S10-2Diagnostics and Modeling of Microplasmas
マイクロプラズマの診断とモデリングKunihide Tachibana
橘 邦英Department of Electronic Science and Engineering, Kyoto University,
Kyoto-daigaku Katsura, Nishikyo-ku, Kyoto 61508510, Japan
京都大学工学研究科電子工学専攻，〒615-8610 京都市西京区京都大学桂Optical emission spectroscopy (OES), laser absorption spectroscopy (LAS) and laser Thomson scattering(LTS) methods have been applied to the diagnostics of dynamic behaviors of microplasmas in a PDP cell.
A method is also being performed for analyzing accumulated charges on the surface by using Pockels
effect. In the simulation parts, a phenomenon of electron bunching in laser produced plasma has beenanalyzed by a particle code. Phenomena in super critical fluid plasma are also being analyzed theoretically.

Address electrode(a) OES(b) LAS

Fig. 1 Spatiotemporal behavior of excited Xe*(2p,1s) atoms in PDP discharge by 3D observation.

a)6.E+194.E+19

2.E+190.E+00-1-0.75

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2. Diagnostics
2.1. Dynamic behaviors of excited species
Excited Xe atoms in unit PDP discharge cell has
been diagnosed by optical emission spectroscopy(OES) and laser absorption spectroscopy (LAD) for
their spatiotemporal behavior. A special cellstructure of realistic scale was developed for thethree-dimensional observation, and the effect of theauxiliary third-electrode was investigated for theimprovement of the production efficiency ofexcited atoms as shown in Fig. 1 [1]. Anotherdiagnostics has been performed on an integrated
structure of microplasmas to realize a large area
plasma source operated at around atmosphericpressure for various material processing. In addition
to the LAS measurements, a mm-wave
transmittance technique is used for the estimation ofelectron density ne.
2.2. Measurements of plasma parameters
Laser Thomson scattering （LTS） technique has
been used by Uchino et al to measure directly neand electron temperature Te profiles in a plasma of
PDP-like discharge [2]. The spatial resolution ofLTS has been improved to be around 50 µm, andthe measurements have been performed at heights
as close as 60 µm from the electrode surface. The
configuration of the electrodes used in this study
were similar to those of sustain electrodes of
coplanar ac-PDPs. Coplanar electrodes of 1 mm
in width were built up on a glass substrate of 50
Electron density (m )1. Introduction
Properties of microplasmas can be characterized
not only by their down sizing from mm to µm but
also by their relatively high plasma densities whichrange over 1013 cm-3 in collision dominant
atmospheres. In addition, the plasma-wallinteraction becomes important, given that thecharges accumulated on the surface can strongly
affect the discharge and plasma behaviors.
Diagnostics and modeling should aim to clarify
those specific properties of microplasmas. Several
measurements have been performed using
spectroscopic methods, only by which such small
spaces can be accessible. Simulations in fluid andparticle codes have also been developed. An
overview of those approaches in our subgroup ofthe MEXT microplasma project will be introduced.

b)
21AnodeCathode0-1-0.75-0.5-0.25

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1from the gap)

Fig.2 (a) Electron density and (b) electron temperaturein PDP-like discharge measured by LTS.

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KDPmm in length and 2 mm in width. The LTS
measurement was performed at a height of 100 µm
above the electrode surface, because vertical
distributions of ne at almost all positions along theelectrode had peaks at around this height for thedischarge in the gas mixture of Ne/Ar (10%) at apressure of 200 Torr. The results are shown in Fig.
2. From the figure, modulations in ne and Te areclearly observed above the anode side. Five striatedpeaks can be easily distinguished. The distances
between the density and temperature peaks are
100-150 µm. It is noted that the modulation in Te isout of phase from the modulation in ne.
2.3. Diagnostics of plasma-surface interactions
The wall charges accumulated on the dielectricsurface play an important role in a gas discharge,
especially in dielectric barrier discharge (DBD).
Sakurai et al have developed a technique to measure the wall voltage in a DBD using an electro-optic
nonlinear crystal as follows [3]. One of thedielectric barriers was made by a z-cut KDP crystal
with a rectangular cross section of 25 x 30 mm anda thickness of 2 mm. An actual discharge volume is
20 x 10 x 3 mm. A pure Ar gas was filled in thedischarge space at a pressure of 3 Torr. A pulsed
power supply was connected to metal conductors
which were in contact with ITO. A laser beam witha circular polarization passes through the barrierdischarge and the KDP in the direction normal tothe surface. The polarization of transmitted laser
beam changes due to a surface voltage on KDP. The
dynamic behavior of the wall voltage due toaccumulated charges is observed from a spatial andtemporal measurement of the polarization change.
An example of the results is shown in Fig. 3. The
positions 0 and 10 mm indicate the edge of theconductor ITO and the end of discharge space basedon a boundary, respectively. It is found that thedischarge progresses in a local space at region from
2 to 6 mm during time from 6 to 6.5 µs. After theend of discharge, the charges accumulate on theKDP surface uniformly.

010POSITION[mm]

Fig. 3 Time evolution of wall(mm)voltage according toaccumulated charge on cathode surface.

Laser Pulse

e-BunchT=Tbegin

T=0

Fig.4 Mechanism of electron bunch compression andacceleration

3. Modeling3.1. Modeling of micro bunch formation
A phenomenon of electron bunch generation wasstudied Kawata et al using a 3-dimensional particle
simulation. A focused short pulse laser of TEM(1,0)+TEM (0,1) mode has two intensity peaks in
the radial direction, so that the transverse
ponderomotive force can trap electrons between thetwo peaks [4]. At the same time the longitudinalponderomotive force accelerates electrons at thehead of the laser pulse, when the laser is focused(see Fig. 4). When the electrons move to the laser
tail, the laser diverges and the electron deceleration
becomes relatively weak. Their numerical analysis
demonstrated that electrons are effectively trappedby the laser potential well, and that at the same time
the acceleration by the longitudinal ponderomotive
force induces the electron bunch compression. This
trapping and compression mechanism is unique: theelectron bunch can be compressed to the scale ofthe laser pulse length. Thus, the electron bunch isconfined well in transverse and compressed
remarkably in longitudinal
3.2. Modeling of supercritical fluid (SFC) plasma
It was found by Terashima et al that thebreakdown voltage drops drastically in CO2 near
the super critical condition. In order to explain thisphenomenon theoretically, our theoretical groups
are working on the modeling of the discharge. In
the first step, clusters of CO2 molecules arestudied by Senda et al using quantum chemistrycalculation to derive the cluster structures andionization potentials of (CO2)n. In the second step,
the electron swarm characteristics in SCF is studied
by Hamaguchi et al by taking into account thequantum statistical fluctuation in the density whichcauses the electron channeling in SCF media.
References[1] K. Tachibana et al: J. Phys. D 36 (2003) 2928.
[2] K. Uchino et al: J. Appl. Phys. 91 (2002) 613.
[3] T. Sakurai et al: J. Phys. D 36 (2003) 2887.
[4] S. Kawata et al: Phys. Rev. E 69 (2004) 056502.