maher i. boulos, pierre l. fauchais, and emil pfender · plasmas. separate sections deal with...

The Plasma State

Maher I. Boulos, Pierre L. Fauchais, and Emil Pfender

Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2 The Plasma State, Fourth State of Matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.1 What Is a Plasma? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.2 Plasma Temperature(s) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3 Different Types of Plasmas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.1 Natural Plasmas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.2 Man-Made Plasmas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4 Nonequilibrium, Man-Made Cold Plasmas (Te � Th) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4.1 Glow Discharges and Some of Their Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4.2 Corona Discharges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

4.3 Dielectric-Barrier Discharges (DBD) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

4.4 Thermal, Man-Made Plasmas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

4.5 Basic Concepts Used for the Generation of Thermal Plasmas . . . . . . . . . . . . . . . . . . . . . . . 26

4.6 Thermal Plasma Sources and Their Fields of Applications . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Nomenclature and Greek Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

M.I. Boulos (*)

Department of Chemical Engineering, University of Sherbrooke and Tekna Plasma Systems Inc.,

Sherbrooke, Québec, Canadae-mail: [email protected]

P.L. Fauchais

European Center of Ceramics, University of Limoges, Limoges, France

e-mail: [email protected]

E. Pfender

Department of Mechanical Engineering, University of Minnesota, Minneapolis, MN, USA

e-mail: [email protected]

# Springer International Publishing Switzerland 2016

M. Boulos et al. (eds.), Handbook of Thermal Plasmas,DOI 10.1007/978-3-319-12183-3_1-2

1

mailto:[email protected]



List of Abbreviations

AC Alternating current

CLTE Complete local thermodynamic equilibrium

CVD Chemical vapor deposition

DBD Dielectric barrier discharges

DC Direct current

DLC Diamond-like carbon

EAF Electric arc furnace

EB-PVD Electron beam-physical vapor deposition

EM Electromagnetic

ER Erosion rate

GM General motors

HID High intensity discharge

HSW Hospital solid waste

ICP Inductively coupled plasma

ICP-MS Inductively coupled plasma mass spectrometry

ICP-OES Inductively coupled plasma optical emission spectroscopy

LLRW Low-level radioactive waste

LTE Local thermodynamic equilibrium

LUX Lux per Watt

MIG Metal inert gas

MIPs Microwave-induced plasmas

MSW Municipal solid waste

NASA National Aeronautics and Space Administration

PACVD Plasma-assisted chemical vapor deposition

PAPVD Plasma-assisted physical vapor deposition

PLTE Partial local thermodynamic equilibrium

PS-PVD Plasma sprayed-physical vapor deposition

PVD Plasma vapor deposition

RF Radio frequency

SSW Sewage sludge waste

TIG Tungsten inert gas

TPH Ton per hour

TWD Traveling wave discharges

UV Ultraviolet

VOCs Volatile organic compounds

1 Introduction

This chapter serves as an introduction to the vast and rapidly growing field of

plasma science and technology. A brief introduction defining what is a plasma and

the different types of plasmas, whether natural or man-made, is presented. This isfollowed by a review of the main characteristics and applications of man-made

2 M.I. Boulos et al.

plasmas. Separate sections deal with nonequilibrium, man-made plasmas, whether

glow discharges, corona discharges, dielectric barrier discharges (DBD), or micro-

wave plasmas and their respective industrial applications. Thermal, man-made

plasmas are discussed next, identifying the basic concepts for the generation of

such plasmas followed by a description of principal thermal plasma sources and

their respective industrial applications. Whenever applicable, forward referencing

is made to subsequent chapters in the handbook where further details are given on

the different subjects.

2 The Plasma State, Fourth State of Matter

The plasma state is frequently referred to as the fourth state of matter. The definition

is derived from the concept that the solid state is a ground state or first state of

matter, which exists at the lowest end of the temperature and specific enthalpy

scale. As illustrated in Fig. 1, the increase of the specific enthalpy of matter results

in corresponding increase of its temperature, until its melting point is reached, at

which it passes to the liquid state, the second state of matter. The further increase of

the specific enthalpy of matter results in the corresponding increase of its temper-

ature until its passage, at the vaporization temperature, to the vapor phase, the third

state of matter. With the subsequent increase of the specific enthalpy of matter, it

eventually reaches the plasma state, the fourth state of matter, at which molecules

start to dissociate and atoms to ionize forming a mixture of molecules, atoms, and

ions resulting from ionization of molecules and/or atoms and electrons all in local

electrical neutrality. This implies that plasmas are electrically neutral with negative

electrons and negatively charged ions, if any, balanced by positively charged ions.

Molecules, atoms, and ions are either in fundamental or excited states. The lifetime

of excited species is low (varying from a few ns to the μs range). Plasmas also

contain photons emitted as a result of transition of molecules, atoms, and ions from

an excited state to lower energy levels or their corresponding ground states. Photons

Solid

Liquid

Vapor

Temperature (K)

Spe

cific

ent

halp

y (M

J/kg

)

Tm Tv

THE PLASMA STATE

IonsAtoms

Molecules

Electrons

Fig. 1 Schematic

representation of the four

states of matter as function of

temperature and the specific

enthalpy of matter

The Plasma State 3

can also be emitted by electromagnetic radiation produced by the deceleration(bremsstrahlung) of a charged particle when deflected by another charged one,

typically an electron by an atomic nucleus. The moving particle loses kineticenergy, which is converted into a photon because energy is conserved. Theseprocesses are at least partially responsible for the luminosity of plasmas. Of course

if excited atoms or molecules emit photons when de-exiting, reciprocally photons

can excite atoms or molecules in lower excitation states or in ground state and

plasmas can also be produced by photoionization.

This classification of plasma as a “state of matter” is justified by the fact that

more than 99 % of the known universe is in the plasma state. A typical example is

the Sun, whose interior temperatures exceed 107 K. The high energy content of a

plasma compared to that of solids, liquids, or ordinary gases lends itself to a number

of important applications. Many textbooks have been devoted to plasma physics

and engineering such as Cobine (1958), Brown (1959), Cambel (1963), Mitchner

and Kruger (1973), Peratt (1992), Bittencourt (2004), and Fridman (2004, 2008).

2.1 What Is a Plasma?

As mentioned earlier, plasmas consist of a mixture of electrons, ions, and neutral

particles. Since the masses of ions and neutrals are much higher than the electron

mass (mH=me ¼ 1836, where mH = mass of the hydrogen atom, lowest atom mass,

and me = electron mass), neutrals and ions are classified as the heavy particles or

the heavy component in plasma. The mixture of electrons, ions, and neutrals in the

ground state, excited species, and photons can be designated as plasma only if the

negative and positive charges balance each other, i.e., overall plasma must be

electrically neutral. This property is known as quasi neutrality.

In contrast to gas at ambient temperature and pressures, plasmas are electrically

conducting due to the presence of free charge carriers. In fact, plasmas may reach

electrical conductivities exceeding those of metals at room temperature. For exam-

ple, hydrogen plasma at atmospheric pressure (100 kPa) and a temperature of 106 K

has approximately the same electrical conductivity as that of copper at room

temperature. Plasmas generally have a low enough density to obey

Maxwell–Boltzmann statistics rather than Fermi–Dirac or Bose–Einstein ones.

However, the dynamical behavior of plasmas is generally more complex than that

of gases and fluids, due to:

(i) Weak Coulomb scattering, resulting in mean free paths of the electrons and

ions often larger than the plasma’s macroscopic length scales. This allows the

particles’ momentum distribution functions to deviate significantly from their

equilibrium Maxwellian forms and, in particular, to be highly anisotropic.

(ii) The long-range electromagnetic fields allowing charged particles coupling to

each other electromagnetically and acting in concert as modes of excitation

(plasma waves) that behave like single dynamical entities.


A more rigorous definition of the plasma state, taking second-order effects into

account, will be given in chapter 4, “▶ Fundamental Concepts in Gaseous Elec-

tronics” of this part.

2.2 Plasma Temperature(s)

The temperatures of a plasma, as in any gaseous medium, are defined by the average

kinetic energy of its constituents or particles (molecule, atom, ion, or electron),

which according to Maxwell–Boltzmann statistics is given by the equation:

1

2mv2 ¼ 3

2kT (1)

where m is the mass of the particle,ffiffiffiffiffiv2

pis its root mean square (rms) or effective

velocity, k is the Boltzmann constant, and T represents the absolute temperature

(K). Equation 1 results from Maxwell–Boltzmann distribution, which can be

expressed by

dnv ¼ n f vð Þdv (2)

with the distribution function f(v) defined as f vð Þ ¼ dnv=dvð Þ and

f vð Þ ¼ 4ffiffiffiπ

p 2kT

m

� �32

v2exp �mv2

2kT

� �(3)

As shown in Fig. 2, f(v) reaches a maximum at vm ¼ffiffiffiffiffiffi2kTm

q. The number density of

molecules with velocities between v and vþ dv is given by dnv. From this

distribution it follows that the average velocity is

Fig. 2 Maxwellian

distribution function

evolution with velocity

(Boulos et al. 1994)

The Plasma State 5

Fundamental Concepts in Gaseous Electronics


v ¼ð10

vf vð Þdv ¼ffiffiffiffiffiffiffiffi8kT

πm

r(4)

And, the mean square velocity is

v2 ¼ð10

v2f vð Þdv ¼ffiffiffiffiffiffiffiffi3kT

m

r(5)

The establishment of a Maxwell–Boltzmann distribution among the particles in

plasma or in an ordinary gas depends strongly on the interaction between the

particles, i.e., on the collisional frequency and on the energy exchange during

each collision. By applying the conservation equations to an elastic binary collision

of particles with mass m and m0, one can show that on the average kinetic energy

exchanged is given according to Howatson (1965) and Chang and Pfender (1990)

by

ΔEkin ¼ 2mm0

mþm0ð Þ2 (6)

This result implies that for particles of the same mass m ¼ m0ð Þ, ΔEkin ¼ 1=2, and

therefore any distortion of the Maxwell–Boltzmann distribution among particles of

the same mass will be eliminated by fewer than 10 successive collisions.

These considerations demonstrate that in a collision-dominated plasma, we can

assume that heavy species and electrons among themselves will have a

Maxwell–Boltzmann distribution that permits the definition of a corresponding

temperature for these species.

If the subscript, r, designates the various components (such as electrons, ions,

and neutrals) in a plasma, then the Maxwell–Boltzmann distribution for each of

these components can be written in terms of their kinetic energy Er ¼ 1=2mv2r (see

chapter 3, “▶Kinetic Theory of Gases” for details) as

dnEr¼ 2nrffiffiffi

πp kTrð Þ�3=2

E1=2r exp � Er

kTr

� �dEr (7)

where Tr represents the temperature of the component r. As the following discus-

sion will show, the temperatures of the various components of plasma may or may

not be the same.

Let’s consider the energy exchange between electrons and heavy species. With

m0 ¼ me (electron mass) and m ¼ mh (mass of the heavy species), we find from

Eq. 6 that

ΔEkin ¼ 2me

mh

(8)


Kinetic Theory of Gases

Since me � mh, a large number of collisions (>103) are required to equilibrate the

temperatures of the electrons and heavy particles and eliminate energy

(or temperature) differences between them.

