st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Temperature Evolution in a Large Volume Planar Type
of Modulated Thermal Plasmas
M. Akao1, K. Kuraishi1, Y. Tanaka1,2, Y. Uesugi1,2, T. Ishijima2, T. Yoshida3
1
Faculty of Electrical and Computer Engineering, Kanazawa University,
Kakuma, Kanazawa 920-1192, Japan
2
Research Center for Sustainable Energy and Technology, Kanazawa University,
Kakuma, Kanazawa 920-1192, Japan
3
Department of Materials Engineering, the University of Tokyo,
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
Abstract: A novel planar type of modulated induction thermal plasma (planar-MITP) system has
been developed using a rectangular quartz vessel, instead of a conventional cylindrical tube, for
large area processings. Two-types of a planar-MITP system were tested using an air-core coil or a
high-frequency ferrite-core coil. The coil current modulation was adopted to control the
temperature field temporally and spatially in the planar-MITP.
Keywords: Thermal plasma, Modulation, Planar torch, Linear torch
1. Introduction
The inductively coupled thermal plasma (ICTP) has
been widely used effective heat and chemical species
sources for various materials processings such as
syntheses of diamond films, thermal barrier coatings
and surface modification or etching of substrates etc
since it has some features of high temperature and high
radical densities without any contamination [1]. As
well known, the conventional ICTP is established in a
cylindrical tube with a coil located around the tube.
However, such conventional ICTP is unsuitable to a
large area processings because it requires a large
volume with a more input power to be sustained.
In this paper, we introduce a large volume planartype of ICTP system with an air-core coil or a ferritecore coil [2]. The planar rectangular quartz vessel was
adopted instead of a cylindrical tube to produce a
confined ICTP in a planar shape. In addition, the coil
current modulation technique was tested to control
behavior of a planar-ICTP in time domain; it is called a
planar modulated induction thermal plasma (a planarMITP). For a fundamental study of a planar-MITP, we
measured responses of the effective power and
distribution of Ar excitation temperature TArex along the
longer side of a planar-MITP.
2. Concept of a planar type of modulated induction
thermal plasma torch
Fig.1 compares three types of ICTP torches: (a) a
conventional cylindrical ICTP torch; (b) a planar-ICTP
torch that we developed in this paper; and (c) a tandem
type of planar ICTP torch for advanced applications.
The conventional ICTP is formed in a cylindrical
dielectric tube by axial magnetic field and then
resultant rotational electric field generated from the RF
current in a coil, which is wounded around the tube as
illustrated in Fig.1(a). This type of the ICTP torch has
been widely used for various applications. However, it
is difficult to adopt this cylindrical type of ICTP
torches for a large area processings. On the other hand,
we have developed a planar-ICTP torch as indicated in
Fig.1(b)[2]. The planar-ICTP torch is composed of a
rectangular quartz vessel and a coil located in parallel
to the plane of the vessel. The alternative magnetic
field is generated perpendicularly to the vessel plane to
produce induced rotational electric field in the vessel.
This electric field forms a donut ICTP in parallel to the
plane. One progress approach of such a planar-ICTP is
indicated in Fig.1(c): A tandem type of planar-ICTP. It
is possibly established in a rectangular vessel by plural
coils for a wide processings.
In addition, we so far developed the pulse modualted
induction thermal plasma technique [1]. Our
modulation technique for input power to ICTP was
applied to the planar-type of ICTP in the present paper:
a planar type of modulated induction thermal plasma (a
planar-MITP). This was done to study controlling the
uniformity of a planar thermal plasma.
3. Experimental setup and conditions
3.1. Configuration of the planar torch systems
We newly developed two types of planar-MITP
torches: one is operated with an air-core coil, the other
with a high-frequency ferrite-core coil. Fig.2 shows the
configuration of the planar-MITP torches developed.
These two systems have the same quartz vessels but
with different coils. The vessel has an inner dimension
of 120 mm x 20 mm, and the inner height is 100 mm.
The vessel is sandwiched with the air-core coil or the
ferrite-core. The advantage of the ferrite core is that it
enhances the flexibility of the coil location and the
localization of applied magnetic field. The vessel wall
was cooled by cooling water to keep the wall
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
20 mm 120 mm
H
H
100 mm
Coil
I
I
H
E
Plasma
(a)
E
Plasma
(b)
Coil
E
Planar plasma
(c)
Fig.1 Configurations of ICTP torches: (a) a
conventional ICTP torch; (b) a planar-ICTP torch; (c)
a tandem type of planar-ICTP torch.
temperature around 300 K. Sheath gas can be supplied
along the torch wall from the torch head made of
stainless steel.
