Role of confinement in the development of a helium impacting
plasma jet at atmospheric pressure
T. Gaudy1,2, J. Iacono1, A. Toutant1, P. Descamps2, P Leempoel2, F. Massines1
1 : Laboratoire PROcédés, Matériaux et Energie Solaire (PROMES) – CNRS, Tecnosud, Rambla de la
thermodynamique, 66100 PERPIGNAN, France
2 : DOW CORNING EUROPE, SA Parc Industriel - Zone C, Rue Jules Bordet 7180 Seneffe, Belgium
Abstract: The aim of this work is to study low temperature point to plane discharges
development in an atmospheric pressure helium plasma jet impacting a surface and confined
in a 16 mm tube. Two discharge regime directly connected to the gas flow dynamic have been
pointed out. In most of plasma jets, the flow dynamic influence the discharge power, the
length of the jet, or interaction with air. But in the PlasmaStream® configuration, not only
power and length are affected, but the flow dynamic lead to the existence of two different
discharge modes, influencing quality of deposited films. These two modes have been defined,
and studied by short exposure time pictures, current and voltage measurement, and flow
dynamic modeling with FLUENT. It has been confirmed that the two discharge modes are
linked to two jets flow modes. One is laminar and allows air entrance along the tube wall. The
other avoids air entrance in the tube and can be turbulent.
1 : INTRODUCTION:
Today, numerous plasma jets are studied as
plasma source for surface treatment (1). Most
of them flow in the air before impacting the
surface. The peculiarity of the studied
configuration is that two impacting jets are
confined in a single tube of 16 mm diameter.
This leads to some interactions between the
two jets, and between each jet and the
confinement tube. It also prevents the
discharge to interact with surrounding air
before reaching the surface. The electrodes
configuration is point to plane with a thick (12
mm) dielectric on the plane. Two very
different visual aspects of the discharge have
been observed during deposition experiments,
which will be called mode 1 and mode 2 in this
paper. This study is dedicated to the
understanding of these two modes and the
parameters controlling the transition from one
mode to the other. At the first stage, visual
observation is considered to be enough to
study the influence of the experimental
parameters on the discharge mode switching.
Then short exposure time pictures have been
taken to characterize each discharge mode and
understand the differences between them.
Simultaneously,
current
and
voltage
measurements
allow
highlighting
the
specificities of the two discharge modes.
Finally, flow dynamic modeling with FLUENT
has been carried out to try to confirm the link
between the discharge appearance and the gas
flow modes.
2 : EXPERIMENTAL SETUP:
All experiments
PlasmaStream®.
are
carried
out
with
Figure 1 - Configuration of the plasma head
The PlasmaStream® belongs to the family of
atmospheric pressure plasma jets, excited by a
low frequency high voltage source in the range
of 20 kHz. The particularities of this set-up
are:
- The electrodes are needles, while in
traditional jets the electrodes are concentric
and set in a parallel or perpendicular field
arrangement. Needles possesses the unique
benefit of (i) creating a gas breakdown using a
lower voltage source because of the enhanced
Transition from mode 1 to mode 2 (75 to
55mm)
possible to change the regime by adjusting the
flow rate. The space between the two modes is
hysteresis like when we switch from mode 1 to
mode 2.
3
Gap (mm)
2,5
2
1,5
75 mm
55 mm
1
0,5
0
0
5
10
15
He flow (L/min)
Figure 4 – Gap associated to the transition between
mode 1 and mode 2 as a function of the gas flow for
two tube lengths
Figure 2 - Standard pictures of:
a) Mode 1
b) Mode 2
Transition from 1 to 2 appears for higher
values of gap than transition from 2 to 1. Some
seconds are necessary to stabilize mode 2. This
observation tends to show that mode 1 is more
stable.
Speed (m/s)
0
10
0
2
20
30
40
50
60
2,4
2,2
2,0
1,8
Gap (mm)
electric field at the sharp extremity of the
needles (ii) giving the possibility of limiting
the discharge extinction to the surface because
of the drastic field decrease with the distance
to the point.
- 2 separate plasma jets of 1.6 mm diameter
open to a confined space having a diameter 10
times larger.
