Electron number density measurement on a DC argon plasma jet by

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Electron number density measurement on a DC argon plasma jet by
ARTICLE IN PRESS
Vacuum 81 (2006) 347–352
www.elsevier.com/locate/vacuum
Electron number density measurement on a DC argon plasma jet by
stark broadening of Ar I spectral line
S. Yugeswaran, V. Selvarajan
Plasma Physics Laboratory, Department of Physics, Bharathiar University, Coimbatore 46, India
Received 2 March 2006; received in revised form 6 May 2006; accepted 1 June 2006
Abstract
In thermal plasma processing, input power and gas flow rate play a major role in controlling the plasma jet temperature, velocity and
density. Emission spectroscopy study is an important method for plasma diagnostics. A DC atmospheric plasma spray torch was
operated at different power levels and flow rates of plasma gas (argon). Electron number density of the plasma jet, the corresponding
temperature and the degree of ionization were determined using stark broadening of the Ar I (430.010 nm) line, the atomic Boltzmann
plot method and the Saha equation, respectively. In the present work, we have investigated the effect of input power, axial position of the
plasma jet and gas flow rate on the electron number density in the plasma jet. While an increase in input power considerably increased the
electron number density, gas flow rate did not show any significant effect on the same.
r 2006 Elsevier Ltd. All rights reserved.
Keywords: Plasma jet; Emission spectroscopy; Electron density; Stark broadening; Degree of ionization
1. Introduction
Thermal plasma technologies have been widely used for
material processing such as plasma spraying, plasma
synthesis of ultrafine powders, plasma cutting and hazardous waste treatment. The knowledge of plasma characteristics like electron density temperature and flow velocity is
required for the application of plasma spraying and
deposition, and synthesis where the plasma properties can
greatly affect the conversion and quality of the products
[1]. Good knowledge of the fundamental physical processes
inside the plasma is necessary to improve the quality of
application. The first step in plasma modeling and
diagnostics is the determination of their electron temperature and electron density [2]. Accurate determination of
electron density, temperature and velocity of the plasma
plume is of primary interest in the study on thermal plasma
jets [3]. Electron density and temperature measurements in
plasma torch systems have been made using both probe [4]
and spectroscopic methods [5–7].Emission spectroscopy is
Corresponding author. Fax: +91 0422 2425706.
E-mail address: [email protected] (V. Selvarajan).
0042-207X/$ - see front matter r 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.vacuum.2006.06.001
one of the classical methods, which is simple and
inexpensive. This method requires the validity of local
thermodynamic equilibrium (LTE) assumptions and a
standard source for calibration. The optical emission
spectroscopic methods are based on measuring the
intensity of spectral lines, continuum spectrum, half widths
and shifts of spectral lines. The plasma parameters can be
determined from spectral line widths. The broadening of
spectral lines is a complicated function of the environment
of radiating atoms and ions. Although temperature and
velocity data for DC plasma spray torches are available in
the literature, only limited studies have been made for the
electron number density determination using stark broadening of Ar I lines.
Bakshi and Kearney [8] used stark broadening of seven
Ar I transitions (826.5, 794.8, 750.3, 720.7, 714.7, 703.0 and
696.5 nm) to measure the electron density and the electron
impact parameter of a direct current argon plasma jet.
Joshi et. al. [9] measured the electron density (ranged from
1016 to 1017 cm3 ) in a DC plasma spray torch using stark
broadening of Hb and Ar I (430 nm) lines. Konjevic [10]
investigated the use of non-hydrogenic spectral line profiles
for plasma electron density diagnostics. Experimental
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studies on plasma broadening and shifting of nonhydrogenic neutral atom and positive ion were reviewed.
The results from theoretical and experimental methods
were derived and a set of data for plasma diagnostic
purposes was proposed. But extensive work has been done
on electron density profile from Hb line broadening and
other spectroscopic techniques.
In this paper, the electron number density of the plasma
jet for selected values of input power and gas flow rate is
reported. The electron density of a thermal plasma jet was
calculated from the stark broadening of Ar I spectral line
(430 nm). At a fixed argon gas flow rate (20 lpm), the axial
variation in electron number density of the plasma jet was
calculated for different input power levels. The atomic
Boltzmann plot method has been used to determine the
excitation temperature of the plasma jet [11–14]. The
temperature was correlated to electron density values.
