1166 IEEE Transactions on Electrical Insulation Vol. 27 No. 6, December 1QQ2

Radiation-induced Electrical Discharges in Complex Structures

A. R . Frederickson, Leon Levy and c. L. Enloe

Space Physics Division, Geophysics Laboratory, Hanscom AFB, MA

ABSTRACT Polyethylene terephthalate (PET) foils were exposed to 25 keV electron beams of 3 nA/cma which has been known to charge the PET surface to -15 kV or more and to produce rapid pulsed discharges sufficient to lower the magnitude of the surface po- tential significantly. The major experimental finding is that the pulse discharges often propagate to nearby mutually bi- ased electrodes and can create a full vacuum breakdown be- tween stainless steel electrodes, rarely if biased at as little as 100 V, but often if biased at 300 V. The phenomena appear to be associated with the pulsed creation of dense plasma from the discharging dielectric, and the pulsed volume of gas evolved is equivalent to roughly 0.1 to 0.4 mm3 at 1 atmosphere for nomi- nally 50 cm2 samples. The plasma could evolve to interconnect more than 2 electrodes. Additionally, PET pulse rates were strongly dependent upon the proximity of sharp edges, either metallic or dielectric.

1. I N T R O D U C T I O N

PACECRAFT have experienced anomalous discharge S events for which i t is difficult to determine a cause. The events are more numerous during periods of increased high energy electron fluxes on some satellites in high alti- tude orbits, where high energy electron fluxes are relative- ly large. It is therefore presumed that keV or higher en- ergy electrons may cause anomalies on spacecraft [l] and many measurements have been performed to assess the situation [2,3]. I t appears that irradiated dielectrics are a likely cause for many anomalies [4]. Spacecraft design- ers have studied many of their materials in keV electron environments to find that discharges can be numerous [5].

The emission of a plasma has been recorded during some of these electron caused discharges under laborato- ry conditions [6]. The work reported here investigates the

possibility that the plasma emitted might be sufficient to cause further problems. We find that the plasma not only ‘grounds’ the initially keV charged surface of the dielec- tric, it also induces full breakdown discharge on nearby metal electrodes spaced by - 1 to - 2 cm and biased by a battery as low as 100 V. Higher voltages increase the probability of occurrence of an induced discharge.

When the irradiated dielectric surface spontaneously discharges, the nearby biased electrodes sometimes ex- perience a full breakdown diacharge where the current is limited only by the ability of the biasing power supply to provide current to the discharge. Thus the irradiat- ed dielectric will induce discharges a t electrodes not even associated with the dielectric. The discharge of the ir- radiated dielectric alone is not a full breakdown, it only alters the near-surface static charge which is not part of the electric circuit operations.

0018-9367 $3.00 @ 1992 IEEE

IEEE Transactions on Electrical Insulation

I IBO i

10-9 A c" 25 keV e-

E - 1 5 kV

IGURE 1 is a simplified schematic cross section of the F basic experiment. In Figure 1 the electron beam of 25 keV is stopped within the first 6 p m of the 125 p m thick dielectric sample [6]. The irradiated surface rises to roughly -15 or -20 kV. A discharge pulse sponta- neously forms a t the dielectric surface and produces a plasma which polarizes in response to the high electric field. The ambipolar diffusion and electric field drift of the plasma electrons and ions in the vacuum form a cur- rent J B O , which rapidly grounds the surface [2,3]. The grounding corresponds t o a rapid pulse of current of order 1 to 1000 A of electrons and negative ions flowing away from the dielectric surface to the vacuum ground. This current is seen in the 'blowoff' BO, probe current trace, and passes through the insulator as capacitive displace- ment current. Negligible real current passes through the insulator.

The plasma current probe monitors current J p flowing between parallel plates which are above the sample sur- face. The batteries are actually charged storage capac- itors which store more charge than does the irradiated sample surface. These capacitors have low internal in- ductance and low internal resistance so that the plasma current can rapidly attain large values.

It is critical that the measurements be able to separate the plasma current which flows between the plates from

Vol. 27 No. 6 , December 1882 1167

the currents due to the -20 kV discharging surface. This requirement should be kept in mind when considering the experimental results. The actual experimental circuit is an attempt to achieve this goal. The currents are moni- tored by meters in the electrical wires, there is no direct monitor of the plasma current flows themselves. We must be able to interpret the meter currents in order that plas- ma flows may be understood. Only by understanding the plasma flows will we be able to predict the electrical re- sponse of a different arrangement of electrodes and biases to a radiation induced discharge pulse.

INS U L A T 0 R S 4 RING

k-

. 0-30 kV

UPPER 4 k ACTUAL

LAYOUT ~ a FILAMENT -LOWER * 9

& %

Figure 2. The 0 to 30 keV electron beam source.

3. EXPERIMENTAL APPARATUS

3.1 HIGH ENERGY ELECTRON SOURCE

HE experiment was performed in a 0.5 m diame- T ter glass bell jar vacuum chamber a t a pressure of lxlO-* Pa. The electron source and the sample, which are shown in Figure 2, are contained within a ground- ed copper screen in order to electrically shield the ap- paratus from any inadvertent charging of the glass bell jar , but still allow one to inspect the apparatus visual- ly while under vacuum. In order to obtain a large-area, uniform beam, the emitting tungsten filament is placed inside a carbon-coated metallic can, the top of which is covered with a conducting screen called the source screen. The source screen then forms the planar cathode of the high energy electron accelerator. This cathode assembly

1168 Frederickson et al.: Radiation-induced Electrical Discharges in Complex Structures

is biased versus the (nominally) grounded sample region which is held 15 cm above the source screen. This ar- rangement provides a diffusion region to allow the elec- trons inside the can to achieve a uniform density prior to their being accelerated. A current-collecting ring mount- ed near the sample is used to monitor the beam intensity. The source is capable of producing - 5 /.LA of current over the sample aperture a t voltages of up to 30 kV.

