Plasma Shield For In-air Beam Processes
Plasma shield for in-air beam processesAdy HershcovitchCitation: Phys. Plasmas 15, 057101 (2008); doi: 10.1063/1.2837052View online: http://dx.doi.org/10.1063/1.2837052View Table of Contents: ed by the American Institute of Physics.Related ArticlesEnhancement of ion generation in femtosecond ultraintense laser-foil interactions by defocusingAppl. Phys. Lett. 100, 084101 (2012)Electric field-perturbation measurement of the interaction between two laser-induced plasmasRev. Sci. Instrum. 83, 023504 (2012)Optimizing conversion efficiency and reducing ion energy in a laser-produced Gd plasmaAppl. Phys. Lett. 100, 061118 (2012)Compression and focusing a laser produced plasma using a plasma optical systemRev. Sci. Instrum. 83, 02B701 (2012)Laser heating of finite two-dimensional dust clusters: A. ExperimentsPhys. Plasmas 19, 013705 (2012)Additional information on Phys. PlasmasJournal Homepage: http://pop.aip.org/Journal Information: http://pop.aip.org/about/about the journalTop downloads: http://pop.aip.org/features/most downloadedInformation for Authors: http://pop.aip.org/authorsDownloaded 21 Feb 2012 to 130.199.3.165. Redistribution subject to AIP license or copyright; see http://pop.aip.org/about/rights and permissions
PHYSICS OF PLASMAS 15, 057101 共2008兲Plasma shield for in-air beam processesa Ady Hershcovitchb兲Collider-Accelerator Department, Building 901A, Brookhaven National Laboratory, Upton,New York 11973, USA共Received 12 November 2007; accepted 26 December 2007; published online 12 February 2008兲A novel concept/apparatus, the Plasma Shield, is introduced in this paper. The purpose of the PlasmaShield is designed to shield a target object chemically and thermally by engulfing an area subjectedto beam treatment with inert plasma. The shield consists of a vortex-stabilized arc that is employedto shield beams and workpiece area of interaction from an atmospheric or liquid environment. Avortex-stabilized arc is established between a beam generating device 共laser, ion or electron gun兲and a target object. The arc, which is composed of a pure noble gas, engulfs the interaction regionand shields it from any surrounding liquids like water or reactive gases. The vortex is composed ofa sacrificial gas or liquid that swirls around and stabilizes the arc. The successful Plasma Shield wasexperimentally established and very high-quality electron beam welding with partial plasmashielding was performed. The principle of the operation and experimental results are discussed inthe paper. 2008 American Institute of Physics. 关DOI: 10.1063/1.2837052兴I. INTRODUCTIONIn current art, many industrial processes like ion materialmodification by ion implantation, dry etching, and microfabrication, as well as electron beam processing, like electronbeam machining and electron beam melting is performedexclusively in vacuum, since electron guns, ion guns, theirextractors, and accelerators must be kept at a reasonably highvacuum, since chemical interactions with atmospheric gasesadversely affect numerous processes. Various processes involving electron, ion, and laser beams can, with the PlasmaShield, be performed in practically any environment. For example, electron beam and laser welding can be performedunder water, as well as in situ repair of ship and nuclearreactor components. The Plasma Shield should result in boththermal 共since the plasma is hotter than the environment兲 andchemical shielding. The latter feature brings about invacuum process purity out of vacuum, and the thermalshielding aspect should result in higher production rates.As the name suggests, the Plasma Shield is designed tochemically and thermally shield a target object by engulfingan area subjected to beam treatment with inert plasma. Theshield consists of a vortex-stabilized arc that is employed toshield beams and workpiece area of interaction from an atmospheric or liquid environment. A vortex-stabilized arc isestablished between a beam generating device 共laser, ion orelectron gun兲 and the target object. The arc, which is composed of a chemically inert gas 共like a pure noble gas兲, engulfs the interaction region. This arc then shields the interaction region from any surrounding liquids like water orreactive gases. The vortex is composed of a sacrificial gas orliquid that swirls around and stabilizes the arc, which displaces the environmental fluid.The Plasma Shield had its origin as an extension of thePlasma Window.1,2 The latter has shown to be a rather effeca兲Paper TI2 3, Bull. Am. Phys. Soc. 52, 275 共2007兲.Invited speaker.b兲1070-664X/2008/15共5兲/057101/5/ 23.00tive interface between vacuum and atmosphere that facilitated unprecedented effective transmission of ion, electron,and x-ray beams from vacuum to