IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 37, NO. 9, SEPTEMBER 2009 1863

Mechanisms and Predictors of Insulator Degradationand Erosion Produced by Pulsed High-Current

Surface DischargesThomas G. Engel, Senior Member, IEEE, and Magne Kristiansen, Life Fellow, IEEE

Abstract—The mechanisms and predictors of insulator degra-dation and erosion by pulsed high-current surface dischargesare presented and discussed. Erosion and degradation depend onthe insulator material, the electrode material, the ambient gas,and the presence of UV stabilizers in, or on the surface of, theinsulator. Insulator degradation is the result of material decom-position into conductive metal or carbon species and is measuredby a decrease in the surface breakdown voltage. Insulator erosionis measured by the material’s mass loss. The performance ofa large group of ceramic, polymeric, and elastomeric materialstested with graphite and molybdenum electrodes is presented inthis investigation. The insulators are exposed to repetitive 300-kA20-μs-long surface discharges. Tests are performed in atmosphericair and pure nitrogen. Various methods to rank insulators interms of holdoff voltage degradation, mass erosion, and holdoffvoltage conditioning (HVC) using the material’s thermochemicalproperties are presented and discussed. HVC is characterized byan initial increase in surface holdoff voltage. The ranking methoddeveloped by the authors characterizes the insulator accordingto the holdoff degradation resistance (HDR), mass vaporizationcoefficient (MVC), and HVC figures of merit calculated by the ma-terial’s thermochemical properties. The investigation also showsthe relationship between the HDR, MVC, and HVC figures ofmerit.

Index Terms—Coilguns, insulator contamination, insulators,pulse power systems, railguns.

I. INTRODUCTION

INSULATOR erosion and degradation produced by high-current surface discharges is a complex physical phenom-

enon involving both gaseous electrical breakdown, plasmachemistry of the arc channel, and the thermochemistry ofthe insulator. The surface discharge switch (SDS) is used totest insulator materials for erosion and degradation in thisinvestigation. The SDS is shown in Fig. 1 and consists oftwo electrodes spaced a short distance apart on an insulator’ssurface. The SDS is operated in a self-break (i.e., nontriggered)manner in this investigation. In the self-break operation mode,the voltage is increased until a spark is produced between

Manuscript received March 26, 2009; revised May 5, 2009. First publishedJuly 28, 2009; current version published September 10, 2009.

T. G. Engel is with the Department of Electrical and Computer En-gineering, University of Missouri, Columbia, MO 65211 USA (e-mail:engelt@missouri.edu).

M. Kristiansen is with the Center for Pulsed Power and PowerElectronics, Texas Tech University, Lubbock, TX 79409 USA (e-mail:m.kristiansen@ttu.edu).

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

Digital Object Identifier 10.1109/TPS.2009.2025886

Fig. 1. SDS is used to test insulator erosion and degradation. Two electrodesare spaced a short distance apart on the insulator’s surface. The electrodevoltage is increased until electrical breakdown occurs. Dimensions are forreference and are typical values only.

the electrodes. Breakdown in nontriggered operation is almostexclusively a single channel event. The SDS can be madeto produce multiple discharge channels by applying a suffi-cient electrical or optical trigger pulse. The interested readershould consult [1], [2] for a more detailed description of theSDS and its operation. The SDS is the basis of many impor-tant pulse power devices including intense light sources [3],lasers [4], laser preionization sources [5], e-beam sources [6],X-ray sources [7], UV sources [8], plasma sources [9], and fastclosing switches [10]. Insulator degradation and erosion is alsoan important issue in electromagnetic launchers [11]–[13] andthe electric power industry [14], [15]. To function correctlyin these applications, the insulator’s surface holdoff voltage(SHV) must be stable and predictable. The insulator should notdecompose into conductive metal or carbon or semiconductivematerials. The insulator should not erode material sufficientlyto cause appreciable changes in the electrode geometry or to beentrained in the arc channel. An understanding of the processesoccurring in the surface discharge is necessary to understandthe mechanisms of the degradation and erosion processes.

