NASA Contractor Report 178323 

Breakdown Characterist ics of an 

Isolated Conducting Object in a Uniform Electric Field 

by 

M. G. Grothaus 

T. F. Trost 

1 

Report on 

N.A.S.A. Grant NAG-1-28 

December 1986 

(NASA-CB- 178x23) EBEAKDCMI CEIARACTERISTXCS lit2 7-26 490 

CE AK ISCLATED CCbDUCTIING CEJECT IN A 

GYIYOEB ELEC3;WIC FXXLC (Texas lecbnological OEiv-) Sb p Avail: E'IIS fC AC4jHF A01 Unclas 

CSCL 04B G3/47 008762C 

Department of Electrical EngineeringlComputer Science 

Texas Tech University 

Lubbock, Texas 79409 
BREAKDOWN CHARACTERISTICS OF AN ISOLATED CONDUCTING OBJECT 

IN A UNIFORM ELECTRIC FIELD 

by 

M. G. Grothaus 

T. F. Trost 

Department of Electrical Engineering/Computer Science 

Texas Tech University 

Lubbock, Texas 79409 

December, 1986 

Prepared for 

National Aeronautics and Space Administration 

Langley Research Center 

Hampton, Virginia 23665 

Under Grant No. NAG-1-28 

"Lightning Sensors and Data Interpretation" 
ABSTRACT 

A laboratory experiment has been conducted to determine the 

physical processes involved in the electrical breakdown of a 

particular spark gap arrangement. The gap consists of an isolated 

conducting ellip8oid located midway between two large flat 

electrodes. Gradual increase of the applied electric field, E, in 

the gap produces corona on the ellipsoid tips followed by 

flashover in a leader-arc sequence. The leader phase consists of 

the abrupt formation of ionized channels which partially bridge 

the gap and then decay prior to the arc. Measurements of dE/dt 

and of current were made, and photographs were taken with an image 

converter camera. Experimental parameters are as follows: 

Gap length = 25.4 cm 

Ellipsoid length = 10.0 cm 

Gap voltage for flashover = 98 kV 

Gap electric field strength (far from ellipsoid) = 

386 kV/m 

Gap capacitance = 78 pF 

Gap electric field energy (estimated) = 0.58 J 

Peak positive-corona-streamer current = 25 mA 

DC corona current = 0.37 - 0.40 mA 

Peak arc current = 200 A 

Arc channel diameter C 1 mm 

Duration of flashover event (leader + arc) = 1 ps 
CONTENTS 

LIST OF FIGURES ........................................... iii 

SECTION 

I . INTRODUCTION ........................................ 1 

Previous Work at Texas Tech ......................... 1 

New Research at Texas Tech .......................... 4 

I1 . EXPERIMENTAL APPARATUS .............................. 10 

Data Acquisition .................................... 11 . 

I11 . BIPOLAR CORONA ...................................... 21 

Positive Corona ..................................... 22 

Negative Corona ..................................... 25 

IV . FLASHOVER ........................................... 37 

Flashover Characteristic6 ........................... 37 

Current Measurement ................................. 41 

V . CONCLUSIONS ......................................... 55 

REFERENCES ................................................ 59 

APPENDIX .................................................. 61 
LIST OF FIGURES 

1 . 

2 . 

3 . 

4 . 

5 . 

6 . 

7 . 

a . 

9 . 

10 . 

11 . 

12 . 

13 . 

14 . 

15 . 

16 . 

Spark Gap Configurations ............................. 

Leader Propagation end Arc Transition in a 

Positive Point-Plane Gap ............................. 

Arc Transition in a Negative Point-Plane Gap ......... 

Leader Propagation and Arc Transition in a Gap 

Containing an Isolated Conducting Ellipsoid .......... 

Experimental Set-up .................................. 

Data Acquisition System .............................. 

D-dot Sensor ......................................... 

I-sensor ............................................. 

Bipolar Corona ....................................... 

The Positive Glow Discharge .......................... 

Coexistence of Positive Streamers and Positive 

Glow Corona .......................................... 

I-sensor Response to Positive Corona Streamers ....... 

I-sensor Response to Individual Positive Corona 

Streamer ............................................. 

I-sensor Response to Negative Corona in the 

Upper Gap ............................................ 

Negative Corona Discharge ............................ 

I-sensor Response to Trichel Pulses with a 

Positive Glow Discharge Present at the Ellipsoid 

Anode ................................................ 

6 

7 

a 

9 

15 

17 

19 

20 

27 

2a 

29 

30 

31 

32 

33 

34 

ili 
iv 

17 . 

10 . 

19 . 

20 . 

21 . 

22 . 

23 . 

24 . 

25 . 

26 . 

27 . 

Modulation of the Trichel Pulses when Positive 

Streamers are Present at the Ellipsoid Anode ......... 

I-sensor Response to an Individual Trichel Pulse ..... 

D-dot Response to Flashover .......................... 

The First Pulse of a D-dot Waveform .................. 

Fine-structure in the Leading Edge of the First 

Pulse of the D-dot Waveform .......................... 

Oscillogram of the Extinction Period Preceding 

Arc Transition ....................................... 

Transition to Arc .................................... 

Color Photo of Flashover ............................. 

Integrated D-dot Waveforms of Flashover .............. 

Equivalent Circuit of the Experimental Set-up ........ 

Simultaneous Oscillogram of Both I-sensor and 

D-dot Sensor Responses to Flashover .................. 

35 

36 

45 

46 

47 

4a 

49 

50 

51 

53 

54 
I. INTRODUCTION 

Due to the increased use of composite materials in modern 

aircraft, on-board digital electronic systems are increasingly 

susceptable to electrical interference from lightning. In order 

to provide guidelines for the design and test of future systems 

which will be immune to this type of disturbance, understanding 

the actual lightning threat is eseential. 

In 1980, the National Aeronautics and Space Administration 

initiated a program for studying the response of aircraft to 

direct lightning strikee. In-flight lightning measurements were 

accomplished with an instrumented F-106B Delta Dart aircraft (1). 

