Symbiosis between low temperature plasma and high temperature
chemistry
Alexander Gutsol
Chevron Energy Technology Company, 100 Chevron Way, Richmond, CA 94802
Abstract: The influence of high temperature chemistry on plasma is not well
recognized. Meanwhile, smart utilization of this interaction can substantially
improve industrially important plasma chemical processes. Intensive chemical
processes result in the formation of excited molecules and chemical ionization.
These phenomena are most obvious for flames with good electrical conductivity
and light emitted by excited molecules. Even if there is no chemical process but
gas temperature is above the threshold of radical appearance, the reactions of
radicals create electronically excited molecules. The appearance of these excited
molecules without direct electron energy consumption stabilizes the discharge
and reduces the required electron energy for ionization. When temperature
becomes even higher and reaches the threshold of thermal ionization, the role of
radicals in ionization processes becomes insignificant. Discharges can be
classified as warm discharges if the discharge temperature is within the
temperature range, in which the role of chemistry and/or heat in the discharge
“life support” is comparable to that of the electric field.
Keywords: warm discharges; step-wise ionization; plasma chemistry; plasma
catalysis, discharge stabilization.
1. Introduction
This work was inspired by experiments with an
elongated atmospheric pressure glow discharge in
the reverse vortex (tornado) flow. It was possible to
pass continuously through the whole range of
available currents (1 – 530 mA) only using helium
discharge (Fig. 1).
even when available voltage provided by a power
supply was much higher than that necessary to
support a discharge above the threshold. The shorter
the discharge, the lower is the current achievable.
Millimeter scale contracted glow discharges can be
stable at currents as low as 1.5-2 mA [1]. What is the
reason for this dependence and how it is possible to
reduce the current and power of long discharges?
Study of this topic led to a discussion of the related
phenomena of “warm” discharges and to the role of
radicals (means chemistry) in discharge support.
This paper is an attempt to clarify all these issues.
2. Contraction of a glow discharge
Figure 1. Current – Voltage – Power characteristics obtained in
several runs for the discharge with 8.5 L/min flow of helium.
Discharges in all other gases were unstable at
currents below a certain threshold (around 100 mA)
Before discussing issues with the stability of
contracted glow discharges (CGDs), it is useful to
clarify what causes the contraction. Much of the
literature on this subject suggests that a glow
discharge is unstable with respect to contraction due
to thermal or ionization-overheating instability (IOI),
and the development of this instability can be
presented by the following chain of causal links [2]:

ne(jE)TNE/NTene (1)
A local increase in electron concentration ne results
in a greater power release jE (j is current density,
and E is electric field) that causes the gas
temperature T to rise and the gas concentration N to
drop. This, in turn, leads to an increase in the E/N
ratio, which results in an electron temperature
increase, which causes the electron concentration to
rise further. However, this logic is imperfect at least
with respect to the glow discharge contraction.
According to this logic, the E/N ratio after
contraction should be higher than that in a diffuse
glow discharge, but this is not the case. The authors
of paper [3], who observed a drop in electron
temperature during the glow discharge contraction,
did not discuss the reason for this transition. Yuri
Raizer discussed these experiments in his book [2, p.
243]: “the thermal mechanism was not included in
the interpretation of the experiments although the
overheating of the gas was tremendous, by a factor
of 3.” In a special review on plasma instabilities [4]
and in a special book on glow discharge [5], the
authors named IOI as the major reason for
contraction, and as secondary reasons they named
the maxwellization of EEDF with ne/N growth that
lead to ionization rate growth for atomic gases, as
well as the vibrational-translational (V-T) relaxation
with explosive heat release in molecular gases. The
authors of book [5] separate optical contraction,
which occurs when the electrons and the current are
present in the whole volume because of surface
recombination at lower pressures, from “volumetric”
contraction, which occurs when the volume
recombination dominates. In other publications [2, 4,
6], the authors consider only the second option and
state that volume recombination is the major
condition for the appearance of contraction.
