Vijay Nehra, Ashok Kumar and H K Dwivedi
International Journal of Engineering, Volume (2) : Issue (1) 53
Atmospheric Non-Thermal Plasma Sources
Vijay Nehra nehra_vijay@yahoo.com
Deptt of Electronics & Communication
Guru Jambheshwar University of Science & Technology
Hisar-125001, India
Ashok Kumar ashokarora123@yahoo.co.in
YMCA Institute of Engineering & Technology
Faridabad-121006, India
H K Dwivedi harish1147@gmail.com
R & D Head (PDP)
Samtel Color Limited
Ghaziabad-201001, UP, India
Abstract
Atmospheric non-thermal plasmas (ANTPs) have received a great deal of
attention in the last two decades because of their substantial breakthrough in
diverse scientific areas and today technologies based on ANTP are witnessing
an unprecedented growth in the scientific arena due to their ever-escalating
industrial applications in several state-of-the-art industrial fields. ANTPs are
generated by a diversity of electrical discharges such as corona discharges,
dielectric barrier discharges (DBD), atmospheric pressure plasma jet (APPJ)
and micro hollow cathode discharges (MHCD), all having their own
characteristic properties and applications. This paper deals with some
fundamental aspects of gas discharge plasmas (GDP) and provides an
overview of the various sources of ANTPs with an emphasis on dielectric
barrier discharge.
Keywords: Atmospheric plasma, Non-thermal, Dielectric barrier discharge.
1. INTRODUCTION
Since the past two decades, considerable efforts have been made by the scientific and
technological community to generate, sustain and utilize ANTP because of their numerous
scientific and industrial applications. Growth and importance of atmospheric cold plasma
technology can be realized by the fact that the scientific and technological utilization of ANTP
has multiplied by several factors and its applications have expanded into a large number of
fields such as in environmental engineering, aeronautics and aerospace engineering,
biomedical field, textile technology, analytical chemistry, and several other areas too. The
enormous promise of atmospheric non-thermal technology stems from its remarkable potential
for being environment friendly & energy-saving, its flexibility & capability for creation of new
products and its clear ecological advantages. Unique features, diversified applications and a
vast array of opportunities offered in a large number of diverse and unrelated fields has made
it indispensable enough to harness their potential in the scientific and industrial areas.
Keeping in view the huge potential of GDPs in several diversified fields, this article serves to
Vijay Nehra, Ashok Kumar and H K Dwivedi
International Journal of Engineering, Volume (2) : Issue (1) 54
provide an overview of atmospheric non-thermal plasma. Most of the earlier studies on manmade
plasmas were focused at low pressure, but the last two decades have witnessed a
growing attention to generate GDP at elevated pressure, preferably close to atmospheric
pressure. This is a challenging task due to instabilities of glow to arc transitions. At present,
several approaches are being used to produce ANTP, out of which a few common approaches
have been briefly discussed in this paper [1-11].
The paper is mainly divided into two parts. For a better understanding of the phenomena of
GDP, one should have an acquaintance of the various parameters that one has to deal with in
the gas discharges. Some of these fundamental aspects of GDP are discussed in the first half
of the paper. The succeeding section mainly focuses on commonly used ANTP schemes such
as corona, APPJ, MHCD, and DBD.
2. FUNDAMENTAL ASPECTS OF GDP
2.1 Plasma: Introduction
Plasma, a quasi-neutral gas, is considered to be the fourth state of matter, following the more
familiar states of solid, liquid & gas and constitutes more than 99% matter of the universe. It is
more or less an electrified gas with a chemically reactive media that consists of a large
number of different species such as electrons, positive and negative ions, free radicals, gas
atoms and molecules in the ground or any higher state of any form of excited species (fig. 1).