I P + h 2

Ihl

I Ip - 2

" c n

I,,, = 1 0 - 6 A V = 20 k V

" "-" I,,,, = ~ o - ~ A

r

" " I,,, = lO-'OA

-5 - 4 -3 - 2 - 1 0 1 2 3 4 5

POSITION [cm]

Figure 3. Beam current density across the sample face using 1 cm diameter detector.

Current density across the sample faces is shown in Fig- ure 3 for a 2 0 kV beam and a variety of beam currents. The data were taken with a 1 cm diameter Faraday cup a t various positions along a diameter across the sample aperture. Computer simulation of electron trajectories, and in-chamber tests, indicated that the beam intensity on a fully charged sample dropped 50% at the center of the sample, but remained nearly constant a t the edges. The sample is fully charged when the arriving electrons cause an equal loss of electrons from the surface. This oc- curs when the arriving electrons impact the surface with an average of roughly 5 keV, depending on incident angle and local electric field.

3.2 SAMPLE AND PULSE DETECTOR CON FlGU RATION

Figure 4 illustrates the electrical arrangement of the sample, the electrodes, and the pulse detection circuits. The electron beam irradiated the sample a t an intensity

POSITIVE PLASMA

ELECTRODE

NEGATIVE PLASMA

ELECTRODE

REAR ELECTRODE

- -

Figure 4. Discharge pulse detection circuitry. Note the dif- ferential transformer for Ip which helps cancel IBO current from the Ip datum.

of the order of 3 nA/cm2, and the sample surface rose to roughly - 2 0 kV in a few minutes. For the rest of the discussion the beam current is not a parameter of interest. It is the pulse currents and voltages which are of interest, and the circuit is designed t o measure these.

The current monitor IBO measures the pulsed currents flowing between ground and the sample rear electrode. In Figure 4 this current is shown to pass equally, I B O / 2 , to both plasma electrodes, but only approximate equality is observed. In addition, current can bypass the electrodes and go directly to ground elsewhere in the chamber. Thus not all of IBO passes t o the plasma electrodes. The ideal situation where all of IBO passes equally to each plasma electrode is shown in Figure 4 for instructional reasons only.

If I B 0 / 2 flows to each plasma electrode then these two currents cancel in the Ip toroidal transformer and do not contribute to the signal at the Ip current monitor as shown in Figure 4. The two batteries providing bias to the plasma electrodes will polarize any plasma which re- sides between the plates and, thus, a current will be seen in the Ip monitor. Any plasma produced by the dielectric discharge will produce a current I p which is independent of IBO to first order.

Having removed the contribution of IBO to I p , then the plasma current between the plates is either excess plasma

IEEE !lYansactions on Electrical Insulation Vol. 2 7 No. 6, December 1992 1169

produced by the sample which passes between the plates where some is collected, or a secondary discharge between the plates which is induced by the dielectric discharge, or perhaps both phenomena together. No plasma pulse is seen in such irradiations if no dielectric sample is present to be charged, even if there is - 300 V on the plates. I t is clear that significant plasma is produced when a dielec- tric pulses because the vacuum chamber pressure monitor records a pulse of gas each time there is a discharge.

A plasma current pulse I,, whose total net charge (flu- ence) exceeds the total charge in IBO must be caused, a t least partially, by a plasma between the plasma elec- trodes. Even if all of IBO flows to one of the plasma electrodes alone, if the charge in I, exceeds that in IBO in magnitude, then there must have been a source for I, which is separate from IBO. To take advantage of this fact experimentally, the batteries in Figure 4 must be ca- pable of rapidly delivering a charge exceeding the charge stored a t 20 kV on the sample surface. This is achieved by placing several capacitors across the batteries (actual- ly adjustable voltage power supplies) to a total of - 2 pF. Each such capacitor must have low internal inductance.

The pulse transformers are standard wideband ferrite toroidal core units where the one turn primary wind- ing is the wire lead itself. These match the 50R lines which are terminated and drive the pulse recorders. The pulse recorders have been, a t various times, Tektronix oscilloscopes, Biomation transient digitizers, and LeCroy transient digitizers. The system response is tested using known pulses from pulse generators as well as by discharg- ing coaxial lines of various lengths through a mercury- wetted switch and known resistors. The transformers are always used in their linear region, they are not driven into saturation.

The total circuit length, from ground to the rear elec- trode, then through the sample and across to the plasma electrodes and through the ‘batteries’ and finally back to the original ground is of the order of 30 cm. Thus one can expect ringing on IBO with a period of the order of 1 ns. The discharge pulses of interest are temporally longer than this so we need not make the system more compact. All wiring resistive impedances are kept low so that large currents can be monitored.

Figure 5 describes the actual plasma electrodes. They are made of stainless steel M 0.030 cm thick. They are actually 18 interleaved electrodes (half are positive and half are negative) so that the plasma polarization current can be more sensitively measured. They are spaced an adjustable distance h from the sample surface, typically from 0.3 to 1.4 cm.

REAR ELECTRODE

Figure 5. Detailed plasma electrode geometry. There is a total of 18 electrodes.

