a thesis submitted to the college of science / university of baghdad … · 2012-09-23 · college...

Republic of Iraq Ministry of Higher Education

& Scientific Research University of Baghdad College of Science

A Thesis

Submitted to the College of Science / University of Baghdad

In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in Physics

BY

Ala' Fadhil Ahmed Al-Rashidy (B.Sc. 1999) (M.Sc. 2002)

Supervisor

Dr. A. S. Hasaani Asst. Professor

2011 AD 1432 AH

Supervisor Certification

I certify that this thesis was prepared under my supervision at the

Physics Department, College of Science, University of Baghdad, as a partial

fulfillment of the requirements for the degree of doctor of philosophy in

Physics / Plasma

Signature : Name : Dr. A. S. Hasaani (CPhys, MInstP)

Title : Asst. Professor Address : College of Science, University of Baghdad Date : / / 2011 In view of the available recommendations, I forward this thesis for

debate by the examination committee.

Signature : Name : Prof. Dr. Raad M. S. Al-Haddad Title : Chairman of Physics Department Address : College of Science, University of Baghdad Date : / / 2011

Examination Committee Certification

We certify that we have read the thesis entitled " Experimental Study of

Impedance Characteristics in Pulsed Electrical Discharges" and examined the student,

" Ala' Ahmed Al-Rashidy " in its content ,and that in our opinion it is adequate for the

Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in Physics

.

Approval by the Deanship of Department of physics in collage of science, University of Baghdad

Signature:

Name : Dr. Saleh Mahdi Ali Title : Professor Address: Dean of the College of Science University of Baghdad Date: / / 2011

Signature:

Name : Dr.Ali A-K.Hussain Title: Professor Address: Dept. of Physics College of Science University of Baghdad (Chairman)

Signature:

Name: Dr. Dhia H. Al-Amiedy Title: Professor Address: Dept. of Physics College of Science for Women University of Baghdad (Member)

Signature:

Name:Dr. Abdulla M.Suhail Title: Asst. Professor Address: Dept. of Physics College of Science University of Baghdad (Member)

Signature:

Name: Dr. Raad A.Khamis Title: Asst. Professor Address: Laser Unit, Department Of Applied Science University of Technology (Member)

Signature:

Name: Dr.Abdul-Hussain K.Iltaif Title : Chief Researcher Address: Center of Applied Physics, Ministry of Science & Technology (Member)

Signature:

Name: Dr. A.S.Hasaani Title: Asst. Professor Address: Dept. of Physics College of Science University of Baghdad (Supervisor)

DEDICATION

To my grandmother who stood beside me and encouraged

me, I dedicate this effort with love and gratefulness

And

To my first teacher who gave me strength …. My father.

To her who planted love in my heart ………. My mother.

To the symbols of love and fait fullness …. My brothers and sisters

To noble spirit for my dear life partner Dr. Falah and my child Saif

ACKNOWLEDGEMENTS

Thanks to God the Compassionate, the Merciful and my God bestow peace on Prophet Mohammed, member of his family and his followers.

I would like to express my deep gratitude to my supervisors. Dr. A. S. Hasaani, who suggested this project and generously gave guidance throughout this work,.

My gratitude is due to the head and staff of the Physics Department in the College of Science for their assistant and support during the years of my study and research.

I would like to express my deepest gratitude to my colleagues and of the My Department Astronomy head Dr. Kamal M. Abood and staff, especially, Dr. Ali Talib , Sundos Albakry ,Maha Ahmed, Huda Sh. Ali , Raaid N.Hassany and Walid Ibrahim for their patient assistant and wonderful companionship.

I feel grateful to My Uncle Saleh S. Abbas , Abd Alhsain Abass , Eng. Mazin , Eng. Ali, Dr. Qusay Adnan, Fadhil Yousif, Emad Abd Alrazak Mohanad Azawy ,Ahmed Hmeed and Ahmed Kelo for supporting me along the experimental work and the valuable advice. Finally, my thanks go to the members of my family for help and encouragement for various kinds of assistance, and to anyone who helped in one way or another in bringing out this work.

My God bestow health and happiness to all of them.

Ala' Fadhil Al-Rashidy

Abstract A number of pulsed experiments has been carried out using a high-voltage

circuit containing R, L and C in certain arrangements. Three configurations

(spherical, rod-sphere and cylindrically-tipped) of the spark gap electrodes,

which were made of either steel or copper, have been used as a high-current

switch operated by a voltage of up to 8 kV and triggered in both self- and a

third-electrode modes. Current measurements are carried out using both current-

viewing resistors and Rogowski coils designed and constructed for this purpose.

Typical current waveforms have shown obvious dominating inductance

effect of the circuit components in an underdamped oscillation. The behavior of

the circuit impedance is studied by recording both pulsed current peaks and

charging voltages when currents of up to 4.5 kA are recorded. The duration of

these current pulses are found to extend between 0.1 μs and 0.3μs depending on

the values of the circuit components as well as the spacing of the spark gap

electrodes along which the plasma propagates with certain speeds of an average

value of 5.6 m ms-1 at atmospheric pressure. Over the whole range of

experimental conditions, the average nominal impedance values ranged between

2 Ω and 30 Ω depending on the gap and circuit parameters. Typical damage

patterns were observed in the spherical electrodes with average diameters of up

to 8.3 mm on the high voltage electrode and 10.5 mm on the grounded sphere

resulting from accumulative discharges and power dissipation within the gap.

Results of the three spark gap configuration were compared with each other

to establish an optimum conditions and parameters, which were capable of

imposing such features on all experiments.

The application of an external longitudinal magnetic field (27G) on the

cylindrically-tipped electrodes showed a slight change in the discharge voltage

and an alteration in the discharge current density due to physical effects

including lateral diffusion of electrons and their energy over successive

collisions.

IV

Sample Meaning Unite in This Thesis

a Minor mean radius of the coil cm A Constant of the Paschen Law B Magnetic field intensity G b Constant of the Paschen Law d Distance between two electrodes mm dz Elementary length of the solenoid df Damping factor Cb Capacitance of Capacitors bank μF E Electric field intensity kV/mm Eo External electric field intensity kV/mm i Current A Id Discharge current kA i0 The current leaving the cathode A k Boltzmann constant (1.38×10-23 J/K0) Ks Spark constant l Length of the coil cm L Inductance H N Number of turns n Neutral particle density cm-3

ne Electron density cm-3 ni Ion density cm-3 no Number of primary electrons density cm-3 P Gas pressure atm r Coil radius cm R Resistance Ω

Rex External resistance Ω Rsh Current shunt resistance Ω Tg Gas temperature K Te Electron temperature K, eV Ti Ion temperature K

τfwhm Time of full width at the half maxima ns τr Rise time ns t1 Time constant ms, μs

List of symbols and Abbreviations

V

Sample Meaning Unite in This Thesis

tch Charging time s Vi Ionization potential V Vb Breakdown voltage kV Vch Charging voltage kV v Charged particle velocity s/m Z Electric impedance Ω Zs Impedance of the spark gap Ω

ф(t) Instantaneous value of electric flux

λDe Electron Debye length m µ0 Permeability of free space

( m/H104 7-´p )

oe Permittivity of free space (8.85´10-12 F/m)

εg Dielectric strength of the gas α Townsend’s first ionization

coefficient,

γ Townsend’s secondary ionization coefficient

CVR Current viewing resistance PED Pulsed electrical discharge RLC Resistance, inductance and

capacitance

II

Page ITEM I Abstract II List of Contents

IV List of Symbols and Abbreviations CHAPTER ONE : Introduction

1 1.1 Introduction 2 1.2 The Plasma State 2 1.2.1 Types of Plasma 5 1.3 Gas Discharge 6 1.3. The Dc Discharge 8 1.4 Electrical Breakdown Mechanism 9 1.4.1 Townsend Mechanism 12 1.4.2 The Gaseous Electrical Conductor 13 1.4.3 Sparks and Streamers 17 1.5 Paschen’s Law 19 1.6 Atmospheric Pressure Discharges 20 1.6.1 Breakdown at high pressure 21 1.7 Pulsed Electrical Discharge (PED) 23 1.8 Plasma Switching 25 1.9 Damping Conditions of PEDs 26 1.10 The Effect of The Electrical Impedance 27 1.11 Uniform and Non-Uniform Fields 30 1.13 Applications of PED 33 1.14 Literature Review 35 1.15 The Objective of This Thesis

CHAPTER TWO : Diagnostics 36 2.1 Introduction 36 2.2 Plasma Diagnostics 36 2.3 Electrical Measurement 36 2.4 Voltage Measurements 37 2.4.1 Voltage Dividers (Resistive Dividers) 40 2.5 Current Measurements 40 2.5.1 Rogowski Coils 44 2.5.2 Current Viewing Resistance (Shunt) (CVR)

CHAPTER THREE: Pulsed Electrical Discharge Experiments 46 3.1 Introduction 46 3.2 Design of the PED Circuit 48 3.3 Design and Construction of Two Rogowski Coils 49 3.4 Design and Construction RC- Integration Circuit 50 3.5 Earthing System (Design and Installation) 51 3.6 Current-Limiting Resistance

List of Contents

III

52 3.7 Spark Gaps (Plasma Switches) 54 3.8 CVR Designed and Construction 57 3.9 Capacitors Bank 58 3.10 Experimental Inductance 62 3.11 Trigger Circuit (Third Electrode) 63 3.12 Voltage Divider 64 3.13 Commissioning Experiment Setup

CHAPTER FORE : Results Analysis and Discussion 67 4.1 Introduction 67 4.2 Spherical Spark Gap 68 4.2.1 Current-Voltage Characteristics 70 4.2.2 The Performance of the PED Circuit 72 4.2.3 Spatial Behaviors of the Spark Gap Voltage 73 4.2.4 Temporal Characteristics of the Impedance 76 4.2.5 Electrode Erosion Under Pulsed Discharge 78 4.3 Rod - Sphere Spark gap 78 4.3.1 Current-Voltage Characteristics 80 4.3.2 The Performance of the PED Circuit 82 4.3.3 Spatial Behaviors of the Spark Gap Voltage 83 4.3.4 Temporal Characteristics of the Impedance 86 4.4 Cylindrically–Tipped Spark Gap 87 4.4.1 Current-Voltage Characteristics 89 4.4.2 The Performance of the PED Circuit 91 4.4.3 Spatial Behaviors of the Spark Gap Voltage 92 4.4.4 Temporal Characteristics of the Impedance 95 4.5 Comparison of Results of Various Gap Configurations

95 4.5.1 Current-Voltage Characteristics 96 4.5.2 The Performance of the PED Circuit 97 4.5.3 Spatial Behaviors of the Spark Gap Voltage 98 4.5.4 Temporal Characteristics of the Impedance 100 4.6 Magnetic Field Effect on the Cylindrically-Tipped Spark

Gap 102 4.6.1 Current-Voltage Characteristics 104 4.6.2 The Performance of the PED Circuit 105 4.6.3 Magnetic Field Effect on the Impedance 109 4.7 Conclusion 110 4.8 Future Work

REFERENCES 111 References

Chapter One Introduction

1.1 Introduction

Pulsed power technology has been used for a wide range of typical

research facilities in plasma physics and fusion devices. Under certain

conditions and experimental requirements, pulsed high voltages are

applied to various gases to generate plasmas with particular parameters

via electrical discharges of these gases.

Depending upon the energy range level, which can be delivered by

these pulsed discharges, their pulse durations and modes of operation, a

number of research interests has been developed for applications in

medicine [1], environment [2], generation of relativistic electron beams

for high power microwaves [3], lasers [4], and industry [5] .

In order to generate a pulsed voltage, a capacitor or a group of

capacitors charged to a certain voltage level and then discharged into the

ground through an arbitrary load such as a resistance or an electrode

system. Upon discharging the capacitors, a pulse of the returned current

can be monitored and recorded by a current viewing resistance (shunt)

(CVR) or a Rogowski coil.

According to the basic physics of gas discharges, pulsed gas discharges

can be operated along a wide range of physical parameters including operating

pressure of the plasma devices over a variety of discharge modes such as

Townsend, glow, or arcs as discussed later.

In this chapter, a brief description of basic theoretical concepts of plasma

physics and electrical discharges of gases is presented.


1.2 The Plasma State

A plasma is an ionized gas containing charged and neutral species

including some or all of the following: electrons, positive ions, negative

ions, atoms, and molecules [6]. On average a plasma is electrically

neutral, because any charge imbalance would result in electric fields that

would tend to move the charges in such a way as to eliminate the

imbalance. As a result, the density of electrons plus the density of

negative ions will be equal to the density of positively charged ions. An

important parameter of a plasma is the degree of ionization, which is the

fraction of the original neutral species (atoms and/or molecules) which

have become ionized. Plasmas with a degree of ionization much less than

unity are referred to as weakly ionized. In fully ionized plasmas, the

degree of ionization approaches unity, and neutral particles play little or

no role [7].

The present thesis is concerned with a typical non-thermal plasma

generated by nonuniform electric field at atmospheric pressure under

pulsed condition [8].

1.2.1 Types of Plasma

Plasmas can be distinguished into two main groups i.e., high-

temperature or fusion plasmas [9] and low-temperatures or gas discharges

plasma [10]. A typical classification and parameters of different kinds of

plasmas is given in table (1-1). A high temperature plasma implies that all

species (electrons, ions and neutral) are in a thermal equilibrium state.

Low temperature plasma is further subdivided into thermal plasma, also

called quasi-equilibrium plasma, which is in a local thermal equilibrium

state, and non-thermal plasma, also called non-equilibrium plasma or cold

plasma [6].


Thermal plasmas are characterized by an equilibrium or near equality

between electrons, ions and neutrals. Commonly thermal plasma is

generated in typical devices such as plasma torches, and microwave

devices. These sources produce a high flux of heat and are mainly used in

areas such as in plasma material processing and plasma treatment of

pollution. High temperature of thermal plasmas can be exploited in many

environment applications such as waste material treatment [11].

Table (1-1) : Types of Plasma [11].

Cold plasmas refer to the plasmas where most of the electrical energy

is primarily coupled to the electron component thereby producing

energetic electrons instead of heating the entire gas stream; while the

plasma ions and neutral components remain at or near room temperature.

Because the ions and neutrals remain relatively cold, this characteristic

feature provides the possibility of using cold plasmas for low temperature

Plasma State Example High temperature

plasma (Equilibrium

plasma)

Te ≈ Ti ≈ Tg , Te=106 – 108K ne ≥ 1020 m-3

Laser fusion plasma

Low temperature plasma

Thermal plasma (Quasi-equilibrium

plasma)

Te ≈ Ti ≈ Tg ≤

2×104K

ne ≥ 1020 m-3

Arc plasma, plasma torches, RF inductively coupled

discharges

Non-thermal plasma

(Non-equilibrium plasma)

Te >> Ti ≈ Tg = 300….103 K ne ≈ 1010 m-3

Glow, corona, atmospheric pressure plasma jet, dielectric

barrier discharges, micro hollow cathode discharges,

plasma needle etc


laboratory plasma physics, plasma chemistry and for the treatment of heat

sensitive materials including polymers and biological tissues. The

remarkable characteristic features of cold plasma that include a strong

thermodynamic non equilibrium nature, low gas temperature, presence of

reactive chemical species and high selectivity offer a tremendous

potential to utilize these cold plasma sources in a wide range of

applications.[11,12].

Figure (1-1) identifies different kinds of plasmas on a log ne (electron

density) versus logTe (Temperature of electron) diagram. There is an

enormous range of densities and temperatures for both laboratory and

space plasmas. Two important types of processing discharges are

indicated on the figure. Low-pressure discharges are characterized by Te ,

Ti << Te ≈ 1-10 eV, and ne = 108-1013 cm-3 [13]. This values of ne and Te

are usually implied in the calculation of Debye screening length of each

plasma kinds as follows [12].

λDe=2/1

2e

eo

enkT

÷÷ø

öççè

æ e…………………(1-1)

where ne is the density of the electrons, k is Boltzmann constant and, Oe

is the permittivity of free space .


Figure (1-1) Space and laboratory plasmas on a log ne versus log Te

diagram [13]

1.3 Gas Discharge

The term "gas discharge" typically originates with the process of

discharge of a capacitor into a circuit incorporating a gap between

electrodes. If the voltage is sufficiently high, electric breakdown occurs in

the gas and then an ionization forms. The circuit is closed and the

capacitor discharges. Later the term "discharge" was applied to any flow

of electric current through an ionized gas, and to any process of

ionization of the gas by the applied electric field. As gases ionize to a

sufficient degree, they emit energy in the form of light [10].


The mode of the electric field can be AC, DC, or pulsed depending

on the required experimental interests and applications as will be seen

below.

1.3.1 The Dc Discharge

An electrical discharge across an electrode gap can either be partial

breakdown, where corona effect is observed where the electrical field is

the highest, or a breakdown. Refer to figure (1-2) below. For an electrode

gap with no external voltage supplied, there will be a background

ionization in the air due to cosmic rays and radiation. Close to Earth’s

surface, there are approximately 1000 ion-electron pairs per cubic

centimeter. If the gap voltage is slightly increased to maybe a few tens of

volts, a very small amount of current will flow. This is because the free

electrons will drift in the air towards the anode before they can

recombine. Further voltage increase will produce no more current. The

current will be saturated because the rate of ionization in the air is

constant. The current can only be sustained by an external ionizing

mechanism. For this reason, the current is said to be non-self sustaining.

More electrons need to be ionized to get an increase in current [14, 15].

As voltage is increased beyond the saturation regime, there will be an

exponential rise in current, and it will be approximately micro-amps. This

is known as the Townsend regime. More increase in voltage will cause a

corona discharge, where the electrode surfaces may glow at the areas of

highest electric field. This effect is sometime observable on high voltage

power transmission lines and is generally not desirable since it represents

a power loss. In the corona region, there is a point at which a further

increase in voltage will cause what is referred to as a breakdown. The

voltage across the gap will suddenly drop and a larger current, on the

order of 1mA, will flow. Conductive, electrically neutral, plasma will


form between the electrodes and this region is known as the glow

discharge region. The first part of the region is normal glow.

Figure (1-2): Voltage-current relationship for gaseous dc discharge [16]

The small variation in voltage in this region will produce a large

change in current because the “cathode fall” will tend to regulate the

voltage to a constant value in a stabilization mechanism.

