22E10
LIF measurement of OH radicals in coaxial cylinder barrier discharge forclarification of toxic gas decomposition
有害ガス処理の機構解明に向けた同軸円筒バリア放電下のOH-LIF計測
Yusuke Nakagawa, Ryo Ono and Tetsuji Oda
中川雄介, 小野亮，小田哲治
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
Behaviour of radicals in non-thermal plasma is worth clarifying in order to interpret the chemical reaction
processes in the plasma gas treatment. Inside the coaxial cylinder dielectric barrier discharge (DBD) reactor,
the spatial distribution of relative density and rotational temperature of OH radicals are measured in pulsed
barrier discharge under atmospheric pressure using laser-induced fluorescence (LIF) method. OH radicals
are produced in the secondary streamer region. The OH rotational temperature increases withintens-hundreds of microseconds, caused by vibrational energy relaxation. The temperature in the coaxial
DBD reactor rises slower and reaches lower than that in needle-plane corona discharge. There is adifference in temperature rising pace between near the anode and center of the discharge gap. It indicatesthat the acceleration of vibrational energy transfer mainly occurs in the secondary streamers.

1. Introduction
Since air pollution with gaseous contaminants is aserious problem today, it is desirable that
decomposition technique of toxic gas with
non-thermal plasma (NTP) be put to practicaluse[1,2]. Our group has been developed the NTP
technique for decomposition of VOCs (volatileorganic compounds). In this field, recent reports
have shown that radicals play a major role inchemical reactions inside NTP. However, the radical
measurement in the coaxial cylinder DBD reactor isunder development yet. In this study, laser-induced
fluorescence (LIF) measurement of OH(X) isperformed in the coaxial cylinder DBD reactor with
the injection of decomposing target, evaluating theeffect of OH radicals on VOC decomposition.

2.2 Experimental setup
Experimental system is represented in Fig. 1.
Optical parametric oscillator (OPO laser) generates
562 nm fundamental laser beam, which is converted
to 282 nm by SHG. The trigger of the laser pulse is
obtained from pulse generator and delayed τ [s]from the trigger of the discharge so that we canobserve the OH density in afterglow period. The
fluorescence passes through frequency filter and is
detected by photon-multiplier tube (PMT).
2.3 Structure of the reactor
The reactor consists of high voltage (HV) central
trigger
Pulse generator

HV generating circuit

trigger

Time delay τPlasma reactor
SHG

Optical Parametric
Oscillator

Collimator lensFrequency filter
Focus lens

PMT

2. Experiments
2.1 Laser-induced fluorescence
An optical parametric oscillator (OPO) is used toexcite OH radicals in the X 2Σ (v=0) ground state tothe A2Π (v=1) excited state by 282 nm transition[3-5]. We measure the fluorescence of 309 nm and315nm. The fluorescence intensity ILIF is
proportional to the incident laser intensity IL andground state density n0,AQA (1)I LIF = cBI L n0  0 v + 1  Γ1Γ0 Γ1 
where c is experimental constant, B represents
the absorption coefficient of X2Π(v=0)-A2Σ(v=1)
transition, A and Γ represent emission andrelaxation coefficients. the indices 0 and 1 arerelated to A2Σ(v=0) and A2Σ(v=1), respectively. Qv
is vibrational relaxation constant. With thesecoefficients of each state [5-8], we can determine
the OH density from fluorescence intensity.

Fig.1

oscilloscope

Experimental setup.

Quartz glass and ground

5mm

HV center electrodeFig.2

Relative OH density [arb. unit]

1010

Photograph of barrier discharge.

0TCE: 0ppm
50ppm
100ppm200ppm500ppm

−1

1001

2

10
10
Postdischarge time [µs]Fig.3 Time evolution of OH radicals(V = 28kV, r = 2.5 mm and humidity is 2.3%).

electrode, discharge space, quartz glass tube andouter ground electrode. The pulsed high voltage
V[kV] is supplied with spark-gap switch. The
stainless M8 screw shaft is used for the central
electrode in order to be favourable for discharge.
The quartz tube has 18 mm inside diameter, the gap
between anode and glass is 5 mm, the length of thereactor is 190 mm. The 282 nm excitation laser
beam is parallel to the electrodes and has
rectangular cross section of 1 mm × 3.5 mm. The
fluorescence is collected perpendicularly to thelaser beam direction. The distance between the HVelectrode and the laser beam is defined as r [mm].
The spatial distribution of OH can be obtained bymeasuring LIF signal at various r. Humid air(O2(20%)/N2) flows in the reactor at a rate of2L/min. Decomposing target is injected to the gas
flow with its boiling temperature. The discharge
emission photograph at V = 28 kV, humidity: 2.3%
is shown in Fig. 2.

3. Results and discussions
Previous study often reports that humidity in thedecomposition
backgroundsuppresses
thedecomposition efficiency of trichloroethylene(TCE)[9]. Therefore it may be suggested that OH
radicals does not decompose TCE. In order toclarify the OH influence upon TCE decomposition,
we measured the time evolution of OH radicals
with various TCE densities in the background gas.
The results are shown in Fig. 3. The discharge
condition is V = 28kV, r = 2.5 mm and humidity is2.3%. The OH decay rate is quickened withincreasing TCE concentration. This indicates OH
radical definitely reacts with TCE.
The reason for suppression of TCE
decomposition with increasing humidity isestimated below;
1. OH radicals decompose TCE.
2. However, OH radicals strongly react with O
radicals which significantly decompose TCE.
3. As a consequence, O reduction by OH has
moreinfluence
thandirect
TCE
decomposition by OH, leading to suppressionof decomposition efficiency.
4. Conclusion
Time evolutions of OH density are measured by
LIF in pulsed barrier discharge under atmosphericpressure. The practical coaxial cylinder DBD
reactor is used in order to simulate the discharge
form in the gas treatment. LIF using 282nm
excitation reveals that OH radicals react with TCE.
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