atmosphere. However,once the beams exited to atmosphere and struck a target object, the process performed was subjected to adverse environmental effects. Electron beam welding performed 共with aPlasma Window兲 in the atmosphere resulted in welds withvisible oxidation, even though welds were performed at anunprecedented stand-off 共and low power兲 with excellent penetration. To rectify this shortcoming, the Plasma Shield wasdeveloped. Recently partial plasma shielded electron beamwelding experiments were performed resulting in the expected high quality in-air electron beam welding. The principle of operation and experimental results are described anddiscussed in this paper.II. INITIAL CONSIDERATIONS AND OPERATIONPRINCIPLESIt is relatively easy to surround an object with plasma byeither injecting plasma from a plasma source to engulf theobject, by biasing the object, and creating a discharge to itssurrounding, or with an rf discharge. However, in order togenerate an effective shield, the plasma must be dense andstable. The plasma must displace all the environmental fluid,requiring high pressure and density. Thus, arc discharges areneeded for most foreseen applications, since they have therequired density and pressure. The objective is to develop anarc that can be extended onto a target object and cover anarea to be treated, while displacing the environmental fluid.Another crucial requirement is that the arc-generating devicemust have hollow geometry in order to facilitate unimpededbeam propagation.Stabilizing arc plasmas can be accomplished by a varietyof techniques:3 wall stabilization,4 transpiration cooling,5vortex stabilization,6,7 electrode stabilization,8 and magneticstabilization. The first three stabilization techniques arebased on cooling the outer boundary of a plasma column.15, 057101-1 2008 American Institute of PhysicsDownloaded 21 Feb 2012 to 130.199.3.165. Redistribution subject to AIP license or copyright; see http://pop.aip.org/about/rights and permissions
057101-2Phys. Plasmas 15, 057101 共2008兲Ady HershcovitchFIG. 1. 共Color online兲 Schematic of the plasma shield concept.Wall stabilized arc is an arc enclosed in a tube consisting ofa stack insulated water-cooled conducting disk 共usually madeof copper兲. In transpiration-cooled arcs, the cooled wall isreplaced by a transpiration-cooled constrictor. And in avortex-stabilized arc, a whirling cold fluid cools the arcboundary. Any accidental outward excursion of an arc column results in an increase in radial heat loss. Consequently,the plasma temperature is reduced, and hence, the plasmaconductivity in that location. Since electricity flows in thepath of least resistance, the arc is forced to return to itsequilibrium axial position.Electrode stabilization and magnetic stabilization are notpractical, since the first is restricted to extremely short 共nolonger than 1 mm兲 arcs, while the latter requires very largemagnetic fields; magnetizing atmospheric pressure plasmainvolves magnetic fields that are in the order of 20 Tesla. Forcompleteness sake it should be mentioned that free burning,self-stabilized arcs are also impractical due to their high intensity 共will damage the workpiece兲. Wall and transpirationcooling stabilized arcs are also not good plasma shield candidates, because the arcs must be surrounded by a solid object 共wall or constrictor兲. Thus, the best candidate seems tobe a vortex-stabilized arc.Literature search of vortex-stabilized arcs revealed thatpreviously vortex-stabilized arcs were confined in a solidchamber. In all these arcs, the vortex generating fluid is injected tangentially to generate a vortex, whose centrifugalforce drives the cold fluid against the chamber wall. An axially stable arc can then be established. These arcs operatedwith water6,7,9 or gas10 vortices.However, as our Plasma Shield concept is illustrated inFig. 1, none of these arcs satisfy the requisite free-standingplasma 共without surrounding walls兲. Thus, a crucial objectiveof this work is to develop free-standing 共not enclosed in achamber or surrounded by any walls兲 stable arcs in atmosphere or in water between a beam generator and a targetobject 共to be treated by the beam兲. Once a stable vortex isestablished, an arc can then be struck to target abject. Additionally, the length of the free-standing arc should be maximized for cases where beam treatment must be performed increvices. Thus, it is important to maximize vortex length.Literature search and consulting with one of the pioneers inthe field of vortex stabilized arcs,11 failed to reveal any previous research and/or scaling that could help in fluid injectordesign that could have helped in vortex generator design.In the absence of either prior experimental or theoreticaldata, the adapted approach