Interpretation of some of the lightning signatures obtained is 

presented in (2) and (3). Analysis of UHF radar data recorded 

concurrently with direct strikes to the F-106B suggests that the 

aircraft actually triggers the strikes (4). In an effort to 

increase our understanding of the basic physical processes 

involved, it was decided to investigate a similar type of 

electrical breakdown phenomenon in the laboratory. A8 a result, 

we have constructed a laboratory experiment in which an isolated 

conducting object triggers the breakdown of a uniform-field spark 

gap. T h e g a p is stressed by a high-voltage DC power supply. 

Previnue,work,et,Tnxas,Ten9 

Initial measurements of laboratory spark initiation were 

accomplished at Texas Tech in 1985 using the three spark gap 

1 

\ L\ 
2 

configurations of Figure 1 (5). These configurations include not 

only the isolated-object arrangement, which is similar to the 

aircraft-triggering case, but also the simpler cases of positive 

and negative point-plane gaps. Data was gathered from a D-dot 

sensor and an fast framing camera, with the D-dot waveforms being 

integrated by a 10 ps analog integrator. 

Measurement of flashover voltage for the three configurations 

versus length of the gap was carried out first. The main result 

was that the flashover voltage in the negative point-plane gap was 

typically two to three times higher than it was in the positive 

case as previously observed by Hutzler (6). 

A 1.0x10.0-cm aluminum ellipsoid was used as the isolated 

object in configuration 3, and one-half of this ellipsoid was 

attached to the underside of the high-voltage electrode in 

configurations 1 and 2 (Figs. la, lb) to act as the point. The 

point-plane arrangement allows one to study the breakdown behavior 

of a single point for comparison with the more complex two-point 

breakdown of the isolated ellipsoid. The point-plane case may 

ale0 have some relevance to the aircraft-lightning interaction 

problem because it is similar to the situation where one lightning 

channel is already attached to a particular location on the 

aircraft and then a new channel develops from some other location. 

The half-ellipsoid together with the entire high-voltage electrode 

simulate the aircraft, and the high-voltage lead and reeistor 

coming from the power supply simulate the attached lightning 

channel. 
3 

In configuration 1, the plate separation (distance from highvoltage electrode to ground plane) was 21.6 cm (8.5 in) with a 

power eupply voltage of 130 kV. A framing photo for the event is 

shown in Figure 2. The sequence of f i v e f r a m e s is from right to 

left. The propagation of the leader through the gap was found to 

be associated with a relatively slow, near-linear rise in the 

integrated D-dot waveform. When the leader reaches the ground 

electrode, arc transition occurs, and a sharp increase in the 

integrated D-dot waveform results (i.e., collapse of the electric 

field in the gap). The velocity of the leader was found to be 3.3 

x lo4 m/s, which is comparable to that reported by Levesque (7). 

For configuration 2, plate separation was 17.1 cm (6.75 in) 

with a power supply voltage of -168 kV. The framing photo 

corresponding to this case ie shown in Figure 3, with f i v e f r a m e s 

from right to left. Since the event was quite rapid, only the 

later stages could be seen. The decay of the arc is readily 

identifiable but no leader propagation prior to the arc is 

observed. 

Configuration 3 was set up by suspending the ellipsoid such 

that it was oriented vertically in the center of the gap 

(Figure IC). The plate eeparation was 25.4 cm (10 in), and 

power supply voltage was 152 kV. Note that i n t h i s c a s e t h e r e 

were actually two gaps in series, each 7.7 cm in length. The 

framing photo for the event is shown in Figure 4, with three 

frames from right to left. The ellipsoid itself ie not visible, 

but it liee in the center between the ionized channele. The 
4 

completion of a channel in t h e t o p gap and partial propagation of 

a leader across the bottom gap, as shown very faintly in the first 

frame, resulted in an initial rise in the corresponding integrated 

D-dot waveform. The closure of the bottom gap and subsequent arc, 

frames 2 and 3, caused the appearance of a second rise in the 

integrated D-dot waveform. It is interesting that a leader-arctransition delay was present and varied in length for different 

firings. Aging of ellipsoid tips was also observed and was found 

to have a significant effect on the breakdown voltage of the gap. 

New_Researc4_nt_Texaa_Tech 

The focus of present research efforts in our laboratory has 

turned exclusively to studying the breakdown on an isolated object 

in a uniform electric field, configuration 3 above. Further 

understanding of this case requires a more in-depth look at the 

individual breakdown processes which are involved. An improved 

experimental arrangement, described in Section 11, along with 

comparisons made with the results reported by other workers, 

enable us to accomplish this goal. 

Bipolar corona was found to have a profound effect on the 

sequence of events leading to the high current arc, or flashover. 

As such, Section I11 explores the basic nature of the corona, 

which occurs near the tips of the ieolated object. Both 

qualitative and quantitative information is presented for each 

corona polarity and some of the intrinsic differences which exist 

between t h e t w o are pointed out. 
5 

Section IV concentrates on the subject of flashover in the 

gap. Several preliminary stages of breakdown which necessarily 

lead to flashover of the gap are revealed through the use of very 

fast camera techniques and concurrent analysis of sensor 

waveforms. Direct measurement of peak current in the arc is given 

as well as an inferred measurement obtained from the response of a 

D-dot sensor located in the uniform field region. 

