22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Microwave interferometry of the diffuse coplanar surface barrier plasma device
J. Faltýnek and V. Kudrle
Department of Physical Electronics, Masaryk University, Brno, Czech Republic
Abstract: The diffuse coplanar surface barrier discharge (DCSBD) has vast potential for
plasma treatment as it provides atmospheric non-thermal plasma in a thin planar layer.
Until now the plasma density was only estimated. The microwave interferometer
measurements and a numeric model quantify the concentration of free electrons for this
industrially successful plasma source.
Keywords: microwave interferometry, plasma density, DCSBD, finite element method
1. Introduction
The DCSBD presents a type of atmospheric nonequilibrium plasma particularly suitable for surface
plasma treatment, with high concentration of chemically
reactive species along with a room temperature of the
neutrals [1,2]. Visually, it also exhibits a high spatial
uniformity of the discharge over a large surface.
However, at higher spatial and temporal resolutions, the
microfilaments can be observed.
While the small thickness (approx. 0.3 mm) of the
plasma layer may be useful for economic operation during
the industrial use, it also makes many standard
diagnostics methods quite challenging [3]. For example,
the plasma density in the DCSBD device, while being one
of most fundamental plasma properties, was until now
mostly estimated, not measured.
This contribution presents a microwave interferometry
approach to the measurement of plasma density, which is
quite straightforward compared to other methods. The
peculiar geometry of the plasma dictates the use of
evanescent field around dielectric waveguide [4].
As this approach doesn’t follow the simplified classical
theory for wave propagation in infinite plasma slab [5] a
numerical simulations were used for refinement of
experimental results.
2. The method
Despite its age, the microwave interferometry is still
routinely used for plasma diagnostics and dielectric
material analysis. The advances in computation power
and numeric modelling allow us to treat problems that
cannot be solved analytically or by simple
approximations. The diagnostics of discharges in nonstandard geometries and/or conditions is such a case.
Figure 1 illustrates the changes in electromagnetic field
induced by the presence of plasma in the proximity of the
dielectric waveguide. The phase shift dependence on the
plasma density in Fig. 1c is non-monotonous due to the
existence of losses in plasma.
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(a)
(b)
(c)
Fig. 1. The electric field norm in linear and logarithmic
scales for dielectric waveguide section in free space
(without plasma) (a) and fully immersed in infinite
plasma (b). The phase shift of the transmitted wave
depends on the plasma density (c).
1
From the point of view of electrodynamics, the plasma
dielectric properties can be described by the complex
permittivity. Under certain assumptions the relation
between plasma parameters and dielectric behaviour can
be derived from the Boltzmann equation, resulting in
𝑛𝑒 𝑒 2 (𝜔 − i𝜈𝑚 )
(1)
𝜀̃𝑟 = 1 −
𝑚𝑒 𝜀0 𝜔(𝜈𝑚 2 + 𝜔 2 )
where 𝑚𝑒 denotes mass of electron, 𝑒 elemental charge,
𝜀0 permittivity of vacuum and 𝜔 the angular frequency of
propagating waves. The plasma parameters are the plasma
density 𝑛𝑒 and the electron-neutral collision frequency for
momentum transfer 𝜈𝑚 .
3. Experimental set-up
DCSBD device with area of 140 cm2 is powered by
high voltage source delivering up to 200 W at 20 kHz. It
produces thin (0.3 mm) plasma layer over its surface.
Although the plasma is filamentary, the macroscopic
appearance is quite homogeneous.
