J. Inst. Electrostat. Jpn.

静電気学会誌，34, 2 (2010) 99-104

Emission Spectroscopy of Microplasma Driven by a Pulsed Power Supply*
Marius BLAJAN,**,1 Shuichi MURAMATSU,** Tatsuya ISHII,**
Hidenori MIMURA*** and Kazuo SHIMIZU**(Received August 3, 2009; Accepted March 22, 2010)

Emission spectrum of the microplasma discharge in N2 or air with water droplets was analyzed. Twoexperimental Marx Generators were used as pulsed power supplies, with negative and positive pulse
respectively. Emission spectra for N2 gas with water droplets show N2 second positive band system (N2 SPS),
N2 first negative band system (N2 FNS), H-γ band system, and OH peaks. For air with water droplets emission
spectrum shows N2 SPS, N2 FNS and H-γ band system with lower intensities comparing with microplasma
discharge in N2 gas with water droplets. Emission signal of the photomultiplier, show short lifetime emission
signal for N2 SPS of 337.1 nm. Vibrational temperature calculation was also carried out using the N2 second
positive system (C3Π - B3Π band).
1. Introduction
Microplasma can be found in many applications. In the lastyears, the technology was used in applications such as NOx
removal, surface treatment and sterilization or inactivation ofbacteria. Although there is an interest for application driven
research, microplasma phenomena are not fully understood.
Emission spectroscopy is one of the methods to analyze
plasma process.1,2)
The high intensity electric field generated between
microplasma electrodes, charged particles, reactive groundstate species, excited species, UV photons can affect thebacteria. Atmospheric microplasmas have been shown toinactivate a wide spectrum of microorganism including spores,
Gram-negative bacteria, Gram positive bacteria and viruses.3)
Research works showed that better results of bacteria
inactivation using non thermal plasma were obtained with H2O
addition in comparison with dry processes.4) One of theprocesses of bacteria inactivation using microplasma consists
in spraying water droplets containing bacteria throughmicroplasma electrodes. Air or nitrogen is used as carrier gas.5)
The high intensity electric field generated between
microplasma electrodes, charged particles, reactive groundstate species, excited species, UV photons can affect thebacteria. The analysis of these factors could be done byemission spectroscopy method. The aim of this paper is to
Key Words：microplasma, pulsed power, emission spectroscopy．
* Marius BLAJAN, 村松秀一, 石井達也, 三村秀典, 清水一
男：パルス電源を用いた大気圧マイクロプラズマの発光分
析** Innovation and Joint Research Center, Shizuoka University, 3-5-1,
Nakaku, Hamamatsu 432-8561, Japan．
** * Research Institute of Electronics, Shizuoka University, 3-5-1,
Nakaku, Hamamatsu 432-8561, Japan1blajanmarius@yahoo.com

analyze the emission spectrum of the microplasma in air andnitrogen containing water droplets.
2. Experimental Setup
2.1 Experimental Setup
The experimental setup is shown in Fig. 1. Emission spectra
were measured by an ICCD camera (Ryoushi-giken,
SMCP–ICCD 1024 HAM-NDS/UV), a spectrometer(Ryoushi-giken, VIS 351) and by a photomultiplier tube
(Hamamatsu Photonics, R3896). A pulse generator (Tektronix,
AFG 3021B) was used to trigger the Marx Generator consistedof semiconductor switches and the ICCD camera. Obtained
data were transferred to a computer.
The discharge voltage and the corresponding discharge
current were measured by a high voltage probe (Tektronix,
P6015), an AC current transformer (Tektronix, P6021) and adigital oscilloscope (Tektronix, TDS 2014B).
The resolution of the ICCD measurements was carried out
with FWHM 3ch. 3ch was set to 0.087 nm at narrow gratings
carried out with 2,400 grooves. Spectrometer has three mirrors
which are reflecting the light with a calculated reflectance of
90%. The measured intensity was 26% at 225 nm, 25% at 250

Fig. 1 An experimental setup.