Conclusions are drawn in Section V along with suggestions 

related to future research areas which will contribute to 

continued progress in understanding aircraft-lightning 

interaction. 
6 

HIGH-VOLTAQE 

ELECTRODE 

i HV+ 

HI Q H -VOLTAQE 

ELECTRODE 

1 ELLIPSOID GROUND / PLANE 

Figure 1 : Spark Gap Configurations : (a) Configuration 1 : half-ellipsoid attached to positive hish-voltage electrode. (b) Configuration 2: half-ellipsoid attached to 

negative high-voltage electrode. (e) Configuration 3: full ellipsoid centered between positive high-voltage electrode and 

the ground plane. 
7 

Figure 2: Leader Propagation and Arc TraRsition in 

a Positive Foint-Plane Ga2. 
ORIGNAE PEE IS 

OF POOR QUAMW 

Figure 3: Arc Transition in a Negative Point-Plane . Ga? 
9 

QRIGINAL' PKGE E# 

POOR QUALITY, 

Cigure 4: Leader Propagation and Arc Transition in 

a Gap Containing an Isolated Conducting 

Ellipsoid. 
I I. EXPERIMENTAL APPARATUS 

The basic experimental setup, a spark gap containing an 

isolated conducting object in atmospheric air, is shown in 

Figure 5. An aluminum prolate ellipsoid with a 10.0 cm major axis 

and a 1.0 cm minor axis is suspended in the parallel plane gap by 

an adjustable dielectric stand. The ellipsoid represents a field 

enhancement factor of fifty, roughly approximating that of the F106B aircraft as discussed in (5). The high-voltage electrode is 

a 91.4 cm (36 in) diameter aluminum plate with a turned up edge. 

The large, 12.7 cm (5 in), radius of curvature of the plate’s edge 

tends to limit its observable contribution to the corona current. 

The ground electrode consists of six 1.2 m x 3.66 m (4 ft x 12 ft) 

aluminum sheets connected on a steel girder frame to create a 

3.66 m x 7.32 m (12 ft x 24 ft) ground plane. Electrode 

separation is varied by moving the prusik knot on the nylon rope 

suspending the top plate. With a plate separation of 25.4 cm 

(10 in), the gap capacitance was measured at 78 pF. Connected 

across the parallel plate geometry is a Hypotronics model 8000 

series 240 kV variable DC power supply with a 4 mA current 

capability. The supply is equipped with a control unit which 

displays the output voltage and current on two meters on the front 

panel. The meters are accurate to within plus or minus two 

percent full scale. In series with the supply and the highvoltage electrode is a 100 megohm, 220 %, power 

resistor has corona inhibitors attached to each 

10 

resistor. The 

of its ends and 
11 

limits the supply current to a maximum of 2.4 mA. The resistorcorona inhibitor geometry produces a capacitance of 26 pF. The 

output impedance of the power supply was measured to be six 

megohms and a ripple factor of less than 1 X at full load was 

observed. These figures of merit indicate that the supply can be 

considered close to ideal since it is always lightly loaded by the 

100 megohm series resistor. 

Pntn-!!nsuieitiss 

The system used for data acquisition is depicted in Figure 6. 

Data was obtained from two sensors: a D-dot sensor located 

48.2 cm (19 in) from the axis of symmetry of the gap and an Isensor located in-line with the axis. Both sensors are of "flush 

plate dipole" design in order to minimize field distortion in the 

gap. The D-dot sensor is a model FPD-1A manufactured by EG&G with 

a bandwidth of 390 MHz and a sensitivity of 0.01 square meters, 

and is shown in Figure 7. The I-sensor of Figure 8 was designed 

and constructed in our lab. It is essentially the same a s t h e 

EG&G sensor but is connected to a sampling (or shunt) resistor and 

is used to measure the current flow in the spark. The sampling 

resistor can be easily changed to give a wide variety of output 

ranges. If no conduction current contacts the I-sensor, then it 

operates as a D-dot sensor. The dimensions of the sensor were 

chosen to give the desired bandwidth and sensitivity in accordance 

with the results of (81, (9) and (10). Using a Tektronix type 

7S12 Time Domain Reflectometer (TDR), the sensor risetime, with no 
12 

sampling resistor, was found to be 0.09 ns, giving a bandwidth of 

390 MHz. Using a sampling resistor, and thus reducing the load 

resistance, results in a larger bandwidth due to the RC-dependence 

of the risetime. The sensitivity of the sensor is 0.015 square 

meters. 

The sensors are connected to a Tektronix 7834 mainframe 

storage oscilloscope with two 2.74 m (9 ft) sections of 50 ohm 

semi-rigid coaxial cable and Type N coaxial connectors. The plugins used with the s c o p e a r e a Tektronix type 7A26 dual-channel 

amplifier with 200 MHz bandwidth and a Tektronix type 7B53A time 

base. A single scope trace of the outputs of the two sensors is 

accomplished by delaying one of the sensor signals with respect to 

the other and setting the scope to the add mode. The delay was 

obtained by connecting a 250 ns delay line (with 3.68 dB 

attenuation) to one of the input channels on the 7A26. 

In addition to oscillograms of the sensor signals, 

photographic data was obtained with a TRW model 1D Image Converter 

Camera complete with a model 68 high speed framing plug-in. 

Exposure t i m e s a r e 20 ne and interframe delay is 50 ne. The 

300 volt pulse required to trigger the camera is provided by a TRW 

model 46A Trigger Delay Generator (TDG) at a risetime of 10 ns. 

The 50 ohm trigger input of the TDG is driven by a high-speed JFET 

source follower amplifier connected to the gate output of the 

oscilloscope. Unfortunately, t h e t i m e between trigger input to 

the TDG and the "zero-delay" output from the TDG is 115 ns, so 

that this amount of delay is always present before the first 
13 

camera frame. The scope is externally triggered by the signal 

from the D-dot sensor. Additional visible-light photographic data 

wee obtained through the use of a Pentax KlOOO 35 mm camera with a 

75 to 200 mm Albinar zoom lens and ASA 200 Kodacolor film. 

To provide relative immunity to the effects of 

electromagnetic interference from the discharge in the gap, the 

scope and trigger generating components were located in a screen 

box. In addition, the AC power strip inside the box was filtered. 

Appropriate attenuation of sensor signals was provided by Narda 

(DC-3 GHz, 0.3 kW peak) 20 dB attenuators and a Weinechel (5 kW 

peak) 20 dB attenuator. 
14 

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16 

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L 

I m-----T- 
18 

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19 

Figure 7: D-dot Sensor (Photo courtesy ZG&G Inc.) 