In an electric discharge, which is one of the most common ways to generate and

maintain plasma, the high-mobility electrons pick up energy from the applied electric

field and then transfer part of this energy to the heavy particles through elastic

collisions. Even with an excellent collisional coupling (high collision frequency)

between electrons and heavy particles, there will always be a difference between the

electron temperature and the temperature of the heavy species in the plasma. The

energy transferred from an electron to a heavy particle in a single elastic collision

may be expressed by 32k Te � Thð Þ 2me

mh

� �, where Te and Th represent, respectively, the

electron and the heavy particle temperatures. The energy that an electron acquires

from the electric field (E) between collisions is given by e E vd τe , where vdis the average drift velocity (see chapter 4, “▶Fundamental Concepts in Gaseous

Electronics” for definition) and τe the average time of flight between collisions. With

τe ¼ ‘eve

; (9)

ve ¼ffiffiffiffiffiffiffiffiffiffi8kTe

πme

r(10)

and ‘e ¼ mean free path (mfp) of the electrons (see chapter 3, “▶Kinetic Theory of

Gases” for definition), it follows that for steady-state situation,

Te � Th

Te

¼ 3πmh

32me

e‘eE32kTe

!2

(11)

According to Eq. 11, kinetic equilibrium Te ¼ Thð Þ requires that the energy that theelectrons acquire in an electric field between collisions must be very small com-

pared to the average kinetic energy of the electrons. Another interpretation of

Eq. 11 considers the fact that, ‘e � 1p, with p as the absolute pressure; thus

Te � Th

Te

¼ ΔTTe

� E

p

� �2

(12)

This relation shows that the parameter (E/p) plays a governing role for determining

the kinetic equilibrium situation in plasma. For small values of (E/p), the electron

temperature approaches the heavy particle temperature, which is one of the basic

requirements for the existence of local thermodynamic equilibrium (LTE) in

plasma. Additional conditions for LTE include excitation and chemical equilibrium

as well as certain limitations on the gradients in the plasma. Details of LTE

requirements for plasma will be discussed in chapter 4, “▶ Fundamental Concepts

in Gaseous Electronics.” The (E/n) ratio between the electric field, E, and the

The Plasma State 7







concentration of neutral particles, n, is often used instead of (E/p), because the

mean energy of electrons (and therefore many other properties of discharge) is a

function of (E/n). Increasing the electric filed intensity, E, by some factor, q, has the

same consequences as lowering gas density, n, by factor, q. The SI unit for (E/n) is

V� m2, but the Townsend unit (Td = 10�21 V . m2) is frequently used.

Plasma that is in kinetic equilibrium and simultaneously meets all other LTE

requirements is classified as thermal plasma. In contrast, plasmas with strong

deviations from kinetic equilibrium Te � Thð Þ are classified as nonequilibrium

plasmas, which are often, though not necessarily, nonthermal. Both types of

plasmas will be further discussed in the following section.

3 Different Types of Plasmas

Plasmas can be classified based on a wide range of criteria. One of the simplest is

their classification based on their origin or nature that is as natural or man-madeplasmas. The main difference is in the way the plasma manifests itself in nature,

whether it is the result of a natural phenomenon such as lightening and auroraborealis or that of a human activity such as in arcs and different types of electricaldischarges. One of such classifications is illustrated in Fig. 3 in which the different

types of plasmas are shown as the number density of charged particles (m�3) versus

the electron temperature (in eV or K) diagram. The range of electron densities

indicated varies from 108 to 1024 (m�3) which corresponds to pressure variations

from high vacuum to pressures above atmospheric. The electron temperatures are

expressed in terms of either units of eV (l eV corresponds to 7,740 K for a

Maxwell–Boltzmann plasma) or in terms of degrees K.

Ionosphere

Nebula and Solar corona

Solar core and Thermonuclear fusion

plasmas

Glow discharges

104

103

102

10−1

10−2

10

1

108 1012 1016 1020 1024

Number density of charged particles (m−3)

Ele

ctro

n Te

mpe

ratu

re (

K)

Ele

ctro

n Te

mpe

ratu

re (

eV)

microwaveand DBD

High pressure arcs and RF discharges

107

106

105

104

103

Lightning

Flames

Fig. 3 Classification of plasmas


At the low end of the pressure or number densities of charged particles, one

identifies two naturally existing plasmas, the ionosphere with relatively low energy

density or temperature levels, less than 0.1 eV or 103 K, and the nebula and solar

corona at energy levels of 102 up to 103 eV (around 106 K). At pressures close to

atmospheric and above (charged particle densities of 1024), we have lightning,which is one of the most frequently observed manifestations of natural plasmas.

Most man-made plasmas also fall in this range such as atmospheric and high-

pressure arcs and RF discharges with energy levels around 1 or 2 eV and temper-

atures in the range of 10,000–20,000 K. At considerably higher electron energies

and temperatures, we also have solar core and thermonuclear fusion plasmas. Inbetween we find a number of other man-made plasmas with different energy levels

such as in flames, glow discharges, microwave plasmas, and dielectric barrierdischarges (DBD). The degree of ionization defined as ξ ¼ ne

neþnvaries widely

from as low as 10�10 in flames to a few percentages in the range of 5–10 % in

high-pressure arcs, up to full and multiple ionization levels in thermonuclear fusion

plasmas. In the following specific examples, the principal characteristics of the

different types of natural and man-made plasmas are briefly discussed.

3.1 Natural Plasmas

As mentioned previously, natural plasmas comprise more than 99 % of the universe

known today. At sea level, in air three-body recombination (see chapter 3,

“▶Kinetic Theory of Gases” for definition) of free electrons with molecular

oxygen limits their lifetimes to only about 16 ns, and thus no natural plasma can

exist under these conditions (Becker et al. 2005). On the other hand, at high

altitudes the pressure decreases, with a corresponding rise of the lifetime of free

electrons. At an altitude of 30,000 ft, the lifetime of a free electron is 119 ns, while

at 60,000 ft, it reaches 1.83 μs. When reaching 60 km above the sea level (beginning

of ionosphere), high fluxes of extreme ultraviolet (solar photons) and also collisions

with free electrons maintain free electron densities in the range of 105–1010 m�3

which is characteristic of the charged particle densities in the ionosphere and

supports the notion that 99 % of the universe is in a plasma state (Suplee 2009).

In fact all of the observed stars in interstellar and interplanetary media as well as the

outer atmospheres of planets are essentially in a plasma state. The Sun is the closest

star to the Earth and is the center of our solar system. In a star the plasma is bound

together by gravitational forces, and the enormous energy it emits originates in

thermonuclear fusion reactions within the interior. The temperature is more than

106–107 K at the core: fully ionized plasma. Heat is transferred from the interior to

the exterior by radiation in the outer layers. In the Sun’s outer layer, the temperature

is lower than in the radiative zone and heavier atoms are not fully ionized. As shown

in Fig. 4, the density of gases is low enough to allow convective currents to develop

a giant, spinning ball of a temperature of about 6,000 K on the surface. A transition

layer, the tachocline, separates the radiative zone and the convective zone. The

The Plasma State 9


light from the Sun heats our planet and makes life possible on it. The Sun is also an

active star that displays sunspots, solar flares, erupting prominences, and coronal

mass ejections. These phenomena, which are all related to the Sun’s magnetic field,

impact our near-Earth space environment and determine our “space weather.”

In the vicinity of a hot star, the interstellar medium consists almost entirely of

completely ionized hydrogen, ionized by the star’s ultraviolet radiation. A corona

surrounds the Sun and other celestial bodies (Aschwanden 2004). The Sun’s corona

extends millions of kilometers into space and is most easily seen during a total solar

eclipse. Associated to the Sun, the solar wind is a stream of charged particles

(plasma) released from its upper atmosphere. It mostly consists of electrons and

protons with energies usually between 1,500 and 104 eV. The stream of particles

varies in density, temperature, and speed over time and over solar longitude.

These particles can escape the Sun’s gravity because of their high kinetic energy

and the high temperature of the corona.

Two of the earliest known natural plasma phenomena are lightning and the auroraborealis. The aurora borealis, shown in Fig. 5, is a natural light display in the sky

particularly in the Arctic and Antarctic regions, caused by the collision of energetic

charged particles with atoms in the high-altitude atmosphere (thermosphere). The

charged particles are directed by the Earth’s magnetic field into the atmosphere. The

aurora borealis appears as a diffuse, widespread (of astronomic dimensions),

low-luminosity event. Lightning, on the other hand, as shown in Fig. 6, is a massive

electrostatic discharge between the electrically charged regions within an agglomer-

ation of clouds or between a cloud and the surface of the Earth. The charged regions

within the atmosphere temporarily equalize themselves through a lightning flash. In

the case of lightning, narrow, high-luminosity channels are observed with numerous

streamers branching out of the main core of the lightning channel.

Fig. 4 Photograph of the Sun


3.2 Man-Made Plasmas

Plasmas started to be studied in the middle of the nineteenth century. They were

essentially produced using a discharge tube at low pressure as shown in Fig. 7.

A direct-current, DC, voltage was applied between parallel electrodes creating an

electric field, E, inside the tube E = V/d, where V is the applied voltage and d being

the distance between both electrodes. As the applied voltage is increased over a

Fig. 5 Aurora borealis

Fig. 6 Lightning

The Plasma State 11

certain value, depending on the nature of the gas and its pressure, breakdownoccurs between the two electrodes and the gas becomes conducting.

With the increase of the applied voltage and consequently of the current passing

through the discharge, the electric discharge passes through the following three

regimes, with distinct current–voltage characteristics as shown in Fig. 8 after Chang

and Pfender (1990) and Pfender (1978).

3.2.1 Townsend DischargeTownsend discharge occurs below the breakdown voltage (A to E in Fig. 8). At low

voltages, the only current is that due to the generation of charge carriers by external

Cathode (–)

Discharge tube

E=V/d

d

Anode (+)

Fig. 7 Discharge tube

Breakdown voltageCorona

Saturation regime

Glow dischargedark discharge

Townsend regime

Normal glowAbnormal

glow

Non-thermal

Thermal arcA BK

C

D E

F G

H

J

Glow-to-arc transition

arc discharge

I

Background ionization

10−10 10−8 10−6 10−4 10−2 102 104

Current (A)

Vol

tage

(V

)

1

Thermal arcJ

Normal glowAbnormal

glow

Non-thermal

IF G

Fig. 8 Evolution of discharge characteristics as function of the discharge current in a tube

discharge


sources such as cosmic rays or other sources of ionizing radiation (A to B). When

increasing the applied voltage, the free electrons carrying the current gain enough

energy to cause further ionization, resulting in an electron avalanche. In this regime,

the current increases from below 10�10 to over 10�6 A, for very little further

increase in voltage C to D in Fig. 8. In this area of V–I characteristics, except for

corona discharges D to E in Fig. 8, and the breakdown itself at E in Fig. 8, the

discharge remains invisible to the eye. However, if one electrode is a very thin wire

or a point, extremely nonuniform electric fields prevail close to it. With sufficiently

high potential V applied to such an electrode, breakdown of the gas near the wire

surface occurs at potentials below the spark breakdown potential. The resulting

discharge is also known as the corona discharge if operated at high (atmospheric)

pressures. Of course nonequilibrium conditions prevail in the Townsend discharge

regime.

3.2.2 Glow DischargeGlow discharge takes place in the current region between E and H in Fig. 8. The

breakdown avalanche is a cascade reaction involving electrons in a region with a

sufficiently high electric field in a gaseous medium that can be ionized. Following

an original ionization event, caused by such events as ionizing radiation, the

positive ion drifts toward the cathode, while the free electron drifts toward the

anode of the discharge tube. If the electric field is strong enough, the free electron

gains sufficient energy to liberate a further electron when it collides with another

molecule or atom. The two free electrons then travel toward the anode and gain

sufficient energy from the electric field to cause further ionization as a consequence

of subsequent collisions. This chain reaction depends on the free electrons gaining

sufficient energy between collisions to sustain the avalanche. As the current further

increases, space charges develop, leading to a sudden drop of the voltage across the

electrodes (subnormal glow discharge) shown in Fig. 8 as the range E to F. Between

F and G in Fig. 8, the voltage across the tube is almost current independent as well

as the electrode current density, j. When the current I increases (F to G in Fig. 8), the

fraction of cathode surface occupied by plasma increases until plasma covers the

entire cathode surface at G. For the formation of glow discharge, the mean free path

of the electrons has to be reasonably long but shorter than the distance between the

electrodes; glow discharges therefore do not readily occur at both too low and too

high gas pressures. At higher currents, the normal glow turns into abnormal glow

(G to H in Fig. 8), the voltage across the tube gradually increases, and the glow

discharge covers more and more of the surface of the electrodes. Electrons striking

the gas atoms and ionizing them facilitate glow discharge. The breakdown voltage

for the glow discharge depends nonlinearly on the product of gas pressure and

electrode distance according to Paschen’s law:

V ¼ a pdð Þln pdð Þ þ b

(13)

The Plasma State 13

where p is the absolute pressure in the discharge tube and d the gap distance (m),

while a and b are constants linked to gas composition. For a certain pressure �distance value (p . d), there is a lowest breakdown voltage. The increase of the

breakdown voltage with decreasing electrode gap is a consequence of the reduced

ionization efficiency of electrons, because the mfp of electrons becomes larger than

the electrode gap. All plasmas produced under these conditions are out of

equilibrium.