It can exist over an extremely wide range of temperature and pressure. It can be produced at
low-pressure or atmospheric pressure by coupling energy to a gaseous medium by several
means such as mechanical, thermal, chemical, radiant, nuclear, or by applying a voltage, or
by injecting electromagnetic waves and also by a combination of these to dissociate the
gaseous component molecules into a collection of ions, electrons, charge-neutral gas
molecules, and other species. It is thus an energetic chemical environment that combines
particles and radiations of a diverse nature, an incredibly diverse source of chemistry that is
normally not available in other states of matter. Parallel to the generation of plasma species,
loss processes also take place in the plasma. In fact, all energy ends up as heat with a small
fraction invested in surface chemistry.
Figure 1: Constituents of plasma
2.2 Classification of plasma
Broadly speaking, plasmas can be distinguished into two main groups i.e., the high temperature
or fusion plasmas and the so called low temperatures or gas discharges. A typical classification
and parameters of different kinds of plasmas is given in table 1. High temperature plasma implies
that all species (electrons, ions and neutral species) are in a thermal equilibrium state. Low
temperature plasma is further subdivided into thermal plasma, also called quasi-equilibrium
plasma, which is in a local thermal equilibrium (LTE) state, and non thermal plasma (NTP), also
called nonequilibrium plasma or cold plasma.
Positive Ions Negative Ions Electrons Metastables Atoms Free Radicals Photons
Plasma
Vijay Nehra, Ashok Kumar and H K Dwivedi
International Journal of Engineering, Volume (2) : Issue (1) 55
Plasma State Example
High temperature
plasma
(Equilibrium plasma)
KTTTT pgie
86
1010, −=≈≈
320
10 −
≥ mne
Laser fusion plasma
Low temperature plasma
Thermal plasma
(Quasi-equilibrium
plasma)
KTTT gie
4
102×≤≈≈
320
10 −
≥ mne
Arc plasma, plasma torches, RF
inductively coupled discharges
Non thermal plasma
(Non-equilibrium
plasma)
KTTT gie
3
10.........300=≈>>
310
10 −
≈ mne
Glow, corona, APPJ, DBD, MHCD,
OAUGDP, plasma needle etc
Table1: Classification of plasma
Thermal plasmas (TP) are characterized by an equilibrium or near equality between electrons,
ions and neutrals. Commonly employed thermal plasma [12-20] generating devices are those
produced by plasma torches, and microwave devices. These sources produce a high flux of heat
and are mainly used in areas such as in plasma material processing and plasma treatment of
waste materials. High temperature of TPs can process even the most recalcitrant wastes
including municipal solids, toxic, medical, biohazard, industrial and nuclear waste into elemental
form, ultimately reducing environmental pollution caused due to them. But for several
technological applications, the high temperature characteristic of TPs is neither required nor
desired, and in some cases it even becomes prohibitive. In such application areas, cold plasmas
become more suited.
Cold plasmas refer to the plasmas where most of the coupled electrical energy is primarily
channeled to the electron component of the plasma, thereby producing energetic electrons
instead of heating the entire gas stream; while the plasma ions and neutral components remain at
or near room temperature. Because the ions and the neutrals remain relatively cold, this
characteristic provides the possibility of using cold plasmas for low temperature plasma chemistry
and for the treatment of heat sensitive materials including polymers and biological tissues. The
remarkable characteristic features of cold plasma that include a strong thermodynamic nonequilibrium
nature, low gas temperature, presence of reactive chemical species and high
selectivity offer a tremendous potential to utilize these cold plasma sources in a wide range of
applications.
2.3 Plasma chemistry and origin of species
The chemistry [10, 21-22] which takes place in a plasma is usually quite complex and involves
a large number of elementary reactions. The main types of reactions occurring in volume
plasma are divided into homogenous and heterogenous reactions. Homogenous reactions
occur between species in the gaseous phase as a result of inelastic collisions between
electrons and heavy species or collisions between heavy species; whereas, heterogenous
reactions occur between the plasma species and the solid surface immersed or in contact with
the plasma. These typical reactions have been listed in table 2 (a) and (b). The heterogenous
reactions are particularly important in the processing of semiconductor materials.