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Quantity (x 1000)

30
25
20
15
10
5
0
0.3

0.5
1
3
Droplets size (um)5(b)

Fig. 2 The size distribution of water particles generated
inside the nebulizer at a gas flow rate of 5 L/min.
nm and 23% at 275 nm respectively, with narrow gratings dueto the reflectance of the mirrors and grating.
At wide grating settings with 200 grooves, the measured
intensity was 53% at 360 nm, 63% at 400 nm and 68% at 440
nm, respectively.
Compositions of gases used in experiments were N2, N2 andwater droplets, air and air with water droplets. Experiments
were carried out in atmospheric pressure and the gas flow rate
was set at 5 L/min. Water droplets were added in the carrier gas
using a medical nebulizer. The water droplets sizes anddistribution is presented in Fig. 2. The diameters of the waterparticles formed by the nebulizer were measured by a laser
particle counter (Kanomax, 3886). With water added to thenebulizer, water droplets with diameters ranging from 0.5 µm
to 5 µm were generated at the applied gas flow rate of 5 L/min.
The spectrometer settings can be set at wide gratings for awide range of spectrum or narrow settings for measuring a
specific range with greater accuracy.
ICCD camera functional elements sensitivities such as microchannel plate (MCP) and phosphor screen can be varied inorder to measure the spectrum for different intensities.

Fig. 3 An experimental Marx Generator: (a) withnegative pulses; (b) with positive pulses.
(a)

(b)

2.2 Marx Generator

A Marx generator with 4 stages MOSFET switches shownin Fig. 3a, was developed to be used as high voltage negative
polarity supply to microplasma electrodes. It has an output
voltage up to －1.8 kV peak (negative pulse, rise time 80 ns,
pulse width 500 ns -1 µs, frequency 1-24 kHz). Trigger signal
for ICCD camera was set at 1 µs (Fig. 4).
Capacitors were charged in parallel connection at a given
voltage V (500 V in this case). When the MOSFET switches
were closed, capacitors were discharging in series connection
thus results an output voltage 4V (2 kV in this case).
A Marx generator with 5 stages MOSFET switches forgenerating high voltage positive pulses was developed (Fig.
3b). An output voltage was up to 1.8 kV peak (positive pulse,rise time 80 ns, pulse width 1.5 µs, frequency 1-24 kHz).

Fig. 4 Waveforms of discharge voltage, discharge
current and gate signal for ICCD camera: (a) negative
pulse; (b) positive pulse.
Trigger signal for ICCD camera was set at 1 µs (Fig. 4).
Figure 4 shows the typical waveforms of both negative (Fig.
4(a)) and positive (Fig. 4(b)) pulse circuit.

Emission Spectroscopy of Microplasma Driven by a Pulsed Power Supply (BLAJAN et al.)3. About Microplasma
Microplasma electrodes used in this study were metallic
electrodes covered with a dielectric layer (Fig. 5). Due to small
discharge gaps (0-100) μm and to the assumed specificdielectric constant of εr = 104, a high intensity electric field(107-108 V/m) could be obtained with relatively low
discharge voltages around 1 kV. Electrode size was 20 mm
versus 40 mm..
Electrode has holes to flow for gas treatment, whichdiameter is Ø 2 mm and its aperture ratio of 36%. Discharge
gap was set at 50 μm in this study.
Emission spectra of microplasma discharge was observed
from the side part of electrodes. Thus the observed
microplasma area had 40 mm versus 50 μm.