At the glow discharge region, if the voltage is further increased, the

discharge will enter the abnormal glow region. Here, the current increase

is not exponential and requires a considerable increase in voltage. This is


because the plasma starts to cover the entire cathode, so the plasma is not

restricted to just the gap in between the electrodes [17]. The voltage may

increase up to a point where if the applied voltage is increased any more,

the gap voltage will abruptly fall to a very small level and a highly

conductive arc will form across the channel. The voltage across the gap

becomes very low as the current reaches into amperes and even hundreds

or thousands of amperes, depending on the power supply output

capability and the value of the current limiting resistor [14, 16].

At higher pressures, the discharge takes apparently different forms

for different parts of the characteristic V−I curve. Corona discharges are

equivalent in certain of their aspects to the Townsend discharges and

spark may replace it depending on the circuit condition. The arc is still

the ultimate form of discharge if the external circuit is capable of

sustaining it. The means by which this state is reached in a gas at ~ 1 atm.

is not always clear. The glow to arc transitions, and sparks as preliminary

stages of the discharge have not yet been fully assessed [36]. However,

more research studies are extensively required for understanding the

physics of these transitional stages.

1.4 Electrical Breakdown Mechanism

Electrical breakdown is a colloquial term used to describe the

process by which a non-conducting medium such as a gas becomes

conductive through the application of a sufficiently strong electric field.

There exist many comprehensive literature sources describing the

characteristics of electrical breakdown mechanisms [10,14,18]. The

mechanisms leading to gaseous breakdown, studied by Townsend, will be

discussed in the proceeding section [19].


1.4.1 Townsend Mechanism

A state of equilibrium exists in an ordinary gas between electron and

positive ion rate of generation and loss. However, when an external

electric field is applied, this equilibrium is altered. Townsend firstly

studied the current generated in gases between two parallel electrodes

[21].

The I-V characteristic curve for an ordinary gas between parallel plate

electrodes is shown in figure (1-2). As the gap voltage increases from zero

to V1, the current increases linearly. For a gap voltage between V1 and V2

the current remains constant at a value I0. This current Io, is known as the

saturation current and is the current generated when the cathode is

irradiated by a sufficient amount of energy.

Above a voltage V2, the electrons leaving the cathode are accelerated

to a certain level enough to cause ionization by collision with gas

molecules. Townsend defined the number of electrons produced per unit

length as the quantity α, known as the Townsend's first ionization

coefficient the incremental increase of electrons is given as [20]

dn = αn dx ……………………(1-2)

where n is the number of electrons at a distance x away from the cathode.

Integrating this equation over the distance, d, from cathode to anode gives

d0enn a= …………… (1-3)

where no is the number of primary electrons generated at the cathode. In

terms of current at the anode

d0eii a= …………… (1-4)


Figure (1-3): Current vs. voltage relationship developed by Townsend

[20].

where i0 is the current leaving the cathode.

The ionization coefficient is actually dependent on the electron

energy distribution in a gas, which depends only on E/P, where E is the

applied electric field and P is the gas pressure. Therefore can be written as

÷øö

çèæ=

aPEf

P ………………. (1-5)

This dependence between α/P and E/P has been confirmed

experimentally [22].

A number of other secondary processes contribute to the breakdown

process. Some of these include secondary electrons produced at the

cathode by positive ion impact, secondary electron emission at the

cathode by photons, and ion impact ionization of the gas. In order to

account for these processes, the Townsend second ionization coefficient,

γ, is introduced. The steady state current, equation (1-4), accounting for

both Townsend coefficients, can be rewritten as [23]:


)1e(1eii d

d

0 -g-= a

a

…………….(1-6)

Experimental values for γ have been determined from equation (1-6)

for known values of E, P, gap distance, and α. Values for γ are highly

dependent on cathode surface properties. Low work function materials

will produce greater emissions. The value of γ is small at low values of

E/P and higher at greater values of E/P. This is to be expected since at

high values of E/P there will be a greater number of positive ions and

photons with energies high enough to eject electrons from the cathode

[20].

)1e(1

eii)

pdv(f)pd(

)pdv(f)pd(

0

-g-

= ………………. (1-7)

As the gap voltage increases, the electrode current at the anode

increases according to equation (1-6). The current will increase until at

some point the denominator of equation. (1- 6) becomes zero, or

1)1e( d =-g a ……………………… (1-8)

At this point, equation (1-6) predicts that the electrode current

becomes infinite. This is defined as the transition from self-sustained

discharge to breakdown.

Theoretically, the value of the current becomes infinite, but in

practice it is limited by the external circuit and voltage drop across the

gap. A self-sustaining discharge occurs when the number of ion pairs

produced in the gap by the passage of one electron avalanche is large

enough that the resulting positive ions, on bombarding the cathode, are

able to release one secondary electron and cause a repetition of the


avalanche process. The discharge may also be self-sustaining as a result of

the secondary electron photoemission process [20].

1.4.2 The Gaseous Electrical Conductor

In metals, there exist large concentrations of free electrons, so that

the application of small voltages causes considerable current flow with

only a minimum resistance being imposed by the metallic atom lattice. A

gas, normally accepted as electrically insulating is from one aspect not

dissimilar to a metal it always contains a small number of free electrons,

which by the application of a voltage can be caused to flow through the

gas the flow being impeded by collisions with the neutral gas atoms as in

figure (1-4)a. If the current is restricted only to the free electrons

available in the gas, it is negligible, and the gas is an insulator. If a

sufficiently high electrostatic field exists between the electrodes, the free

electrons can attain high kinetic energies. These high-energy electrons

colliding with neutral gas atoms can be caused a splitting of the electronic

structure of the atoms and hence produce further free electrons and new

positive ions, as shown in figure (1-4) b. This type of process is

cumulative, producing an electron avalanche and finally a spark

discharge. The gas under these conditions is electrically conductive and

can quite easily be maintained in a state of stable conduction. It is

possible also with varied gas conditions and current values to obtain

widely different forms of conduction, typified for example by the well-

known spark, glow and arc discharges.

The properties of these gaseous conductors differ considerably form

those of metallic conductors. The impedance of metal, for example, is

constant over a wide current range, while the impedance in a gas depends

upon the type of discharge, and in general, it decreases markedly with


increase in current. Many other unique discharge properties exist, which

are of importance in relation to arc interruption [24].

Figure (1-4) Electron conduction in a gas (a) Electron- atom collision

with no ionization. (b) Electron- atom collision with ionization and

production of electron avalanche [24]

1.4.3 Sparks and Streamers

Since the plasma created during the electrical discharge mechanism

process is a spark, it is worthwhile to describe this type of discharge in

more detail. Note that lightning shows beautiful examples of giant spark

discharges.

The breakdown phenomenon leading to the creation of a spark may

be complicated. The breakdown is too fast to be explained by repetitive

electron avalanches through secondary cathode emission, as in low-

pressure discharges. It consists rather of a very rapid growth of a thin

weakly ionized channel called a streamer, from one electrode to the other

[25].


A streamer is formed from an intensive primary electron avalanche,

starting from the cathode as shown in figure (1-5)a. A space charge field

is associated with this avalanche, due to the polarization of charges inside

it. This electric field increases with the avalanche propagation and

growth. The avalanche has to reach a certain amplification before it can

create a streamer. As soon as the space charge field is comparable or

exceeding the applied external field, a weakly ionized region can be

created due to this amplification of the electric field, the streamer is thus

initiated.

Once the streamer is initiated, it then grows and propagates, following

a zigzag and branched paths due to the random nature of the propagation

mechanism. The speed of this propagation is extremely high, typically

reaching 106 m/s. The propagation can be directed towards both the anode

and the cathode, depending on the gap distance and voltage.

In moderate gaps and with moderate voltages, the avalanche-to-

streamer transition occurs only when the primary avalanche has crossed

the gap and reached the anode. The avalanche has not grown enough and

the space charge field is not high enough to create an ionized region,

before the avalanche has reached the anode. Then, the streamer starts

from the anode and grows towards the cathode. This kind of streamer is

cathode-directed or positive. The streamer growth is caused by secondary

avalanches, created near the positive head as shown in figure (1-5)b.

These secondary avalanches are initiated by electrons that are released by

photo-ionization. The electrons of the secondary avalanches are rapidly

attracted into the streamer, neutralizing the streamer positive head and

leaving behind them the positive ions of the secondary avalanches (ions

move much slower than electrons). These positive charges become the


new head of the extended streamer. This is how the positive streamer

grows. [10].

Figure (1-5) Breakdown mechanisms leading to a spark discharge propagation of: (a) the primary electron avalanche; (b) a positive

streamer; (c) a negative streamer [10]

In large gaps and/or with strong gap voltages, the space charge field of

the primary avalanche can be sufficiently high to create the streamer even

before reaching the anode. Thus, the avalanche-to-streamer transition

occurs in the gap. Then, the streamer propagates towards both electrodes

at the same time. If the avalanche-to streamer transition occurs while the


avalanche has not yet moved far from the cathode, the streamer grows

mostly towards the anode. In this case, the streamer is called anode-

directed or negative. The growth mechanism towards the cathode remains

the same as described above, but the growth towards the anode is slightly

different. Here, the electrons of the primary avalanche form a negative

head for the streamer. These electrons rapidly neutralize the positive ions

of secondary avalanches, also initiated near the streamer head by photo-

ionization and by moving electrons as shown in figure (1-5)c. The

electrons of the secondary avalanches then form the new head of the

extended streamer. Thus, for both positive and negative streamers, the

streamer is “feeding” on charges created ahead of its tip by secondary

avalanches.

When the electrode gap is closed by a streamer, the breakdown

phase is completed and the discharge phase begins. The transition from a

weakly ionized channel (the streamer bridging the gap) to a highly

ionized channel (the spark itself) is poorly understood. It is probably

caused by a “back streamer”, similar to the well-known “return stroke” in

lightning discharges [10,25].

If it is assumed that a streamer is perfectly conducting, the head of a

positive streamer, for example, is at the same potential as the anode.

When the streamer head is approaching close to the cathode, all the

potential fall is located over a very short distance, the distance between

the cathode and the streamer head. The electric field is so intense in this

region that electrons are emitted in great number from the cathode and

from atoms near the cathode. Once the gap is closed by the streamer,

these electrons, multiplied at enormous intensity, are accelerated towards

the anode in the initial streamer channel, causing strong ionization. The

formation of the true spark channel probably caused by this back


streamer, which strongly increases the degree of ionization in the original

streamer channel.

The plasma composing the spark channel is highly ionized and

conductive, capable of sustaining a large current (in the order of kilo-

amperes). The spark is accompanied by a cracking sound (the thunder in

the case of lightning), resulting from the shock wave created by the rapid

and localized heating of the gas surrounding the plasma channel. The

channel radially expands with time, because the surrounding gas is

gradually ionized, by heat conduction and by the shock wave.

If the power source is capable of delivering the discharge current

over a certain period of time, the spark will naturally transform into an

arc, since the spark is only a transient process [10].

1.5 Paschen’s Law

Friedrich Paschen, was the first to state in 1889, that the breakdown

voltage of parallel plates in a gas is a function of the product of pressure

and gap distance.The dependence of the probability of ionization on the

number of gas molecules between electrodes has been formally

developed and studied for different gases and is known as Paschen’s law.

Paschen found that breakdown voltage can be written as [26]:

÷÷÷÷÷

ø

ö

ççççç

è

æ

÷÷ø

öççè

æ+

=

g11ln

ln APd

bPdVb (1-9)

or V b=f (Pd) …………….(1-10)

where γ secondary electron coefficient, A and b are constants and

values for various gases can be found in table (1-2). This method of

relating the breakdown voltage as a function of pd is known as Paschen’s


law. It means that the breakdown voltage is a function of the gas pressure

and gap distance.

A Paschen curve for atmospheric air is shown in figure (1-6). Note

that the breakdown voltage goes through a minimum value at a particular

(Pd)min value. This Vbmin can be explained qualitatively. For Pd > (Pd)min,

electrons crossing the gap make more frequent collisions than at (Pd)min,

but the energy gained between collisions is less [23]. This results in a

lower ionization level for a given gap voltage.

Table (1-2): Selected ionization constants and ranges of applicability.

T = 20 ºC [20].

Gas

A ion pairs

cm-1 .Torr-1

b V.cm-1.Torr-1

E/p V.cm-1 .Torr-1

Vi Volts

H2 5 130 150–600 15.4

N2 12 342 100–600 15.5

Air 15 365 100–800 _

CO2 20 466 500–1000 12.6

He 3 34 20–150 24.5

For Pd < (Pd)min, electrons crossing the gap make less frequent collisions

than at (Pd)min. Therefore, (Pd)min corresponds to the highest ionization

frequency depending on the mean free path of electrons and the

probability of ionization [22].

This shows that unless initial electrons are provided, the electrical

breakdown can not occur, because an avalanche can not be started then.

In the case of slowly varying fields, there is usually no difficulty in

finding an initiatory electron from natural sources, i.e., cosmic rays,

detachment from gaseous ions, etc.


Figure (1-6): Measured and calculated Paschen curves for air [27]

1.6 Atmospheric Pressure Discharges

Atmospheric pressure gas discharges have received an increasing

amount of attention both from academic groups and companies. Low

pressure plasma reactors are widely utilized for surface treatment of

polymers. The possibility to use plasma sources at higher pressure is of

great interest. Despite their tendency to turn into hot filaments and to be

more difficult to control than low pressure discharges. Atmospheric

pressure plasmas present many advantages for industrial applications [28,

29]:-

• Existence of effective particle density

• Absence of expensive and power consuming vacuum systems

• Absence of pumping down time

• Absence of vacuum compatibility issues

These four factors drastically reduce all the operational costs of

manufacturers. Some atmospheric pressure reactors are open to the


ambient air for surface treatment. The plasma is not turned off between

each sample making the treatment process even faster and cheaper. There

are many applications that benefit from the possibility of stable

atmospheric pressure plasmas [6,30]. For instance, depollution,

sterilization, medical interventions, and more recently, flow control or

combustion control.

Different geometrical configurations and excitation schemes can be

utilized to obtain an atmospheric pressure discharge in various gases.

Many studies have been made about Dielectric Barrier Discharges, corona

discharges, plasma jets, pulsed electrical discharges and micro discharges

[25, 31, 32].

1.6.1 Breakdown Voltage of Air

The breakdown in air (spark breakdown) is the transition of a non-

sustaining discharge into a self-sustaining discharge. The buildup of high

currents in a breakdown is due to the ionization in which electrons and

ions are created from neutral atoms or molecules, and their migration to

the anode and cathode respectively leads to high currents. As discussed

earlier, Townsend theory and streamer theory are the present two types of

theories, which explain the mechanism of breakdown under different

conditions as temperature, pressure, nature of electrode surfaces,

electrode field configuration and availability of initial conducting

particles. Normally, air is widely used as an insulating medium in

different electrical power equipments underground and overhead lines as

its breakdown strength is sufficient in such applications.

Typical breakdown voltage of air for a parallel plate electrode

geometry separated by 1cm gap is 30kV [33].


1.7 Pulsed Electrical Discharges (PED)

Pulse power engineering is the science and technology of storing

energy over a relatively long period of time and releasing it in a relatively

short time aiming at increasing the instantaneous power. It is obviously a

pulsed power system includes an energy storage stage, a load, and a pulse

forming stage between these two stages.

Pulsed discharges are used in plasma-technological applications.

Pulsed sources have the following advantages:

• Operation at higher power;

• Additional performance control by a variable duty cycle of active plasma

regime and plasma after glow;

• Variations in the neutral gas composition between the plasma boundary

and the plasma centre (due to plasma chemical reactions) may cause, for

example, inhomogeneous thin film deposition in a continuous dc plasma,

pulsed operation in conjunction with rapid gas exchange between pulses

can prevent or minimize such effects [6, 15].

In addition to its power and energy, pulsed electric field has another

important characteristic that is the shape of pulse, defined by its rise and

fall times, duration, and flatness of its plateau region. Usually, the

duration of a power pulse lies between 10-9 – 10-6 seconds, depending on

the application. The typical pulse shape shown in figure (1-7) may be

characterized by the following temporal parameters:


Figure (1-7) Pulse Shape [34]

Rise Time: The time taken by the voltage to rise from 10% to 90% of its

peak voltage.

Decay Time: The time taken by the voltage to decrease from 90% to

10% of its peak voltage, it is also called fall time.

Pulse Duration: There is no unique definition, sometimes it means the

full time width between rise and decay half maximum of the pulse

(FWHM). However, for some applications, it is defined as the time it

remains at 90% of its maximum value [33].

Energy can be stored in several forms, namely, chemical, mechanical

and electrical. By using appropriate switches, the desired shape, rise and

decay time of the pulse could be achieved. An impedance matching

network may also become necessary for optimal energy transfer to the

load. Electrical energy can be stored either capacitively in an electric field

or inductively in a magnetic field.


1.8 Plasma Switching

Closing plasma switches are “open” naturally and are “closed” with

an application of external trigger or as a result of its own over voltage.

Different high voltage switches [35] have been used as key components

to transfer electrical energy from the storage unit to the load. Plasma

switches are among these switching devices, which can potentially

transport high electrical currents at relatively low power dissipation with

controllable repetition rate. These plasma switches cover thyratrons,

pesudospark, and spark gaps [36-38].

Because of their simple design and construction, low cost, and

capability of current level control, spark gaps have stimulated interests in

a number of research activities [39-44]. In these studies, a number of

operating parameters was investigated including repetition rate, discharge

region of operation, electrode erosion, gas pressure, and damping

conditions of the output signals [39].

Different types of plasma switches operate in their specific pressure

ranges. Figure (1-8) shows the range of operating pressure and voltage for

some of the most common types of plasma switches. These include

thyratrons, pseudospark switches, ignitrons, krytrons, and spark gaps

switches.

Spark gap switches working in high-pressure gas such as air and

nitrogen have been very widely used in high- power pulsed technology.

They are known to permit very large currents (hundreds of kA to MA), to

have a short current rise time of a few nanoseconds. In comparison with

other switches, the main advantages of spark gap switches are a high

voltage, large conducting current, high energy efficiency, low cost [45].

In addition, Paschen curve for air with the fixed gap width of 3 mm is

overlaid as reference to the breakdown voltage of gaseous media. As seen


in figure (1-8), all the plasma switches operate below the Paschen curve.