Name Reactions Description
Excitation of atoms or
molecules
eAAe +→+ ∗
22
eAAe +→+ ∗
Leads to electronically excited
state of atoms and molecules by
energetic electron impact.
Vijay Nehra, Ashok Kumar and H K Dwivedi
International Journal of Engineering, Volume (2) : Issue (1) 56
De-excitation hveAAe ++→+
∗
22
Electronically excited state emits
electromagnetic radiations on
returning to the ground state.
Ionization eAAe +→+ +
22
Energetic electrons ionize neutral
species through electron
detachment and positively charged
particles are formed.
Dissociation eAAe +→+ 22
Inelastic electron impact with a
molecule causes its dissociation
without ions.
Dissociative
attachment
eAAAe ++→+ +
2
Negative ions are formed when
free electrons attach themselves to
neutral species.
Dissociative ionization eAAe +→+ 2
Negative ions can also be
produced by dissociative ionization
reactions.
Volume
recombination
BABAe +→++ Loss of charged particles from the
plasma by recombination of
opposite charges.
Penning dissociation MAAM +→+∗
22
Penning ionization eMAAM ++→+ +∗
Collision of energetic metastable
species with neutral leads to
ionization or dissociation.
Charge exchange ABBA +→+ ++ Transfer of charge from incident
ion to the target neutral between
two identical or dissimilar partners.
Recombination of
ions
ABBA →+ +− Two colliding ions recombine to
form a molecule.
Electron–Ion
recombination
MAMAe +→++ +
22
Charge particles are lost from the
plasma by recombination of
opposite charges.
Ion-ion recombination MABMBA +→++ −+ Ion-ion recombination can take
place through three body
collisions.
Table 2 (a): Gas phase reactions involving electrons and heavy species
Name Reactions Description
Etching
vapoursolid BCACAB +→+ Material erosion.
Adsorption
sg MSM →+
sg RSR →+
Molecules or radicals from a
plasma come in contact with a
surface exposed to the plasma
and are adsorbed on surfaces.
Deposition
solidBAAB +→ Thin film formation.
Recombination
2ASAAS +→+−
MSRRS +→+− 1
Atoms or radicals from the
plasma can react with the
species already adsorbed on
the surface to combine and
form a compound.
Metastable de-excitation AAS →+ ∗ Excited species on collision
with a solid surface return to the
ground state.
Vijay Nehra, Ashok Kumar and H K Dwivedi
International Journal of Engineering, Volume (2) : Issue (1) 57
Sputtering ABSABS ++→+− ++ Positive ions accelerated from
the plasma towards the surface
with sufficient energy can
remove an atom from the
surface.
Polymerization
ssg PRR →+
,
ssg PRM →+
Radicals in the plasma can
react with radicals adsorbed on
the surface and form polymers.
Table 2 (b): Surface reactions
2. 4 Electrical breakdown of gases
2.4.1 Conditions for self sustained discharge
To sustain plasma, the applied voltage must exceed the breakdown voltage for the gases. When
this voltage is reached, the gases lose their dielectric properties and turn into a conductor. The
criteria for self sustaining is given as
( ) 011 =−− d
eα
γ






+=
γ
α 1
1d
e
2.4.2 Paschen breakdown criteria
The breakdown voltage in gas discharge plasma is given as
)]}
1
1ln[ln(){ln(
γ
+−
=
Apd
Bpd
Vb (1)
)(pdfVb =
, which is the Paschen law.
Figure 2: Paschen curve for breakdown voltage versus pd
From (1) it is obvious that breakdown voltage depends only on the product pd for a given gas and
the cathode material, regardless of the individual values of p and d. The Paschen curves (fig. 2)
(Vb)min
(pd)Optimum (pd)
High pressure insulation
Vacuum insulation
Vb
Vijay Nehra, Ashok Kumar and H K Dwivedi
International Journal of Engineering, Volume (2) : Issue (1) 58
for different gases have roughly the same shape but are shifted from one another. From the
curve, it is clear that there is a minimum breakdown voltage at a certain pd product and the
breakdown potential is large for both small and large values of pd. At low pd values, the
breakdown voltage is high because of too few collisions and at high pd values; the breakdown
voltage is high because of too many collisions. The physical significance of this minimum voltage
is that no matter how small the gap or pressure, it is impossible to strike a discharge at a voltage
less than the minimum breakdown voltage [22-23].
