Vacuum/volume 47/numbers 6-a/pages 541 to 54411996                                                                        ...
M Onozuka eta/: Dust removal system using static electricity                              Electrode                 //////...
M Onozuka eta/: Dust removal system               using static electricity             Particles                          ...
M Onozuka et al: Dust removal system using static electricityReferences                                                   ...
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Electrostatic dust removal


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Electrostatic dust removal

  1. 1. Vacuum/volume 47/numbers 6-a/pages 541 to 54411996 Copyright 0 1996 Published by Elsevier Science Ltd Pergamon Printed in Great Britain. Ali rights reserved 0042-207X/96 $15.00+.00 PII: SOO42-207X(96)00016-4Dust removal system using static electricityM Onozuka,B* Y Ueda,” K Takahashi,” Y Seki,” S Ueda,’ I Aoki,b aMitsubishi Heavy Industries, Ltd, AdvancedTechnology Development Department, 3-3- 1, Minatomirai, Nishi-ku, Yokohama 220-84, Japan; bJapan AtomicEnergy Research Institute, Toukai-mura, Naka-gun 379- 11, JapanDevelopment of a dust removal system using static electricity has been conducted. It is envisioned that thesystem can collect and transport dust under vacuum. In the system, the dust is charged by dielectricpolarization and floated by an electrostatic attraction force that is generated by a dc electric field. The dust isthen transported by the electric curtain formed by a three-phase ac electric field. Experimental investigationhas been initiated to examine the characteristics of the system. It was found that carbon and copper particlesmeasuring 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 transportationVacuum devices, such as fusion experimental reactors, film depo- mechanisms.sition devices and material processing devices, require a cleanand high vacuum environment. However, the operation of suchdevices occasionally generates unnecessary by-products like dust. Dust floatation using static electricity.In this study, metallic andFor example, in the Joint European Torus (JET) fusion exper- non-metallic particles (dust) are considered under a vacuumimental device, airborne and deposited erosion dust was observed environment. The dust is initially grounded on the bottom surfaceinside the vacuum vessel after a number of plasma operations.’ 4 of the vacuum chamber. Under such conditions, when the electricThe 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 thefirst 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 chargedsize 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 inof millimetres. The amount of graphite collected inside the vessel Figure 2. Therefore, using the static electricity, the dust can bewas 91.1 g, while that of metal was 7.5 g. Because such mobile removed from the bottom of the vacuum chamber to the requireddust 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 Electrodedust under vacuum. This report presents the research results. +System description and background theoryIn the system. the dust is charged by dielectric polarization and Electricfloated by electrostatic attraction (Coulomb) force that is gen- Fielderated 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. 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 solvingthe following equation of motion: of the interactions between the polarized particle and the non- uniform electric field. For example, as shown in Figure 4, thewz$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 therespectively. E is the electric field, and F is the gravitational force. following equation:E is formulated by the electrostatic potential V asE = -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 spherev2v= 0. particle with a radius of rThe 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 asFigure 3 shows the simulation result. It was shown that theparticles 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 travellingelectric 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 dustFigure 3. Trajectories of particles for dust floatation system floatation are presented.542
  3. 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 6Figure 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 floatationis shown in Figure 6. The dust is placed on the bottom of thevacuum chamber, which is electrically grounded (grounded elec-trode). Above the dust, a circular electrode plate measuring 40mm in diameter is positioned. High voltage electricity up to 2400 again. Both sizes of carbon dust showed similar results. ThisV is applied from the dc power supply to the electrode. The finding is evaluated as follows. For the case of a high electricpressure 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 beingof the carbon were 5 and 44 pm, while that of the copper was adhered to the electrode, the particles are again polarized on40 pm. Experimental parameters were the distance between the the electrode and travel back to the ground. This movement iselectrode 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 electricFigure 7 presents the result, showing the relation of the electric field above the electrode is also strong, the particles easily travelfield (the supplied voltage divided by the distance between the above it. After neutralization, the particles fall on the top surfaceelectrode 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 pariclesto the electrode. from the top surface of the electrode. When the electric field is low, the particles are collected on the bottom surface of theDiscussion 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 haswas 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 atFigure 6. Experimental apparatus of dust floatation test. these objectives. 543
  4. 4. M Onozuka et al: Dust removal system using static electricityReferences “A T Peacock, J P Coad, K J Dietz and A P Knight, Proc 17fh .S~rnp Fusion Technol(l992).‘J Charuau and H Djerassi, Proc 15lh Symp Fusion Technol, 743 ‘Ph Charruyer, H Djerassi, L Giancarli and M Maupou, Fusion Engng(1988). Design, 10, 263 (1989).2J Roth, J Ehrenberg, K Wittmaack, P Coad and J B Roberto, J Nucl “R E Lyon and D F Holland, Proc 9th Symp Fusion Engng, 1482Mater, 383, 145-147 (1987). (1992).3J Charuau, Y Belot, Ph Cetier, L Drezet, L Grivaud, A T Peacock and ‘5 D Jackson, Classical Electrodynamics, 2nd ed. John Wiley & Sons,C H Wu, Proc 17th Symp Fusion Technol, 1700 (1992). New York (1975).544