Above this curve, the normal operating voltage will exceed the

breakdown strength of the gaseous, and will cause unexpected breakdown

events [27, 45].

Figure (1-8) Rang of gas pressures and operating voltage for plasma

switches [27, 34]

Nearly all types of plasma switches are operated on the basic

principle of ionization and breakdown of gases. Under normal conditions

(below its breakdown voltage), gas is an insulator and becomes

conducting when ionized.

At particular gas pressure P and electrode spacing d, the product pd is

known as the sparking parameter and is characterized by the Paschen

curve for each gas and geometry [40]. The mechanism involves the

production of plasma and the propagation of plasma particles between the

electrodes of the spark gap giving rise to a high current flow through the


circuit. Characteristic current-voltage curves can then be deduced from

the output signals after recording the charging voltage of the capacitor or

the energy storage unit. 1.9 Damping Conditions of PEDs

The energy stored in a capacitor bank of C capacitance that charged

to a voltage V0 is 0.5 C V02 joules. When this energy is discharged

through a circuit containing inductance and resistance, the current will be

either a damped periodic function or an aperiodic function depending on

the damping.

A damping factor df is defined in equation (1-11) [46]:-

L4CRd

2

f = ………….(1-11)

If 0 < df < 1 the circuit is underdamped and oscillatory in nature.

If df = 1 the circuit is said to be critically damped, and current will

always be positive.

If df > 1 the circuit is over-damped, and the use of the energy is not as

efficient as in the critically damped or under-damped case .

Figure (1-9) Critically Damped Current


The circuit must be critically-damped is shown in figure (1-9). Such

circuit conditions were chosen to carry out the calibration of Rogowski

coils since the energy is more efficient than the overdamped circuit and

safer than the underdamped case [46, 47].

1.10 The Effect of The Electrical Impedance

The characteristics impedance of the gap depends upon the geometry,

spacing and the gas of interest. One of the main parameters, which

determines the gap impedance is the gap spacing, which basically

determines the Paschen minimum voltage for the gas filling the gap at a

particular pressure. Nevertheless, the current flowing in the circuit can be

described in terms of this impedance and other component in the external

circuitry.

The breakdown voltage of the gas is related to the gas pressure with

in the gab by the flowing [48]:

P

Eg =e …………….(1-12)

where E is the electric field of breakdown, εg dielectric strength of the

gas and p gas pressure.

The impedance of the spark Zs in the spark gap can be written as :

Q

d.KZ s

s = …………… (1-13)

where Ks spark constant, d spark length, Q number of ampere-second

transmitted through the spark.

The speed of the switch, or the voltage pulse rise time, can be

determined by gas pressure and the spark length as [36]

31

s

21

r

EZ

P44=t ……………….. (1-14)


where P is the pressure in atmosphere, E in kV/mm and τr is the rise time

in nsec.

1.11 Uniform and Non-Uniform Fields

All the effects of breakdown discussed can happen for electrodes with

uniform or non-uniform fields. The difference is where the effects are

most likely to happen. Getting an idea of the fields for different electrode

configurations will help predict where a breakdown is most likely to

occur.

Consider two electrodes separated by some distance. Their electric

field lines may look something like that shown in figure (1-10). The

electric field lines are perpendicular to the electrode surfaces and parallel

to each other. They point from the positive plate to the negative plate.

This figure ignores fringing effect that may take place at the edges [49].

Figure (1-10): Electric field lines for parallel plate electrode configuration [49]

Not many real-life situations have electrodes that produce a uniform

electric field. It is more likely that for a practical electrode configuration,

there will be a non-uniform field. A point to point, point to plane and

sphere to sphere will be explained. The electric fields for these electrodes

are not uniform. To get an idea of what the field will look like, computer


simulation was performed [50]. Results obtained for each electrode are

shown in figure (1-11).

In figure (1-11)a, the lines represent the equipotential lines and the

shades of color represent the electric field strength, with blue

corresponding to lower E field and red corresponding to the highest E

field. Notice that the highest E field occurs where the equipotential lines

are most closely spaced. This corresponds to the red region between the

tip of the anode and the surface of the cathode. Sharp points and edges

tend to produce the highest electric fields because the gradient of the

equipotential lines is very high at such areas. One can also notice a little

peaking of E field around the corners of the cathode and anode.

Figure (1-11)b is the simulation for the point-to-point electrode

configuration. Here, the electric field is highest in between the electrode

tips where the equipotential line gradient is the highest. During

experiments, the areas on the electrodes with high fields tend to have the

brightest glow [50].

Figure (1-11)c show the maximum electric field between the sphere

electrodes in the standard sphere gap method. It is observed from the

electric field is maximum in vertical axis where the curvatures of the

sphere electrodes more or less uniform and it is decreasing gradually in

non uniform fields. That is observed that the electric field distribution is

non-uniform for sphere-sphere electrode arrangement [51].


Figure (1-11) Computer simulation of electric fields for (a) - point- plane (b) - point- point (c) - sphere-sphere [50,51]

There are various other types of electrode arrangements and circuits

for used to spark gaps to obtained high voltages and currents such as

sphere-plate, sphere-rod, rod-rod, rod-Plate, and cylinder-cylinder. In this

experimental study used three types of electrodes sphere-sphere, sphere-

rod and cylindrically-tipped.

(a) (b)

(c)


1.12 Applications of PEDs

Several physical phenomena are correlated in PED experiments.

Depending on the physical processes arisen in such experiments,

theoretical studies have been carried out establish reasonable agreements

between theory and experiments to meet the technological applications.

A part from these versatile research activities, the pulsed discharge has

witnessed a wide area of applications .In this section, some of the major

areas which involves the plasma are discussed as follows: -

1- Medical and feeding applications. PED have been used in medical

research such as:- to investigate plasma characteristics of repetitively-

PED in saline solutions used for surgical procedures [1]. Ultra short

electric pulse induced change in cellular dielectric properties [52].

Optically used emission spectroscopy in UV–visible regions for the

diagnostics of atmospheric pressure plasmas and discharges relevant to

bio-medical applications was investigated [53].

To improve the food quality and safety, including extended shelf life,

some of the alternative technologies that have been considered to have

high potential for commercialization are high pressure processing by

used PED. Treatment has gained increasing interest, which is mainly

caused by some attractive advantages in relation to conventional thermal

treatments [54, 55].

2- Material and surface processing;- The insulated surface discharge

treatment under atmospheric pressure operated with high voltage is

investigated [56-60]. Metal release in a stainless steel pulsed electric field

system affect different pulse shapes; theory and experimental met became

plain by Bart Rodenburg [61]. On the other hand, when research work to

the ion beam and plasma technology development for surface can be


achieved modification and post discharge phenomena on surfaces [62,

63].

3- Laser applications :- The use of nanosecond duration repetitively

pulsed discharges has formed the basis for several recent studies of

oxygen-containing plasmas for Plasma induced laser, Electric Discharge

Chemical Oxygen Iodine Laser [64 - 66].

4- Environment and pollution application The increases in the demand

for clean potable water and clean air with stricter environmental

regulations have raised interest in the development of safe water

treatment technologies as well as emission control. Disinfection of water

is highly essential in order to reduce the number of waterborne diseases.

Disinfection methods for potable water, that are in practice today range

from use of chemicals like chlorine to ultraviolet (UV) light. But,

pathogens like Cryptosporidium are resistant to conventional drinking

water disinfectants, including chlorine. The pulsed high voltage

discharges generated inside water, or on the surface initiate a variety of

physical and chemical processes such as the formation of chemically

active species like OH radicals and H2O along with O and H molecules,

UV radiation, ozone formation and shockwaves. Through advanced

oxidation, it is possible to break toxic chemicals without using any

harsher chemical treatment. [67-75].

The use of nonthermal plasmas for the development of efficient,

compact pulsed corona sources for pollution has been investigated [76 -

78]. While a high temperature pulsed corona plasma system for fuel

cleaning [79].

5- Thin film technology the deposited films can be altered and the

processing rate can be obtained under even lower average power. Also


sputtering, plasma etching and thin film technology rate controlled by the

pulse duration of the discharge. [80-83].

The process of electro pulse sintering of ferrous and high speed steel

powder materials by powerful pulse current and external pressure was

investigated [84].

6- High vacuum instrument and requirements in which the vacuum cycle

can be reduced by discharge cleaning [85, 86].

7- The Marx generator is widely used as a high voltage pulse generation

device. [87,88].

8- The dense plasma focus as a high intensity pulsed neutron source [89-

93].

9- Conversion of Methane to Hydrogen via pulsed corona discharge [94,

95]. The use of a pulsed high voltage discharge for removal of organic

compounds in aqueous solution can be reported investigation [96].

10- Generation of relativistic electron beams for high power microwaves.

The production of pulsed high-density electron beam by channel spark

discharge [97].

11- Industrial Application of pulsed power technology was investigated.

[5, 98, 99].

12- Fast closing plasma switching devices [100-103] and pseudo sparks

[37, 104-106] where high current rates (dI/dt) and voltage can be

remarked.


1.13 Literature Review

The following is a summary of the previous experimental and

theoretical work related to the present research in the field of pulsed

electrical discharge.

Both theoretical and experimental studies in the PED may be

classified into some group.

The first group is the high-voltage breakdown in gases at high

pressure (atmosphere) when PED are produced a result of transitions in

the discharge mode [25,31,32, 107-111]. In such studies, various gases

are used under certain pressures to make good understanding of the

breakdown mechanism with the gap voltage collapse. Arguments involve

the major role of the external circuitry on the breakdown mechanism.

The second group of research studies is devoted to the diagnostics

[30, 112-116] of the plasma, the discharge, the electron and ion density

and temperature are measured after the discharge had been established

under the required conditions including gas pressure, electrode geometry

and the nature of the applied voltage in addition to the proposed

technique of diagnostics. While other researches study the characteristics

of the impedance and magnetic effect on the spark gap to investigate the

rise time and pulse duration and other parameters [39,47, 117-123].

The third group is to investigate in nonequilibrium plasmas using

nanosecond duration and high voltage pulses [64-66, 124-131]. The basic

idea is to create large volume ionization in a gas flow by application of

20-50 kV, and (10~100 nsec) duration pulses at a (10-100 kHz) pulse

repetition rate. Between the pulses, the plasma can be sustained, if

necessary, by application of a relatively low voltage, sub-breakdown DC

or RF field. This approach has two distinct advantages.


First, since the ionizing pulse duration, ~10-8 sec, is much shorter than

the characteristic time scale for development of Joule heating/ ionization

instability, ~10-4- 10-3 sec [64,131], which leads to glow-to-arc transition,

stable repetitively pulsed plasmas can be sustained at much higher

pressures and power loadings compared to other types of non-

equilibrium plasmas.

Second, unlike conventional self-sustained discharges, with this

technique, the sustainer voltage can be independently controlled making

it very efficient, as the self-sustainer discharge accounts for as much as

90-95% of the total input power.

The use of nanosecond duration repetitively pulsed discharges has formed

the basis for several recent studies of oxygen-containing plasmas for

plasma assisted combustion, electric discharge chemical oxygen iodine

laser development, and magneto hydrodynamic supersonic flow control

[65,66,127-129,132].

The fourth group is to investigate in plasma switch (spark gap).

Previous experiments [41-44,133,134] were carried out to study the effect

of the breakdown conditions and the plasma characteristics where

breakdown could be preceded by corona stabilization in non-uniform

electric field geometry. Such studies established a correlation scheme

between charged particle density and its governing parameters. Moreover,

these experiments intended to select a method for triggering for a single

and multiple spark gaps. However, electrode surface flashover was found

to be adequate to operate typical spark gaps with certain geometry and

gas pressure.


1.14 The Objective of This Thesis

The prime objective of this research work is to design, construct, and

to test a high voltage circuit consisting of three main components such as

resistance, capacitance, and inductance capable of delivering various

damping conditions.

The effect of the pulse shape on a gas discharge at atmospheric

pressure is studied in terms of the circuit components and other inherently

profound parameters in the discharge itself resulting from the behavior of

the current–voltage characteristic curves. Other prime objective of this work is to investigate the temporal

behavior of the impedance of an atmospheric pressure spark gap with two

electrodes (sphere-sphere, cylindrically-tipped and rod -sphere).

Such experiments can sufficiently demonstrate the behaviour of the

plasma generated between the electrodes of the spark gap where the

electric field is nonuniform and the energy delivered to the collisional

processes is controlled.

Chapter Two Diagnostics

2.1 Introduction

In plasma physics experiments, PEDs impose particular diagnostic

tools for measurements and monitoring as the durations of the physical

processes are of great importance. These processes can be inspected to

some extent by electrical and magnetic measurements in this type of

experiments. Among such methodological techniques are the

measurements of currents and voltages and then the construction of

current-voltage characteristic curves, which establish a subsequent

monitoring of plasma behaviour in the discharge.

In this chapter, an overview of the essentially required tools is

presented for these PED experiments.

2.2 Plasma Diagnostics

In general the plasma of discharges is studied either via direct

electrical measurements of the space of the plasma or indirectly by

spectroscopic observation of the emitted light during the discharging

process. In each case the principles, the techniques and in some times if

not otherwise, the objectives are different [33]. In this research electrical

measurement are used.

2.3 Electrical Measurement

Two basic electrical diagnostics were used in our experiments, in

different configurations. One was for the measurement of the discharge

current and the other was for the voltage measurements. These two

diagnostics are discussed in the following sections.


2.4 Voltage Measurements

The measurement of the high voltage pulse shape and amplitude is of

importance. A high voltage divider is required to reduce the voltage to a

level, which can be measured by the oscilloscope. Each divider used in

measuring such a high voltage is restricted by several considerations,

which include:

1. The voltage divider must present negligible load on the pulse power

supply. This means that the divider must have a very high impedance.

2. The output of the divider is normally connected to the input of the

oscilloscope by a co-axial cable. Matching between the output impedance

of the divider and the characteristic impedance of the cable is required in

order to avoid unwanted reflections.

3. The divider should have a uniform transient response over a wide

range of frequencies so that it produces negligible distortion on the

measured waveform.

There are several well-developed techniques for voltage

measurements. These include [135,136]:

1- Spark gaps.

2- Electrostatic meters.

3- Capacitive dividers.

4- Voltage dividers (Resistive dividers).

5- Mixed RC dividers.

6- Electro- optic effect.

2.4.1 Voltage Dividers (Resistive Dividers)

High voltage resistive dividers are still considered to be the most

appropriate devices for the measurement of fast transient voltages, such


as lightning impulses. In AC or pulse measurements, a number of

problems related to [33, 20]:

1- The residual inductance in any resistance or capacitance element.

2- Stray capacitance: (a) from any section of the divider to the high

voltage lead, (b) from any section of the divider to ground and (c)

sections of the divider.

3- Impedance drop in the connecting lead between the divider and the test

object.

4- Impedance drop in the ground return lead from the divider resulting

from extraneous ground currents flowing in the lead.

5- Oscillations in the divider circuit caused by capacitance from divider

high-voltage terminal to ground and lead inductance.

Probably the biggest problem is associated with stray capacitances.

Ohms law provides a method to reduce high voltage to measurable

quantities, i.e. adequate currents or low precisely measurable voltages.

The simplest method, often used for the low voltage measurements

to extend a voltmeter range, employs an ammeter in series with a resistor

R of sufficiently high value to keep the loading of the high voltage source

as small as possible is shown in figure (2-1)a. The voltage drop across the

meter is neglected, which is usually allowable due to the small terminal

impedance of such instruments. For DC voltage measurements, average

current- indicating instruments such as moving coil meters are used to

give the arithmetic mean value of V according to the equation above.

Fundamentally also the time-dependency V(t) according to equation (2-1)

could be measured by, for instance, an oscilloscope. The difficulties,

however, in treating the resistance R as a pure resistance are limiting this

application.


The main difficulties encountered in this method are related to the

stability of the resistance R [20].

Figure (2-1) Measurement of high DC and AC voltages by means of:

(a) Ammeter in series with resistor R. (b) Voltage divider R1, R2 and

voltmeter of negligible current input. OP: output over voltage

protection [20]

All types of resistors are temperature- dependent and may often

show some voltage-dependency. Such variations are directly proportional

to the voltage measured and impede the accuracy. If the output voltage of

the voltage divider is measured with instruments of negligible current

consumption (i → 0 or 2i/i <<1) as in figure (2-1 (b)), the high voltage

will be computed by:

( ) ( ) ÷÷ø

öççè

æ+=

2

12 R

R1 tVtV ……………. (2-1)


A part from the accuracy of the output voltage measurement (V2 or V2(t)),

the magnitude of the high voltage will be influenced only by the

ratio 21 RR , as both resistors pass the same current 21 ii = [20, 135].

2.5 Current Measurements

In many technical fields and scientific research problems there is

often a need to ascertain the peak value and the waveform of high, rapidly

changing currents, as in discharging energy-storage capacitor bank in

plasma physics, lightning research, and so on. Current peak value may

vary from ten to millions of amperes, with rise times ranging from

nanoseconds to many microseconds. There are many measurement

methods available, like [136];

1- Rogowski Coil.

2- Current viewing resistance (Shunt) (CVR)

3- Hall generators.

The first and second types have been used in this research and will be

discussed as follows:

2.5.1 Rogowski Coils

Rogowski coils are used for measuring an alternating current. They

work by sensing the magnetic field caused by the current without the need

to make an electrical contact with the conductor. These coils have been

used in various forms for detecting and measuring electric current for

decades [46, 137].

They operate on a simple principle. Rogowski coil is placed round the

conductor in a toriodal fashion as shown in figure (2-2) so that the

alternating magnetic field produced by the current induces a voltage in the


coil. The coils are effectively a mutual inductance coupled to the

conductor being measured and the voltage output is proportional to the

rate of change of current. To complete the transducer this voltage is

integrated electronically.

Figure (2-2) Rogowski coil [141] Many of the useful features of Rogowski coils systems result from

their linearity. They have a wide dynamic range in that the same coil can

be used to measure currents ranging from a few milliamperes to several

million amperes and they respond accurately to transient currents which

makes them an excellent choice for use in protection systems and for

measuring current pulses. The following are the Rogowski coils

advantages [138 - 140]:

i) The frequency response of the Rogowski coil sensor is very wide.

ii) There is no conductive coupling between the coil sensors and the high

voltage test circuits. Furthermore, the coil installation does not necessitate

disconnection of the grounding leads of the test objects and therefore

becomes a non-intrusive sensor, which is a very important aspect for on-

site, on-line monitoring.


iii) It has the advantage of possessing high signal to noise ratio with wide

frequency bandwidth.

iv) There is no saturation due to air-cored coil; therefore, it is not

damaged by over current.

v) It has very good linearity due to the absence of magnetic materials.

vi) The Rogowski coil based electrical discharge measurement system is

a low cost solution and can be easily implemented on-site due to its light

weight.

