Wire Explosion by Electromagnetic Induction

IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 40, NO. 7, JULY 2012 1891

Wire Explosion by Electromagnetic InductionRyan van Herel, Rowan Sinton, Wade Enright, and Pat Bodger

Abstract—A conductive plasma channel in the shape of a 360◦

helical arc has been realized via the mechanisms of electromag-netic induction and exploding wire (EW). This paper is focusedon documenting the restrike (RS) mechanism for a wire explodedvia electromagnetic induction. The apparatus is a pair of mu-tually coupled helical coils, which are arranged on cylindricalpolyvinylchloride formers, with a capacitor bank as the energysource. Voltage and current waveforms are presented. The wireexplosion by induction exhibits key features that exist in conven-tional wire explosion by conduction, namely, wire fragmentation, adwell period, and RS phenomena. Exponential damping envelopeswere graphically applied to distinguish between RS and non-RSoutcomes of the wire explosions. Photography, visual observa-tion, and copper oxide residue patterns also helped to determinewhether an RS took place. Lichtenburg figures in the form of dusttrees were found to have formed at some time during the inducedEW discharge.

Index Terms—Electromagnetic induction, exploding wire (EW),plasma generation, restrike (RS).

I. BACKGROUND

EXPLODING WIRE (EW) research to date has been en-tirely concerned with the direct discharge of electricity

from a storage element (typically a capacitor bank) into a wire.That process is regarded herein as “EW by conduction” (CEW).When a wire explodes in CEW, it undergoes several physicalchanges. Chace and Moore summarized the EW process [1].The “first phase” of an EW refers to the initial current conduc-tion period (or initial current pulse) incorporating the fragmen-tation of the wire. The “dwell period” (or “current pause”) ofthe EW is the period of ionizing growth of plasma. The currentconduction during the dwell period is small compared with thecurrent magnitude during the first phase. At the conclusion ofthe dwell period is the possibility of restrike (RS), i.e., the EWresponse that forms a conductive plasma channel.

Vlastos later demonstrated with short EWs that several dif-ferent types of RS exist and that the type of RS dependson the average electric field (AEF) applied to the wire [2],[3]. Generally, the AEF is the voltage applied to the wiredivided by the wire length. The RS mechanisms were achievedthrough AEFs much higher than 10 kV/m. Several authors have

Manuscript received November 24, 2011; revised March 21, 2012; acceptedApril 21, 2012. Date of publication June 11, 2012; date of current versionJuly 5, 2012. The research was made possible with the generous support ofthe Electric Power Engineering Centre, Christchurch, New Zealand.

The authors are with the Department of Electrical and Computer Engineer-ing, University of Canterbury, Christchurch 8140, New Zealand.

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TPS.2012.2198245

noted “plasma beads” forming during an EW [4]–[7]. Taylorintroduced large inductances, which reduced the dwell periodduration, but did not preclude plasma beads from instigatingan RS. Sinton et al. created channels of plasma up to 36 m longusing a lower AEF RS mechanism, which relies on plasma beadformation [8]. Sinton et al. later used the same mechanism toachieve plasma channels up to 60 m in length [9].

Since 1998, the phenomenon of EW RS has been investigatedat the University of Canterbury. The research has been appliedprimarily to experimental high-voltage engineering education,with a view to applications in the investigation of atmosphericelectricity and EW plasma-knot making, as a test bed for ball-lightning studies. The basis for these studies is an understandingof how the EW behaves in simple geometric structures suchas straight lines, helices, and rings, which are conducive tostraightforward mathematical conception and modeling.

Related to this paper, research on inductively coupled plas-mas has been carried out by Shpanin et al. to create and propelplasma rings under atmospheric conditions [10]. In that work,formation of the azimuthal electric arc was instigated by rapidlyseparating a pair of annular electrodes.

Other prior research demonstrated a plasma winding createdvia the EW RS mechanism, to investigate the use of plasma ina helical coil shape. That winding was formed by CEW, and aconcentric solid conductor winding was used to couple with theplasma winding to flash a sphere gap [11].