3.2.3 Arc DischargesArc discharges occur at higher currents, over the range from H to K in Fig. 8. There

is an important voltage drop in the transition region due to the much lower cathode

fall in the arc region. The cathode is heated to temperatures sufficient for thermionic

electron emission, which is a much more efficient electron emission mechanism

compared to that in the glow discharge regime (gamma mechanism). The cathode

fall in the arc region is typically around 10 V, whereas in the glow discharge,

cathode falls are typically around 100 V. The transition region from I to J in Fig. 8

represents a hybrid between the glow and arc discharge with strong deviations from

equilibrium. As the current further increases (J to K in Fig. 8), the arc plasma

approaches LTE.

A more detailed discussion of the principal characteristics and major areas of

industrial applications of man-made plasmas will follow. This is divided into two

sections depending on the nature of the plasma state, one dealing with

nonequilibrium man-made cold plasmas and the other on man-made thermal

plasmas.

4 Nonequilibrium, Man-Made Cold Plasmas (Te � Th)

Nonequilibrium plasmas are frequently classified as “cold” plasmas, because of the

low temperature of the heavy species Te � Thð Þ. The plasmas or discharges can be

classified according to their time dependence (transient or steady state), importance

of space charge effects or of heating of the neutral gas species, and presence of a

surface close to the discharge (Nijdam et al. 2012). Nonequilibrium plasma systems

can be generated at pressures ranging from 10 kPa or less up to atmospheric

pressure. According to the previously discussed (E/p) criterion, substantial devia-

tions from kinetic equilibrium are expected for large values of (E/p). Typical values

of (E/p) or (E/n) are several orders of magnitude higher for nonthermal plasmas,

compared to thermal plasmas. A typical value, of a nonequilibrium glow discharge

operated at a pressure of 0.1 Pa, is on the order of E/p = 107 V/m.kPa.

4.1 Glow Discharges and Some of Their Applications

Figure 9 shows the basic physical structure of a nonequilibrium discharge, gener-

ally run in Ar or other noble gases with Te = 104 K, while Th ’ 300 K.


The different regions of the discharge shown in Fig. 9 are not always found in any

given condition. These depend on the dimensions, pressure and voltage, and nature

of the exciting field. Glow discharges can be generated in direct-current (DC) mode,

which requires that the cathode must be conductive. The pressure, voltage, and

current are interrelated, but only two parameters can be controlled, the third being

dependent on the two variable parameters. Glow discharges may also be generated

by alternating current (AC), radio frequency (RF), and other source applied in order

to establish a negative bias voltage on the electrode surface. Capacitively coupled

RF discharges are still the most common plasmas used in dry etching in the

electronic industry. In this case nonelectrical conductive materials can be used.

Both DC and RF glow discharges can be operated in pulsed mode allowing higher

instantaneous powers to be applied without excessively heating the electrodes.

As shown earlier in Fig. 8, in the case of a DC glow discharge, current can

increase by several orders of magnitude at constant voltage. It is controlled by

resistive ballast. Most of the voltage fall takes place close to the cathode. In the

cathode glow, electrons are energetic enough to excite the neutral atoms with which

they collide. The negative glow, shown on the RHS of Fig. 9, the brightest intensity

of the entire discharge, has a relatively low electric field and is long compared to the

cathode glow. Electrons that have been accelerated in the cathode region to high

speed produce ionization, and slower ones produce atom excitations. These slower

electrons are responsible for the negative glow. The electron number density in the

negative glow discharge is typically in the range of 1015 to 1016 e/m3 with energies

varying between 1 and 2 eV. The positive column has a small positive electric field

(about 100 V/m). Glow discharges can be run at very low pressure, as low as 10 Pa,

with voltages of about 100 V. At atmospheric pressure, voltages of a few kV are

needed combined with considerably shorter distance between the electrodes (Chap-

man 1980; Marcus and Broekaert 2003; Hippler et al. 2008).

Applications of glow discharges include elemental analysis of solids, liquids,

and gases. In the case of elemental analysis of solids, which is the most common,

the sample is used as the cathode (Marcus and Broekaert 2003). Gas ions striking

the sample surface sputter it. The sputtered atoms, in the gas phase, are detected by

atomic absorption, atomic emission, and mass spectrometry. When sample atoms

are ionized, the ions can then be detected by mass spectrometry. Both bulk and

Anode (+)

Cathode glow

Aston dark space

Cathode (−)

Negative glow

Faraday dark spacePositive column

Anode glow

Anode dark space

Cathode dark space

Fig. 9 Basic physical structure of a glow discharge

The Plasma State 15

profile analysis of solids may be performed with glow discharge (Marcus and

Broekaert 2003). Glow discharge is used for surface cleaning and modification of

metallic biomaterials (Aronsson et al. 1997). Low-pressure glow discharges are

used in lamps as well as in plasma-assisted physical or chemical vapor depositionprocesses, while atmospheric pressure glow discharges are also used for decon-

tamination, cleaning, preparation, and modification of biomaterial and implant

surfaces.

4.1.1 LampsLamps represent an important application of glow discharges. Among the different

types of lamps shown in Fig. 10, florescent lamps are probably the best-known

gas-discharge lamps. They mostly use a noble gas such as argon, neon, krypton, and

xenon or a mixture of these gases with additional doping materials, such as

mercury, sodium, and metal halides. Gas-discharge lamps offer higher energy

conversion efficiency, lux per watt (LPW), more than most conventional incandes-

cent lamps.

Fig. 10 Range of lamps commonly used in industrial, residential, and commercial applications


4.1.2 Plasma-Assisted Physical Vapor Deposition (PAPVD)Plasma-assisted physical vapor deposition (PAPVD) corresponds to a group of

vacuum processes for the deposition of layers composed of (primarily) metals,

alloys, nitrides, oxides, carbides, borides, sulfides, silicides, fluorides, and mixtures

of these. Layer thickness ranges from a few 10 nm to a few tens μm (Frey and Khan

2013). Evaporated or sputtered particles can reach the substrate only if their mean

free paths are in the few meters range, which requires vacuum levels lower than

1 Pa (typical of glow discharge). The process is a line of sight one, and the substrate

must be moved accordingly to achieve a coating with a uniform thickness. The

deposited layer growth rate is influenced by evaporation or sputter process with the

assistance of excited atoms and molecules and/or ions generated in plasmas (Erkens

et al. 2011; Bunshah 2001). At least one part of the particles involved in layer

growth possesses larger energy than the thermal energy of evaporation.

The simplest system represents cathode sputtering: an electric field is generated

between the material to be deposited (the target), necessarily a metal or an alloy

(cathode) and the substrate (anode). Voltages up to 1,000 V are used. The sur-

rounding atmosphere is argon; the mass of its ions, accelerated toward the cathode,

should be high enough for sputtering (mechanical process). For ceramic materials,

the corresponding metal is sputtered and the reactive gas, which is responsible for

the formation of the ceramic, is injected in the chamber (process close to chemical

vapor deposition). To increase the process efficiency, “magnetron sputtering” is

used with a magnetic field applied behind the target, and parallel to its surface, to

intensify the sputtering process. Due to the magnetic field, the electrons from the

glow discharge no longer move parallel to the electric field lines, but instead along a

spiral track. They are thus able to ionize more gas molecules on their longer path to

the target. Electron and ion densities are highest in this zone. However, because of

their high mass, ions are hardly deflected by the magnetic field, and the greatest

erosion of the target occurs below this zone.

Arc PVD is also used, with an arc initiated between the cathode (the material to

be evaporated) and an anode (the chamber wall in most cases). The plasma ignition

is generated by laser evaporation or a high-voltage and high-frequency spark. With

arcs of 10–100 A, the cathode is melted locally at different points, which are in

continuous motion over its surface. The evaporated cathode material is present in

the form of highly ionized plasma. The ions are accelerated (to velocities ranging

from a few 104 to a few 105 m/s) and frequently manifest multiple ionizations

(Erkens et al. 2011). This process is generally used for the evaporation of metals

such as Ti, Al, or Cr and their alloys such as AlTi but also carbon, for deposition of

extremely hard amorphous carbon layers. Reactive gases can also be added, for

example, to deposit nitrides. According to Erkens et al. (2011), arc PVD is used for

all types of tools (cutting, primary shaping, shaping and forming, and machining of

plastics), for car and engine components (including piston rings, bucket tappets, and

power train components), for hydraulic components (such as pistons), for medical

tools and instruments (e.g., bone punches), and for turbine blades, in machine parts

(e.g., collet chucks) and in decorative coatings (bathroom fittings, for instance).

The Plasma State 17

Glow discharge-based techniques are extensively used for coating tools, for

example, with TiN and diamond-like carbon (DLC), using PVD and PAPVD,

respectively, as shown in Figs. 11 and 12.

4.1.3 Plasma-Assisted Chemical Vapor Deposition (PACVD)The CVD process is used for the deposition of a layer on a substrate from the gas

phase by means of reactions of gas phase constituents. Compared to PVD process,

they are not a line of sight process allowing for a more uniform layer deposition on

three-dimensional surfaces of complex shape with undercuts or hollow parts. In the

conventional CVD process, the reaction occurs in the high-temperature zone

(900–1,400 �C) where the substrate is typically located, which provides the energy

necessary to activate the reaction. In the PACVD process, the energy necessary for

the activation of the gases is supplied by means of high-energy electrons in the

plasma (Frey and Khan 2013). Plasma excitation (the generation of effectively free

electrons, ions, radicals, and excited species) is accomplished by means of glow

discharges (DC voltage, pulsed DC voltage, medium frequency, and radio fre-

quency) or by means of microwaves (Erkens et al. 2011; Bunshah 2001). Conven-

tional hard coatings such as TiN, TiCN, or Al2O3 are achieved using a combination

of plasma-assisted and thermal CVD in the 400–600 �C temperature range. Hard

amorphous carbon layers, known as DLC layers, are achieved at temperatures

below 200 �C with pulsed glow discharges or high-frequency discharges using

precursors such as C2H2, hexamethyldisiloxane (HMDS), or tetramethylsilane

(TMS) (Erkens et al. 2011).

Fig. 11 TiN-PVD-coated tools (Erkens et al. 2011)


4.2 Corona Discharges

Corona discharges use nonsymmetric pair of electrodes, for example, a flat or a

slightly curved surface for the cathode and a pointed surface or a wire as anode.

This arrangement results in an increased intensity of the electric field near the

pointed element or wire, with a corresponding increase in the gas ionization level.

In Fig. 13a two such electrode configurations are illustrated. Figure 13a shows the

case for a point electrode used as the anode and the flat electrode as the cathode,

while in Fig. 13b the electrodes are coaxial with the central wire being anode and

the outer cylindrical shell cathode. The discharge develops in the high field region

near the sharp electrode and it spreads out toward the cathode (Jogi 2011; Nijdam

et al. 2012). The main risk is the transition of the discharge to an arc, which can be

avoided if:

• The voltage is low enough to stop the spreading of the discharge before the

cathode is reached.