3. ATMOSPHERIC NON-THERMAL PLASMA SCHEMES
Low pressure glow discharge plasmas are of great interest in fundamental research as well as in
the microelectronic industry and material technology. But, these plasmas must be contained in
costly air tight enclosures (massive vacuum reactors) making them highly expensive and time
consuming. Also, the density of activated particles is relatively low. Therefore, one of the recent
trends focuses on developing new plasma sources, which operate at atmospheric pressure, but
retain the properties of low pressure media. The economic and operational advantages of
operating at 1 atm have led to the development of a variety of atmospheric plasma sources for
several scientific and industrial applications.
Thus, in the last two decades, ANTPs have attracted more attention due to their significant
industrial advantages over low-pressure discharge. Non-thermal atmospheric plasma may be
obtained by a diversity of electrical discharges such as corona discharge, micro hollow cathode
discharge, atmospheric pressure plasma jet, gliding arc discharge, one atmospheric uniform glow
discharge, dielectric barrier discharge, and plasma needle, all having important technological
applications. The characteristic of all these atmospheric plasma sources in terms of plasma
properties is shown in table 3. A brief description of commonly used forms of ANTP is illustrated
in the succeeding sections.
Table 3: Plasma properties of atmospheric discharge schemes
Parameters Corona
Discharge
DBD APPJ Atmospheric
glow MHCD
Method and
Type
Sharply
pointed
electrode
Dielectric barrier
cover on
electrodes
RF
capacitvely
coupled
DC glow with
micro hollow
cathode electrode
Excitation Pulsed
DC
AC or RF RF 13.5
MHz
DC
Pressure
(bar)
1bar 1bar 760 torr 1bar
Electron
energies (eV)
5 variable 1-10 1-2 …………..
Electron
Density, cm
-3
10
9
-10
13
variable
≈10
12
-10
15
10
11
-10
12
Breakdown
Voltage (kV)
10-50 5-25 0.05-0.2 ………….
Scalability
& Flexibility
No Yes Yes Yes
Tmax Temp
T (K)
Room Average gas
Temp (300)
400 2000
Gas ……….. N2+ O2+ NO+
Rare gas/Rare
gas halides
Helium,
Argon
Rare gas Rare
gas/Rare gas
halides
Vijay Nehra, Ashok Kumar and H K Dwivedi
International Journal of Engineering, Volume (2) : Issue (1) 59
3.1 Corona discharge
The first scheme that was used to generate ANTP was corona discharge [1, 2, 24-26]. It exists in
several forms, depending on the polarity of the field and the electrode geometrical configuration.
This type of discharge is the characteristic of an asymmetric electrode pair and results from the
electric field that surrounds inhomogenous electrode arrangements powered with a continuous or
pulsed dc voltage. In a highly non-uniform electric field, as for example, point plane gap or wire
cylindrical gap, the high electric field near the point electrode or wire electrode far exceeds the
breakdown strength of the gas and a weakly ionized plasma is created. Coronas are thus
inherently non-uniform discharges that develop in the high field region near the sharp electrode
spreading out towards the planar electrode. This phenomenon of local breakdown is called
corona discharge. Fig. 3 shows a schematic of point to plane corona. It is a positive corona when
the electrode with the strongest curvature is connected to the positive output of power supply and
a negative corona when this electrode is connected to the negative terminal of power supply. The
development of a corona discharge progresses sequentially through the following steps: (1) an
asymmetric electrode configuration is made; (2) a high voltage is applied, and some free electric
charge is made available; (3) an avalanche builds up and leaves behind a space charge area; (4)
photons from the avalanche create new charge carriers outside the space charge area; (5) a new
avalanche develops closer to the cathode. The most important large-scale application of corona
discharge is in electrostatic precipitators (ESP), which are used for dust collection in many
industrial off gases. In addition to ESP, corona discharges are also used in water purification,
electrophotography, copying machine, printers and liquid spray gun and in powder coating.
