S10-1

Generation and Control Techniques for Microplasmas
マイクロプラズマの生成と制御
Shozo Ishii
石井彰三
Department of Electrical and Electronic Engineering, Tokyo Institute of Technology,
2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan
東京工業大学理工学研究科電気電子工学専攻，〒152-8552 東京都目黒区大岡山2-12-1

Solid, liquid, and gaseous materials with a small volume were used to generate microplasmas by electricaldischarges. Atmospheric glow discharge developed in a dc discharge with a miniature gas flow from anozzle electrode. Pulsed discharge mode appeared also in the discharge. When a microdrop of ethanolcame into the gap between the needle electrodes, where dc voltage was applied, ultra-fast pulsed discharge
developed and the spherical microdrop deformed to spray or a filament of liquid. The efficiency of energy
input to powder particle discharges was improved by increasing the rate of current rize.

1. Introduction
Microplasmas are generated by electricaldischarges between the electrodes where a material
with a small volume is placed in open space. We
name it a “mass-limited mode” of the discharge inwhich the volume of microplasmas is determinedby both the mass of initial material and theseparation of the electrodes, because there is nosurrounding wall. Where spatially isolated small
structure is obtainable, any state of matter such as
solid, liquid, or gas can be used in this mode. To
generate microplasmas, we used a powder particle
for solid, a microdrop for liquid, and a miniature
gas-flow. We characterize these three methods ofmicroplasma generation.
2. Experimental Set-up
In the powder particle discharge[1], a copper
particle with a diameter of 100 µm was placed
between tungsten electrodes of 300 µm in diameter.
The discharge was powered by a capacitor of 200
nF charged to 3 kV. In the microdrop discharge[2],a microdrop of ethanol generated by a microdispenser (MJ-020 by MET Co.) was dropped into
the discharge space. The droplet was sphere withthe diameter of 300 µm at the minimum volume of
10 nl. A needle-to-needle and a sphere-to-sphere
electrode system were used. The discharge
appeared when the microdrop came near to theelectrodes. In the miniature gas-flow discharge[3],helium or argon flowed longitudinally from astainless nozzle with a 200 µm in inner and 350 µmin outer diameter to stainless mesh of 350
mesh/inch. Both the nozzle and the mesh worked as
the electoredes, of which separation was varied
from 100 to 1000 µm. The discharge was poweredby a regulated dc power supply with a ballast
resistor ranging from 0.1 to 5 MΩ  connected inseries to the nozzle electrode. All of theexperiments were carried out in air.
3. DC Glow Discharge in Air
An atmospheric glow discharge is observed in
the dc discharge with the helium gas flow. There areboth bright and dark regions near the mesh cathode.
A uniform bright region appears between the anodeand the dark space. Its length increases withincreasing the electrode separation. On the contrary,
the lengths of the bright and dark region near thecathode do not change. The profile shows the glow
discharge structure, which consists of a cathodeglow, a Faraday dark space, and a positive column.
When the electrode separation is varied from 200 to1000 µm, the length of the positive column
increases linearly.
The sustaining voltage increases with increasingthe discharge current. The average power of thedischarge ranges from 0.2 to 0.5 W. The voltage
increases and the current decreases with increasing
the electrode separation. The intensity of helium
atomic lines emitted from the cathode glow isstronger than that from the positive column. At both
the regions, the emission spectra of nitrogenmolecules are observed. The gas flow affects boththe lengths of the positive column and the cathoderegion. We observed their polarity effect by thedirection of the gas flow. The change in length ofthe positive column is not clear for the electrodeseparation of 500 µm, when the helium flows
toward the mesh cathode. However, it ispronounced for the discharge with the shorter
electrode separation.

4. Self-driven Pulsed Mode of Microdischarges
Pulsed discharges develop in the gas flow and themicrodrop discharge without any high-voltage pulse
generator. The dc discharge with the gas flow wasoperated in such a way that the applied voltage wasincreased always from zero. When gas breakdown
is initiated during the voltage increase, the voltage
across the electrode shows a sudden drop and then apulsed current with a peak of tens milliampere
flows. The pulse-width of the current is tens
nanosecond. The voltage starts to increase
exponentially after the pulsed discharge has ceased.
When the voltage recovers to the breakdown level,
the pulsed discharge appears again. When theapplied voltage is kept constant which is more thanthe breakdown voltage, the successive process
mentioned above takes place repeatedly. The pulsed
discharge is powered by parasitic capacitance,
which is charged through the ballast resistance withan RC time constant related to the repetition period.
The parasitic capacitance is determined to be 12 pF
in our experiment. Fig.1 shows the dependence ofthe peak current and the repetition period on thecapacitance and the ballast resistance. The
repetition period becomes longer as the capacitanceand the resistance increase.
In the microdrop discharge, pulsed discharge
develops between the electrodes, to which a
capacitor is connected in parallel. Ultra-fast pulsed
discharges are possible because no switching device
is necessary; therefore the residual inductance canbe reduced substantially in the discharge circuit.

system as small as possible to reduce the residualinductance.
The power loss in the ballast resistor is adrawback of dc discharges. The loss is reduced in
the pulsed mode observed in the gas flow discharge,
because the current is used only to charge theparasitic capacitance.
6. Control of Initial Structure of Small Matter
The initial shape of the materials is usually asmall cylinder or a sphere in our experiments. We
found that microdrop deform to spray or a filament
by an electrostatic force acting between theelectrodes. The deformation develops from thesurface of electrode on which a small amount ofliquid exists. Fig.2 shows a filament of ethanol
bridging the gap between sphere electrodes. The
filament protrudes from the tip of the Taylor-cone.
When the electric field is higher, the electro-sprayappears in a needle-to-needle electrode system. One
can make an electrical discharge by using these
initial shapes of liquid state.

Fig.2. Filament of ethanol protruding from thetip of the Taylor-cone formed on the sphere
anode; Gap separation is 700 mm.

7. Conclusion
The generation of microplasmas with the“mass-limited mode” using gas, solid, and liquidshows the features that are specific to themicroplasma. Employing these features, a numberof future applications are possible.
Fig.1. Dependence of peak currents andrepetitive periods on capacitance and ballast
resistance. Helium gas flow rate: 100 sccm.

5. Efficient Power Input to Microplasmas
The microdrop and the powder particle areheated by electrical discharges so that they show
the phase transition from liquid or solid to gas andare finally ionized. The input energy is transferredto the materials in different ways according to theirphase transition processes. The faster the rate ofcurrent rise is and the higher the accumulated
energy between the electrodes is supplied before thevaporization of powder particle. The fast current
discharge is realized by making the discharge

Acknowledgments
This work was supported by a Grant-in-Aid for
Scientific Research from the Ministry of Education,
Culture, Sports, Science and Technology.
References[1] T. Amano, et al.: Papers Technical Meeting
Plasma Science & Technology, IEE Japan,
PST-04-97 (2004) [in Japanese].
[2] N. Shirai, et al.: Papers Technical Meeting Plasma
Science & Technology, IEE Japan, PST-04-29(2004) [in Japanese].
[3] T. Yokoyama, et al.: Papers Technical Meeting
Plasma Science & Technology, IEE Japan,PST-04-50 (2004) [in Japanese].