4. Emission Spectrum of Microplasma
4.1 Microplasma by negative pulses
Emission spectrum of the microplasma discharge in N2 withwater droplets showed peaks of N2 second positive bandsystem (N2 SPS) and N2 first negative band system (N2 FNS)
(Fig. 6).
This spectrum was obtained at discharge voltage －1.4 kV,
rise time of 80 ns, pulse width of 500 ns, and dischargefrequency of 1 kHz.
N2 SPS (337.1, 357.7, 375.5, 380.5 nm) was observed due tothe electron collision (Table 1). N2 FNS peak was also
observed at 427.8 nm as shown in Fig. 6.
Emission spectrum of the microplasma discharge in air withwater droplets, showed peaks of N2 SPS and N2 FNS.
Intensities of peaks were lower than those measured fornitrogen, due to the quenching effect of O2.11,12) Theseelementary processes described the radiation kinetics for thesecond positive band system of nitrogen, wavelength 337.1 nm,
at atmospheric pressure:13)
Excitation of nitrogen molecules in the ground state by directelectron impact:
e + N2(X1Σg+)v=0 → N2(C3πu)v’=0 + e (ΔE=11eV)(1)
Spontaneous radiation of formed excited state of nitrogen:N2(C3πu)v’=0 → N2(B3πg)v’’=0 + hν(τoC=40 ns)
(2)
18000
N2 with H2O
Air with H2O

16000
14000
Intensity (a.u.)

Discharge current was confirmed at rising point of discharge
voltage. About -2 A was obtained at -1.4 kV for negative pulse.
Discharge current was also confirmed at rising point ofdischarge voltage for positive polarity. Discharge current was
lower for the positive pulse (about 1 A at 1.4 kV). The
MOSFET used in the positive pulse Marx Generator had
longer rise time. Thus a difference occurred between discharge
current peaks for negative pulse and positive pulse respectively
at the same discharge voltage.
The use of pulsed power in various applications such as
NOx removal or biomedical applications is advantageous
because of the low cost of equipment and insignificant heating.
Pulsed atmospheric plasma has a higher energy efficiency than
the sinusoidal plasma due to the long time plasma off periodthat reduces power consumption.6,7) In the microplasma
processes such as NOx removal, is necessary less power toremove the same quantity of NO from exhaust gases by using apulsed power supply in comparison with the case where AC
voltage is used,8,9) due to the reduced thermal load on electrodes
at higher currents.10)

101(55)

12000
10000
8000
6000
4000
2000
0
280

300

320

340 360 380 400
Wavelength (nm)

420

440

Fig. 6 Emission spectrum of microplasma discharge in N2
with water droplets and air with water droplets for negative
pulses.
Table 1 List of detected systems and peaks for the
microplasma in N2 gas and air with water droplets at wide
gratings settings of the spectrometer.
Transition

Peak Position(nm)

N2 second
positive

C3Π→B3Π

315; 337.1; 357.7;
375.5; 380.5; 400.0

N2+ first negative

B2Σu+→X2Σg+

427.8

Species (system)

Fig. 5 Microplasma electrodes.

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第 2 号 (2010)
10000

Air with H2O
Air

9000

Intensity (a.u.)

8000
7000
6000
5000
4000
3000
2000
1000

Fig. 7 Waveforms of discharge voltage, discharge
current and emission signal of microplasma (N2 SPS 337.1nm) in N2 with water droplets for negative pulses.

0
303 304 305 306 307 308 309 310 311 312 313 314 315
Wavelength (nm)Fig. 9 Emission spectrum of microplasma discharge in air
and in air with water droplets for negative pulses.

15000

14000

N2 with H2O
N2
12000

10000
Intensity (a.u.)

Intensity (a.u.)