From the result, the degree of ionization was calculated
using the Saha equation. The measured electron density
values are time averaged and line of sight averaged. Under
the assumption of LTE, the average excitation temperature
can be taken as the ionization temperature.
Different spectral lines are scanned from 400 to 452 nm.
When the peaks are well resolved, the spectral lines are
recorded in the X-T recorder. The focusing point of the
optical fiber was varied along the axial direction of the
plasma jet. The average intensity values of two spectra at
the same point were used for calculation to minimize the
effect of intensity fluctuations.
The spectrum of non-hydrogenic lines exhibit quadratic
stark effect and half-width (Dl1/2) is proportional to the
electron density:
Dl1=2 ¼ 2 10 16 o ne ½1 þ 1:75 104 n1=4
e
1=2
af1 0:068 n1=6
e =T e g,
where o is the electron impact parameter and a is the ion
impact parameter. The values of o and a for an Ar I line
(430 nm) have been given by Griem [12]. Although various
Ar I lines have been obtained in the wavelength region of
400–452 nm, the Ar I 430 nm line is well resolved and
separated from other lines. A curve-fitted empirical
formula for calculating the electron density ne (cm3) using
the half-width of the Ar I line (Dl1/2) is given by [9]
log ne ¼ 17:432 þ 0:662 log Dl1=2 .
2. Experimental setup and methodology
A non-transferred plasma spray torch was used in this
experiment. It consists of a water-cooled thoriated tungsten
cathode and a copper anode nozzle. The arc is vortex
stabilized by the injection of plasma-forming gas. Argon
was used as the plasma-forming gas. The torch was
operated at 8.12, 10.8, 12.8, 14.52 and 16.32 kW power
levels at argon flow rates of 15, 20 and 25 lpm. The typical
operating parameters are given in Table 1.
A schematic diagram of the experimental setup for
measuring the electron density and temperature is shown in
Fig. 1. The diagnostic arrangement consists of a monochromator (Thermo Oriel, 14M) along with a photomultiplier tube (PMT), power supplies, oscilloscope, X-T
recorder and computer for running the monoutility
program and optical fiber with xyz-positioner for collecting
radiation from the plasma jet.
The PMT has a photocathode of diameter 25 mm and
the PMT output signal is amplified by a preamplifier. The
amplified output from the PMT is fed into the oscilloscope.
The output is fed into the X-T recorder for recording the
spectral intensity as a function of wavelength.
Table 1
Typical operating parameters
Arc current
Arc voltage
Electrode gap
Primary gas flow rate
Anode cooling water flow rate
Cathode cooling water flow rate
240–400 A
27–31 V
12 mm
15, 20 and 25 lpm
11.75 lpm
10.15 lpm
ð1Þ
(2)
The excitation temperature of plasma jet along the line
of sight can be determined by the atomic Boltzmann plot
method from the following equation:
logðIl=gk Ak Þ ¼ C ð625E k =TÞ
(3)
or
T ¼ 625=slope value:
(4)
A plot between log (Il/gkAk) and Ek yields a straight line
by the best fit method. The inverse of the slope of the line
gives the excitation temperature of the plasma jet along the
line of sight, were I is intensity and l is the wavelength of
the line; C is the intercept on the ordinate axis; Ak is
transition probability; Ek and gk are the excitation energy
and statistical weight of atomic state k, respectively. The
values of I were measured and those of Ak, Ek and gk at
various wavelengths of Ar I lines were taken from the
literature [15].
The degree of ionization in plasma jet was calculated
using the Saha equation in terms of electron density (cm3)
and ionization temperature (eV):
Degree of ionization ðDIÞ
¼ ð3 1021 =ne ÞðTÞ3=2 expðV i =TÞ,
ð5Þ
where ne is the electron density, T is the ionization
temperature and Vi is the ionization energy of argon atom
[16].