3.2. RF power supply for planar-MITP system
For a planar-MITP, a metal-oxide semiconductor
field-effect transistor (MOSFET) RF power supply was
used with a matching transformer and an LC series
circuit. Its rated power is 30 kW, and its driving
frequency can be regulated from 40 kHz to 420 kHz
according to load impedance. It also has an insulated
gate bipolar transistor (IGBT) dc-dc converter circuit,
by which it can also modulate the coil current
amplitude [1]. To perceive response of the rf power
supply for planar-MITPs, the instantaneous current
iinv(t) and voltage vinv(t) were measured at the output
terminal of the inverter circuit. The active power P(t)
was calculated by iinv(t) and vinv(t). The root mean
square values were estimated by integrating
instantaneous value in each fundamental cycle.
3.3. Measurement of Ar excitation temperature
One important key point for a planar-MITP is the
uniformity of the plasma temperature. In this work, the
Ar excitation temperature was determined from the
spectroscopic observation. The observation was carried
out on four lines along a longer side of the rectangular
vessel as indicated in Fig.2. Four observation lines are
designated by A, B, C and D, each of which has
different height. Lines A, B, and C correspond to ones
at 5 mm above the air-core coil top, at the center of the
coil and at 5 mm below the coil end, respectively. Line
D is 20 mm below the ferrite-core coil. At each
observation position, the intensities at 703.0 nm and
714 nm for Ar atomic lines and that at 709 nm were
measured with three photomultiplier tubes. Use of this
system enables us to measure time evolution in these
radiation intensities at the above three wavelengths
simultaneously. The radiation intensities yield to
determine Ar excitation temperature TArex between the
specified levels by the two-line method. The TArex was
calculated in real-time in a Digital Signal Processor
(DSP). The optical emission intensity from planarMITPs was also observed with a high-speed video
camera to find the dynamic behavior of the planerMITPs.
3.4. Experimental condition
Table 1 summarizes the experimental conditions
adopted in this work. The sheath gas was Ar, and its
gas flow rate was set to about 20 slpm. The pressure in
the chamber was fixed at 30 torr. The driving
frequency of the air-core coil current was set to 369
kHz and that of the ferrite-core coil current was 140
kHz. The lower frequency was adopted for ferrite-core
coil because of its higher self-inductance. The coil
current was modulated in a rectangular waveform with
a duty factor of 50%. The shimmer current level, which
is defined as a ratio of lower current level to higher
current level, was set to about 70% [1].
A
B
D
C
(a) Air-core coil
(b) Ferrite-core coil
9 turn
Coil
Ferrite
A
B
C
5 turn
Lens
Optical
fiber
D
5 turn
Plasma
Observation position
Monochromator
PMT
DSP
Fig.2 Configuration of two types of planar-MITP torches:
(a) air-core coil type ; (b) ferrite -core coil typ e
Spectroscopic observation system is also indicated.
Table 1 Experimental conditions.
Pressure [torr]
Coil current frequency [kHz]
Ar gas flow rate [slpm]
Modulation signal form
Modulation frequency [Hz]
On time [ms]
Shimmer current level [%]
Duty factor [%]
Inverter effective power [kW]
Inverter output voltage [Vrms]
Inverter output current [Arms]
Air-core coil
MITP
30
369
23
Rectangular
20
25
68
50
4.2 to 12.5
104
80
Ferrite-core
coil MITP
30
140
20
Rectangular
20
25
67
50
2.8 to 10.3
140
47
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
4. Results and discussions
Fig. 3 presents plasma images of the two-types of
planar-MITPs with an air-core coil or a ferrite core coil.
We confirmed that the plasma was successfully
sustained stably in the planar torches with either coil.
4.1. A planar-MITP with air-core coil
Fig.4 depicts results for a planar MITP with air-core
coil, including (a) the external signal for coil current
modulation, (b) the inverter current iinv(t) and voltage
vinv(t), (c) the active output power P(t) from the
inverter power supply and (d) Ar excitation
temperature TArex for different positions x along a
longer side of the rectangular vessel. The TArex in
Fig.4(d) is the one measured on observation line C
below the coil. As seen in Fig.4(b), the iinv(t) and vinv(t)
can be modulated following the modulation signal (a).