A schematic drawing of the equivalent circuit for such a magnetic

pickup coil, with a simple RC integrating network is shown in figure (2-

3). The circuit equation for this arrangement is [142, 47]

dtiC1iR

dtdiL

dtd t

0ò++=

f…………………(2-2)

where i is the (small) current flowing in the measuring circuit L is the

inductance of the coil , R and C are the resistance and capacitance of the

passive integrating network respectively.

When the impedance of the coil is negligible in comparison with R,

R >> ωL; where is the highest frequency component of ф (t) or i ,

equation (2-3) become

dtiC1iR

dtd t

0ò+=

f …………..(2-3)

and for t << RC , the second term in the right hand side of equation (2-3)

is negligibly small. Therefore, equation (2-2) can be re written as

dtd)

R1(i f

» ……………(2-4)

Since the output voltage of coil (Vc) is given by

dtiC1V

t

0c ò= ……………. (2-5)


From equations (2-4) and (2-5) Vc will written as

RC

)t(Vcf

= ………… (2-6)

Figure (2-3) The equivalent circuit of a Rogowski coil with a passive RC

integrator [142] The flux ф(t) at any time is related to the main current I(t) which flows

in the electrical discharge circuit and produces the magnetic induction by

the following equation [47]

)t(KNI)t( =f ………………..(2-7)

where N is the total number of turns in the Rogowski coil and K is a

proportionality constant depending upon the coil geometry and current

distribution .

Generally K is a measure of the approximate coil sensitivity for which the

DC

SAK 0m= ………………(2-8)

where A = πa2 and S = 2πr ; a being the minor mean radius of wining and

r is the major mean radius of the coil . From equation (2-6) and (2-7), the

coil output voltage Vc is related to the main current I(t) by [47]:

dtdf Vc


)t(IRCKNVc = ……………(2-9)

Since Rogowski coils are used to measure rapidly, time varying

current the factor L/R of the coil is of great importance. When L/R is very

short in comparison with the pulse width of the current to be measured,

idtdi

RL

áá equation (2-9) gives the output voltage of the coil, which can be

written as

dtdiR f

= ………………….(2-10)

The equation means that the current flowing in the coil is proportional

to the flux dtdf . Therefore, the resulting voltage must be integrated to give

an output proportional to the main flowing current to be measured I(t).

Hence, Rogowski coils operated in the differentiating mode require an

integrator after which the integrated output signal will be a measure of the

main pulsed current [138, 136].

2.5.2 Current Viewing Resistance (Shunt) (CVR)

The second important method of measuring the current a plasma

involves measuring the potential drop which results when the current in

question flows through a precisely known resistance. Since the currents

involved are often very large, the resistance must be very small indeed, in

order that the voltage drop be of manageable size. Such low resistance

elements are usually called shunts.

The major problem in the use of shunts results from the fact that the

current to be measured is often rapidly changing, so that the inductive

reactance becomes comparable to or larger than the resistance unless

special precautions are taken. The frequencies and permissible dimensions


are often such that the only way to ensure that R>>Lω is to arrange the

geometry so that none of the magnetic flux produced by the current to be

measured is sensed by the voltage measuring circuit [136].

An example of such a geometry is that shown in figure (2-4). Here the

current to be measured flows through a cylinder of resistive material such

as nichrome foil and returns via a highly conducting coaxial outer

cylinder. The voltage drop along the inner cylinder is sensed by means of

connection made to its ends. In this geometry, the magnetic field produced

by the main current I is entirely confined to the interior region between

the two cylinders, so that there is no dφ/dt in the voltage sensing circuit.

The voltage observed is then simply IR [143].

Figure (2-4) Current Viewing Resistance (CVR) [143]

Chapter Three Pulsed Electrical Discharge Experiments

3.1 Introduction

This chapter is devoted for describing the high-voltage circuit and the

full experimental procedure and measurements. A pulsed high-voltage

circuit has been designed and constructed to include three main

components; inductances, capacitors and resistances. One of the main

objectives of this work is to investigate the temporal behavior of the

impedance of an atmospheric pressure spark gap (spherical, rod-sphere

and cylindrically-tipped,) electrodes at air atmospheric pressure. Analysis

of the discharge current oscillograms under various damping conditions

was carried out as two triggering techniques were followed after being

designed for these experiments. In addition, Rogowski coils have also

been designed to measure the current pulse as well as current shunt

resistors for comparison.

The measurements were rounded to a proper degree of precision

within the capability of the instrumentations providing that pulsed

parameters could not be precisely controlled in terms of reproducibility of

shot-to-shot output signals.

3.2 Design of the PED Circuit

A pulsed electrical discharge circuit is basically designed in which a

capacitor is charged from a DC power supply to a certain voltage through

a resistance and discharged through a spark gap or on appropriate switch.

In this section, the description of the PED circuit design is given.

Figure (3-1) shows a photograph of this system. This circuit illustrates the

following parts.


Figure (3-1): The pulsed electrical discharge circuit setup used in the present work (a) Equivalent circuit (b) Photograph

(a)

(b)

(a)


3.3 Design and Construction of Two Rogowski Coils Two Rogowski coils of insulating material are designed shaped as

toriodal in figure (3-2) and used copper wire to wrap. These coils were

designed, constructed, and calibrated to record the pulsed discharge

currents as shown in chapter two (2.5 paragraph).

Inductance, resistance, measure of approximate coil sensitivity and

time constant of the coil can be calculated using the following relations

(equations 2-10, 3-1, 3-2, 3-3).

l

2o AN

Lm

= …………….. (3-1)

l

l

ALR r

= ……………… (3-2)

RLtl = ..................... (3-3)

Figure (3-2): Photograph of the Rogowski coils (a) First (b) Second

Where Rogowski coils are connected to 50 Ω matching coaxial

impedance and then to input of the oscilloscope to any possible mismatch

in signal propagation

a b


The following table (3-1) contains the measurement parameters of

Rogowski coils.

Table (3-1) The measurement parameters of Rogowski coils.

3-4 Design and Construction RC-Integration Circuit

Two circuits of RC-integration are designed and built, which consist

of resistance with the first value R = 10 kΩ and capacitor value C = 1 μf

and the second value is R = 10 Ω, C = 1 μf as shown in the figure (3-3). It

is both parties are linked to the PNC coaxial cable and connected to

oscilloscope and then the integration process of the signal emerging from

a Rogowski coil starts in order to find the value of the current pulse

passing through the wire. The value of electric current equals the value of

Parameters First Rogowski coil Second Rogowski coil

External diameter

105 mm

50 mm

Internal diameter 31 mm 25 mm

Major mean radius = r 36.5 mm 22.3 mm

Minor mean radius= a 20.9 mm 9.8 mm

Length of the coil = l 22.9 cm 14 cm Number of turns = N 86 turns 84 turns

Inductance = L 55.7 μH 19 μH

Resistance 0.6 Ω 0.3 Ω

Time constant = tl 92 μsec 63 μsec

Coil Sensitivity = K 7.5 ×10-6 2.7×10-6

Length of the wire=L1 9.7 m 4.6 m

Wire diameter 0.6 mm 0.6 mm


the voltage multiplied by a certain constant, as explained in chapter two

(2.2 paragraph).

The time constant is given in the equation (3-4)

t = RC .................(3-4)

which is equal

t1= 10 msec

t2 = 10 μsec

Figure (3-3): RC-integration circuit a- Schematic, b- Fabrication

3.5 Earthing System (Design and Installation)

Earth stick is designed and used for safety and discharge circuit if

there is any residue electric charge in capacitors. In figure (3-4), it consists

of 50 cm of copper rod. The part of the rod is covered with Teflon

insulation material of thick wire 50 cm long and 6 cm in diameter, linked

to thick wire to the earth ground, in delta form.

In all methods of providing protection against dangers associated with

the use of electricity, earthing plays a very important role. Earthing means

making a connection to general mass of earth.

The value of earth ground resistance is verified by using digital earth

resistance tester is shown in figure (3-5).

(a) (b)


Figure (3-4): Earth stick

Figure (3-5): Digital Earth Resistance Tester.

3.6 Current-Limiting Resistance

Current-limiting resistance (charging resistance) is designed to

protect the power supply and the charging process will be gradual. It

consists of thermal resistances of high value of impedance connected in

parallel and series to obtain high sufficient circuit and to avoid any over

heating as shown in the figure (3-6). The value of resistances used is:

R1= 9.1 MΩ, R2= 2.2 MΩ, R3= 40 kΩ

as a result the total value of Current-limiting resistance obtained is :

Rtotal= 541 kΩ.


Figure (3-6): The current-limiting resistance (a) Diagram (b) Construction

3.7 Spark Gaps (Plasma Switches) The high voltage of high power discharge circuits require special

switching devices, commonly gap switches, at most spherical gaps, are

used for switching. The selection of the switch depends upon the peak

current passing through it during the discharge, and the charging voltage

of the capacitor, the breakdown voltage of the sphere gap is less than the

peak value of the supply. Spark gaps or plasma switches were designed in three shapes : -

spherical, cylindrically-tipped and rod-sphere electrodes as shown in

figure (3-7). Practically, a spherical spark gap of steel electrodes was

used, 70 mm in diameter (r = 35mm), that can be adjusted by a screw

threaded to each sphere as shown in figure (3-7)a.

(a)

(b)


Figure (3-7): Spark gap schematic: - a- Spherical b-Cylindrically-

Tipped c- Rod-Sphere electrodes

The cylindrically-tipped spark gap electrode is made of Copper. The

measurement of each of the two electrodes:- the ground electrode internal

(a)

(b)

(c)


and external radius are r1=9 mm, r2 =9.5 mm respectively. The thickness

equals 0.5 mm and the length is 46 mm while the high voltage electrode,

the internal radius is r1 = 9 mm and external radius is r2 = 9.5 mm,

thickness equals 0.5 mm and the length is 76.5 mm. This is shown in

figure (3-7)b.

The third type of the plasma switches is rod- sphere electrodes with

ground electrode is a rod made of steel with curved shape as shown in

figure (3-7)c. Specifications and measurements is r = 7.5 mm, curvature is

20 mm and length is 60 mm. The high voltage electrode is the same

sphere as in figurer (3-7)a.

In either case, it is important that the spark gap (electrodes) should be

so placed that the space between spark gap is free from external electric

fields and from bodies, which may affect the field between the spark gap.

3.8 CVRs Designed and Construction

Three types of CVR is designed and constructed which are used to

measure the amount of voltage and current passing through, as shown in

figure (3-8). They have very little impedance so as to pass most or all of

the power without any current loss.

The value of each resistance is measured during applying certain

voltage and calculated the current value of the return of its resistance

figure (3-9). Hence, the value of each resistance is obtained according to

Ohm's law and the slope is calculated by the following relation :

IVR

DD

= …………….(3-5)

Figure (3-10) shows the relationship between current and voltage in

order to calculate the value of resistance.


The specifications and measurement of each three resistance are as

follows (table (3-2)).

Table (3-2) The specifications and measurement of three types CVR

Parameters First Resistance Second Resistance Third Resistance

Length 14.5 cm 25 cm (30×4) 120 cm

Diameter 25 mm 20mm 15 mm

Resistance 0.2 mΩ 29 mΩ 6.4 mΩ

Thickness 0.5 mm 1.2 mm 0.2 mm

Figure (3-8): Photograph of the Current Shunt Resistance a- First, b- Second c-Third

(a) b) (

c) (


Figure (3-9): Measurment the value of the CVR. (a) ) Equivalent circuit (b) Photograph of the CVR

Figure (3-10) Graph shows the relationship between current and voltage in order to calculate the value of CVR a- First b- Second

c- Third resistance

a

(b)

(a)

b a

c


3.9 Capacitors Bank

High voltage capacitor consists of 16 big size chemical capacitors

type, each one with a length of 60 cm, width 34.5 cm and thickness 12.5

cm as shown in figure (3-11). All are connected in parallel to obtain the

total capacitor Ctotal= 39 μF. Each accepts the maximum voltage is 30 kV.

The stored energy can be calculated for each capacitor by the following

relationship:-

2CV21E = …………………(3-6)

The maximum energy storage in the capacitor bank is E = 17.6 kJ

Figure (3-11) The capacitor bank a- Diagram of one capacitor b- Photograph 16 capacitors

(a) (b)


Figure (3-12) Storage energy of capacitor bank for different voltages

3.10 Experimental Inductance

Two coils of insulating material are designed shaped as solenoid

(figure (3-13)) and used copper wire to wrap. The first coil is designed

and constructed for the purpose of controlling the damping factor and the

value of the time constant (t = L/R).

The damping factor in present work is defined as :

L4

)RR(Cd

2shexb

f+

= ……………(3-7)

where Cb is the capacitor bank, Rex is the external resistance and Rsh is the

current shunt resistance. It is equal's df @ 0.93 . The second coil is designed to study the effect of the magnetic field on

the parameters of the experiment (breakdown voltage, discharge current,

charging time and others).

The solenoid is composed of a number of circular current loops

having the same axis and the same current passes through them . The

magnetic field is obtained by summing up the fields of the separate

current loops.


Figure (3-13) Inductance coils a- First coil, b- Second coil (used with cylindrically-tipped electrodes)

To calculate the magnetic filed at any arbitrary point P on the axis of

a solenoid, consider a solenoid may be described as N turns uniformly

wound on a cylindrical form of radius (a) and length ( l ).

Such a configuration is shown in figure (3-14). The magnetic induction at

point z0 is found by dividing the length l into elements dz as shown in

the equation [133].

)83(..........................dz)az(

aNI2

dB 2322

20 -

+

m=

l

Applying equation (3-8) to each element and summing the results.

[ ])93(.........................dz

a)zz(

1aIN2

dB2

3220

0

20 -

+-

m= ò

l

l

The change of variable, z - z0 =a tanθ, leads to

)103........(..........2

sinsinNIdcosaIN

2dB 120

20 2

2-úû

ùêë

é q-qm=qq

m= ò

q

q ll

(a)

(b)


where θ1=-tan-1(z0/ a) and θ2=tan-1( l -z0)/ a. The fact that sines appear

rather than just ones as in the elementary formula, represents end

correction. To help understand the approximation that is usually made,

namely, B=μ0NI/ l it is convenient to introduce the angles α1 and α2 (both

positive) shown in figure (3-14). In terms of these angles, equation (2-10)

becomes

)113(...................2

)cos(cosNIB 210 -úû

ùêë

é a+am=

l

Figure (3-14): Axial magnetic field of a solenoid [49]

If the solenoid is long compared with its radius and z0 is not too close to

either zero or l , than α1 and α2 are both small angles and may be

approximated by [49]:-

)123(....................z

a,za

02

01 -

-@a@a

l

The magnetic field was measured by a Teslameter model

magnetfeldmeβgerat which made by Phywe company (made in Germany)

as shown in figure (3-15). The behavior of the magnetic field that

measured by Teslameter along the coil length at different currents are

shown in figure (3-16). The magnetic field has maximum values in the


center region. The magnitude of this field increases with the increasing

coil current.

Figure (3-15): The circuit of the measurement of the magnetic field

a-Schematic b- Photographic

Figure (3-16): The magnetic field distribution along the axis of the coil

(a)

(b)


The Inductance coils figure (3-13) has the following specifications shown

in table (3-3).

Table (3-13) The measurement parameters of coils

3.11 Trigger Circuit (Third Electrode)

A trigger circuit has been designed for accelerating the electrical

discharge between two electrodes of the spark gap. This circuit increases

the number of charge carriers in the dielectric medium by various

mechanisms depending on the type of the switch. Two types of trigger

circuit are designed : electronic and mechanical. The first one is designed

by using electronic lighter, while the second trigger circuit consists of

relay, power supply and push bottom switch as shown in figure (3-17).

Parameters First coil Second coil

Length of the coil l

17.5 cm

10 cm

Diameter of the coil

7.5cm 6 cm

Number of turns = N

22 turns 26 turns

Inductance = L 15.4 μH 24 μH

Resistance 33 mΩ 35 mΩ

Length of the wire= L1

6 m 5 m

Wire diameter 2 mm 1.8 mm


Figure (3-17) Trigger circuit a- Mechanical triggering diagram, and construction b- Electronic triggering

3.12 Voltage Divider

A divider for recording high transient voltages may consist of

resistance, capacitors, or combinations of both. The essential requirement

is that the measuring of the voltage wave shape should faithfully be

reproduced on the oscilloscope with an accurately known reduction ratio

as explained in chapter two (2.4) paragraph.

Voltage divider was designed and constricted to number of high and low

value resistance, that is connected in series to experience high voltage and

(a)

(b)


current passing through it as shown in figure (3-18) . Measurements can

be done by the potential divider on the low impedance section by a

storage oscilloscope or scale device as follows:

÷÷ø

öççè

æ+

=21

2inout RR

R V V ……………….(3-13)

R1 = 500 Ω

R2 = o.5 Ω

Vout = 10-3 Vin

if Vin = 1kV then Vout =1V

Figure (3-18) Schematic of the voltage divider

3.13 Commissioning Experiment Setup

A group of capacitors was stacked into a bank to deliver a

capacitively stored energy of variable level. This energy is controlled by

the amount of charging voltage, which is applied from a DC-power

supply as shown in figure (3-19). The experimental procedure starts by


setting the electrode spacing of the spark gap (d) measured by a calibrated

filler gauge.

To establish a proper damping conditions in the circuit, the capacitor

bank (Cb = 39 μF) was charged through a current-limiting resistance to

variable voltages. For each specified voltage, a discharge current was

recorded when the spark gap had been triggered via a particular mode. To

make sure that there was no additional inductance coupled to the circuit

from the triggering device, a number of experiments was carried out by

spark gap self-triggering (self breakdown). In such conditions, the bank

erected to a certain voltage level enough to establish self-breakdown

followed by the transport of discharge current.

