SCIENCE

Similarities between electron emissionand consequent breakdown processes in

high-pressure gases and in vacuumProf. A.E. Guile, Ph.D., D.Sc.(Eng)., C.Eng., F.I.E.E., R.V. Latham, Ph.D.,D.Sc, F.lnst.P., and A.E.D. Heylen, Ph.D., D.Sc.(Eng).. C.Eng., M.I.E.E.

Indexing terms: Breakdown and gas discharges, Electric discharges, Gas discharges, Vacuum breakdown,High-pressure gas breakdown

Abstract: The paper exhibits and considers most of the similarities and some of the differences between electronemission and breakdown processes in compressed gases and in vacuum.

1 Introduction

It has been the general practice to treat electron emissionmechanisms and their connection with breakdown pro-cesses in high-pressure gases as entirely separate from anddifferent to those occurring in vacuum.

In recent years, however, it has become well recognised,at least for a range of low-pressure to high-pressure gases(of a few bars, 1 bar = 105 Pa), that the electron emissionand resulting emission sites in arcing on nonrefractorycathodes are dependent on complex and highly localisedvariations in the cathode material and its surface condi-tions, including charges within or on the surface of oxidefilms [1, 2]. In the interpretation of these data, possiblemechanisms suggested as influencing the electron emissionincluded the switching of conducting filaments throughsemiconducting or insulating oxide films and energy bandbending [3, 4], and it was recognised [2] that these pro-cesses were also being examined in electron emission fromcathodes in vacuum, particularly with respect to impurityinclusions [5, 6].

In particular too, it has been observed that the electricfields at which electron emission occurs in vacuum andthose at which departures from Paschen law occurs inhigh-pressure gases are very similar, and also that, in bothcases, the electron emission rate follows a Fowler-Nordheim type of field dependence.

When a new technique was used to measure single-electron emission from cathodes almost undamaged byprevious sparking in high-pressure nitrogen/ethanemixture [7], and in later work in which all sparking wasavoided [8], it was recognised that the Fowler-Nordheim-type plots which resulted could no longer be interpreted ina simplistic manner, because of the evidence, which hadbeen accumulating for cathodes in vacuum and in high-pressure gases, of the importance of oxide films and oxide-type impurities of micrometre sizes which may besemiconducting or insulating.

Very recently, further experimental evidence has beenfound which connects electron emission in the two differ-ent regimes. By viewing a cathode through the anode,which consists of a glass disc coated with a transparent tinoxide film, it has been discovered [9] that there is direct

Paper 4626A (S3), received 18th July 1986

Dr. Heylen is, and Prof. Guile was formerly, with the Department of Electrical &Electronic Engineering, University of Leeds, Leeds LS2 9JT. Prof. Guile resides atShalom 7, The Garth, Norton, Stockton-on-Tees, Cleveland TS20 IAD. Dr. Lathamis with the Department of Mathematics & Physics, Aston University, BirminghamB4 7ET, United Kingdom

correlation between the spatial distribution of emissionsites in vacuum and those which occur later in sulphurhexafluoride at 10 bar (106 Pa) pressure, provided that aninterval of at least 18 hours is introduced between the twoexperiments.

The point has now been reached where it is importantto examine, as this paper does, the similarities betweenprocesses in the two regimes, so as to stimulate new think-ing and progress. In particular this should improve theunderstanding of mechanisms of electron emission and itsrole in breakdown in high-pressure gases and in arc initi-ation, where some of the techniques available in vacuumsystems cannot directly be applied at high pressures.

2 Total gap characteristics

The literature abounds with reports of 'external' measure-ments of the physical properties of both high-pressure andvacuum-insulated high-voltage gaps. The aim here is toabstract this information with a view to highlighting theexistence of a range of experimental features that arecommon to the two regimes. For more detailed informa-tion, readers are referred to the extended review byCookson [10], for high-pressure processes, and by Latham[5, 11], for vacuum processes.

