23D03
Analysis of dielectric properties of atmospheric-pressure negative ion plasmas
大気圧負イオンプラズマ現象の誘電特性解析
Osamu Sakai, Yuu Hiraoka, and Keiichiro Urabe
酒井 道, 平岡 悠、占部継一郎
Department of Electronic Science and Engineering, Kyoto University
Kyoto-daigaku Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
京都大学 工学研究科電子工学専攻 〒615-8510 京都市西京区京都大学桂In the ambient air around us, collective phenomea mainly composed of positive and negative ions may
exist, and we refer to it as “atmospheric-pressure negative ion plasma” hereafter. Unlike collisonlessnegative ion plasmas and ionic plasmas, the charged particles in an atmospheric-pressure negative ion
plasma suffers heavy collisions against neutral particles in the air, and its collective properties mightmuch different from classical understandings of the plasmas. Dielectric properties as well as transportparameters are derived theoretically and numerically, and new findings on its behaviors are addressed.

1. Introduction
Plasma sources in non-thermal states at
atmospheric pressure in the ambient air haveattracted much attention since they do not requirehuge vacuum systems for conventional
fabrication processes in the semiconductor
industry. Recently, in addition to this point,reaction fields induced by such plasmas in or on
liquids are a new emerging area of plasmaengineering. Also plasma-biomaterial interaction
is a very promising research target since plasmas
have not been able to access it when they are
generated at low pressure.
In our recent report [1], we measured lifetimeof electrons in the non-thermal atmosphericpressure. In the similar condition to the ambient
air, they survive, at most, for 1 s. Addition ofoxygen to nitrogen lessens their lifetime, partlybecause electrons attach oxygen molecules,
resulting in negative ion formation. However,after that, negative ions and remaining positive
ions have much longer lifetime when we assume
that their losses are attributed to recombination.
As we will mention later, although their densities
are not so high, they can survive for seconds or
more, and we refer to such a state as “atmosphericnegative ion plasma” or “atmospheric ionicplasma.”
When we consider their governing equations,
we will notice that their transport properties aremuch different from those of the classicalplasmas, since they are collisional and they havea number of mass spectra. In this report, wedemonstrate their significant features stressed intheir unusual conditions that have not been
investigated so far.

2. Theoretical Prediction
Starting point of theoretical analysis on theatmospheric-pressure negative ion plasmas are
momentum balance equation, given asvit ni qi E  pi  Mi ni vi ig
M p,m, i 1 , 2 , 3 ,...ni(1)where M is the mass of an ion, n is its density,
v is its fluid velocity, q is its electric charge, pis its pressure,  g is its elastic collision frequency,n is the electric field, and t is the time. Thesubscript  denotes positive ion (“p”) or negative(“n”) ion, and the subscript i is the sequential
number of ions with different mass numbers. Thatis, we have to solve a number of equations,expressed in Eq. (1), for each ion species.
At the first glance, the set of equations based on
Eq. (1) are not summarized in the Drude model,
although there is certainly frequency dependence.
The second point we will notice that the dielectric
constant deduced from the set of the equations are
not linear. These two points indicate that thedielectric properties are much different from theconventional dielectric functions for usual plasmas
composed of electrons and positive ions.
3. Numerical Results
We performed numerical analysis based on thefluid model; from the original numerical code [2],we replace electrons by negative ions, and weassumed that negative ions have similar mass topositive ions (Xe+) with negative charge. We also
set suitable recombination coefficient between
positive and negative ions at atmospheric pressure[3].
Assuming certain initial density profiles, weobtain their decaying properties. First of all, when
we assume that negative ions are only present, thelifetime of the ions is quite short; no ions aredetected after 0.1 s. On the other hand, when we
assume that uniform and equal density profiles forboth ions as initial states, we observed quite long
lifetime; for atmospheric–pressure negative ionswith densities ranging from 105-106 cm-3, thelifetime is several seconds. During such a long
lifetime, the ions can drift outwards to get touchwith other boundaries, such as solids, liquids, andbiomaterial surfaces. Some modification of thenumerical results indicates that this losses of ions
are mainly attributed to recombination in the space,
not to transport to the wall boundary.
4. Conclusion
A regime of the collective motions by negativeions and positive ions is described theoretically.
The features will lead to new understandings of theplasma-related phenomena, and may reveal
unknown effects induced by atmospheric-pressureplasmas.
Acknowledgments
This work was partly supported by aGrant-in-Aid for Scientific Research from theJapanese Ministry of Education, Culture, Sports,
Science and Technology.
References[1] Y. Ito, O. Sakai and K. Tachibana: Plasma Sources
Sci. Technol. 19 (2010) 025006.
[2] O. Sakai and K. Inoguchi, J. Phys. D: Appl. Phys. 36(2003) 2891.
[3] Y. P. Raizer, Gas Discharge Physics (Springer,Berlin, 1987).