3.3 ANALOG PULSE EVENT DETECTORS

Costly experience proves that single pulses are difficult to capture in all their detail. We have learned that a simple set of analog event occurrence indicators are highly advantageous. A chart recorder monitors the samples during the run to indicate the magnitudes and time-of- occurrence of pulses. In addition to the transient pulse waveform digitizers shown in Figure 4, several circuits may be monitored. The pulses being monitored come from

1. the system vacuum gauge tube, 2. the voltage supplied to the charged capacitors which

provide the plasma pulse currents, 3. the beam current monitor ring, and 4. the HV (25 kV) power supply over-current trip circuit.

Each of these provides valuable information beyond the actual da t a from I, and IBO pulses.

3.3.1 VACUUM PULSES

Every time a discharge occurs, the vacuum gauge tube also produces a pulse on its analog line, which is contin- ually monitored by the chart recorder, even though the gauge tube is not close to the sample. We have calibrated the size of these pulses in terms of total mass or volume of gas liberated by two methods. First, small sealed glass vials of atmospheric air were broken by a magnetically ac- tuated iron hammer inside the vacuum chamber and the deflection of the vacuum gage reading was recorded. The deflection was correlated to the volume of air in the sealed glass vial. Second, the vacuum pump was valved off from the chamber allowing the pressure in the chamber to rise slowly. The valve is then rapidly opened and the chamber

1170 Frederickson et ai.: Radiation-induced Electrical Discharges in Complex Structures

L

5 0 ,

GLASS VIALS

BREAKDOWN INDUCED - (36 cm2)

I 2 3 v) W O

a n ATMOSPHERIC VOLUME, mm3

Figure 6. Calibration of the pressure pulse detector.

pressure drops, in analogy to the pressure drop after the pulsed introduction of gas. Since the mass of gas is known prior to the rapid opening of the valve, the pressure drop produces a chart recorder deflection which indicates the amount of gas initially in the chamber. Note that both methods produce a calibration valid for neutral gas, but tha.t the real discharge pulses contain ionized species for which the vacuum gauge is more sensitive. The results of the pressure gauge pulse calibration are given in Figure 6. Here the chart recorder was run rapidly and the arbitrary unit is the area under the pressure decay curve over an 8 s time interval, long enough to nearly fully recover the vacuum. The correct result probably lies somewhere be- tween the two straight lines in Figure 6 for two reasons. The glass vials may have lost some of their air to the epoxy sealer or after they were inserted into the vacuum. On the other hand, the opening/closing valve technique suffers because it uses gases which naturally evolve from the vacuum walls. The air from the vial may be pumped briefly by the walls themselves a t a different rate.

The pressure pulse calibration is then used in the fol- lowing way. The pressure pulses produced by a discharg- ing sample are recorded on the chart recorder. For exam- ple, as shown in Figure 6, a sample of size 36 cm2 pro- duced pulses of sizes from 1 t o 6 arbitrary units. There- fore the pulses caused the evolution of an equivalent at- mospheric volume of from 0.1 to 0.4 mm3.

3.3.2 THE PLASMA BIAS VOLTAGE

The plasma discharge pulse current is provided by the voltage stored on the capacitors which are - 3 p F or larger. If a plasma discharge occurs between the plasma electrodes, then the voltage on these capacitors will drop

and the drop is monitored by a voltmeter which drives the chart recorder. The voltage is replenished slowly through the series resistors which charge the capacitors and pre- vent the plasma discharge from becoming a ‘permanent’ arc. This voltage drop technique is highly reliable because it records all substantial events.

3.3 .3 THE BEAM CURRENT MONITOR

This electrode shown as a ring in Figure 4 is a sensi- tive monitor of most currents, real or displacement, in the vacuum chamber. The size and algebraic sign of the pulses on this monitor are a helpful diagnostic. It can be a tricky job to determine the source of pulsed signals in a HV chamber. For example, small partial discharges within the polyethylene dielectric of the HV power supply cable were being seen on the sample electrodes and on the current monitor ring when using electrometers to moni- tor for prebreakdown events in the sample. We changed the HV cable to eliminate this. A chart recording of the beam current monitor can help to determine the source of events.

3.3.4 THE HV TRIP CIRCUIT

A full dielectric discharge would sometimes trip this power supply protection circuit and would also produce a pulse in the vacuum gauge. Tripping the power sup- ply protector by drawing an arc external to the vacuum chamber would not produce a vacuum gauge pulse. Thus, the HV monitor is a useful indicator for plasma-initiated discharges which propagate far from the sample and con- nect the vacuum HV wires t o ground.

4. SAMPLE PREPARATION

HE dielectric samples were sheets of Mylarm polyeth- T ylene terephthalate of thickness 125 pm. One side of the sheet was aluminized with optically thick aluminum and this side was electrically in contact with the rear elec- trode. The samples were clamped to the rear electrode: samples 3 and 4 were clamped a t a few points along their edges leaving most of the edges exposed to the electron beam, and sample 5 was clamped by Kaptonm tape along its entire edge. This Kapton tape was tested separately to prove that it did not produce discharge pulses itself.

The samples were cut to a size which is slightly larger than the collimator plate. Some scattered electrons could irradiate the edges of the samples, and beyond. The colli- mator plate was positioned between the cathode and the

r

IEEE nansactions on Electrical Insulation

-1 -

i

2 3 0 5 5 s

-2 LL---> .o 1 .1 1 .

Micrometer

Figure 7. Electric field near front surface of 100 pm thick Mylar P E T after irradiation by 1 nA/cm2 20 keV electrons for two time spans. Steady state is achieved a t 3000 s. Figure reprinted from [7] where charge distribution (the spatial derivative of electric field) is also provided.

sample and was held a t ground potential which helps to keep the beam intensity uniform across the face of the charging sample.