2.1 I/V characteristics

2.1.1 High-pressure gas insulation: With continuoussparking at atmospheric pressure, it was found [12, 13]that electron emission of some 105 electrons per secondcommenced at an electric field as low as 3.7 MV/m;however, recent work [14], also at atmospheric pressure,has shown that for virgin, unsparked copper and alu-minium cathodes no emission at all could be observed forfields up to 6 MV/m, and that, using compressed gas insu-lation, electron emission of only some tens of electrons persecond commenced at fields of 8 MV/m for occasionallysparked copper surfaces [7]. The latest work [8], whichcompletely avoids any sparking, shows that for hardmaterials (small Fermi energy) field emission only com-mences at 14 MV/m and for soft materials such as alu-minium (high Fermi energy) it starts only at 20 MV/m,providing the surfaces are very carefully polished. Theinitial field range covered for copper was [7] 8 to22 MV/m and, more recently [8], 17 to 25 MV/m; higherfields could not be reached as they lead to breakdown ofthe gas. The emission rate varied from 1 to 10 electrons

280 IEE PROCEEDINGS, Vol. 133, Pt. A, No. 5, JULY 1986

per second only. It might have been expected, because ofthe very low electron emission rates from completelyunsparked virgin surfaces, that very steady electron emis-sion would be measured, especially from the very carefullypolished samples which were flat to 0.05 fim. However,surprisingly, the emission rate has been found [8] to beerratic and subject to switching, particularly as the oxidelayer gets thicker and the emission rate increases. This canhardly be due to local joule heating producing filamentarychannels at emission rates up to only 10 electrons persecond.

For the very limited field range of 10 to 12.5 MV/m,Llewellyn-Jones et al. [12, 13] obtained apparentlyremarkably accurate Fowler-Nordheim plots; however,Morant [15] was able to show that, because of the smallfield range, a Richardson-Schottky relationship could alsobe obtained from the same results. No such ambiguityexists for our results [7] because of the much wider fieldrange covered, but there was more scatter when theFowler-Nordheim law was applied. It is significant that thenew derived work functions for copper [7, 8], based onwhat seems to be reasonable field enhancement factors notgreatly exceeding 10, are some ten times smaller than thoseof Llewellyn-Jones et al. [12, 13] and, while they derivedemission areas of atomic dimensions, the later resultsapproach the cross-section of the electron [3 x 10~28 m3],although it is now clear that the physical model on whichthe Fowler-Nordheim law is based does not apply to realelectrodes. A change from the natural (2-3 nm) oxide layerto a medium thickness one (20-30 nm) produced thebigger change in the Fowler-Nordheim derived constants,after which a further increase in oxdide thickness (say to athick layer of 380 nm for copper) produced less change.

2.1.2 Vacuum insulation: Under good vacuum condi-tions, i.e. <10~ 8 mbar (10 " 6 Pa), a pair of virgin elec-trodes will exhibit a switch-on phenomenon, whereby, atsome threshold field in the range 10-20 MV/m, the currentwill rise discontinuously from an effective zero value( ^ l O " 1 1 A) to a value that is typically in the range 10" 7 -10"5 A. Subsequent voltage cycling of the gap will yieldreasonably smooth and reversible current/voltage (I/V)characteristics, although on occasions secondary switch-onprocesses are observed as the applied field is progressivelyincreased. If these I/V data are presented in the form of aFowler-Nordheim (F-N) plot, it is typical to obtain alinear characteristic up to fields c^20 MV/m, with a fall-offat higher fields. The slopes of such plots have in the pastbeen conventionally characterised in terms of the fieldenchancement factor /?, with typical values in the range100 < /? < 2000; correspondingly, the intercepts have beenrelated to the area from which the current is emitted andfrequently indicated values lying in the unrealistically widerange of 10" i — 10"16 m2. In fact, these prebreakdowncurrents originate from microscopically localised electronemission processes, but, as will be discussed in a laterSection, there remains speculation as to the precise physi-cal mechanism responsible for the emission, which givesrise to a relationship of the Fowler-Nordheim form.

2.2 Influence of environmental and electrode materialand surface conditions

2.2.1 High-pressure gas insulation: It has been found [8]that the presence of the gas affects the emission rate andleads to an hysteresis effect in the Paschen characteristics.Normally emission rates were measured with increase ingas pressure to enable higher electric fields to be applied;

if, however, the gas pressure was reduced then, for thesame field, the emission is less at the reduced pressure thanbefore. This effect seemed to increase with increase in oxidethickness. It was also discovered that for copper [7] thethicker the oxide layer, the more the emission, and thisphenomenon has been verified for a range of othermaterials [8].

