23G05
Measurement of OH radicals in atmospheric pressure helium plasmausing laser induced fluorescence
レーザー誘起蛍光法による大気圧ヘリウムプラズマジェット中のOHラジカル計測
Seiya Yonemori1, Yusuke Nakagawa2, Ryo Ono1, Tetsuji Oda2
米森星矢1, 中川雄介2，小野 亮1，小田哲治2
Advanced Energy, The University of Tokyo1, Electrical Engineering, The University of Tokyo 2
5-1-5 Kashiwanoha, Kashiwa, Chiba1, 7-3-1 Hongo, Bunkyo-ward, Tokyo2

Two-dimensional distribution of OH radical and air-helium mixture ratio in atmospheric pressure
helium plasma jet were measured using laser induced fluorescence (LIF). The air-helium mixture
ratio in the plasma jet was measured using the quenching rate of OH on excited state. The plasma
jet was generated by an ac high voltage (10 kV peak-to-peaks, 7 kHz). Maximum density of OH
radical was 0.1 ppm and density distribution of OH radicals in the plasma was similar to the shapeof the plasma jet. The results suggest that OH radicals were generated inside of the plasma jet.
1. Introduction
Atmospheric-pressure helium plasma jet is one of
the non-thermal plasma. This plasma is gatheringmuch attention because of its high energy efficiency
and extremely low heat load. These days, many
biological researches were done using this plasma
such as sterilization or cell activation[1.2]. In theseplasma processes, active species generated byplasma such as radicals or ions play important role.
It is thought that this plasma can generate various
active species in air in the reaction of plasma with
ambient air[3]. However, generative mechanism ofactive species and their quantity have not beenmeasured because quenching effect is complex allalong with the mixed air in the plasma.
OH radical plays important role in many plasma
processes because it is chemically reactive. In thisstudy, we measured OH radical density and their
two-dimensional distribution in the helium plasma
jet by laser induced fluorescence (LIF).
2. Experiment2-1. Atmospheric pressure helium plasma jet
Schematic diagram of atmospheric pressure helium
plasma jet is shown in Fig. 1. This shrinking plasma
jet is generated by applying high voltage from thesurface of glass tube in which helium gas flows.
Microscopically, this plasma jet is made of theintermittent propagation of ionization front calledplasma bullet[4]. Plasma bullet is usually ring shapeor sphere and it changes its shape and speed ofpropagation in relation to electrical polarity andwave profile of applied voltage[5]. This uniquepropagation has strong relation with mixed air into

Fig. 1 Schematic diagram of atmosphericpressure helium plasma jet

the plasma and generative mechanism of active
species in the air[6]. In this study, plasma wasgenerated by ac high voltage (10 kV peak-to-peak, 7kHz).
2-2. LIF investigation
LIF investigation has high resolution of time andspace and is used widely in plasma diagnostics.
Energy diagram of OH radical in LIF investigationis shown in Fig. 2. Most excited OH radicals lose
their energy by the quenching effect by mixed air in
the plasma and the decay time of fluorescence ofOH radical strongly depends on air-helium mixture
ratio in the plasma jet. The equation of relation
between air-helium mixture ratio and the decay timeof fluorescence of OH radical is.

(1)

The ILIF is signal intensity of fluorescence of OH, xFig. 2 Energy diagram of OH radical

is air-helium mixture ratio, Q is quenching rate
coefficient and Qv is vibration energy transfer (VET)
rate coefficient. In equation(1), x is the only variable
number, therefore, air-helium mixture ratio can be
estimated from the decay time of ILIF. In this study,
air-helium mixture ratio can be simultaneously
measured with the density of OH radicals in eachposition.
3. Experimental conclusion
Two-dimensional distribution of OH radical isshown in Fig. 3. It shows that OH density decreases
with increasing the distance from center of theplasma jet. Shrinking distribution area of OH radical
matches shape of the plasma jet, therefore, it also
shows most of OH radicals exist inside of plasma jet.
Maximum OH density in the plasma jet wasapproximately 0.1 ppm.
Fig.4 shows two-dimensional distribution of
air-helium mixture ratio. The mixture ratio increases
with increasing the distance from center of theplasma jet as well as OH density distribution. On theinside surface of the plasma jet, it revealed thatapproximately 80 percent of composing gas ofplasma is the mixed air. This result qualitatively
matches the result of fluid calculation in previousresearch[7].
4. Conclusion
Two-dimensional distribution of OH radical andair-helium mixture ratio were successfully measured
by LIF investigation. The air-helium mixture ratio is
one of the most important factors for this plasma jet
because it has strong relationship with generation ofvarious active species and propagation of plasmabullet in the air.
The method used in this study revealed useful
because two important parameters, the air-helium
mixture ratio and absolute density of active species,
can be measured at once.

Fig. 3 Two-dimensional distribution ofOH radical(The unit in a right chart is the number of OH radicals
per cubic centimeter)

Fig. 4 Two-dimensional distribution of air-helium
mixture ratio(The number in right chart is the air ratio versus
helium in plasma)

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