- The jets do not flow out in the ambient air but
are confined in a dielectric plasma tube, whose
length can vary between 35 and 90 mm.
- The plasma tube is located above the
substrate that placed on a grounded metallic
electrode covered by a dielectric. The space
between the bottom of the confinement tube
and the substrate (the gap) is adjustable from 0
to several centimeters. In standard conditions,
the tube is 75 mm in length with and a gap
between 0.5 mm to 3 mm from the grounded
electrode. The helium flow is controlled by a
mass flow ranging from 0 to 15 slpm.
1,6
1,4
1,2
1,0
0,8
0,6
0,4
0,2
4
6
8
10
12
Flow (L/min)
3 : RESULTS:
3 – a) Experimental observations :
As shown figure 2 and 3, two strongly
different discharge modes are observed, named
mode 1 and mode 2 in this paper. In the mode
1, the discharge fills all the tube in an almost
homogenous manner. In the mode 2, discharge
is localized in two jets, with few micro
discharges around. The color is also different:
white for mode 1 and pink for mode 2. The
most influencing parameters of the discharge
mode are the gap, the total flow rate and the
tube characteristics. Figure 4 shows the
influence of the total flow rate in the
confinement tube and the gap. Mode 2 occurs
for larger gap. Whatever the flow rate is, the
mode can be changed by adjusting the gap, and
whatever the gap is, it is almost always
Figure 3 - gap associated to the transition from one
mode to the other as a function of the gas flow. Red
curve: when the gap is increased, black curve when the
gap is decreased
Some experiments have been done with
different length and diameter of confinement
tube. With a shorter confinement tube (55 or
35 mm), mode 1 is more difficult to be
obtained because gap has to be smaller,
whereas with a thinner tube (8 mm diameter)
it's more easy. But in all of the cases, there are
always two discharge and flow modes that can
be obtained by changing gap or flow.
3 – b) Short exposure time pictures
To try to understand the behavior of these two
modes and the origin of the transition, 500 ns
exposure time pictures synchronized with the
voltage have been taken over one period for
the two discharge modes. Pictures are the
results of 70 accumulations. The absolute
value of the light intensity is changed from one
picture to another using the camera software
auto-contrast function. So it is not possible to
compare light intensity just comparing the
pictures. But comparison of the spatial
distribution of the energy injected in the two
discharge modes is possible. In mode 1 the
discharge close to the point is intense, then
there is a rather dark area followed by a light
uniformly distributed along the tube radius and
again an intense discharge between the tube
bottom and the plane surface which extend far
away from the tube. In mode 2, two filaments
are visible from the point to the surface. They
follow the gas jets and the light between the
tube bottom and the surface is limited. In both
the cases, there is always light at the sharp
extremity of the needles (top of the
confinement tube). An other point that
differentiate the two regimes is the breakdown
mechanism when the electrode points are
positive. In mode 2 and in that mode only,
plasma bullets propagate towards the surface
(Figure 7). This phenomenon appears before
the main positive breakdown. Plasma bullet
initiates the main discharge which begins when
the "plasma bullet"(2) reaches the surface.
It's very specific to the mode 2 as we never
observe this phenomenon in mode 1
Figure 5 – 500ns exposure time picture of the "plasma bullet"
independent of the flow rate or the gap values.
So with these short exposure time pictures we
have seen that the discharge is somewhat
different when the flow mode changes. Energy
localistion is always more homogeneous in
mode 1, and some discharge mecanisms exist
only in mode 2. Simultaneously with these
experiments voltage and current have been
acquired of the discharge.
3 – c) Current and voltage analysis :
Figure 8 presents the 2 modes current for the
same voltage. Current is the superposition of
capacitive and discharge current. The dielectric
surface charged by the discharge and thus the
capacity in series with the discharge, depends
on the discharge mode and development.
Therefore it is not possible to remove the
capacitive component from the current
measurement. The voltage shape is not fully
sinusoidal as an oscillation is superrimposed to
the negative part.The consequence is that the
capacitive current becomes positive during the
negative half cycle of the voltage. The
discharge current being in phase with the
voltage it decreases the total current in the
zone where the capacitive current polarity is
opposite to that of the voltage. So according to
figure 8, during a cycle of the excitation, there
is one positive and two negative breakdowns.