Fig. 2. The schematic drawing of DCSBD. 1 – generator,
2 – electrodes, 3 – cooling and insulating oil, 4 – oil inlet
and outlet, 5 – dielectric (alumina) barrier, 6 – plasma
filaments, 7 – surrounding air
A Gunn diode driven 34.5 GHz interferometer is used
in Mach-Zehnder configuration (see Fig. 3). Microwave
signal is divided into reference and probing arm and
finally mixed via hybrid-tee which allows the quadrature
detection by two Spacek Labs, Inc. diodes. This feature
ensures that the phase shift and changes in the amplitude
can be separated from each other. Therefore it should be
theoretically possible to simultaneously determine both
𝑛𝑒 and 𝜈𝑚 . In practice however, the collision frequency
needs to be determined by some other method or
estimated. Part of the probing arm is a rectangular teflon
section with ends fitting into standard WR28 metallic
waveguide. This dielectric waveguide is placed 1.5 mm
above the ceramic DCSBD surface. The interaction
between the plasma and microwave evanescent field takes
place over the whole width of the DCSBD device, i.e.
7.5 cm.
4. Results and discussion
The electromagnetic model of dielectric waveguide
immersed in DCSBD plasma was developed using finite
element method based COMSOL Multiphysics, namely
its RF module. The device (see Fig. 4) has input and
output ports placed on the teflon waveguide ends, the
DCSBD is simulated as ceramics over metal layer and the
infinite space around is simulated using perfectly matched
layers. The phase shifts important to interferometry are
determined from the scattering matrix of this two port
system.
(a)
(b)
Fig. 4. Relative permittivity (real part) in all simulation
domains (a) side view and (b) front view (cyan – air, blue
– plasma, orange – teflon, brown – alumina)
The calculated electromagnetic field is shown in Fig. 5.
The evanescent field is present in the air above and below
the waveguide, in the thin plasma layer but it does not
penetrate the ceramics. The standing wave pattern in the
dielectric waveguide is caused by mismatching.
Fig.5. Logarithm of the electric field norm in the side
view
In experiment, it was observed that with increasing the
input power, the number of plasma filaments increased,
too. As a result, effectively larger area of the DCSBD
device become plasma covered
The phase shifts observed experimentally were
compared/fitted to the modelled shifts of 0.3 mm thick
homogeneous plasma layer. This procedure yields mean
plasma density, shown in Table 1 and Figure 6.
Table 1. Experimental results and comparison of mean plasma density with
approximate filament density (based on geometric considerations) with
increasing power (assuming 𝜈𝑚 = 5𝜔)
Fig. 3. The Mach-Zehnder interferometer probing the thin
plasma layer above the DCSBD.
2
Discharge power
[W]
Mean plasma density
[1019m-3]
Filament plasma density
[1020m-3]
125
1.9
1.7
140
2.7
1.7
170
3.9
1.8
185
4.3
2.0
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Fig. 6. The mean plasma density as a function of the
discharge power.
With the knowledge of the ratio between total area of
DCSBD and area occupied by filaments we can estimate
the plasma density in the individual filament. This
approach however will have to be verified as nonlinearities can negatively affect the outcome.
5. Conclusion
We developed enhanced interferometric technique
using evanescent electromagnetic field to measure the
plasma density, which is suitable for large area planar
plasma sources such as DCSBD. The measured phase
shifts were recalculated to a mean plasma density, using
numerical factors obtained from the electromagnetic FEM
model. Depending on discharge power, the typical results
are 2×1019 m-3 or higher.
The plasma density inside the filament is then well
above 1020 m-3 at minimum and increases only slightly
with the applied discharge power. The highest uncertainty
of this estimation lies in the fine interaction between high
density filaments and the evanescent field.
6. Acknowledgement
This research was supported by European regional
development fund, project CZ.1.05/2.1.00/03.0086 and by
Ministry of Education, Youth and Sports, project LO1411
(NPU I).
7. References
[1] Černák M., et al., Plasma Phys. Control. Fusion 53
(2011), 124031
[2] Černák M., et al., Eur. Phys. J. Appl. Phys. 47, 2
(2009), 22806
[3] Čech J., et al., Eur. Phys. J. 54 (2009), 259
[4] Bonaventura et al., Czech. J. Phys. B-56 (2006), B651
[5] Heald M. A., Wharton C. B., Plasma diagnostics with
microwaves, Wiley, 1965
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