Electrical breakdown across an insulator’s surface is relatedto the Townsend breakdown process in a gas medium. A suffi-ciently energetic electron starts an avalanche in the gas abovethe insulator surface leading to conductive streamer formationsin the gap. Surface charging of the insulator alters the Townsend

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1864 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 37, NO. 9, SEPTEMBER 2009

Fig. 2. Typical SHV response versus number of discharges is shown with thevarious response phases indicated. An ideal insulator would have a constantSHV response. HVC causes an increase in the holdoff voltage and is not presentfor all insulator materials. Degradation (decomposition) of the insulator intoconductive materials causes a decrease in the holdoff voltage.

process by producing an electric field component normal to theinsulator surface. The normal electric field causes electrons toimpact the insulator surface producing secondary electrons andreleasing adsorbed gases. If conditions are suitable, the stream-ers will increase in length to bridge the gap and form an arcor spark discharge. Thus, the electrical breakdown process isaffected by the geometry of the electrodes, the ambient gas, andthe insulator’s surface charge distribution. Conductive metal orcarbon deposits on the insulator’s surface from degradation willalso affect the breakdown process by altering the applied elec-tric field. Initially, the arc is located a short distance above theinsulator surface but becomes compressed against the insulatoras eroded insulator material is ionized and entrained by thedischarge. The discharge column expands laterally from hydro-dynamic forces. The lateral expansion velocity is faster thanthe normal expansion velocity yielding an arc channel with anelliptical cross section that is approximately one-half the cross-sectional area of a comparable free arc discharge [16], [17].

The action of the high-current arc on the insulator surfacenot only decomposes the insulator but also erodes the insulator.Eroded insulator material entrained in the arc channel can havea feedback effect on erosion and degradation by causing spec-tral enhancements at wavelengths dictated by the insulator’schemical composition. The spectral output of the dischargecan be tailored by choice of insulator material or by dopantsmixed with the insulator [8]. The surface discharge blackbodytemperature reaches a maximum of 40 000 K to 60 000 K fora 30-kA discharge [18]. It is generally accepted that the arctemperature of a surface discharge is similar to that of the freearc, which is to say that it is only a weak function of the peakcurrent in the 10- to 100-kA level [19] and reaches a maximumtemperature of 60 000 K to 70 000 K for peak discharge currentsof approximately 300 kA.

Fig. 2 shows a generic SHV response versus the numberof discharges. An ideal insulator would have a constant SHV.Two phases of SHV response are noted for a typical insulatormaterial: 1) the conditioning phase and 2) the decomposi-tion phase. This investigation reports the holdoff degradation

resistance (HDR), mass vaporization coefficient (MVC), andholdoff voltage conditioning (HVC) figures of merit developedto characterize the insulator’s SHV response. The conditioningphase of the SHV response is experimentally measured by thequantity NC, which is termed the “conditionability” of the in-sulator and is the number of discharges which occur at voltageshigher than the initial breakdown voltage VI. The conditioningphase is not present in all insulators and can be masked if theinsulator degrades too rapidly. The decomposition phase of theSHV response is experimentally measured by the quantity, NL,which is termed the “lifetime” of the material and is the numberof discharges required to reduce the breakdown voltage to thehalf-power level (i.e., VI/

√2) for three consecutive discharges.

Other factors such as electrode material, ambient gas, and theuse of UV stabilizers affect an insulator’s SHV response andare demonstrated in this investigation using the G-9, G-10, andG-11 insulator materials.

II. THEORY

A. Holdoff Voltage Degradation

Insulator degradation is produced when the material is ex-posed to high temperatures causing it to decompose or dis-sociate into its gas and metal (if the material is ceramic) orgas and carbon (if the material is polymeric or elastomeric)components. Conductive deposits of metal or carbon remain onthe insulator surface unless they are removed by ablation. Theseconductive materials distort the electric field and decrease theSHV. While a small amount of electrode material can also beeroded and deposited on the insulator’s surface and furtherreduce of the SHV [20], these effects are usually insignificant.This investigation will only be concerned with degradation ofthe insulator material itself. Interested readers should consult[21] for information regarding high-current electrode erosion.

Insulator degradation is the complex process by which itsatoms gain sufficient energy to rupture molecular bonds andis; therefore, a thermal stability problem. There are severalsemiempirical relationships reported in the literature regardingthe thermal stability of materials. Ellingham [22] reported thethermal stability of ceramic materials to loss of oxygen (ornitrogen) to be expressed by

Thermal stability =ΔGf

g(1)

where ΔGf is the Gibb’s free energy of formation [23], [24] andg is the number of gas atoms in the molecule. For example, alu-mina (Al2O3) has three gas atoms and silicon nitride (Si3N4)has four gas atoms.

Parr and Scarisbrick [14] developed a comparative trackingindex (CTI) for insulators used in the electric utility industry.They found the carbon-forming tendency for a variety of syn-thetic insulators in dust–fog tests related to

CTI =ΔHf

ΔHc(2)

where ΔHf is the enthalpy of formation and ΔHc is the sumof the energies of those molecular bonds that, when broken,produce free carbon.

