S3-1

Characteristics and Application of Atmospheric-Pressure
High-Density Non-thermal Plasma Produced in Microgap
微小ギャップにより生成される高密度大気圧非平衡プラズマの特性と応用
A, Kono, J. Wang and M. Aramaki
河野明廣，王 剣亮，荒巻光利
Department of Electrical Engineering and Computer Science, Nagoya University,
Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
名古屋大学工学研究科電子情報システム専攻，〒464-8603 名古屋市千種区不老町
The basic characteristics of a high-pressure high-density non-thermal plasma produced in a microgap by
microwave excitation are discussed on the basis of optical diagnostics and heat transport simulation. Whenconsidering an application to a VUV excimer light source, the gas temperature is one of the most importantparameters. The simulation indicates that a strong non-gravitational convection is induced in the gas
because of a very large temperature gradient between the plasma center and the wall. The convention
appears to be an essential feature of small-sized high-pressure high-density plasmas.

1. Introduction
Recently atmospheric-pressure non-thermal
plasmas attract considerable attention because ofvarious potential applications arising from
non-necessity of vacuum systems and from possiblevery high active species densities in the plasma.
Microwave excitation is one of the effective
methods of producing such plasmas. We are
studying a high-pressure high-density non-thermal
plasma produced by microwave excitation in the
microgap between two knife-edge electrodes,
aiming at a VUV light source application. In thispresentation, we report basic diagnostic results ofthe discharge (electron density, electrontemperature, and gas temperature) and the results ofheat transport simulation. Clarifying the heattransport mechanism and the gas temperaturecharacteristics should be essential for understanding
the properties of high-pressure high-density plasmas.
2. Microgap plasma excited by microwave
Diffuse high-pressure high-density plasma can beproduced continuously in a microgap between two
knife-edge electrodes. Figure 1(a) shows thedimension of the electrodes (made of copper) weare currently using, which are attached at the end of
the microwave strip-line structure [1]; the length (inthe z direction indicated in the figure) of theelectrodes is 10 mm. In 1-atm air discharge, dc andrf (13.56MHz) excitation lead to constricteddischarge and microwave excitation was essential
to obtain a plasma uniformly extending over theentire electrode width. We suppose that thedifference is caused by the impedance effect; in the
microwave frequency (2.45 GHz) the impedance ofthe constricted discharge path is large and should

not be suitable for sustaining the discharge.
For the discharge in ambient air at a microwave
power of 100 W, Thomson scattering measurements
indicated that the electron density profile in the x
direction has a FWHM of ~200 µm, with the peakelectron density as high as 1.8 × 1015 cm-3 [2]. Notethat a total power of 100W, if divided by themicrogap volume of 100 µm × 100 µm × 10 mm,
corresponds to a power deposition density of 1
MWcm-3. The gas temperature in the microgap
estimated from the optical emission spectrum of theN2 C3Πu-B3Πg band was ~2000K. For atmosphericpressure He discharge (containing 5% N2) at amicrowave power of 100W, the gas temperature
was reduced to ~1200K. The dependence of the gastemperature on the microwave power as well as on
the forced gas flow rate through the microgap was
not well understood when only the diffusive heat
conduction was considered as the heat transportmechanism [3].

Fig. 1 Microgap discharge configuration and thesimulation space for heat transport study.

Fig. 2 Flow vector diagram in the central part of thesimulation space for He without forced gas flow (Q0=0.5 MW/cm3).

Fig. 3 Gas temperature at the center of the heating zone
as a function of Q or Q0 for He and N2 gas in cases of (a)static gas, (b) flowing gas with Q=constant, and (c)flowing gas with Q=Q0ng/n0. Experimental results for air
discharge in the microwave power range of ~30~100W
are also shown, where the horizontal position of the plotis arbitrary chosen.

3. Heat transport simulation
To understand the gas temperature characteristicsof the microgap discharge, a heat transport
simulation based on a complete set of fluid dynamic
equations consisting of mass, momentum, andenergy balance equations was carried out. Figure1(b) shows the simulations space, in which thecentral rectangular zone is the heat source
simulating the plasma. It is assumed that in thiszone the gas is given either constant heat Q(W/cm3) or heat proportional to the local gasdensity, Q=Q0ng/n0 (ng is the local gas density, n0Fig. 4 Maximum and central gas temperatures as afunction of the forced gas flow rate for He ( =0.5
MW/cm3), together with experimental values for
He/N2(5%) discharge at a microwave power of 100W.

the gas density in the standard condition [1atm, 273K],
and Q0 the constant). Temperature dependence of theheat conductivity and viscosity is taken into account.
Figure 2 shows the flow vector diagram in thesteady state without forced gas flow, indicating theexistence of strong natural convection; since we do
not include gravitational force in the governing
equations, the convection has a purely thermal origin.
Figure 3 shows the dependence of the gas
temperature on the heat source intensity, together withthe experimental gas temperature for air discharge as afunction of the microwave power. The figure indicatesthat the natural convection affects the gas temperaturesignificantly and the experimental results are betterexplained by the assumption Q=Q0ng/n0.
Figure 4 shows the dependence of the gas
temperature on the gas flow rate. For flow rates above
1 L/min, the simulation reasonably well explains theexperimental results. For low flow rates (<1 L/min)the simulated gas temperature is nearly constantbecause of the effect of natural convection; thediscrepancy with the experimental results in thisregion is probably due to ambient air admixture in the
discharge because of low flow rates.
Thus the simulation with the assumption Q=Q0ng/n0
reasonably well explains the experimental results,indicating a significant effect of gas dynamics on theheat transport. This appears to be an essential feature ofsmall-sized high-pressure high-density plasmas. (Worksupported by a Grant-in-Aid for Scientific Research on
Priority Areas [15075205] from MEXT Japan)[1] A. Kono, T Sugiyama, T. Goto, H. Furuhashi, and Y.
Uchida: Jpn. J. Appl. Phys. 40 (2001) L238.
[2] A. Kono and K. Iwamoto: Jpn. J. Appl. Phys. 43 (2004) L1010.
[3] A. Kono, J. Wang and M. Aramaki: submitted toThin Solid Films (Proc. 7th APCPST, Fukuoka, 2004).