. 
20 

k 

0 
111. BIPOLAR CORONA 

Due to enhancement of the local electric field near the 

ellipsoid, visual manifestations of ionization and excitation 

processes, known as "partial breakdown" or coronas, can be 

observed and studied long before complete breakdown of the gap. 

These coronas will be bipolar in nature, i.e., emanating from both 

the positive and negative tips of the ellipsoid (Figure 9a). For 

the positive high-voltage case, the ellipsoid takes on the 

polarities shown in Figure 9b, with negative corona appearing in 

t h e t o p gap and positive corona in the bottom gap. Keeping in 

mind the symmetry resulting from t h e t w o flat plates, reversal of 

the ambient field yields the same phenomena with the locations of 

the positive and negative coronas being transposed (Figure 9c). 

Application of a DC voltage to the high-voltage electrode, 

rather than a pulsed voltage, will allow accumulation of space 

charge near the ellipsoid tip6 which can assume a transient or a 

stationary form. If condition6 are such that this space charge 

becomes stable, considerable distortion of the original "pointplane" fields of the upper and lower gaps will occur, resulting in 

the appearance of different corona modes. 

Positive and negative coronas exhibit remarkably different 

characteristics and will therefore be treated individually in the 

following sections. Since we are primarily concerned with 

conditions conducive to complete breakdown of the gap, we will 

focus our attention on corona at voltages slightly below breakdown 

21 
22 

potential for the gap. In thie situation the DC corona current, 

as meaeured by the panel meter on the power supply, is in the 

range 0.37 to 0.40 mA. 

Esmlt&ve-SnEnse 

We have observed two distinct modes of positive corona. The 

first mode, henceforth referred to as glow corona, consists of a 

thin region near the positive tip (anode) of the ellipsoid. The 

steady glow occurs when sufficient space charge of negative ions 

accumulates around the anode (11). A quasi-uniform field is set 

up between this space charge and the anode, as shown in Figure 10. 

If the space charge is of ample deneity, the locally confined 

field will be large enough to cause "breakdown" in this gap. 

Because of the quasi-uniform nature of the field, thie breakdown 

is Townsend with photoionization of the gas as the main source of 

the secondary electrons (11). As the voltage on the high-voltage 

electrode is increased, another mode of corona appears in the form 

of a pre-breakdown streamer. This mode d e v e l o p s f r o m the glow 

when the field is adequately non-uniform. Glow corona may exist 

simultaneously with the streamers as seen in Figure 11. In this 

photograph, the glow is recognized a s t h e bright region near the 

tip of the ellipsoid (top) and the streamers are the readily 

apparent blue column extending far into the gap and eventually 

contacting the gap electrode. 

With the I-sensor mounted i n t h e ground plane directly 

beneath the ellipsoid, ae previously described in Section 11, 
23 

several oscillograms of positive streamer activity were recorded. 

A typical trace, for an ambient electric field of 3.86 kV/cm*, is 

shown in Figure 12. The positive polarity of the pulses indicates 

that positive charge Is flowing from the sensor plate through the 

sampling resistor to ground, a8 it must for downward streamer 

currents. The frequency of the pulses was typically found to be 

20 to 30 kHz and was, for the most part, regular, with slight 

variations in pulse amplitude. 

A closer look at the structure of the individual pulses 

yields the waveform of Figure 13. From this figure, we can see 

that the record consists of five distinct regions: an initial 

flat region of very small amplitude, a steeply rising front and 

associated current peak, a decay, a steady current which may flow 

for varying lengths of time, and, finally, a second current peak 

and eventual decay to zero. Recalling that the sensor behaves 

like a D-dot sensor until the streamers make actual contact with 

the sensor plate (at which time the record represents conduction 

current), the uniform propagation of a streamer from the anode 

should be associated with a constant D-dot level for approximately 

150 ns. The tail end of this response is seen as the flat region 

of minute amplitude at the extreme left of the waveform. The 

*This field value is obtained by dividing gap voltage by gap 

length. T h e g a p voltage equals the power supply voltage (read 

from panel meter) minus the drop i n t h e 100 megohm resistor due to 

the DC corona current (read from panel meter). 
24 

sharp rise in the leading edge of the waveform should correspond 

to the bridging of the gap by the streamer. The linking of the 

ellipsoid anode and the sensor by the streamer produces a current 

maximum of typically 25 mA. The positive streamer current then 

decays to a level determined by the electron attachment 

coefficient, v, for air (v = 106/~). The current flowing after 

this decay, about 10 mA, is referred to as a "dark current" and is 

probably due to electrons generated within the channel itself, 

some detachment process, or contribution from the cathode (12). 

This dark current, which flows through the low conductivity 

channel of the bottom gap (about 3 megohms/cm as measured by 

Bicknell (12)), seeks to charge up the capacitance of the "pointplane" geometry in the top gap. The capacitance, estimated at 

1 pF, would have a charging time of about 23 ps, accounting for 

the flatness of the current waveform in this region. However, a 

second current peak occurs which cuts short this charging process 

and leads to the immediate decay of the current. The appearance 

of the second current maximum creates a waveform which appears 

somewhat similar to the two-peak signature of the flashover 

described in the next section. In the present case, the second 

peak may be due to some phenomenon occurring in t h e t o p gap or to 

increased ionization in the lover gap causing a rise in the 

conductivity of the channel. Unfortunately, no image converter 

photos are available of this event. 

Further increase in the sensitivity of the s c o p e t r a c e s h o w s 

evidence of a fine-structure contained in the waveform of 
2s 

Figure 12. This fine-structure is illustrated in the oscillogram 

of Figure 14. The pulses represent the D-dot response of the 

sensor to negative corona in the upper gap. 

Nesnrive-coronn 

Unlike the positive corona, the observed negative corona 

remained completely localized (Figure 151. The solid angle 

encompassed by the corona increases with increasing ambient field. 

In order to enhance the sensitivity of the I-sensor to the 

negative corona, the high-voltage electrode was made negative, 

thus producing the negative corona in the bottom gap. 

Oscillograms show that the negative corona consists of the usual 

characteristic Trichel pulses. No negative glow mode was 

observed. 

