaerts et al., 2002), as shown in Figure 1.1.

electrode

(a) : £wn. dischorg.

. ,- . + dielectric

electrode

(b) s'lllface surf'ac e dis chargeelectrode ,C==:c==":''!~&~"!"=.,e~'·'!:'=.b:=::3+ dielectnc. countere1ectrode

Figure 1.1: Schematic representation of two dielectric barrier discharges: (a) Volume DBD and (b) Surface DBD (Bogaerts et aI., 2002)

Volume DBD (VDBD) can be formed between two parallel plates (Figure 1.1a)

which makes a vertical gap design. One of the electrodes is attached to the dielectric

layer. The microdischarges that happen in this thin channel across the discharge gap are

normally distributed randomly over the surface of the electrodes. The number of

microdischarges per period is proportional to the amplitude of the applied voltage. On

the other hand, surface DBD (SDBD) consists of an electrode on the dielectric layer and

a counter electrode on its reverse side (Figure 1.1 b). There is no clearly defined vertical

gap. The generated microdischarges in this design can be considered homogeneous over

certain distances on the dielectric surface. Any voltage increments will widen the

discharge area on the dielectric layer.

Soon after the generated EHD force has proven effectively to manipulate the

aerodynamic boundary layer and to re-attach the flow to airfoils, SDBD design is

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commonly applied to the construction of DBD plasma actuator due to the limitation to

place the VDBD design on the airfoil. Therefore, the DBD plasma actuator in this study

always refers to the SDBD design. Typically, a DBD plasma actuator consists of two

offset electrodes separated by a dielectric material. The upper electrodes of the actuator

will be connected to a high voltage AC power supply whilst the lower electrodes will be

grounded, as demonstrated by Figure 1.2. There were two general configurations,

namely capsulated design and non-capsulated design. The latter design is analysed in

this study.

GND GND

(a) encapsulated design (b) non-encapsulated design

Figure 1.2: Typical SDBO plasma actuator configurations

1.2 PROBLEM STATEMENTS

DBO plasma actuator has advantages of no movmg parts, performable at

atmospheric conditions and devising complex control strategies through the applied

voltage. However, the mechanism of the momentum coupling between the plasma and

the fluid flow is still inadequately understood (Jayararnan et ai., 2007). Multiple

parameters including electrode arrangement are identified to have impact on the

performance of the plasma actuator. A number of computational research activities and

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attempts are still being carried out to improve the present designs. Therefore, this study

is crucial in order to quantify the EHD force and understand the parameters required to

optimize this force.

Potential parameters m a plasma actuator include the electrode spacmg, the

width of the electrode, the thickness of the dielectric layer, the type of dielectric

material used as well as the electrode gap between the trailing edge of the top electrode

and the leading edge of the bottom electrode (Mohan, 2004). These geometries are

widely used in the applications of plasma actuators due to the geometric flexibility.

Figure 1.3 shows the optimization parameters of a DBD plasma actuator.

Electrodes8ectrode Spacing

/] Dielectric Thickness I

I I I I I I

~ Bectrode Type of Dielectric

I

:<f( ~ 8ectrode Wklth

Electrodes

gap

Figure 1.3: Potential optimization parameters of a DBD plasma actuator (Mohan, 2004)

DBD constitutes lot of non-equilibrium, transient and unstably triggered plasma

filaments, which are generally known as microdischarges (Eliasson and Kogelschatz,

1991). One of the greatest challenges nowadays is to understand the plasma formation

process originated from the microdischarges. In order to enhance the efficiency of the

plasma actuator, it is important to investigate the DBD operating parameters by

examining the principal physical mechanisms.

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The dielectric and the solid-gas interface played a fundamental role in the

plasma discharge mechanisms. When one increases the time-averaged transferred

charge, either by increasing the voltage or the frequency, the charge build-up can raise

so high until the discharge becomes unstable. This was mainly due to the filaments

appeared between different points on the surface where space charge has not been

relaxed as described by Moreau (2007). This study also aims to inspect the maximum

voltage the plasma actuator can withstand before leading it to the discharge instability.

1.3 OBJECTIVES OF THE THESIS

The objectives of the thesis are shown as following:

i) To define the principles of a Dielectric Barrier Discharge (DBD) plasma actuator.

ii) To characterize the electrical performances of DBD plasma actuator by

analyzing the following parameters: dielectric configurations (i.e. dielectric

permittivity and thickness), electrode configurations (i.e. width, gap spacing and

amount) together with the factors from the supplied voltage and frequency.

iii) To study the material degradation on the plasma actuator surfaces.

1.4 OUTLINE OF THE THESIS

This is the first of four chapters, which explains the research background and

purposes of this study. The general design of the DBD plasma actuator as well as the

research problems are briefly introduced in this chapter. The discharge regions, the

breakdown theory and the descriptions of some related formulae are also discussed here.

The approaches or methods required to run the experiment are qualitatively narrated in

Chapter 2. The specifications of experimental equipments and the potential design

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parameters are elucidated in this chapter. In Chapter 3, the mechanisms of plasma

formation and deformation processes are introduced with the aids of a high speed

thermal imaging camera. The required ignition voltage for the actuator to start

generating plasma is investigated.