According to the modulation in iinv(t), P(t) could be
also modulated from 4.2 kW to 12.5 kW in a
modulation cycle as shown in Fig.4(c). The transition
time from the lower to the higher power was estimated
to be 2 ms. The TArex at any observation position
changes with P(t), having an almost rectangular form
with amplitude about 1500 K.
Fig.5 presents the emission intensity variation in the
planar-MITP torch with an air-core coil in rise-up
duration of the power. The intensity is shown here with
color fringe, where higher intensity is shown in red
while lower intensity is in blue. Increasing power from
t=50 ms to 56.6 ms grows up the thermal plasma in the
torch in an ellipsoid ring form, although a lower half of
the ellipsoid plasma-ring may be located into the
chamber downstream of the torch. This is possibly
attributed to the Ar gas flowing from the upper side of
the torch, which pushes down a plasma-ring which is
generated by the air-core coil.
The MITP has a periodically-modulated temperature
fields. Curves in Fig.6 are the time-averaged TArex on
lines A, B and C. It is noted that the time-averaged
TArex distributions are approximately uniform within
500 K in range of x=-50 to 50 mm at lines A and C,
although the TArex at line B has non-uniform
distribution because of a donut shape of the plasma.
Fig.7 shows the maximum, minimum and mean
values in TArex for a planar-MITP measured on line C.
This figure also contains TArex for a non-modulated
planar-plasma at 9.2 kW. As seen, the MITP has more
uniform time-averaged temperature distribution than
the non-modulated thermal plasma. This means that the
modulation enhances uniformity in temperature field.
4.2. A planar-MITP with ferrite-core coil
Use of a ferrite-core coil also made it possible to
sustain a planar-MITP. Fig. 8 represents (a) external
signal, (b) iinv(t) and vinv(t), (c) P(t), and (d) TArex. The
P(t) could be changed from 2.9 kW to 10.1 kW in a
modulation cycle. The transition from the lower to the
Coil
Ferrite
Plasma
(a)
(b)
Fig.3 Pictures of planar-MITP:
(a) with air-core coil; (b) with ferrite-core coil.
Fig.4 Time evolutions in electric parameters and Ar
excitation temperature at different positions on line C in
a planar-MITP with an air-core coil.
A
B
C
50.0 ms
51.7 ms
ms
55.0 ms
56.6 ms
x=0
Observation line
53.3 ms
Fig.5 Emission from a planar-MITP with air-core coil.
higher power was in 5 ms, longer than those with an
air-core coil. The TArex roughly follows P(t), having a
amplitude of 2000 K. However, a difference is seen in
decaying rates in TArex during off-time for observation
positions. Fig.9 shows high-speed camera images for a
planar-MITP with a ferrite core coil in rising duration
of the power. The ferrite core applies magnetic field on
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Fig.6 Time-averaged temperature distributions along the
longer side of a planer-MITP with an air-core coil.
Fig.8 Time evolutions in electric parameters and Ar
excitation temperature at different positions on line D
in a planar-MITP with a ferrite-core coil.
D
x=0
25.0 ms
25.5 ms
ms
56.5 ms
27.0 ms
Observation line
Fig.7 Maximum, minimum and mean temperature
distributions on line C in a modulation cycle for a
planar-MITP with an air-core coil.
the local region. Around that region, a half-ring of the
plasma can be seen to grow-up in the vessel.
The TArex distributions for the planar-MITP with a
ferrite-core coil are indicated along the longer side of
the vessel in Fig.10 with maximum, minimum and
mean values in a modulation cycle. It also includes the
TArex for non-modulated ICTP at 6.4 kW. As seen, the
time-averaged TArex is in range of 9000 to 10000 K at
x=-35 mm to 40 mm, which is similar to that of nonmodulated ICTP in this case.
5. Conclusion
In this paper, a planar modulated induction thermal
plasma (a planar-MITP) system has been developed
using rectangular quartz vessel with an air-core coil or
a ferrite-core coil. Both coil types of the planar-MITP
could be established successfully. It was found that Ar
excitation temperature in the planar-MITP could be
controlled by power. Uniform temperature distribution
was obtained for wide area below the coil for air-core
coil MITP. These features of the planar-MITP can be
useful for various materials processings.
26.0 ms
Fig.9 Emission from a planar MITP with ferrite-core
coil.
Fig.10 Maximum, minimum and mean temperature
distributions on line D in a modulation cycle for a
planar-MITP with a ferrite-core coil.
6. References
[1] Y.Tanaka, et al., J. Phys. D: Appl. Phys., 43,
265201 (2010)
[2] M.Akao, et al., ISPlasma2013, p.74, P1011A
(2013)