All known types of gas discharges require an electric field to initiate them. We are reporting on a unique type of gas breakdown in explosively driven generators that does not require an electric field

Altgilbers, Larry L.

Baird, Jason

Shkuratov, S. I.

Talantsev, E. F.

English

Missouri University of Science and Technology (Missouri S&T): Scholars' Mine

Missouri University of Science and Technology Scholars' Mine Mining and Nuclear Engineering Faculty Research & Creative Works Mining and Nuclear Engineering 26 Jul 2010 Electric Field-Free Gas Breakdown in Explosively Driven Generators S. I. Shkuratov Jason Baird Missouri University of Science and Technology, jbaird@mst.edu E. F. Talantsev Larry L. Altgilbers Follow this and additional works at: https://scholarsmine.mst.edu/min_nuceng_facwork  Part of the Explosives Engineering Commons Recommended Citation S. I. Shkuratov et al., "Electric Field-Free Gas Breakdown in Explosively Driven Generators," Physics of Plasmas, vol. 17, American Institute of Physics (AIP), Jul 2010. The definitive version is available at https://doi.org/10.1063/1.3460349 This Article - Journal is brought to you for free and open access by Scholars' Mine. It has been accepted for inclusion in Mining and Nuclear Engineering Faculty Research & Creative Works by an authorized administrator of Scholars' Mine. This work is protected by U. S. Copyright Law. Unauthorized use including reproduction for redistribution requires the permission of the copyright holder. For more information, please contact scholarsmine@mst.edu. Electric field-free gas breakdown in explosively driven generatorsSergey I. Shkuratov,1,a Jason Baird,1,2 Evgueni F. Talantsev,3 and Larry L. Altgilbers41Loki Incorporated, Rolla, Missouri 65409, USA2Department of Mining and Nuclear Engineering, Missouri University of Science and Technology, Rolla,Missouri 65409-0450, USA3Pulsed Power LLC, Lubbock, Texas 79416, USA4U.S. Army Space and Missile Defense Command/Army Forces Strategic Command,Huntsville, Alabama 35807, USAReceived 13 April 2010; accepted 15 June 2010; published online 26 July 2010All known types of gas discharges require an electric field to initiate them. We are reporting on aunique type of gas breakdown in explosively driven generators that does not require an electricfield. © 2010 American Institute of Physics. doi:10.1063/1.3460349All types of gas discharges previously described1–3 re-quire an electric field for their initiation. The electric fieldstrength in a gas filled interelectrode gap is the main param-eter that determines the probability of ionization of the gasand the generation of low-temperature plasma.1–3 Metallicelectrodes can play an important role in the gas breakdownprocess providing free electrons in the gas gap due to thefield electron emission, secondary electron emission, or pho-toelectron emission. In the early 1960s, it was discoveredthat the laser beams can induce gas breakdown.4,5 It wasexperimentally demonstrated that high-power laser beamscause plasma formation and the propagation of the plasmafront in gases with no metallic electrodes in the system. Theionization of gas and the formation of plasma occur due toinelastic collisions between electrons and atoms in which theinteraction energy is drawn from the electromagnetic field ofthe light waves. The laser-beam-induced gas breakdown wassuccessfully used to initiate gas discharges in ultrahigh volt-age spark gap switches.5 In this brief communication, wereport on experimental results that demonstrate that gasbreakdown can occur with no electric field under the uniqueoperational conditions occurring in explosively drivengenerators.The electrical operation of helical flux compression gen-erators FCGs is based on the magnetic flux compressioninside the helical coils stators due to the explosively drivenexpansion of a cylindrical metallic armature loaded with ahigh explosive HE charge along its axis.6–8 A schematic ofa FCG is shown in Fig. 1a. Consider the traditional modelof the gas breakdown phenomenon in FCGs most recentpublications9,10. The model is based on the assumption thatgas breakdown is caused by a high pulsed electric field, E,generated in the system due to the expansion of the armatureof the FCG and the compression of the magnetic flux insidethe stator Fig. 1a,d/dt = I · dL/dt + L · dI/dt , 1where , I, and L are magnetic flux, electric current, andinductance of the FCG, respectively.In helical FCGs, the maximum electric fields, Emax, islocated roughly 12 to34 of the distance from the contact pointof the expanding armature with the stator to the beginning ofthe armature expansion Fig. 1a. In accordance with theaforementioned model, the gas breakdown within the FCGwill occur in the region of highest electric field strength, EmaxFig. 