22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Electron density and temperature distributions in a supersonic dc plasma jet
generated at reduced pressure
J. Cao1,2, H. Huang1, W. Pan1 and C. Wu1
1
State Key Laboratory of High Temperature Gas Dynamics, Institute of Mechanics, Chinese Academy of Sciences,
100190 Beijing, P.R. China
2
Department of Nuclear Technology Application, China Institute of Atomic Energy, 102413 Beijing, P.R. China
Abstract: In this paper, two dimensional distributions of electron density and electron
temperature of a plasma jet produced by a fixed arc-length torch are diagnosed by
electrostatic triple probes.
Knot-like distributions of the electron density and
correspondence electron temperature distributions are observed. Possible mechanisms of
such kind of distribution are discussed.
Keywords: electron density, electron temperature, dc plasma jet, fixed arc-length
1. Introduction
Being able to generate high enthalpy plasma jet, nontransferred dc plasma torch has been widely used in
thermal spraying and coating technology [1]. However,
conventional dc plasma torch with turbulent jet has large
amplitude of fluctuations, which limited its application in
tailoring fine coating structure [2, 3]. A stepped structure
plasma torch with fixed arc-length has been developed in
our group, which facilitates generating stable laminar
plasma jet in a wide range of operating parameters [4-10].
In this study, electron density and electron temperature
distributions of the plasma jet generated at reduced
pressure are diagnosed by an electrostatic triple probe
system to understand the characteristics of the plasma
better.
2. Experimental methods
measurements start at Z=11mm when the arc current is
80A and 90A, and starts at Z=27mm when the arc current
is 100A.
3. Results and discussion
Measured distributions of the electron density are
shown in Fig. 2. A knot-like structure is clearly shown in
all the measured distributions. With the increase of the arc
current, the high density domains expand. There are no
noticeable position change of the “knots”. Numerical
simulations of the plasma show that under the
experimental conditions, the plasma jets are in supersonic
state, which leads to Mach-disks because of compressible
flow. Although the shock diamonds are hardly
distinguished in normal photographs of the jet under the
present experimental conditions, they could easily been
detected by the sensitive electrostatic probes.
80A
90A
0
0
0
20
20
20
100A
Ne (/m3)
40
40
60
Z (mm)
Z (mm)
Z (mm)
40
60
80
80
100
100
1.5E+20
1.4E+20
1.3E+20
1.2E+20
1.1E+20
1E+20
9E+19
8E+19
7E+19
6E+19
5E+19
4E+19
3E+19
2E+19
60
80
Fig. 1 Schematic diagram of diagnosing system.
100
0
10
20
r (mm)
Fig. 1 shows the schematic diagram of the diagnosing
system.The chamber pressure is kept at 200Pa during
experiments. Pure Argon is used and the flow rate is 8.5
slm. The arc current varies from 80A to 100A. The axial
(z direction) and radial (r direction) spatial intervals are
8mm and 4mm respectively. The anode exit locates at
Z=0mm. To prevent over heating of the probe tip, the
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0
10
20
r (mm)
0
10
20
r (mm)
Fig. 2 Distribution of electron density at 80A, 90A and
100A. The anode exit locates at Z=0, and r=0 is the center
axis of the torch and the jet.
Fig. 3 shows the distributions of the electron
temperature with different arc current. Comparing Fig. 2
1
and Fig. 3, it is clearly seen that some areas along the
center axis with higher electron density have relatively
lower electron temperature, and vice versa. While the
outer region of the jet has low electron density and
electron temperature. Possible mechanisms causing such
kind of distribution include:
80A
90A
0
100A
0
0
Te (eV)
20
40
40
40
60
80
80
100
100
0
Z (mm)
Z (mm)
20
Z (mm)
20
60
10
20
r (mm)
0
2.4
2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6
60
80
100
10
20
r (mm)
0
10
20
r (mm)
Fig. 3 Distributions of electron temperature at 80A, 90A
and 100A.
First, the deviation from LTE (Local Thermodynamic
Equilibrium) state in the plasma jet causes the electron
temperature decreasing at the positions with higher
density where collisions between electrons and heavy
particles are more frequent. Usually in non-LTE state,
electron temperature is higher than that of heavy particle,
which means the collision will decrease the electron
temperature, which leads to “Collision Cooling”.
The second mechanism is ambipolar diffusion. The net
charge current in the jet is zero everywhere, so the
electron diffusion flux must equal to ion diffusion flux.
Since electrons move far faster than ions, an electric field
is induced with opposite direction with electron density
gradient. An electron diffusing along the density gradient
will be cooled down gradually due to the electric field,
which is called “Diffusion Cooling”. This effect was
studied in [11], and it was shown that the electron
2
temperature could be cooler than heavy particle
temperature if the collision is not frequent.
According to the radial electron density distribution as
demonstrated in Fig. 1, the “Collision Cooling” effect
cools the regions near the center, while the “Diffusion
Cooling” effect cools the peripheral regions. This
combination of this two effects results in the distribution
as shown in Fig. 3.
4. Acknowledgement
This work is supported by the National Natural Science
Foundation of China (No. 11175226).
5. References
[1] P. Fauchais, Journal of Physics D-Applied Physics,
37, pp. R86-R108 ( 2004).
[2] Z. Duan and J. Heberlein, Journal of Thermal Spray
Technology, 11, pp. 44-51 (2002).
[3] S. Ghorui, M. Vysohlid, J. V. R. Heberlein, and E.
Pfender, Physical Review E, 76 (2007).
[4] H. Huang, W. X. Pan, Z. Y. Guo, and C. K. Wu,
Journal of Physics D:Applied Physics, 43, (2010).
[5] H. J. Huang, Z. Q. Fu, W. X. Pan, and C. K. Wu,
"Fast plasma sintering deposition of nano-structured
silicon carbide coatings," 18th International Vacuum
Congress (Ivc-18), 32, pp. 598-604 (2012).
[6] H. J. Huang, W. X. Pan, Z. Y. Guo, and C. K. Wu,
IEEE Transactions on Plasma Science, 36, pp. 10521053 (2008).
[7] H. J. Huang, W. X. Pan, and C. K. Wu, IEEE
Transactions on Plasma Science, 36, pp. 1050-1051,
(2008).
[8] H. J. Huang, W. X. Pan, and C. K. Wu, Chinese
Physics Letters, 25, pp. 4058-4060 (2008).
[9] H. J. Huang, W. X. Pan, and C. K. Wu, Plasma
Chemistry and Plasma Processing, 32, pp. 65-74
(2012).
[10] W. X. Pan, Z. Y. Guo, X. Meng, H. J. Huang, and C.
K. Wu, Plasma Sources Science & Technology, 18,
(2009).
[11] M. A. Biondi, Physical Review, 93, pp. 1136-1140
(1954).
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