• The voltage is lowered when the cathode is reached, which requires a complex

power supply.

The corona is said to be positive when the highly curved electrode is connected

to the positive output of the power supply and a negative corona when it is

connected to the negative output of the power supply. When voltages are relatively

low, the space charge near the sharp electrode disappears due to diffusion and

recombination and the discharge dies out. The process is self-repetitive. Discharge

Fig. 12 DLC steel gear

components for the aviation

industry coated by PAPVD

(Erkens et al. 2011).

The Plasma State 19

properties depend strongly on the polarity of the sharp electrode, as illustrated in

Fig. 14. With DC power sources, the power input in continuous coronas is rather

limited by the voltage range that could be used while still avoiding the development

of sparks. The use of pulsed power supplies offers means of avoiding the develop-

ment of streamers by limiting the duration of voltage pulses to less than the

streamer development and propagation time, which is typically in the 100–300 ns

ranges for a 10–30 mm gap. A small ball of light around the point electrode

characterizes the corona discharge, the ball expands and forms a shell, and then

eventually, the expanding shell breaks up into multiple streamer channels, as shown

High electric field ionization zone

Ion drift to the other electrode

a b

Fig. 13 Corona discharges principle. (a) Positive point, (b) positive wire (Jogi 2011)

Ionization

• Large volume,• Higher electron concentration• Lower energy electrons

• smaller volume,• lower electron concentration• Higher energy electrons

a b

Fig. 14 Comparison of positive and negative corona discharges (Jogi 2011)


in Fig. 15. Corona discharges are used in many applications, the most important of

which is the manufacture of ozone and in electrostatic precipitators. Air ionizers are

commonly used for sanitization of pool water, removal of unwanted volatile

organics, such as chemical pesticides, solvents, or chemical weapons agents, from

the atmosphere. Corona discharges are also used for improvement of wettability of

polymer films to improve compatibility with adhesives or printing inks, inactivation

of bacteria, removal of unwanted electric charges from the surface of aircraft in

flight and thus avoiding the detrimental effect of uncontrolled electrical discharge

pulses on the performance of avionic systems, etc. (Parvulescu et al. 2012; Fridman

2004; Dobrynin et al. 2011).

4.3 Dielectric-Barrier Discharges (DBD)

They are somewhat similar to corona discharges, but a dielectric layer covers one or

two of the electrodes in the discharge gap. At a sufficiently high voltage between

the electrodes, the discharge starts in the gas volume. It spreads out until it reaches

the electrodes, but at the dielectric it builds up a space charge that cancels the

applied electric field. At that moment the discharge stops. With dielectric barrier

(electrical insulator), these discharges require alternating voltages for their opera-

tion. Typical planar DBD configurations are schematically shown in Fig. 16.

According to Kogelschatz (2003): “The dielectric constant and thickness, in com-

bination with the time derivative of the applied voltage, dV/dt, determine the

amount of displacement current that can be passed through the dielectric(s). To

transport current (other than capacitive) in the discharge gap the electric field has to

be high enough to cause breakdown in the gas. In most applications the dielectric

limits the average current density in the gas space. It thus acts as a ballast which, in

the ideal case, does not consume energy.” Dielectric barrier materials are glass or

silica glass and in special cases also ceramic materials, thin enamel, or polymer

Fig. 15 Discharges in a 40 mm gap in atmospheric air with a 54 kV pulse, 30 ns rise time, and

half-width of about 70 ns. The images are acquired with (a) short (50 ns) and (b) long (1,800 ns)

exposure times (Nijdam et al. 2012)

The Plasma State 21

layers. At very high frequencies, the current limitation by the dielectric becomes

less effective. For this reason DBDs are normally operated between 50–60 Hz to

about 10 MHz. When the electric field in the discharge gap is high enough to cause

breakdown, a large number of micro-discharges are observed in most gases when

the pressure is of the order of 105 Pa. The number of micro-discharges per unit of

electrode area and time depends on the power density and their strength (energy

density, transferred charge) and is determined by the gap spacing, pressure, and

dielectric properties. Figure 17 shows micro-discharges in a 1-mm gap containing

atmospheric pressure air, photographed through a transparent electrode. According

to Kogelschatz (2003), the ionic and excited atomic and molecular species initiate

chemical reactions that finally result in the synthesis of a desired species (e.g.,

ozone, excimers) or the destruction of pollutants (e.g., volatile organic compounds

(VOCs), nerve gases, odors, NH3, H2S, NOx, SO2, etc.).

DBDs can be used to:

• Generate optical radiation by the relaxation of excited species in the plasma,

mainly the generation of UV or vacuum UV radiation. Those excimer ultraviolet

lamps can produce light with short wavelengths. Many excimers have been

produced that way (Kogelschatz 2003). UV photons can also be utilized to excite

phosphors that convert UV radiation to visible light. This principle is used on a

large scale in fluorescent lamps and energy-saving lamps.

• Industrial ozone generation is performed by DBD using oxygen or air, at 0.1 and

0.3 MPa. Generally ozone is produced with cylindrical discharge tubes of about

20–50 mm diameter and 1–3 m length. A typical configuration is presented in

Fig. 18. The Pyrex glass tubes are closed at one side, mounted inside slightly

wider stainless steel tubes to form annular discharge gaps of about 0.5–1 mm

High voltage ACgenerator

Ground electrode

Discharge

Dielectric barrier

High voltage electrode

Fig. 16 Common planar and cylindrical dielectric barrier discharge configurations (Kogelschatz

et al. 1999)


radial width. Large ozone installations reach input powers of several MW, the

ozone production reaching 100 kg/h.

• Based on the mature ozone generation technology, large atmospheric pressure

gas flows, with negligible pressure drop, can be treated for pollution control.

DBC discharges are used to treat diesel exhaust gases.

• Plastic and other polymer materials have nonpolar chemically inert surfaces

making them non-receptive to bonding, printing inks, coatings, and adhesives.

DBC discharges substantially increase the surface energy of different substrates

that can be treated at speeds over 10 m/s. These discharges are also used for

treatment of textiles at atmospheric pressure and room temperature. The

Fig. 17 End-on view of

micro-discharges in

atmospheric pressure dry air

in a plate ozonizer (original

size, 6 � 6 cm; exposure

time, 20 ms) (Kogelschatz

2003)

HV fuse

Discharge gap Outer steel tube

Gas flow

Metal coating Glass tube

Gas flow

Cooling water flow

∼

Fig. 18 Configuration of

discharge tubes in a technical

ozone generator (not to scale)

(Kogelschatz 2003)

The Plasma State 23

treatment can be used to modify the surface properties of textile to improve

wettability, absorption of dyes, and adhesion and for sterilization.

4.3.1 Microwave PlasmasIn microwave-induced plasmas (MIPs), the plasma receives its sustaining energy

from electromagnetic (EM) microwaves called surface waves or traveling wave

discharges (TWD) for sustaining the plasma at frequencies in the range

300 MHz–300 GHz. The plasma can be created at pressures between 10–3 Pa to

several 100 kPa, in discharge dielectric tubes with diameters from 0.5 to 150 mm,

without the presence of electrodes, which prevents its contamination by electrode

material. The microwave systems that are used to sustain MIPs are comparable to

those found in domestic microwave ovens. A typical reactor is presented schemati-

cally in Fig. 19. The wave generated by the magnetron travels to a coupler system,

which drives it into a cavity where the heated target is placed; some shielding protects

the user from the microwave radiation. The MIP is created if the target is a dielectric

container filled with a gas. The energy is coupled to the charged particles of the

plasma. Electrons play a key role in the energy transfer because they oscillate with the

frequency of the EM field and collide with heavy particle to which they transfer

energy. The electrons with higher energies are responsible for ionization processes,

which maintain the discharge. Many different ways allow creating and operating

MIPs. The size and shape of the plasma depend on the gas chemical composition and

the pressure in the container. Many applicators exist: cavity microwave plasma

generators, waveguide microwave plasma generators, surface wave plasma genera-

tors, slow wave plasma generators, wave beam microwave plasma generators, etc.

For more details see Moisan and Pelletier (1992) and Lebedev (2010).

MIPs can be used for (Uhm et al. 2006; Fridman 2008):

Gas flow

Waveguide

Sliding short resonator

Fig. 19 Schematic diagram

showing the microwave

system components and the

plasma torch (Jogi 2011)


• Production of carbon nanotubes with C2H2 (carbon source) and using iron

pentacarbonyl as the source of metal catalyst particles.

• Diamond synthesis: crystalline diamond predominantly composed of {100} and

{111} faces was grown on a non-diamond substrate from a gaseous mixture of

hydrogen and methane under microwave glow discharge conditions.

• Deposition of hydrophobic or hydrophilic layers on surfaces via plasma

polymerization.

• Abatement of fluorinated compound gases (hydrofluorocarbons, perfluor-

ocarbons, SF6) resulting from semiconductor fabrication systems.

• Decontamination of chemical and biological warfare agents.

• Plasma Assisted Chemical Vapor deposition (PACVD).

4.4 Thermal, Man-Made Plasmas

Thermal, man-made plasmas are also classified occasionally in the North American

and European literature as hot or equilibrium plasmas. In contrast, they have been

classified in the Russian literature as low-temperature plasmas in order to distin-

guish them from thermonuclear plasmas.While, by definition, thermal plasmas are in, or close to, LTE, it has become

increasingly clear that the existence of LTE in thermal plasmas is the exception

rather than the rule. Many plasmas that are classified as thermal plasmas do not

meet all requirements for LTE, i.e., they are not in complete local thermodynamicequilibrium (CLTE). As will be discussed later in more detail, one of the main

reasons for deviations from CLTE is the lack of the excitation equilibrium

(Boltzmann distribution). In particular, the lower-lying energy levels of atoms

may be under populated due to the high radiative transition probabilities of these

levels, resulting in a corresponding overpopulation of the ground state. Because of

the small contribution of excited species to the enthalpy of plasma, this type of

deviation from CLTE is immaterial for most engineering applications. For this

reason, such plasmas are still treated as thermal plasmas or, more accurately, as

plasmas in partial local thermodynamic equilibrium (PLTE).Caution must be exercised, however, if emission spectroscopy is used for

diagnostics in such plasmas. Substantial errors may be incurred if energy levels

that deviate from excitation equilibrium are used. More serious deviations from

LTE may be expected in the fringes of plasma or in the vicinity of walls or

electrodes. Deviations from both kinetic Te 6¼ Thð Þ and chemical (composition)

equilibrium may be found in such regimes. In high-speed plasma flows, deviations

from chemical equilibrium are likely because chemical reactions cannot follow the

rapid macroscopic motion of the species; a chemically “frozen” situation results. In

this case electron densities may be substantially higher than one would expect from

the prevailing temperatures. A more detailed discussion of such deviations from

LTE will follow in chapter 4, “▶Fundamental Concepts in Gaseous Electronics.”

The Plasma State 25


4.5 Basic Concepts Used for the Generation of Thermal Plasmas

The generation of man-made, thermal plasmas is possible using two broad ranges of

physical concepts. These are:

• Electric arcs which result from the passing of an electric current (DC or AC)

through a gas between two electrodes.

• Capacitive or inductive coupling of the energy in the plasma. These are often

also referred to as electrodeless plasma generation.

4.5.1 Thermal ArcsPassing an electric current through a gas between two electrodes may generate

thermal arcs. Since gases at room temperature are excellent insulators, a sufficient

number of charge carriers must be generated to make the gas electrically

conducting. This process is known as electrical breakdown, and there are many

possible ways to accomplish this breakdown. Breakdown of the originally

nonconducting gas establishes a conducting path between a pair of electrodes.