N2 with H2O
Air with H2O
N2
Air

12000

9000

6000

8000
6000
4000

3000

2000
0

0
303 304 305 306 307 308 309 310 311 312 313 314 315
Wavelength (nm)Fig. 8 Emission spectrum of microplasma generated bynegative pulse in N2 gas and in N2 gas with water dropletsfor negative pulses.
Figure 7 shows waveform of discharge voltage, discharge
current and emission signal of photomultiplier corresponding
to N2 SPS wavelength of 337.1 nm..
The lifetime of photomultiplier signal of N2 SPS was about
40 ns as shown in Fig. 7 and according with reaction (2).
The gratings of spectrometer were set at narrow settings inorder to achieve a higher sensitivity of the MCP settings of
ICCD camera on a distinct range of wavelengths.
In Fig. 8 is presented the emission spectrum of microplasma
discharge in N2 and in N2 with water droplets. The spectrum
for the N2 gas containing water droplets showed the presenceof the OH wavelengths at 306.4, 307.8 and 308.9 nm. An
interference optical filter was used for measurements. The filter
has the center wavelength at 309.5 nm with a peaktransmission of 16.52%, and FWHM coordinates of 304.315
nm and 314.676 nm respectively.
Electron impact dissociation of H2O leads to the productionof H and OH radicals: 14)
e + H2O → e + H + OH
(3)
Air which used as carrier gas with water droplets, leads to

425

430

435
440
Wavelength (nm)

445

450

Fig. 10 Emission spectrum of microplasma discharge
in N2 or air with water droplets for negative pulses.
the generation of active species and ozone by microplasma:14, 15)
e + O2 → e+ O(3P) + O(1D)(4)
O + O2 + M → O3 + M(5)The excited state O(1D) O2 dissociated H2O to generate OH:9)
O(1D) + H2O → 2 OH
(6)
The emission spectrum for air and air containing water
droplets are presented in Fig. 9. With oxygen in the carrier gas,any peaks of OH were not observed due to the quenchingeffect of oxygen atom and ozone:12,14)
OH + O → O2 + H(7)
OH + O3 → HO2 + O2
(8)
Emission spectrum of microplasma in N2 or air with water
droplets for 425 nm to 450 nm range is presented in Fig. 10.
The peak of the hydrogen Balmer line H-γ is shown at 434.1
nm.16,17) An interference optical filter was used formeasurements. The filter has the center wavelength at 432.22
nm with a peak transmission of 55.32%, and FWHM
coordinates of 432.747 and 441.7 nm respectively.
Hydrogen radicals react with oxidative species to generate
OH or HO2 radicals:
H + O* → OH*(9)

Emission Spectroscopy of Microplasma Driven by a Pulsed Power Supply (BLAJAN et al.)Table 2 List of detected systems and peaks for the
microplasma in N2 gas and air with water droplets at narrowgratings settings of the spectrometer.
Transition

N2 second positive

C3Π→B3Π

N2+ first negative
OH (3064 – Å
system)H-γ

B2Σu+→X2Σg+
A2Σ+→X2Π

Peak Position (nm)315; 337.1; 357.7; 375.5;
380.5; 400427.8
306.4; 307.8; 308.916, 17)

2 0

P → 2D

14000

9000

6000

434.1 16, 17)

3000

N2 with H2O
Air with H2O0
303 304 305 306 307 308 309 310 311 312 313 314 315

12000
Intensity (a.u.)

N2 with H2O
N2

12000
Intensity (a.u.)

Species (system)

15000

103(57)

Wavelength (nm)10000

Fig. 12 Emission spectrum of microplasma generated bypositive pulse in N2 gas and in N2 gas with water dropletsfor positive pulses.

8000
6000
4000

8000

2000

7000

N2 with H2O
Air with H2O
N2
Air

6000
280

300

320

340

360

380

400

420

440

Wavelength (nm)Fig. 11 Emission spectrum of microplasma discharge inN2 gas and in N2 gas with water droplets for positive pulses.
H + OH → H2O(10)*
H + O2 → HO2(11)
Thus the peaks of hydrogen could not be detected withoxygen.
The list of detected system and peaks for microplasma in N2
gas and air with water droplets at narrow gratings settings ofthe spectrometer is showed in Table 2.
4.2 Microplasma by positive pulses
Emission spectrum of the microplasma discharge in N2 withwater droplets show peaks of N2 second positive band system(N2 SPS) and N2 first negative band system (N2 FNS) (Fig. 11).
This spectrum was obtained at discharge voltage 1.4 kV, risetime of 80 ns, pulse width of 1.5 µs, and discharge frequencyof 1 kHz.
The emission spectrum is similar in shape with spectrum
obtained at negative pulses.
The peaks of OH for the microplasma discharge at positive
pulses in N2 gas with water droplets are presented in Fig. 12.
An interference optical filter was used to observe the weaksignal.
The filter has the center wavelength at 309.5 nm with a peaktransmission of 16.52%, and FWHM coordinates of 304.315
and 314.676 nm respectively. The results were similar to that ofnegative pulse. Production of H and OH radicals byIntensity (a.u.)

