25G13
Spatial diffusion of charged dust particles
in complex plasma in a vapor of liquid helium
極低温コンプレックスプラズマ中の熱泳動力による帯電微粒子拡散とその抑制
Megumi Chikasue, Masashi Masuda, Masako Shindo and Osamu Ishihara
近末恵美, 増田政史，眞銅雅子，石原 修
Faculty of Engineering, Yokohama National University79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
横浜国立大学工学府 〒240-8501 横浜市保土ヶ谷区常盤台79-5
Experimental study on non-equilibrium plasma is presented. A complex plasma in the vapor of liquid
helium was produced by applying an rf voltage. The plasma is characterized by cold ions with as low as afew meV and hot electrons with a few eV. Dust particles were introduced into the rf plasma and spatial
diffusion of charged dust particles under the thermophoretic force is studied in detail.

1. Introduction
A complex plasma in a vapor of liquid helium
has been studied [1]. The plasma is characterized
by a non-equilibrium state with T e >> T i, where T e
is a few eV and T i is a few meV to 10 meV [2].
The collisional effects on charges of dust
particles [3] and thremophoretic force on charged
dust particles [2] were studied. Thermophoretic
force is found to play a major role in thedynamics of dust particles in the presence ofliquid helium. Thermophoretic force andgravitational force extract dust particles from theplasma through diffusion toward the surface ofliquid helium. In this study, we measure neutral
temperature to estimate ion temperature, and set adevice with a ring electrode and mesh electrodes.
Spatial diffusion of charged dust particles areobserved in the plasma.
2. Experiment
Our experimental setup is described in Fig. 1. We
produce an rf plasma in helium gas at three
conditions, one at room temperature, one cooled byliquid nitrogen stored in the outer bottle and one ina vapor of liquid helium. Liquid helium is stored in
the bottom of the bottle, and the vapor pressure is
reduced to as low as 0.1 kPa to produce a stable
plasma. A movable thermocouple measure theneutral temperature in the plasma. An rf plasma is
produced between upper two mesh electrodes,
where the top electrode is grounded and an rf power(13.56 MHz, 1~10W) is applied at the lower
electrode. The plasma is diffused toward the lowest
electrode. A ring electrode is set between the rf
electrode and the lowest electrode, to make radial
electrostatic potential produced by applied DC

Fig. 1 (a) Experimental apparatus. (b) Structure of theelectrodes

voltage Vr, from -80 to 80 V. Distance between thering electrode and the lowest grounded electrode, hcan be changed from 5 to 9 mm. Acrylic particles of
3 μm in diameter, dropped from the top flange, areintroduced into the plasma. Dust particles areirradiated by green sheet laser (532 nm), which isreflected at vertical direction by a small mirror at 13cm below the rf electrode. Particles are recorded byhigh-speed CCD camera of 400 or 500 fps.
2.1 He gas at room temperature and cooled byliquid nitrogen
Plasmas are produced in helium gas at room
temperature and cooled by liquid nitrogen.
Experiments are done at various conditions, such as
h = 5 mm, 7 mm, and at 100, 200 and 500 Pa.
Neutral temperatures between the ring electrode andthe lowest mesh electrode are ~300 K and ~ 80 K in

0y[mm]without discharge
with discharge

1010y [mm]

y[mm]

90V
6V-20V-4-202x [mm](a)125V50V-50V

4-4-2

1402x [mm](b)43040T n [K]
(a)

50

20

without discharge
with discharge

00.02

0.04

0.06
t [s]
(b)

0.08

0.1

Fig. 4 (a) Space dependence of neutral temperature ath = 9 mm 120 Pa, and (b) time dependence of
positions of dust particles with and without discharge
in the vapor of liquid helium at h = 9 mm and 120 Pa.

Fig. 2 Dust particles levitating between the ring
electrode and the lowest grounded electrode atroom temperature helium gas, 130Pa, power
supply 7W and h = 5 mm.
1011121314201016Fig. 3 (a) Positions of dust particles at 130 Pa, power
supply 7 W and h = 5 mm at room temperature. (b)Positions of dust particles at 100 Pa, power supply 3W and h = 7mm, cooled by liquid nitrogen.

helium gas at room temperature and cooled byliquid nitrogen, respectively. At room temperature,
dust particles are observed between the ringelectrode and the lowest electrode at all conditions,while dust particles are observed in the region at
100 Pa at h = 5mm, and at 100, 200 Pa at h = 7 mmin the helium gas cooled by liquid nitrogen. Figure
2 shows a dust cluster at room temperature and x
and y axis, where x axis is in a horizontal direction,y axis is set at a center of mesh electrodes in a
vertical direction toward downward, and y = 0 is set
at the rf electrode. Figure 3 shows the position ofdust particles at the lowest edge of the cluster inhelium gas at room temperature and cooled byliquid nitrogen.
2.2 In the vapor of liquid helium
In the vapor of liquid helium, same experiment isdone as 2.1. The background neutral temperature iskept to be cryogenic in the presence of thetemperature gradient of 3 K/cm. Dust particles all
diffuse toward liquid helium surface. Figure 4 (a)shows that neutral temperature increases after
discharge, while neutral temperature gradient iskept constant about 3 K/cm, and (b) shows that dust
particle are not accelerated in the presence ofdischarge. The time t = 0 is set when dust particles
are at y = 0.

3. Discussion
Ion temperature in the vapor of liquid helium isestimated to be a few meV from the neutral
temperature. The plasma is shown to be a
non-equilibrium plasma with the ratio of electron
temperature to ion temperature ~ 103. Spatial
diffusion of charged dust particles is observed in
the vapor of liquid helium. On the other hand, thediffusion is suppressed in helium gas at room
temperature and cooled by liquid nitrogen. Spatial
diffusion in the vapor of liquid helium becomes
possible by the presence of thermophoretic force
toward downward and the decreasing electrostatic
field.
4. Conclusion
We produce an rf plasma in the vapor of liquidhelium. Diffusion of charged dust particles isachieved in the vapor of liquid helium in thepresence of thermophoretic force, while nodiffusion of charged dust particles is observed inhelium gas at room temperature and cooled byliquid nitrogen. The diffusion is caused by thepresence of thermophoretic force and decreased
electrostatic field on the lowest electrode in thevapor of liquid helium. Neutral temperature
measurement estimates the ratio of electron
temperature to ion temperature ~ 103.
Non-equilibrium plasma is successfully produced.
Acknowledgments
This work is supported by Asian Office ofAerospace Research and Development underaward number 104158 and JSPS Grant-in-Aid for
Scientific Research (A) under Grant 23244110.
References[1] O. Ishihara, J. Phys. D: Phys. 40 (2007) R121.
[2] M. Chikasue, M. Shindo and O. Ishihara: AIP
conference proceedings, in press.
[3] N. Uotani et al.: J. Plasma Fusion Res. Series 9(2010) 404.