When a positive glow is present at the ellipsoid anode, the 

Trichel pulses are regular with a frequency of approximately 

60 kHz (Figure 16). The frequency of the Trichel pulses depends 

on the time needed to clear the gap of the negative ions produced 

in the previous pulse (13). Figure 17 illustrates the case when 

positive streamers are present at the anode. The D-dot response 

of the seneor to these streamers is marked by the larger periodic 

pulses in the oscillogram, and the Trichel pulses are the finestructure In between. The Trichel pulses ehow marked frequency 

and amplitude modulation as previously noted by Bicknell (14) 

whose experiment, using an isolated conducting cylinder, was 

similar to ours. In the present case, the pulses exhibit a 
26 

frequency ranging between 150 and 400 kHz. 

Examining the individual Trichel pulses more closely, we 

obtain the waveform of Figure 18. The pulse consists of a quick 

rise to a D-dot maximum and then a decay to zero. Pulse width was 

roughly 150 ns. 

I 
i 

Figure 9: Bipolar Corona: (a) Bipolar corona emanating from the ellipsoid tips. (b) Corona polarities for positive high-voltage 

electrode case. (c) Corona polarities 

for negative high-voltage electrode case. 

27 
28 

t 

Fizure 10: The Positive Glow Discharge 
ORIGWAE PAGE Is 

OF POOR QUm 

29 

Figure 11: Coexistence of Positive Streamers and 

Positive Glow Corona. 
ORIGINAL' PAGE IS 

OF FOOR QUALITY 

Figure 12: I-sensor Response to Positive Corona 

Streaners. 
ORIGINAL i-’?,GZ IS 

OP POOR QUALITY 

Figure 13: I-sensor 3esponse to an Individual Tositive Corona Streamer. 
32 

Figure 14: I-sensor 3esponse to Negative Corona in 

the Upper Gap. 
33 

Figure 15: Negative Corona Discharge 
34 

Figure 16: I-sensor Response to Trichel Pulses with 

a Positive Glow Discharge ?resent at the 

Zllipsoid Anode. 
35 

Figure 17: Modulation of the Trichel Pulses when 

Pcsitive Streamers are PreseKt zt the 

Zllipsoid Anode. 
34 

F-igure 18: I-sensor Response to an Individual Trichel Pulse. 
IV. FLASHOVER 

Having discussed partial breakdown, attention will now be 

turned to the high current phenomenon of complete breakdown of the 

gap, namely flashover. High speed photographic data and recorded 

sensor responses are correlated in order to develop a scenario for 

the flashover process. In addition, two methods of arc current 

measurement will be presented and compared. The required power 

supply voltage is 136 kV. 

Fleahnver-chsrecfer4stics 

A representative waveform obtained from the D-dot sensor 

during flashover is shown in Figure 19. The waveform is composed 

of three distinct parts: an initial pulse of roughly 350 ns 

duration, a time delay which may be of widely varying length and 

is nonexistent in Figure 19, and a second intense pulse of 40 to 

50 ns width. Correlation of the image converter camera 

photographs with the D-dot waveforms is achieved by superimposing 

the camera monitor pulses on the waveform. 

Investigation of the first pulse of the D-dot waveform proved 

to be a very difficult problem. Since the experiment involved DC 

voltages and relied on the spontaneous occurrence of flashover 

rather than a triggered occurrence, the time delays inherent in 

the diagnostic system prevented us from photographing the 

discharge during the initial rise of the first pulse in the D-dot 

waveform. Various techniques were employed to reduce this "event37 
38 

to-photo" delay as discussed in the Appendix. Despite this 

limitation, information about the latter part of the first pulse 

was obtained. Figure 20a shows an oscillogram of the first pulse 

of the D-dot waveform. The corresponding framing photo was quite 

faint so a representative drawing of the results is given instead 

of the actual photo (Figure 20b). From this figure, the following 

can be observed: 

(a) The camera monitor pulses indicate that the first frame 

of the photo coincides very nearly with the maximum of the 

waveform shown. At this point, a leader has already formed acrose 

the top gap and a leader, with its associated corona bush tip, is 

partially bridging the bottom gap. The fact that the negative 

"point-plane" (top) gap breaks first is interesting in that, as 

previously noted, it takes two to t h r e e t i m e s the voltage to break 

a negative point-plane gap as it does to break its positive 

counterpart. In our case, the presence of the relatively 

conductive corona streamers in the lower gap effectively lowers 

the acquired potential of the ellipsoid, thus increasing the field 

strength in the top gap, causing it to break first. This 

dominance of the early stage of breakdown by the negative pointplane gap is further illustrated by moving the ellipsoid down from 

the midplane of the gap towards the ground plane. The larger the 

negative point-plane separation, the larger the required power 

supply potential for flashover. Framing photos showing negative 

leader propagation were not obtained due to the extremely high 

speed of closure of t h e t o p gap; thus we do not know precisely 
39 

where within the top gap the leader began. Some fine structure is 

evident in the leading edge of a few of the acquired D-dot 

waveforms, as illustrated by Figure 21. 

(b) The second frame shows a decay in the luminosity of the 

established channel. The completion of a channel in t h e t o p gap 

leads to the appearance of a high electric field in the bottom 

gap, causing a positive leader to propagate towards the I-sensor. 

The upper channel, not having made the transition to the high 

conductivity conditions required for the high current arc, is 

still of relatively low conductivity. Consequently, the current 

drawn across the high impedance of t h e t o p gap by the positive 

leader in the bottom gap precipitates a drop in the potential of 

the ellipsoid and extinction of the positive leader. This 

relaxation type of extinction phenomenon has been previously 

observed by Labaune (15). 

(c) The third frame indicates complete loss of luminosity by 

the channel and a corresponding decay in the D-dot waveform. This 

marks the beginning of a period of extinction (typically less than 

1 ps) preceding the actual transition to arc. The oscillogram of 

Figure 22 is typical of those obtained in this period and the 

related framing photo (not shown) gives no real evidence of 

luminosity in the gap. 