1a Ref. 10 contains detailed explanations and re-lated references. This assumption about the location of gasbreakdown at the maximum electric field region is in goodagreement with known mechanisms of gas breakdown inconventional electric systems.1–3It follows from the aforementioned model that with theabsence of a magnetic field, B, and magnetic flux, , in theFCG, the gas breakdown should not occur because of theabsence of an electric field Eq. 1. Researchers apparentlydid not question this conclusion prior to the work reportedherein, but we examined the hypothesis and believed it to befalse for the following reason.A schematic of an experiment with a FCG armature thatexplosively expands without any electric and magnetic fieldin the system is shown in Fig. 1b. We loaded the oxygen-free high-conductivity copper cylindrical armature outer di-ameter, o.d.=50.8 mm, inner diameter, i.d.=46.2 mm, andlength, h=170 mm at one end with a 225 g cylindrical ex-plosive charge of desensitized RDX explosive Chapman–Jouguet state pressure of 22.4 GPa, and we initiated theexplosive charge using a single RP-501 detonator. The arma-ture was placed inside a clear polycarbonate tube o.d.=82.7 mm, i.d.=77.0 mm, and h=150 mm that servedas a model of the stator of the FCG. There were no electricpotentials, electric currents, or electric and magnetic fields inthe system Fig. 1b. We ignored the presence of Earth’smagnetic field in these experiments due to the field’s negli-gibly low intensity in comparison with typical values ofmagnetic flux density, B, in actual FCGs.10,11 The clear plas-tic tube made it possible to observe, in detail, the processesoccurring during explosive expansion of the armature. Thetube confined the gas between the explosively expanding ar-mature and the inner wall of the tube, in the same way thatthe expanding armature and stationary stator of an actualhelical FCG traps gas between them. In addition, the tubeaElectronic mail: shkuratov@lokiconsult.com.PHYSICS OF PLASMAS 17, 074504 20101070-664X/2010/177/074504/3/$30.00 © 2010 American Institute of Physics17, 074504-1simulated the wire insulation of the coil forming the stator ofa helical FCG Figs. 1a and 1b. We performed high-speed photography of the device’s operation with a Cordin10 A high-speed framing camera and obtained 26 framesexposed at 500 000 frames/s. To avoid shock-induced glowin the atmosphere around the device Fig. 1b, we purgedthe test chamber with pure helium immediately before pho-tographing the shots. Cameral synchronization and othertechnical details were given in Ref. 12.Series of high-speed photographs taken during explosiveoperation of the device Fig. 1b are shown in Fig. 1c; thesix photographs had a 2 s frame-to-frame interval. The op-eration of the system Fig. 1 started with the point detona-tion of its HE charge at the end of the copper armature. Thespherically expanding detonation shock within the explosivecharge reached the inner wall of the armature, went throughthe armature thickness as an overdriven acoustic shock, andemerged from the armature outer surface. As a result of theFIG. 1. Color a Diagram illustrating the explosively driven expansion of the metallic armature within the stator of helical FCG and location of themaximum electric field. b Schematic of the experimental setup and cut-away view of the device used for investigation of initiation of the electric dischargein gas within the FCG. c Series of high-speed photographs taken during explosive operation of the device shown in Fig. 1b.FIG. 2. Color Diagram of the experimental setup for high-speed photography of explosive operation of the shock wave FMG and series of high-speedphotographs taken during explosive and electrical operation of the FMG.074504-2 Shkuratov et al. Phys. Plasmas 17, 074504 2010action of the shock wave and high-pressure gases from thedetonation of the HE charge inside the armature, the arma-ture started its expansion. In the series of photographs, onecan see the dynamics of an explosively expanding copperarmature. Note that the explosive charge length within thearmature was half the overall length of the armature so theexplosive expansion stopped approximately midframe ineach photograph. The radial velocity of expansion of thearmature as determined by measurements from the high-speed photographs was 3.00.1 km /s. When the explo-sively expanding armature approached the inner wall of theplastic tube, intense plasma formed around the expandingarmature. The phenomenon documented by these photo-graphs is direct experimental evidence of the formation ofplasma and the propagation of the plasma front in the FCGpurely due to the explosive motion of conductors; there wereno electric or magnetic fields in the system. This means thatin actual explosive driven generators the gas breakdown doesnot depend on the electric field strength in the system. There-fore, even with no electric field in the system or with anextremely low electric field level, it is possible to have aninitial plasma in the FCG.To demonstrate that this effect is common to all explo-sively driven