However, the restricted area and the inherent non-uniformity have limited their application in
material processing.
Figure 3: Schematic of corona discharge
3.2 Atmospheric-pressure plasma jet
Another kind of discharge capable of generating non-thermal plasmas at atmospheric
pressure is atmospheric-pressure plasma jet. A schematic of the APPJ is shown in fig 4. The
APPJ [27-32 ] developed by Jeong et al. (University of California, Los Angeles) in
collaboration with Park et al. (Los Alamos National Laboratory) consists of two concentric
electrodes through which a mixture of helium, oxygen or other gases flows. In this
arrangement, the inner electrode is coupled to 13.56 MHz radio frequency power at a voltage
between 100-250 V and the outer electrode is grounded. By applying RF power, the discharge
is ignited and operates on a feed stock gas, which flows between an outer grounded,
cylindrical electrode and a central electrode and produces a high velocity effluent stream of
highly reactive chemical species. Central electrodes driven by radio frequency power
accelerate free electrons. These energetic electrons undergo inelastic collisions with the feed
gas, producing excited state molecules, atoms, free radicals and additional ion-electron pairs.
Once the gas exits the discharge volume, ions and electrons are rapidly lost by recombination,
but the fast flowing effluent still contains neutral metastable species and radicals. The key
operational features of APPJ are as follows: (1) it produces a stable, homogenous and uniform
Drift region
Cathode wire Corona discharge
Anode
Corona Discharge
Vijay Nehra, Ashok Kumar and H K Dwivedi
International Journal of Engineering, Volume (2) : Issue (1) 60
discharge at atmospheric pressure; (2) operates at radio frequency (RF) power of 250 W and
frequency of 13.56 MHz ; (3) the ionized gas from the plasma jet exits through the nozzle
where it is directed onto the substrate and hence utilized in downstream processing; (4) it
operates without a dielectric cover over the electrode, yet is free from filaments, streamers
and arcing; (4) The gas temperature of the discharge is as low as 50°C, allowing it to treat
delicate surfaces without damage, or as high as 300°C, allowing it to treat robust surfaces
much more aggressively. (5) it exhibits a great similarity to low-pressure DC glow discharge.
This technology shows promises for being used in material application that are now limited to
vacuum. These features give APPJ the potential to be utilized in a large number of
applications. It has major utilization in material processing, for example, applications ranging
from etching polyamide, tungsten, tantalum and silicon dioxide as well as to deposit silicon
dioxide film by plasma assisted chemical vapor deposition. The fast flowing effluent of reactive
species in APPJ technology is also utilized in applications such as decontamination of
materials having chemical and biological warfare agents and in the removal of radionuclide
from surfaces and equipments. In addition, it is also used to clean large industrial parts more
effectively than solvents; sterilization of surgical and dental equipments and hospital surfaces
and removal of paint from brick, making it effective for graffiti removal; and in the textile
industry.