The transition to arc of the entire gap is marked by the 

second pulse of the D-dot waveform. A D-dot oscillogram is 

illustrated in Figure 238 with its corresponding framing photo 

given in Figure 23b. The first f r a m e s h o w s no luminosity since it 
40 

occurs in the period of extinction. In the second frame, however, 

visible evidence of activity in the gaps is readily apparent. A 

very faint reillumination of the channel in the upper gap is 

present, but, as can be seen, the transition to spark occurs first 

in the bottom gap. This identification of the actual location of 

the beginnings of the arc with a framing photo has not been 

reported by previous workers. A time exposure photograph of the 

entire arc events given in Figure 24. 

For the event of Figure 23, arc transition occurs without 

being preceded by leader propagation from the positive point of 

the lower gap. In some instances, however, there is evidence that 

indicates that positive leader propagation does precede arc 

transition. Figure 25 consists of two integrated D-dot waveforms 

of separate events. Aside from the decays due to the time 

constants of the analog integrators used, the waveforms provide a 

picture of the collapse of the electric field in the gap. 

Comparing t h e t w o waveforms, we see that the upper is a relatively 

short event while the lower is quite a bit longer. The region of 

interest is that section just preceding the second peak. Any 

evidence of positive leader propagation will show up as a slow 

linear rise in the electric field record. Figure 25a gives no 

indication of a leader. Figure 25b, though, has the 

characteristic ramp indicating leader propagation. Measurement of 

t h e t i m e involved produces an approximate leader velocity of 1.7 x 

lo7 cm/s, which is fairly consistent with the results stated 

previously. 
41 

currest,!!ensuremest 

In addition to employing the direct method of current 

measurement accomplished by the I-sensor, the current flowing in 

the gap can also be inferred from the D-dot sensor by making a few 

assumptions. A simplified equivalent circuit of the experimental 

setup is shown in Figure 26a. Extensive ringing of the circuit 

was not observed in the experimental results so inductances were 

ignored in the circuit. The initial charging polarities of the 

capacitances are indicated. The circuit can be further simplified 

by making the following observations: 

(a) Since the power supply has a relatively low output 

impedance, it can be considered to be fairly ideal. As such, the 

voltage across capacitor C1 is essentially that of the power 

supply 

(b) Capacitor C2 appears as a small impedance for the time 

scales of interest, enabling us to neglect the large parallel 

resistance, R2. 

Accordingly, the circuit of Figure 26b remains. This circuit 

shows that the energy necessary for flashover comes from the 

energy stored in the gap capacitance C3 and directly from the 

power supply through capacitance C2. Analysis of the circuit 

gives 

i(t) = -C3 dV/dt + C2 d(Vi-V)/dt 

and with Vi = constant, 
42 

We now assume that the gap can be modeled as a simple parallel 

plate capacitor with 

V = Ed 

Since d = constant and Er = 1, 

(3) 

(3’) 

If the D-dot sensor is placed within the uniform field region 

between the plates of the gap capacitor, its output will 

essentially give u s t h e D-dot term contained in Equation (4). The 

output of the D-dot sensor le 

with Rs corresponding to the sensor load impedance and As denoting 

the sensor equivalent area or sensitivity. So substituting (5) 

into (4), we have 
43 

This final equation provides us with a measure of the current in 

the gap from known constants and the output voltage of the D-dot 

sensor. 

The accuracy of Equation (6) can be examined by comparing the 

signals from the D-dot sensor and the I-sensor during the same 

event. A typical result is shown in Figure 27. Both waveforms 

are displayed in the same trace, where the D-dot waveform lags the 

I waveform by 250 ns due to a delay line inserted in the D-dot 

sensor data line. Calibration of t h e t w o signals is achieved by 

considering total attenuation introduced in the data lines, load 

conditions on the lines, and Equation (6). For the I-sensor, 

and for the D-dot sensor, 

Table 1 contains a series of four measurements which are 

representative of the type of results achieved. The D-dot method 

of current measurement typically underestimates the actual 

measured current from the I-sensor by 8 %. This is not an 

unusually large discrepancy and can probably be attributed to the 

fact that the D-dot sensor was not located in a uniform field 

region or to the circuit approximations made in the calculation 

above. 
44 

t 

183.60 167.58 

204.00 190.86 

168.30 167.58 

224.40 186.20 

1 

TABLE 1 

COMPARISON BETWEEN CURRENT MEASUREMENT METHODS 

I 

Arc Current, Amperes 

(I-sensor) 

Arc Current, Amperes 

(D-dot Sensor) 
45 

Figure 19: D-dot Response to Flashover 
1 1 

I A 

3 2 1 

Figure 20: The First Pulse of a D-dot Yaveform: 

(a) Oscillogram of the first pulse with 

camera monitor pulses superimposed. 

(b) Framing photo of the first pulse. 
47 

Figure 21: Tine-structure in the Leading 3age of 

the First Pulse of the D-dot Waveform. 
Figure 22: Oscillogram of tne Extinction Period 

Preceding Arc Transition. 
49 

2V I zoons -- 

. . . __ . ... ._ .._ .. . . . 

3 2 1 

Figure 23: Transition to Arc: (a) Oscillogram with 

camera monitor pulses superimposed. (b) 

Corresponding framing photo. 
Figure 24: Color Photo of Flashover 
Figure 25: Integrated D-dot Waveforms of Flashover: 

(a) Transition to arc without leader 

propagation (1 OmV /div, 1 OOns/div) . 
Figure 25: (Continued) (b) Transition to arc with 

leader propagation. 
R2 RI 

VI 

RI = 

CI = 

R2 = 

c2= 

c3 = 

R3= 

S= 

- 

--. 

