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Plasma Sources Science and Technology
Since plasma science mostly deals with charged particles, the
gravitation force, which is much weaker than the electromagnetic
force, is often neglected. Although this might be just
and reasonable for many kinds of discharges, there are cases
where gravity plays a crucial role through its action on neutral
particles. Typical examples include astrophysical plasmas,
dusty plasmas, arcs, plasma torches and others.
In these discharges the alteration of real or apparent gravity
(g-force) could significantly change the plasma properties
with consequences for both fundamental and applied
science. Study of the electric discharges in altered gravity
could be important for electrical and fire safety precautions at
high accelerations (space flight, aircraft, racing), ion thruster
design, or understanding the plasma related processes in the
atmospheres of other planets, where the gravity is significantly
stronger or weaker than that on Earth. One of the discharges
where the gravity has to be taken into account is a gliding arc.
A gliding arc is similar to well known stable arc discharge
[1]. It differs in the electrodes geometry, which in the case
of a gliding arc enables the vertical movement of the plasma
channel. This movement is caused by: (i) gravity-dependent
buoyant force originating from the temperature difference
between the hot plasma channel and cold surrounding
atmosphere and/or (ii) the drag of gas, which typically
comes from below. In a standard gliding arc configuration
with divergent electrodes, the length of the discharge
Gravity effects on a gliding arc in four noble
gases: from normal to hypergravity
L Potočňáková1, J Šperka1,2, P Zikán1, J J W A van Loon 3, J Beckers4
and V Kudrle1
1
  Department of Physical Electronics, Masaryk University, Kotlářská 2, CZ-61137, Brno, Czech Republic
2
  The Central European Institute of Technology (CEITEC), Masaryk University, Kamenice 5, CZ-62500,
Brno, Czech Republic
3
  Dutch Experiment Support Center (DESC), ACTA-VU-University and University of Amsterdam,
Amsterdam, The Netherlands and European Space Agency (ESA), ESTEC, TEC-MMG, Noordwijk, The
Netherlands
4
  Faculty of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven,
The Netherlands
E-mail: georgejewel@gmail.com
Received 20 October 2014, revised 3 February 2015
Accepted for publication 20 February 2015
Published 19 March 2015
Abstract
A gliding arc in four noble gases (He, Ne, Ar, Kr) has been studied under previously
unexplored conditions of varying artificial gravity, from normal 1 g gravity up to 18 g
hypergravity. Significant differences, mainly the visual thickness of the plasma channel,
its maximum elongation and general sensitivity to hypergravity conditions, were observed
between the discharges in individual gases, resulting from their different atomic weights and
related quantities, such as heat conductivity or ionisation potential. Generally, an increase
of the artificial gravity level leads to a faster plasma channel movement thanks to stronger
buoyant force and a decrease of maximum height reached by the channel due to more intense
losses of heat and reactive species. In relation to this, an increase in current and a decrease in
absorbed power was observed.
Keywords: gliding arc, noble gases, hypergravity
(Some figures may appear in colour only in the online journal)
Fast Track Communication
0963-0252/15/022002+6$33.00
doi:10.1088/0963-0252/24/2/022002Plasma Sources Sci. Technol. 24 (2015) 022002 (6pp)
2
channel increases as the arc is moving upwards. When the
length exceeds a critical value, the discharge extinguishes
and a new one is ignited at the minimum distance between
the electrodes at the same moment [2]. This repetitive character
of a gliding arc lifecycle with a characteristic time
period is one of its most distinguishing attributes. At low
current density, i.e. in a so-called glowing arc [3] mode,
gliding arc properties are similar [2] to non-equilibrium
atmospheric pressure glow discharges.
The gliding arc has been studied intensively from the point
of view of fundamental research [4–6], as well as an outstanding
plasma source for various industrial applications [7, 8].
The gliding arc is usually studied in a flow regime with a
relatively high flow rate, which governs the discharge dynamics,
especially the vertical speed of the plasma column [5, 9,
10]. In such a case, the buoyancy plays only a minor role and
the gas drag dominates. So to study the buoyancy effects, the
gravity level should be increased to emphasise the buoyancy
compared to the gas flow effects. Several experiments have
been performed to explore the gravity effects on standard arc
discharges [11–13]. Only recently have the first studies [6, 14,
15] about the influence of increased gravity on the gliding arc
behaviour been published. In higher gravity levels the plasma
channel glided faster and the whole life cycle was shorter.