The charging voltage was monitored by a (1000:1) standard high

voltage probe, which was connected to a digital multimeter. The

discharge current was recorded on a digital double beam storage

oscilloscope by using current-viewing resistance (CVR) which was

designed and constructed for this purpose. For cross-correlated

measurements of the discharge current, Rogowski coils also threaded into

the grounded connection of the RLC circuit. These coils were calibrated

to record the pulsed discharge currents.

A 50Ω matching coaxial impedance was connected to each input of

the oscilloscopes to avoid any possible mismatch in signal propagation. A

proper RC-integration circuit was connected to the output of the

Rogowski coil to measure the current rather than its rate of change.


Figure (3-19) Experimental setup Schematic photograph of the pulsed RLC circuit.

Chapter Four Results Analysis and Discussion

4.1 Introduction

Various PED experiments with different configurations have been

carried out for constructing a scope of understanding of the behaviour of

the impedance of the plasma switch as a part of the high-voltage circuit

constructed and operated for this purpose.

For three electrode configurations; sphere-sphere, sphere-rod, and

cylindrically-tipped, results were analyzed in terms of the current-voltage

characteristics, temporal and spatial behaviour of the plasma switch, and

the erosion rate of the electrode surface. By considering these parameters,

the behaviour of the impedance was thoroughly investigated and

associated phenomena have been justified.

This chapter presents these results analyses and the discussion

necessary for each group of experimental conditions. 4.2 Spherical Spark Gap

Exploring the current-voltage and current-time characteristics of the

operational pulsed circuit, a number of preliminary experiments carried

out using the circuit shown in figure (3-1). By operating the spark gap

with self-breakdown mode, measurements of the time required for

charging the capacitor bank up to a voltage sufficient for self-breakdown

(charging time) were necessary to test the reliability of the circuit as well

as the range of reproducibility. Current measurements were carried out by

both CVR and Rogowski coils. Therefore, for each charging voltage,

there was a corresponding time-varying discharge current signal to be

inspected.

Figure (4-1) shows four typical oscillograms obtained for a spark

gap spacing of 0.5 mm using self-triggering mode (a and b ) and third-

electrode triggering mode (c and d). With a charging voltage of 2.8 kV, a


typical peak discharge current of (1.2±0.05) kA could be obtained for

self-triggering while a peak current value of (1.35±0.05)kA was obtained

with a 3 kV charging voltage in the other mode of triggering with longer

Figure (4-1) Typical discharge current oscillograms recorded by CVR and Rogowski coil; (a, b) self- triggering and (c, d) third-electrode

triggering.

spacing (2mm). The features of the signals illustrate the effect of both

triggering mode and gap spacing on the damping conditions of the circuit,

which explained in the chapter three, section (3.11).

4.2.1 Current Voltage-Characteristics

A typical discharge current-charging voltage characteristic curve is

shown in figure (4-2), where both CVR and Rogowski coil measurements

of the current are demonstrated.

(a) (b)

VCharging=2.83kV d=0.5mm CVR

VCharging=2.83kV d=0.5mm Rogowski

VCharging= 3kV d=2mm CVR

VCharging= 3kV d=2mm Rogowski

(c) (d)


Figure (4-2) Typical discharge current-charging voltage characteristic

curve for d=2mm under third-electrode triggering mode Two regions may be distinguished in this curve; one region shows a

gradual increase in the current with raising the charging voltage and the

other, i.e., after a charging voltage of about 7 kV, shows a very slight

change in the current which illustrates that there is no additional source of

charged particles including that from secondary emission for the 2 mm

gap spacing where the electrical power is dissipated. Meanwhile figure

(4-3) shows a typical discharge current–charging voltage characteristic

curve where both CVR and Rogowski coil measurements of the current

are demonstrated with self-breakdown (self-triggering). It is clear from

figure (4-3) that the gap of the spherical electrodes varies in the range

from 0.5 mm to 2.5 mm and the corresponding breakdown voltage ranges

from 2.8 kV to 9.0 kV. Although these observations are expected but

under pulsed conditions they are reproducible within less than 10% of the

measurements under nonuniform field configuration.

In other recent experiments using an AC voltage and standard

spherical electrodes (25 cm in diameter), increase of the breakdown

voltage with lengthening the gap spacing was also reported [51].



curve for different distance at self- breakdown 4.2.2 The Performance of the PED Circuit

A graphical representation of the charging and discharging times is

shown in figure (4-4). The performance of the pulsed circuit may be

envisaged by the ratio (discharging time/charging time) for the self-

breakdown condition.

Differences in current values may be imposed by the

(inductance/resistance) ratio of Rogowski coil, which is reasonably

acceptable in pulsed techniques where a number of readings are averaged

over their corresponding experimental conditions [47].

In the course of these experiments, a peak discharge current of 1.2 kA

was found to have a rise time of 150 ns at d=0.5mm while increasing

current up to 3.4 kA was recorded with a rise time of 161ns for the CVR

measurements at self-breakdown at d= 2.5 mm.


Figure (4-4) Rise time of the pulse as a function of charging time for

Self-breakdown triggering

Figure (4-5) represents the rise time of the triggering (third electrode)

breakdown as a function of the discharge current for the same distance.

Figure (4-5) The rise time versus the discharge current for the same distance (d=2mm) by used third electrode triggering mode

Taking the full-width at the half maxima of current signals obtained

for gap spacing ranging from 0.5 mm to 2.5mm, a graphical

representation demonstrates that the time required for short-circuiting the


gap shows slight fluctuations as shown in figure (4-6). Such behaviour

illustrates the effect of damping conditions created within the gap

throughout the time interval of plasma expansion, i.e., oscillation

conditions need more than τfwhm to damp out for this circuit which is

slightly underdamped.

Figure (4-6) Variation of time of full width at the half maxima τfwhm

with gap spacing 4.2.3 Spatial Behaviors of the Spark Gap Voltage An almost linear dependence of the breakdown voltage on the spark

gap spacing is shown in figure (4-7). This behavior demonstrates that

these experiments were run along the right side of the Paschen curve with

a nonthermal plasma generated within the gap at atmospheric pressure.

At this pressure the plasma is highly conductive as spark ignition

proceeds to arcing which may impose a shrinkage of both normal and

abnormal glow regions in the current-voltage characteristic curve of the

discharge [29].


Figure (4-7) Charging voltage versus gap spacing for self-breakdown

4.2.4 Temporal Characteristics of the Impedance To investigate the time-evolution of the impedance of the spark gap

under study, a number of parameters were considered including the

damping conditions, gap spacing and pulse duration. Two approaches

were adopted to analyze the results of the output current signals. The first

was to take a number of oscillograms for various gap spacing. A number

of current values was extracted from each oscillogram that corresponds to

a certain distance (d). For a number of d-values ranged from 0.5mm to

2.5mm, the nominal impedance Z (charging voltage / discharge current)

was plotted as a function of the rise time of each pronounced current peak

on the oscillogram under self-breakdown conditions as shown in figure

(4-8).

The same trend of behaviour is obvious in both CVR and Rogowski

coil measurements of the current, as shown in figure (4-8). The values of

Z increases with enlarging d while the impedance shows a prompt

increase with rise time as a result of current degrading as the signal


damps out due to the finite closure time of the spark gap [39]. The peak

current values showed a periodic damp out as a result of the behaviour of

the external circuitry which must be accounted for in results analysis as

will be discussed below. Typical value of Z for the resistance (figure (4-

8)a ) took the rang from 2.9 Ω to 77 Ω over a time period between 161ns

and 600 ns for a gap distance of typically 2.5mm .

Figure (4-8) Typical impedance behaviour for self-breakdown mode at different distance (a) CVR (b) Rogowski coil, extracted from various

current oscillograms

(b)

(a)


Figure (4-8)b shows typical value of Z for the Rogowski coil took

the range from 2.8 Ω to 77.4 Ω over a time period between 202 ns to 620

ns and for a gap distance of typically 2.5 mm.

The other way of analysis was to take the first half cycle of each

current signal in a number of oscillograms and record the peak value and

its rise time. Through a similar manner to that discussed above, the values

of Z were deduced and plotted as a function of the corresponding value of

the rise time as shown in figure (4-9).

Figure (4-9) Impedance as a function of current rise- time for self-breakdown mode (a) CVR (b) Rogowski coil, extracted from various

current oscillogram

(a)

(b)


It can be concluded from figures (4-8 and 4-9) that time-varying

impedance exists due to the inductance and resistivity of the spark gap as

well as the expansion of plasma channels along the spacing d. The gap

closure time may be taken as the time required for the plasma to expand

between the two gap electrodes resulting in an impedance collapse.

Over the whole range of experimental conditions being undertaken, the

average values of an overall nominal impedance were found in the range

(2-23)Ω depending on the gap and circuit parameters. Table (4-1)

illustrates typical Z-values corresponding to the operating parameters. Table (4-1) Illustrates typical Z-values for CVR and Rogowski coil for

different distance

d(mm)

CVR

Z(Ω)

Rogowski

Z(Ω)

0.5 2.4 2.3

1 2.5 2.5

1.5 2.6 2.6

2 2.7 2.6

2.5 2.9 2.8

4.2.5 Electrode Erosion Under Pulsed Discharge Over a number of experiments conducted with the spark gap and by

having an accumulative number of shots exceeding one thousand, power

dissipation within the gap resulted in an erosion on the surfaces of both

electrodes as shown in figure (4-10). Erosion rate may increase

nonlinearly with upgrading current levels [44].

The damage pattern created on the two surfaces has the dimensions

of about 8.3 mm on the high voltage electrode and 10.5 mm on the


grounded electrode. The expansion of plasma particles towards the

grounded electrode and space charge effect may be responsible for this

difference in pattern dimensions.

Figure (4-10) A photograph of the experimental spark gap showing the

formation of spark and damage patterns

It was found that the rate of electrode erosion was affected by the gap

length and the corresponding applied voltage. This may be attributed to

the energy dissipated within the spark gap. At low currents, the cathode is

exposed to erosion while the anode is not significantly affected. As the

current increases, anode spots are formed and anode erosion begins. At

high currents, both electrodes erodes experience significant erosion rate

[144].


4.3 Rod-Sphere Spark Gap

In order to study the effect of the degree of field nonuniformity, one

of the spherical electrodes in the spark gap was replaced by a rod. The

PED circuit was operated with this switch over a wide range of

conditions. Results were analyzed in an approach similar to that discussed

above. A typical discharge plasma flare is demonstrated in figure (4-11)

when the circuit was discharged through the rod-sphere gap.

Figure (4-11) Rod-Sphere electrodes during the self-breakdown

4.3.1 Current-Voltage Characteristics

A typical discharge current-charging voltage characteristic curve is

shown in figure (4-12), when both CVR and Rogowski coil

measurements of the current are demonstrated for triggering mode (third

electrode). It is the same behavior as the spherical electrodes but different

in charging voltage which is less than the rod-sphere because the shape of

the spherical electrode is large unformed electrical field than rod sphere

electrodes.


Figure (4-12) Typical discharge current-charging voltage characteristic curve for d=2mm under third-electrode triggering mode

Figure (4-13) shows a typical discharge current-charging voltage

characteristic curve for self-breakdown when both CVR and Rogowski

coil measurements of the current are demonstrated with self breakdown

(self-triggering).

Figure (4-13): Typical discharge current-charging voltage

characteristic curve for different distance at self- breakdown


4.3.2 The Performance of the PED Circuit

In order to evaluate the transfer of energy from the capacitor bank

into the load in these experiments, a sufficient charging time must be

elapsed as a measure for a corresponding discharge time over a pulse-

shape the rise-time of which is sufficient to be considered for the circuit's

performance. A graphical representation of the charging and discharging

times is shown in figure (4-14) for self-breakdown condition. The

performance of the pulsed circuit may be envisaged as the ratio

(discharging time/charging time) for self-breakdown condition. The

differences in current values may be imposed by the


acceptable in pulsed techniques.

A peak discharge current of 1.2 kA was found to have a rise time of

139 ns for d=0.5 mm while increasing current up to 3.2 kA was recorded

with a rise time of 156 ns for the CVR measurements at d=2.5 mm for

self-breakdown.





breakdown to the discharge current for the same distance. In this shape

the rise time decreases when charging voltage increases due to increased

ionization energy, which in turn causes increased the speed of the

electrons which leads to decrease of pulse rise time.

Figure (4-15) The rise time versus the discharge current for the same

distance (d=2mm) by used third electrode triggering mode Taking the full-width at the half maxima of current signals obtained

for gap spacing range from 0.5 mm to 3 mm, a graphical representation

demonstrates that the time required for short-circuiting the gap shows

slight fluctuations as shown in figure (4-16). These fluctuations may be

reduced by having more sampling measurements before having the

averages and standard errors. Temperature increase of the materials of

circuit's resistors may also raise their resistance to shift the damping

conditions towards an over damping mode [47]. Such behaviour






Figure (4-16) Variation of time of full width at the half maxima τfwhm with gap spacing

4.3.3 Spatial Behaviors of the Spark Gap Voltage

In the present spark gap, voltage and current may show variations in

both spatial and temporal modes. Figure (4-17) depicts a typical

dependence of the breakdown voltage on the spark gap spacing. This

behavior makes it clear that these experiments are run along the right side

of the Paschen curve with a nonthermal plasma generated within the gap

at atmospheric pressure. At this pressure, there is highly conductive

plasma channel as spark ignition proceeds to arcing, which may impose a

shrinkage of both normal and abnormal glow regions in the current-

voltage characteristic curve of the discharge.

These observations are familiar in PED experiments as the rise-time

of the pulse is equal or slightly longer than that required for glow


discharge transitions, i.e. from normal to above-normal glow or from the

later to an arc [8].


4.3.4 Temporal Characteristics of the Impedance

Over a wide range of experimental conditions the dynamic correlation

between the voltage and current may be established by investigating the

impedance of the circuit over the whole pulse for many pulsed

discharges. The impedance behaviour of the resent spark gap, covers a

number of parameters including the damping conditions, gap spacing and

pulse duration. Two methods were adopted to analyze the results of the

output current signals. The first was to take a number of oscillograms for

various gap spacing. A number of current values was extracted from each

oscillogram that corresponds to a certain distance (d).

For a number of d-values ranged from 0.5 mm to 3 mm, the nominal

impedance Z (charging voltage / discharge current) was plotted as a

function of the rise time of each pronounced current peak on the

oscillogram under self-breakdown conditions as shown in figure (4-18).


Figure (4-18) Typical impedance behaviour for self-breakdown mode at different distance (a) CVR (b) Rogowski coil, extracted from

various current oscillograms The same trend of behaviour can obviously be noticed in both CVR and

Rogowski coil measurements of the current, (figure (4-18)). The values of



damps out due to the finite closure time of the spark gap with its certain

(a)

(b)


spacing and field geometry which governs the energy of particles, i.e.,

power time. Typical value of Z for the resistance (figure (4-18)a) took the

range from 2.2 Ω to 57 Ω over a time period between 156 ns and 600 ns

for a gap distance of typically 2.5 mm . Figure (4-18)b shows typical

value of Z for the Rogowski coil that has taken the range from 2.1 Ω to

55 Ω over a time period between 200ns to 620ns and for a gap distance of

typically 2.5mm.


current oscillogram

(b)

(a)


As discussed in section (4-2-4), the other way of analysis was to take

the first half cycle of each current signal in a number of oscillograms and

record the peak value and its rise time. Through a similar manner to that

discussed above, the values of Z were deduced and plotted as a function

of the corresponding value of the rise time as shown in figure (4-19). It

can be concluded from figures (4-18 and 4-19) that time-varying

impedance exists due to the inductance and resistivity of the spark gap as

well as the expansion of plasma channels along the spacing d. The gap

closure time may be taken as the time required for the plasma to expand

between the two gap electrodes resulting in an impedance collapse. Over

the whole range of experimental conditions being undertaken, the average

values of an overall nominal impedance were found in the range (1.5-

19.4) Ω depending on the gap and circuit parameters.

4.4 Cylindrically-Tipped Spark Gap

As discussed in section (1-11), the density of equipotential surfaces

and electric flux density can be altered by the two electrode surfaces

subtended the electric field lines. In these experiments, a spark gap with a

cylindrically-tipped electrode was incorporated within the high-voltage

circuit to establish a good understanding of the geometry effect which

may alter the values of the current density rather than the peak current.

The cylindrically-tipped electrodes were used in these experiments to

form a spark gap as shown in the photograph of figure (4-20) during a

typical breakdown stage after a number of shots.

A number of experiments were carried out with this configuration by

using both self-breakdown and trigger-electrode modes. By doing so, the

effect of an external inductance possessed by the triggering electrode

circuit can be observed in the output signal after the circuit is being

discharged.


Figure (4-20): Cylindrically-tipped electrodes during a typical self-

breakdown stage

4.4.1 Current-Voltage Characteristics A typical discharge current-charging voltage characteristic curve is

shown in figure (4-21) where both CVR and Rogowski coil

measurements of the current are demonstrated for triggering mode (third

electrode).

The effect of the electric field lines on the applied voltage may be

significant and can be altered when the degree of uniformity is

considered. This can be simply understood from the basic relationship

between the voltage and the electric field for any two electrodes. This

degree of field uniformity may be determined by the ratio of the electrode

dimension and the gap spacing. In figure (4-22) a typical discharge

current–charging voltage characteristic curve for triggering breakdown is

shown where both CVR and Rogowski coil measurements of the current

are demonstrated. Typical peak current values ranged from 0.8 kA to 4.7

kA were recorded when voltages between 1 kV and 9 kV were applied

for changing the C-bank.



curve for d=2mm under third-electrode triggering mode

Although the discharge mechanism is governed by a streamer, see section

(1-4), such behaviour looks linear in a part of it because the voltage

considered is the charging voltage. However, more future measurements

of discharge voltage may put forward more understanding of the current-

voltage characteristic curves.


curve for different distance at self breakdown


Comparison of figures (4-21) and (4-22) may significantly present the

values of the parameters corresponding to the two modes of triggering the

spark gap.

4.4.2 The Performance of the PED Circuit Similarly, a graphical representation of the charging and discharging

times is shown in figure (4-23). The performance of the pulsed circuit

may be envisaged by the ratio (discharging time/charging time) for self-

breakdown condition. The differences in current values may be imposed

by the (inductance/resistance) ratio of Rogowski coil, which is reasonably

acceptable in pulsed techniques. In the present experiments, a peak

discharge current of 1 kA was found to have a rise time of 163 ns while

increasing current up to 4.3 kA was recorded with a rise time of 174 ns

for the CVR measurements at self-breakdown.



