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International Journal of Plasma Environmental Science & Technology, Vol.5, No.1, MARCH 2011
Radiated Radiofrequency Emission from the Plasma of Compact
Fluorescent Lamps
G. Schmidt1 and I. Berta2
1
General Electric Appliances & Lighting - Technology, Hungary
2
Budapest University of Technology and Economics, Hungary
Abstract—International EMC standards describe the measurement environment (setup, equipment, frequencies) and
applicable limits for the electromagnetic emission testing of lighting equipment, including compact fluorescent lamps
(CFLs). These standards consider the compact fluorescent lamp a ‘black box’. In order to better understand the emissions
from this type of light source, the operation of the equipment needs to be analyzed not just at system, but also at
component level. The paper presented the compact fluorescent lamps’ operation from electromagnetic compatibility
(EMC) point of view, examined their operation theoretically and experimentally, analyzed the arc discharge from
electromagnetic emission point of view. The electric and magnetic field emission originated from the plasma of the CFL
was shown. The independent sources of emission within the system were identified.
Keywords—Arc discharge, compact fluorescent lamps, effect of plasma, electric field, electromagnetic compatibility,
electromagnetic radiation, EMC, EMI magnetic field, radiated disturbances, radiofrequency emission
I. INTRODUCTION
One important demand of the development of
modern discharge lamps is to satisfy the ever-increasing
electromagnetic environmental regulations. Fulfilling the
EMC (electromagnetic compatibility) conditions is an
essential criterion of producing marketable products. A
light source, which meets EMC considerations on the one
hand does not emit more electromagnetic disturbances
than the permissible level, on the other hand it operates
reliably under a well-defined disturbance level.
Electric, magnetic and electromagnetic disturbances
spread in a conducted or radiated way. Discharge lamps
produce and radiate high-frequency disturbances to their
surroundings during their operation. The corresponding
EMC standards [1-3] describe the testing circumstances,
including the measurement setup with all the testing
equipments, frequencies and distances. Also, they define
the applicable limits for the device under test.
Standardized EMC tests related to compact
fluorescent lamps and other lighting equipment consist of
radio frequency radiated (9 kHz - 300 MHz) and
conducted emission (9 kHz - 30 MHz) measurements;
harmonic current emission measurements (50 or 60 Hz 2 or 2.4 kHz); and a set of various immunity tests.
Several works are to be found investigating the
radio-frequency emission from CFLs; however, these
publications treat them at system level, without the
intention to understand the physical processes behind the
emission [4-5].
The reason of disturbance emission can be explained
by understanding the physical processes of the operation.
All of these aspects concerning EMC have become
more important with the phase-out of incandescent lamps
Corresponding author: Gabor Schmidt
e-mail address: gabor.schmidt@ge.com
Received; August 4, 2010
in the European Union. The legislation is expected to be
taken over in other parts of the world.
II. COMPACT FLUORESCENT LAMPS
The replacement of incandescent lamps will be
realized at a significant scale by compact fluorescent
lamps (CFL). Today’s CFLs are very close to
incandescent lamps in aesthetics and are much (4-5times) more energy efficient than those.
There are some special requirements that are very
easily solved with incandescent lamps and in order to
gain the satisfaction of the customer, they need to be
fulfilled by CFL lamps as well. Here we can think of among other features - a reliable starting, a stable light
output with no flashing/flickering, or even the regulation
of the light output (so called dimming) of the lamp.
Meeting these requirements has been allowed by using
electronic control gears (ballasts) for driving the lamps.
The system consists of the wirelamp, the ballast and the
wiring between them (Fig. 1). Fluorescent or compact
fluorescent lamps are low-pressure gas discharge lamps,
Fig. 1. Compact fluorescent lamp.
Schmidt et al.
85
where the discharge is normally an arc discharge (the
emission of electrons is achieved thermionically). The
wirelamp is essentially the discharge tube with the
emission-mix (oxides of alkaline earth metals used for
decreasing the work function of bare tungsten) coated
electrodes (cathode and anode), containing an inert gas
fill (usually Ar or a mixture of Ar, Ne or Kr) and Hg
vapor. Hg is dosed mostly in the form of amalgam today.