Here, an extension and improvement to our previous papers
is made by carrying out the experiments in four noble gases—
He, Ne, Ar and Kr. These were chosen both for their similarities
(they are all monoatomic and non-reactive gases) as
well as for their differences (their atomic masses cover a wide
range, which results in a wide spread of dependent quantities,
e.g. thermal conductivity).
The experiments in hypergravity (up to 18  g) were performed
by using the Large Diameter Centrifuge (LDC) at
ESA-ESTEC centre in Noordwijk, the Netherlands [16].
The whole experimental setup (except the spectrometer) was
placed inside the centrifuge gondola. Given that the detailed
description of the experimental apparatus and methods has
been presented in [14], only a brief overview follows.
The discharge at atmospheric pressure was operated
between slanted copper electrodes with a minimum distance
of 4.5 mm and an initial angle of 36° between them. The electrodes
were enclosed in a non-conductive discharge chamber
with 2 litre inner volume with holes for gas inlet and outlet.
The discharge voltage was supplied via a variable autotransformer
(0–250  V, 50  Hz AC) by high voltage transformer
(0–10 kV). Three multimeters were used to measure the RMS
voltage, current and power on the primary winding of the high
voltage transformer.A high voltage probe and a Rogowski coil
current probe were used to measure the instantaneous electric
parameters of the discharge. The discharge visual appearance
was recorded as a video (up to 1000 frames/s) and as standard
photographs by a digital camera.
Although the experimental device is essentially the same
as in [14], the sealing of the discharge chamber was improved
and the longer exhaust tube suppressed possible air backstreaming.
Altogether, these changes significantly improved
the purity of noble gas inside the discharge chamber. The
amount of air admixtures in the noble gas was estimated
from spectroscopic measurement to be in the order of 0.1%
or better.
The main features of the gliding arc appearance will now
be discussed based on figure 1, where long exposure (0.1–
1 s) photographs of the gliding arcs at various conditions in
He, Ne, Ar and Kr are shown. Each row corresponds to one
gas and each column to different experimental conditions.
The photographs in columns 1–4 were taken remotely from
inside the centrifuge gondola and they show the changes that
the gliding arc in each gas undergoes with the increase of
artificial gravity. The experimental conditions were deliberately
set differently for each gas to enable gliding of the
plasma channel already in 1  g conditions and simultaneously
not to mask the gravity effects by too high a gas flow.
Unfortunately, due to severe complications (including the
influence of hypergravity on the camera lenses, shallow
depth-of-field, impossibility to remotely control the camera
during experimental runs at LDC), the quality of some of the
photographs was affected. The rightmost column shows the
photographs taken in laboratory 1 g conditions with gas flow
increased by 1 slm compared to the columns 1–4 for each
gas. The changes in brightness between the images are not
to be attributed to plasma processes but to different exposure
times. In the majority of the photographs (except for Ar and
Kr, both at 1 g and 2 g), rather long exposure time and high
gliding frequency caused an overlaying of many glides of the
plasma channel.
The most pronounced differences among gases in the first
column of figure  1 are: apparent colour of the gliding arc
plasma, which is naturally determined by dominant spectral
lines of each gas, the thickness of the plasma channel and its
maximum elongation.
The thickness of the plasma channel was observed to be
visually wider for lighter gases (helium and neon) and thinner
for heavier gases (argon and krypton). The phenomenon
of radial contraction can be attributed to the inhomogeneous
heating in the radial direction due to finite thermal conductivity
of the gases, which is more than 20 times lower for krypton
than for helium at 300 K and this difference increases even
more at higher temperatures [17]. As a result, the gliding arc
in helium and neon had a rather diffusive character, while in
argon and krypton the sharp edges of individual filaments
could be distinguished [15]. Unfortunately, quantitative analysis
of the filament thickness is difficult: for the long exposure
photographs in figure 1, the unceasing movement of the
plasma channel causes motion blur of several millimeters and
the low resolution of our high speed camera leads to comparably
high uncertainty.