electric generators, we performed high-speedphotography of the operation of an explosive driven shockwave ferromagnetic generator FMG.13,14 For these experi-ments, we designed and constructed miniature FMGs withbodies made of a clear polycarbonate so that we could ob-serve the processes which occurred inside the devices duringexplosive and electrical operations. The impact of an explo-sively accelerated flyer plate Fig. 2 on the cylindricalNd2Fe14B element initiated a longitudinal shock wavepropagates along the magnetization vector M shock wavethat demagnetizes the magnet.13 We loaded the FMG with a19 g charge of desensitized RDX and a RP-501 detonator.The diameter of the Nd2Fe14B ferromagnet was 22.2 mm andits length was 25.4 mm. The 6061 aluminum flyer plate hada diameter of 25.0 mm and a thickness of 2.5 mm. Series ofhigh-speed photographs taken during explosive operation ofthe FMG are presented in Fig. 2. Intense plasma formationexists ahead of the explosively accelerated flyer platebetween the impacting faces of the plate and the ferromag-netic element at t=2 s and later at t=4 s Fig. 2. Itfollows from our comparison of the high-speed photographsand waveforms of the voltage and current pulses producedby the ferromagnetic generator that the plasma formed priorto initiation of the shock wave in the ferromagnetic elementand prior to the generation of electric signals in the FMG.This is additional experimental evidence of the formationof plasmas in explosive electric generators, in the absenceof external electric fields, by explosively driven metallicelements.Based on observed experimental fact that an electricfield does not have to be present to initiate gas breakdown,which can, in turn, cause the development of intense gasdischarges even when low electric fields are generated in aFCG armature-stator circuit, this breakdown phenomenon isdifferent from all described types of gas breakdown.1–5Similar gas breakdown phenomena may take place inshock wave ferromagnetic and ferroelectric generators de-signed with explosively accelerated metallic flyer plates. Infact, in one series of our experiments, we had difficulty ob-taining signals from the first of several single-turn diagnosticcoils placed along the ferromagnetic element of a FMG.13 Asreported in Ref. 13, it was practically impossible to obtainclear electrical signals from diagnostic coils placed close tothe impact face of the magnet.In conclusion, we demonstrated that there is a heretoforeunexplained type of gas breakdown that occurs in explo-sively driven electric generators and that the resulting plasmamay be the cause of the development of gas discharge inthese generators.1L. B. Loeb, Fundamental Processes of Electrical Discharge in GasesWiley, New York, 1947.2A. D. MacDonald, Microwave Breakdown in Gases Wiley, New York,1966.3G. A. Mesyats, Pulsed Power Kluwer/Plenum, New York, 2005.4R. G. Meyerand and A. F. Haught, Phys. Rev. Lett. 11, 401 1963.5C. G. Morgan, Rep. Prog. Phys. 38, 621 1975.6C. M. Fowler, W. B. Garn, and R. S. Caird, J. Appl. Phys. 31, 588 1960.7A. D. Sakharov, Sov. Phys. Usp. 9, 294 1966.8L. L. Altgilbers, J. Baird, B. Freeman, C. S. Lynch, and S. I. Shkuratov,Explosive Pulsed Power Imperial College Press, London, 2010.9P. Appelgren, G. Bjarnholt, M. Elfsberg, T. Hurtig, A. Larsson, and S. E.Nyholm, IEEE Trans. Plasma Sci. 36, 2673 2008.10J. V. Parker, T. C. Cavazos, C. E. Roth, J. H. Degnan, G. F. Kiuttu, F. M.Lehr, and S. K. Coffey, in Proceedings of the 2006 IEEE InternationalConference on Megagauss Magnetic Field Generation and Related Topics(Megagauss Magnetic Fields and High Energy Liner Technology), editedby G. F. Kiuttu, P. J. Turchi, and R. E. Reinovsky IEEE Service Center,Piscataway, NJ, 2007, p. 265.11R. J. Faehl, B. G. Anderson, D. A. Clark, C. A. Ekdahl, J. H. Goforth, I.R. Lindemuth, R. E. Reinovsky, P. T. Sheehey, T. Peterson, L. J. Tabaka,V. K. Chernyshev, V. N. Mokhov, V. N. Buzin, O. M. Burenkov, A. M.Buyko, V. V. Vakhrushev, S. F. Garanin, B. E. Grinevich, G. G. Ivanova,V. A. Demidov, V. I. Dudoladov, V. V. Zmushko, A. I. Kuzyaev, A. I.Kucherov, B. M. Lovyagin, P. N. Nizovtsev, A. A. Petrukhin, A. I.Pishurov, V. N. Sofronov, S. S. Sokolov, V. P. Solovyev, A. I. Startsev, V.B. Yakubov, and E. V. Gubkov, IEEE Trans. Plasma Sci. 32, 1972 2004.12S. I. Shkuratov, J. Baird, E. F. Talantsev, and L. L. Altgilbers, Appl. Phys.Lett. 94, 171502 2009.13S. I. Shkuratov, E. F. Talantsev, J. C. Dickens, M. Kristiansen, and J.Baird, Appl. Phys. Lett. 82, 1248 2003.14S. I. Shkuratov, E. F. Talantsev, J. Baird, L. L. Altgilbers, and A. H. Stults,AIP Conf. Proc. 845, 282 2006.074504-3 Electric field-free gas breakdown… Phys. Plasmas 17, 074504 2010

Electric Field-Free Gas Breakdown in Explosively Driven Generators

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Electric Field-Free Gas Breakdown in Explosively Driven Generators

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