Evaluation of microplasma discharges as active components for reconfigurable antennas

Abstract

This paper presents an experimental setup for the wideband RF characterization of plasma Micro Hollow Cathode Sustained Discharge (MHCD). This microdischarge is studied as a candidate for integrated active component in antennas, using the change of permittivity caused by the presence of plasma. The measurement setup consists of a 50 Ω microstrip line with a single MHCD placed in its center. The measurement of the scattering parameters of the experimental device permits the evaluation of the complex permittivity and conductivity of the MHCD.

Full text

Evaluation of microplasma discharges as active
components for reconfigurable antennas
Francisco Pizarro, Romain Pascaud, Olivier Pascal, Thierry Callegari, Laurent
Liard
To cite this version:
Francisco Pizarro, Romain Pascaud, Olivier Pascal, Thierry Callegari, Laurent Liard. Evalua-
tion of microplasma discharges as active components for reconfigurable antennas. Antennas and
Propagation (EuCAP), 2012 6th European Conference on, Mar 2012, Prague, Czech Republic.
pp. 117-119, <10.1109/EuCAP.2012.6206117>.<hal-00905021>
HAL Id: hal-00905021
https://hal.archives-ouvertes.fr/hal-00905021
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To cite this document: Pizarro, Francisco and Pascaud, Romain and Pascal, Olivier and
Callegari, Thierry and Liard, Laurent Evaluation of microplasma discharges as active
components for reconfigurable antennas. (2012) In: Antennas and Propagation (EuCAP),
2012 6th European Conference on, 26 March 2012 - 30 March 2012 (Prague, Czech
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Evaluation of Microplasma Discharges as Active
Components for Reconfigurable Antennas
Francisco Pizarro, Romain Pascaud
Universit´e de Toulouse: ISAE, DEOS
Toulouse, France
Email: f.pizarro-torres@isae.fr
Olivier Pascal, Thierry Callegari, Laurent Liard
Universit´e de Toulouse: UPS, INPT, LAPLACE
CNRS: LAPLACE
Toulouse, France
Abstract—This paper presents an experimental setup for the
wideband RF characterization of plasma Micro Hollow Cathode
Sustained Discharge (MHCD). This microdischarge is studied as
a candidate for integrated active component in antennas, using
the change of permittivity caused by the presence of plasma. The
measurement setup consists of a 50 Ωmicrostrip line with a single
MHCD placed in its center. The measurement of the scattering
parameters of the experimental device permits the evaluation of
the complex permittivity and conductivity of the MHCD.
I. INT ROD UCTION
Due to their physical characteristics, plasma discharges
appear as interesting candidates in the research of new devices
in the RF domain. A plasma medium is an ionized gas, with the
particularity that it can behave as a low permittivity material
or as a conductor with low conductivity at several frequencies,
and under certain conditions. On the other hand, when the gas
is not ionized (so the discharge is not present), the medium
remains electromagnetically transparent, like vacuum. In fact,
the complex permittivity of the plasma medium depends on
the plasma density and collision frequency, which depend on
the type of gas, its pressure and the power injected to the
discharge.
The use of plasma discharges on the RF domain has been
case of several studies. One example is the plasma filled tubes
used as radiating elements [1], [2]. Good results in this topic
have been obtained for a wide frequency range (500 MHz
to 20 GHz) with efficiencies comparable to classic copper-
wire antennas [3]. Other devices such as RADAR Transmit-
Receiver tubes use also high power plasma discharges for their
operation [4].
The possible control of the complex permittivity of the
plasma discharge makes it interesting for RF components
reconfiguration [5], [6]. Under this perspective, the Micro
Hollow Cathode Sustained Discharge (MHCD) seems to be
a good candidate for a possible integration in a RF printed
circuit due to its small size (around 1 mm) [7].
Before using the MHCD in a RF circuit, it is necessary to
characterize its complex permittivity behavior. A measuring
device consisting of a 50 Ωmicrostrip line with a single
MHCD placed in its center is proposed. This device permits
the wideband evaluation (1 to 15 GHz) of the S parameters of
the MHCD as a function of several parameters (gas pressure,
electric current and applied voltage).
II. PL AS MA DI SC HA RG E
A. Plasma characteristics
Aplasma is an ionized gas, macroscopically neutral, with
the characteristic of being a conducting medium. One way
to create a plasma at ambient temperature is by applying an
electromagnetic field to a gas. This will cause an acceleration
of the free electrons present in the gas, providing the enough
kinetic energy to produce collisions and so, ionize other
molecules. This phenomenon, called avalanche breakdown,
leads to the generation of an electric discharge and therefore,
the formation of the plasma media.
One parameter that defines the type of plasma discharge is
the ionization degree δdefined as:
δ=ne
ne+n0
(1)
where neis the electron density and n0is the neutral density.
The MHCD device creates plasmas with low ionization degree,
typically δ≤10−3, that are called non-equilibrium plasmas.
Thus, this kind of plasmas allows heavy particles to keep their
temperature close to room temperature whereas electron are
warmed enough to ionize.
According to the Drude’s model, the complex permittivity
ǫof the plasma and so, the complex conductivity σcan be
defined as:
ǫ= 1 −ωp2
ω2−jνp·ω(2)
σ=ωp
2·ǫ0
ν2
p+ω2·(νp−jω)(3)
where ω(rad/s) is the RF pulsation of the electromagnetic
wave interacting with the plasma, νpthe collision frequency
(s−1) and ωpthe plasma pulsation (rad/s) defined as:
ωp=sne·q2
ǫ0·me
(4)
where neis the electron density, qthe electron charge and me
the electron mass.
As exposed in (2) and (3), the complex permittivity and
conductivity of the plasma depend on the collision frequency
and the electron density (both dependent on the type of gas, the