In the spectral range of 400–451 nm, 10 Ar I peaks were
well resolved and distinct (Fig. 2). The values of intensities
were calculated from the spectrum, and gk, Ak and Ek
values of corresponding lines were taken from literature
[15]. Using a software, log(Il/gkAk) vs. Ek were plotted.
The slope was obtained from the straight line of the best fit
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349
Monochromator
Input
slit
Torch
Port 2
Optical fiber
Plasma jet
Port 1
Photo multiplier
tube
Computer
Oscilloscope
PMT power supply
Fig. 1. Schematic diagram of the experimental setup.
10
Gas flow rate
15,20 lpm
ne ( X 1017/ cm3 )
9
25 lpm
8
7
6
5
4
Fig. 2. Emission spectra of Ar I lines at 2 mm below the nozzle exit. Argon
gas flow rate: 20 lpm, input power: 14.52 kW.
8
10
12
14
16
Input Power (kW)
and the correlation coefficient was more than 0.85 to give
the best fit for random points.
Fig. 3. Effect of input power on electron density ne of argon plasma jet at
2 mm below the nozzle exit.
3. Results and discussions
3.1. Electron number density
3.1.1. Effect of input power
Fig. 3 shows the electron density ne in the argon
plasma jet as a function of input power for three argon
flow rates at 2 mm below the nozzle exit. The electron
density increases with increasing input power delivered
to the torch. The electron density ranged from 4 1017 to
9 1017 cm3, depending on the input power. Electron
densities and line widths are almost proportional to
each other. If we increase the torch input power the
width of Ar I spectral line at 430 nm increases due to the
collision and ionization of atoms. On the other hand,
increasing the input current, the arc voltage decreases due
to higher conduction, resulting in temperature increase
[17]. Fig. 4 shows the relationship between the electron
number density and the excitation temperature of the
plasma jet.
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350
9
10
8
9
7
8
ne ( X 1017/cm3)
ne ( X1017/cm-3)
Gas flow rate: 20 lpm
6
5
Input power (kW)
8.12
14.52
16.32
10.8
12.8
7
6
5
4
6500
7000
7500
Temperature (K)
8000
4
2
4
6
Axial position (mm)
Fig. 4. Electron density vs. temperature.
Fig. 6. Electron density ne of a plasma jet as a function of axial position.
Argon gas flow rate 20 lpm.
10
ne ( X 1017/ cm3)
9
Input power (kW)
8.12
14.52
10.8
16.32
12.8
8
7
6
5
4
15
20
Gas flow rate (lpm)
25
Fig. 5. Effect of gas flow rate on electron density ne of argon plasma jet at
2 mm below the nozzle exit.
3.1.2. Effect of gas flow rate
Fig. 5 shows the electron density as a function of gas
flow rate. Electron density is slightly higher for higher flow
rates. At lower power level (8.12 kW), electron density
increases from 4.3 1017 to 4.45 1017 cm3 with increasing gas flow rate from 15 to 25 lpm. The electron density
variation is small but in higher power level (16.32 kW), it
increases from 8.55 1017 to 9.45 1017 cm3 with increase
in gas flow rate from 15 to 25 lpm. The electron density is
the same for 15 and 20 lpm, but slightly increases for
25 lpm. This behavior is in good agreement with other
reports [18,19]. An increase in gas flow rate constricts the
gas axially, resulting in increase of gas density leading to an
increase in electron density.
3.1.3. Effect of axial position
Fig. 6 shows the electron density ne in the argon plasma
jet as a function of axial position. The gas flow rate was
20 lpm and input power was varied from 8.12 to 16.32 kW.
As shown in Fig. 6, the electron density decreases with
increasing distance from the nozzle exit. However up to
6 mm from the nozzle exit, the variation is only marginal.
Between 4 and 6 mm, the decrease is 40.5% at power level
10.8 kW and 38.5% at power level 12.8 kW. At 16.32 kW,
there is no difference in electron number density values.
The jet length was 23 mm.