This allows us to have only 1 mg Hg in the discharge
tube and prevents the penetration of free mercury into the
soil in case of a lamp breakage. The mercury atoms are
excited in the discharge by the accelerated electrons,
through inelastic collisions. The excited electrons return
back to their normal (low energy) state and during this
process they radiate the excess energy in the UV (and at
a smaller extent in the visible) range. The UV energy is
converted to visible light by the fluorescent powder at the
inner wall of the tube.
III. OPERATION OF THE ELECTRONIC CONTROL GEAR
A block diagram of a compact fluorescent lamp
system can be seen in Fig. 2. The CFL is operated from
an AC mains network at 50 or 60 Hz, at various mains
voltages. This AC input voltage is converted to DC by a
full-wave rectifier. The rectifier charges an electrolytic
capacitor, which stores the DC energy. The capacitor
feeds a half-wave inverter circuit, which produces a highfrequency (typically 40-80 kHz) square wave on its
output.
A resonant RCL output stage is used to drive the
fluorescent lamp. The resonant behavior of the circuit is
used to preheat, ignite the lamp and control its current.
During preheat, the lamp is not conducting and the circuit
is a high-Q series L and C. The frequency is held
constant and above resonance for a fixed time to preheat
the filaments with a given current. After preheat, the
frequency is swept down smoothly towards resonance to
generate a high voltage for ignition (Fig. 3). After
ignition, the lamp is conducting and the circuit is an L in
series with a parallel R and C. The frequency continues
to decrease to the final frequency where the nominal
lamp current is reached. For dimming, the frequency is
increased to decrease the lamp current and the Q-factor
of the circuit changes depending on the lamp resistance
[6]. Additional cathode heating shall be used during
dimming in order to maintain the stability of the arc and
to protect the e-mix on the filament from the sputtering,
thus preventing early life issues.
As we have seen, the output waveform of the
inverter is a square wave, which is rich in spectrum lines.
Fig. 2. Block diagram of a CFL system.
Fig. 3. Bode-plot of resonant tank operation.
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International Journal of Plasma Environmental Science & Technology, Vol.5, No.1, MARCH 2011
Fig. 4. Half-wave inverter output voltage and lamp current.
The Fourier-spectrum of a square wave can be depicted
by the following equation:



 sin 2ft  



4
sin 2k  12ft  4  1

x square (t )  
  sin 6ft   
2k  1
 k 1
 3


 1 sin 10ft   ... 



5
(1)
As it can be seen, it contains the fundamental
harmonic and its odd harmonics with gradually
decreasing amplitudes. However, as the switching
elements are not ideal (their switching speed is limited),
the output of the half-wave inverter is not perfect square
wave (see Fig. 4), and the real Fourier-spectrum contains
even harmonics as well. One of the resonant tank
circuit’s functions is to transform the square wave to sine
wave voltage for feeding the wirelamp. As the Q-factor
of the resonant circuit is limited, theoretically all of the
original harmonics are present in the lamp voltage and
lamp current.
IV. PLASMA IN COMPACT FLUORESCENT LAMPS AND EMC
A light source which meets EMC considerations on
the one hand does not emit more electromagnetic
disturbances than the permissible level (emission); on the
other hand it operates reliably under a well-defined
disturbance level (immunity). At the frequencies in
which the electronic CFL operates, the electric and
magnetic fields behave similarly to static fields in terms
of their sources (quasi-static electric and magnetic fields).
Also, they can be treated independently. The concerned
frequency bands are SLF, ULF, VLF, LF, MF, HF and
VHF [7].
The radiation pattern of a noise source can be broken
down to the emitted field of a set of short electric dipoles
or current elements [8], (Fig. 5). The emission of the
source shall be measured in 3 orthogonal positions. This
Fig. 5. Electromagnetic field of a short electric dipole [8].
is done similarly in case of the standard measurements
with the 2 m loop antenna.
The components of the electric and magnetic field
strengths are the following [9]:
60Idl  1
1 
j
(2)
 
 sin e j t  r 

E  j
  r r 2  2 r 3 
H  j
j
j 
1 60Idl  1
  2  sin e j t  r  
Z 0   r r 
(3)
Il  1
j 
 
 sin e j t  r 
2  r r 2 
120Idl   j
1 
(4)
 2  2 3  cose j t  r 
  r
 r 
where I [A] is the current,  [m] is the wave length,  =
2/ [1/m] is the phase constant and Z0 = 120 [Ohm] is
the characteristic impedance of free space.