The passage of an electrical current through the ionized gas leads to an array of

phenomena known as gaseous discharges. In most cases such discharges are

produced by direct current between the cathode and the anode. Alternating current

sources working at low frequency (less than 100 Hz) produce arcs that resemble

direct-current ones: on each half cycle, the arc is initiated by breakdown, and the

electrodes interchange roles as anode and cathode as current reverses. As the fre-

quency of the current increases, there is not enough time for all ionization to disperse

on each half cycle, and the periodic breakdown of the gas is no longer needed to

sustain the arc; the voltage vs. current characteristic becomes nearly ohmic. Such

gaseous discharges are the most common though they are not the only means for

producing thermal plasma. Finally, heating gases (vapors) in high-temperature fur-

nace can also produce plasmas. However, because of inherent temperature limita-

tions, this method is restricted to metal vapors with low ionization potentials.

Because of space limitations, this section will be limited to a presentation of the

most common arc-generated, steady-state thermal plasmas (including those produced

with low-frequency alternative current). Transient plasmas and plasmas that are not

produced by electrical discharges such as those produced by laser beams, high-energy

particle beams, shock waves, or heating in a furnace will not be included in this book.

The potential distribution in an arc shows a peculiar behavior, as indicated in

Fig. 20. Steep potential drops in front of the electrodes and relatively small potential

gradients in the arc column suggest dividing the arc into three parts:

• The cathode region• The anode region• The arc column


The arc column is a true plasma that will approach a state of LTE in a

high-intensity arc.

High-intensity arcs, with which we are mainly concerned in the following, are

defined as a discharges operated at current levels, I > 50 A, and pressures p > 10

kPa (0.1 atm). These are characterized by strong macroscopic flows induced by the

arc itself. Any variation of the current-carrying cross section of the arc gives rise to

a pumping action of the type shown in Fig. 21. The effect is a consequence of the

interaction of the arc current with its own magnetic field. At sufficiently high

currents (I > 100 A) and axial current density variations, flow velocities of the

order of 100 m/s are produced. The cathode jet phenomenon, also known as the

Maecker effect (Finkelnburg and Maecker 1956; Boulos et al. 1994; Pfender 1999),

is a typical example. Temperatures and charged particle densities, which are the

most important properties of arc plasma, can vary over a wide range. These

properties are determined by the arc parameters, including the arc geometry.

For arc applications, it is useful to classify arc columns in terms of their methods

of stabilization. There is a direct link between the method of stabilizing the arc

column and the options available for the design of arc devices. For stable operation,

most electric arcs require some kind of stabilizing mechanism that must be either

provided externally or produced by the arc itself. Here the term stabilization refers to a

particular mechanism that keeps the arc column in a given stable position, i.e., any

accidental excursion of the arc from its equilibrium position causes an interaction with

the stabilizing mechanism such that the arc column is forced to return to its equilib-

rium position. This stable position is not necessarily a stationary one; the arc may, for

example, rotate or move along rail electrodes with a certain velocity. Stabilization

Arc column

AnodeCathode

Vol

tage Va’

Vc’

dc’ da’

Fig. 20 Typical potential

distribution along an arc

(Boulos et al. 1994)

The Plasma State 27

implies in this case that the arc column can only move in a well-defined pattern

controlled by the stabilizing mechanism (Finkelnburg and Maecker 1956).

Free-burning arcs, as the name implies, have no external arc stabilization

mechanism imposed on the arc. This does not exclude the possibility that this arc

generates its own stabilizing mechanism. Although high-intensity arcs may be

operated in the free-burning arc mode, they are frequently also classified as self-

stabilized arcs if the induced gas flow due to the interaction of the self-magnetic

field with the arc current is the dominant stabilization mechanism. Arcs operated at

extremely high currents (up to 100 kA) known as ultrahigh-current arcs should also

be mentioned in this category. Although most experiments in this current range

utilize pulsed discharges, the relatively long duration 10 msð Þ of the dischargejustifies classifying them as arcs. There is considerable interest in such arcs for

applications involving melting and steelmaking, chemical arc furnaces, and high-

power switchgear. Visual observations of ultrahigh-current arcs in arc furnaces

reveal a rather complex picture of large, grossly turbulent plasma volumes, vapor

jets emanating from the electrodes, and parallel current paths with multiple, highly

mobile electrode spots. In this situation there is no evidence for any dominating

stabilizing mechanism. Induced gas flows and vapor jets exist simultaneously,

interacting with each other in a complicated way. For certain polarities of the arc

and certain electrode materials, stable vapor jets that are able to stabilize the arc

column have been observed. Thus, the generation of vapor jets by the arc represents

another possible mechanism for self-stabilization of arcs (Edels 1973).

Induced flow

Induced flow

Diaphragm

Fig. 21 Induced flow

resulting from arc

constriction using a water-

cooled diaphragm (pumping

action) (Boulos et al. 1994)


The integration of an arc in a plasma-generating device is possible using any of

the arc configurations illustrated schematically in Fig. 22. These can be grouped as

one of the following two setups:

• Blown arc configuration (plasma torch) represented on Fig. 22a, c for a hot or

cold cathode, respectively. In each of these two cases, the arc is struck between

an internal cathode and anode integrated in the plasma torch, with an adequate

flow of plasma-forming gas in the torch in order to insure the stabilization

mechanism of the arc and the continuous motion of the arc root on the surface

of the cold electrodes (anode in the case of Fig. 22a and both cathode and anodes

in the case illustrated in Fig. 22c).

• Transferred arc configuration represented in Fig. 22b, d, e. In cases b and d, the

plasma torch comprises only the cathodic electrode, while the anode is external

to the plasma torch and can be the workpiece as in the case of plasma welding

and cutting, or a molten metal bath, as in the case of metallurgical applications of

thermal plasmas. Alternately the arc polarity can be inversed, as shown in

Fig. 22e, with the internal electrode in the plasma torch acting as anode, while

the external electrode being the cathode. Such a configuration is often identified

as a reversed polarity mode of operation.

Photographs of laboratory scale transferred arcs are shown in Figs. 23 and 24.

The transferred arc shown in Fig. 23 is formed between a hot cathode and a molten

copper anode. The arc length is 100 mm, with a 750 A arc current. A photograph

of a transferred arc between two plasma torches is shown in Fig. 24. In this case

one of the two torches is acting as cathode (right-hand side), while the second one

Pla

sma

gas

Plasma gas

Pla

sma

gas

Plasma gas

a b c d e

Plasma gas

Plasma gas

Fig. 22 Arc configurations commonly used for the integration of an arc in a plasma-generating device

The Plasma State 29

(left-hand side) is acting as anode. It may be noted that in this case the current flows

from the first torch to the molten metal bath and back from the molten metal bath to

the second torch.

Fig. 23 Photograph of an

Ar/H2 transferred arc,

100 mm long, 750 A

Fig. 24 Photograph of a transferred arc struck between two plasmas torched over a molten

metal bath


4.5.2 Inductively Coupled DischargesWhile the concept of electrodeless induction heating of gases was recognized as

early as 1945, by Babat (1947), the first demonstration of the continuous operation

of an inductively coupled radio frequency (RF) discharges can be traced back to

Reed (1961a, b). The basic concept used is illustrated on the left-hand side of

Fig. 25. General reviews on the subject are given by Eckert (1974) and Boulos

(1997). The plasma is generated through the electromagnetic coupling of the energy

from an induction coil into the plasma discharge maintained in a coaxial plasma

confinement tube. A high frequency current is applied to the coil generating an

alternating magnetic field in the discharge region, in which it induces an alternating

annular electric field, which sustains the discharge through ohmic heating. A

photograph of pure argon, inductively coupled discharge, confined in an open-

ended, air-cooled, quartz tube is shown on the right-hand side of Fig. 25.

Typical radial profiles of the induced electric field, electrical current density, and

electrical conductivity at the middle section of the induction coil are given in

Fig. 26 after Eckert (1971) and Eckert (1972). These show the radial variation of

the induced electric field, E (V/cm) and the radial profile of the electrical conduc-

tivity of the plasma, σ (mho/cm), and of the induced electric current density,

j (A/cm2). The radial distribution of the local power generation can be calculated

as (j � E) which gives rise to an off-axis maximum in the annular region known as

the skin depth δ. The value of the skin depth is typically in the tens of mm range. It

is function of the frequency of the applied magnetic field, the electrical conductivity

of the plasma, and the permeability of the medium as given by Eq. 14:

Powder

Sheath gasCentral gas

RF Electrical Supply (MHz)

Fig. 25 Basic concept for energy transfer in an inductively coupled plasma torch (left) and

photograph of a 12 kW open-air, inductively coupled argon discharge (right)

The Plasma State 31

δ ¼ 1ffiffiffiffiffiffiffiffiffiffiffiffiffiπξo σ f

p (14)

4.6 Thermal Plasma Sources and Their Fields of Applications

A summary of the range of the principal, man-made, thermal plasma sources

considered is presented in Fig. 27 as function of their nominal power rating

(kW) and the plasma velocity range (m/s). With the exception of plasma sources

for ballistic and aerospace reentry testing, most of these sources operate at essen-

tially atmospheric pressure or soft vacuum conditions.

4.6.1 Plasma Torches with Hot CathodesThese are among the most commonly used thermal plasma sources for a wide range

of applications including plasma cutting and welding and plasma spraying of

protective coatings and near-net-shaped parts. In these devices, an electric arc

generates the high-temperature plasma through resistive energy dissipation by the

current flowing through the partially ionized gas within the torch. In order to allow

the current to flow through the gas, the gas temperatures must be sufficiently high to

have an appreciable degree of ionization resulting in sufficiently high electrical

conductivity. For most plasma-forming gases, the required temperatures are around

8,000 K and above at atmospheric pressure. A schematic of a typical dc plasma

torch is shown in Fig. 28. The arc is struck between a central cathode (usually a rod-

or button-type design) and a cylindrical annular anode nozzle. The plasma gas is

Fig. 26 Radial distribution

of the magnitude of the

induced electric field (E),

current density (J), and

electrical conductivity (σ) inan argon induction discharge,

f = 2.6 MHz, Po = 25 kW

(After Eckert (1972))


injected (axially or as a vortex) at the base of the cathode, heated by the arc, and

exits the nozzle as a high-temperature, high-velocity plasma jet. In plasma torch

designs used for plasma spraying, electrons are supplied through thermionic emis-

sion from the hot cathode material. A mixture of tungsten doped with 1 % or 2 % by

weight of a material with a low work function value, such as ThO2, La2O3, or LaB6,

is widely used as torch cathode. The consequence is a significantly reduced cathode

operating temperature and longer cathode life. The arc column characteristics are

determined by the energy dissipation per unit length, i.e., by the arc current, the

plasma gas flow and composition, and the arc channel diameter.

The torch anode usually consists of a water-cooled copper channel, sometimes

lined with a tungsten or tungsten copper sleeve. Numerous nozzle designs are used

to maintain the anode arc root attachment in continuous motion over the annular

anode surface and to optimize the stability and characteristics of the plasma jet

Segmented dc plasma torches for ballistic and aerospace

reentry testing104

103

102

10−1

10

1

1 10 100 1000 10 000

Plasma power rating (kW)

Pla

sma

velo

city

(m

/s)

Hot cathode DC plasma torches for plasma spraying,

cutting and weldingCold cathode DC plasma

torches for plasma metallurgical applications

and waste treatment

R.F. induction plasma torches for materials

synthesis and processing

Fig. 27 Classification of man-made thermal plasma sources according to their nominal power

rating and plasma velocity range

Plasma gas

Cooling Water in

Cold gas Boundary layer

Anode attachmentArc column

Cooling Water out

Plasma jet

Hot Cathode

Anode nozzle

Fig. 28 Schematic representation of a DC plasma torch with a hot cathode design

The Plasma State 33

emmerging from the plasma torch. This jet is typically highly turbulent with

continuous motion as shown by the results of three-dimensional arc modeling

studies shown in Fig. 29. The turbulent nature of the flow is also observed in optical

photographs of the emerging plasma jet shown on the right-hand side of the same

figure.

