Emission Spectroscopy of Microplasma for NOx Removal Process
Marius Blajan ,Tatsuya Ishii, Hidenori Mimura, and Kazuo Shimizu
Innovation and Joint Research Center, Shizuoka University, 3-5-1 Jyouhoku, Hamamatsu, 432-8561, Japan, E-mail：shimizu@cjr.shizuoka.ac.jp

Introduction
Microplasma can be found in many applications. In thelast years, the technology was used also for NOx
removal. Although there is an interest for application
driven research, microplasma phenomena are not fully
understood. Emission spectrometry is one of themethods to analyze plasma process.
The aim of this paper is to analyze the emission
spectrum of the microplasma in a simulated exhaust gas.
Emission spectra of the microplasma discharge in N2,NO and O2 gas mixture was analyzed. The diagnosis of
microplasma discharge was performed in order tounderstand the processes of NOx removal. An
experimental Marx Generator was used as a pulsed
power supply.
Temperature measurements are also carried out, whichshow the temperatures are specific for non-equilibrium
plasmas.

Experimental setup

Fig.4 Microplasma electrodes.

In NO 950 ppm and O2 5% in N2 balance. intensities ofpeaks are lower than those measured for nitrogen, due
to the quenching effect of O2 .Weak peaks were alsomeasured for the NO γ band. That could be explained bypresence of O2 molecules to oxidize to remove NOmolecules.
Excitation of nitrogen molecules in the ground state bydirect electron impact:►the lifetime of photomultiplier signal of N2 SPS was
about 40 ns
NO γ band is excited by the collision of N2 metastablestates:►process of light emission of NO γ band (260.1 nm) islonger, thus the lifetime of photomultiplier signal for NO γband was about 2 µs (Fig. 8).

Emission spectrum of microplasma
Emission spectrum of the microplasma discharge wasmeasured in N2 gas, in NO 1000 ppm/N2 as balance gas
and in NO 950 ppm and O2 5% in N2 balance.
Spectrum were obtained at discharge voltage -1.4 kV,
rise time of 80 ns, pulse width of 500 ns, and dischargefrequency of 1 kHz.
(a)Fig.1 An experimental setup.
Emission spectra were measured by an ICCD camera,a spectrometer and by a photomultiplier tube. A pulse
generator was used to trigger the Marx Generator andthe ICCD camera.
Compositions of simulated gases used in experiments
were N2, NO 1000 ppm in N2, and NO 950 ppm, O2 5%
in N2. Experiments were carried out in atmosphericpressure and the gas flow rate was set at 2 L/min.
Concentration of NO and NOx were measured by a NOxanalyzer.

(b)

Fig.8 Waveforms of discharge voltage, discharge
current and emission signal of microplasma (N2SPS 337.1 nm and NO γ band 260.1 nm) in N2,NO 1000 ppm / N2 as balance gas and in NO
950 ppm and O2 5% in N2 balance .

(c)

Fig.2 An experimental Marx Generator.
A Marx generator with 4 stages MOSFET switches:
output voltage up to -1.8 kV peak (negative pulse, risetime 80 ns, pulse width 500 ns ～ 1 µs, frequency 1 kHz～24 kHz).
Trigger signal for ICCD camera was set at 1 µs (Fig. 3).
Capacitors are charged in parallel connection at a given
voltage V (500 V in this case).
MOSFET switches are closed, capacitors aredischarging in series connection：►output voltage 4V (2 kV in this case).

CalculatedExperimental

Fig.5 Emission spectrum of microplasma
discharge (a) in N2; (b) in NO 1000 ppm/N2 as
balance gas; (c) in NO 950 ppm and O2 5% in N2balance.
Table 1. List of detected systems and peaks for themicroplasma in N2 gas

Species (system)

Transition

N2 second positive

C3Π→B3Π

N2+ first negative

B2Σu+→X2Σg+

Peak Position (nm)315; 337.1; 357.7;
375.5; 380.5; 400
427.8

Table 2. List of detected systems and peaks for themicroplasma in NO 1000 ppm/N2 as balance gas

Fig.3 Waveforms of discharge voltage, discharge
current and gate signal for ICCD camera.

About microplasma

Microplasma electrodes are metallic electrodes covered
with a dielectric layer (Fig. 4).
Small discharge gaps (0∼100) μm and assumed specificdielectric constant of εr = 104：►a high intensity electric field (107 ∼ 108 V/m) around 1kV.
Electrode size was 20 mm versus 40 mm. Electrode has
holes to flow for gas treatment, which diameter is Ø 2mm
and its aperture ratio of 36%. Discharge gap was set at
50 μm.

Species (system)

Transition

N2 second positive

C3Π→B3Π+2+2

N2 first negative

B Σu →X ΣgNO γ bandA2Σ+→X2Π+Peak Position (nm)315; 337.1; 357.7;
375.5; 380.5; 400427.8
226.9; 237; 247.9;
259.6;
260.1;
271.5; 285

Table 3. List of detected systems and peaks for themicroplasma in in NO 950 ppm and O2 5% in N2balance.

Species (system)

Transition

N2 second positive

C3Π→B3Π

N2+ first negative

B2Σu+→X2Σg+

Peak Position (nm)315; 337.1; 357.7;
375.5; 380.5; 400427.8Fig.9 Determination of microplasma
temperatures for N2 SPS (337.1 nm).
Table 4. List of temperatures for microplasma
discharge in N2 gas.
Temperature
Rotational
Vibrational
Translational
Electronic

Value (K)3603100
360
8200

Conclusion
1)Analysis of emission spectrum shows
N2 SPS, N2 FNS for microplasma in N2 gas.
N2 SPS, N2 FNS and NO γ band. for the microplasma
in NO 1000 ppm in N2 and N2 SPS, N2 FNS, withlower intensities comparing with emission spectrum ofmicroplasma in N2 for NO 950 ppm and O2 5% in N2.
2)Lifetime of emission signal measured by aphotomultiplier tube corresponding to wavelength337.1 nm of N2 SPS was about 40 ns. Lifetime ofemission signal of NO γ band was about 2 µs, due tothe excitation of N2 metastable states.
3)Temperature estimation of microplasma discharge
show high electron and vibrational temperatures andlow rotational and translational temperatures. Thesetemperatures are specific for non-equilibrium plasmas.