In the above equations the r-1 term stands for the
radiation field (far field), while the r-2 and r-3 terms stand
for the near field. The far field term is present only in E
and H. Close enough to the short dipole near fields can
be described through the Biot-Savart and Coulomb laws.
So, the field strengths at an r distance are:
B
(5)
H
Er  j
0
 0 Idl  r
B
4 r 3
(6)
1 q
(7)
4 0 r 2
At a small distance from the dipole, near fields are
larger than the far fields. In case of increasing distances,
the near fields decrease at a higher scale than the far
fields. Far field begins where the near fields can be
neglected besides the radiated field, where:
E

(8)
2
At a frequency of 30 MHz, the wavelength is 10 m,
while it is more than 30 km at 9 kHz. Thus,
r
Schmidt et al.
87
A. Cathode oscillation
Fig. 6. Electromagnetic disturbances generated by fluorescent
lighting systems.
measurements in this frequency range (9 kHz-30 MHz)
by the standard method and the method used by the
authors (described in V), are considered to be near-field
measurements. So the simplified equations (5), (6) and
(7) might be applied instead of (2), (3) and (4). In our
measurements, we focused on the electric field generated
by the compact fluorescent lamp.
How can the plasma of a compact fluorescent lamp
cause EMI? Discharge lamps produce and emit LF and
radiofrequency noise to their surroundings continuously.
The reason of this emission can be explained by
understanding the physical processes of the lamp
operation.
A CFL lighting system, including the electronic
ballast, may theoretically emit disturbances in 4 main
ways (Fig. 6):
 The wirelamp/discharge may generate electric,
magnetic or electromagnetic fields (1)
 The wires that connect the ballast to the lamp
may
generate
electric,
magnetic
or
electromagnetic fields around itself (2)
 The switch mode power supply of the ballast
may radiate electromagnetic disturbances to its
surroundings (3)
 The wires from the electronic ballast conduct
harmonic (LF) and RF (HF) currents to the
power network (4)
(2) and (3) are not significant in case of an integrated
CFL (CFLi - where the ballast is assembled together with
the wirelamp) due to the given structure, small size and
the given operating frequencies. Conducted disturbances
are not in the focus of this paper. The physical processes
in the arc tube will be introduced in the following.
As we have seen, the lamp is driven by electronic
ballast, the electrodes are hot enough to emit the
electrons thermionically, thus operating stable as an arc
discharge. RF radiated emission is expected from the
system at discrete frequencies (defined by the inverter
and the resonant tank circuit); and as a random-type,
broadband noise originated from the electronic
components of the ballast. Some physical processes in
the discharge similarly generate broadband noise. These
processes are the following [10-15]:
During the normal operation of fluorescent lamp, the
electron current that operates the lamp (the discharge) is
produced by thermionic emission of the cathode. The hot
spot is the region of the cathode (4-5 turns of the spiral),
where the discharge leaves the coil. The temperature of
the hot spot, which is also the hottest point of the cathode,
must be high enough to ensure the emission of sufficient
number of electrons that supplies the discharge. If the
temperature of the cathode is not high enough, the
cathode fall increases automatically and positive ions
accelerate passing through the cathode sheath. As they
hit the surface of the cathode, particles from emission
mix may be knocked out. This process is called
sputtering, and causes a limitation in lamp life. The aim
of dimming is to decrease the luminous flux produced by
lamp. This is done through decreasing the discharge
current. As a result of this, the number of excitations in
mercury atoms will also be lower. On the other hand, if
we decrease the current, which flows through the cathode,
the heating effect of the current may become insufficient
to ensure the optimal temperature of the cathode, the hot
spot to emit thermionically. Therefore we must apply an
additional heating current, which flows only through the
cathode - it does not feed the discharge. As we decrease
discharge current, we must increase the additional
heating current. However, if we apply too high heating
current at a given dimming state, the number of emitted
electrons can be as high as the discharge cannot transfer
them towards the anode. So a negative space-charge
region forms in front of the cathode. That means that no
ion production is required for the operation of the
cathode, the cathode fall will drop, the positive ions
produced in the main body of the discharge slow down,
and get trapped in the potential minimum made by the
negative space charge. When they do, they oscillate and
generate an RF emission in the frequency range of
several hundred kHz (see test results in Fig. 15).