Contrary to expectation [8], it has been discovered thatthe harder the material, the more the electron emission.Thus the emission rate for a fixed field increases in goingfrom pure aluminium to OFHC copper, commercialcopper and aluminium, stainless steel, and mild steel toniobium. A feature of this sequence is that in going fromaluminium through copper to niobium, the Fermi energydecreases and the Fowler-Nordheim law correctly predictsthat the higher the energy, the less the emission, but thelarger variation observed (over a 1000 to 1) is muchgreater than the 1.5 to 1 change predicted.

2.2.2 Vacuum insulation: For this regime, the two mostimportant environmental factors are the residual pressureand the component gas species. Thus, an increase ofambient pressure above ~ 108 mbar generally leads to arapid increase in the noise level of the prebreakdown cur-rents, coupled with the onset of microdischarge events andcurrent ignition processes. There is, however, evidence toindicate that emission may be suppressed by an increase inthe partial pressure to ~10~ 5 mbar of such relatively inertgases as N 2 , H e and SF6; oxygen on the other hand,which is chemically very active through chemisorptionprocesses, tends to produce unpredictable effects at thispressure. The single most important electrode surfacerequirement for suppressing electron emission, irrespectiveof electrode material, is cleanliness. Thus, to obtain thebest performance from a given material, it is necessary todevelop specific chemical polishing and rinsing processes,together with a 'clean-room' assembly procedure. Apartfrom this general dictum, there is some statistical evidenceto suggest that certain materials (e.g. Ti, Mo and stainlesssteel) have unspecified surface properties that make themparticularly suitable for use as high-voltage electrodes. Thedeliberate oxidation of electrodes generally tends tosuppress emission; however, if a breakdown is allowed tooccur, the subsequent performance of the gap is likely tobe considerably worse than that found with ambientoxided electrodes. Lastly, it should be noted, for reasons tobe discussed later, that the use of ion etching as an elec-trode cleaning procedure is generally unreliable, in that itleads to unpredictable and apparently random changes inthe emissivity of electrodes.

2.3 Electrode conditioning

2.3.1 High-pressure gas insulation: A feature of high-pressure gas-insulation systems has been that the initialspark breakdowns have generally occurred at a relativelylow voltage, and that as successive spark breakdowns takeplace the breakdown voltage tends to increase, reachingperhaps twice the initial value. The cathode surface is thensaid to have been conditioned. In experiments with sparksrepeated 50 times per second, the electron emission fellfrom an initial rate of about 106 electrons per second at6 MV/m for freshly polished cathodes of various materials,until after about 15 minutes no electron emission wasobserved and the cathode surface was said to have beenconditioned [12, 13]. It seems that the most favourablesites for electron emission were put out of action by spar-king.

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2.3.2 Vacuum insulation: A number of techniques havebeen developed, mainly for industrial appplications, whichprovide the possibility of improving the insulating capabil-ity (i.e. reducing the emissivity) of preset modular electrodeassemblies. The most widely used of these techniques are'current', 'glow-discharge', 'gas' and 'spark' conditioning.In all cases, they rely on systemically destroying or sup-pressing dominent emission sites until the applied field canbe progressively raised to an acceptable margin above itsrequired operational value.

3 Emission processes leading to breakdown

3.1 High -pressure gasIt has long been suspected [16] that electron field emis-sion, in the broadest sense of the word, plays a leading rolein the determination of the breakdown voltage of com-pressed gas insulation and is responsible for departuresfrom Paschen's law. The difficulty has always been toseparate the initiatory process of electron emission fromthe primary ionisation mechanism leading to avalancheformation and the secondary process of electron regener-ation of the cathode by photon radiation from excited gasmolecules. By using a gas mixture of 10% ethane added toanother gas [14], typically nitrogen, the secondary processis completely suppressed and the initiatory electrons canreadily be observed on an oscilloscope and counted by apulse height analyser [8]. For this gas mixture, it wasinvariably found that those cathodes giving the least elec-tron emission (say typically aluminium) yielded Paschencharacteristics which lay above those for cathodes givinghigher emission such as mild steel and stainless steel;however, in pure nitrogen, where secondary ionisationtakes place, the difference in the Paschen curves is reducedand, in air, where the secondary process is predominant, itwas found that at the highest gas pressure used (13.8bar = 1.38 MPa), the Paschen characteristic for the alu-minium cathode fell below that for the stainless-steel one,in agreement with the findings of Trump, Cloud, Mannand Hanson [16]. It was found that very close to the spar-king voltage, switching mechanisms in the cathode surfacetook place which resulted in a large emission of electrons;it was difficult to separate this switching process from thebreakdown one.