The effects created by edges can be very important as the results will show. Discharges will often, but not al- ways, be associated with edges. The edges of these sam- ples were left as they were created when they were cut from large sheets of Mylar using a scissor. Since we are studying the nature of the discharges themselves on nor- mal materials, and since we are not addressing the mech- anism of initiation of the discharges, we chose to leave the edges in this condition. The discharges involve the full sample surface, not the edges alone, even though the edge may be the most common point of initiation. In addition, controlled edges were introduced to investigate their role in causing discharge events; they are discussed below. Sample 5 had no edges of itself which were irradi- ated.

5. ESTIMATED ELECTRIC FIELDS

Vol. 27 No. 6, December 1992 1171

E propose, but do not prove, that high electric fields W in the insulator initiated the pulses. Nothing in this study is inconsistent with the concept that dielectric

breakdown is caused by the application of large electric fields. In this case the electric field is due to trapped electrons or holes in the dielectric. The electrons or holes are introduced by the radiation, each 25 keV electron produces roughly 700 temporarily mobile electrons and holes in the sample before being slowed to a stop a t the end of the 25 keV electron range. The electric fields and charge distributions which result from this process have been calculated in the one dimensional approximation [7] and are reproduced here in Figure 7 as a baseline for discussion. There should be no important conceptual dif- ference between 25 keV irradiations and 20 keV in these experiments. It is the electric field near the surface of the sample (depth = 0) which is responsible for the dis- charges which can be monitored in these experiments. The large negative fields in Figure 7, which extend to the rear electrode, probably produce prebreakdown puls- es which are too small to be monitored in the arrangement of this experiment. There is no room in this paper to ex- plain the role played by the large negative field a t the rear electrode, but it should not be active in this exper- iment except a t the edges of the samples. The positive near-surface field in Figure 7 is barely large enough to initiate breakdowns so one would expect to see very few breakdowns initiated near the center of the front surface of good samples. I t has been found that bulk fields must exceed lx105 V/cm in order that breakdowns initiate in such materials, and that higher fields are needed to as- sure that a breakdown will occur in a reasonable time period [4]. However, by placing a large flaw or discon- tinuity a t the surface, one can enhance the front surface field strength in a localized region and produce a regular discharge pulse every few minutes. Introducing a flaw of the type commonly found in electronic materials is part of the sample preparation in this experiment. Scratches, conductive edges, and dielectric discontinuities are briefly investigated.

The field enhancement which, we presume, induces the discharge pulses is not calculable in the one dimension- al model. The points and edges require a two or three dimensional model to predict electric field strengths. In most cases, one would expect the enhancement to be in the Y or 2 directions as well as in the X direction, de- pending on the shape of the flaw. Based on the literature concerning electric field enhancement a t points and edges, it is safe to say that 'good' flaws will enhance the field as much as a factor of 10 to 100, producing local fields > 1 x 1 0 ~ V/cm.

6. RESULTS

A N Y pulses were created and many hours of irradi- M ation were required in order to investigate the phe-

I

1172 Frederickson et al.: Radiation-induced Electrical Discharges in Complex Structures

nomena. We report the consistent results in a framework suitable for our explanation of the phenomena. One must remember that our explanation is not the results them- selves. We encourage the reader to consider the results and to attempt to develop alternative explanations.

6.1 SIMPLE SAMPLES

The simplest sample is one which extends to infinity without edges and with a perfect surface. We approached this ideal by using a collimator to irradiate 6 cm2 a t the center of a 49 cm2 sample, sample 5 discussed below. This arrangement produced nine pulses during 8 h of ir- radiation a t x 3 nA per cm2. One can not characterize the pulses when so few occur. Presumably the pulses were rare because in this arrangement the electric field is roughly as shown in Figure 7, and such fields a t the front surface will only rarely produce pulses.

The next simplest sample is the same semi-infinite sam- ple with scratches on the irradiated surface. A sharp metal scribe was used to create visible scratches in the 6 cm2 irradiated surface prior to irradiation. A negligible number of pulses were seen in this arrangement so that pulses could not be statistically characterized. Howev- er, the few pulses that did occur on the simple sample ha.d the same kind of IBO and Ip responses as the pulses discussed below on other samples.

So far in this research, we have found no reason to be- lieve that the discharges which result on the simple sam- ple are any different in nature than on the more complex samples. At this time, it would be wise to presume that the simple samples have pulse phenomena similar to the more complex samples below, but that the pulse rate on simple samples is substantially less. Since simple sam- ples are not easily achieved in practice, there is no loss of practical information in this turn of events. There is a valuable lesson in this result: electric field enhancement effects may be very important considerations for the de- sign of non-pulsing structures.

6.2 A METAL STRIP ON DIELECTRIC SAMPLE

The pulse rate of a simple sample can be substantially increased by placing a metal strip on the irradiated por- tion of the dielectric. Figure 8 describes the geometry. Even though the pulse rate was substantially higher, the pulse currents were similar to the simple sample without the metal strip. Presumably, the metal strip produced local field enhancement which initiated discharges often.

25 keV e- COLLIMATOR ) / ( / / )

I I 'I'II'I I OPTIONAL METAL STRIP

POLYIMIDE TA"' \.I

A MYLARPET L

Figure 8. Illustration of the sample surface structure and plasma electrodes. The polyimide tape prevents the plasma from electrically connecting the plas- ma electrodes to the rear electrode. When used, the copper tape was z 1 x 2.5 cm2 in size.