It looks like the effect of contraction is analogous to
inability to support several parallel discharges with
negative differential resistance using one power
supply. A diffuse discharge can be considered a
combination of many parallel discharges. Instability
development in this case can be represented by the
following simplified chain of causal links: 
T   (jE) T
(2)
A local increase in gas temperature T results in an
increase in local electric conductivity . This leads
to an increase in local power release w = jE, that
causes the gas temperature to rise further. There are
multiple factors (or combinations of factors) that
may cause an increase in conductivity in response to
rising temperature. In addition to the reasons
mentioned above, there is one more notable reason:
T N  en  Vd  
(3)
A temperature increase means a drop in gas
concentration N that results in a drop in frequency of
collisions of electrons with neutral molecules en,
that is equivalent to the electron drift velocity Vd
increase, means an increase in local electric
conductivity . Thus, the contraction does not
require an increase in the E/N ratio and electron
temperature. So, it is a thermal instability, but not
necessarily IOI, and this is consistent with
publication [6], which argues that a sharp growth in
w(T) is a necessary condition for contraction. In
paper [6], the radius of a contracted channel re where
most of the power is released was theoretically
estimated from the thermal balance equation:
re2   (T0)/(dw/dT)
(4)
Here, T0 is the tube wall temperature, and  is gas
thermal conductivity. Diffuse discharge in helium
contracts smoothly because it has very high thermal
conductivity, and also because its ionization
mechanism (Penning ionization) does not have a
strong dependence on gas temperature.
3. What causes the electron temperature
to drop after the discharge contraction?
In most cases stepwise ionization is a major
ionization mechanism in a positive column of a
diffuse glow discharge even before contraction.
However, after contraction, the concentration of
electrons in the discharge column increases manifold
(by factor of 40 in work [3]), and therefore the
concentration of excited molecules in the column
also increases, in spite of the lower concentration of
neutral molecules. The higher concentration of
excited molecules results in a decrease in the
required average electron energy for ionization, and
this means that the E/N parameter drops. Sure, other
effects, e.g. EEDF maxwellization, can have their
own contributions to the E/N parameter drop;
however, growth in concentration of excited
molecules is important to emphasize for this paper
discussion.
4. Why a long CGD cannot “live” at a low
current?
The energy balance of the elongated CGD is defined
mostly by thermal losses to the surrounding gas,
similar to that of an arc discharge. Step-wise
ionization in this discharge is also similar to that in
thermal plasmas. However, because gas temperature
is not high enough for thermal ionization, the role of
electric field is also important, and the E/N
parameter is just a little smaller than that in diffuse
glow discharges. According to conventional logic,
contracted discharge should be stable with respect to
parameter fluctuations along the discharge. Negative
feedback formation in response to a small
perturbation can be presented by the following chain
of causal links: 
T N  (I2/) T 
(5)
A local temperature increase and a drop of gas
concentration result in an increase in local electric
conductivity. Current I through the discharge
channel is the same in all cross-sections; therefore,
power release I2/ drops locally resulting in a
temperature decrease. What can happen with this
discharge when the current drops slowly? If a power
supply can provide necessary voltage, the discharge
should survive, but it dies out.
Paper [7] discusses a method of stabilization of an
atmospheric glow micro-discharge at low current. It
assumes that the discharge oscillation was caused by
IOI. This assumption may be incorrect, as the
discharge can be considered a contracted one all
time because the discharge cross-section was not
restricted by a chamber wall. Nevertheless, it was
experimentally demonstrated that the stray
capacitance reduction helps to stabilize the
discharge. A key idea of paper [7] was that the
constant electric field assumption for the chain (1) is
not always correct: if an electric circuit has a small
response time e = RpCs (Rp is the plasma resistance,
and Cs is the stray capacitance), the conductivity
growth results in fast voltage and electric field drop
that help to stabilize the discharge. Applying the
same logic to the chain (5), we can conclude that if
the response time is long enough, a drop in the local
temperature and electric conductivity can result in a
current drop, promoting positive feedback that
results in the discharge extinguishing. And the larger
the discharge length, the longer is the response time.
This is the reason why a long CGD is less stable that
a similar but shorter discharge. Discharge stability is
additionally challenged by the fact that in a system
with a gas flow, discharge length fluctuations are
unavoidable due to the electrode spot motion, and
this causes strong fluctuations in the discharge
voltage and other parameters.
5. What can help to stabilize a CGD?
Some methods of the CGD stabilization became
evident from the previous section: (a) increase of
current and discharge temperature that reduce
discharge resistance; and (b) circuit and/or power
supply modification to reduce the response time. In
this paper, another option is emphasized:
stabilization with the help of chemical reactions,
means radicals, and gas preheating. Radicals are
always present in molecular gas plasma. However, if
the plasma temperature is low, an electric field is
solely responsible for radical production through
energy transfer to electrons, and they spend
significant portion of their energy to support a stable
concentration of radicals that persistently disappear
in chemical reactions. Also, electrons spend a lot of
their energy on supporting some level of electronic
excitation, and this level is the backbone of stepwise
ionization. If the temperature is above some
threshold, the energy of the gas itself can support an
equilibrium concentration of radicals, radicals
participate in chemical reactions, and a “byproduct”
of these reactions is a stable concentration of excited
molecules. At these conditions, conductivity of the
discharge becomes more stable, and does not depend
so much on small fluctuations of other parameters.
