1. Vacuum/volume 47/numbers 6-a/pages 541 to 54411996
Copyright 0 1996 Published by Elsevier Science Ltd
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Dust removal system using static electricity
M Onozuka,B* Y Ueda,” K Takahashi,” Y Seki,” S Ueda,’ I Aoki,b aMitsubishi Heavy Industries, Ltd, Advanced
Technology Development Department, 3-3- 1, Minatomirai, Nishi-ku, Yokohama 220-84, Japan; bJapan Atomic
Energy Research Institute, Toukai-mura, Naka-gun 379- 11, Japan
Development of a dust removal system using static electricity has been conducted. It is envisioned that the
system can collect and transport dust under vacuum. In the system, the dust is charged by dielectric
polarization and floated by an electrostatic attraction force that is generated by a dc electric field. The dust is
then transported by the electric curtain formed by a three-phase ac electric field. Experimental investigation
has been initiated to examine the characteristics of the system. It was found that carbon and copper particles
measuring 5-44 pm were successfully removed from the bottom of the chamber under a vacuum environment.
Copyright 0 1996 Published by Elsevier Science Ltd.
Key words: Dust, dust removal, electric curtain, electrostatic force, static electricity.
Introduction electric curtain formed by the three-phase ac electric field. The
following outlines the dust floatation and transportation
Vacuum devices, such as fusion experimental reactors, film depo-
mechanisms.
sition devices and material processing devices, require a clean
and high vacuum environment. However, the operation of such
devices occasionally generates unnecessary by-products like dust. Dust floatation using static electricity.In this study, metallic and
For example, in the Joint European Torus (JET) fusion exper- non-metallic particles (dust) are considered under a vacuum
imental device, airborne and deposited erosion dust was observed environment. The dust is initially grounded on the bottom surface
inside the vacuum vessel after a number of plasma operations.’ 4 of the vacuum chamber. Under such conditions, when the electric
The composition of the dust was found to primarily be C, Be, field is applied to the particle, the particle is dielectrically pol-
Co, Cr. Fe and Ni. It is thought that C and Be came from the arized as shown in Figure 1. Since the particle is grounded to the
first walls made of graphite and beryllium, and that Co, Cr. Fe chamber, the positive charge on the ground surface is neutralized,
and Ni came from the vacuum vessel made of inconel steel. The so the particle is charged negatively. The negatively charged
size of the airborne particles was found to be a few microns in particle is then floated by the Coulomb force due to the inter-
mean diameter, while the size of debris particles was of the order actions between the charge and the electric field, as shown in
of millimetres. The amount of graphite collected inside the vessel Figure 2. Therefore, using the static electricity, the dust can be
was 91.1 g, while that of metal was 7.5 g. Because such mobile removed from the bottom of the vacuum chamber to the required
dust becomes radioactive and accumulates tritium, it must be position.
removed from the vessel for safety reasons.5.6
A dust removal system using static electricity has been inves-
tigated. It is expected that the system can collect and transport
Electrode
dust under vacuum. This report presents the research results. +
System description and background theory
In the system. the dust is charged by dielectric polarization and Electric
floated by electrostatic attraction (Coulomb) force that is gen- Field
erated by the dc electric field. The dust is then transported by the
* Correspondence to: Masanori Onozuka, Mitsubishi Heavy Indus-
tries, Ltd, Advanced Technology Development Department, 3-3-1, Mina-
tomirai, Nishi-ku. Yokohama 220-84. Japan. Tel: 81(Country Code)-45-
224-9587, Fax: 8 l-45-224-9963. Figure 1. Dielectric polarization of particle
541