ENGEL AND KRISTIANSEN: MECHANISMS AND PREDICTORS OF INSULATOR DEGRADATION AND EROSION 1865

Wall [25] found that the thermal stability of vinyl polymersto scission or rupture reactions upon irradiation could be ex-pressed by

Thermal stability = ΔHp (3)

where ΔHp is the enthalpy of polymerization.Engel et al. [26] developed a relationship that could be used

for ceramic, polymeric, or elastomeric materials. This thermalstability figure of merit is the specific enthalpy of formation(i.e., the bond energy density) of the material and given as

Thermal stability =−ΔHeff

wc(4)

where wc is the molecular (or formula) weight of the ceramic ormonomer and ΔHeff is the effective enthalpy for the material,equal to ΔHf (enthalpy of formation) for ceramic materialsand equal to ΔHf(monomer) + ΔHp (sum of the monomer’senthalpy of formation and the enthalpy of polymerization) forpolymeric or elastomeric materials.

While (1)–(4) were satisfactory indicators for a single classof molecularly simple materials, they were unsatisfactory forthe different classes of insulator materials, particularly if thematerial is molecularly complex (e.g., containing more thanone gas or metal specie). Engel [27] developed a more generalindicator called the HDR figure of merit given as

HDR =

⎛⎝1 +

k∑j=i+1

mj

mi

⎞⎠

−1

exp

⎡⎢⎢⎢⎢⎣

−ΔG

RT

(k∑

j=i

mj +�∑

j=i

gj

)⎤⎥⎥⎥⎥⎦

×

⎧⎨⎩wmi

Mi

⎡⎣ k∏

j=i+1

(mj

mi

)2mj

wmj

Mj

⎤⎦

×

⎡⎣ �∏

j=i

(gj

mi

)2gj

wgj

Gj

⎤⎦

⎫⎬⎭

1

2

(k∑

j=i

mj+

�∑j=i

gj

)(5)

where R is the gas constant and T is the temperature. Thevariables mi and gj are the stochiometric coefficients of theith metal and jth gas specie of the decomposition reaction.The variables wMi

and wGjare the molecular weights of the

ith metal and jth gas specie of the degradation reaction. Thesummation and product operators are indexed over k total metalspecies and � total gas species. For example, the balanceddegradation reaction for cordierite ceramic is given

2Mg(v) + 4Al(v) + 5Si(v) + 9O2(g) ↔ Mg2Al4Si5O18(s)(6)

where v denotes vapor phase, g denotes gas phase, and s de-notes solid-phase reactants. In (6), there are k = 3 metal speciesand � = 1 gas species. The stochiometric coefficients for (6)are m1 = 2, m2 = 4, m3 = 5, and g1 = 9. In using (5), Engel[27] assumes the decomposition temperature to be 3000 K(for ceramics) and 600 K (for polymers and elastomers). The

exact decomposition is unknown for several insulator materials.These two temperatures are average decomposition tempera-tures for those materials whose decomposition temperatures areknown. Additionally, when analyzing ceramic materials

ΔG ∼= ΔGf (7)

and when analyzing polymeric or elastomeric materials

ΔG ∼= ΔGf(monomer) + ΔGp. (8)

The ΔG values in (7) and (8) should be selected at the decom-position temperatures of 3000 K and 600 K above.

There are several factors found to affect an insulator’s SHVresponse including ambient atmosphere, electrode material, UVstabilizers, and even discharge repetition rate. These effects areusually evaluated on a comparative basis since they are difficultto quantify theoretically.

B. Insulator Mass Erosion

Heat is transferred from the arc channel to the insulatorsurface through radiative [18] and conductive [28] heat transfer.Dashuk et al. [18] reported mass erosion rates for various peakcurrent levels with pulses 20 to 30 μs in duration. Those resultsshow that at currents above 14 kA, erosion scales proportionallyto the current. At currents of 35 kA, the erosion rate is 5 to8 μg/J for G-10 and Delrin (polyformaldehyde), respectively.Engel [29] reported erosion rates of 4.5 and 4.4 μg/J for G-10and Delrin, respectively, when exposed to 300-kA currentpulses of 30-μs lengths. The erosion rates for a large groupof ceramic insulators were 0.6 to 3.3 μg/J. The Leidenfrosteffect [30] is a vapor shielding process that complicates thermalmodeling for SDS devices. Vaporized insulator material formsa layer between the arc channel and the insulator surface thatblocks conductive heat flow. It is unknown if the layer restrictsradiative heat transfer.