0

5000
4000
3000
2000
1000
0
425

430

435

440

445

450

Wavelength (nm)Fig. 13 Emission spectrum of microplasma discharge in N2
gas or in air with water droplets, and without water dropletsfor positive pulses.
electron impact dissociation of water, according with reaction(3), did not occur without water droplets. Thus OH peaks were
not observed.
H are generated by microplasma discharge in N2 gas and air
with water droplets according to relation (3). Emission
spectrum of hydrogen Balmer line H-γ is shown at 434.1 nm(Fig. 13), using the interference optical filter withcharacteristics described for the emission spectrum presented
in Fig. 10. It shows the peak of H-γ at 434.1 nm only for N2 gas
with water droplets and air with water droplets.
The data showed in Figs. 8 and 12 were measured with thesame sensitivity of ICCD camera (micro channel plate (MCP)and phosphor screen) and also emission spectrum presented in
Figs. 10 and 13 respectively. The peak intensities for the OH
wavelengths and H-γ band system are lower for positive pulses.

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第 34 巻That could be explained by the lower value of peak discharge
current for the positive pulse of 0.9 A (Fig. 4(b)), comparing
with the peak value of discharge current for negative of －2 A
(Fig. 4(a)) at the same discharge voltage. Rise time of negative
discharge pulse voltage is about 3 times faster than that of
positive discharge pulse voltage (about 67 ns for negative pulse,
about 210 ns for positive pulse). Difference of rise timedepends on the MOSFET used in each circuit. It could makethe difference of discharge current peak, even at the samedischarge voltage.
4.3 Estimation of vibrational temperature formicroplasma
The spectral bands of the 0–0 transitions of the second
positive system of N2 (λ=337.1 nm) and the first negative
system of N2+(λ=391.5 nm) could be used for calculating the
reduced electric field strength E/N, where E is intensity ofelectric field and N the concentration of neutral particles, as afunction of their intensities ratio IB(λ=391.5 nm)/IC(λ=337.1
nm).
The reduced electric field strength in air with water droplets
has the value around 500 Td (IB/IC = 0.12) (1 Td = 1 × 10-17Vcm2) for negative pulses (Fig. 6) and around 310 Td (IB/IC =0.04) for positive pulses (Fig. 11) according with Ref. 18).
The vibrational temperature Tvib was evaluated using the N2
second positive system (C3Π - B3Π band). We used the ratio
0-2 and 1-3 bands (λ = 380.5 and 375.5 nm) to calculate thetemperature according to the following formula:ln (I0-2A1-3/I1-3A0-2) = E1-3-E0-2/Tvib(12)
where I0-2, I1-3, A0-2, A1-3, E0-2 and E1-3 are the intensities, thetransition probabilities, and the energy levels for the 0-2 and1-3 bands respectively. The parameters for the calculation
were obtained from reference.19) According to equation (12) wehave obtained for the emission spectra of the microplasma
discharge in N2 gas with water droplets using a positive pulse
power supply (Fig. 11) a value of vibrational temperature Tvib
= 1,450 K.
5. Conclusions
Diagnostics of microplasma are carried out by using a Marx
generator, an ICCD camera and a photomultiplier. Estimationof vibrational temperature was also carried out to investigate
the characteristics of microplasma. The experiments andcalculation data conclude shown bellow:
1) Analysis of emission spectrum shows N2 SPS, N2 FNS,
OH 3064 – Å system and H-γ band system for the
microplasma discharge in N2 gas with water droplets.
2) Emission spectrum of air with water droplets show N2 SPS,N2 FNS, and H-γ band system with lower intensities
comparing with emission spectrum of microplasma discharge