2. M Onozuka eta/: Dust removal system using static electricity
Electrode
////////I/
Figure 2. Induced forces on floated particle.
Figure 4. Gradient force on particle
The forces acting on the particle are gravitational and Cou-
lomb forces.7 The trajectory of the particle is calculated by solving
the following equation of motion: of the interactions between the polarized particle and the non-
uniform electric field. For example, as shown in Figure 4, the
wz$r = qE+F particle is polarized by the electric field. Since the polarized
charge and the electric field strength are different, a gradient force
F works on the particle.
where m, II and q are the mass, velocity and charge of the particle,
In general, the gradient force on the particle is given by the
respectively. E is the electric field, and F is the gravitational force.
following equation:
E is formulated by the electrostatic potential V as
E = -VI/.
V is given by the following Poisson’s equation:
F=
0
“2” VE2
where 3 is the induced polarization, L’is the particle volume, and
E is the electric field. This equation is rewritten for the sphere
v2v= 0.
particle with a radius of r
The polarized charge q of the particle is found to be
c-1
F = 2&S--E ,VE2.
Es+2
This gradient force is used to transport the collected dust from
the vacuum chamber to the required location.
where t0 and L, are the permittivity of the vacuum and relative To produce an electric curtain of travelling wave type, three-
permittivity of the particle, respectively. phase ac voltage will be used. The three-phase ac voltages on
The trajectories of the particles were analyzed as an example. each electrode (U, V and W) are expressed as
Figure 3 shows the simulation result. It was shown that the
particles travel from the bottom (ground) toward the upper elec- V” = V,cos(ot) (7)
trode.
Vv = V,cos(wt - 2n/3) (8)
Dust transportation using an electric curtain. Alternating-current
(ac) lines with different phases produce a non-uniform travelling
electric field (electric curtain), i.e. the potential distribution tra-
VW = V,cos(ot-47q3) (9)
vels in space. When the electric field is not uniform in space,
polarization force (gradient force) acts on the particle as a result where V,, is the peak voltage and w is the angular frequency.
Therefore, this three-phase ac voltage produces a travelling
potential with a velocity of 3wp/27c (p is the pitch of the elec-
trodes).
Particle: Electrode:
Size: lpm Diameter: 40mm The trajectory of the particle is calculated by the above equa-
Density: 1Sg/cc Applied Voltage: 9ooV tions as well as eqns (I) and (2).
Relative permittivity:
5.5 Distance between electrodes: 7mm
The particles’ trajectories were simulated for the electric cur-
tain produced by a spiral-type electrode. As shown in Figure 5,
theoretically, particles can be transported toward the top through
a tube with spiral electrodes.
Experiments of dust floatation
Experimental investigation has been initiated to examine the
system’s characteristics. In this report, the experiments on dust
Figure 3. Trajectories of particles for dust floatation system floatation are presented.
542

3. M Onozuka eta/: Dust removal system using static electricity
Particles
o : Carbon5um
W
Size: ljfm A : Copper 40 cI m
Density: 1Z&c i 0.08 - q : Carbon 44 ~1m
Relative perrnittivity: 5.5
2
Electrode: H
Inner diameter of tube: 16mm tij
Diameter of electrode: 6mm
Pitch of electrodes: 1Omm 6 0.04-
0
Applied Voltage: 30kV !?!
al
Ground 6
Figure 5. Trajectories of particles for dust transportation system.
a I , I I
0 4
Electric Field E (kV/cm)
Figure 7. Experimental result of dust foatation test.
A schematic of the experimental apparatus for dust floatation
is shown in Figure 6. The dust is placed on the bottom of the
vacuum chamber, which is electrically grounded (grounded elec-
trode). Above the dust, a circular electrode plate measuring 40
mm in diameter is positioned. High voltage electricity up to 2400 again. Both sizes of carbon dust showed similar results. This
V is applied from the dc power supply to the electrode. The finding is evaluated as follows. For the case of a high electric
pressure inside the chamber is around 0.01 Pa (IO-“Torr). field. the piled dust particles under the upper electrode are floated
Carbon and copper particles were tested. The mean diameters toward the bottom surface of the electrode. However, after being
of the carbon were 5 and 44 pm, while that of the copper was adhered to the electrode, the particles are again polarized on
40 pm. Experimental parameters were the distance between the the electrode and travel back to the ground. This movement is
electrode and the ground, and the applied voltage. The flowing repeated. Therefore, it is difficult to collect particles on the bot-
current was found to be about 10 mA through the experiments. tom surface of the electrode. On the other hand, since the electric
Figure 7 presents the result, showing the relation of the electric field above the electrode is also strong, the particles easily travel
field (the supplied voltage divided by the distance between the above it. After neutralization, the particles fall on the top surface
electrode and the ground) and the adhered dust mass on the of the electrode and stay there. as simulated in Figure 3. Thus,
electrode. Both materials were successfully floated and collected the collected dust for the high electric field is mainly those paricles
to the electrode. from the top surface of the electrode. When the electric field is
low, the particles are collected on the bottom surface of the
Discussion electrode.
As for the carbon dust, it was found that when the electric field For the case of copper dust, the particles were only collected
on the top surface of the electrode. This is because copper has
was low. the dust was collected efficiently. As the electric field
high electrical conductivity and approximately infinite permit-
increased. the collected dust mass decreased, then increased
tivity. Therefore, copper particles are easily floated due to the
dielectric polarization. However, they are again polarized on the
surface of the electrode and travel back to the ground. Thus, they
are only collected on the top surface of the electrode.
Power Supply An efficient dust floatation method and system configuration
should be considered according to the above results.
In the experiments, as the electrode and ground were not elec-
trically insulated. the electrical discharge could have occurred
between them. Therefore, the induced electrons or ions may have
neutralized the polarized particles. thereby lowering the amount
of collected dust. So, an electrical insulation, at least on the
electrode, is required to avoid unnecessary discharge between the
electrode and the ground and to produce only the electric field in
space for efficient dust collection.
Although development is still in progress, current research
Electrode results indicate that the dust removal system using static elec-
tricity can be used to collect and transport the dust under a
vacuum environment. To confirm the characteristics of the
system, further experimental investigation will be required.
- including dust transportation tests. Current research is aimed at
Figure 6. Experimental apparatus of dust floatation test. these objectives.
543

4. M Onozuka et al: Dust removal system using static electricity
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