Alternately, arc rotation on the surface of the anode could also be enhanced

through the addition of a field coil surrounding the anode as shown in Fig. 30.

Through this arrangement, the interaction of the generated axial magnetic field,

with the radially oriented arc current near the arc root attachment point on the anode

surface, gives rise to a tangential electromagnetic force J!� B

!� �which further

enhances the velocity of rotation of the arc root, provided, of course, that the

direction of the force is in the same direction as that of the aerodynamic vortex

Fig. 29 Results of three-

dimensional simulation of arc

inside plasma torch (left)(Reproduced with kind

permission of Juan Pablo

Trelles) and photographs of

the emanating plasma jet

(right) (Fauchais et al. 2014)

Fig. 30 Schematic of a plasma torch with stick-type hot tungsten cathode and a water-cooled cold

anode, with magnetic field rotation of the arc root on the anode surface


motion in the torch. It should be mentioned that, as will be further discussed in

part II, chapter 6, “▶ Inductively Coupled Radio Frequency Plasma Torches,” the

use of a strong vortex motion, with or without magnetic field enhancement, has

been the subject of extensive studies in the former Soviet union (Zhukov 1979),

applied to reduce electrode erosion in a wide range of cold-electrode, plasma torch

designs.

Using hot cathode, arc-generated thermal plasmas for metal cutting and welding

is a well-established technology (Finch 2007). Both TIG (tungsten inert gas) and

MIG (metal inert gas) welding processes are in wide use today (Lucas 1990). In TIG

welding, a transferred arc configuration is used where the non-consumable tungsten

electrode serves as the cathode and the workpiece is the external anode. Inert gas or

gas mixtures (Ar, He) are blown along the cathode to prevent contamination from

the surroundings. In MIG welding, the arc is maintained, also in the transferred arc

mode, between the workpiece and a consumable wire electrode that is fed contin-

uously through the torch at controlled speeds. Inert gas is fed simultaneously

through the torch into the weld zone, protecting the weld from the contaminating

effects of the atmosphere (Wilden et al. 2006). Other welding methods make use of

submerged arcs.

Plasma cutting is also a well-established technology Thomas (2012). An exam-

ple of a typical plasma cutting torch design is shown in Fig. 31. Torches with stick-

or button-type cathodes (made of tungsten or hafnium) with laminar or vortex

injection of plasma-forming gas (oxygen) are used (Nemchinsky and Severance

2006; Zhou et al. 2008). In plasma cutting, the specific heat fluxes at the workpiece

are at least one order of magnitude higher (typically in the range from 100 to

200 kW/cm2) than for arc welding. This difference implies that the arc for plasma

cutting must be extremely constricted, resulting in high current densities and

correspondingly high temperatures in the axis of the arc, even at relatively low

currents (20 A).

New developments in this area were reported over the past few years, including

air-operated and air-cooled low-amperage cutting torches that are extensively used in

automotive repair shops. Another more recent development considers highcurrent

cutting torches for underwater cutting. This technology may become important in the

dismantling of nuclear power plants that are beyond their useful life span.

Carbon-arc cutting, also known as air arc cutting, is an arc cutting process where

metal is cut and melted by the heat of a graphite electrode. A compressed air jet

removes molten metal. This process is most often used for cutting and gouging

aluminum, copper, iron, magnesium, and carbon and stainless steels. Because the

air jet blows the molten metal away, oxidation is rather low. The main purpose for

air carbon arc is gouging and removal of old or defective welds so that they can be

redone or the equipment dismantled. It allows removing the minimum possible

amount of material so that the joint can be re-welded. For more information see

part IV, “Industrial Applications of Thermal Plasma Technology”, chapter 2,

“▶ Plasma Cutting and Welding.”

In plasma spraying, which is one of the most important applications of these

torches, the material to be sprayed is in the form of a fine powder (typically with a

The Plasma State 35

Inductively Coupled Radio Frequency Plasma Torches

Plasma Cutting and Welding

mean particle diameter between 20 and 100 μm), suspended in a carrier gas, and is

injected into the plasma jet, where the powder particles are accelerated, heated, and

melted. A schematic representation of the plasma spraying process is shown in

Fig. 32. As the molten powder particles impinge at high velocities (from 100 to

300 m/s) on the surface of a substrate, they form splats, which superimpose on each

other resulting in a more or less dense coating. Any material, provided its melting

and vaporization or decomposition temperatures are separated at least by 300 K,

can be sprayed. Plasmas are mainly used to spray ceramic materials either in air

(oxides) or controlled atmosphere (carbides, borides, etc.) but also metals or alloys

in air, if oxidation is not a problem, or under soft vacuum, especially, for example,

to achieve diffusion adhesion for bond coats (Fauchais et al. 2014). To coat big

parts, such as big rolls in the paper industry, plasma torches up to 250 kW have been

developed with particle feed rates up to 20 kg/h (Morishita et al. 1991). Recently,

Oerlikon Metco has developed a new process called PS-PVD using a high-power

plasma spray torch (180 kW–3000 A, gas flow rate up to 200 slm) working at a

Hot cathodeholder

Anode-nozzle

Vortex gas injectionAxial gas injectiona b

Tungsten stick type cathode

Button type cathode

Plasma gas

Hot cathode

Exit nozzle

Fig. 31 Typical design of a plasma cutting torch (Eliot 1991)


pressure as low as 0.05–0.2 kPa (0.5–2 mbar). Under such low-pressure conditions,

the plasma jet reaches more than 2 m in length and up to 0.4 m in diameter. The

coating obtained exhibits a microstructure similar to that of EB-PVD coatings (von

Niessen.and Gindrat 2011). These coatings are used against wear, corrosion and

oxidation, thermal protection, clearance control and bonding. Other applications

include freestanding spray-formed parts, medical applications, replacement of hard

chromium, and electrical and electronic applications. Industries using them are

aerospace, land-based turbines, automotive, electrical and electronic, land-based

and marine applications, medical applications, ceramic and glass manufacturing,

printing, pulp and paper, metal processing, petroleum and chemical, electrical

utilities, textile and plastic, polymers, reclamation, and nuclear. For example,

Fig. 33 presents ceramic coating plasma sprayed on a large drum for manufacturing

paper. Plasma spray coating has also been traditionally used extensively in the

aerospace industry. Figure 34 shows components in a typical jet engine, which are

plasma coated with different thermal barrier coatings, wear resistance, and abrad-

able coatings. Globally, plasma spraying represents about 50 % of the thermal spray

market which was evaluated to be about $ 4.6 billion in 2012. For more details see

part IV, chapter 4. “▶Thermal Spray Coating”.

4.6.2 Plasma Torches with Cold ElectrodesThe cold electrodes used in these torches usually consist of a water-cooled metal

sleeve, mostly copper, steel, silver, or an alloy of these materials (Heberlein 1999).

Such electrodes can easily work with oxidizing or reducing plasma-forming gases.

Plasma torch

Air engulfment

Plasmajet core

Plasma plume

Particlesinjector

Substrate

Coating

Details of coatingSplats

Unmeltedparticles

Pores

Fig. 32 Schematic representation of plasma spray process: the plasma gas entering the torch at

the base of the cathode is heated by the arc between the cathode and the cylindrical anode. Spray

particles are injected into the plasma jet and transported to the substrate

The Plasma State 37

Thermal Spray Coating

Fig. 34 Plasma sprayed components in a typical jet engine (Reproduced with kind permission of

Oerlikon-Metco AG, Switzerland)

Fig. 33 Plasma spray applications in the paper and printing industry (Reproduced with kind

permission of Sulzer-Metco AG, Switzerland)


For the cathode, electron emission is the result of evaporation and ionization of the

metal in very small cathode spots with the consequence that it is necessarily

associated with materials loss. Studies of cathode spots under plasma conditions

show spot diameters of 1–2 mm for currents ranging from 800 to 1,400 A, with

current densities of 108–109 A/m2. The erosion rate (ER) in (μg/C) has been found

to follow the general equation:

ER ¼ A � Im (15)

where I is the arc current. The constants A and m depend on the plasma-forming gas

and the operating conditions.

A critical design feature of such torches is the introduction of a strong vortex

motion in the flow pattern of the gases in the discharge cavity driving the arc roots

over the electrode surfaces leading to a more uniform distribution of the electrode

erosion over as large an area as possible. The motion of the arc root can be enhanced

by the superposition of a magnetic field coil around the electrodes, cathode, and/or

anode, The interaction of the generated axial magnetic field with the arc at its point

of attachment to the electrode gives rise to a tangential electromagnetic force acting

on the arc which is responsible for the rotation of the arc root over the inner surface

of the electrode. An example of such an arrangement was given in Fig. 30 for the arc

root rotation on the surface of a cold anode using a superimposed magnetic field.

Similar designs using electromagnetic fields for the rotation of the arc root

attachment on the surface of a cold electrode were adopted by Aerospatiale in

France in their complete line of high-power, cold-electrode plasma torches (Fig. 35)

where only the back electrode, normally the anode, had a field coil for the rotation

of the arc root attachment. The need for the generation of a strong vortex motion in

the discharge cavity required the use of relatively high plasma gas flow rates (about

200–300 m3/MW) resulting in relatively low enthalpies of the plasma jets emerging

from these torches (<10 kWh/m3). A typical power rating of these torches is in the

1–3 MW range with arc currents up to 1,000 A and voltages between the electrodes

in the range of 1.0–3 kV.

Commercial use of this concept was also undertaken by Westinghouse, in the

USA (Fig. 36), with the development of their complete line of plasma torches in

which both front and back electrodes had appropriate field coils in addition to the

injection of the plasma gas in the discharge cavity with a strong vortex motion. The

most commonly used Westinghouse torches were the Marc 3 and Marc 11 for

operation at power levels up to 2 MW mostly with air as the plasma gas. The

reported energy efficiency was in the 70–90 % range with a specific enthalpy of the

plasma at the torch exit, h = 8–19 MJ/kg.

Through the use of insulated segments between the cathode and anode coupled

with the injection of cold plasma-forming gas between them, to renew the cold

boundary layer surrounding the arc column, SKF in Sweden could increase the

length of the arc between the cathode and anode. This gave rise to a significant

increase of the arc voltage for the same arc current with a corresponding increase of

the torch power to the 6 MW range. Because of the increase of the plasma-forming

The Plasma State 39

gas flow rate (15,000 m3/h), the specific enthalpy of the plasma, h0, remains rather

low (h0 < 3 kWh/m3). For more details see part II Generation of thermal plasmas,

chapter “▶High Power DC Plasma Torches and Transferred Arcs.”

Such plasma-generating devices have been mostly used in the 1970s and 1980s

for large-scale applications of plasma sources in the metallurgical industry, as a

heat source for melting, remelting, and extractive metallurgical applications

Plasma jet

Rear electrode

Front electrode

Plasma gas

Marc 3

Model Power kWAir flow rate

m3/h

75-150 10-20

50-400

300-800

600-2000

250-2000

1000-3000

3000-10000

Marc 11

Marc 31

Marc 100

Fig. 36 Schematic of the cold-electrode plasma torch developed by Westinghouse

Power 0.2-3 MW•Current •Voltage

1000 A3000 V

•Air flow rate 200 to 1200 Nm3/h•Pressure 0.1-1.0 MPa•Enthalpy of Air 0.5 to 4 kWh/kg•Efficiency ≥ 90%

Fig. 35 Schematic of the cold-electrode plasma torch developed by Aerospatiale


DC to be consistant with the abreviations

(Feinman 1987; Neuschutz 1992; Mishra 2012; Rao et al. 2013). There are several

advantages to this approach, including:

• High-temperature operation under well-controlled processing conditions

• High throughputs and process energy efficiency

• Compact systems with lower thermal inertia allowing for easier and flexible

start-up and shutdowns

• Elimination of graphite electrodes

• Reduction of noise level and of volumes of process gases

A typical example of a shaft furnace heated by six (6) 2 MW plasma torches used

by GM for scrap steel remelting in the early eighties is shown in Fig. 37.