But the metal strip cannot substantially alter the aver- age surface voltage and that is why the pulse current was comparable.

6.3 A DIELECTRIC ON DIELECTRIC SAMPLE

Alternatively, by placing a different dielectric strip on the irradiated region of a sample, a copious pulse rate was also achieved. The sample structure is illustrated in Figure 8 but in this instance the metal strip was replaced by a polyimide strip of the same size. A polyimide was chosen because it is known not to pulse by itself under these irradiations [8]. The electrical conductivity of poly- imides exceeds that of Mylar P E T by several orders of magnitude, especially under irradiation, because the co- efficient of radiation-induced conductivity is much larger for polyimide. We tested this material t o prove that it does not pulse.

Presumably, the edge of the polyimide has created an enhanced field in its vicinity within the Mylar PET, and pulses are initiated there. There are several reasons for field enhancement but there is no space to discuss them here. They include: sharp corners, electrons stopping in polyimide, the rear field in polyimide a t its edges, con- duction in polyimide parallel to the surface producing high charge density a t its edges, and the difference in the electron beam-induced steady state voltage between the polyimide surface and the Mylar P E T surface. It would be a difficult task to ascertain the importance of each of these.

IEEE Transactions on Electrical Insulation Vol. 27 No. 6, December 1992 1173

W K

0 2 2 a

9 n

a y-1 d. -

Discharges follow one after the other spaced by only - 3 min. This time period is of the order for electric fields to re-establish themselves in Mylar P E T as seen in Figure 7. The details of the calculation of this process are complicated and will not be discussed. It is a firm experimental result that the pulses followed one another with a roughly constant time spacing, *50%. In this way we captured hundreds of pulses.

40 3 L

3 2 2 60

4 > 80

Sample 5 was 125 pm thick Mylar P E T cut to 10.2 x 6.3 = 64.3 cm’. This thickness of Mylar provides - 2 . 2 6 ~ 10-l’ F/cm’ between the front and rear surface. The edges were covered by polyimide tape so that the exposed surface was 9.8 x 5.0 = 49.0 cm’. Two sizes of collimators were used, 36 and 6 cm2.

SAMPLE 5

the plasma velocity is roughly (8 x 10-3)/(8 x lo-*) = 1 x lo5 m/s. Inclusion of fringing field effects would de- rive a slightly smaller plasma expansion velocity. (3) The blowoff currents can go preferentially to either of the plas- ma electrodes, the potential difference between the plas- ma electrodes is small compared to the charged sample surface potential (-10 to -20 kV). This causes a non ze- ro current to register in Ip even if true plasma current as depicted in Figures 1 and 4 is not occurring. Thus, the plasma currents which flow during the time of the surface discharge itself can be a t most weakly ascribed, if a t all, to the bias applied between the plates. Normally one must wait for the surface potential and IBO to abate before plasma currents Ip unambiguously have the mean- ing which was argued in the beginning of this paper. In Figure 9 only zero or negligible true plasma currents I p are seen.

In the first series of pulses, the collimator produced a beam whose area is 8.0 x 4.5 = 36.0 cm’, but a few scattered electrons certainly irradiated the entire exposed surface. No polyimide strip is needed at the middle of the sample in this irradiation to induce discharges because the edge of the polyimide, which covers the edges of the sample, initiates the discharges. Similarly, no copper tape was required. 165 pulses occurred with a spacing h = 8 m m and 35 pulses occurred a t a spacing h = 3 mm before the tests on this sample were arbitrarily ended. Typical pulses are shown in Figure 9.

The points to notice in Figure 9 are the following. (1) The blowoff current removes a charge of order 10 to 20 pC which is less than, but of the order of, the charge initial- ly on the sample surface acting as a capacitor charged a t roughly -20 kV. (2) The plasma associated with the blowoff current arrives between the plasma electrodes 0.05 to 0.1 ps after the pulse begins. The transient digitizer data was much more detailed than in Figure 9, it has a resolution of - 1 ns. Since the plates are 8 m m above the sample surface, if one neglects fringing field effects,

Figure 10. Plasma current after the blowoff pulse ceases (at the arrow). Prior to the arrow, it is possible that the signal is either positive or negative since im- balance in the fraction of I B O that arrives at the two plasma electrodes can be random. Zero cur- rent is seen on each curve at its initial ( 1 = 0) and final ( t = 3.5 ps) values.

6.4 TRUE PLASMA PULSES

It is not clear whether we have seen simple plasma puls- es in the results. The plasma pulses that we have mea- sured are illustrated in Figure 10. The true plasma cur- rent measurement begins after the blowoff current ends, a t the arrow in Figure 10. The maximum true plasma current seen to date is of the order of several amperes and decays in several ps for a total charge of order 5 pC. This is small compared to the blowoff pulse so it provides no new information. The biased plasma electrodes need more area and higher bias in order to produce more plas- ma current. When the bias is raised, a new phenomena

1174 Frederickson et al.: Radiation-induced Electrical Discharges in Complex Structures

(secondary discharges) sets in and prevents further ef- forts in this direction. Full breakdown between the plas- ma electrodes often dominates the response a t Vp above 100 V. When full breakdown does not occur, the currents between the plasma plates are composed of both plas- ma particle motions and of secondary electrons (or ions) generated at the plasma plates by impacting plasma par- ticles. In Figure 10 one can not determine how much of the plasma current comes directly from the plasma creat- ed by the dielectric discharge alone, nor how much comes from secondary processes occurring between the plasma electrodes.