Discharges with temperatures that can support a
stable concentration of excited molecules and thus
stabilize stepwise ionization can be distinguished as
“warm” discharges. It is obvious that the same
temperature is high enough to initiate and support
chemical reactions inside and in the vicinity of the
discharges. It is clear that this temperature can be
reached not only with the help of the discharge
power, but with gas preheating, or with additional
external or internal (e.g. combustion) heating. Gas
heating reduces temperature gradients on the
discharge periphery, and thus reduces the power
necessary to support CGD.
6. What is a “warm” discharge?
The process of thermal formation of radicals has a
threshold that depends on gas composition; however,
it is usually at temperatures about 1500 K. This can
be considered a lower temperature limit of “warm”
discharge in non-reacting gaseous mixtures. In
reacting mixtures (e.g. combustible), excited
molecules can appear earlier, as soon as the
temperature reaches the level of chemical reaction
activation (ignition temperature). Experiments show
that an increase in current (power) converts CGD
(warm discharge) into an arc (thermal discharge)
without any instability. Is there any boundary
between these discharges? Yes, it is possible to find
this boundary, assuming that in thermal ionization
the role of electric field is insignificant. This role
becomes insignificant as soon as thermal ionization
becomes important, and this process also has an
appropriate threshold in the temperature range of
3500-4000K. Thus, a warm discharge can be defined
as a discharge with non-thermal stepwise ionization,
in which a stable concentration of electronically
excited molecules is supported not only by electrons,
but also by the gas temperature and/or by chemical
processes. Ionization in the warm discharges has
comparable sensitivity to the gas temperature as well
as to the E/n parameter. Simple thermodynamic
equilibrium simulation shows two temperature
thresholds for a non-reacting gas mixture. Though
these thresholds are rather sharp, it is still unclear
what level of the radical concentration and
ionization degree can be considered the point of
“appearance” of radicals and electrons. Plasma is
very conductive at ne >1011 cm-3; for atmospheric
pressure and temperature of about 3000 K, this
corresponds to the ionization degree of 310-8. To
have significant influence on stepwise ionization
near the low temperature threshold, the
concentration of excited molecules should also be on
the order of 1011 cm-3, and it means that the
concentration of radicals should be about two orders
of magnitude higher, at the level of 1013 cm-3.
7. Why is this significant?
An understanding of discharge stabilization methods
is important for many practical processes especially
related to fuel conversion and to plasma catalysis
[8]. Many such processes are defined by chemical
reactions that take place at temperatures of 15002500 K. Electric discharges can heat any gas to these
temperatures; however, electric energy is about 3
times more expensive than the chemical energy of
natural gas. Therefore, if it is possible to make a
discharge “warm” using chemical energy, it is
almost always preferable. Especially this is
important in plasma-catalytic processes, where the
major role of plasma is to provide radicals that
initiate or direct the desirable chemical process. In
these cases usually, it is desirable to have the least
powerful (but stable) plasma possible. Let’s consider
experimental data for the plasma assisted process of
methane partial oxidation using different low-current
gliding arcs (or CGDs) [9]. Preheating of the
reagents resulted in plasma Specific Energy
Requirements (SER) on the level of 0.1 kW-h/m3 of
synthesis gas. In a rotating gliding arc reactor
without preheating [10], SER obtained is at least 10
times higher. The “reverse” assistance of plasma by
thermal and chemical energy in the former cases can
produce a large difference in fuel conversion, and it
can also be very influential in plasma assisted
ignition and combustion processes. A more detailed
analysis shows two key reasons in obtaining so
different SERs: (1) Heat recovery is equivalent to an
additional SER of 0.35 kW-h/m3; (2) When the
discharge is placed in non-preheated flow of
reagents, the discharge is still supported by the
chemical reaction inside the discharge column;
however, the combustion area around the column is
much smaller, and the temperature gradient between
the column and the surrounding gas is larger than in
the case of preheated reagents. This higher thermal
gradient requires higher power to support the
discharge itself and results in a SER increase.
Thus, warm electrical discharges and plasma
chemical processes can support each other’s
existence, and this “perfect match” is similar to the
symbiosis in biological systems.
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