B. Anode oscillation
Another oscillation takes place at the opposite
electrode, and it is called anode oscillation. Electrons
arriving at the anode surface obtain considerable energy.
As a result of this energy gain, there may be electrons
with sufficient energy to produce a significant excess
ionization, either in the anode sheath itself or in the presheath volume of the lowered plasma density. If there is
an excess ionization, the plasma density will suddenly
increase and become high enough for the anode to collect
the necessary electron current without a positive anode
fall. The anode fall will then abruptly drop to zero and
the excess ionization cease. The excess plasma density in
front of the anode will slowly relax and the ions diffuse
away by the process of ambipolar diffusion. As the
plasma density at the anode sheath surface decreases, the
anode fall increases until it becomes high enough for
beginning the ionization again. The critical anode fall
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International Journal of Plasma Environmental Science & Technology, Vol.5, No.1, MARCH 2011
Fig. 7. Anode oscillation.
equals to the ionization potential of mercury, when that
level is reached, the ionization build-up takes place
abruptly, and the anode fall collapses quite suddenly [10].
As a consequence of these relaxation oscillations, the
anode fall has a sawtooth-shaped variation in time, with a
frequency of about 10-20-times the lamp operation
frequency and an amplitude of about 10 V. Anode
oscillation is influenced by the anode design, the rare gas
pressure, impurities in the gas filling and the ambient
temperature.
Anode oscillation resulting in lamp voltage and
current distortions can be seen on Fig. 7 at a lamp current
of 190 mA and lamp voltage of 62 V (RMS). The
oscillation has amplitude of up to 10.4V (this is the
ionization potential of mercury).
V. RADIATED EMISSION FROM COMPACT FLUORESCENT
LAMPS
We have seen the potential noise sources in the
compact fluorescent lamp system and based on the
physical processes and operation we have expectations
about the emission’s frequency range as well.
The CISPR15 standard requires the magnetic field
to be tested by a 2 m diameter, 3-axis loop antenna (Van
Veen-loop), the measurement unit is dbμA, as the
induced current is measured in the loop in the three (X, Y
and Z) axis. None of the measured emission curves shall
exceed the (same) limit line.
It can be seen in Fig. 8, where the X-axis emission
curve is shown, that the sensitivity of this method is low,
the spectrum line at the nominal operating frequency can
be found only. We don’t have any information about the
other spectrum lines. The abrupt change at 150 kHz is
caused by the fact that the quasi-peak detector’s
resolution bandwidth is set from 200 Hz to 9 kHz here.
In order to achieve a higher sensitivity, an ETSEmco
5407
GTEM
(Gigahertz
Transversal
Electromagnetic) test cell was used to be able to analyze
the emitted spectrum. The GTEM cell is a pyramidshaped, doubly-terminated 50 Ω transmission line. At the
front end, which is an input in case of immunity testing
and an output in case of emission testing, a normal 50 Ω
Fig. 8. Radiated measurement according to the standard (X-axis).
Fig. 9 Measurement setup inside the GTEM cell (orientation: X).
coaxial line is physically transformed to a rectangular
cross section. The cross sectional dimensions are in a
ratio of 3:2 horizontal to vertical dimensions. The centre
conductor, known as the septum, is a flat, wide conductor.
By the theory of reciprocity, both radiated emissions and
immunity testing are conducted in the test volume. The
septum is physically terminated in a resistive array
having a total value of 50 Ω for matching the current
distribution of the septum. The volume fields, either
applied to immunity test item or produced by the
equipment under test (EUT) during emissions testing, are
terminated in free-space foam RF absorber [16]. The
emission is measured as a voltage between the septum
and the floor of the cell and it depends on the septum
height at which the EUT is located inside the cell (as it
was mentioned, the cell has got a pyramid shape and the
septum height increases from the input in the direction of
the end of the cell). The voltage is transferred to the
EMC receiver through the coaxial connection. In our
case the septum height was kept constant at a value of
0.74 m (see Fig. 9). More details on the test system can
be obtained from [17] and [18].
