Emission spectrum of the electric arc discharge in CO2 between
copper electrodes
T. Billoux1, V. Boretskij2, Y. Cressault1, A. Gleizes1, Ph. Teulet1, A. Veklich2
1
Université de Toulouse; UPS, INPT; LAPLACE (Laboratoire Plasma et Conversion d’Energie); 118 route de
Narbonne, F-31062 Toulouse cedex 9, France, E-mail: [email protected]
2
Taras Shevchenko Kyiv National University, Radio Physics Faculty, 64, Volodymyrs’ka Str., Kyiv, 0133, Ukraine,
E-mail: [email protected]
Abstract: This paper characterizes an electric arc discharge in CO2 (3.5A) comparing two
types of results: registered experimental spectra and simulated emission spectra. The
plasma’s temperatures and the plasma’s compositions were carried out by optical emission
spectroscopy. Theoretical investigations were realized to simulate the corresponding
emission spectrum in order to validate the experimental techniques or the LTE assumption.
1. Introduction
There are many diagnostic methods in the arc plasma
investigations. The most popular of them is the optical
emission spectroscopy, mainly because it is not a disturbing
manner. This method allows obtaining plasma temperature,
electron density, rotational and vibrational temperatures of
molecules in plasma, etc [1]. The method is based on the
registration and analysis of plasma emission spectrum. If we
have a more complex plasma composition, we obtain a more
complicated emission spectrum. Therefore, the analysis of
the real spectrum emitted by multicomponent plasma is very
complicated and special algorithms must be developed to
simplify such procedure, namely, to obtain plasma
parameters from this spectrum. The well-known special
software “specair” is developed for such purposes, which
allow an analysis of air plasma spectrum (emission of C, C2,
CN, CO, e-, N, N2, N2+, NH, NO, O, O2, OH) by comparing it
to real and modeled spectra [2, 3]. However, if some
impurities are present in the plasma (air-Cu) or in another
working gas used (CO2-Cu), other spectrum analysis
procedures must be developed. An additional problem can be
the knowledge of appropriate spectroscopic data for these
procedures.
The aim of this work is to develop and to validate
spectrum modeling approaches by characterizing the
emission spectrum of electric discharges established in a
CO2 flow between copper electrodes at arc current 3.5 A.
2. Experimental set-up and techniques
The experimental investigations of plasma parameters of
discharge in CO2 flow of 6.4 slpm at arc current 3.5 A were
carried out by optical emission spectroscopy. The arc was
ignited between rod copper electrodes of 6 mm in diameter.
The discharge gap was of 8 mm. Plasma emissions in
average cross sections of the discharge were registered by a
diffraction spectrometer coupled with CCD camera
(see Fig.1). Tungsten ribbon lamp as an etalon light source
was used to obtain sensitivity curve of the experimental
device. Plasma temperatures were calculated by Boltzmann
plot method. Spectral lines of CuI 465.1, 510.5, 515.3, 521.8,
570.0, 578.2 nm were used in this case. The spectroscopic
data for these lines were used from [4]. Laser absorption
spectroscopy was used to obtain the radial distribution of
copper concentration NCu [5]. Copper vapor laser “Kriostat 1”
CCD-camera
Arc
Condenser
Entrance slit
Collimator
Diffraction grating
Mirror
Fig.1. Optical scheme of the experimental setup.
CCD-matrix
Laser emission
CuI 510.5 nm
Arc
Copper vapor
laser
“Kriostat 1”
Fig.2. Optical scheme of linear laser absorption spectroscopy.
was used as source of probing emission on wavelength
510.5 nm (see Fig.2), which is absorbed by copper atoms in
arc plasma volume. Measuring of the probing (without
discharge) and absorbed by plasma laser emission, it is
possible to obtain the population of absorbing atomic level
2
(in this case D5/2), which can be recalculated to the copper
atom concentration by means of the Boltzmann distribution.
The plasma composition was calculated using experimental
values of temperatures and NCu in the assumption of local
thermodynamic equilibrium (LTE) [6].
3. Simulation of Emission spectrum
The plasma’s radiation was modeled on the basis of the
real experimental CO2-Cu mixtures. The atomic continuum
(attachment, recombination, electron-ion and electronatom bremsstrahlung), the molecular continuum (CO, C2,
O2, CO+, CO2), the atomic lines (Cu, Cu+, Cu2+, Cu3+, C,
C+, C2+, C3+, O, O+, O2+, O3+), the main electronic systems
of the diatomic molecules (CO, C2, O2, CO+) and the
rovibrational lines of the polyatomic molecules CO2 were
included in the modeling.
The fundamental data for atomic lines, continuum and
molecular lines have been described in a previous paper
dealing with CO2-Cu mixtures [6]. Some additional data
and developments concerning molecular bands were
described in previous papers [7] dealing with air mixtures.
In the present work, we considered some additional
electronic systems in our database: CO (Hopfield-Birge,
Angström and 3rd positive); C2 (Phillips, DeslandresD’Azambuja, Ballik-Ramsay, Fox-Hertzberg) and CO+
(Comet-Tail, Baldet-Johnson and 1st negative). The escape
factor approximation [6] was replaced by a systematic line
by line description of the atomic and molecular lines in
order to take into account the overlapping phenomenon.