Figure 4: Schematic of atmospheric pressure plasma jet
3.3 Microhollow cathode discharge
A third approach to generate ANTP relies on the use of micro-hollow cathode electrode
concept. The general idea is that the modification of cathode shapes in linear discharge lead
to an increase in the current density by several orders of magnitude as compared to linear
discharge. This kind of discharge with modified cathode is known as hollow cathode discharge
(HCD). HCD [33] consists of a cathode, which contains some kind of a hole or a cavity or it
may be a hollow cylinder, spherical segment or simply a pair of plane parallel plates, and an
arbitrary shaped anode. The hollow cathode effect was originally used as a high current
density electron source at low gas pressure for the development of high power pseudospark
switches. The chief causes of electron generation in hollow cathodes include: (1) secondary
electron emission from cathode due to ions and ultraviolet photons; (2) secondary electron
emission due to bombardment of metastable atoms on the cathode, however, their flux flowing
to the cathode is much smaller than that of ions; (3) pendulum or pendel effect for oscillatory
motion of electrons between the opposite cathode surfaces under the influence of positive
plasmas. In a specific range of values for the product pD, where p is the pressure and D the
diameter of cathode bore, the current is enhanced by the pendulum motion of electrons. From
a fundamental perspective, high-pressure operation of HCD can be accomplished by reducing
the diameter of the bore to the values of the order of a few tens of micrometers; this hollow
cathode effect can be obtained at atmospheric pressure. Thus, for atmospheric pressure
discharges in hollow cathode, the typical hole diameter should be in micrometer range and
RF electrode
Plasma gas Grounded electrode Plasma jet
Water cooling
Vijay Nehra, Ashok Kumar and H K Dwivedi
International Journal of Engineering, Volume (2) : Issue (1) 61
hence the term microhollow cathode discharge [34-38] arises because of the required small
size of the cathode opening for high-pressure operation.
Figure 5: Typical electrode geometries of micro hollow cathode discharge [38]
The MHCD plasma may be operated in either direct current or pulse mode and two scaling laws
largely determine its properties. The product (pd) of the pressure,p, and anode-cathode
separation d obeys the well-known Paschen law, which determines the required breakdown
voltage for a given value of p and d as well as the identity of the operating gas. A second scaling
law, unique to the HCD involves the product (pD) in which D is the dimension of an aperture to
the cathode. The similarity law for HCD is the basic effort to extend the pressure range for the
hollow cathode discharge operation. The typical electrode geometries of MHCD are shown in
fig.5. In fact, MHCDs are direct current high pressure, gas discharges between two plane parallel
electrodes separated by thin layers of a dielectric material with a central borehole in each
electrode. The thickness of the electrode material and the dielectric layers is in the range of
100µm, while the hole diameter varies between 100-200µm. MHCD plasmas have been utilized
for several applications including remediation of gaseous pollutants, medical sterilization and
biological decontamination, cleaning of metallic surfaces, diamond deposition etc. In addition to
these applications, energetic electrons created by pendel effect efficiently generate excimers in
MHCD. Such excimer sources can be operated over a wide range of wavelength in the ultraviolet
and vacuum ultraviolet region.
3.4 Dielectric barrier discharge
Dielectric barrier discharge, also referred to as barrier discharge or silent discharge is a specific
type of AC discharge, which provides a strong thermodynamic, non-equilibrium plasma at
atmospheric pressure, and at moderate gas temperature. It is produced in an arrangement
consisting of two electrodes, atleast one of which is covered with a dielectric layer placed in their
current path between the metal electrodes. The presence of one or more insulating layer on/or
between the two powered electrodes is one of the easiest ways to form non-equilibrium
atmospheric pressure discharge. Due to the presence of capacitive coupling, time varying
voltages are needed to drive the DBD. One of the major difference between the classical and a
DBD discharge is that in a classical discharge, the electrodes are directly in contact with the
discharge gas and plasmas, and therefore during the discharge process, electrode etching and
corrosion occurs. On the contrary, in DBDs the electrode and discharge are separated by a
dielectric barrier, which eliminates electrode etching and corrosion. Another fundamental
difference is that the DBDs cannot be operated with DC voltage because the capacitive coupling
of dielectric requires an alternating voltage to drive a displacement current. An AC voltage with
amplitude of 1-100 kV and a frequency from line frequency to several megahertz is applied to
DBD configurations. DBD cold plasma can be produced in various working mediums through
ionization by high frequency and high voltage electric discharge. The DBDs unique combination
of non-equilibrium and quasi-continuous behavior has motivated a wide range of applications and
fundamental studies.
