Download Atmospheric-pressure piezo-driven microplasma source for bio-medical applications

Atmospheric-pressure piezo-driven microplasma source for
bio-medical applications
P. Rajasekaran, N. Bibinov and P. Awakowicz
Institute for Electrical Engineering and Plasma Technology,
Ruhr-Universitaet Bochum, Germany
Abstract: The microplasma source using a piezo-electric transformer is
investigated for bio-medical applications. It comprises of a rod-shaped piezo
transformer with a rectangular cross-section. The piezo element is housed in the
front end of a metal tube; the power and gas supply enter the tube through its rear
end. A quartz tube is coaxially fitted to the metal tube for experimental purpose
and to protect the active plasma from perturbation by ambient air. Helium is
generally used as the working gas. Nitrogen is used as admixture for diagnostics.
Plasma conditions are characterized by determining gas temperature and plasma
parameters namely electron density and electron distribution function. Gas
temperature in the plasma volume is determined using optical emission
spectroscopy and numerical simulation. The excitation emission of N 2(C-B, 0-0)
at 337 nm is fitted with rotational distributions simulated for different gas
temperatures. Electron velocity distribution function (EVDF in eV -3/2) is
determined by comparing the ratio of excitation emission of N2(C-B, 0-0) to that
of N2+ (B-X, 0-0) with the measured emission intensities. Electron density (ne in
m-3) in the active plasma volume is determined from the excitation emission of
N2(C-B, 0-0) taking into account quenching and rate constant for this excitation
emission.
Keywords: piezo, optical diagnostics, biomedical, emission spectroscopy,
electron density
1. Introduction
Non-thermal plasma devices for biomedical
applications have been developed by a number of
research groups. These devices are based on
different operating principles including the dielectric
barrier discharge (DBD)[1] and are operated from
low frequencies (in the order of kHz) to microwave
frequencies[2]. Recently, plasma sources comprising
of a piezo transformer have been introduced and
characterized profoundly[3,4] in order to identify
potential low-temperature applications, especially in
the biomedical field.
In this report, a piezo-driven microplasma source is
introduced and the microplasma produced in a
mixture of helium-nitrogen is characterized in order
to identify potential applications. The working and
characteristics of piezo-driven plasma sources have
been profoundly discussed in the literature[3,4].
Hence, in this report more focus is laid on the
characterization of the microplasma produced a
piezo device in order to determine the plasma
conditions.
The microplasma is characterized by the
determination of gas temperature in the active
plasma volume and plasma parameters such as
electron density and electron distribution function.
Experimental methods namely optical emission
spectroscopy, current-voltage measurements and
microphotography are used. Numerical simulation
complements experimental methods and most
parameters are determined comparing the
experimental measurements and simulated values.
2. Piezo driven microplasma source
The piezo-driven microplasma source comprises of a
piezo element with a rectangular cross-section (6 x 2
mm). It is 7.5 cm long and is held in position by two
halves of a copper ring which are fastened to each
other using polymer screws. The rear side of each
half of the copper ring is provided with a point
contact that arises from a control-circuit which
converts low frequency of the applied voltage to a
high frequency which is the resonant frequency of
the piezo element. The resonant frequency is
measured to be 140 kHz.
The piezo arrangement is confined in a metal tube
through which the working gas flows. In the
experiments reported here, helium-nitrogen (95%5%) mixture is used as the working gas. Nitrogen is
used in small concentration for diagnostic purpose.
To protect the helium-nitrogen plasma from
perturbations caused by ambient air, a quartz tube is
coaxially fitted to the metal tube. The quartz tube
also facilitates optical diagnostics of the plasma. The
microplasma produced in the working gas mixture is
shown in figure 1.
center of one of the edges where plasma with low
emission is observed (as shown in figure 2).
Figure 2: (Side-on view of the piezo element). Spots on the
piezo element chosen for optical emission spectroscopy.
3. Experimental arrangement
The microplasma is characterized to determine the
plasma conditions which includes determination of
plasma parameters namely the electron density and
electron distribution function. For plasma
characterization, experimental methods such as
optical emission spectroscopy, current voltage
measurements and microphotography are used. In
addition, numerical simulation is used to determine
the plasma conditions.
Optical diagnostics is performed using a relatively
and absolutely-calibrated Echelle spectrometer [5]
which is provided with an optic fiber fitted with a
diaphragm to enhance spatial resolution.
The volume of active plasma in the corner of the
piezo element is determined from the image
captured using microphotography. A high speed
sensitive camera with a minimum exposure time of
500 ns is used for microphotography. Figure 3
shows the inverse microphotograph of the piezodriven microplasma taken with an exposure time of
50 ms.
Figure 1: Piezo-driven microplasma
Plasma is ignited on the four corners of the piezo
element where the electric field is high. In addition,
plasma with low emission is also observed on the
edges of the face of the piezo element. In this
preliminary investigation of the piezo-driven
microplasma, optical diagnostics are performed on
one of the corners of the piezo element and at the
Figure 3: Side-on inverse microphotograph of piezo-driven
microplasma. Exposure time = 50 ms.
4. Plasma characterization
The averaged plasma parameters are determined
using optical emission spectroscopy and numerical
simulation. First, the averaged gas temperature (Tg)
in the active plasma channel is determined using
emission of second positive system of neutral
nitrogen molecules N2(C-B, 0-0) at 337.1 nm with
the assumption that the rotational temperature of
diatomic molecules (Trot) is equal to the gas
temperature at atmospheric pressure condition.
Emission spectra for various rotational temperatures
are simulated with the same spectral resolution (∆λ =
25 pm) as in the experiment. The measured spectrum
is compared with simulated spectra and averaged Tg
with confidence interval is determined by fitting
procedure.
The density of nitrogen molecule and helium atoms
at Tg is calculated using the ideal gas law p =
[M]kBTg where p is pressure (in Pa), [M] is density
of nitrogen molecules or helium atoms (in m-3) and
kB is the Boltzmann constant. For determination of
plasma parameters, photoemission of neutral (N2(CB)) and ionized (N2+(B-X)) nitrogen molecules are
used, under the assumption of direct electron-impact
excitation from ground state of nitrogen molecule
N2(X). The intensities of nitrogen molecules IN2 and
IN2+ (in photons s-1), as shown in (1), are obtained by
integrating the measured spectrum
corresponding wavelength range (1 -2).
I N  ( B X , 0  0 )
2
I N 2 ( C  B, 00 )
in
the
2
QN
2 (C)