POWER SUPPLY VOLTAQE 

POWER SUPPLY OUTPUT IMPEDANCE 

POWER SUPPLY CAPACITANCE 

CURRENT LIMITING RESISTOR 

RESISTOR-CORONA INHIBITOR STRUCTURE 

CAPACITANCE 

GAP CAPACITANCE 

GAP RESISTANCE 

GAP BREAKDOWN 

53 

VI 

VI 

Figure 26: Equivalent Circuit of the Experimental 

Set-up: (a) Simple circuit representation. (b) Reduced circuit representation. 
54 

Figure 27: Simultaneous Oscillogram of Both I-sensor 

and D-dot Sensor 3esponses to r’lashover 

(9-dot sensor response delayed by 25Ons) 
V. CONCLUSIONS 

The individual processes involved in the breakdown of an 

isolated object in a uniform electric field have been 

examined through photographic techniques and electromagnetic 

sensor waveform analysis. A detailed characterization of bipolar 

corona was established by identifying unique properties of both 

positive and negative corona and their effect on subsequent arc 

phenomena. The negative corona remained localized near the upper 

tip of the ellipsoid while positive corona streamers extended from 

the lower tip and completely bridged the lower gap. A scenario 

for the flashover process was developed and can be summarized as 

follows: 

(a) The initial phase of flashover invariably occurred in 

the negative point-plane (top) gap due to the shorting action of 

positive corona streamers traversing the entire length of the 

positive point-plane (bottom) gap. 

(b) Connection of the ellipsoid t o t h e high-voltage 

electrode by an ionized channel, or leader, in t h e t o p gap 

activitated the development of a positive downward leader in the 

bottom gap which produced a strong response at the D-dot sensor. 

(c) The voltage drop caused by the current drawn through the 

high impedance channel of the top gap by the positive leader then 

produced a period of extinction in the both gaps of typically less 

than 1 ps. 

(d) Arc transition was then initiated, causing a second 

55 
56 

etrong response at the D-dot sensor. The arc occurred in one of 

two modes: direct transition without leader propagation in the 

bottom gap, and transition following leader propagation in the 

bottom gap. The origin of transition to arc was placed in the 

lower region of the bottom gap, in the manner of a lightning 

return stroke starting near the ground. 

Two methods of current measurement were employed and tested 

for consistency. One method placed a sensing element directly in 

the current path and the other relied on using a D-dot sensor for 

an indirect measurement of the current. Good agreement between 

the two techniques was demonstrated. Peak positive corona 

currents were 25 mA, and peak arc currents were 200 A. From 

photographs, we estimate the arc channel diameter to be on the 

order of 0.5 mm, which gives a current density of 100 kA/cm2-- 

about the same as lightning. 

From the results of our experiment, we can conclude that the 

negative point-plane gap largely determines the development of the 

arc. The establishment of a resistive channel in the upper gap, 

and resulting positive leader propagation in the lower gap, cause 

significant variation in the potential of the isolated object and 

set the gap up for a period of extinction preceding arc 

transition. This phenomenon is of value in the modeling of 

aircraft-lightning interaction. Labaune (15) and Moreau (16) have 

demonstrated previously that the strongest variations in the 

surface electric field of an isolated object occur due to current 

flow through this highly resistive pre-discharge channel. In 
57 

addition, they have suggested a correlation between this 

laboratory phenomenon and data provided by instrumented aircraft. 

Turner (17) has found that the impedance of the attachment channel 

can have a profound effect on the natural resonant response of an 

aircraft to discharge phenomena. Further laboratory investigation 

of the nature of the resistive channel preceding arc transition, 

therefore, is certainly warranted. It is also interesting to note 

that the relaxation process described in (a)-(c) above is very 

similar to that observed in the large-gap experiments of Labaune 

(15) and Moreau (16). 

It is known that the presence of rain will substantially 

lower the flashover voltage of a negative point-plane gap but not 

that of a positive point-plane gap (18). Brief tests in our 

experimental set-up where a fine water mist was injected into the 

gap showed that a large drop in flashover voltage was obtained. 

This merits detailed examination in the future since here 

different processes may come into play, with the negative pointplane gap no longer controlling the breakdown process. 

-Improvements in our present experimental set-up might consist 

of the addition of water and air motion to the gap, order of 

magnitude increaee in the gap dimension, and the inclusion of 

streak photography in the data acquisition system. An attempt 

should be made to measure the time-dependent impedance of 

predischarge ionized channels, or leaders, which form in the gap. 

The use of isolated objects of different shapes is also of 

importance. 
58 

The behavior of the leaders is especially interesting because 

it is probably the main distinguishing factor between aircrafttriggered lightning strikes and aircraft-intercepted strikes. TO 

study this behavior further in the laboratory, a much larger gap 

is needed, so that the leaders can develop freely without 

contacting the electrodes. The key question seems to be, "what 

are the conditions needed to obtain a propagating leader from an 

isolated body?" 
REFERENCES 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

10. 

11. 

Pitte, F. "Electromagnetic Measurement of Lightning Strikes 

to Aircraft," Journal of Aircraft, vol. 19, no. 3, Mar. 1982, 

pp. 246-250. 

Trost, T. "Interpretation of a Class of In-flight Lightning 

Signatures," Proc. Intl. Aerospace and Ground Conf. on 

Lightning and Static Electricity, Dayton, OH, June 1986, 

pp. 6-1, 6-9. 

Rudolph, T., et, a&. "Development and Application of Linear 

and Nonlinear Methods for Interpretation of Lightning Strikes 

to In-Flight Aircraft," NASA CR 3974, Sept. 1986. 

Mazur, V., et, a&. "Conditions Conducive to Lightning 

Striking an Aircraft in a Thunderstorm," Proc. Intl. 

Aerospace and Ground Conf. on Lightning and Static 

Electricity, Ft. Worth, TX, June 1983, pp. 90-1, 90-7. 

Cesani, J. "Flashover Test of an Isolated Conducting Object 

in a 10-inch Gap," Master's Thesis, Texas Tech Univ., 

Dec. 1985. 

Hutzler, B., E&. "High Voltage Laboratory Test and 

Lightning Phenomena," Proc. Intl. Aerospace and Ground Conf. 

on Lightning and Static Electricity, Paris, France, June 

1985, pp. 191-196. 