The maximal elongation of the plasma channel before
extinguishing was also different for each gas. Generally, the
gliding arc in krypton was able to rise much higher than the
gliding arc in lighter gases. In typical conditions (low g-level,
the gas flow low enough to maintain regularly shaped individual
plasma channels) the gliding arc in krypton and argon
was even able to easily ascend above the electrodes—meaning
that the end points of plasma channels reached the top of
electrodes, yet the centre of the arc continued to rise up (see
images in [15]). This can probably be attributed to the lower
Plasma Sources Sci. Technol. 24 (2015) 022002
3
ionisation potential of heavier gases, which makes sustaining
the discharge easier than in helium or neon.
The influence of increasing hypergravity can be observed
on photographs in columns 2–4 of figure 1. With the increase
of gravity, the gravity-dependent buoyant force acting on the
plasma channel gets stronger, making its upward movement
faster. However, due to faster convective cooling, the losses
of excited atoms and heat become more intense and cause the
decrease in maximum height. This decrease is the most significant
for krypton and the least significant for helium. This
different sensitivity of each gas to an increase in gravity is the
result of various factors, one of them being the gas atomic
weight. The resulting gravity-dependent force (the difference
between downward gravitational force and upward buoyant
force) acting on the heated arc channel is proportionally
dependent to the mass density difference between a cold and a
heated noble gas. This difference is proportional to the atomic
weight of the gas (4, 20, 40 and 84 for He, Ne, Ar and Kr) and
so the hypergravity effects are much more profound for krypton
than for lighter gases. Also, the thin filament of krypton
transfers the heat to the colder environment more slowly than
the wide diffusive helium plasma channel. This effect is intensified
by the fact that helium is a good heat conductor compared
to krypton. Thus, the temperature difference between
the heated and cold gas is bigger for heavier noble gases,
therefore enhancing the hypergravity effects. Finally, the thinner
the plasma filament, the less gas aerodynamic resistance it
has to overcome during its upward movement.
In the gliding arc device with the gas feed from below,
the plasma channel is lifted up by a combination of two
forces: buoyant force and gas drag. Increasing either of
them will make the resulting upward force acting on the
Figure 1.  Long exposure photographs of the gliding arc in helium, neon, argon and krypton. Experimental conditions for each of the
columns 1–4 (dependence on artificial gravity level) were set to enable gliding of the arc already in 1 g: helium—1.37 slm, 8 kV; neon,
argon and krypton—0.4 slm, 4 kV. Conditions in column 5 differ in the gas flow, which was increased by 1 slm for each gas. Exposure
times of photographs in columns 1–4 were: He—0.1 s, Ne—1 s, Ar—0.1 s, Kr—0.3 s; exposure times of photographs in column 5 were:
He—0.1 s, Ne—0.1 s, Ar—0.1 s, Kr—1 s.
Plasma Sources Sci. Technol. 24 (2015) 022002
4
plasma channel become stronger. Which component of the
force will dominate depends on the g-level and on the gas
flow rate. The high gravity case was already discussed and
the high-flow case (approximately 1  slm above the normal
flow conditions presented in columns 1–4 of figure 1:
helium—1.37  slm  →  2.37  slm; neon, argon and kryp-
ton—0.4 slm → 1.4 slm) is presented in column 5 of figure 1.
The high gas flow influences the motional characteristics of
the gliding arc as the plasma channel is dragged by the fast
moving gas. It was found that even though this force is of
a different nature to the buoyant force the main effect on
the gliding arc remains the same—the speed of the plasma
channel increases, convective cooling also increases and as
a consequence the maximum height of the arc decreases and
new ignitions occur more frequently.
The recapitulation of maximum heights in four gases in
1 g versus 18 g versus high gas flow from figure 1 can be
seen in figure 2. As discussed above, the increase of the artificial
gravity level did not cause similar changes in all four
gases. The decrease of maximum height in helium and neon
is only small (around 10%), while in argon and krypton the
decrease is significant (over 50%). Krypton is the only gas
in this experiment, for which the increase of artificial gravity
caused a bigger decrease of the maximum height than did the
increase of the gas flow.
In the above-described experiment, the experimental
conditions for individual gases differed in order to maintain
the gliding motion of the plasma channel. If the conditions
were set the same for each gas, then the situation
might be drastically changed, as will be shown in figure 3.
Here, the gas flow rate was 0.15 slm for each gas and the
discharge voltage, 4  kV, was also the same for each gas.