1 v, = 120

L b- V, = 180

C A 4 v, = 200

t n l

~~~ ~ ~

0 1 2 3 4 MICROSECOND

Figure 11. Typical secondary discharges.

6.5 SECONDARY DISCHARGES

When the plasma electrode voltage Vp > 100 V we frequently observed that a discharge evolves into a sec- ondary discharge between the plasma electrodes. Most often, when the plasma electrode current rose above that described in Figure 10, i t continued to rise and created a full secondary discharge arc which discharged the voltage stored on the plasma electrode bias capacitors. Figure 11 illustrates the typical plasma electrode secondary break- down pulses. The total charge in the Ip current pulse equaled that which was stored on the plasma electrode bias capacitors.

Eighty secondary pulses on this sample have been seen and fully recorded. The secondary discharge piilaes can begin during the blowoff pulse event, but they often be- gan more than a half ps after the initial blowoff event had abated. This is evidence that a remnant plasma re- mains between the plasma electrodes after the insulator has been discharged. Since breakdown is not expected between stainless steel plates biased a t - 300 V in vacu- um, even in the presence of the 25 keV electron beam, it seems that the plasma generated by the insulator is the likely cause of this secondary discharge arc.

Secondary discharges have initiated as late as 2 ps after the blowoff has ended. If the plasma sheath drift velocity is lx105 m/s, then the plasma which participated in IBO has mostly dispersed in less than 0.5 ps and it is remnant plasma which initiates the discharge. The remnant plas- ma can have three sources: secondary ions and electrons from the plasma electrodes sustained by the bias, remains of the blowoff plasma, or continued issuance of plasma from the dielectric sample which, though its surface is discharged, still has an internal electric field sufficient to produce continued dielectric breakdown. We have not tried to study these three causes separately. All three have interesting consequences for practical applications.

The secondary discharge peaks in Figure 11 have not been further analyzed. Some overshoot is evident a t the end of the larger pulses. The circuit should be investigat- ed to determine its response to a ‘perfect’ short circuit by measuring the current-time waveshape created by a mercury-wetted contact closure or similar ideal short cir- cuit. Then one could determine the rates of rise and fall of conductivity in the discharge path. Having done this, one might then be able to estimate the time dependence of the plasma density between the electrodes. At this stage in the research we report only the measurement (in Figure 11) of peak currents of roughly 1.5 A per initial applied V on a 2 p F plasma bias capacitor. Peak slew rates for Ip were of the order of 500 A/ps a t the high- er applied voltages - 200 V. The IBO slew rates were roughly 1000 A/ps.

6.6 O T H E R SECONDARY DISCHARGES

The electrical discharge path can expand inside the vac- uum to other electrodes. Probably it is the plasma which expands and envelopes the other electrodes and thereby initiates further discharges among the other electrodes. Sample 4 of these experiments shows a commonly record- ed secondary discharge which connects the plasma elec- trodes to the rear electrode (Figures 4 and 5).

IEEE Tkansactions on Electrical Insulation Vol. 27 No. 6 , December 1992 1175

2 - 2 - 0

v)

Sample 4 was configured as shown in Figure 4; there was no polyimide tape covering the edges. This sample also experienced hundreds of pulses. When the plasma bias voltage was small, the IBO blowoff pulses were the simple surface discharge pulses often reported in the liter- ature. However, when the plasma bias voltage exceeded 100 V, IBO would often experience large excursions tem- porally coincident with the I, plasma pulse. The missing polyimide tape allowed the plasma electrode voltages to discharge directly to the rear electrode.

V, = 200 SAMPLE 4

I I I 1 I 1 0 1 2 3 4 5

MICROSECOND

Figure 12. Secondary discharge connects to the rear electrode.

Referring to Figure 4, if either the positive or the neg- ative plasma electrode discharges through space to any ground, including the rear electrode, the I , signal will have positive polarity in either case. If either plasma electrode discharges to the rear electrode, then the IBO signal can be positive or negative depending on which electrode discharges most strongly to the rear electrode. These facts form a diagnostic tool which is used to un- derstand Figure 12.

Figure 12 exhibits the connection of the plasma elec- trodes to the rear electrode, presumably via the expand- ing plasma. In Figure 12 we see, as we saw before, the blowoff pulse which begins a t 0.6 ps followed by the ap- pearance of the blowoff pulse on the plasma electrodes a t 0.7 ps. The true blowoff IBO pulse ends a t 0.8 ps, but there is a slow rise in both IBO and I, from 0.8 to 1.6 ps and a faster rise from 1.6 to 2.3 ps. This rise might be due to the growth in the density of a plasma bridge between the positive plasma electrode and the rear electrode. At 2.3 ps the plasma electrodes became directly connect- ed to each other, and a large I , current flows between them in this path while a smaller current IBO continues to flow, now from each plasma electrode, to the rear elec- trode. IBO is first positive and then negative because

initially the positive electrode is partially discharged ( to the rear electrode) before the negative electrode begins to discharge.

On sample 4, IBO was often seen to be involved in the discharge of the plasma electrodes; pulse records similar to Figure 12 were noted often on sample 4 with uncov- ered rear electrode. They were never seen on sample 5 which had a covered rear electrode. Presumably this was because the radiation induced discharge initiated a t the Mylar P E T sample and the plasma spread to connect the nearby rear electrode to the plasma electrodes on sample 4 only, never on sample 5.