3.2 Vacuum emission sitesOver the past decade, a number of specialised analyticaltechniques have been developed for locating and studyingthe properties of individual emission sites on broad-areavacuum-insulated electrodes. The findings of these investi-gations have been comprehensively reviewed elsewhere[5, 11], but their central conclusions can be summarisedfor the present overview as follows:

(a) Anode probe techniques [5, 11, 17-19] have estab-lished that emission sites are broadly of two types, namelypoint and extended sites. Point sites are associated withmicrometre-sized surface inclusions that are electricallyinsulated from the electrode surface: however, in the fieldrange 10-30 MV/m, only a tiny fraction (less than 1 in 108)of such inclusions act as emitters. The material composi-tion of these field-emitting micro-inclusions has beenfound to involve either the element of the substrate elec-trode, or the 'foreign' elements of aluminium, silver andcarbon. Extended sites, in contrast, can have dimensions ofseveral tens of micrometres, and are generally found toinvolve carbon deposits that are often situated along grainboundaries or cracks.

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(b) All sites exhibit an initial 'switch-on' phenomenon[20], and thereafter give a reversible current/voltage char-acteristic that can be represented by an approximatelylinear Fowler-Nordheim plot.

(c) All sites exhibit nonmetallic electron spectral charac-teristics where both the spectral halfwidth (FWHM) andshift are strongly field-dependent [21, 22]. It has also beenestablished that the emission of electrons is accompaniedby the emission of electroluminescent optical photons, [23,24], which further confirms the nonmetallic nature of theemission mechanism. In fact, this evidence has led to a newphysical interpretation of the phenomenon that is basedon the concept of a field-induced hot-electron-emissionmechanism that occurs in conducting channels that are'formed' in the dielectric medium [6, 11, 20]: i.e. similar tothe process operating in metal-insulator-metal switchingdevices [25, 26]. This model has not only provided a suc-cessful explanation for the field-dependent properties ofthe electron spectrum, but has also given a new physicalsignificance to the slope of the Fowler-Nordheim plot ofthe I/V characteristic.

(d) From recent emission image studies [27], it has beenestablished that sites typically consist of a group of inde-pendent subsites (i.e. emission channels) that individuallyswitch on and off randomly with time and give rise to aflickering of the image and associated fluctuations of thesite current. The flicker frequency of an individual subsiteis highly pressure-dependent, varying from <0.01 Hz at10"1 0 mbar(10"8 Pa) to > 1 Hz at 10"8 mbar(10"6 Pa),and provides an explanation why the noise level of preb-reakdown currents similarly increases rapidly with press-ure.

(e) The general properties of emission sites appears to beindependent of the substrate electrode material: however,their properties are strongly influenced by electrodesurface treatments [11,27]. Oxidation, for example, canseverely modify the emission characteristics of an individ-ual site (e.g. by increasing the complexity of the subsitearray and hence changing the I/V characteristic of thetotal site current), but it rarely results in the emission froma site being completely suppressed. Argon-ion etching, incontrast, can not only eliminate known sites, but alsounearth randomly located new emitting structures thatwere previously buried below the surface of the electrode.

3.3 Spatial correlation between vacuum andhigh-pressure processes

In a recent investigation [9], a new experimental techniquewas developed for determining whether there was a spatialcorrelation between the vacuum emission sites and the firsthigh-pressure arcs in a plane-parallel electrode geometry inthe field range 10-40 MV/m. A 40 mm diameter diamond-polished copper disc formed the test cathode and was setat a separation of 0.5 to 1 mm from an optically transpar-ent anode, so that an open-shutter camera could sequen-tially record the anode spots [6,23] of the vacuumemission processes and the subsequent HP arcs when thegap was insulated by SF6 at a pressure of ~ 10 bar(106 Pa). The main conclusions of the investigation can besummarised as follows:

(i) For over 60% of the specimens studies, a spatial cor-relation was observed between the first high-pressure arcand one of the vacuum sites

(ii) In all cases of correlation, the arc was initiated froma point rather than an extended site [11]

(iii) The gas breakdown initiating event is thought tostem from the burst of charge injected into the gas follow-ing the switch-on of a site [20]: i.e. the dark currents that

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were observed in ~ 30% of experiments are not thought tobe responsible for gas breakdown events

(iv) High-pressure breakdown events are preferentiallyinitiated at artificially introduced carbon emission sites[28, 29].