to generating a free-standing vortex stabilized arc was by trial and error. Hence, the first experiments were performed with configurations described inthe patent application12 for this technology 共since then, thepatent was granted兲. Though very recent and subsequent toexperimental results presented in the next section, a verycrude simulation13 共using Fluent software兲 of a water swirl共generated in a 4 cm long, 1 cm radius cylindrical tube witha 150 l / min water flow at an angular velocity of 30 rad/ s兲showed that a 4 cm long free-standing water vortex could begenerated.III. EXPERIMENTAL RESULTSTo proceed experimentally, one of the embodiments described in the patent12 covering this technology, as shown inFig. 1 was fabricated and experimented. Components weredesigned and fabricated to generate a free-standing arc stabilized by a gas vortex. Figure 2 shows the top view of thevortex generator. Though a Plasma Window is not necessaryto generate a Plasma Shield, a Plasma Window was used as aplasma source. Basically, a vortex generator was mounted onFIG. 2. 共Color online兲 Top view of the vortex generator sketch.Downloaded 21 Feb 2012 to 130.199.3.165. Redistribution subject to AIP license or copyright; see http://pop.aip.org/about/rights and permissions
057101-3Plasma shield for in-air beam processesPhys. Plasmas 15, 057101 共2008兲FIG. 4. 共Color online兲 Photo of the Plasma Window and vortex generator共inside the bottom plate兲 forming a Plasma Shield generator with a plasmaplume protruding out 共courtesy of Acceleron Inc.兲.FIG. 3. 共Color online兲 Diagrams of the experimental setup 共not to scale兲. 共a兲Cascade arc that serves as a Plasma Window is the plasma source. Switch isused to electrically float the vortex generator. 共b兲 Electron beam welder withthe Plasma Window and vortex generator as used during weldingexperiments.a Plasma Window, which was to be mounted on an electronbeam welding column. The Plasma Window 共whose description can be found in Refs. 1 and 2兲 function was to separatethe electron gun vacuum from the atmosphere where the target object is located as well to be a plasma source. As it canbe seen in Fig. 1, the plasma generator was followed by aventuri-shaped plasma injector, whose purpose was to “suck”plasma from the plasma generator through the vortex generator and onto a target object.Diagrams of the vortex generator are displayed in Fig. 2,which was made of two sections: a plate, i.e., the main bodysurrounding a tube for gas feed, a plenum, and an insertcontaining two tangential injection slits. The inner 共beam兲channel diameter is 2.49 mm, and plenum length 共along thebeam direction兲 is 4.6 mm, which is also the maximum slitsize. Slots with downward tilt seem ideal for vortexgenerating tangential injectors. However, due to a manufacturing limitation, two rows of 4 holes of 0.8 mm diametereach were used for tangential injection instead of slots.Numerous attempts were made to initiate arc dischargein the plasma generator 共plasma window兲 of the Fig. 3共a兲configuration, extend the arc through the plasma injector andthe vortex generator onto a target object in atmosphere. Allattempts failed; the arc could not be extended beyond theplasma injector. Therefore, the plasma injector was eliminated.In this modified configuration after firing the plasmawindow arc and optimizing gas pressure input for the vortexgenerator, a plasma plume extended into the atmosphere as itcan be seen in Fig. 4. The working gas was argon; arc currentwas 45 A, arc voltage was 85 V. The white bright portion ofthe plume was 6 mm long. This mode of operation was accompanied by a strong ozone smell. Attempts to extend thedischarge onto a target object located at a distance of 1 cmfailed. This mode, with a plasma plume, shall be referred toas a partial shield.After some trial and error a technique was developed fordischarge extension onto a target object. After a discharge isestablished as shown in Fig. 4, a water-cooled target object,with bias identical to the vortex generator, is brought within1 – 2 mm of the vortex generator. Like the previously described plasma window operation, arc current and voltagewere 45 A and 85 V, respectively. A switch is opened toelectrically float the vortex generator, which in this caseserved as an anode. Now the target object becomes the anodewith a free-standing discharge. It is important to note that inthe case of a stationary target, the target must be cooled toprevent its melting and/or uncontrolled arcing. Next the target is pulled away to extend the free-standing discharge asshown in Fig. 5, which was taken through very dark weldingglass, since the free-standing arc was extremely bright. Amaximum length of 2.5 cm was obtained for the freestanding extended arc. Arc current was