It has been seen that the AEF metric is central to the obser-vations made in studies where wires are directly exploded via acapacitor discharge. The AEF metric is certainly applicable inthose cases. It is herein demonstrated that similar effects, suchas plasma bead formation and the RS channel, can be obtainedin a dissimilar circuit topology, namely, wire explosion viaelectromagnetic induction.

II. INTRODUCTION

A conductive plasma channel in the shape of a 360◦ helicalarc has been realized via the mechanisms of electromagneticinduction and the EW, in the High Voltage Laboratory at theUniversity of Canterbury, Christchurch, New Zealand, and isshown in Fig. 1.

This paper introduces results of current impulses applied to asolid copper winding, producing a rapidly changing magneticfield used to explode a wire by induction. Consequently, aplasma channel is formed from the ensuing RS of the EW.

The RS mechanism in wire explosion by electromagneticinduction (IEW) is documented here. It was found that anumber of physical features of IEW are common to CEW, suchas wire fragmentation, the dwell period (also known as currentpause), and RS. The IEW apparatus used to facilitate creationof plasma by induction via the EW RS is described in this

0093-3813/$31.00 © 2012 IEEE

1892 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 40, NO. 7, JULY 2012

Fig. 1. Explosion of a copper wire by electromagnetic induction.

paper. Enameled copper wires, which are 0.2 mm in diameter,were exploded in the magnetically coupled coil system foreight different discharge voltages. Wires, which are 0.2 mm indiameter, were used because that diameter is currently beinginvestigated in concurrent studies of CEW at the University ofCanterbury. Observation methods and waveforms are presentedfor these discharges, with accompanying descriptions of theEW response. The plasma formed by IEW has the advantageof being magnetically coupled to but electrically isolated fromthe discharge circuit. However, IEW still requires connectionto a receiver coil structure and suffers flux losses due to the aircore of the magnetically coupled coils.

III. METHOD

A. Circuit Description

The high-voltage energy supply for the discharge is a21.4-μF bank of oil-insulated capacitors C. This is connectedto the L1 winding through a three-electrode triggered sparkgap (TSG) [12]. A water resistor R of 30 kΩ is connected ina shunt across the L1 winding to maintain minimum currentflow through the TSG once it has triggered. In Fig. 2, thecircuit elements are shown, and the winding sets are laid outto show the relationship between magnetic flux density B andthe voltages and currents that are generated in the machine.

B. Machine Description

The IEW machine is an air-cored helical concentricallywound set of mutually coupled coils. The windings are arrangedon cylindrical polyvinylchloride (PVC) formers, in order tomaximize the magnetic flux coupling between them. The crosssection of the IEW machine, depicting the layout of the con-ductors, is shown in Fig. 3. The inside winding L1 is 140 mmin diameter and is wound from 20 turns of a 2-mm-diameterenameled copper wire, with a winding pitch of 18 mm. Theoutside winding is composed of two receiver coils and the IEWturn, spanning a diameter of 160 mm. The receiver coils areeach wound from ten turns of a 0.63-mm-diameter enameledcopper wire, with a winding pitch of 11 mm. These are situated

Fig. 2. Circuit diagram with windings arranged in exploded view, depicting asingle axis of magnetic flux B through an air core and corresponding senses ofvoltage and current. Coils L1 and L2 represent the mutually coupled coils. L1

is connected to the capacitor bank C via the TSG. R is the shunt water resistor.V1 is the voltage across the L1 coil. L2 is comprised of two coil sections LA

and LB , with the EW straddling the two. VA and VB are the voltages acrosscoils LA and LB , respectively. VEW is the voltage across the EW. I1 and IEW

are the currents flowing through each of the mutual inductors.

at the top and bottom of the device, with the EW straddling theintervening space. The EW is a single turn of a 0.2-mm enam-eled copper wire with a winding pitch exceeding 56 mm. There-fore, there is enough electrical clearance to avoid shorting theterminals out before the EW has an opportunity to form an RS.The inside winding is connected to the capacitor bank, whereasthe outside windings are connected together in short circuit.