Levesque, P., et_ a&. "A Study of the Physical Mechanism and 

Perturbations Created by the Attachment of an Arc to a 

Conducting Cylinder," Proc. Intl. Aerospace and Ground Conf. 

on Lightning and Static Electricity, Paris, France, June 

1985, pp. 29-36. 

Baum, C., et a&. "Sensors for Electromagnetic Pulse 

Measurements Both Inside and Away From Nuclear Source 

Regions," IEEE Transactions on Antennas and Propagation, 

vol. AP-26, no. 1, Jan. 1978, pp. 26-27. 

Baum, C., ed. "Electromagnetic Pulae Sensor Handbook, " EMP 

Measurement, vol. 1-1, sec. 3, June 1971. 

Latham, R., and Lee, K. "Capacitance and Equivalent Area of 

a Disk in a Circular Aperture," Sensor and Simulation Note, 

no. 106, May 1970. 

59 
60 

12. Bicknell, J., and Humood, B. "The Streamer to Spark 

Transition, " Proc. Intl. Aerospace and Ground Conf. on 

Lightning and Static Electricity (Appendum), Ft. Worth, TX, 

June 1983, pp. 42-1, 42-6. 

13. Nasser, E. Eundarnentsla,of_9aee~~p-~~n~~~t_~~~-and Plasma_ 

Electrqqicp, Wiley-Interscience, New York, 1971, p. 346. 

14. Bicknell, J., and Humood, B. "Bipolar Corona and AircraftTriggered Discharges," Proc. Intl. Aerospace and Ground Conf. 

on Lightning and Static Electricity, Orlando, FL, June 1984, 

pp. 4-1, 4-4. 

15. Labaune, G., et, e&. "Experimental Study of the Interaction 

Between an Arc and an Electrically Floating Structure," Proc. 

Intl. Aerospace and Ground Conf. on Lightning and Static 

Electricity, Dayton, OH, June 1986, pp. 27-1, 27-9. 

16. Moreau, J., and Alliot, J. "Analysis of the First 

Milliseconds of Aircraft Lightning Attachment," Proc. Intl. 

Aerospace and Ground Conf. on Lightning and Static 

Electricity, Paris, France, June 1985, pp. 18-1, 18-6. 

17. Turner, C., and Trost, T. "Laboratory Modeling and Analysis 

of Aircraft-Lightning Interaction," NASA CR 169455, Aug. 

1982. 

18. Knudson, N., et_ a&. "Flashover Test on Large Gaps with DC 

Voltage and with Switching Surges Superimposed on DC 

Voltage, " IEEE Trans. Power Apparatus and Systems, vol. PAS89, no. 5/6, May/June 1970, pp. 781-788. 
APPENDIX 

A certain amount of delay is present in our Image Converter 

Camera trigger system. As mentioned earlier, the Trigger Delay 

Generator (TDG) is responsible for producing the 300 V pulse 

needed to trigger the camera. The trigger input of the TDG has a 

minimum threshold of 1 V. The gate output of the scope will 

deliver, at best, only 1 V into the 50 ohm load presented by the 

trigger input, or 10 V if the load is 1 megohm. Therefore, a 

high-speed JFET source-follower amplifier was added to the gate 

output of the scope. With this addition, the gate voltage 

transferred to the TDG was well above the 1 V threshold and thus 

more reliable triggering was obtained. To insure minimum trigger 

delay, all cable lengths were minimized. Unfortunately, the TDG 

has a measured internal delay of 115 ns, which is much higher than 

its 50 ns rated value. (We did not have time to repair or modify 

the TDG.) Adding up all of the delays in the system yields an 

"event-to-photo" delay of approximately 150 ne. As such, we are 

rather limited in how quickly we can photograph an event. 

61 
Standard Bibliographic Page 

. Report No. 

NASA CR-178323 

2. Government Accession No. 

'. Author(s) 

M. G. Grothaus and T. F. Trost 

1. Performing Organization Name and Address 

Dept. of Electrical Engineering/Computer Science 

Texas Tech University 

Lubbock, TX 79409 

.. 

2. Spoqsorin A ency Name +nd Addre s Nati ona? fieronauti cs and Space Administration 

Washington, DC 20546-0001 

.5. Supplementary Notes 

Langley Technical Monitor: Felix L. Pi tts 

3. Recipient's Catalog No. 

5. Report Date 

Decembe r 1986 

6. Performing Organization Code 

8. Performing Organization Report No. 

10. Work Unit No. 

505-66-21-04 

11. Contract or Grant No. 

NAG- 1-28 

13. Type of Report d Peri d Covered 

or Feeport t?%8- 1986 

14. Sponsoring Agency Code 

.6. Abstract 

A laboratory experiment has been conducted to determine the physical 

ptocesses involved in the electrical breakdown of a particular spark gap 

arrangement. The gap consists of an isolated conducting ellipsoid located 

midway between two large flat electrodes. Gradual increase of the applied 

electric field, E, in the gap produces corona on the ellipsoid tips 

followed by flashover in a leader-arc sequence. The leader phase consists 

of the abrupt formation of ionized channels which partially bridge the gap 

and then decay prior to the arc. 

made, and photographs were taken with an image converter camera. 

Experimental parameters are as follows: 

Gap length = 25.4 cm 

Ellipsoid length = 10.0 cm 

Gap voltage for flashover = 98 kV 

Gap electric field strength (far from ellipsoid) = 386 kV/m 

Gap capacitance = 78 pF 

Gap electric field energy (estimated) = 0.58 J 

Peak positive-corona-streamer current = 25 mA 

DC corona current = 0.37 - 0.40 mA 

Peak arc current = 200 A 

Arc channel diameter < 1 rnm 

Duration of flashover event (leader + arc) = 1 ps. 

Measurements of dE/dt and of current were 

17. Key Words (Suggested by Authors(s)) 

Electri cal breakdown processes 

Laboratory experiment 

Li g h tni ng 

Spark gaps 

Image converter camera 

18. Distribution Statement 

Unclassified-Unl imi ted 

Subject category 47 

I 

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