maintained at a constant 45 A; the arc voltage increased roughly linearly withlength of the free-standing discharge up to 195 V at themaximum length. The resultant discharge is a cathode-totarget-arc, where the cathode to vortex generator portion is a3.9 cm long wall stabilized cascade arc, and the vortex generator to target portion is a 2.5 cm long free-standing vortexstabilized arc. From the voltage characteristics 共85 V across3.9 cm vs 110 V across 2.5 cm兲, it is obvious that the twoportions of the arc have different characteristics, suggestingthe atmospheric portion to be denser and/or cooler and/orDownloaded 21 Feb 2012 to 130.199.3.165. Redistribution subject to AIP license or copyright; see http://pop.aip.org/about/rights and permissions
057101-4Ady HershcovitchPhys. Plasmas 15, 057101 共2008兲FIG. 5. 共Color online兲 Photo of a free-standing arc between the PlasmaShield generator and a water-cooled copper plate. Photo taken through awelding mask 共courtesy of Acceleron Inc.兲.having a lower ionization fraction. This mode, with an arcextended to the target, shall be referred to as Plasma Shield.With an extended free-standing arc, the ozone smell waspractically gone. Measurements with an ozone meter wereperformed for the two operating modes partial shield andPlasma Shield. All measurements indicated that the ozonelevel is deceased in the case of Plasma Shield operation.Since the experiments were carried out in a large industrialsetting with varying ventilation, measurements were not reproducible on a daily basis; hence no quantitative numberswill be presented. Ozone generation is due to exposure ofatmospheric oxygen to electrical discharge. Decrease inozone level implies that the plasma discharge is shieldedfrom atmospheric oxygen. Therefore, the observed qualitative decrease in ozone level strongly suggests that a swirl ofunionized argon surrounds the free-standing arc in the casePlasma Shield operation; a fact consistent with the freestanding arc stability.Finally, the Fig. 3共a兲 apparatus, with the anode containing the built in vortex generator, was mounted on an electronbeam 共EB兲 welder 关Fig. 3共b兲兴. Since no provisions weremade to allow the welding table to function as the experimental target object, only partial shielding was utilized during welding. For clarification 共again兲, partial shielding is provided by plasma that extends beyond the anode into air dueto the low pressure generated by the vortex 共the plasmaplume partially displaces atmospheric gases兲. Full shieldingoccurs when vortex stabilized plasma is projected 共driven byvoltage兲 from the plasma window anode to target object.With this setup, welding experiments with partial shieldingwere performed, and compared to previous nonvacuum electron beam welding with a Plasma Window. Figure 6 showsthe previously obtained welding result.14,15 The dark color ofthe weld beads indicates oxidation. In Fig. 7, a weld withpartial shield is shown. Welding electron beam energy andcurrent were 150 keV and 20 mA, respectively. Though theFig. 6 nonvacuum welds are considered of good quality16共except for the oxidation兲, the Fig. 7 results are indicative ofFIG. 6. 共Color online兲 Pictures of an unshielded nonvacuum electron beamweld. Dark colors on the beads 共lower photos兲 indicate oxidation. Top photoshows the weld cross section.cleaner welds. Quantitatively, this observation is confirmedby weld analysis.17Additional significant results were obtained in this setupcompared to pure plasma window operation:共1兲 Welding with pure argon operation was achieved, i.e.,welding at even lower plasma window power ispossible.共2兲 Superior electron beam propagation was observed:propagation in atmosphere, for about 7.5 cm, was ob-FIG. 7. 共Color online兲 Photo of a partially shielded nonvacuum electronbeam weld, which indicates plasma shielding effectiveness 共courtesy of Acceleron Inc.兲.Downloaded 21 Feb 2012 to 130.199.3.165. Redistribution subject to AIP license or copyright; see http://pop.aip.org/about/rights and permissions
057101-5Phys. Plasmas 15, 057101 共2008兲Plasma shield for in-air beam processesserved with the arc operating in pure argon 共compared to1.5 cm with plasma window only and some helium兲.共3兲 Upstream pressure was lower by a significant factor 共ofabout 2兲. It means that addition of the plasma shield tothe plasma window greatly improved vacuumseparation.IV. DISCUSSIONResults presented in the experimental section of this paper do suggest that plasma shielding may accomplish theexpectations set forth in the Introduction. Stable freestanding arcs between a Plasma Window and a target objectwere established. Nonvacuum electron beam welding, performed with partial plasma shielding 共with argon plasmaplume兲 covering welded workpieces, produced much cleanerwelds. Unlike previous attempts14 to displace atmosphericsgases by using a venturi with a Plasma Window, which faileddue to blowing-off the molten pool, vortex flow had no adverse affect on the welding process. The most likely reason isthe fact that the vortex generated a lower pressure region,which is filled by plasma from the cascade arc without causing violent flow.Comparison of nonvacuum welds, without any shielding共Fig. 