C. Instrument Description and Observation Methods

Voltage V1 was measured with a 600-kV Ferranti capacitivevoltage divider, and current I1 was measured using a 50-kAPearson ferrite-cored high-frequency current transformer, asshown in Fig. 2. The EW voltage VEW was measured witha Tektronix 40-kV dc high-voltage probe. Due to the smallaperture size of the Pearson coil, sufficient voltage isolationfrom the receiver coil conductors could not be achieved tosafely measure the current flow through the EW using thatcurrent probe. Therefore, the EW current IEW was measuredwith a Fluke i3000 flexible current clamp.

A Canon 400D digital single-lens reflex camera was usedto capture still photographs in synchronism with the TSG’striggering signal. The camera was fitted with two ND8 dark-ening filters, and the exposure time for the camera was 500 ms.The photographs help to visually confirm the outcome of theexperiment, whether there was a full plasma discharge or onlypartial plasma formation. There are qualitative methods usedto record experimental outcomes, for instance, that of listeningto the loudness of the discharge, observation of bright flashes,observation of winding condition, and inspection of metallicoxide residue patterns left behind by the wire explosion.

VAN HEREL et al.: WIRE EXPLOSION BY ELECTROMAGNETIC INDUCTION 1893

Fig. 3. Sectional elevation of the EW by an induction machine.

IV. RESULTS

A. Experimental Results

Eight experiments of wire explosion by induction were car-ried out. Each test was performed at a different value of initialcapacitor voltage V0. The initial capacitor voltages used were14, 16, 18, 20, 21, 23, 25, and 27 kV in dc. The tests beganfrom the lowest voltage and finished with the highest voltage inthe series. For the preparation of each test, a new Nomex sheathwas fitted around the exterior of the receiver coil former, andthe EW test specimen was wrapped around the Nomex sheathand connected to the receiver coil terminals. The capacitor bankwas charged up to the test voltage, and the TSG was triggeredto execute the experiment. The capacitor discharged into theL1 winding, creating a rapidly changing magnetic flux. TheL2, being magnetically coupled with the L1 winding, producesvoltage and current via mutual induction. The wire connected tothe receiver coils, i.e., LA and LB , explodes, producing plasma.There were two different outcomes for the EW test wire: thatof physically continuous plasma channel formation, which is

Fig. 4. Outcomes of IEW. (a) Partial plasma formation in IEW with 14-kVdischarge. (b) RS of IEW.

known as RS, and that which does not produce a continuousplasma channel, which is called non-RS (NRS). The experi-ments V0 = 14 kV up to and including V0 = 21 kV producedNRS behavior, which is shown in Fig. 4(a). The V0 = 23 kVup to and including V0 = 27 kV experiments produced an RS,which is the objective of this paper and is shown in Fig. 4(b).

B. Electrical Characteristics

In the capacitor voltage waveform V1 in Fig. 5, an oscilla-tion is consequent, which is typical of an underdamped RLCresponse. This is similarly true for the capacitor current I1, asshown in Fig. 6. Between the NRS and RS experiments, thereappears to be a difference in the exponential damping envelope.As a simple first-order approximation, the coupled coil systemmay be assumed to be an underdamped parallel RLC circuit, asshown in Fig. 7. C is the capacitance of the capacitor bank, Lis the leakage inductance of the mutually coupled coil system,and the resistance of the EW REW is represented by a lumpedplasma resistance, which is assumed to be static with respect totime. In the parallel RLC circuit, the exponential damping coef-ficient α is dependent upon the capacitance and the resistance.With the capacitance of the circuit known to be constant, theremaining free variable is the plasma resistance. The dampingfactor α for a parallel RLC is α = (2REWC)−1. The parallelRLC is a convenient approximation because there is no rela-tionship between the exponential damping coefficient and thecircuit inductance. Once the EW forms an RS, it is known thatthe plasma resistance drops from a nearly open circuit to a lowresistance. This resistance change is detectable by observingthe change in the exponential envelope of the underdampedRLC response in the inside winding, between RS and NRSoutcomes. This observation is useful as a first reference, in thecase that there is no possible means of directly measuring thevoltage or current of the EW (for instance, if the experiment


Fig. 5. Voltage V1 across the inside winding L1 with exponential dampingenvelopes displayed.