Besides the maximum height, the frequency of new ignitions
(or gliding frequency) (figure 3(a)) is also a valuable
parameter for the study of hypergravity influence because
it effectively combines both the maximum height reached
by the plasma channel (figure 3(b)) and the velocity of its
movement (figure 3(c)). For these particular experimental
conditions, the lightest of the gases—helium—appeared in
the form of stable arc-like plasma channel for all g conditions
(zero gliding frequency) because the upward force
was not strong enough to lift the arc up. The second lightest
gas—neon—also started as a stable plasma channel but
was able to undergo a transformation to a gliding arc after
an increase of the artificial gravity level above 8  g. The
buoyant force for the discharge in argon and krypton was
strong enough to initiate gliding already at 1 g, although the
frequency was quite low, since the plasma channel moved
rather slowly and quenched (and reignited) after travelling
a relatively long distance. With increasing gravity, the gliding
frequency increased to values many times higher for all
three gases for which the discharge appeared in the form of
a gliding arc. However, one must keep in mind that the gliding
frequency is a time-averaged quantity, so it exhibits a
statistic behaviour and, hence, the individual glides may differ
a lot. Moreover, the gliding arc is also influenced by the
system history (e.g. preheated electrodes, gas circulation)
and shows signs of hysteresis. We repetitively observed that
the transition from stationary into gliding mode happens at
a higher gas flow rate than reverse transition (from gliding
into stationary mode), probably thanks to the positive
feedback between the ambient gas circulation and plasma
channel gliding.
The electrical parameters that were directly measured on
the primary winding of high voltage transformer are plotted
in figure 3(d). Since the discharge in helium was not in a form
of a gliding arc but a stationary arc-like discharge, it is not
included in the graphs, the same goes for neon in g-levels
below 8 g. The curves for neon and argon are almost constant,
while for krypton they exhibit a clear dependence.
In krypton, as the g-level increases, the power input into
the primary winding (see figure 3(d) solid symbols) decreases
as a consequence of the lowered power consumption in the
gravity-shortened plasma channel [2]. Simultaneously, the
primary winding current increases (see figure 3(d) open symbols),
albeit slightly. This counter-intuitive behaviour is most
probably caused by the non-ideality of the transformer, i.e. its
transformation matrix is not diagonal, it is complex and may
even be non-linear. The substantial primary current error bars
at low g-levels would then be a consequence of the sensitivity
of the current to the phase variations for a given voltage and
power at low power factors.
The plots of primary current and RMS power for neon and
argon are almost constant with g-level because their height
decreased insignificantly.
In this paper, we have shown that the increased artificial
gravity (up to 18 g) spectacularly influences the properties of
the gliding arc discharge between diverging electrodes. Four
noble gases of atomic weight in wide range (He, Ne, Ar and
Kr) were used to reveal how the differences in gliding arcs
observable at 1 g affected their peculiar behaviour at higher
g levels.
At the elementary level of the complex interplay of physical
phenomena evoked by hypergravity, there is an increase
of the gravity-dependent buoyant force. The stronger buoyancy
initiates faster movement of the plasma channel, which
Figure 2.  The maximum heights of gliding arcs calculated from
image analysis of figure 1 in 1 g, 18 g and high gas flow conditions.
He Ne Ar Kr
0
2
4
6
8
10
12
14
maximumheight[cm]
1g,
18g,
1g, high gas flow
normal gas flow
normal gas flow
Plasma Sources Sci. Technol. 24 (2015) 022002
5
increases the losses of excited species and heat and thus
results in the decrease in maximum height reachable by the
plasma channel. These changes of the plasma properties are
then reflected in the electrical parameters of the discharge. All
of these effects of the varying gravity on the gliding arc discharge
are strongly interconnected and create a complex system
that is very sensitive to all of the experimental conditions,
even with signs of hysteresis behaviour.
Acknowledgments
This work was supported by European Space Agency
SpinYourThesis! 2013 programme, Masaryk University
Rector’s Scholarship, project CZ.1.05/2.1.00/03.0086 funded
by European Regional Development Fund and project LO1411
(NPU I) funded by Ministry of Education Youth and Sports of
Czech Republic. We would also like to thank Mr A Dowson
from ESA-TEC-MMG for his support of these studies.
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0 2 4 6 8 10 12 14 16 18 20
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glidingfrequency[Hz]
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0 2 4 6 8 10 12 14 16 18 20
30
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power:
neon
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g
primarypower[W]
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