Under these conditions when the dimensions of the electrodes are altered,

both inductance (L) and capacitance (C) of the gap will be varied

consequently because both L and C are geometry-dependent. Such


variation of both L and C of the gap will introduce a change in the total

impedance of the circuit giving rise to an alteration in the rising and

decaying parts of the pulsed discharge current signal. This may be

understood by the ratios (L / R) of the circuit for the rise-time and (RC)

for the time after the peak current toward the end of the signal tail.


breakdown to the discharge current for the same distance.

Figure (4-24) The rise time versus the discharge current for the same distance (d=2mm) by used third electrode triggering mode

Taking the full-width at the half maxima of current signals obtained

for gap spacing that ranged from 0.2 mm to 1.8 mm, a graphical

representation demonstrates that the time required for short-circuiting the

gap shows slight fluctuations as shown in figure (4-25). Such behaviour






Figure (4-25) Variation of time of full width at the half maxima τfwhm with gap spacing


The dependence of the breakdown voltage on the spark gap spacing is

shown in figure (4-26) at small distance of (0.2, 0.4, 0.6, 0.8, 1, 1.2, 1.4,

1.6, and 1.8) mm. This behavior demonstrates that these experiments are

run along the right side of the Paschen curve with a nonthermal plasma

generated within the gap at atmospheric pressure. At this pressure, the

plasma is highly conductive as spark ignition proceeds to arcing, which

may impose a shrinkage of both normal and abnormal glow regions in the

current –voltage characteristic curve of the discharge [29].



4.4.4 Temporal Characteristics of the Impedance

A number of parameters were considered including the damping

conditions, gap spacing and pulse duration. Two approaches were

adopted to analyze the results of the output current signals. The first was

to take a number of oscillograms for various gap spacing. A number of

current values was extracted from each oscillogram that corresponds to a

certain distance (d). For a number of d-values ranging from 0.5mm to 1.5

mm, the nominal impedance Z (charging voltage / discharge current) was

plotted as a function of the rise time of each pronounced current peak on

the oscillogram under self-breakdown conditions as shown in figure (4-

27). The same trend of behaviour is obvious in both CVR and Rogowski

coil measurements of the current as shown in figure (4-28). The values of



damps out due to the finite closure time of the spark gap. The peak

current values showed a periodic damp out as a result of the behaviour of


the external circuitry which must be accounted for in results analysis as

will be discussed below. Typical value of Z for the resistance (figure (4-

28)a) took the range from 2.2 Ω to 80 Ω over a time period between 167

ns and 600 ns for a gap distance of typically 1.5 mm. Figure (4-8)b

shows a typical value of Z for the Rogowski coil taking a range from 2

Ω to 79 Ω over a time period between 205 ns to 620 ns and for a gap

distance of typically 1.5 mm.

Figure (4-27) Typical impedance behaviour for self-breakdown mode at

different distance (a) CVR (b) Rogowski coil, extracted from various current oscillograms

(a)

(b)


From a number of oscillograms the values of Z were obtained and

plotted as function of the rise-time as demonstrated in figure (4-28).

It may be concluded from figures (4-27) and (4-28) that the time-

evolution of Z depends on the inductance and resistivity of the spark gap

as well as the expansion of plasma channels along the spacing d.


current oscillogram

(a)

(b)


The gap closure time may also be taken as the time required for the

plasma to expand between the two gap electrodes resulting in an

impedance collapse. Over the whole range of experimental conditions

being undertaken, the average values of an overall nominal impedance

were found in the range (2-30)Ω depending on the gap and circuit

parameters. Also, this range implies the approximation of consider the

charging voltage in the calculation.

4.5 Comparison of Results of Various Gap Configurations

Results of the three spark gap configuration were compared with

each other to establish an optimum conditions and parameters, which

were capable of imposing such features on all experiments. Typical

values were selected for such comparison can be seen below.


Figure (4-29) is a typical discharge current–charging voltage

characteristic curve showing current both CVR and Rogowski coil

measurements with self-breakdown (self-triggering) for three electrodes

configurations. In this figure, one can see that the discharge current for

cylindrically-tipped electrodes is slightly greater than that of spherical

and rod-sphere electrodes. These observations can be attributed to the

nature of the electric field and the electric flux density within the gap.


Figure (4-29) Typical discharge current-charging voltage characteristic curve for different distance at self-breakdown for three electrodes

configurations (a) CVR (b) Rogowski coil

4.5.2 The Performance of the PED Circuit A graphical representation of the charging and discharging times is

shown in figure (4-30) under self-breakdown condition for the three

electrodes. The performance of the pulsed circuit may be envisaged by

the ratio (discharging time/charging time) for self-breakdown condition.

(b)

(a)


Figure (4-30) Rise time of the pulse as a function of charging time for Self-breakdown triggering (a) CVR (b) Rogowski coil

The differences in current values may be imposed by the


acceptable in pulsed techniques.


The breakdown characteristics between three type electrodes (sphere-

sphere, rod-sphere and cylindrically-tipped) are observed with variations

(a)

(b)


in electrode arrangements, both in size and spacing. It is concluded that

with the increase of gap between spheres the breakdown voltage and

electric field strength are increased.

Figure (4-31) represents the behavior of the three types while

cylindrically-tipped electrode which need large voltage to obtain self-

breakdown whereas the spherical and rod-sphere electrodes need less

breakdown voltage.

As indicated in the figure, the cylindrical-tipped electrodes behaviors

like almost plane electrodes because of its thickness and the gap spacing.

However, more analysis of the electric field lines and equipotential

surface are substantially required to figure out the degree of field

uniformity [146].

Figure (4-31) Charging voltage versus gap spacing for self-breakdown to the three electrodes for different distance (0.5, 1, 1.5mm)

4.5.4 Temporal Characteristics of the Impedance The impedance characteristic of the spark gap can be compared of the

three electrodes (spherical, rod-sphere and cylindrically-tipped) at

distance 0.5 mm for self-breakdown shown in figure (4-32).


Figure (4-32) Typical impedance behaviour for self-breakdown mode at distance 0.5mm for CVR extracted from various current oscillograms

for three electrodes

Compression charging voltage to the discharge current of the peak

for pulsed electrical discharge and the impedance at this region for three

electrodes at distance 0.5 mm for self-breakdown for CVR measurement

is shown in table (4-2)

Table (4-2) Compression of results three electrode configurations at distance 0.5 mm spacing for self-breakdown (CVR measurements)

Electrodes Geometry Vch /kV Id /kA Z/ Ω Sphere-Sphere 2.8 1.3 2.2

Rod-Sphere 2.1 1.2 1.7 Cylindrically-Tipped 3.5 1.9 1.9

The other way of comparison analysis is to take the first half cycle of

each current signal in a number of oscillograms and record the peak value

and its rise time. The values of Z are deduced and plotted as a function of

the corresponding value of the rise time as shown in figure (4-33).


Figure (4-33) Impedance as a function of current rise- time for self-breakdown mode for CVR extracted from various current oscillogram

(d=0.5mm) for three electrodes 4.6 Magnetic Field Effect on the Cylindrically-Tipped Spark Gap

The behavior of the magnetic field can be explained in this section.

The solenoid coil is used whose specifications is explained in chapter

three, section (3.10). Figure (4-34) shows the cylindrically-tipped

electrode, which rounded to solenoid coil during the self-breakdown.

The magnetic field effect can be visualized as the interaction of

charged particles of the plasma with this field within the spark gap

region. The arrangement of the coil is made in such a way that the peak of

the magnetic field strength occurs at mid-way of the gap spacing resulting

a region of a slight magnetic confinement of the particles, i.e., at a region

of magnetic field of strength 27G along the coil axis.

Under these conditions electrons will be guided between the two

electrodes producing more ionization along the gap axis in addition to a

reduction of lateral diffusion of these electrons [40].


Figure (4-34) Inductance coil used with cylindrically-tipped electrodes during the self- breakdown

Figure (4-35) shows two typical oscillograms obtained for

cylindrically-tipped electrodes with magnetic field and a spark gap

spacing of 0.5 mm using self-triggering mode (a and b ). With a charging

voltage of 3 kV, a typical peak discharge current of (2.2±0.05) kA could

be obtained for self-triggering.

Figure (4-35) Typical discharge current oscillograms recorded by CVR and Rogowski coil; (a, b) self- triggering to the cylindrically-tipped

electrodes with magnetic field

VChar=3.1kV d=0.5mm B = 27G

Rogowski

VChag=3.1kV d=0.5mm B= 27G

CVR

(a) (b)



Figure (4-36) a typical discharge current–charging voltage

characteristic curve is shown where both CVR and Rogowski coil

measurements of the current are demonstrated with self-breakdown (self-

triggering). The effect of the magnetic field is to show that with its

increase in our experiment the maximum magnetic field used is 27G. The

effect of the magnetic field on the discharge current is about 10% for

increasing the current.

Figure (4-36) Typical discharge current-charging voltage characteristic curve for different distance at self-breakdown (a) CVR (b) Rogowski

(a)

b) (


Figure (4-37) represents the charging voltage and the magnetic

field, used for different distances for self-breakdown. In this part can be

shown that the magnetic field caused decreasing of the charging voltage

at the same distance.

Figure (4-37) Charging voltage versus magnetic field for self-breakdown

Figure (3-38) represents the behavior of the discharge current to the

magnetic field for self-breakdown and shows that at increasing the

magnetic field the discharge current increases.

A typical discharge current–charging voltage characteristic curve for

self-breakdown using a magnetic field of 27G is shown when both CVR

and Rogowski coil measurements of the current are demonstrated.

Typical peak current values ranged from 2.2 kA at distance 0.5 mm to 4.3

kA at distance 1.5 mm were recorded when voltages between 3.2 kV and

8.1 kV were applied for changing the C-bank.


Figure (4-38) Discharge current versus magnetic field for self-breakdown

4.6.2 The Performance of the PED Circuit A graphical representation of the charging and discharging times is

shown in figure (4-39). The performance of the pulsed circuit may be

envisaged as the ratio (discharging time/charging time) for self-

breakdown condition. The behavior is demonstrated when increasing

magnetic field the rise time increases and decreasing the charging time. In

the present experiments, a peak discharge current of 2.3 kA is found to

have a rise time of 175 ns at d = 0.5 mm while increasing current up to

4.3 kA at d = 1.5 mm is recorded with a rise time of 188 ns for the CVR

measurements at self-breakdown.


Figure (4-39) Rise time of the pulse as a function of charging time for Self-breakdown triggering (a) CVR (b) Rogowski coil

4.6.3 Magnetic Field Effect on the Impedance

To investigate the impedance behaviour of the spark gap, a number

of parameters were considered including the damping conditions, gap

spacing and pulse duration. Two approaches were adopted to analyze the

results of the output current signals. The first was to take a number of

(b)

(a)


oscillograms for various gap spacing. A number of current values was

extracted from each oscillogram that corresponds to a certain distance (d).

For a number of d-values that ranged from 0.5 mm to 1.5 mm, the

nominal impedance Z (charging voltage / discharge current) was plotted

as a function of the rise time of each pronounced current peak on the

oscillogram under self-breakdown conditions as shown in figure (4-40).

Figure (4-40) Typical impedance behaviour for self-breakdown mode at different distance (a) CVR (b) Rogowski coil, extracted from various

current oscillograms

(a)

(b)


Typical value of Z for the CVR as shown in figure (4-40)a took the rang

from 1.4 Ω to 38 Ω over a time period between 175 ns and 600 ns for a

gap distance of typically 0.5 mm. However typical value of Z for the

Rogowski coil took the range from 1.3 Ω to 37 Ω over a time period

between 216 ns to 620 ns and for a gap distance of typically 0.5 mm.

The same trend of behaviour is obvious in both CVR and Rogowski

coil measurements of the current shown in figure (4-41)a and b. The

values of Z increase with enlarging d while the impedance shows a

prompt increase with rise time as a result of current degrading as the

signal damps out due to the finite closure time of the spark gap.

The other way of analysis was to take the first half cycle of each

current signal in a number of oscillograms and record the peak value and

its rise time. Through a similar manner to that discussed above, the values

of Z were deduced and plotted as a function of the corresponding value of

the rise time as shown in figure (4-41).



current oscillogram

The speed of plasma propagation along the spark gap electrodes

were deduced from the output waveforms by considering the value of

τfwhm and the electrode spacing . A nominal value of this speed was found

to be 5.6 m μs-1.

(a)

(b)


4.7 Conclusion In order to establish a scope of understanding of the temporal

behaviour of the impedance featured by a PED circuit, it was essential to

study the plasma characteristics within the circuit's switching device.

The plasma behaviour is elucidated by constructing a group of

curves relating the governing parameters such as current, voltage, gap

spacing, charging and discharging times, electric field configuration, and

external circuit elements.

Regardless of the complication of the physics behind the operation

of pulsed plasma devices, these experiments have demonstrated a

number of potentially practical conclusion, which can be summarized as

follows:-

1- The temporal behaviour of the total circuit impedance showed a

strong dependence on the behaviour of the nonthermal plasma which

is generated during the breakdown process of the air between the

spark gap electrodes.

2- All the discharge current waveforms showed damped oscillations

from which results analyses were accomplished. These modes of

oscillations may be governed by a dominating inductance in the

circuit as well as the electronic components included in the methods

of triggering.

3- The first cycle of the discharge current signal may be considered as

that resulted from the spark gap closure by the plasma expansion the

time of which is measured by the full width at the half maxima of

this part of the signal.

4- Due to the existence of the external inductance in the circuit, damped

pulses occur in the discharge current signals over a certain period of


time which can be figured out from the RC time constant of each

corresponding current signal.

5- Slight alteration in the dependence of the discharge voltage on the

product (Pd) was observed when the gap was immersed in a

longitudinal magnetic field due to the slight deviation of Paschen

curve on its right hand side.

6- The triggering method was found to have an effect on the discharge

current pulses monitored by both current viewing resistance and

Rogowski coil.

7- aDamage patterns on the surfaces of the gap electrodes were

photographed as they resulted from power dissipation within the gap

controlled by discharge current levels.

4.8 Future Work

1- The present PED experiments can be extended for higher magnetic

field generation for immersing the spark gap. In doing so, the

electromagnetic drift forces may be more pronounced and their

effects can modify the operation of the spark gap.

2- Higher stored energy in the C-bank can be achieved by raising the

charging voltage (more than 10kV) which requires a modification

of the whole circuit components including the spark gap itself.

(a)

Paper published from this thesis

1- A. S. Hasaani, Ala’ F. Ahmed and A. A. Khdayeir "Impedance

Characteristics of Pulsed Atmospheric Electrical Discharge in

Spherical Plasma Switch" Baghdad Science Journal, Vol.8, No.2,

pp.630-637 (2011).

المستخلص

الجراء تجارب حاثةمتسعة ومومقاومة نبضية عالية تحتوي على فولتية دائرةتستخدمأ

تفريغ كهربائي في مفتاح بالزما في ثالثة اشكال هندسية هي كروي ، قضيب-كرة واسطواني حيث

. kV 8 صنعت من مادة الفوالذ والنحاس .وقد وصلت فولتية الشحن فيها الى اكثر من

لقياس تيار التفريغ النبضي عند انهيار مفتاح البالزما وفسكي غروات وملفمجزئ مقاومة ت ستخدمأ

4.5kAبنمطين هما التفريغ الذاتي و باستخدام قطب ثالث للتفريغ حيث وصلت قيمة الذروة للتيار الى

اعتمدا على مكونات الدائرة والمسافة بين االقطاب μs 0.3 و μs 0.1تراوحت بين زمن النبضة و

تحت تاثير الضغط 5.6m/secحيث كان متوسط سرعة البالزما على طول الفجوة بين القطبين هو

الجوي االعتياديز

اوجدت التجارب وجود سلوك حثي سائد في الدائرة معتمدا على قيمة مكوناتها وخصائص

البالزما المتولدة بين قطبي فجوة المفتاح وانعكاس ذلك على ممانعة الدائرة النبضية . وبسبب

التفريغات الكهربائية المتراكمة وتفاعل الطاقة مع سطح القطبين لوحظ وجود اثار للتفريغ الكهربائي

mm 10.5 وةقطب الفولتية العالي على mm 8.3بالنسبة لالقطاب الكروية الشكل وصلت اقطاره

تحت جميع الظروف التجريبية المعطاة وجد ان .على قطب مفتاح البالزما الموصول باالرضي

اوم معتمدة على ظروف التذبذب وطريقة تحليل نتائج 30 اوم و2القيمة المطلقة للممانعة تتراوح بين

اشارات تيارالتفريغ الكهربائي.

قورنت نتائج استخدام االقطاب الثالثة ( الكروي ،االسطواني والقضيب- الكرة ) في حالة

االنهيار الذاتي حيث لوحظ ان االقطاب االسطوانية النحاسية تحتاج الى فولتية اعلى ووقت اطول لكي

يحدث االنهيار الذاتي وذلك باالعتماد على المساحة السطحية التي يقع بها تأثير المجال الكهربائي

بالمقارنة مع االقطاب الكروية والقضيب و الكرة .

استخدم مجال مغناطيسي بين االقطاب االسطوانية حيث كان هناك تأثيرطفيف على قيمة تيار التفريغ

في الفجوة علما ان الملف موضوع بشكل تكون (27G)وخاصةعند زيادة شدة المجال المغناطيسي

فية اعلى شدة للمجال المغناطيسي في منتصف المسافة بين قطبي الفجوة وهناك ايضا تأثيرات على

الفولتية التي يحد ث عندها االنهيار او التفريغ الكهربائي النبضي حيث يؤدي الى تقليلها وكذلك تقليل

الزمن الذي يحدث عنده االتفريغ الكهربائي بين نبضة واخرى عند نفس الظروف المستخدمة في

التجربة في حالة عدم استخدام مجال مغناطيسي .

References

References

[1] Woloszko, Kenneth R. Stalder, and Ian G. Brown "Plasma Characteristics of Repetitively-Pulsed Electrical Discharges in Saline Solutions Used for Surgical Procedures" IEEE Transactions on Plasma Science, Vol. 30, No. 3, pp.1376-1383 (2002).