Cold-electrode plasma torches have also been used in the transferred arc mode

for tundish heating and other melting/remelting applications. The molten pool

serves in this case as one of the electrodes and the major energy input is at the

arc root to the surface of the molten bath. A typical example of such a torch used for

ladle heating is shown in Fig. 38. The main objective in this application is to hold

the molten steel in the ladle at constant or slightly increasing temperature levels.

Extractive metallurgy refers to the extraction of metals in either pure or alloyed

form from their respective ores. Thermal plasma reactors for smelting, melting, or

Rotary drum chip preheater

Wet chips

To A.P.C.

Temperingchamber

Dust

Dust

Natural gas burnerRecycle fan

CO gas burned inCupola stack

Plasma air 95 m3/min

Electric power 24 MWDried chips, 30 TPH, 538 oC

510 m3/min, 870 oCCO2+N2+O2

397m3/min infil. AirCoke 2.6 TPHSteel 18.0 TPHStone 0.8 TPHChips 12.0 TPH

Slag 0.6 TPH

Iron 60 TPH

113

m3 /

min

(35

% C

O)

69 m

3 /m

in14

9 o C

Fig. 37 Schematic of a plasma shaft furnace used by GM for scrap remelting

The Plasma State 41

refining operations represent highly concentrated heat sources that allow for high

processing rates per unit reactor volume. In spite of this interesting feature, there

have been very few industrial-scale applications of plasma reactors in extractive

metallurgy (MacRae 1988). This fact may be explained by the required production

size of smelting systems. A modern economy steel smelter, for example, must

operate at approximately 30 t/h; this rate requires a plasma furnace capacity of close

to 100 MW. Even with multiple torch operation, this power level is beyond present

technology. Process economics has also been responsible for limiting the large-

scale industrial applications in the metallurgical industry to specialized small-

volume applications.

Large-scale industrial applications of high-power, cold-electrode plasma torches

have been quite successful for treatment of hazardous waste materials. It is to be

noted that any material exposed to thermal plasmas will be decomposed, possibly

into its elemental constituents. This decomposition process is of interest for the

breaking down of extremely stable chemical compounds in the field of waste

destruction. The main challenge, however, is to control the quench of the decom-

position products in order to avoid forming undesirable side recombination prod-

ucts which are carcinogenic or representing a serious health hazard. Compared to

competing technologies, thermal plasma destruction of toxic waste offers a number

of advantages:

• The high temperatures cause fast and complete pyrolysis of organic hazardous

wastes, as well as melting and possible vitrification of inorganic wastes,

resulting in a volume reduction and encapsulation of non-destructible wastes.

Plasmatorch Graphite

electrode

Gasexhaust

port

Slide gate

Plasmaarc

Meltline

Fig. 38 High-power plasma

torch arrangement for ladle

heating (Camacho 1986)


• The high energy density obtainable in a plasma reactor allows the use of smaller

installations for comparable waste throughputs, thus reducing capital costs and

favoring the construction of small mobile units.

• The use of electric arcs to generate the high-temperature gas reduces the total gas

throughput and with it the need to heat excess gas for combustion; the conse-

quence is a reduction in the required capacity for off-gas treatment systems. A

wider choice of process gases is also possible, thus improving control over the

process chemistry.

• The reduced system size and the high energy density allow rapid start-up and

shutdown times.

• The ultraviolet radiation emitted by the plasma can lead to process enhancement

in some waste-destruction processes such as pyrolysis of organic chlorides.

Thermal plasmas are now being used on a commercial basis to destroy hazard-

ous chemicals and other wastes [Murphy and McAllister (2001), Heberlein and

Murphy (2008)]. The process technology developed consists essentially of the

following:

• Plasma pyrolysis, i.e., thermal breakdown of chemical components without

oxidation.

• Plasma gasification, i.e., incomplete oxidation of organic components of the

waste and generation of a combustible gas, which is often used for energy

generation. The approach is often referred to as waste-to-energy transformation.

• Plasma compaction and vitrification of solid wastes by gasifying organic mate-

rial, melting inorganic material eliminating voids, and binding hazardous metals

in a ceramic matrix (e.g., a silicate) by adding appropriate fluxes.

The main wastes stream that could be plasma treated are (Heberlein and Murphy

2008):

• Municipal solid waste (MSW), the largest waste stream, usually with low levels

of contaminants, traditionally deposed of in landfills. Incineration is widely used

because of the heating value of the waste, for example, see Fig. 39.

• Hospital solid waste (HSW), with a wide range of contaminated material. The

high heating values favor incineration.

• Incinerator residues, i.e., bottom ash and fly ash, frequently containing heavy

metals as contaminants.

• Contaminated soil, usually with hazardous organic materials.

• Sewage sludge waste (SSW) and other sludge wastes, with a range of organic

contaminants, and high moisture content.

• Low-level radioactive waste (LLRW), ranging from contaminated structural

materials to clothing.

• Military waste and other “problem” wastes, ranging from nerve gas ordinance to

asbestos materials.

The Plasma State 43

• Manufacturing wastes that lend themselves to materials recovery, such as elec-tric arc furnace (EAF) dusts and aluminum dross.

• Recovery of valuable materials from waste, e.g., platinum from discardedexhaust catalysts.

For more details, see part IV, chapters 7, “▶Thermal Plasmas in the Metallur-

gical Industry,” and 8, “▶ Plasma Treatment of Waste.”

4.6.3 Segmented Plasma Torches for Materials Testing Under ReentryConditions

The testing of materials under reentry flow conditions has been one of the major

drivers that stimulated the development of large-power thermal plasma sources in

the 1960s and 1970s. Specifically there was a need for a source that could replicate

in a research environment, the conditions to which materials in space vehicles were

exposed under reentry condition. This meant that it should reproduce the very high

velocities, high temperatures, and gas compositions, a combination that could only

be met through thermal plasma technology. The concept of the segmented plasma

torch illustrated in Fig. 40 was developed during this period as a means of scaling

Air blastWinddrum

Feed stock

Pyrolyzingarea

Winddrum

Plasma torches

Coke bed

Slag Cupola well Dust

Exhaustgas

1500°C

Fig. 39 Municipal solid

waste (MSW) destruction

developed by Westinghouse


Thermal Plasmas in the Metallurgical Industry

Thermal Plasmas in the Metallurgical Industry

Plasma Treatment of Waste

up the plasma source power through the increase of the arc length and consequently

its voltage. As illustrated, this required the addition of multiple intermediate

segments between the hot stick-type cathode and the annular water-cooled anode.

Each of these segments needed to be individually water cooled and electrically

insulated from the adjacent segments. Moreover, a stream of sheath gas had to be

introduced between each segment in order to maintain a cold boundary layer

surrounding the central arc channel. The concept was successfully developed to

power levels up to 60 MW as illustrated in Fig. 41 representing a schematic of one

of such torches developed by NASA during this period. A photograph of one of

these high-power arc heaters (70 MW) at Arnold Engineering Development Center

D.C. Source

Plasma jet

Anode

Cathode

Plasma gas

Insulating rings +Gas channels

Fig. 40 Conceptual design of a segmented DC plasma torch

Argon start /sheath gas

Air to segments, 0.1-1.4 kg/s Argon shield gas

To nozzleinlet

Cathode6 electrodes

Anode6 electrodes

CoolingWater

Load switch13 constrictor modules30 discs / modules

D.C. Power supply6 rectifier units

L= 3900 mmD= 80 mm

Fig. 41 Wall-stabilized segmented high-power arc heater (NASA), power rating = 60 MW,

pressure = 12 bar, current = 5.4 kA, voltage = 11 kV, air flow rate 1.4 kg/s

The Plasma State 45

in the USA is shown in Fig. 42. It is also to be noted that while such plasma sources

could satisfy the power requirement for the testing purposes, their typical run time

was rather short in minutes, compared to hundreds of hours expected of other

commercial torches for other applications such as plasma spraying, material

processing, and metallurgical or waste treatment. A detailed discussion of this

family of plasma sources used for aerospace reentry studies is presented in

part IV, chapter 1, “▶Thermal Plasmas in the Aerospace Industry”.

4.6.4 RF Inductively Coupled PlasmasAs mentioned earlier in section 4.5.2, RF inductively coupled discharges are

electrodeless plasma sources in which the energy is transferred from the electric

circuit into the discharge through electromagnetic coupling. It is accordingly

characterized by being a high-purity discharge environment with a relatively

large-volume and low-energy-density discharge. Induction plasma discharges can

be generated in inert, oxidization, or reducing atmosphere at pressures ranging from

soft vacuum up to a few atmospheres (10 kPa to 0.5 MPa). The technology is widely

used at low powers (5–10 kW) for trace element spectrochemical analysis of

materials using either inductively coupled plasma-optical emission spectroscopy

(ICP-OES) or mass spectrometry (ICP-MS). A typical plasma source used for

ICP-OES is shown in Fig. 43 with a quartz-wall, air-cooled open discharge.

Induction plasma technology has also been widely used over the past four

decades in the fiber optics industry for the cladding of the fiber optics preforms.

A photograph of a typical setup showing the preform horizontally mounted on the

coating lathe with the induction plasma directed toward it in an open discharge

configuration is presented in Fig. 44. The silica grain used for the cladding of the

preform is externally injected into the plasma plume, melted in-flight before being

deposited on the surface of the preform. The process is run in a batch mode with

Fig. 42 High-power aerospace wind tunnel at Arnold Engineering Development Center in the

USA (AEDC 30–70 MW)


Thermal Plasmas in the Aerospace Industry

typical run time of a few hours for the deposition of a 50 mm cladding thickness

over a one meter long preform.

The successful use of induction plasma for materials synthesis and processing

was very much dependent on the availability of reliable, industrially worthy,

RF induction plasma source (Boulos 1985). The induction plasma torches

developed at the University of Sherbrooke and at Tekna plasma systems Inc. in

Sherbrooke, Quebec, Canada, over the eighties and nineties, are characterized by the

Fig. 44 Typical setup used for the cladding of fiber optics preforms using inductively coupled

plasmas

Induction coil

Magnetic field

Plasmagas

Aerosol +Nebulizer gas

Sheath gas

Fig. 43 Photograph of a typical ICP-OES plasma source used for trace element analysis

The Plasma State 47

use of a high-performance ceramic as a plasma confinement tube surrounded by a

high-velocity water film cooling which insures the adequate protection of the plasma

confinement tube against the high-energy fluxes to which it is exposed. A schematic

representation of the Tekna plasma torch is shown in Fig. 45. This shows the torch

assembly with a gas distribution head on its top of a polymer matrix composite body

and a water-cooled exit nozzle. The torch allows for the central axial injection of the

material to be treated at the center of the induction coil using a water-cooled injection

probe. The location of the tip of the probe can be axially adjusted for optimal

processing conditions depending on the nature the material to be treated.

The design of the Tekna induction plasma torch was scaled up for operation at

different power levels ranging from 15 up to 350 kW. Research and development

(R&D) and industrial-scale installations were developed for a wide range of

applications such as:

• Plasma deposition of protective coatings and near-net-shaped parts

• Metallic and ceramic powder purification, densification, and spheroidization

• Plasma melting and atomization of high-purity materials

• Plasma synthesis of high-purity nano-powders

Induction plasma sources have also been used for the plasma spraying of high-

purity refractory materials (Boulos 1992) and the in-flight heating melting and

purification of metals, alloys, and ceramics (Boulos 1997). A typical example of

induction plasma deposition of refractory metals on X-ray targets (left) and

Central gas

Sheath gas

Exit nozzle

Torch bodyIntermediatetube

High velocitywater-film cooling

Powder injectionprobeInduction

coil

Powder injection probe

Fig. 45 Schematic drawing of the Tekna induction plasma torch


cylindrical tubes (right) is shown in Fig. 46. Electron micrographs of molybdenum

powders before and after processing in an induction plasma are given in Fig. 47.