A
C
A
C
A
C
Vijay Nehra, Ashok Kumar and H K Dwivedi
International Journal of Engineering, Volume (2) : Issue (1) 62
3.4.1 DBD Structure
The discharge burning [39-43] between two electrodes, at least one electrode insulated with a
dielectric layer can be operated in a wide range of geometrical configurations such as the
classical volume discharge, surface discharge, and coplanar discharge.
Figure 6: Typical electrode arrangements of DBD configurations
Dielectric
High
Voltage
AC
Grounded
Electrode
Dielectric
High
Voltage
Dielectric
High
Voltage
Grounded
Electrode
Planar DBD Electrode Arrangements
AC
Dielectric
Grounded Electrode
High voltage
AC
Grounded Electrode
AC
Dielectric
Grounded Electrode
High Voltage
Cylindrical DBD Electrode Arrangement
AC
Dielectric
Grounded
Electrode
Surface Discharge
Coplanar Discharge
Vijay Nehra, Ashok Kumar and H K Dwivedi
International Journal of Engineering, Volume (2) : Issue (1) 63
Volume discharges can also have either planar or coaxial arrangements. In planar electrode
arrangements, the two electrodes are parallel to each other, and one or two dielectric barriers are
always located either (i) on the powered or the ground electrode, or (ii) on both the electrodes, or
(iii) in between the two metal electrodes. The electrodes in DBD can also be arranged in a coaxial
manner having one electrode inside the other with at least one or two dielectric barriers located
either (i) on the outer side of the inner electrode/on the inner side of the outer electrode, or (ii) on
both the electrodes facing each other, or (iii) in between the two cylindrical electrodes. Besides
the volume discharges, other designs also exist that use either surface or coplanar discharge
geometry. Surface discharge [44] device have a thin and long electrode on a dielectric surface
and an extended counter-electrode on the reverse side of the dielectric. In this configuration, the
discharge gap is not clearly defined and so the discharge propagates along the dielectric surface.
There also exist combinations of both volume and surface discharge configuration such as the
coplanar arrangement [45-46] used in plasma display panel. The coplanar discharge device is
characterized by pairs of long parallel electrodes with opposite polarity, which are embedded
within a dielectric bulk nearby a surface. In addition to these configurations, other variants of DBD
[47] are also used in various applications. The typical arrangements of DBD are shown in fig. 6.
DBD can exhibit two major discharge modes [48-49], either filamentary mode, which is the
common form of discharge composed of many microdischarges that are randomly distributed
over the electrode surface; or homogenous glow discharge mode, also known as atmospheric
pressure glow discharge mode due to similarity with dc glow discharges.
3.4.2 Applications of DBD
DBD technologies have an incredible potential [50-55] and are widely used in a large number of
technical applications. The advantage of DBD over other discharges lies in having the option to
work with non-thermal plasma at atmospheric pressure and a comparatively straightforward
scale-up to large dimensions. Initially, this technology was utilized for ozone production for the
treatment of drinking water. Since then the number of industrial applications of this type of
discharge have shown a tremendous growth. Besides ozone synthesis, today the phenomenon
of DBD in gases is widely used in the generation of excimer radiation in the UV/VUV spectral
regions, surface treatment, in the field of environment protection, for pumping CO2 lasers,
pollution control, various thin film deposition processes, in the textile industry, and more recently
in plasma display panel and in several other technological processes in science and industry.
Out of all these applications of DBD, the excimer formation, one of the significant application area
of DBD technology gained major impetus in the last decade. Excimer UVR optical source [56-64]
is a particular configuration of DBD, specifically; a volume discharge. The acronym excimer refers
to complexes with weakly bound excited state of molecules that under normal conditions do not
possess a stable ground state. These excimers when they come to their ground state convert
their binding energy into VUV/UV radiation. In the forthcoming section, the application of barrier
discharge in this area has been elaborated.