4 Discussion

The research of recent years has shown an increasingnumber of similarities between electron emission and itsrole in breakdown in high-pressure gas and in vacuum.The most significant of these are shown by the following

(a) With improvements in measuring and electrodepreparation techniques, it is now known that a significantelectron current begins to flow in the same range of electricfield of about 10-25 MV/m and this is where departuresfrom the Paschen law occur in both cases

(b) In both cases the current/voltage characteristics ofthe gap yield Fowler-Nordheim-type plots, but for both ithas been recognised that they cannot be interpreted in asimplistic way

(c) In both cases the cathode material and surface con-ditions, such as oxide and nature and degree of polishing,greatly affect the electron emission, and this occurs incertain microscopically localised parts of the surface

(d) In both cases some types of switching phenomenaare occurring in the cathode surface, which are as yetimperfectly understood but where similar mechanisms arebeing examined for each of the two regimes

(e) In both cases, starting from certain initial surfaceconditions, the electron emission can be reduced and thebreakdown voltage increased by 'conditioning', by repeat-ed sparking, as electron emitting sites are destroyed orsuppressed

(/) In vacuum cathodes the role of micrometre-sizeddielectric-type impurities in electron emission sites is wellestablished, and there are indications in certain high-gas-pressure arcing studies, notably of cathode erosion in archeaters and in arc welding, that a few parts per million ofimpurities can play a significant role

(g) In both cases charges associated with surface statesor trapping centres within surface layers, such as oxide orimpurity inclusions, can affect electron emission

(h) In both cases there is now evidence that small emis-sion areas frequently consist of a group of independentemitting sites which individually switch on and off andcause the electron current to fluctuate (These tiny areasand sites are often termed sites and subsites in vacuumliterature)

(i) Spatial correlation has now been established betweenvacuum emission sites and the first high-pressure arcs onthe same cathode, provided that a delay of at least 18hours exists between the two processes. It is thought thatcharge transfer processes occur in the cathode surfaceduring this interval.

A major difference, however, is that in compressed gas,breakdown may ensue from one initiatory electron, whilein vacuum the current has to reach at least microamperes.

5 Conclusions

Many points of similarity have now emerged in electronemission and its connection with breakdown, betweenhigh-pressure gas and vacuum systems, which warrantfurther examination and study, not least because sometechniques which can be applied in vacuum are not avail-able at high gas pressures. It has not, for example, been

possible to examine the spatial resolution of emitting sitesin high-pressure gases as it has in vacuum, except when thecurrent following breakdown has built up to the extentthat an arc occurs and craters are left on the cathodesurface at the sites which were previously emitting. Norhas it been possible to obtain Fermi-level evidence forhigh-pressure gas emitting sites as has been obtained forvacuum, and it is important to discover what rolemicrometre-sized dielectric or semiconducting impuritiesplay in high-pressure gas systems on the electron emissionfrom cathodes with varying oxide thickness. This has con-siderable potential significance to a number of other engin-eering applications beside breakdown, e.g. in arc heatercathode erosion [2] and in arc welding [30].

6 References

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23 HURLEY, R.E., and DOOLEY, P.J.: 'Electroluminescence producedby high electric fields at the surface of copper cathodes', J. Phys. D,1977, 10, pp. L195-L201

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27 BAYLISS, K.H., and LATHAM, R.V.: 'The spatial distribution andspectral characteristics of field-induced electron emission sites onbroad-area high voltage electrodes', Vacuum, 1985, 35, pp. 211-217

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29 ATHWAL, C.S., BAYLISS, K.H., CALDER, R., and LATHAM,R.V.: 'Field induced electron emission from artificially producedcarbon sites on broad-area copper and niobium electrodes'. Proc.Xlth Int. Symp. on Discharges and Electrical Insulation in Vacuum,Berlin, 1984, pp. 77-80

30 GUILE, A.E.: 'Electric arcs: their electrode processes and engineeringapplications', IEE Proc. A, 1984,131, (7), pp. 450-480

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