6, where oxidation is obvious兲 and nonvacuum weldswith partial shielding 共Fig. 7, that shows a rather clean weld兲,are indicative that considerable shielding was accomplished.Although a water vortex simulation predicts that a stable fewcm long vortex can be established is encouraging, it shouldbe regarded as very preliminary work. Superior electronbeam propagation in atmosphere and greatly improvedPlasma Window vacuum separation with partial plasmashielding, is, at least in part, most likely due to heating andrarifying of the atmosphere. With full shielding electronbeam propagation in atmosphere 共through the free-standingarc兲 should be as good as through the plasma window due tothe strong focusing effect of the arc current.1,2,14 In thesepapers, it was shown that plasma current generates an azimuthal magnetic field which exerts a radial inward Lorentzforce on beam electrons, which overcompensates for scattering by gas atoms and ions.Future plans are to perform electron beam welding withfull plasma shielding in-air and underwater.ACKNOWLEDGMENTSMany thanks to members of the Acceleron personnelwho participated in the experiments.This work was supported by Acceleron, Inc. 共EastGranby, CT兲, Connecticut Light & Power Co., and Connecticut DEP. This work was performed under the auspices of theU.S. Department of Energy NICE3 Grant No. DE-FG4101R110925.Notice: This manuscript has been authored byBrookhaven Science Associates, LLC under Contract No.DE-AC02-98CH1-886 with the U.S. Department of Energy.The Untied States Government retains, and the publisher, byaccepting the article for publication, acknowledges a worldwide license to publish or reproduce the published form ofthis manuscript, or others to do so, for United States Government purposes.A. Hershcovitch, J. Appl. Phys. 78, 5283 共1995兲.A. Hershcovitch, Phys. Plasmas 5, 2130 共1998兲.3E. Pfender, Gaseous Electronics, edited by M. N. Hirsh and H. J. Oskam共Academic, New York, 1978兲, Vol. 1, Chap. 5.4H. Maecker, Z. Naturforsch. A 11a, 457 共1956兲.5E. Pfender, G. Gruber, and E. Eckert, “Experimental investigation oftranspiration-cooled constricted arc,” in Proceedings of the InternationalSymposium on High Temperature Technology I.U.P.A.C. 共Butterworth,Washington, D.C., 1969兲, p. 3.6O. Schoenherr, Z. Elektrochem. Angew. Phys. Chem. 30, 365 共1909兲.7H. Gerdien and A. Lotz, Wiss Veröff. Siemens-Konz 2, 489 共1922兲.8G. Ecker, Ergeb. Exakten Naturwiss. 33, 1 共1961兲; S. I. Braginskii, Reviews of Plasma Physics 共Consultants Bureau, New York, 1965兲, Vol. 1,pp. 205–311.9E. Pfender, “Generation of an almost fully ionized, spectrally clean, highdensity hydrogen plasma,” in Proceedings of the International Conferenceon Ionization of Phenomenological Gases, 6th ed., Paris, France 共NorthHolland, Amsterdam, 1964兲, Vol. 34, p. 369.10R. Krichel, S. Druxes, and G. Schmitz, Z. Phys. 217, 336 共1968兲.11E. Pfender, private communication 共2004兲.12A. Hershcovitch and R. Montano, “Shielded beam delivery apparatus andmethod,” U.S. Patent No. 7,075,030 issued on 11 July 2006.13E. Foroozmehr and A. Dimitrovska, “Modeling of vortex column used inshielding of plasma beam” final project in Computational Fluid Dynamicscourse, taught by B. Anthohe in the Department of Mechanical Engineering, Southern Methodist University, 2007 共unpublished兲; R. Kovacevic,private communication 共2007兲.14A. Hershcovitch and the Acceleron Team, Phys. Plasmas 12, 057102共2005兲.15A. Hershcovitch, Nucl. Instrum. Methods Phys. Res. B 241, 854 awindow/plasmawindow.htm#;where results of independent evaluation are posted by Acceleron Inc., 21Lordship Rd., E. Granby, CT 06026.17Report No. 06-15743 of Quali-Tech, Inc. to Acceleron Inc., 28 February2006 in NICE3 Acceleron final project report 共unpublished兲; archived atthe Brookhaven National Lab Office of Intellectual Property, M. Fury,private communication 共2007兲.12Downloaded 21 Feb 2012 to 130.199.3.165. Redistribution subject to AIP license or copyright; see http://pop.aip.org/about/rights and permissions
vortex generator. Though a Plasma Window is not necessary to generate a Plasma Shield, a Plasma Window was used as a plasma source. Basically, a vortex generator was mounted on FIG. 1. Color online Schematic of the plasma shield concept. FIG. 2. Color online Top view of the vortex generator
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