For efficient excimer formation in non-thermal plasma (DBD & MHCD), three conditions need to
be satisfied: (1) the bulk gas has to be provided with a large concentration of energetic electrons
with energies above the threshold for the metastable formation or ionization; (2) since the
formation of excimers is a three-body process, the gas pressure needs to be high, close to
atmospheric in order to have sufficiently high rate of three body collisions. The high pressure is
needed to ensure that the excimer formation reaction is faster than any quenching processes of
the excited precursors; (3) the gas temperature has to be cold since excimers are thermally
unstable. These conditions can only be effectively achieved in electron driven high-pressure nonthermal
plasma processes occurring in DBD plasma.
The mechanism of excimer formation takes place with ionization and excitation of rare gas
species by high-energy electrons. The dominant plasma chemical reactions for excimer
formation can be described as follows:
Vijay Nehra, Ashok Kumar and H K Dwivedi
International Journal of Engineering, Volume (2) : Issue (1) 64
A. Electron impact ionization and excitation
The high-energy electrons in DBD ionize and excite the rare gas species. In case of rare gas
halides, the high energy electrons ionize and excite the rare gas atom, and at the same time, the
halogen molecules are split by a dissociative attachment reaction
*
RgeRge +→+
+
+→+ RgeRge 2
∗∗∗
+→+ RgeRge
−
+→+ XXXe 2
B. Formation of excimers and exciplexs
The excimer molecule is formed by three body reactions of an electronically excited rare gas
atom Rg* with other rare gas atoms or with a buffer gas in the ground state.
MRgRgMRgRg ++→++ *
2
*
*
2RgeRge +→+ ∗∗
In case of rare gas/halogen mixtures, most RgX
*
exciplexes are formed either by a three body
ionic recombination of the positive rare gas ions and the negative halogen ions or by the
Harpooning reaction in which the excited rare-gas species transfers its loosely bound electron to
the halogen molecule or halogen containing compound to form an electronically excited state of
*
RgX .
MRgXMXRg +→++ −+ *
XRgXXRg +→+ *
2
*
C. Emission of UV/VUV photon
These excimer or exciplex molecules are not very stable and once formed decompose within a
few nanoseconds giving up their excitation energy in the form of UV or VUV photons.
hvRgRg +→ 2*
2
hvXRgRgX ++→*
Depending on the optical working media, a large number of different excimers can be generated
in ANTP such as in DBD and MHCD.
4. CONCLUSION
In the present paper, a review of commonly used atmospheric non-thermal plasma sources has
been presented. The unique features of non-thermal plasma have made possible substantial
breakthroughs in many growth areas of modern technology and newer applications are
continuously emerging, more recently in the vastly growing areas of nanotechnology, which
indicate that the non thermal plasma has become an important player in several up-coming
technologies. On the other hand, the prospect of using plasmas in numerous industrial
applications without the need of any vacuum equipment has been driving the search for methods
to generate atmospheric pressure non-thermal plasmas. While there is still more to go in the
development and utilization of these plasma sources, no doubt that low temperature atmospheric
pressure gas discharge plasma is a promising technology, not only for the future, but also for
today’s processes and applications. Looking ahead, still many opportunities remain to be
harnessed for further research and development in order to meet the demand of various diverse
plasma technological applications.
Note: Te = electron temperature, Ti = ion temperature, Tn = neutral temperature, Tp = plasma
temperature, ne= electron density, Rg represents the ground state of rare gas species (eg., Ar, Kr,
Xe, etc), X is a halogen species (eg., F, Cl, Br), Rg
*
is a metastable state of neutral gas atom,
Vijay Nehra, Ashok Kumar and H K Dwivedi
International Journal of Engineering, Volume (2) : Issue (1) 65
Rg
*
2 is the excimer, M is a collisional third partner, which in many cases can be an atom or
molecule of the active species or even of the buffer gas. The symbols, A, B stand for atoms, A2,
B2 for molecules, and e stands for an electron, M is a temporary collision partner, and species
marked by + or – are ions, R, for a simple radical, and P, for a polymer formed in the plasma. The
excited species are marked by asterisk (*), S-A indicates an atom adsorbed on the surface. The
subscripts g and s indicate, respectively a species in the gas or solid phase. The term hv
indicates release of radiation energy.
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