22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Shadowgraphy investigations of a filamentary atmospheric plasma jet
M. Teodorescu, E.R. Ionita and G. Dinescu2
National Institute for Laser, Plasma and Radiation Physics, Magurele, PO Box Mg36, Bucharest, 077125, Romania
Abstract: A long and thin filamentary plasma jet was generated at atmospheric pressure
via radiofrequency power. The behaviour of the plasma jet was investigated by means of
shadowgraphy, at different gas flow rates and applied RF powers, in order to determine the
operation domains for which the flow is stable. The turbulent behaviour was also
investigated and its dependence with the applied power and gas flow.
Keywords: atmospheric pressure, filamentary plasma jet, shadowgraphy
1.Introduction
Low temperature atmospheric pressure plasma jets are a
practical means of processing materials that require either
controlled atmosphere and low treatment temperatures or
a large surface area treatment. The complex shape of
some substrates represents a problem that can be solved
with localized treatments employing long and thin plasma
jets. Usually this kind of jets can be generated in Helium
but the treatment temperatures may be too high, and thus
damage the substrate (like for example for polymeric
foils).
The requirements of such specific plasma jets can be
resolved partially via the device described in the
following presented work. The results here focus on the
gas interaction with the ambient air and can lead to
conclusions regarding the optimum discharge parameters
and nozzle-to-substrate distance for direct use in
treatment procedures.
2.Experimental
The investigated plasma source consists of a glass tube
of 6.5mm outer diameter and 3.5mm inner diameter. The
discharge is initiated and maintained inside the tube,
while the plasma column exits the glass tube as a plasma
jet.
electrode is absent altogether. The discharge
configuration uses a single-electrode geometry, placed on
the outside of the glass tube, and thus the tube itself acts
as a dielectric barrier [1]. The power to the discharge is
supplied via a computer controlled AD-TEC AX-600 III
radiofrequency generator (13.56MHz) and an automatic
matching network. An additional movable electrode is
used for initiating the discharge by placing it inside the
glass tube near the RF electrode. After ignition it is
retracted from the tube. The experiments were performed
in open atmosphere using argon as the discharge gas. The
experimental parameters are as follows: forwarded RF
power in the range of 50-130W and argon gas flow in the
range of 500 – 3000sccm.
In order to investigate the dependence of the filament’s
length with the gas flow, a simple yet effective
shadowgraphy investigation setup was designed, using a
He-Ne laser, a beam expander and an imaging lens
(Figure 2). The lens has a 500mm focal length and the
Focusing
Laser beam
Expanding
Laser beam
He-Ne Laser
Beam
expander
Collimating
lens
Plasma
source
DSLR
Camera
Fig. 2. Setup for shadowgraphy measurements
Fig. 1. Image of the filamentary plasma jet. Discharge
parameters: RF power- 100W, 3000sccm Argon 5.0.
The plasma jet itself has a dual nature: a filamentary
discharge is surrounded by a diffuse one (Figure 1). The
jet can have lengths in the range of 30 to 60mm,
depending on the operating parameters. The aluminium
RF electrode is of a cylindrical shape, while the ground
P-I-2-13
diameter of the lens is 65mm, such that it can cover the
entire filament and also a large part of the outward gas
flow. The plasma jet was placed immediately after the
lens, and at 700mm a DSLR camera without the objective
was used to image the object. Due to the designed setup,
the RAW images were of low contrast, so a digital
subtract method was used in order to enhance the final
data. The RAW images were all corrected using an initial
no-plasma source image (in which only the laserilluminated field of view was imaged) which was
1
afterwards subtracted from the data, leaving only the gas
flow images.
3.Results and discussion
The difference between the no-plasma laminar flow and
plasma-on images are shown in Figure 2. There is also a
normal image of the mono-filament at the same scale,
showing that the turbulence zone of the gas flow actually
starts about half-way from the jet outwards. This
observed that the turbulence start point gets closer to the
nozzle with the increase of the gas flow (Figure 3).
The power increase also plays a major role in the
turbulence evolution in the flow range of 1500-3500
sccm. Increasing the RF power also moves the turbulence
starting point towards the nozzle but this is less obvious
then for the gas flow change (Fig. 4). Also the stability of
the discharge filament deceases with the power increase.
This is important, again, for the precise localization in the
case of substrate treatments.
However, it seems that except for the higher RF powers
and gas flows, the two parameters do not actually
influence greatly the stability of the monofilament, as in
the flow domain of 500-1500sccm and throughout the
entire range of the applied power values, the filamentary
discharge remains largely unaffected.
48
36
24
12
Distance from nozzle [mm]
60
0
0W
Fig. 2. Images showing shadowgraphy and normal
imaging aspects of the filamentary plasma: Leftgas flow without discharge; Center-gas flow with
discharge; Right- discharge filament in normal
operation. Operating parameters: gas flow
1800sccm Ar, RF power 130W.
70W
80W
90W
100W
]
[W
er
ow
P
110W
120W
130W
500
1000
1500
2000
2500
3000
3500
Gas flow [sccm]
Fig. 4. Turbulence dependence with the gas flow
and the forwarded RF power.
Fig. 3. Turbulence evolution with the gas flow
increase in NO-plasma mode (first row) and with
plasma (second row) at 130W forwarded RF
power.
sequence shows just how important this type of
investigation is for determining the optimum distance
from the glass tube end at which a substrate can be placed
in order to get a uniform treatment by using the laminar
flow characteristics of the discharge.
The turbulence start point was investigated for different
RF forwarded powers and different gas flow rates. It was
2
4.Conclusions
One of the advantages of our device compared to
other plasma sources is the possibility to generate a
plasma jet with lengths of more than 40mm, and also the
narrowness of the discharge column, which can be very
useful for fundamental studies or highly localized plasma
treatments.
The shadowgraphy measurements showed that
despite the apparent stable discharge aspect, the gas
turbulence is present on at least half of the plasma jet. The
starting point of the turbulence shift with the two main
parameters, getting closer to the nozzle with the power
increase, and also with the increase of the gas flow rate.
The measurements allow for the optimization of the
treatment setup in the case where a laminar gas flow is
needed.
5.Acknowledgements:
The financial support of the Ministry of National
Education National Authority for Scientific Research
Financing
contract:
PN-II-RU-PD-2012-3-0583
44/30.04.2013 is gratefully acknowledged.
6.References
[1] M. Teodorescu, M. Bazavan, E. R. Ionita, G.
Dinescu, Plasma Sources Science and Technology,
Submitted.
P-I-2-13