In a few cases, even the electron gun HV power supply protection circuit was tripped. The HV was over 10 cm distant in the vacuum from the Mylar P E T sample and the plasma electrodes. The plasma and its associated neutral gas form a cloud which spreads more than 10 cm, and bends around corners to short the 25 kV supply to ground.

7. STATISTICAL RESULTS

HE earlier portions of the paper described the general- T ized concepts and showed the form of typical results. In this Section we gather together some statistical results for many pulses. Each pulse can have a unique signature which, taken alone, can be misleading. The averaged data taken over many pulses provides a different picture which helps our understanding.

7.1 PRESSURE MONITOR PULSES

The pressure pulses were collected from a number of the discharge events and are indicated in Figure 13. It is interesting to see that the pressure level is larger, on aver- age, for the discharges where the plasma electrodes had a secondary discharge. But the increase is not proportional to the integral discharge current increase. This leads one to the conclusion that the dielectric discharge produces more gas than does a metal to metal arc of the same total charge transfer. This fact agrees with the thesis that the dielectric discharge alone produces substantially more gas or plasma than that which is necessary to discharge the insulator surface.

7.2 TOTAL CHARGE PER PULSE

Figure 14 describes the total charge per pulse as a func- tion of the plasma electrode bias for sample 5. The re- sults show that blowoff charge is independent of v, but

1176

m a s W v) -I 2

W

2 v) v) W

n

a

a n

Fkederickson et al.: Radiation-induced Electrical Discharges in Complex Structures

WITH SEC DISC 0 FIRST RUN, NO SEC DISC 6 SECOND RUN, NO SEC DISC

PET# 5, h = 8 mm, 36 cm2 0 0 100 200 300

PLASMA ELECTRODE VOLTAGE, V,

Figure 13. Pressure pulses as a function of plasma bias volt- age. Note that the secondary discharges increase the pressure, but not substantially. Each point is the average of several (usually 5) pulses. The arbitrary unit is the instrument response without calibration. The larger pulses usually occurred early in the sample irradiation history and V, was low then.

m E

3

0 K

I

s 8 0

15

10 .

0 100 200 300 -5 ' I

'P

Figure 14. Blowoff charge and plasma charge vs. plasma volt- age. Here QPR is the integrated plasma current beginning a t the time of the arrow shown in Fig- ure 10 and continuing for 2 ps.

plasma charge increases with V,. Each point is the aver- age of several (usually 5) discharges. Simple function- al fits were made by least squares, and are: QBO = 13.4 - O.O014V, pC, QPR = 0.12 + 0 .0125 pC. With- out removing the imbalance in Qp due to QBO the same kind of plot has been made and its functional fit is similar, Qp = -0.18 + 0.02Vp pC. Thus, removal of the imbalance did not distort our conclusions.

Because the rear electrode is covered by Kapton tape in this experiment, the plasma could not connect the plasma electrodes to the rear electrode. The data for blowoff charge shows that the blowoff charge is not dependent on the plasma electrode bias. There is no surprise here.

The dependence of plasma polarization current on plas- ma bias voltage is positive but weak on this scale. This data needs to be compared to a theory for plasma po- larization current in this plasma which is expanding and relaxing and flowing away. Then, perhaps, one could esti- mate the charge density of the plasma which results from the dielectric discharge.

It is disappointing that we did not have a good diagnos- tic for measuring the discharge pulse plasma density. But the major finding of this paper, that secondary discharges are so easy to create with the radiation-induced dielectric discharge, has prevented us from measuring the plasma density. We are unable to raise the plasma electrode bias significantly beyond 100 V, where better plasma current data would be available, because the secondary break- downs swamp the results.

1 2 3 4 5 6 7 8 ARBITRARY EVENT NUMBER

Figure 15. Blowoff pulses for simple sample with and without floating copper strip.

7.3 FLOATING METALIZATION

It has been reported in the literature that floating met- alization causes pulses, and that either grounding the metal or removing i t can help to prevent discharge puls- es. Our results modify this concept. Changing conditions amongst grounded, floating, or missing metalization will vary the probability of occurrence of pulses, but can not be depended on to eliminate them. For example, Fig- ure 15 describes the few pulses which were measured on the simple sample with only 6 cm' of irradiated area, for both the clean surface and the clean surface with floating copper strip added. In this case the clean surface pro- duced nine larger pulses, but that may be because there

IEEE Bansactions on Electrical Insulation Vol. 27 No. 6, December 1992 1177

" x 2 ' 0 -

=- P

was more time, N 1 h between pulses, for scattered ra- diation to charge more surface area. Also, not shown, the clean surface produced only one secondary discharge even though the plasma bias was 300 V for all nine pulses, and i t occurred after the electron beam was turned off. No secondary discharges occurred with the ten recorded pulses associated with the copper strip and with 300 V plasma bias.

-Q,?

** * ++

I + I 1

w 0. + + 0 . + + +

*** ** +," +++ rC* +

The important concept is that edges, whether they are dielectric or conductor, which produce a sharp change in conductivity will significantly increase the pulse rate in irradiated insulators. Kapton on Mylar P E T is the case investigated here. An electrical device without such edges is a rare structure. Nearly all devices have edges built in to them, and removal of inadvertent floating conductors by grounding will not prevent pulses.