Finally, for trial temperature, concentration and pressure,
we can simulate the corresponding high-resolution (5 106
points) absorption and emission spectra.
These spectra could first be used as reference for
approximated models for the radiative transfer but these
spectra were used here to compare the theoretical results
deduced from the experimental parameters (temperature,
pressure and CO2/Cu molar fractions) with the
experimental measurements.
4. Results and discussion
The emission spectrum of discharge established between
copper electrodes in CO2 flow at 3.5 A is shown in Figure 3.
The spectrum was registered with spatial (coordinate r) and
spectral (coordinate ) resolution simultaneously. We can
observe that the most intensive region corresponds to the CuI
spectral lines. Figure 4 shows the radiation intensity if the
discharge integrated along the arc axis line of sight. This part
was registered with overexposure for CuI lines to show
details of the molecular emission bands.
Fig.3. Emission spectrum of plasma of the electric arc
discharge between copper electrodes in CO2 flow at 3.5 A.
The Abel inversion [8] and the Boltzmann plot
procedures were used to obtain the radial distribution of
temperature (see Fig.5). We estimate the error of the
temperature less than 10 %. The radial distance for
temperature profile (r = 1.2 mm) is limited by narrowest
line, which is usually CuI 465.1 nm. The radial distribution
of copper atom concentration obtained by linear laser
absorption spectroscopy is also shown in Figure 5.
-3
Nj, m
1E24
O
CO
1E22
O2
C
CO2
Cu
I, a.u.
300
1E20
C2
200
e
+
Cu
+
CO
-
O
C2 O
CuO
1E18
+
+
O2
CuI
CuI
1E16
0.0
100
O
0.3
C
0.6
0.9
+
1.2
r, mm
CuI
Fig.6. Plasma composition at arc current 3.5 A.
0
400
450
500
550
600
650
, nm
Fig.4. Emission spectrum of plasma of the discharge between
copper electrodes in CO2 flow at 3.5 A (integrated along the
arc axis line of sight).
-3
T, K
5400
NCu, m
NCu
T
1E21
8E20
6E20
The radial distribution of temperature and copper atom
concentration were used to calculate the plasma equilibrium
composition (see Fig. 6). The method of calculation is
detailed in [6]. The result of calculation showed that CO and
O particles are the dominant plasma components.
Then, we simulated the emission spectra following these
plasma’s parameters (temperature and concentration of
copper) and we superposed the simulated spectra with
experimental records in order to identify the presence and
the role of each molecular system in the plasma for the
spectral range observed in Figure 4. For a mean
temperature of 5.5kK, the simulation is given in Figure 7.
We can identify the C2 Swan and the O2 Schumann-Runge
electronic systems. This calculated spectrum is sufficient
to deduce the main species and the main electronic
systems responsible for the emission spectrum in each
spectral region observed.
5200
4E20
5000
0.0
0.3
0.6
0.9
1.2
r, mm
Fig.5. Radial profiles of plasma temperature  and copper
atom concentration .
Unfortunately, the superposition of the two spectra
(experimental and simulated spectra) highlights some
discrepancies and requires further investigations. Indeed,
the O2 Schumann-Runge system was not observed in the
experimental spectrum. It could be explained by an
underestimation of the temperature as the O2
concentration quickly decreases when the temperature
increases. Moreover, some fundamental data were missing
to compute the CuO electronic systems in the theoretical
study and Figure 7 don’t show the presence of these
systems which can be observed near 613nm (A-X) and
470nm (E-X, F-X and G-X) [9].
6. References
C2 Swan
SchumannRunge
Fig.7. Theoretical emission spectrum of a CO2-Cu plasma at
5.5kK, P=1atm.
The comparison of the two spectra leads to comments:
-
the plasma is not really in LTE? In this case, the
equilibrium composition remains not valid.
the Abel inversion is not adapted to this type of
plasma.
Electronic sytems of Cu2 and CuO must be
included in the simulation
5. Conclusions
The Radio Physics Faculty of the Taras Shevchenko
University of Kiev (in Ukraine) has developed an
experimental study to characterize an electric arc
discharge established in a CO2 flow between copper
electrodes at arc current 3.5 A. In parallel, the AEPPT
team of the LAPLACE laboratory (in France) has
developed several tools to simulated CO2/Cu emission and
absorption spectra in order to make possible comparisons
with experimental results.
The comparison of the real and modeled spectra
showed that emissions of CuI lines play an important role
in radiation transfer. The emission of molecular bands of
CO, O2, C2 and CO2 must be also taken into account in
energy distribution consideration but the comparison
highlighted important discrepancies that we have to
understand. To find the reasons of these differences,
several simulations will be done for large ranges of
temperature, molar fractions and pressures. The
superposition of these theoretical spectra with the
experimental spectrum will allows us to validate at first
the LTE assumption, then to validate the temperature of
the plasma deduced from the diagnostic methods.
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(2012).
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