[2] Abou-Ghazala , S. Katsuki , Schoenbach , F. Dobbs and K. Moreira "Bacterial Decontamination of water by means of pulsed Corona Discharges" IEEE Transaction on Plasma Science, Vol.30, No. 4 ,pp. 1449- 1453 (2002).

[3] A. S. Hasaani "On the Temporal Behavior of Pulsed Relativistic Electron Beam Diodes" Engineering and Technology, Vol. 12, No. 4, pp.56-69 (1993).

[4] W. C. Nunnally, R. Lewis, F. Allen, S. Hawkins, C. Holmses, S. Sarnpayan , and G. Capopraso "Experiments with UV laser triggered spark gaps in a stacked Blumlein system" IEEE Int. Pulsed Power Conference (Monterey ,Ca, USA Journal, pp. 14-17 (2005).

[5] H. Akiyama, T. Sakugawa, T. Namihira, K. Takaki, Y. Minamitani, and N. Shimomura "Industrial Applications of Pulsed Power Technology" IEEE Tran. On Diele. And Elect. Insu. , Vol. 14, No. 5, pp.1051-1064 (2007).

[6] Annemie Bogaertsa, Erik Neytsa, Renaat Gijbelsa, Joost van der Mullenb "Review Gas discharge plasmas and their applications" Spectrochimica Acta Part B 57, pp. 609–658 (2002).

[7] Boris M. Smirnov "Physics of Ionized Gases" (Wiley-VCH, New York, 2001).

[8] A. S. Hasaani, Ala’ F. Ahmed and A. A. Khdayeir "Impedance Characteristics of Pulsed Atmospheric Electrical Discharge in Spherical Plasma Switch" Baghdad Science Journal, Vol.8 No.2, pp.630-637 (2011).

[9] A.S. Hasaani "Magnetically Confined Plasmas: The Past and the Present State of The Art" Iraqi J. Sci. and Technol., Vol.3 No.1, pp.111-120 (2006).

[10] Raizer, Y. P. "Gas discharge physics" (Spring-Verlag, New York 1991).

References

[11] Vijay Nehra, Ashok Kumar and H K Dwivedi "Atmospheric Non-Thermal Plasma Sources" International J. of Engineering, Vol.2 Issue (1), pp.53-68 (2004).

[12] Nicholas A. Krall and Alvin W. Trivelpiece "Principles of plasma physics" (New York: McGraw-Hill, 1973).

[13] Michael .A. Liebermann and Allan J. Lichtenberg "Principles of Plasma Discharges and Materials Processing" (John Wiley & Sons, Inc., New York, 1994).

[14] Sanborn C. Brown "Introduction to Electrical Discharges in Gases" (John Wiley and Sons, INC, 1966).

[15] H Conrads and M Schmidt "Plasma generation and plasma sources" Plasma Sources Sci. Technol. Vol. 9, pp. 441-454 (2000).

[16] J. Reece Roth "Industrial Plasma Engineering" Vol.2. (Bristol and Philedelphia: Instute of Physics Publishing, 2001).

[17] Annemie Bogaerts "The glow Discharge an Exciting Plasma Invited Lecture" J. Anal. At. Spectrom. , pp.1375-1384 (1999).

[18] Rees, J. A. "Electric Breakdown in Gases" (John Wiley & Sons, New York, 1973).

[19] A.Ganguli and R. D. Tarey "Understanding plasma sources" Current Science, Vol.83, No.3, pp. 279-290 (2002).

[20] E. Kuffel, W. S. Zaengl, and J. Kuffel "High Voltage Engineering: Fundamentals" (Newnes, New York, 2000).

[21] Donald. H. Hale "The Townsend Ionization Coefficients for Ni and Al Cathodes in an Atmosphere of Hydrogen" Physical Review, Vol. 56, pp.1199-1202 (1939).

[22] J. M. Meek and J. D. Craggs "Electrical Breakdown of Gases" (Oxford at The Clarendon Press, 1953).

[23] E. Kuffel and M.Abdullah "High Voltage Engineering" (Pergamon Press Itd., Oxford, 1970).

[24] H. Edels, F.W. Crawford "Gaseous Electrical Conduction and the Circuit- Breaker" Journal of the Institution of Electrical Engineers, Vol.2, pp. 712-716, Dec. (1956).

References

[25] A. Chirokov, A. Gutsol and A. Fridman "Atmospheric pressure plasma of dielectric barrier discharges" Pure Appl. Chem., Vol. 77, No. 2, pp. 487–495 (2005 ).

[26] Stephen M. Rossnagel , Jerome J. Cuomo and William D. Westwood "Handbook of Plasma Processing Technology , Fundamentals" (Noyes Publications, Park Ridge, NJ, 1990).

[27] H Bluhm "Pulsed Power Systems: Principles and Applications" (Springer, Berlin, 1996).

[28] N. Georgescu "High Voltage Pulsed, Cold Atmospheric Plasma Jets: Electrical Characterization" Romanian Reports in Physics, Vol. 60, No. 4, pp. 1025–1032 (2008).

[29] Andreas Schutze, James Y. Jeong, Steven E. Babayan, Jaeyoung Park, Gary S. Selwyn and Robert F. Hicks "The Atmospheric Pressure Plasma Jet : A Review and Comparison to Other Plasma Sources" IEEE Transitions On Plasma Science, Vol. 26 , No.6 , pp. 1685-1694 (1998).

[30] R.Barni, S.Zanini, R.Siliprandi, P.Esena, and C.Riccardi "Characterization of atmospheric pressure discharges" Proceedings of the 3rd International Conference on the Frontiers of Plasma Physics and Technology (PC/5099), pp. 17-24, (2006).

[31] David Staack, Bakhtier Farouk, Alexander Gutsol and Alexander Fridman "Characterization of dc atmospheric pressure normal glow discharge" Plasma Sources Sci. Technol.Vol.14, pp. 700- 711 (2005).

[32] Tao Shao, Kaihua Long, Cheng Zhang, Jue Wang, Dongdong Zhang, Ping Yan, Shichang Zhang "Electrical characterization of dielectric barrier discharge driven by repetitive nanosecond pulses in atmospheric air" Journal of Electrostatics, Vol.67, pp. 215–221 (2009).

[33] C. L. Wadhwa "High Voltage Engineering" published by New Age International (P) Limited, 2nd edition (2007).

[34] Shaomao Li "Cold Cathode Materials for Pseudospark Switches" M.Sc., Thesis, Auburn University, Alabama (2010). [35] Esin Bengisu Sozer "Gaseous Discharges and their Applications As High Power Plasma Switches for Compact Pulsed Powers Systems" MSc. thesis, University of Auburn (2008).

[36] Hasibur Rahaman "Investigation of a High Power, High Pressure Spark Gap Switch with High Repetition Rate" PhD. Thesis, University Erlangen–Nürnberg (2007).

References

[37] BL Meena, SK. Rai, MS Tyagi, UN Pal, M Kumar and AK Sharma "Characterization of high power Pseudospark Plasma Switch (PSS)" Journal of Physics: IOP Publishing , Conference Series 208, 012110 (2010).

[38] Hasibur Rahaman, Byung-Joon Lee, Jürgen Urban, Robert Stark, Klaus Frank and S.H. Nam "A spark gap switch with very high repetition rate" 28th ICPIG, , Prague, Czech Republic. pp. 1496-1498, July 15-20 (2007).

[39] Horacio Bruzzone, Cesar Moreno and Roberto Vieytes "Measurement of the time evolution of averaged impedances in small atmospheric pressure spark gap" Meas. Sci. Technol. , Vol.4, pp. 952 -956 (1993).

[40] A. S. Hasaani "Correlation of Paschen Parameters in Magnetized argon Plasma" Iraqi , J. Phys. , Vol. 8, No. 11, pp. 95-101 (2010).

[41] Ya. E. Krasika, K. Chirko, J.Z. Gleizer, A. Krokhmal, A. Dunaevsky, and J. Felsteiner "Application of a ferroelectric plasma cathode as a high-current switch" Eur. Phys. J. D, Vol. 19, pp. 89-95 (2002).

[42] W. Frey, M. Sack, R. Wuestner and G. Mueller "Gas-Insulated Self-Breakdown Spark Gaps" Conference on Electrostatic Precipitation, pp.704-708 (2008).

[43] J. R. Beveridge, S. J. MacGregor, M. J. Given, I. V. Timoshkin "A Corona-stabilized Plasma Closing Switch" IEEE Trans. Dielectr. Electr. l Insul., Vol. 16, No.4, pp. 948-955, (2009).

[44] Z. Liu, A. J. M. Pemen, E. J. M. van Heesch, K. Yan, G. J. J. Winands, D. B. Pawlok "A Multiple-switch Technology for High-power Pulse Discharging" 11th International Conference on Electrostatic Precipitation, pp.704-708 (2008).

[45] H.J.Doucet, M.Roche and J.M.Buzzi "Very high Power Plasma Switches Basic Plasma and Switch Technology" 14, International congress on electric contacts Paris (FR), pp. 1-24 (1988).

[46] D. Bruce Montgomery "The Generation of High Magnetic Fields" Rep. Progress. Phys. Vol.26, pp 69-104 (1963).

[47] A.S. Hasaani "Experimental Studies of Differtiating Rogowski Coils" J. Math. and Phys. ,Vol.13, No.2 , pp. 160-176 (1993).

References

[48] Ahmed H. Wanas "Temporal Behaviour of Glow Discharge in Nitrogen" M.Sc. Thesis, University of Baghdad, College of Science (2005). [49] J.R.Reitz, F.J.Milford, and R.W.Christy "Foundations of Electromagnetic Theory" (Addison-Wesley Publishing Company, Inc., California, 1979).

[50] Mark Lawrence Lipham "Electrical Breakdown Studies of Partial Pressure Argon Under KHz Rang Pulse Voltages" M.Sc. Thesis, Auburn University, Alabama (2010).

[51] Paraselli Bheema Sankar "Measurement of Air Breakdown Voltage and Electrical Field Using Standard Sphere Gap Method" M.Sc. Thesis, National Institute of Technology, Rourkela Rourkela- India (2011).

[52] Allen L. Garner, George Chen , Nianyong Chen, Viswanadham Sridhara, Juergen F. Kolb and Karl H. Schoenbach "Ultrashort electric pulse induced changes in cellular dielectric properties" Biophysical and Biophysical Research Communications, Vol. 362, pp. 139- 144 (2007).

[53] Z. Machala, M. Janda, K. Hensel, I. Jedlovsky, L. Lesˇtinska, V. Foltin, V. Martisˇovits and M. Morvova "Emission spectroscopy of atmospheric pressure plasmas for bio-medical and environmental applications" Journal of Molecular Spectroscopy, Vol. 243 , pp.194–201 (2007).

[54] Bart Roodenburg, Johan Morren, H.E. Berg and Sjoerd W.H. de Haan "Metal release in a stainless steel pulsed electric field (PEF) system Part II. The treatment of orange juice; related to legislation and treatment chamber lifetime" Innovative Food Science and Emerging Technologies Vol. 6, pp. 337 – 345 (2005).

[55] Jozef Korolczuk, Jose Rippoll Mc Keag, Jose´ Carballeira Fernandez, Florence Baron, Noe¨l Grosset , and Romain Jeantet "Effect of pulsed electric field processing parameters on Salmonella enteritidis inactivation" J. of Food Engineering Vol.75 , pp. 11-20 (2006).

[56] V. Schulz Gathen "Atmospheric Pressure Glow Discharges for Surface Treatment: Selected examples" XXVIIth ICPIG, Eindhoven, the Netherlandes , pp.1-2, July 18-22 (2005).

[57] D. Korzec, E.G. Finantu-Dinu, A. Schwabedissen, J. gemann, J. Ra´hel, M. Sˇ tefecka, Y. Imahori, and M. Kando "Insulated surface discharge for metastables driven processing at atmospheric pressure" Surface and Coatings Technology, pp. 169 –170 (2003).

References

[58] Fuhliang Wen, Jhenyuan Lin Hungjiun and Kuo-Hwa Chang "Pulsed Atmospheric Pressure Plasma System Applied to PCBs Surface Treatment" The 3rd International Multi-Conference on, Engineering and Technological Innovation (IMETI), –Orlando, Florida, USA, June 29th - July 2nd (2010).

[59] Psarkar, S. Chaturvedi, Raj Kumar, Rajesh Kumar, D. Lathi, ,Ashyam and J. Sonara "Operation of a capacitor bank for plasma metal forming" Pramana J. of physics, Vol. 55, No. 5, pp. 941-945 (2000).

[60] Ananth N. Bhoj and Mark J. Kusshner "Repetitively pulsed atmospheric pressure discharge treatment of rough polymer surfaces: II. Treatment of micro-beads in He/ NH3 /H2Oand He/ O2 / H2O mixtures" Plasma Sources Sci. Technol. Vol.17, pp. 1-15 (2008).

[61] Bart Roodenburg, , Johan Morren, H.E. Berg, and Sjoerd W.H. de Haan "Metal release in a stainless steel Pulsed Electric Field (PEF) system Part I. Effect of different pulse shapes; theory and experimental method" Innovative Food Science and Emerging Technologies Vol. 6 , pp. 327– 336 (2005).

[62] A. Goldman and R. S. Sigmond "Post- Discharge Phenomena on Surfaces" PISP- 7Eindhoven, Vol.2, pp. 454-459 , July (1985).

[63] H.A. Davis, B.P. Wood, C.P. Munson , L.J. Bitteker , D. M. Cotes, and H. M. Schleinitz "Ion beam and plasma technology development for surface modification at Los Alamos National Laboratory" Materials Chemeistry and Phyisics, Vol. 54 , pp. 213-218 (1998) .

[64] Mruthunjaya Uddi, Naibo Jiang, Evgeny Mintusov, Igor V. Adamovich and Walter R. Lempert "Atomic oxygen measurements in air and air/fuel nanosecond pulse discharges by two photon laser induced fluorescence" Proceedings of the Combustion Institute , Vol.32, pp. 929-936 (2009).

[65] D.L. Carroll, J.T. Verdeyen, D.M. King, J.W. Zimmerman, J.K. Laystrom, B.S. Woodard, G.F. Benavides, K. Kittell, D.S. Stafford, M.J. Kushner, and W.C. Solomon "Continuous-Wave Laser Oscillation on the 1315 nm Transition of Atomic Iodine pumped by O2(a1Δg) Produced in an Electric Discharge" Appl. Phys. Lett., Vol. 86, 111104 (2005).

[66] A. Hicks, Yu. G. Utkin, W. R. Lempert, J. W. Rich, and I. V. Adamovicha "Continuous wave operation of a non-self-sustained electric discharge pumped oxygen-iodine laser" Applied Physics Letters, Vol. 89, 241131 (2006) .

References

[67] K. Hirayama, D. Wang, M.Matsuda , X. Lin , T. Namihira, H.Takano , S. Takio and H. Akiyama "Activation of Retrotransposon in Red Alga by Underwater Pulsed Discharge" Acta Physica Polonica A , Vol. 115, No. 6, pp. 1110-1111 (2009).

[68] C.P. Lungu, A.M.Lungu, P. Chiru, O. Pompilian , N.Georgescu, M.Maguranu, and V. Predica "Treatment of Levigated Materials and Saturated Waters" Rom. Journ Phhs., Bucharest, Vol. 54, No. 3, pp. 369- 375 (2009).

[69] Philip Grigorievich Rutberg, Victor Andreevich Kolikov, Vladimir Efimovich Kurochkin, Ludmila Konstantinovna Panina, and Alexander Philipovich Rutberg "Electric Discharges and the Prolonged Microbial Resistance of Water" IEEE Transactions on Plasma Science, Vol. 35, No. 4, pp. 1111-1118 (2007).

[70] S Mededovic and B R Locke "Primary chemical reactions in pulsed electrical discharge channels in water" J. Phys. D: Appl. Phys. Vol. 40, pp. 7734–7746, (2007).

[71] P.D.Starchyk, P.V. Porytskyy "On the Optical Properties of the Nonideal Plasma of Electrical Pulse Discharge in Water" Problem of Atomic Science and Technology, Series Plasma Physics, Vol. 13, No. 1, pp. 182-184 (2007).

[72] S. Gershman, O. Mozgina, A. Belkind, K. Becker, and E. Kunhardt "Pulsed Electrical Discharge in Bubbled Water " Contrib. Plasma Phys. Vol. 47, No. 1, pp.19 – 25 (2007).

[73] Bin Yang, Le Cheng Lei and Ming Hua Zhou "Effects of the Liquid Conductivity on Pulsed High Voltage Discharge Modes in Water" Chinese Chemical Letters, Vol. 15, No.10 pp. 1215- 1218 (2004).

[74] Muhammad Arif Malik1, Abdul Ghaffar and Salman Akbar Malik "Water purification by electrical discharges" Plasma Sources Sci. Technol. Vol. 10 , pp. 82–91 (2001) .

[75] P. Sunka, V.Babicky, M. Clupek, P. Lukes, M.Simek and B.R.Locke "Potential applications of Pulse electrical discharge in Water" XXVIIth ICPIG, Eindhoven , the Netherlands, 18-22, July, (2005).

[76] A. Pokryvailo, M. Wolf, Y.Yankelevich, E.Abramzon, and A. Welleman "Compact High- Power Pulsed Corona Source for Treatment of Pollutants in Heterogeneous Media" XXVIIth ICPIG, Eindhoven , the Netherlands, 18-22, July (2005).

References

[77] Richard A. Korzekwa, Louis A.Rosocha and Z. Falkenstein "Experimental Results comparing Pulsed Corona and for Pollution Control" Eleventh IEEE International Pulsed Power Conference, Balitimore, Maryland, June 29 July 2 (`1997).

[78] Shesha H. Jayaram "Pulse Power Applied to Process Industry and Environment" IEEE Industry Application magazine ,Vol. 6, Issue 4, pp. 34-40, July (2010 ).

[79] S.A. Nair, K. Yan, A.J.M. Pemen, G.J.J. Winands, F.M. van Gompel, H.E.M. van Leuken, E.J.M. van Heesch, K.J. Ptasinski and A.A.H. Drinkenburg "A high-temperature pulsed corona plasma system for fuel gas cleaning" J. of Electrostatics , Vol. 61 pp.117–127 (2004).

[80] Joel M. Goldberg and Kevin and P. Carney "Electrical characteristics of imploding thin film plasma atom cells" Spectrochimica Acta. . Vol. 45B. No. I0. pp. 1167- 1175. (1990).

