30th EPS Conference on Contr. Fusion and Plasma Phys., St. Petersburg, 7-11 July 2003 ECA Vol. 27A, P-4.107
Efficiency of Plasma Energy Transfer to Sample Surfaces
under Materials Processing with Pulsed Plasma Streams
I.E. Garkusha, O.V. Byrka, V.V. Chebotarev, V.I. Tereshin
Institute of Plasma Physics of the National Science Center “Kharkov Institute of Physics
and Technology”, 61108 Kharkov, Akademicheskaya St., 1, Ukraine
1. Introduction
Application of pulsed plasma streams for materials modification requires precise control
of energy load to the treated surfaces [1]. It should be pointed that for material processing
with pulsed plasma streams of 2-5 µs in duration it is necessary to scrutinize possible
screening of sample from contact with plasma stream. Such screening arises due to
formation of transient layer of cold plasma (plasma shield) under the plasma stream
interaction with the surface. In common case only part of plasma stream energy is delivered
to the sample surface. Therefore it is rather difficult to provide a comparison of experiments
on plasma processing with different devices as far as, one possible to have different
screening effects resulting to decrease of energy deposited to the surface.
Very strong shielding effects were observed in ITER disruption simulation experiments,
when material surfaces were exposed to high energy density plasma streams (of an order 1
kJ/cm2 in pulses with length of > 0.1 ms) [2]. Due to the shielding layer formation the
energy deposited to the surface was less than 1% of incident plasma energy [2,3]. In
technological applications the plasma energy loads are significantly lower but effects of
surface shielding can be still essential.
2. Experimental Results
Investigations of formation and dynamics of transient plasma layers were carried out
with coaxial pulsed plasma accelerator (PPA) generating plasma streams with ion energy up
to 2 keV, plasma density 2x1014 cm-3, average specific power up to 10 MW/cm2 and an
energy density of the plasma stream in the range of 5-40 J/cm2 [1]. Hydrogen and nitrogen
were used as working gases. Samples of molybdenum, stainless steel, steel 45, and caprolon
were used as the targets. Diameter of targets was varied from 2.5 cm to 10 cm.
Scheme of calorimetric measurements is presented in Fig. 1. The movable calorimeter
was inserted into the target central hole. It was possible to move the calorimeter along the
plasma stream axis. So, the measurements of energy density on different distances from the
target were done. Typical distributions of energy density near the surfaces of both steel and
30th EPS Conference on Contr. Fusion and Plasma Phys., St. Petersburg, 7-11 July 2003 ECA Vol. 27A, P-4.107
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caprolon targets with diameter of 100 mm are shown in Fig. 2-4 for different values of
energy densities in incident plasma stream.
For all metal targets with diameters less than 5 cm it was found that energy of the
plasma stream is completely delivered to the surface of target even in the case of increasing
the energy density of plasma stream up to 35 J/cm2. Some shielding properties of this
transient layer are registered with increasing the diameter of target up to 10 cm. However
even in this case up to 90% of plasma stream energy density was delivered to the material
surface. Thickness of shielding layer determined from energy density distributions was not
exceeded 7-10 mm and was increased with plasma stream energy density increase.
24
ε , J/cm2
1
2
3
Plasma
stream
20
16
12
8
Steel
Caprolon
4
0
0
2
4
6
8
10
12
14
Distance from the target, mm
Fig.3.
Energy density distribution close to the
targets for 22 J/cm2 plasma
stream impact
Fig.1.
Scheme of experiment.
1- target with ∅ 100 mm,
2- insulator, 3- calorimeter
30
ε , J/cm2
35
12
ε , J/cm2
14
10
8
6
Steel
Caprolon
4
20
15
10
0
0
5
10
Distance from the target, mm
Fig.2.
Energy density distribution close
to the targets for 12 J/cm2
plasma stream impact
Steel
Caprolon
5
2
0
25
15
0
2
4
6
8
10
12
Distance from the target, mm
Fig.4.