Short exposure time pictures confirm this
interpretation. The positive breakdown starts
5µs before origin on the figure 6. The two
other breakdowns visible between 15 and 50
µs are negative breakdowns. Mode 1 (black)
and mode 2 (red) currents are very similar in
their general shape. The number of breakdown
and order of magnitude of the total current are
the same. A more careful comparison with the
pictures allows identifying differences between
the 2 modes electrical characteristic. "Plasma
bullet" is associated to the small shift at 0 µs.
The current amplitude in mode 1 is higher than
in mode 2. This can be due to the capacitive
part of the current that is more important in
mode 1, charged dielectric surface being
bigger in this mode. Overall, the voltage
current characteristics of mode 1 and mode 2
are similar. But it is possible to identify which
mode is active with the little current peak
created by the "plasma bullet" propagation
before the main positive breakdown. This peak
is always visible in mode 2 and never appears
in mode 1. Parameters influencing the
transition from mode 1 to mode 2 depend
strongly on the gas flow. To verify this
hypothesis, that the modification of the gas
flow is at the origin of the differences between
the two discharge modes, flow dynamic
modeling with FLUENT software has been
done.
Air entry
3 – d) FLUENT modeling :
The experimental configuration requires a 3D
modeling. However, as the discharge regime is
controlled by the total flow rate, it is assumed
that 2D ax symmetric simulation can give
helpful results to understand the reason of the
discharge transition. Therefore, we chose to
simulate a "virtual" configuration, purely ax
symmetric, with only one helium injection at
the center of a smaller diameter confinement
tube (8 mm diameter). Dimensions have been
studied to keep the flow dynamic as close as
possible from the original geometry. To
validate the modeled configuration, a real
plasma head with the new geometry has been
manufactured. Reynolds number in the
injection channels is between 2000 and 3500,
so the transition between laminar and turbulent
is possible. The main results given by
FLUENT modeling is that in laminar
conditions, air enters inside the confinement
tube (figure 7). The breakdown voltage of air
is 30 times higher than that of He. So air
introduction along the tube inner wall confined
the discharge in a smaller volume. The other
consequence is a higher ionization level related
to air N2 Penning ionization by He metastable.
This is confirmed by the N2+ emission
increase compare to N2 (figure 9) (3). That can
explain the color change in the discharge with
the diffusion of nitrogen in the plasma zone at
the bottom of the confinement tube, were the
model's streamlines show that outside gas
enter, just over the helium outflow.
Figure 7 - FLUENT in laminar conditions with air entry at the
bottom of the confinement tube
Figure 8 - FLUENT in turbulent conditions without air entry
Figure 6 – Current and voltage oscillograms for mode 1 and 2
Figure 9 - OES in mode 1 and 2 showing evolution of the ration
N2 / N2+
4 : CONCLUSION:
In the PlasmaStream® configuration, the flow
mode (laminar or turbulent) has a very strong
effect on the discharge mechanisms, in
particular on the spatial behavior of the
discharge. Turbulent mode being more
homogeneous, almost like a glow discharge in
some conditions, helps to control the energy in
the confinement tube, mixing the gas and so
avoiding coexistence of low energy and very
high energy zones. It also prevent from
nitrogen diffusion in the plasma zone, so we
can more easily control the energy of the
plasma in this mode.
References:
1. Gas flow dependence of ground state atomic oxygen in
plasma needle. Yukinori Sakiyama, Nikolas Knake, Daniel
Schröder, Jörg Winter. 151501, 2010, APPLIED PHYSICS LETTERS
97.
2. Finite element analysis of ring-shaped emission profile in
plasma bullet. Yukinori Sakiyama, David B. Graves, Julien
Jarrige, and Mounir Laroussi. 041501, 2010, APPLIED PHYSICS
LETTERS 96.
3. Neutral gas flow and ring-shaped emission profile in nonthermal RF-excited plasma needle. Graves, Yukinori Sakiyama
and David B. 025022, 2009, Plasma Sources Sci. Technol., Vol.
18.