Similarly to the standard test method, we have
tested 20W multi-finger compact fluorescent lamps with
Schmidt et al.
89
Fig. 11. The wirelamp substituted with a resistor load - emitted
spectrum.
Fig. 10. Emission from a 20W CFL with an unshielded vs. a
shielded mains cable.
integrated control gear (CFLi) for radiated emission in 3
orthogonal orientations (compared to the GTEM cell,
namely X, Y and Z). This is enabled by the manipulator
built in the cell (Fig. 9). The EUT and the cable are fixed
on the turn-table of the manipulator, so the any change in
the radiation is caused by changing the orientation of
EUT relative to the cell. As the purpose of this work is to
localize the sources of RF emission from the CFLi, all
the test data that are shown in the following are measured
in X orientation (the EUT’s and the cell’s axis are
parallel).
We have seen the potential sources of radiated
emission in Fig. 6. In order to localize the source of
emission, an RF filter circuit (Schurter FSW2-65) was
inserted between the ballast and the mains cable in order
to suppress high frequency signals on the mains cable.
Additionally a metal shielding was used on the cable (l =
1 m), earthed to the floor of the cell. These enabled us to
exclude the cable from the system as a radiating element.
No significant difference was found in the two cases (see
Fig. 10). The source of emission is not the mains cable at
these RF frequencies. The upper trace shows the results
from the quasi-peak detector of the EMC receiver, while
the bottom trace shows the results from the average
detector. This is the case for all following figures that
include two traces. The frequency range is 9 kHz - 30
MHz, and the measured quantity is disturbance voltage in
dBμV. In order to identify the individual spectral lines of
the emission, the resolution bandwidth of the detector
was kept constant (200 Hz) in the full frequency range.
The measured voltage at the fundamental operation
frequency of 50 kHz is 42 dbμV. This refers to 44.6
dbμV/m field strength (corrected with 0.74 m septum
height).
An arc discharge behaves like a resistor in a given
working point. This means it adjusts itself to a defined
operating voltage when driven by a particular current.
Based on this behaviour, the wirelamp (the discharge
tube) was replaced by a low-inductance, low capacitance
resistor of 620 Ohms, while the electronic control gear
was kept the same. The intention was to provide the same
load for the electronic ballast as the wirelamp would be
with an operating arc discharge in a given (normal)
working point. The input and output parameters were
kept the same in both cases, which guaranteed that the
working points were also the same in the two cases (106
V and 170 mA).
The main differences are the following (Fig. 11):
less efficient radiation at harmonics of the operating
frequency (39.6 dbμV/m vs. 47.6 dbμV/m at 150 kHz),
same field strength at the fundamental frequency (44.5
dbμV/m); in addition less broadband emissions. The
reason for the latter is the absence of the oscillations at
the cathode and anode. The result is emission at discrete
frequencies (originated from the inverter and resonance
circuit) sitting on the background noise including the
broadband emission from electronic components
(compare with Fig. 10). We have seen that we were able
to affect the radiated electric field by replacing the
wirelamp (the arc) by a resistor. These points at the lamp
as the main source of radiated emission. To prove this, a
conducting enclosure was used for shielding the arc tube,
similarly to the case when the mains cable was shielded.
Again, the enclosure was earthed to the floor of the cell.
The reader can compare the measured spectrum to the
case, when no lamp was operated in the test cell, so the
shown spectrum is the background noise (change at 150
kHz is caused again by the bandwidth change of the
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International Journal of Plasma Environmental Science & Technology, Vol.5, No.1, MARCH 2011
Fig. 14. The magnetic field.
Fig. 12. Shielded wirelamp and the background noise.
Fig. 15. Effect of cathode oscillation in dimmed operation.
Fig. 13. H-field measurement with the 6 cm loop antenna.
detector). Preventing the emission from the wirelamp
decreases the electric field practically to the level of the
background noise (Fig. 12). The associated field
strengths are 19.4 and 19.2 dbμV/m.