Φ
Dielectric
Anode 1
Anode 2
Cathode
d
Fig. 1: Micro Hollow Cathode Sustained Discharge (MHCD)
configuration.
gas pressure and the power injected to the discharge). There-
fore, it is possible to control the electromagnetic behavior of
the plasma medium, and modify the way it interacts with an
incident RF electromagnetic wave.
B. Micro Hollow Cathode Sustained Discharge
Among the structures that enable the generation of non-
equilibrium, the MHCD has several interesting characteristics
for a possible integration in a RF planar circuit, such as
its small size and the possibility to generate very localized
discharges.
As shown in Fig. 1, the MHCD is a three electrode
configuration discharge. On the first section of the device a
very thin substrate (several hundred micrometers) is placed
between two planar electrodes (Cathode and Anode 1), where
a central cylindrical hole of diameter Φis drilled between
these three layers. The function of the dielectric layer is to
electrically isolate these two electrodes. When a DC voltage is
applied (several hundreds volts) between these two electrodes,
a plasma discharge takes place inside the hole. Finally, a third
electrode (Anode 2) is placed at a distance d(up to several
millimeters) from the Anode 1. This electrode, when positively
DC biased, pulls up the discharge and creates a larger volume
plasma between the Anode 1 and the Anode 2 [8].
In order to characterize the interaction between a RF elec-
tromagnetic wave and the plasma discharge generated between
the Anode 1 and the Anode 2, a measurement device that
includes a MHCD has been designed.
III. MEA SU RE ME NT DE VI CE
A 3D representation of the device is exposed in Fig. 2. The
measurement device consists of a 50 Ωmicrostrip line with a
hole in its center where the plasma discharge takes place. Its
objective is to measure the influence of the plasma discharge
on the S parameters of the microstrip transmission line. The
device configuration is exposed in Fig. 3. All the electrodes
are made of copper, the microstrip line is made of gold and the
dielectrics are made of alumina due to the high temperatures
expected in the discharge area (around 900 K).
In addition to the three electrodes used for the MHCD
generation, a RF section is placed on top of Anode 1 to
characterize the discharge. It consists of a microstrip line
etched on an alumina substrate with a hole of a diameter
Φat its center (Fig. 4). This hole is aligned with the hole
of the MHCD configuration. Note that Anode 1 is also used
as the RF ground plane for the transmission line, and also,
Fig. 2: 3D view of the characterization device
X
Z
Φ
Φcat
Φan2
hal
has han1
han2
hcat
hdif
Dielectric
Dielectric
Anode 1
Anode 2
Cathode
d
50 Ωmicrostrip line
Fig. 3: Cut view of the measuring device.
X
Y
ǫr,tanδ
Φ
Wms Wal
Lal
Hole
50 Ωmicrostrip line
Fig. 4: Top view of the RF section.
that this configuration permits a separation of the DC and RF
excitations. Finally, when the discharge is pulled up by the
Anode 2, it fills the hole placed under the microstrip line,
and the plasma interacts with the RF electromagnetic field
propagating along the microstrip line. This interaction can be
evaluated by measuring the S parameters of the transmission
line.
The final values of the device parameters are: Φcat =
20 mm ;hcat = 5 cm ;han1= 11 mm ;hdif f = 1 mm ;
has = 600 µm ;Lal = 10 cm ;Wal = 5 cm ;hal = 1,57 mm
;Wms = 1,516 mm ;Φ = 1 mm ;d= 7 mm ;ǫr= 9,9;
tanδ = 2.10−4,φan2= 20 mm ;han2= 5 cm.
The measuring device is finally inserted in a vacuum cham-
ber, where the type of gas (argon, helium) and its pressure can
be controlled.