Fig. 6. Current I1 flowing in the inside winding L1.

Fig. 7. Circuit model showing the capacitor bank C, the leakage inductanceL of the air-cored mutually coupled coils L1 and L2, and the lumped staticresistance REW of the EW.

Fig. 8. Voltage V1 across the inside winding L1 in the first phase.

Fig. 9. Current I1 flowing in the inside winding L1 in the first phase.

is arranged such that probes are physically unable to be fittedto the wire due to temperature or size constraints). In practice,the plasma resistance dynamically varies; therefore, the valueof alpha changes with respect to time. This makes it difficult toaccurately apply this lumped model to check the frequency ofthe RLC response. Exponential damping envelopes (±V0 e

−αt)were graphically applied to the V1 voltage waveforms, as shownin Fig. 5. In the NRS case, α = 1200 s−1, whereas in theRS case, α = 3400 s−1. That result indicates that when theEW reaches the RS condition, then the coupled coil systementers a more conductive state than is maintained in the NRSresponse. For the RS case, the EW voltages and currents appearto converge to similar magnitude values, particularly during theRS. This can be explained if the IEW machine is viewed as atransformer with the EW as the load. The load is identical ineach different experiment, i.e., the same length of EW/plasmais used; hence, the load voltage and current are similar.

In the first phase detail of voltage and current in the L1 wind-ing (see Figs. 8 and 9), some effects of the EW fragmentation


Fig. 10. Voltage VEW across the EW.

Fig. 11. Voltage VEW across the EW in the first phase.

are visible. The voltage across the EW VEW (see Fig. 10) showsa large negative polarity initial voltage spike. In the detail ofthe first phase (see Fig. 11), it is observed that the peak of thefirst-phase EW voltage spike increases in height with respect tothe applied capacitor voltage and current. This is because thecorresponding higher current flowing through the EW duringfragmentation (see Fig. 12) drives the resistance change ofthe wire more quickly. The EW voltage was developed viamagnetic induction from coil L1 connected to the capacitorbank. The receiver coils and EW comprise a circuit, i.e., L2,which is electrically isolated from the capacitor bank; therefore,voltages developed in this section of the circuit are isolatedfrom the capacitor voltage. When the wire fragments, thecurrent flow in the wire is abruptly interrupted, and the resultis a large-magnitude inductive voltage pulse, at around 30 μs.The advantage of electrical isolation from the capacitive sourceallows the voltage spike to reach large magnitude values, upto 30 kV for these experiments. After the initial spike, NRSEWs become open circuit; therefore, there is a maximal voltageswing (see Fig. 10). In the RS case, the voltage after the spike

Fig. 12. Current IEW flowing in the EW in the first phase.

Fig. 13. Current IEW flowing in the EW.

continues to increase through its dwell period. Once the dwelltime has elapsed, the EW has formed a complete plasma path.When this EW RS is created, it acts like a short circuit acrossthe L2 winding. This causes the voltage to collapse (see Fig. 10)because high current conducts through the plasma channel (seeFig. 13). In Fig. 12, the peak of the fragmentation current isobserved to increase in magnitude with respect to the initialcapacitor voltage.

In the IEW current waveforms (see Fig. 13), the first phase ofIEW appears similar to the first phase of EW by conduction in[13]. For the NRS case, the EW undergoes fragmentation, andthere is some conduction for a time, before the ionization mech-anism ceases. Upon failure of that mechanism, the EW entersan open-circuit state depicted by the collapse of current, whichindicates loss of conduction. In the RS case, the initial currentconduction period again resembles the first-phase current ofthe straight EW by conduction. After the EW dwell period,the plasma conduction channel is formed, and the current risesto a maximal oscillation. Most notably, the RS current flows


Fig. 14. Copper oxide residues from NRS EW by induction.