Described through the quasi-static approximation,
magnetic fields are assumed to be produced by charges in
motion (in a conductor wire or in a discharge). Thus, the
quantity that is responsible for generating magnetic fields
is electrical current. Voltages oscillating at a particular
frequency generate electric fields oscillating at the same
frequency. This is the same for oscillating currents and
magnetic fields. As a result, the electric and magnetic
fields produced by compact fluorescent lamps are
expected to have the same frequency characteristics as
their operating voltages and currents. The magnetic field
emission was analyzed using a near-field loop antenna
with a diameter of 6 cm (Fig. 13).
Meeting the expectations, the same spectrum is
found as previously shown (Fig. 14). That was related to
the lamp voltage and this one is connected to the lamp
current. The resolution bandwidth of the detector was
kept again constant (200 Hz), so the spectrum lines are
easy to be identified above the broadband noise. The
amplitudes are irrelevant here as these kinds of loop
antennas are used for the localization of sources instead
of exact measurements.
During dimming, the magnetic field is decreased
and the electric field is increased as the working point of
the arc discharge is changed by a shift on the voltagecurrent characteristic curve. The additional heating
current increases the effect of cathode oscillations, which
results in a broadband noise emission in the 100
kHz/MHz range, with the broadband noise amplitude
spreading between 30-45 dbμV/m in this frequency range.
We shall note that this lies in the range of the intensity at
the fundamental frequency (Fig. 15). A detailed analysis
of the dimmed operation of CFL lamps with regards to
the EMC characteristics can be found in [19].
All of the results shown before were taken during
the stable operation of the CFL system. In case the
Schmidt et al.
91
Fig. 16. Emission from an instable arc.
during the end-of-lamp life (EOL), when no emission
mix remained on the cathode. The resulting noise can
easily be observed in Fig. 16 for a 57 W lamp.
Here we can see that the spectrum gets crowded, a
lot of spectrum lines occur besides the harmonics of the
operating frequency (47.6 dbμV/m) and a broadband
emission is present, too. The emission from this type of
operation is stochastic, therefore hard to handle. It shall
be prevented, or in case it already occurred, the
termination of this condition shall be forced as soon as
possible.
The latest revision of the CISPR15 standard has
extended the upper limit frequency of the radiation
measurement from 30 MHz to 300 MHz. In this
frequency range a biconilog antenna was used in a semianechoic room for detecting the radiated electromagnetic
field similarly to the CISPR22 standard’s requirements.
Fig. 17 shows the test results of the same 20W CFLi
from 30 MHz to 1 GHz (10-16 dbμV/m at 10 m). The
source of the detected noise is the mains cable - in
opposition with what we have seen previously -, and the
peaks are caused by cable resonances at the related
frequencies. No emission can be detected from the lamp.
This is in accordance with the conclusions of [20]. The
radiation might be prevented by shielding the mains
cable.
IV. CONCLUSION
Fig. 17. Cable resonance and shielding of the cable prevents
radiation (30 MHz – 1 GHz, 10 m, vertical polarization, measured
with a biconilog antenna).
cathode temperature/the hot spot temperature is not high
enough, the arc becomes instable and the hot spot of the
arc is changing its position rapidly (this is the point
where the arc starts and the electrons are released from
the cathode). This stochastic process will generate a
random-type, broadband RF noise. The phenomenon may
occur in case of an improper cathode design; during
dimming with insufficient additional cathode heating; or
The operation of compact fluorescent lamps was
analyzed from EMC - radiated emission point of view.
Understanding the physical processes, that are
responsible for the radiated emission, makes it possible
to design CFLs, which disturb their surroundings at a
significantly lower scale. The main source of emission
was identified, which is the plasma of the wirelamp.
However, the spectrum is basically defined by the
electronic ballast (inverter and resonant tank circuit)
operation through feeding the lamp voltage and current.
The mains cable that had been previously considered as a
significant contributor to the radiated emission was found
to be negligible with a length of 1 m in the frequency
range of 9 kHz - 30 MHz. This can be explained by the
long wave lengths at these frequencies. The dimming of
compact fluorescent lamps affects their radiated emission
characteristics through the shift in their working point
and the applied additional cathode heating current, which
influences the number of electrons present in the
discharge and might result in cathode oscillations. The
instable operation of a discharge lamp shall be avoided,
as its operation is stochastic from EMC point of view.
92
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
International Journal of Plasma Environmental Science & Technology, Vol.5, No.1, MARCH 2011
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