Energy density distribution close
to the targets for 32 J/cm2
plasma stream impact
14
30th EPS Conference on Contr. Fusion and Plasma Phys., St. Petersburg, 7-11 July 2003 ECA Vol. 27A, P-4.107
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For caprolon target it was registered more essential influence of shielding layer on
process of energy transfer to the sample surface. For instance only 44-50% of plasma stream
energy delivered to the material surface in this case. Thickness of shielding layer achieved
1,2-1,5 cm. It should be noted that decrease of layer thickness was observed with increase of
incident plasma stream energy. Obtained result can be explained by compression of layer
under the plasma stream pressure increase and streamline effect of shielding layer. However
the shielding coefficient was increased with decreasing the layer thickness. Obtained results
are presented in Table 1.
Table 1. Shielding of samples under pulsed plasma stream treatment
Steel sample
Caprolon sample
Plasma stream
Energy density on
Shielding
Energy density on
Shielding
energy density,
the sample surface,
coefficient
the sample surface,
coefficient
J/cm2
J/cm2
12
22
30
32
34
10,5
18,5
24
26
28
J/cm2
1,14
1,19
1,25
1,23
1,21
6
10
13
14
14,5
2
2,2
2,3
2,3
2,34
These results can be interpreted in the following way. Under the pulsed plasma
stream interaction with metal target the shielding layer is not completely formed due to short
duration of plasma stream generation (<5µs), relatively low density of plasma stream (n ~
2*1014 !m-3) and high melting temperature of material. Thus shielding layer consists mainly
from particles of plasma stream stopped under contact with a surface. When caprolon target
is used the shielding layer consists of both stream particles and atoms and ions of target
material because of caprolon melting temperature (t=2250C) is essentially low in comparison
with steel one. At that the layer density and dimensions are increase. This leads to the
shielding coefficient increasing. As result only half of plasma stream energy transmitted to
the material surface. Other part is absorbed by layer and used for layer heating and
expansion and reradiated by the layer. There was found that shielding properties and plasma
density in the layer were not changed practically with changes of working gas kind from
nitrogen to hydrogen.
Thus experiments with steel samples have shown that under the treatment with
streams generated by pulsed plasma accelerator, in contrast to quasi-steady-state plasma
30th EPS Conference on Contr. Fusion and Plasma Phys., St. Petersburg, 7-11 July 2003 ECA Vol. 27A, P-4.107
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accelerator (QSPA) [2], forming transient shielding layer influent only incidentally on value
of energy deposited to the sample surface. Comparing the obtained results with experiments
on QSPA, it should be noted that considerable influence of shielding layer on energy transfer
for treatment by quasi-steady-state plasma streams (even for relatively low values of plasma
stream energy density) conditioned by much long duration of plasma stream and essentially
higher plasma stream density.
It should be pointed that even in case of using the pulsed plasma streams with
duration 2-5 µs for industrial steels processing it is necessary to take into account possible
screening of sample from contact with plasma stream for each concrete case. It is very likely
that under utilization of pulsed streams with higher density (n > 1015 cm-3) forming the
shielding layer will essentially influent on results of plasma processing (see for example [4]).
3. Conclusions
In contrast to the materials processing with quasi-steady-state plasma accelerator [2] the
experiments carried out with PPA have shown that shielding layer formation does not
influent essentially on value of energy delivered to the metal sample surface by plasma
stream. Obtained result is conditioned by considerable low duration of plasma stream
generation (<5 µs) and rather low plasma density of PPA plasma stream (n ~ 2*1014 cm-3).
However formed shielding layer in the case of target treatment with melting temperature
much less than steel melting temperature consist from both plasma and target particles.
Therefore, even for short pulse plasma stream processing the shielding layer can essentially
influent on process of energy transmission.
4. References
[1] V.I. Tereshin et al. Review of Scientific Instruments, V.73, N2 (2002) 831.
[2] V.V. Chebotarev, I.E.Garkusha, V.V.Garkusha et al. Journal of Nuclear Materials
233-237, (1996), 736-740
[3] H. Wuerz, N. Arkhipov, V. Bakhtin et al. Fusion Technology 32, (1997), 45-74
[4] V.V.Chebotarev, J. Langner, I.E. Garkusha et al. Journal of Technical Physics,40, 1.,
(1999) 469-472.