Fig. 15. Dust trees on the surface of the outer PVC former. The trees haveappeared between the turns of the receive coils.

through several zero crossings, in an underdamped oscillatoryresponse. For the 25- and 27-kV discharges, the current magni-tude values in the initial RS maxima were above 5.4 kA, whichexceeded the measuring capability of the Fluke i3000 sensor.Therefore, that detail is omitted in the current traces in Fig. 11.

C. Physical Evidence

1) Bands of Residue: The EW leaves behind copper metaland oxide residues on any surface that the explosion comesinto contact with. Such residue patterns have been captured byresearchers as early as van Marum in 1786 [14]. For the EWby an induction machine, a paper sheath was used to protectthe PVC former from being coated with conductive residues. Inthese experiments, the visual continuity of the band of residuehelped to indicate if an RS had occurred. The residue colorsare suggestive of the type of copper oxides formed, and thesein turn suggest the temperature of the plasma at different partsof the band [15]. The residue contains copper(I) and copper(II)oxides, but it is principally composed of black copper(II) oxide(see Fig. 14).

2) Dust Trees: A dust tree is a type of a Lichtenburg figurethat was noted during the experiments. After a discharge, rowsof dust trees were found to have sprouted perpendicular tothe nonexploding copper conductors of the receive coils (seeFig. 15). In these experiments, the dust trees typically formedin the presence of the copper suspension and must arise dueto the high electric fields along the profile of the receive coils.They appear in neat rows where the receive coils are located.While the main results are not dependent on this observation, itwas a reliable observation made during the IEW experiments.

V. CONCLUSION

This paper has introduced and described new experimentsthat produce plasma from IEW. A conductive plasma channel inthe shape of a 360◦ helical arc was produced. The apparatus wasa pair of mutually coupled helical coils, which are arranged oncylindrical PVC formers, with capacitor discharge as the energysource. Several techniques were employed to make physicalobservations of the wire explosion. Waveforms of voltage andcurrent of the EW were presented. The IEW exhibits keyfeatures that exist in CEW, namely, wire fragmentation, a dwellperiod, and RS phenomena. Copper oxide residue patternshelped to determine whether an RS took place. Lichtenburgfigures in the form of dust trees were found to form sometime during the IEW discharge. A simple RLC approximationwas used to distinguish between RS and NRS outcomes in theIEW by examining the exponential damping coefficient of thevoltage wave in the L1 field winding. A higher value of αsuggested the system had become more conductive, indicatingthe RS condition for the IEW. It was difficult to proceedwith comprehensive modeling of the results in the absence ofinformation about EW plasma resistance, which is an area forfuture investigations. It is also of interest to determine whetherthere is any periodic behavior in the dynamic resistance of theEW plasma in the IEW topology, which could be conducive topulsed power applications.

This paper has presented the first EW by induction andproved that uniform plasma can be created in this manner.Further investigation into the RS mechanism may allow opti-mization of this technique in future.

ACKNOWLEDGMENT

The authors would like to thank the generous support fromJ. Lawrence (Electric Power Engineering Centre) and technicalsupport from D. Healy, K. Smart, and J. Woudberg.

REFERENCES

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[4] H. Nagaoka, T. Futagami, and T. Machida, “Electric explosion ofwires and threads,” in Proc. Imperial Acad., Tokyo, Japan, 1926, vol. 2,pp. 328–331.

[5] R. Dannenberg and A. Silva, “Exploding wire initiation and electricaloperation of a 40-kv system for arc-heated drivers up to 10 feet long,”NASA, Washington, DC, Tech. Rep. NASA TN D-5126, 1969.

