2472 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 42, NO. 10, OCTOBER 2014
Modulation-Induced Filament Tearing in
Microwave Atmospheric Plasma Jet
Jaroslav Hnilica, Lucia Potoˇcˇnáková, and Vít Kudrle
Abstract—A typical 1-D surface wave discharge reacts to
varying power by changing its length. If the temporal change
of incident power is high, the discharge column may not react
sufficiently fast. The effect is demonstrated by high resolution
images of plasma plume tearing-off during rapid power decrease
in amplitude modulated microwave argon jet.
Index Terms—Atmospheric-pressure plasmas, plasma
diagnostics.
LOW temperature plasmas of atmospheric pressure
discharges are widely used in various technological
applications, such as plasma surface modification, activation,
sterilization, plasma-enhanced chemical vapor deposition of
thin films, or plasma assisted destruction of toxic compounds.
Discharges operated using pulsed or modulated power are
of considerable interest for both fundamental and applied
research. They can have simultaneously higher active particle
density and lower gas temperature than the continuous wave
(CW) plasma at the same mean power. These practical advantages
of modulated microwave jet were used, e.g., in our recent
work [1].
The atmospheric pressure plasmas in heavy noble gases
often exhibit substantial radial contraction of the plasma
channel. This effect may negatively influence the homogeneity
in some applications. This problem can be overcome
by adding molecular admixture, which promotes diffusive,
non-contracted character of the plasma. Recently [2], [3],
it was found that filamentary nature, typical for the argon
jet, can be easily affected by small variations in experimental
conditions, namely power and gas flow. In this way, the
plasma inhomogeneity can be partly compensated by an optimization
of temporal delocalization of the filament(s). In this
paper, we report on transient phenomena in power modulated
microwave jet.
The nonthermal microwave plasma jet excited in SAIREM
Surfatron 80 is operated in argon at atmospheric pressure.
The microwave generator SAIREM GMP 20 KED (2.45 GHz,
0–2 kW) is amplitude modulated (AM) by function generator.
Rectangular AM envelope (150 W in minimum, 350 W in
maximum, duty cycle 1:1, 250 W mean power) with frequency
Manuscript received November 7, 2013; revised March 19, 2014; accepted
May 20, 2014. Date of publication June 12, 2014; date of current version
October 21, 2014. This work was supported by the project R&D
Center for Low-Cost Plasma and Nanotechnology Surface Modifications
CZ.1.05/2.1.00/03.0086 funded by European Regional Development Fund.
The authors are with the Department of Physical Electronics, Faculty of
Science, Masaryk University Brno, Brno 61137, Czech Republic (e-mail:
hnilica@mail.muni.cz; nanai@mail.muni.cz; kudrle@sci.muni.cz).
Color versions of one or more of the figures in this paper are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TPS.2014.2327293
265 Hz is used in this paper. The argon plasma is excited
in a fused silica capillary with inner diameter of 1.5 and
4 mm outer diameter by high electric field in the surfatron [4]
launching gap. The capillary ends 25 mm from the launching
gap and the plasma column extends ∼1 cm outside the
capillary end into the surrounding atmosphere. The argon flow
of 1.45 slm is maintained by the mass flow controller. The
surfatron body is cooled by water and the discharge tube by
a compressed air flowing around it.
The tip of the active discharge is imaged by digital
single lens reflex camera Nikon D3100 with objective
Nikon 55–300 mm f/4.5-5.6G AF-S DX VR set to maximal
zoom from 1.2 m distance. The gain is set to ISO 3200 in
order to achieve good signal at shortest exposure times.
In CW operation, the plasma exhibits typical argon filamentary
structure. Depending on incident power and gas flow, the
filaments are either straight and colinear with capillary axis or
they are helically twisting around its inner diameter [2], [3].
After switching on the AM, this filamentary structure
visually disappeared as the discharge appeared filling
the whole capillary cross section. Therefore, a short exposure
imaging was carried out to prove if the filaments really
disappear and the plasma becomes diffusive. Fig. 1(a) and (c)
shows that at certain instants the discharge is still filamentary.
The filaments tend to wind around the inner wall of the
capillary, which is typical for locked modes described in [2]
and [3]. Vice versa, Fig. 1(b) and (d) suggests that the plasma
can be really diffusive, at least when imaged with 250 µs
exposure time.
In Fig. 1(b), one can observe that part of the plasma
column is tearing-off. It is visually similar to plasma bullet [5].
However, detailed electric measurements would be needed
for the confirmation. This happens during the falling edge
of modulation envelope, when the incident power rapidly
decreases. The surface wave discharge reacts to lowering
power by shortening its length. However, it seems that the tip
of the plasma column is not able to retract quickly enough and
the tearing-off takes place. This separated plasma plume then
lives for a certain time and even after, its residual ionization
helps in a propagation of the plasma during the rising part
of AM envelope. The twisting of the filaments continues in
the separated plasma plume as it exhibits strongly helical
appearance.
Fig. 1(c) demonstrates strongly localized inhomogeneities—
in the filament tips, the argon is radiating, whereas
in-between them, the nitrogen from the surrounding air is
excited. To decide, if it is inhomogeneity in gas composition
or an inhomogeneity in electron energy distribution function,
more research is needed.
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et al.: MODULATION-INDUCED FILAMENT TEARING 2473
Fig. 1. Side view of the tip of the microwave plasma jet in argon with AM input power (265-Hz AM frequency, rectangular envelope-150 W in minimum,
350 W in maximum, and duty cycle 1:1). Exposure time 250 µs, ISO 3200. Sequence of the images was taken in different phases of the modulation envelope
[(a) = 1 ms, (b) = 2 ms, (c) = 3 ms, and (d) = 4 ms]. At the falling edge of envelope (second image from the left), the plasma plume end separates from
the bulk plasma.
In conclusion, the photographs demonstrate plasma column
tearing-off caused by the rapid decrease of the incident
power during the AM. This phenomenon has been typically
described as taking place in submicrosecond timescales.
However, we observe it for modulation frequency of 265 Hz.
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