This chapter also evaluates the electrical performances of the DBD plasma

actuator in relation to its electrical limit and discharge power. After operating for about

2 hours, the degradation marks of the dielectric can be seen on the DBD panel. This

phenomenon is thus studied in the same chapter by examining a number of potential

influencing parameters. Finally, Chapter 4 is devoted to conclusions and

recommendations. The overall findings of this study are summarized and some

recommendations are suggested for future improvement work.

1.5 LITERATURE REVIEW

The study of the processes involving dielectric barrier discharge (DBD) plasma

actuator is presented in this section. It attempts to define and describe the basic

principles behind the processes involving with silent discharge. Experimental results

and findings of others that are useful for the designs of DBD plasma actuator are also

presented. This chapter will begin with general and essential aspects of the gas

discharges.

1.5.1 FUNDAMENTAL THEORY OF GAS DISCHARGES

A gas discharge can be created by directing electrical energy through a gas. To

achieve this it is necessary to create and store considerable amounts of electrical charge.

Studies show that gas discharges consist of partially ionized gas, containing neutral and

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p

both positively and negatively charged particles. In 1928, Langmuir introduced the

word 'plasma' to describe ionized gas that is created in a gas discharge. The basic

definition of plasma is as follows. Plasma is a collection of charged particles

sufficiently dense that space charge effects can result in strongly coherent behaviour

(Uberoi, 1988). In addition, plasma can be also defined as the ionized state of matter

that consists of a quasi-neutral mixture of neutral particles, positive ions, negative ions

and electrons (Chang et al., 1995). These particles have a variety of interactions with

each other as well as with surrounding wall materials and with electric and magnetic

fields present in the discharge. This multitude of particles and interactions makes a gas

discharge a complex system which is still not fully understood.

1.5.2 TYPE OF GAS DISCHARGES

Gas discharge phenomena can be classified typically into two categories, namely

low pressure discharge (i.e. Townsend and arc discharges) and high pressure discharge

(i.e. spark and silent discharges).

1.5.2.1 LOW PRESSURE GAS DISCHARGE

Various discharge modes between parallel plate electrodes in a low-pressure gas

environment are schematically shown in Figure 1.4. These different modes or regimes

can be identified by looking at the voltage-current characteristic of such a discharge.

This subdivision in different discharge regimes is applicable for discharges at low

pressures typically below 100 Torr with parallel metal electrodes.

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~ :f- ·i 0..)

0..)

·i:O..):b 0..)

c: >­- ~ 0..) •

> ~A ·i ! ~ 0..) : -Q) 0..) C) ~ co S 'i

~ :

·i0..) 0..) -> 0 j ·i ~

0..)

: 0..) d: e: ~

~

\ 9 a . . . 10.10 10-8 10'& 10" 10-2 100 102

Current (A)

Figure 1.4: The voltage-current characteristic between parallel plate electrodes in a low pressure environment (Wagenaars, 2006)

There are six different discharge regimes, namely non-self-sustaining discharge,

Townsend discharge, subnormal glow discharge, normal glow discharge, abnormal

glow discharge and arc discharge, which can be identified as follows:

i) Regime I: Non-self-sustaining discharge:

When a low voltage is applied to an electrode gap containing neutral gas, an

extremely small current below 10-10 A can be observed. This is caused by electrons in

the gap created by external sources, for instance cosmic rays or a nearby UV lamp.

These few electrons are accelerated towards the anode and create a very small current.

The applied voltage is not high enough to cause ionization of atoms by electron impact,

which is observed as in higher voltages as will be shown later. Since the discharge

needs external sources for the creation of electrons it is not self-sustaining, it will die

out when the electron source is removed.

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ii) Regime II: Townsend discharge:

The Townsend discharge is also known as dark discharge since there is no

appreciable light emission from such a discharge. Increases in the applied voltage cause

a transition from a non self-sustaining to a self-sustaining discharge. The increasing

voltage results in a higher electric field inside the discharge gap. Electrons in the gap

cause ionization of neutral atoms by electron impact. The result is a multiplication of

electrons and ions in the discharge gap. At the cathode surface, new electrons can be

emitted into the gas by secondary emission caused by ion impact. This provides a

feedback mechanism which sustains a current through the discharge gap. The voltage

marking the transition between a non self-sustaining and a self-sustaining discharge is

known as the breakdown voltage. In the case of a Townsend discharge, the applied

voltage is just above the breakdown voltage and the current is limited to low values by a

large external resistance. The number of charged particles in the discharge gap is limited

which gives no significant space charges effects. The applied potential in the gap is not

disturbed. The voltage-current characteristic for the Townsend discharge (Figures l.4b ­

c) is almost flat. This originates from the fact that a small increase in voltage leads to a

higher electron multiplication in the gap. It produces more electrons, ions and secondary

emission at the cathode. This phenomenon leads to even more electrons in the gap and a

further multiplication of charges. It means that for a small increase in voltage, the

current rises considerably.

iii) Regime III: Subnormal glow discharge:

A further increase of voltage leads to significant space charge effects in the

discharge gap. Since there is a big difference between the mobility of the ions and the

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