Electron density at the exit of the nozzle could not be
determined due to experimental difficulties; it has been
estimated by extrapolating the curve between ne and Z/L,
because the average electron density variation along the
axial direction in a plasma jet may be given by an
expression [9]
ne ðZÞ ¼ ne ð0Þ expðZ=LÞ,
(6)
where ne(0) is the average electron density along the line of
sight at Z ¼ 0 and L is the plasma jet length, which was
23 mm, measured from optical method. Fig. 7 shows the
plot of ne and Z/L; by extrapolating this curve, values of
ne(0) ¼ 4.39 1017 cm3 for 8.12 kW, 6.44 1017 cm3 for
12.8 kW and 8.73 1017 cm3 for 16.32 kW input power
are obtained.
3.2. Temperature
Fig. 8 shows the temperature of the plasma jet as a
function of axial distance from the nozzle exit for three
input power levels at a fixed gas flow rate (20 lpm). The
temperature decreases with increasing axial position from
the nozzle exit. The decrease in temperature away from the
nozzle is mainly due to cooling by air entrained by
turbulent mixing. Further, there is no heat dissipation
beyond the nozzle exit. The temperature of the plasma jet
at the exit of the nozzle has been calculated by using the
following equation:
TðZÞ ¼ Tð0Þ expðZ=LÞ,
(7)
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351
9
Gas flow rate
ne (X1017/cm-3)
7
Log (Degree of ionization)
Input power (kW)
8.12
12.8
16.32
8
6
5
4
15 lpm
20 lpm
25 lpm
-7
-8
-9
3
0.10
0.15
0.20
0.25
Z/L
8
Fig. 7. Plot of ne vs. Z/L.
10
12
14
Input Power (kW)
16
Fig. 9. Effect of input power on degree of ionization of argon plasma jet
at 2 mm below the nozzle exit.
8500
Input Power (kW)
8.12
12.8
16.32
8000
Input power (kW)
8.12
12.8
16.32
7000
6500
6000
5500
5000
4500
2
3
4
5
6
Log (Degree of ionization)
Temperature (K)
7500
-6
-8
-10
-12
Axial Position (mm)
Fig. 8. Temperature of a plasma jet as a function of axial position for
different input powers.
-14
where T(0) is the temperature of the plasma jet along the
line of sight at Z ¼ 0. The extrapolating curve in a plot of
temperature and Z/L gives the temperature values at the
nozzle exit. The values of T(0) ¼ 7281 K for 8.12 kW,
8663 K for 12.8 kW and 9239 K for 16.32 kW input power
are obtained.
3.3. Degree of ionization
Fig. 9 shows the degree of ionization in the plasma jet
along the line of sight as a function of input power for three
different plasma gas flow rates. The ionization process of
the plasma mainly depends on temperature and electron
density and gas densities in the plasma jet. If we increase
the plasma torch input power and gas flow rate, the
electron density and temperature of the plasma jet increase,
which in turn increase the degree of ionization.
Fig. 10 shows the degree of ionization of the plasma jet
as a function of axial position of the plasma jet for different
input powers. Along the axial direction, the temperature
2
4
6
Axial position (mm)
Fig. 10. Degree of ionization of a plasma jet as a function of axial position
for different input powers.
and the electron number density decrease due to the
recombination process of ions and electrons with surrounding atmospheric air, so that the degree of ionization
also decreases with increasing axial distance from the
nozzle exit. The log values of degree of ionization at the
nozzle exit are 7.3749 for 8.12 kW, 5.6881 for 12.8 kW
and 5.2061 for 16.32 kW.
4. Conclusions
A spectroscopic setup was used for recording the
spatially integrated spectral intensities emitted from
inhomogeneous, axisymmetric, optically thin plasma column in the wavelength region 400–451 nm. Radially
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averaged electron number density, temperature and degree
of ionization were estimated.
The variation in electron number density due to input
power, gas flow rate and axial position along the line of
sight in thermal plasma jet was measured using stark
broadening of Ar I (430.030 nm) line and the corresponding plasma jet temperature was measured by the atomic
Boltzmann plot method. Using these results, the degree of
ionization was investigated by the Saha equation. It is
concluded that the electron density depends on input
power and axial position of the point of investigation.
Knowledge of electron density profile will be useful for
plasma processing of powder.
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