These show a significant improvement in the sphericity and density of the individ-

ual powder particles. The plasma treatment gives rise to a significant increase in the

Fig. 46 Photographs of plasma deposition of refractory metal coatings on an X-ray target and a

cylindrical graphite substrate

Fig. 47 Electron micrographs of molybdenum powder prior and after treatment in an Ar/H2

induction plasma

The Plasma State 49

tap density of the powder from 2.3 to 6.6 g/cm3. The plasma-treated powder is also

observed to have an improved flow ability with a Hull flow value of 13.0 s/50 g

which is in contrast to the non-flowable nature of the powder before treatment. For

more information on induction plasma torch design and applications, see part

II, chapter 6, “▶ Inductively Coupled Radio Frequency Plasma Torches,” and

part IV, chapter 4, “▶Plasma Spray Coating,” chap. 5, “▶ Plasma Spheroidization

and Densification of powders,” and chap. 6, “▶ Plasma Synthesis of Nanopowders.”

Nomenclature and Greek Symbols

Nomenclature

d Distance between two electrodes (m)

E Electrical field (V/m)

Er Energy of particles of species r (J)

E/n Parameter determining the kinetic equilibrium in plasma (V � m2)

E/p Parameter determining the kinetic equilibrium in plasma (V/m.kPa)

f Frequency (Hz)

f(v) Maxwell–Boltzmann distribution

h Specific enthalpy (MJ/kg or MJ/m3)

h0 Enthalpy (MJ/m3)

I Arc current (A)

j Current density (A/m2)

k Boltzmann’s constant (k = 1.38 x 10�23 J/K)

m Mass of the gas (kg)

me Mass of electrons (kg)

mh Mass of heavy species (kg)

n Number density of particles (m�3)

ne Number density of electrons (m�3)

p Pressure (Pa)

T Equilibrium temperature (K)

Td Townsend unit (Td = 10�21 V . m2)

Te Temperature of electrons (K)

Th Temperature of heavy species (K)

Tr Temperature of species r (K)

v Gas velocity (m/s)

v Mean gas velocity (m/s)

vd Average drift velocity (m/s)

ve Mean velocity of electrons (m/s)

vm Maximum gas velocity (m/s)ffiffiffiffiffiv2

pRoot mean square of the velocity (m/s)


Inductively Coupled Radio Frequency Plasma Torches

Plasma Spray Coating

Plasma Spheroidization and Densification of powders

Plasma Spheroidization and Densification of powders

Plasma Synthesis of Nanopowders

V Voltage (V)

V0 Volume of the gas or of the plasma (m3)

Greek Symbols

δ Skin depth (mm)

ΔEkin Average kinetic energy exchange during elastic binary collision

‘e Mean free path (m)

ν frequency (s�1)

σ Electrical conductivity (mho/m)

ξ Degree of ionization ξ ¼ neneþn

� �

References

Aronsson B-O, Lausmaa J, Kasemo B (1997) Glow discharge plasma treatment for surface

cleaning and modification of metallic biomaterials. J Biomed Mater Res 35(1):49–73

Aschwanden MJ (2004) Physics of the solar corona. An introduction. Praxis Publishing, Berlin

Babat GI (1947) Electrodeless discharges and some allied problems. J Inst Elec Eng 94:27–37

Becker KH, Kogelschatz U, Schoenbach KH, Barker RJ (eds) (2005) Non-equilibrium air plasmas

at atmospheric pressure. IOP Publishing, Bristol

Bittencourt JA (2004) Fundamental of plasma physics. Springer, 678 p

Boulos MI (1985) The inductively coupled RF (Radio Frequency) plasma. Pure Appl Chem

57:1321

Boulos MI (1992) R.F. induction plasma spraying, state-of-the-art review. J Thermal Spray

Technol 1:33–40

Boulos MI (1997) The inductively coupled radio frequency plasma. High Temp Mat Process

1:17–39

Boulos MI, Fauchais P, Pfender E (1994) Thermal plasmas fundamentals and applications. Plenum

Press, New York

Brown SC Jr (1959) Basic data of plasma physics. Wiley, New York

Bunshah RF (2001) Handbook of hard coatings: deposition technologies properties and applica-

tions. Elsevier, Norwich

Cambel AB (1963) Plasma physics and magneto fluid mechanics. McGraw-Hill, New York

Camacho SL (1986) Plasma technology: its technical merits and applications potential, high

temperature process, High Tech. Show, Osaka Japan

Chang CH, Pfender E (1990) Non-equilibrium modeling of low-pressure argon plasma jets, Part I:

Laminar flow. Plasma Chem Plasma Process 10:473–491

Chapman BN (1980) Glow discharge processes: sputtering and plasma etching. Wiley, New York

Cobine JD (1958) Gaseous conductors. Dover Publication, New York

Dobrynin D, Friedman G, Fridman A, Starikovskiy A (2011) Inactivation of bacteria using dc

corona discharge: role of ions and humidity. New J Phys 13:103033

Eckert HU (1971) Measurement of the r.f. magnetic field distribution in a thermal induction

plasma. J Appl Phys 42:3108–3113

Eckert HU (1972) Dual magnetic probe system for phase measurements in thermal induction

plasmas. J Appl Phys 43:2707–2713

Eckert HU (1974) The induction arc: a state of the art review. High Temp 6:99–134

Edels H (1973) Properties of the high pressure ultra high current arc. In: Proceedings of the 11th

international conference on phenomenon in ionized gases. Czechoslovak Academy of Sci-

ences, Institute of Physics, Prague, Czechoslovakia, 18040 Prague 8, Na Slovance 2, CSSR

The Plasma State 51

Eliot D (1991) Technologie et spécificité du découpage par plasma dans “les plasmas dans

l’industrie” (Pub.) Dopée Avon, France, 229–241 (in French)

Erkens G, Vetter J, M€uller J, auf dem Brinke T, Fromme M, Mohnfeld A (2011) Plasma-assisted

surface coating processes, methods, systems and applications, Sulzer-Metco, S€uddeutscherVerlag onpact GmbH, 81677 Munich

Fauchais P, Heberlein J, Boulos M (2014) Thermal spray fundamentals. Springer, New York

Feinman J (1987) Plasma technology in metallurgical processing. Iron and Steel Society,

Warrendale

Finch R (2007)Welder’s handbook, a guide to plasma cutting, oxyacetylene, ARC, MIG and TIG

Welding, Revised HP1513

Finkelnburg W, Maecker H (1956) Electric arcs and thermal plasmas. In: Encyclopedia of physics,

vol 22. Springer, Berlin

Frey H, Khan HR (2013) Handbook of thin film technology. Springer, Berlin, p 550

Fridman A (2004) Plasma physics and engineering. Taylor & Francis, New York

Fridman A (2008) Plasma chemistry. Cambridge University Press, Cambridge

Heberlein J (1999) Electrode phenomena in plasma torches, in heat and mass transfer under plasma

conditions. Ann NY Acad Sci 891:14–27

Heberlein J, Murphy AB (2008) Thermal plasma waste treatment, topical review. J Phys D Appl

Phys 41:053001 (20 pp)

Hippler R, Kersten H, Schmidt M, Schoenbach KH (eds) (2008) Low temperature plasmas:

fundamentals, technologies and techniques, 2nd edn. Wiley, Weinheim

Howatson AM (2011) An introduction to gas discharges. Pergamon Press, Oxford (1965)

Jogi I (2011) Methods of plasma generation and plasma sources. Plasma training course and

Summer school, University of Tartu, Warsaw/Szczecin

Kenneth Marcus R, Broekaert JAC (eds) (2003) Glow discharge plasmas in analytical spectros-

copy. Wiley, Chichester

Kogelschatz U (2003) Dielectric-barrier discharges: their history, discharge physics, and industrial

applications, invited review. Plasma Chem Plasma Process 23(1):1–46

Kogelschatz U, Eliasson B, Egli W (1999) From ozone generators to flat television screens.

History and future potential of dielectric-barrier discharges. Pure Appl Chem 71

(10):1819–1828

Lebedev Yu A (2010) Microwave discharges: generation and diagnostics. J Phys Conf Ser

257:012016

Lucas W (1990) TIG and plasma welding, 1st edn. Woodhead Publishing, Abington

MacRae DR (1988) Plasma reactors for process metallurgy. Pure Appl Chem 60(5):721–725

Marcus RK, Broekaert JAC (eds) (2003) Glow discharge plasmas in analytical spectroscopy.

Wiley, Chichester

Mishra S (2012) Thermal plasma application in metallurgy plasma processing of materials. LAP

Lambert Academic Publishing, Saarbr€ucken, p 148

Mitchner M, Kruger CH (1973) Partially ionized gases. Wiley, New York

Moisan M, Pelletier J (eds) (1992) Microwave excited plasmas, plasma technology, 4th edn.

Elsevier, Amsterdam/London/New York/Tokyo

Morishita T (1991) Coatings by 250 kW Plasma Jet Spray System. In: Blum-Sandmeier S,

Eschnauer H, Huber P, Nicoll A (eds) Proceedings of the 2nd Plasma Technik symposium,

Luzern, Switzerland. (pub.) Plasma Technik, Wohlen, pp 137–145

Murphy AB, McAllister T (2001) Modeling of the physics and chemistry of thermal plasma waste

destruction. Phy Plasmas 8(5):2565–2571

Nemchinsky VA, Severance WS (2006) What we know and what we do not know about plasma

arc cutting. J Phys D Appl Phys 39:R423

Neuschutz D (1992) Plasma applications in process metallurgy. High Temp Chem Process

1:511–525


Nijdam S, van Veldhuizen E, Bruggeman P, Ebert U (2012) An introduction to non-equilibrium

plasmas at atmospheric pressure, Chapter 1. In: Parvulescu VI, Magureanu M, Lukes P (eds)

Plasma chemistry and catalysis, in gases and liquids. Wiley, Weinheim

Parvulescu VI, Magureanu M, Lukes P (eds) (2012) Plasma chemistry and catalysis in gases and

liquids. Wiley, Weinheim

Peratt AL (1992) Physics of the plasma universe. Springer, New York

Pfender E (1978) Electric arcs and arc gas heaters, Chapter 5. In: Hirsh MN, Oskam HJ (eds)

Gaseous electronics, vol 1. Academic, New York, pp 291–398

Pfender E (1999) Thermal plasma technology: where do we stand and where are we going. Plasma

Chem Plasma Process 19(1):1–31

Rao L, Rivard F, Carabin P (2013) Thermal plasma torches for metallurgical applications. In: 4th

international symposium on high-temperature metallurgical processing, TMS meeting San

Antonio TX, USA

Reed TB (1961a) Induction coupled plasma torch. J Appl Phys 32:821–824

Reed TB (1961b) Growth of refractory crystals using the induction plasma torch. J Appl Phys

32:2534–2536

Suplee C (2009) The plasma universe. Cambridge University Press, Cambridge

Thomas GP (2012) Air carbon arc cutting: fundamentals, industrial applications and benefits,

azom.com

Uhm HS, Hong YC, Shin DH (2006) Microwave plasma torch and its applications, Department of

Molecular Science and Technology, Ajou University, Korea

von Niessen K, Gindrat M (2011) Plasma sprayed-PVD: a new thermal spray process to deposit

out of the vapor phase. J Thermal Spray Technol 20(4):736–743

Wilden J, Bergmann JP, Frank H (2006) Plasma transferred arc welding – modeling and exper-

imental optimization. J Therm Spray Technol 15(4):779–784

Zhou Q, Li H, Liu F, Guo S, Guo W, Xu P (2008) Effects of nozzle length and process parameters

on highly constricted oxygen plasma cutting arc. Plasma Chem Plasma Process 28:729

Zhukov MF (1979) Basical calculations of plasmatrons (in Russian; Nauka, Novosibirsk, USSR)

The Plasma State 53

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