[81] Mingmei Wang and Mark J. Kushner "High Energy Electron Fluxes in dc- Augmented Capacitively Coupled Plasmas. II. Effects on Twisting in High Aspect Ratio Etching of Dielectrics" Journal of Applied Physics, Vol. 107, 023309, (2010).

[82] Daniel Lundin , Ulf Helmersson , Scott Kirkpatrick, Suzanne Rohde and Nils Brenning "Anomalous Electron Transport in High Power Impulse Magnetron Sputtering" Plasma Sources Sci. Technol. ,Vol. 17, 025007 (2008).

[83] A. Belkind, A. Freeilich, J. Lopez, Z. Zhao and W. Zhu "Characterization of Pulsed dc Magnetron Sputtering Plasmas" New J. of Physics , Vol. 7, No 90, pp. 1-16, (2005).

[84] E. G.Grigoryev "Kinetics of Densification Processes of Powder Materials under Electro Pulse Sintering" The Arabian Journal for Science and Engineering Vol.34, No.1C, pp.29-33 (2009).

[85] J. B. Javedaniξ , D. A. Goerz, T. L. Houck, E.J. Lauer, R. D. Speer, L. K. Tully and G.E. Vogtlin "Understanding High Voltage Vacuum Insulators for Microsecond Pulses" IEEE International Pulsed Power Conference Albuquerque, NM, United States, June 17-22 (2007).

[86] F. Le Pimpec and R.Ganter "Field Emission Dark Current of Technical Metallic Electrodes" Nucl. Instrum. Methods Phys. Res. Sect A, Vol. 574, p.22 (2006).

References

[87] A.N. Grigoriev1, A.V. Pavlenko, and E.I. Karnaukhov "Characteristics of Multi-channel Surface Discharge Switch for High Current Generator" 14th IEEE, International Pulsed Power Conference, Vol.2, pp. 313-315 (2003). [88] Biswajit Adhikary, Anurag Shyam "Development of 1MV Marx Generator" 10th International Conference on Modification of Materils with Particle Beams and Plasma, pp. 335- 337 (2010).

[89] M. Beg, M. Shabbir, M. Zakaullah and G. Murtaza "Pressure range broadening for a plasma focus operation" Physics Letters A , Vol. 186 , pp.335-338 (1994).

[90] A. Bernard, P. Cloth, H. Conrads, A. Coudeville, G. Gourlan , A. Jolas, Ch. Masisonnier and J. P. Rager "The Dense Plasma Focus A High Intensity Neutron Source" Nuclear Instruments and Methods , Vol. 145, pp.191-218 (1977).

[91] J.S. Brzosko, V. Nardi, J.R. Brzosko and D. Goldstein "Observation of plasma domains with fast ions and enhanced fusion in plasma-focus discharges" Physics Letters A, Vol. 192, pp.250-257, (1994).

[92] S.R. Mohanty, N.K. Neog, R.S. Rawatb, P. Leeb, B.B. Nayak and B.S. Acharya "Self-organized transformation to polyaniline nanowires by pulsed energetic electron irradiation in a plasma focus device" Physics Letters A, Vol. 373, pp.1962–1966 (2009).

[93] M. Habibi, R. Amrollahi, and M. Farrahi "Study of Dence Plasma Surface Interaction by a Filippov Type Plasma Focus Device" Brazilian Journal of Physics Vol. 38, No.2, pp.264-267(2008).

[94] Lekha Nath Mishra , Kanetoshi Shibata , Hiroaki Ito, Noboru Yugami and Yasushi Nishida "Conversion of Methane to Hydrogen via Pulsed Corona Discharge" Journal of Natural Gas Chemistry, Vol. 13, pp. 82-86 (2004) .

[95] Mishra Leka Nath, Shibata Kanetoshi, Ito Hiroaki , Yugami Noboru and Nishida Yashushi "Characteristics of Methane Destruction Using a Pulsed Corona Discharge at Atmospheric Pressure" J. Plasma Fusion Res. Series, Vol. 6, pp.760- 763, (2004) .

[96] Bing Sun , Masayuki Sato and J S Clements "Use of a Pulsed High Voltage Discharge for Removal of Organic Compounds in Aqueous Solution" J. Phys. D: Appl. Phys.Vol.32, pp. 1908-1915 (1999).

References

[97] Yoshiro Nakagawa and Hiroshi Kawauchi "Production of a Pulsed High-Density Electron Beam by Channel Spark Discharge" Electrical Engineering in Japan, Vol. 134, No. 4, pp.10 -18 (2001).

[98] W. H. Tuan , S. M. Liu, C.J. Ho, C.S. Lin, T. J. Yang, D. M. Zhang, Z. Y. Fu and J.K. Guo "Preparation of Al2 O3 – ZrO2- Ni Nano composite by Pulse Electric Current and Pressure less Sintering" Journal of the European Ceramic Society , Vol. 25 , pp. 3125- 3133 (2005).

[99] Yong Hwan Lee, Won Suk Jung, Yu Ri Choi. , Jong Seok Oh , Sung Duckjang , Y oon Gyuson, Moo Hyuncho, Won Namkung ,Dong Jun Koh ,Young Sun Mok and Jae Woo Chung "Application of Pulsed Corona Induced Plasma Chemical Process to an Industrial Incinerator" Environ. Sci. Technol., Vol. 37, pp. 2563-2567 (2003).

[100] T. V. Babu Rajendran , C.S. Lalshminarasimha and M.S. Naidu "Lightning and Switching impulse breakdown of rod – plane gaps in nitrogen and nitrogen/ Freon (CCl2F2) Mixtures" IEE Proc. Vol.130, Pt. A, No. 3, pp.134-139, (1983).

[101] J. Dams, P. Osmokovic and Schwab "Design of Low Jitter Spark Gaps For Impulse Switching Applications" Fifth Intentional Symposium on High Voltage Engineering, Braunschweig, Federal Republic of Germany ,pp.1-4, August 24-28 (1987).

[102] Z. Liu, A. J. M. Pemen, R. T. W. J. van Hoppe, G. J. J. Winands, E. J. M. van "An Efficient, Repetitive Nanosecond Pulsed Power Generator with Ten Synchronized Spark Gap Switches" IEEE Transactions on Dielectrics and Electrical Insulation Vol. 16, No. 4, pp. 918-925(2009).

[103] Horacio Bruzzone, Hector Kelly and Cesar Moreno "The Effect of Transmission Lines and Switching Action on the Electrical Signals in a Powerful Capacitive Discharge" IEEE Transactions on Plasma Science, Vol. 18, No.4, pp.689-694 (1990).

[104] V.D.Bochkov, D.V. Botchkov, V. M.Dyagilev, V.N.Kudinov, V.G.Ushicch V.A.Glouschenkov and R.Yu.Yusupov "High Power Pseudospark Switches for Pulsed Power" Proc International Power Modulator Conference, Hollywood, USA, June 30 - July 3 (2002).

[105] Peter Choi, Hernan Chuaqui, J. Luney, R. Reichle, A. J. Davies, and K. Mttag "Plasma Formation In a Pseudospark Discharge" IEEE Transactions On Plasma Science. Vol.17, No.5, pp. 770-774 (1989).

[106] V.D. Bochkov, D.V. Bochkov, V.M. Dyagilev and V.G. Ushich SN-series "Pseudospark Switches, Operating Completely without

References

Permanent Heating. New Prospects of Application in Pulsed Power" Acta Physica Polonica, A Vol. 115 No. 6, pp. 980-982 (2009).

[107] Zdenko Machala, Emmanuel Marode, Christophe O.Laux and Charles H. Kruger "DC Glow Discharges in Atmospheric Pressure Air" J. Adv. Oxid. Technol. Vol. 7, No.2, pp.133-137 (2004).

[108] Ananth N. Bhoj and Mark J.Kushner "Continuous processing of Polymers in Repetitively pulsed atmospheric pressure Discharges with Moving Surfaces and Gas Flow" J. Phys. D: Appl. Phys., Vol. 40 pp.6953- 6968 (2007).

[109] R. Sosa1, D. Grondona, A. M´arquez, G. Artana and H. Kelly "Electrical characteristics and influence of the air-gap size in a trielectrode plasma curtain at atmospheric pressure" J. Phys. D: Appl. Phys., Vol. 42, 045205, pp.1-7 (2009).

[110] Shao Tao1, Long Kaihua1, Zhang Cheng, Yan Ping, Zhang Shichang and Pan Ruzheng "Experimental study on repetitive unipolar nanosecond-pulse dielectric barrier discharge in air at atmospheric pressure" J. Phys. D: Appl. Phys. Vol.41, 215203, pp.1-8 (2008).

[111] Yasin Khan, Akihito Oda, Shigemitsu Okabe, Junya Suehiro and Masanori "Wire Particle Motion Behavior and Breakdown Characteristics around Different Shaped Spacers Within Diverging Air Gap" IEEJ Trans. PE, Vol. 123, No.11, pp.1288-1295 (2003).

[112] Zdenko Machala , Christophe O. Laux, Xavier Duten , Denis M. Packan, Lan Yu and Charles H. Kruger "Scaled up Nonequilibrium Air Plasmas" American Institute of Aeronautics and Astronautics, (AIAA) , 874, pp.1-15, January 6-9 (2003).

[113] K. Criner1, A. Cessou, and P. Vervisch "A Comparative Study of the Stabilization of Propane Lifted Jet-Flames by Pulsed, AC and DC High-Voltage Discharges" Third European Combustion Meeting ECM (2007).

[114] Kefu Liu, Jiang Qiu, Houxiu Xiao, Qiong Hu, Shengguo Xia "An All-Solid-State Pulsed Power Generator for Non-Thermal Plasma Discharge Applications" 13th International Symposium on High Current Electronics, Tomsk, Russia, pp.157-160 (2004).

[115] S. Pancheshnyi, M. Nudnova, and A. Starikovskii "Development of a cathode-directed streamer discharge in air at different pressures: Experiment and comparison with direct numerical simulation" Physical Review, Vol. 71, 016407 (2005).

References

[116] I. V. Uimanov "Electric Field at a Cathode Microprotrusion Under Intense Field Emission" 13th International Symposium on High Current Electronics, Tomsk , Russia, pp. 63-65 (2004).

[117] Joshua Leckbee, Randy Curry, and Ken McDonald "An Advanced Model of A High Pressure Liquid Dielectric Switch For Directed Energy Applications" 14th IEEE, International Pulsed Power Conference, Vol.2, pp. 1389-1393 (2003).

[118] S.K. Lam, D.Lo, C.E.Zheng C.Shangguan, T.L.Yang and I.V.Kochetov "Parametric Study of Xe2

Dimer in High- Pressure Electrical Discharges" Appl. Phys. B Lasers and Optics, Vol. 75, Issue 6-7, pp. 723-730 (2002).

[119] Xiang Wei, Zhao Weijiang, Zeng Baoqing and Yang Zhonghai "Computer Simulation of Impedance Characteristics For Magnetically Insulated Diodes" Proceedings of the Second Asian Particle Accelerator Conference , Beijing, China, pp.173-175 (2001).

[120] M.P.Desjarlais "Impedance Characteristics of Applied- B Ion Diodes" Physical Review Letters, Vol.59, No.20, pp.2295-2298 (1987).

[121] Sven Bonisch, David Pommerenke and Wilifried Kalkner "Broadband Measurement of ESD Rise times to Distinguish between Different Discharge Mechanisms" J. of Electronics Vol. 56 pp.363-383 (2002).

[122] N. R. Pereira and A. Fisher "Slower Impedance Collapse with Hot Anode and Enhanced Emission Cathode" IEEE Transactions on Plasma Science, Vol. 27, No. 4, pp.1169-1174 (1999).

[123] D.J .Johnson, P.L. Dreike, S.A. Slutz, R.J. Leeper , E.J. T.Burns, J.R. Freeman, T.A.Mehlhorn and J.P. Quintenz,n "Applied -B field Ion Diode Studies at 3.5 TW" J. Appl. Phys. Vol. 54, No.5, pp.2230-2241 (1983).

[124] Gabi D. Stancu, Mario Janda, Farah Kaddouri, Deanna A. Lacoste, and Christophe O. Laux "Time-Resolved CRDS Measurements of the N2 (A3Σu

+) Density Produced by Nanosecond Discharges in Atmospheric Pressure Nitrogen and Air" J. Phys. Chem. A, Vol. 114, pp. 201–208 (2010).

[125] Tsuyohito Ito, Kazunobu Kobayashi, Uwe Czarnetzki and Satoshi Hamaguchi "Rapid formation of electric field profiles in repetitively pulsed high-voltage high-pressure nanosecond discharges" J. Phys. D: Appl. Phys. Vol.43, 062001 (2010).

References

[126] A. Dutta, I. Choi, M. Uddi, E. Mintusov, A. Erofeev, Z. Yin, W.R. Lempert, and I.V. Adamovich "Cavity Flow Ignition and Flameholding in Ethylene-Air by a Repetitively Pulsed Nanosecond Discharge" 47th Aerospace Sciences Meeting and Exhibit, Orlando, FL.(AIAA), 0821, January 5-8 (2009).

[127] Munetake Nishihara, Naibo Jiang, J. William Rich, Walter R. Lempert, and Igor V. Adamovich, and S. Gogineni "Low-temperature supersonic boundary layer control using repetitively pulsed magnetohydrodynamic forcing" Physics of Fluids, Vol. 17, No. 10, 106102 (2005).

[128] David L. Carroll, Joseph T. VerdeyenDarren M. King, Joseph W. Zimmerman, Julia K. Laystrom, Brian S. Woodard, Gabriel F. Benavides, and Wayne C. Solomon "Studies of CW Laser Oscillation on the 1315-nm Transition of Atomic Iodine Pumped by O2Produced in an Electric Discharge" IEEE J. Quantum Electron., Vol. 41 , No. 10, pp.1309-1315 (2005).

[129] Ainan Bao, Yurii G. Utkin, Saurabh Keshav, and Igor V. Adamovich "Methanol and Ethanol Ignition by Repetitively Pulsed, Nanosecond Pulse Duration Plasma1" 45th Aerospace Sciences Meeting and Exhibit, Reno, NV, (AIAA), 1387, January 8-11 (2006).

[130] E. Mintusov, M. Nishihara, N. Jiang, I. Choi, M. Uddi, A. Dutta, W.R. Lempert, and I.V. Adamovich "Nanosecond Pulse Burst Ignition of Ethylene and Acetylene by Uniform Low-Temperature Plasmas" 39 Plasma dynamics and Lasers Conference, Seattle, WA. (AIAA), 3899, June 23-26 (2008).

[131] M. Uddi1, N. Jiang2, I. V. Adamovich3, and W. R. Lempert4, "Nitric Oxide Density Measurements in Air and Air/Fuel Nanosecond Pulse Discharges by Laser Induced Fluorescence" 39 Plasma dynamics and Lasers Conference, Seattle, WA. (AIAA), 3884, June 23-26 (2008).

[132] S.Korenev, V. Efanov and I.Korenev "The Nanosecond Generators of Air Plasma" Pulsed Power Conference IEEE, pp. 860-863 (2005).

[133] Max Chung, and Erich Kunhardt "Novel Trigger Mechanism High-Power Switch: The Electrostatic Plasma Injection Switch" IEEE Transitions On Plasma Science, Vol. 34, No. 5, pp.1626-1639 (2006).

[134] F. E. Peterkin and P.F. Wisiiams "Physical mechanism of triggering in trigatron spark gaps" Appl. Phys. Lett. Vol.53, No.3, pp.182-184 (1988).

References

[135] M.Murano, H.Nishikawa, A.Kobayashi, T.Okazaki and S. Yamashita "Current Zero Measurement for Circuit Breaking Phenomena" IEEE Transactions on Power Apparatus and Systems, Vol.PAS-94, No.5, pp.1890-1900 (1975).

[136] I. M. Podqornyi "Topics in Plasma Diagnostics" (New York, London 1971).

[137] F.Piperno and G. Solaini "On Rogowski Coil as Quantitative Probe for Current Density Measurements in a Linear Pinch" IL Nuovo Cimento , Vol. 29 B, No. 1, pp. 239-245 (1975).

[138] Sanda Pleslic and Zeljko Andreic "Application of Rogowski coil in Fast Pulsed Current Measurements of capillary Discharge" Fizika A(Zagreb) Vol. 16, No. 4, pp. 233-242 (2007).

[139] Jan Hlavacek, Radek Prochazka Karel Draxler and Vladislav Kvasnicka "The Rogowski Coil Design Software" 16th IMEKO TC4 Symposium Exploring New Frontiers of Instrumentation and Methods for Electrical and Electronic Measurements, Florence, Italy, pp.22-26, Sept. (2008).

[140] Donald G. Pellinen, Marco S. , Di Capua, Stephen E. Sampayan, Harold Geribracht and Ming Wang "Rogowski coil for Measuring Fast, High Level Pulsed Current" Rev. Sci. Instrum, Vol. 51, No. 11, pp. 1535-1540 (1980).

[141] D. A. Ward and J. La T. Exon "Using Rogowski Coils for Transient Current Measurements" Engineering Science and Education Journal, pp. 105-113, June (1993).

[142] G. Murtaza Hashmi and Matti Lehtonen "On-line PD Measuring System Modeling and Experimental Verification for Covered-Conductor Overhead Distribution Lines" Proceedings of the 15th Mediterranean Conference on Control and Automation, Athenes- Greece, T21-010, July 27-29 (2007).

[143] Richard H. Huddleston, and Stanley L. Leonard "Plasma Diagnostic Techniques" (Academic Press, Inc., New York, 1965).

[144] Rodney Alan Petr "Erosion Phenomena of Arcing Electrodes" M.Sc. Thesis, Texas Tech University (1980).

[145] Essam Nasser "Fundamentals of Gaseous Ionization and Plasma Electronics" (John Wiley, 1971).

جمهورية العراق

وزارة التعليم العالي و البحث العلمي جامعة بغداد- كلية العلوم

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كلية العلوم / جامعة بغداد وهي جزء من متطلبات نيل درجة الدكتوراه الفلسفة في الفيزياء

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آالء فاضل أحمد الراشدي 1999بكالوريوس

2002ماجستير

بأشراف أ.م.د. عبد الرضا سلمان حساني

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a thesis submitted to the college of science / university of baghdad … · 2012-09-23 · college...

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