Figure 16. Distribution of blowoff pulse sizes. Only one plas- ma pulse connected to the rear electrode in this set at 100 V.

7.4 BLOWOFF PULSE DISTRIBUTION

Figure 16 is a typical distribution of sizes of the to- tal charge QBO transferred during the IBO current pulse. This da t a is for sample 4 where plasma bias can connect to the edge of the rear electrode. Blowoff pulses do not always remove the same amount of charge; the measure- ments do not distinguish whether this is due to differing surface voltages immediately prior to the pulse or to in- complete discharging of the surface by the pulse. The pulses were spaced sufficiently in time that the incident current should have achieved the effective surface charge, QS a t 20 keV, shown in Figure 16 prior to nearly every pulse. Thus we seem t o have learned that the plasma it- self' varies, from event to event, in its ability to discharge the surface.

At V, = 100 only one of the pulses (#71) caused the plasma electrodes to connect t o the rear electrode. From all the other pulses in Figure 16 we find that the plasma electrode bias has no effect upon the blowoff pulse as long as the plasma does not connect the plasma electrode to the rear electrode. This was a n important assumption for the design of the apparatus, and the results indicate that the design was a t least successful a t these lower plasma biases. Similar results accrued for the cases where h = 8 or 3 mm, but these closer spacings enhanced the proba- bility for the rear electrode to become connected to the plasma electrodes. As previously discussed, higher val- ues of Vp > 100 V also increase the probability that such secondary discharge current paths develop.

8. SUMMARY

1. The Mylar P E T samples by themselves responded to 25 keV electron irradiation much like many other highly insulating dielectrics reported in the literature by several groups.

2. The P E T discharge would produce a plasma which moved across the chamber with a velocity roughly lo5 m/ s in an electric field of roughly 20 keV/cm.

3. The experiment was unable to measure the amount or density of this plasma with accuracy.

4. The plasma would induce secondary discharges on stainless steel electrodes not associated with the Mylar PET, but nearby.

5. For nominally 50 cm2 P E T dielectrics, secondary discharges occurred with: low probability (< 10%) for electrode bias of 100 to 150 V, high probability (50%) for bias from 250 to 300 V, and zero probability < 50 V. A 6 cm' dielectric sample had a much lower production of secondary discharges, but the data base is limited.

6. For the nominal 50 cm' samples, the surface dis- charge produced vacuum chamber pressure pulses equiv- alent to the sudden introduction of 0.1 to 0.4 mm3 of atmospheric pressure gas.

7. The secondary discharges appeared to be full vacu- um insulation failure and fully discharged - 4 p F within - 4ps.

8. For a specific amount of charge in a discharge, the dielectric surface produced far greater gas burst than did the metal electrodes. The amount of dielectric plasma is

I

1178 fiederickson et al.: Radiation-induced Electrical Discharges in Complex Structures

far more than that which is necessary to discharge the surface of the radiation-charged dielectric surface.

9. The plasma is dense enough and extensive enough that it can encompass a t least three electrodes, 1 cm apart, within its sheath, and mutually short them to- gether.

10. Slew rates of 500 A / p a t 200 V bias were mea- sured between the plasma electrodes. 1000 A/ps were commonly seen from the dielectric charged to -20 kV. It is not known if the measurement was limited by the circuit.

11. Both dielectric edges and metal edges produced significant enhancement of the radiation-induced pulse rates. With the edges in place, rates of a pulse every 5 min a t 3 nA/cm2 beam intensity were experienced. With- out edges, pulse rates were roughly 20x less on the one sample studied briefly.

REFERENCES

H. B. Garrett, “The Charging of Spacecraft Sur- faces”, Rev. Geophys. Space Phys., Vol. 19, pp. 577- 616, Nov. 1981.

M. Gossland, K. G. Balmain and M. J. Treadaway, “Surfa.ce Flashover Arc Orientation on Mylar Film”, IEEE Trans. Nuc. Sci., Vol. 28, pp. 4535-4540, 1981.

K. G. Balmain, “Surface Discharge Effects”, AIAA Progress in Astronautics and Aeronautics, Vol. 71, in Space Systems and Their Interaction With Earth’s Space Environment, H. B. Garrett and C. P. Pike, ed.. DD. 276-98. 1980.

[4] A. R. Frederickson, “Electrical Discharge Pulses in Irradiated Solid Dielectrics in Space”, IEEE Trans. Elec. Ins., Vol. 18, pp. 337-49, 1983.

[5] L. Levy, in three reports by Center Etudes et Recherches Toulouse (CERT), bp 4025-31055 Toulouse cedex, France: (a) Comportement Elec- trostatique d’elements de Couvertures Externes du Satellite SPOT. CR/CLAQ/05, May 1985. (b) As- sistance Technique a u Projet METEOSAT: ldentifi- cation des Sources de Decharge. Dec. 1986. (c) Lois d’echelle et Caracteristiques de Decbarges Obtenues sur Support Flottant. CR/CLAQ/lO. Dec. 1986.

[6] R. C. Hazelton, R. J . Churchill and E. J. Yadlowsky, “Measurement of Particle Emission From Discharge Sites in Teflon Irradiated by High Energy Electron Beams”, IEEE Trans. Nuc. Sci., Vol. 26, pp. 5141- 5145, 1979.

[7] A. R. Frederickson and S. Woolf, “Electric Fields in keV Electron Irradiated Polymers”, IEEE Trans. Nuc. Sci., Vol. 29, pp. 2004-2011, Dec. 1982.

[8] T. M. Flanagan, R. Denson, C. E. Mallon, M. J. Treadaway, E. P. Wenaas, “Effect of Laborato- ry Simulation Parameters on Spacecraft Dielectric Discharges”, IEEE Trans. Nuc. Sci. Vol. 26, pp. 5134-5140, Dec. 1979.

This paper i s based on a presentation given at the Conf on Electr. Insul. in Vacuum, Santa Fe, N M , September 1990.

Manuscript was received on 26 August 1991, in revised form 28 February 1992.

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