1 2.5 5 7.5 10 12.5 15
−50
−40
−30
−20
−10
0
Frequency [GHz]
|S11| dB
Air
Metal
ne 1013 cm−3 ; νp 1012 s−1
ne 1014 cm−3 ; νp 1012 s−1
ne 1013 cm−3 ; νp 3.1010 s−1
Fig. 5: Simulated reflection coefficient for different micro-
discharges
1 2.5 5 7.5 10 12.5 15
−15
−12.5
−10
−7.5
−5
−2.5
0
Frequency [GHz]
|S12| dB
Air
Metal
ne 1013cm−3 ; νp 1012s−1
ne 1014 cm−3 ; νp 1012s−1
ne 1013cm−3 ; νp 3.1010s−1
Fig. 6: Simulated transmission coefficient for different micro-
discharges
IV. RES ULT S
Several simulations (using Ansoft HFSS [9]) were done to
study the influence of the plasma parameters ωpand νpon the
S parameters of the line (Fig. 5 and Fig. 6).
Three different discharges were simulated considering ex-
pected values for ωpand νp[10]. It concerns two atmospheric
pressure (νp= 1.1012 s−1) discharges with electron densities
of 1013 cm−3and 1014 cm−3(ωp= 1,8.1011 rad/s and
5,6.1011 rad/s respectively) and a low pressure discharge
(νp= 1.1010 s−1) with an electron density of 1013 cm−3.
These discharges are compared with two reference cases: a
metallic filled hole and an air filled hole.
As shown in Fig.5 and Fig.6, the behavior of the discharge
depends on the ωp/νpratio. The results obtained with the at-
mospheric pressure discharges are similar to the ones obtained
with the air filled hole case. This occurs because of the low
conductivity and low relative permittivity values of the plasma
(ǫr≈1). For the low pressure discharge, the ωp/νpratio
value increases and the S parameters are more influenced by
the discharge. This behavior could be interesting for further
studies targeting its use in RF components.
V. CONCLUSION
A wideband RF measurement device for the characterization
of a plasma MHCD has been presented. The device offers a
large possibility in terms of the control of plasma parameters
(gas pressure, injected power, type of gas), that makes it
interesting for the characterization of the discharge.
Simulations show significant changes on the S parameters
depending on the ωp/νpratio. The device permits to explore
the influence of different parameters of the discharge in order
to obtain a suitable value of the ωp/νpratio for RF applications.
In future works, several measurements varying the discharge
parameters will be carried out and compared with simulations
in order to validate the obtained results.
ACK NOW LEDGMEN TS
This work was supported by the PRES and the R ´egion Midi-
Pyr´en´ees.
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