22P071-P
Release of a dust particle in plasma with electron beam component
電子ビーム成分を含むプラズマ中での微粒子の脱離に関する研究	 
Masafumi Ueno1, Noriyasu Ohno1, Yukihiro Tomita2, Gakushi Kawamura2 and Masako Shindo3
上野昌史1, 大野哲靖1，冨田幸博2，河村学思2，眞銅雅子3
1) Department of Energy Engineering and Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8603, Japan
名古屋大学大学院工学研究科	  〒464-8603	  愛知県名古屋市千種区不老町
2) National Institute for Fusion Science, 322-6 Oroshi-cho, Toki 509-5292, Japan
核融合科学研究所	  〒509-5292	  岐阜県土岐市下石町322-6
3) Faculty of Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama, 240-8501, Japan
横浜国立大学大学院工学研究院	  〒240-8501	  神奈川県横浜市保土ヶ谷区常盤台79-5
The release condition of a dust particle from a plasma-facing wall, which is a start point of transport processof dust particles in fusion plasma, is studied theoretically. Analytical model in this study is a spherical andconductor dust particle on a flat and plasma-facing wall. Forces on a dust particle in this model are assumed
as electrostatic force, ion friction force and gravitational force. Furthermore, high-energy electroncomponent is important for dust release. Release condition with high-energy electron component is analyzed
theoretically. Experiment of dust release with electron beam is carried out.

1. Introduction
In fusion devices, dust particles with different
sizes, shapes and compositions are observed inplasma during discharges and they accumulate after
discharges on plasma-facing wall (PFW). The
elements of dust particles originate in structurematerials of divertor plates and first wall. It issupposed that the dust particles are generated byplasma-wall interaction and transported toward thecore plasma as impurities. Therefore, release fromthe PFW and dynamics of dust particles have beenstudied [1,2]. On the other hand, in the experiment,
release of the dust particle from the PFW wasobserved with electron beam exposure [3,4].
High-energy electron component might have played
an important role in dust release.
In this study, simple analytical model, in which aspherical dust particle having a radius much smaller
than the Debye length on the flat and PFW, is used
and the dust release condition is analyzed
theoretically. Forces on a dust particle are analyzed
and the plasma condition for the dust particle
release is derived. Forces on a dust particle areconsidered as electrostatic force, ion friction force
and gravitational force in this study. But actually,other forces may act on a dust particle, for example,
adhesive force. Therefore, the release condition
derived in this paper is minimum required condition
for release of a dust particle. Moreover, releasecondition with high-energy electron component isanalyzed theoretically and release condition withelectron beam component is investigatedexperimentally.

2. ModelThe 1D sheath plasma model is applied. A
spherical dust particle is on a flat wall (Fig.1). Wallpotential is determined by floating condition.
Radius of a dust particle is assumed to be muchsmaller than the Debye length. Sheath profile isassumed to be unaffected a dust particle. At the
sheath entrance, electric potential and electric field
are assumed to be zero. The velocity distributionfunctions of electrons and ions are assumed to be
Maxwellian and shifted-Maxwellian distribution,
respectively. Electron velocity distribution function
is truncated on the high velocity tale of minus,
because of wall effect. Ion shift velocity is assumed
to be ion sound speed. Both charges of the wall anda dust particle are negative, therefore electrostaticforce FE acts on a dust particle. Additionally, ion
friction force Fi due to accelerated ions andgravitational force Fg are considered as forces on
the dust particle. The dust particle is released fromthe wall in a condition of FE > Fi + Fg.

Fig.1 Model3. Result on single Te distribution
In a case of a horizontal wall and tungsten dust
without electron beam component, dust releasecondition depends on τie = Ti/Te and Rd/nseTe (Fig.2),
where Ti and Te are ion and electron temperature
respectively, Rd is dust radius and nse is plasma
density at sheath entrance. For dust release, enoughsmall τie and Rd/nseTe are required. At τie = 0, dust
release condition is expressed in the following
equation:ρ d ,g/ccRd ,µm(1)< 2.80n se,18Te,eV

Where ρd is dust mass density. For example, in thecase of Rd = 1µm, nse = 1018 m-3 and τie = 0, releasecondition€ of a tungsten dust is Te > 7.1 eV.
4. Result on double Te distribution
In plasma with high-energy electron component,
dust particle may be easily released from the wall
due to strong electric field. In a case of double
electron temperature distribution, dust releaseconditions are shown Fig.3 and Fig.4, where Teh, neh,
Tec are electron temperature and density of hotelectron component and cold electron temperature,respectively. A few percent of high-energy electroncomponents play an effective role for dust release.

Fig.2 Critical of dust releaseon single Te distribution

Acknowledgments
This work is performed with the support andunder the auspices of the NIFS Collaboration
Research program (NIFS11KLEF004).
References[1] Y. Tomita, R. Smirnov, H. Nakamura, S. Zhu, T.
Takizuka, D. Tskhakaya, J. Nuclear Materials, 363-365(2007) 264.
[2] Y. Tomita, R.D. Smirnov, T. Takizuka, D. Tskhakaya,
Contrib. Plasma Phys., 48 (2008) 285.
[3] T. M. Flanagan and J. Goree, Phys. Plasmas, 13,123504 (2006).
[4] M. Shindo and O. Ishihara, “Study of Complex
Plasmas Nano- and Micromaterials”, ed. by K. Ohno, M.
Tanaka, J.Takeda and Y. Kawazoe, Springer Science,
pp.311-327 (2008).

Fig.3 Critical of dust releaseon double Te distribution at Ti = 0

Fig.4 Critical of dust releaseon double Te distribution at Teh / Tec = 10