第 2 号 (2010)

in N2 with water droplets due to the quenching effect of oxygen
atom and ozone.
3) Lifetime of emission signal measured by a photomultipliertube corresponding to wavelength 337.1 nm of N2 SPS was
about 40 ns.
4) Emission spectrum intensities of microplasma generated
with positive pulses are lower than the emission spectrum
intensities generated with negative pulses. That could be
explained by the lower discharge current generated at positive
pulses at the same discharge voltage.
5) The calculated value of vibrational temperature of
microplasma was Tvib = 1,450 K.
The factors which contributes to the inactivation processes ofbacteria by using microplasma are known to be high intensity
electric fields, radicals and UV light. Emission spectroscopy
method proved the existence of OH, H radicals and UV lightand could be helpful to understand the processes of bacteria
inactivation.
References
1) K. Shimizu and T. Oda: Sci. Technol. Adv. Material., 2 (2001)
577
2) J. Machala, M. Janda, K. Hensel, I. Jedlovsky, L. Lestinska, V.
Foltin, V. Martisovits, and M. Morvova: J. Mol. Spectrosc., 243(2007) 194
3) F. Iza, G. J. Kim, S. M. Lee, J. K. Lee, J. M. Walsh, Y. T. Zhang,and M. G. Kong: Plasma Process. Polym., 5 (2008) 322
4) M. Tanino, W. Xilu, K. Takashima, S. Katsura and A. Mizuno:
Int. J. Plasma Environ. Sci. Technol., 1 (2007) 102
5) K. Shimizu, M. Yamada, M. Kanamori and M. Blajan: IEEE
Trans. Ind. Appl., 46 (2010) 641
6) T. Heeren, T. Ueno, D. Wang, T. Namihira, S. Katsuki and H.
Akiyama: IEEE Trans. Plasma Sci., 33 (2005) 1205
7) J. L. Walsh and M. G. Kong: Appl. Phys. Lett., 91 (2007)
251504.
8) M. Blajan, M. Kanamori, H. Mimura and K. Shimizu: J. Inst.
Electrostat. Jpn., 33 (2009) 8
9) M. Blajan, T. Ishii, M. Kanamori, H. Mimura, and K. Shimizu:
The 2009 Annual Meeting Record IEE Japan, p.252 (2009)10) R. Foest, M. Schmidt and K. Becker: Int. J. Mass Spectrom., 248(2006) 87
11) E. Kamaratos: J. Quant. Spectrosc. Radiat. Transf., 110 (2009)
26412) S. Zheng, L. Zhang, Y. Liu, W. Wang and X. Wang: Vacuum, 83(2009) 238
13) K. V. Kozlov and H.-E. Wagner: Contrib. Plasma Phys., 47(2007) 26
14) F. Liu, W. Wang, S. Wang, W. Zheng and Y. Wang: J. Electrostat.,
65 (2007) 445
15) R. Ono and T. Oda: Int. J. Plasma Environ. Sci.Technol., 1 ( 2007) 123
16) N. Ezumi, D. Nishijima, H. Kojima, N. Ohno, S. Takamura, S. I.
Krasheninnikov and A.Y. Pigarov: J. Nucl. Material., 266-269(1999) 337
17) C.-H. Tsai, Y.-F. Wang, H.-H. Yang and C.-N. Liao: J. Hazard.
Mater., 150 (2008) 401
18) K.V. Kozlov, H.-E. Wagner, R. Brandenburg and P. Michel: J.
Phys. D: Appl. Phys., 34 (2001) 3164
19) W. Zyrnicki: Spectrosc. Lett., 25 (1992) 175