k depends on the electron velocity distribution
function (EVDF – f(E) in eV-3/2)
and the
corresponding collisional cross-section σ (in m2) as
shown in (2):

k  4 2 
f (E)
0
2e
E   ( E )dE
me
(2)
where, me is the mass of electron (in kg), e is the
elementary charge of electron (in C), and E is the
kinetic energy of electrons (in eV). f(E) is
normalized to fulfill equation (3):

4 2 
f (E)
E dE  1
(3)
0
f(E) is determined by comparing the measured
emission of nitrogen molecules and ions individually
with their calculated emissions. To calculate
emissions, Boltzmann‟s equation is solved
numerically in “local” approximation for a mixture
of 95% helium / 5% nitrogen at atmospheric
pressure condition, using the program code “EEDF”
[7]. In the fitting procedure, f(E) and the
corresponding reduced electric field (E/N in Td) are
determined. The rate constants for electron-impact
dissociation are determined using (2). Electron
density, ne (in m-3) in the active plasma volume is
determined using the expression (4).


N2
He met
Q N ( B)  ([ N 2 ]  k exc
 b1  [He]  k exc
)
2
N2 (C)
Q N2 ( C)  [ N 2 ]  k exc
(1)
ne 
where,
Q N  ( B) 
m3 s-1) is quenching rate constant of the
corresponding excited states during collision with
gas species „M‟.
A
Ak
N 2 ( B )
q1

[ N 2 ]  k qN22 ( B) [He]
A
Ak
N2 (C)
q3
[ N 2 ]  k qN42 ( C ) [He ]
-1
and
,
b1 is the branching factor, A (in s ) is the Einstein
coefficient [6], k (in m3 s-1) is the rate constant for
ionization or excitation of the respective species,
[M] (in m-3) is the density of the gas species, kq (in
IN
G  QN
2
N (C )
 [ N 2 ]  kexc
Vp
2
2 (C )
(4)
where G is the geometric factor, Vp (in m3) is the
volume of the active plasma.
5. Results
Using optical emission spectroscopy and numerical
simulation, gas temperature in the active plasma
volume is determined as 300 + 20 K (c.f. figure 4) at
the corners of the piezo element as well as in the
center of the edge.
Intensity (a.u.)
measured spectrum
simulated spectrum at 300 K
Acknowledgement
336
(nm)
Financial support by the German Research
Foundation (DFG) within the frame of the project
„FOR1123-Physics of Microplasmas‟ is gratefully
acknowledged.
337
Figure 4: Fitting of measured emission spectrum with simulated
spectrum to determine gas temperature in the active plasma
volume. The spectra are shifted for clarity.
E/N in the corner of the piezo element is about 50
Td and ne is 9·1015m-3 while E/N in the center of the
element is about 40 Td and ne is 3.1·1014 m-3. The
rate constants for electron-impact excitation of
neutral nitrogen molecule and nitrogen ion,
respectively, are 0.22·10-15 m3 s-1 and 0.02·10-15 m3 s-1
at the corners, and 0.16 ·10-15 m3 s-1 and 0.01 ·10-15
m3 s-1in the center of the piezo element.
40 Td
50 Td
1
0.1
-3/2
)
0.01
EVDF (eV
Electron density is in the order of 1014-1015 m-3 in
the active plasma volume. Gas temperature is
determined as 300 + 20 K from the experimentallymeasured excitation emission of nitrogen molecule
fitted using numerically-simulated spectra.
References
[1] P. Rajasekaran, C. Oplander, D. Hoffmeister, N.
Bibinov, C. V. Suschek and P. Awakowicz, Plasma
Processes and Polymers 8, 246 (2011)
[2] S. Kühn, N. Bibinov, R. Gesche and P.
Awakowicz, Plasma Sources Science and
Technology 19, 015013 (2010)
[3] H. Kim, A. Brockhaus and J. Engemann, Applied
Physics Letters 95, 211501 (2009)
[4] M. Teschke and J. Engemann, Contrib. Plasma
Phys. 49, 614 (2009)
1E-3
[5] N. Bibinov, H. Halfmann, P. Awakowicz, K.
Wiesemann, Meas. Sci. Technol. 18, 1327 (2007)
1E-4
1E-5
1E-6
[6] S. V. Pancheshnyi, S. M. Starikovskaia and A.
Y. Starikovskii Chem. Phys. 262, 349 (2000)
1E-7
1E-8
1E-9
1E-10
0
10
20
30
40
50
Energy (eV)
6. Summary
The plasma conditions in a piezo-driven
microplasma source in helium-nitrogen mixture are
investigated by plasma characterization where
plasma parameters such as electron density and
electron velocity distribution function are
determined. Experimental methods and numerical
simulation are used for plasma characterization.
Plasma is ignited at the corners of the electrode
where the electric field is high, and low-intensity
emission is also observed along the sides connecting
the four corners of the rectangular piezo element.
[7] Code EEDF, available from A P Napartovich,
Triniti Institute for Innovation and Fusion Research,
Troizk, Moscow Region, Russia