[6] B. K. Bhat and I. B. Jordan, “Explosion of bare and insulated copperwires,” J. Appl. Phys., vol. 42, no. 2, pp. 809–814, Feb. 1971.

[7] M. J. Taylor, “Formation of plasma around wire fragments created byelectrically exploded copper wire,” J. Phys. D, Appl. Phys., vol. 35, no. 7,pp. 700–709, Apr. 2002.

[8] R. Sinton, R. van Herel, W. Enright, and P. Bodger, “A Marx generatorfor exploding wire experiments,” in Proc. APPEEC, Mar. 25–28, 2011,pp. 1–4.

[9] R. Sinton, R. van Herel, W. Enright, and P. Bodger, “Generating extra longarcs using exploding wires,” J. Appl. Phys., vol. 110, no. 9, pp. 093303-1–093303-4, Nov. 2011.

[10] L. M. Shpanin, G. R. Jones, J. W. Spencer, and B. E. Djakov, “Control andpropulsion of an atmospheric pressure plasma ring,” IEEE Trans. PlasmaSci., vol. 36, no. 5, pp. 2795–2800, Oct. 2008.


[11] R. Sinton, C. Hammond, W. Enright, and P. Bodger, “Generating highvoltages with a plasma coil transformer,” in Proc. Techcon Asia Pacific,Sydney, Australia, 2009, pp. 211–219.

[12] R. Sinton, R. van Herel, W. Enright, and P. Bodger, “Design and con-struction of a triggered spark gap for long distance exploding wire exper-iments,” in Proc. 20th AUPEC, 2010, pp. 1–3.

[13] R. Sinton, R. van Herel, W. Enright, and P. Bodger, “Investigating long-distance exploding-wire restrike,” IEEE Trans. Plasma Sci., vol. 38, no. 4,pp. 1015–1018, Apr. 2010.

[14] M. van Marum, “Eerste vervolg der proefneemingen gedaan met Teyler’selectrizeer-machine,” in Library Online Resource. Delft, The Nether-lands: Technische Univ. Delft Library, 1785. [Online]. Available: http://www.library.tudelft.nl/tresor/books/Electrizeer-machine/

[15] M. J. Taylor, “Plasma propellant interactions in an electrothermal-chemical gun,” Ph.D. dissertation, Royal Military College Science,Cranfield Univ., Shrivenham, U.K., 2002.

Ryan van Herel received the B.E. (Hons) degree inelectrical and electronic engineering and the Masterof Engineering degree from the University of Canter-bury, Christchurch, New Zealand, in 2008 and 2011,respectively.

He is currently working in the electric powerindustry in the Netherlands, specializing in the short-circuit testing of high-voltage switchgear and powertransformers.

Rowan Sinton received the B.E. (Hons) degree inelectrical engineering and the Ph.D. degree with athesis titled “Long Distance Exploding Wires” fromthe University of Canterbury, Christchurch, NewZealand, in 2007 and 2011, respectively.

He is currently working in the electric power in-dustry in New Zealand, specializing in high-voltageengineering.

Wade Enright received the B.E. (Hons) and Ph.D.degrees in electrical and electronic engineering fromthe University of Canterbury, Christchurch, NewZealand, in 1992 and 1995, respectively.

In 1996, he was with the Manitoba HVDC Re-search Centre, Winnipeg, MB, Canada. He is cur-rently a Senior Lecturer with the University ofCanterbury. He also offers electrical engineering ser-vices to the industry via his own company “Viva.”His research interests include power transformersand high voltage.

Pat Bodger received the B.E. (Hons) and Ph.D.degrees in electrical engineering from the Universityof Canterbury, Christchurch, New Zealand, in 1972and 1977, respectively.

From 1977 to 1981, he was with the ElectricityDivision, Ministry of Energy, New Zealand. He iscurrently a Professor of electric power engineeringwith the Department of Electrical and ComputerEngineering, University of Canterbury, where he isalso a Director of the Electric Power EngineeringCentre.

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Wire Explosion by Electromagnetic Induction