S16-7
Measurement of Radicals in Plasma for Pollution Control andIts Future Prospects
環境応用プラズマのラジカル計測と今後の展望
Ryo Ono小野亮
The University of Tokyo
7-3-1, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan東京大学 〒113-8656 東京都文京区本郷7-3-1
Measurement of radicals in a pulsed streamer discharge is introduced in this paper. Radicals (O3, OH, O, N,N2(A3Σu+), NO, O2(v), and N2(v)) were measured using laser spectroscopy. The pulsed streamer discharge
is one of discharges used for pollution control. The laser diagnoses show densities, reaction rates, andspatial distributions of radicals. Streamer simulation that can reproduce the measurement results is also
introduced, and future prospects is described.

1. Introduction
In gaseous pollution control using a plasma,
the pollutants are removed by reaction withradicals. The radicals have been measured in theplasma and their densities, reaction rates, andspatial distributions have been investigated. This
paper describes the radical measurements in apulsed streamer discharge, which is often used forthe pollution control, using laser spectroscopy.
Then, a streamer simulation that can reproducethe measurement results is introduced, and future
prospects is described.
2. Measurements of Radicals
The author’s group has measured various radicals
in pulsed streamer discharge. Table I shows the listof our measurements. Laser-induced fluorescence(LIF), Two-photon absorption LIF (TALIF), andcoherent anti-Stokes Raman scattering (CARS)
have been used for the measurements. Temperatureis also measured from rotational temperature ofradicals [4, 5, 15]. Shock waves and density ofambient gas in streamer channels were alsomeasured using schlieren and shadowgraph [16, 17].
Figure 1 shows an example of the measurements. It
shows one-dimensional distributions of OH density
and gas temperature between the needle-plateelectrodes of 13 mm distance. The streamer
discharge occurs in a humid air with 100 ns, 32 kV
pulse operated at 1 pps [4]. The reaction rate of OH
radicals can be estimated from the decay rate of OH
density shown in Fig. 1. The rise in temperature
afterdischarge
pulse
iscaused
byvitration-to-translation energy transfer from N2(v).
The higher OH density near the needle electrodethan near the plate electrode indicates that OH

Table I. Radical measurements in pulsed streamer
discharge by the author’s group.

Radicals
Measurement method
O3Laser absorption
OHLIF
O
TALIF
N
TALIF
N2(A3Σu+) LIF
NO
LIF
O2(v)
LIF
N2(v)
CARS
Temperature
Schlieren and shadowgraph

Refs.
1, 2
3-5
6, 78, 910, 11
12
13144, 5, 1516, 17

radicals are mainly produced in the secondary
streamer, not in the primary streamer. In this way,
the radical production process can be estimatedfrom the LIF results. It was found that the N atom
density is proportional to the square of dischargeenergy, while other radical densities areproportional to the discharge energy [9]. It indicatesthat N atoms are produced via two-step processwith intermediate N2(v) [18].
3. Simulations
The author’s group has developed a simulation ofpulsed streamer discharge [18, 19]. It can reproduce
some of the measurement results: e.g. O, N, and OH
production in the secondary streamer, two-step
production of N atoms, saturation of OH production
against ambient humidity, and streak photograph ofstreamer propagation. It can analyze major
production and reaction paths of radicals [18, 19].

Fig.1. One-dimensional distributions of OH density and gas temperature between needle-plate electrodes [4].

4. Future Prospects
There are some measurements that should beconducted in future work. Measurement andsimulation should be conducted under conditions
similar to actual plasma reactors: the shape ofreactor (coaxial reactor, not a point-to-planeelectrodes), repetitive high-voltage pulses (not asingle-pulse), and use of decomposition target gas
in the ambient gas such as volatile organiccompounds. Measurements in surface discharge [2]is also important because it occurs in dielectricbarrier discharge and packed-bed type reactor.
Clusters and tens of other species than those listed
in table I should be measured. The radial
distribution of radicals in the streamer channels isalso important. Since the electron energy is notradially uniform in the streamer, the radial
distribution of radicals depends on species [20].
5. Conclusions
Radical densities in a pulsed streamer discharge
have been measured using laser spectroscopy. A lotof data have been accumulated and a streamer
simulation has been developed that can reproduce
some of the measured results. The behavior ofradicals has been clarified gradually, but it is not
sufficient. The pollution control technology needs
more measurement data on radicals and a precise
simulation that can reproduce the measurement
results.
References[1] R. Ono and T. Oda: J. Phys. D: Appl. Phys., 37(2004) 730.
[2] R. Ono and T. Oda: J. Phys. D: Appl. Phys., 40(2007) 176.
[3] R. Ono and T. Oda: J. Phys. D: Appl. Phys., 35(2002) 2133.
[4] R. Ono and T. Oda: J. Phys. D: Appl. Phys., 41(2008) 035204.
[5] Y. Nakagawa, R. Ono, and T. Oda: J. Appl. Phys.,
110 (2011) 073304.
[6] R. Ono, Y. Yamashita, K. Takezawa, and T. Oda: J.
Phys. D: Appl. Phys., 38 (2005) 2812.
[7] R. Ono, K. Takezawa, and T. Oda: J. Appl. Phys.,
106 (2009) 043302.
[8] R. Ono, Y. Teramoto, and T. Oda: Jpn. J. Appl. Phys.,
48 (2009) 122302.
[9] Y. Teramoto, R. Ono, and T. Oda: J. Appl. Phys., 111(2012) 113302.
[10]R. Ono, C. Tobaru, Y. Teramoto and T. Oda: Plasma
Sources Sci. Technol., 18 (2009) 025006.
[11]Y. Teramoto, R. Ono, and T. Oda: J. Phys. D: Appl.
Phys., 42 (2009) 235205.
[12]Y. Teramoto, R. Ono, and T. Oda: IEEE Trans. FM,
131 (2011) 553 (in Japanese).
[13]R. Ono and T. Oda: Plasma Sources Sci. Technol., 18(2009) 035006.
[14]Y. Teramoto and R. Ono: J. Appl. Phys., 116 (2014)073302.
[15]R. Ono, Y. Teramoto, and T. Oda: Plasma Sources
Sci. Technol., 19 (2010) 015009.
[16]R. Ono and T. Oda: Jpn. J. Appl. Phys., 43 (2004)
321.
[17]R. Ono, Y. Teramoto, and T. Oda: J. Phys. D: Appl.
Phys., 43 (2010) 345203.
[18]A. Komuro, R. Ono, and T. Oda: J. Phys. D: Appl.
Phys., 45 (2012) 265201.
[19]A. Komuro, R. Ono, and T. Oda: J. Phys. D: Appl.
Phys., 46 (2013) 175206.
[20]A. Komuro and R. Ono: J. Phys. D: Appl. Phys., 47(2014) 155202.