The simplest figure of merit for the erosion resistance of agiven material can be found from the time it takes to melt thematerial. Assuming 1-D heat transfer, Engel [29] used the timeto melt as the arc melting resistance (AMR) of an insulatormaterial which is given as

AMR = Tm

√ktρcp (9)

where kt is the thermal conductivity, ρ is the density, and cp

is the specific heat. The AMR figure of merit is a worst caseindicator and is accurate if the molten material is removed fromthe insulator by some mechanism (e.g., ablation) which is nottrue in all cases. Visual inspection shows the surface of manypolymeric insulators have been melted and resolidified. In mostcases, it is difficult to know the amount of molten materialremoved from the insulator. The molten material can remainon the insulator and be heated to vaporization before removal.Additionally, the AMR does not account for decompositionwhich can occur at any elevated temperature.

Another erosion figure of merit can be obtained if oneassumes that only the gas species from the decomposition

1866 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 37, NO. 9, SEPTEMBER 2009

reaction are responsible for mass erosion. Engel [27] called thisfigure of merit the MVC whose general expression is given by

MVC =gi

miJMi

⎛⎝wGi

+�∑

j=i+1

gj

giwGj

⎞⎠ NL (10)

where JMiis the metal mass flux of the ith metal specie.

The other nomenclature is the same as that given in (5). It isinteresting to note that for an insulator with one gas and metalspecie, (10) reduces to

MVC =[m

g

1wGNL

HDR

]−1

(11)

where HDR is the figure of merit given in (5). Equation (11)clearly shows an insulator with a low HDR figure of merit hasa high MVC figure of merit.

Insulator erosion rates are affected by ambient atmosphere,electrode material, UV stabilizers, and even discharge repeti-tion rate. As in the case of the degradation, although, theseeffects are usually evaluated on a comparative basis since theyare difficult to theoretically quantify.

C. HVC

HVC is characterized by an increase in SHV above the initialbreakdown voltage. The conditioning effects of insulator coat-ings and surface treatments have been reported in the literatureby several investigators [31]–[36]. Most investigators attributethe conditioning effect from coatings and surface treatmentsto a decreased secondary electron emission coefficient fromthe insulator surface which inhibits the breakdown process.Conditioning effects have also been observed as a result ofelectrode conditioning [37], [38] and attributed to either theremoval of surface contaminants or the physical alteration ofthe electrode’s surface finish.

The conditioning effect observed in this investigation isdifferent than that due to surface coatings and electrode condi-tioning and is an intrinsic property of the insulator material. Atthe present time, water absorption by the insulator is believedto be the cause of the conditioning effect and was accidentlydiscovered when a ceramic insulator was dropped in water priorto testing [27]. Even though the sample had been completelydried with a cloth prior to testing, the sample had retainedenough water to render the surface conductive and prevent theapplication of a high voltage. The sample had to be dried atelevated temperature (1000 ◦C) for several hours to return to itsoriginal state.

Water absorption will affect surface charge distribution andpermittivity, both of which affect the SHV. Measurements haveshown the surface conductivity of many polymeric insulatorscan be increased by as much as four orders of magnitude witha change in relative humidity from 40% to 90% [39], [40]. Ab-sorbed water on the insulator’s surface also has beneficial prop-erties including to act as a “sacrificial” layer transferring arcenergy that would otherwise produce degradation and erosionand to act as a vapor shield similar to that from the Leidenfrosteffect [30]. The SHV increases with each successive discharge

Fig. 3. Schematic of the SDS showing the experimental arrangement used totest the degradation and erosion of insulator materials.

since the absorbed water is evaporated from the insulator’ssurface.

Even with absorbed water’s beneficial effects, the arc channelenergy works to degrade and erode the insulator material. Thesecompeting effects lead Engel [27] to propose the HVC figure ofmerit to be expressed as

HVC = αHDR (12)

where α is the insulator material’s water-vapor absorptioncoefficient and HDR is the figure of merit given in (5).

Insulator conditioning is affected by ambient atmosphere,electrode material, UV stabilizers, and discharge repetition rate.As in the case of the degradation and erosion effects, although,these effects are usually evaluated on a comparative basis sincethey are difficult to quantify theoretically.

III. EXPERIMENTAL ARRANGEMENT

Fig. 3 shows a schematic of the SDS experimental arrange-ment used in this investigation to test the degradation and ero-sion of insulator materials. The capacitor bank consists of fiveSyllac-style energy discharge capacitors connected in parallel.Each capacitor is 1.85 μF with a 30-nH series inductance andrated up to 50 kV. A high-voltage power supply charges thecapacitor bank through a 40-kΩ resistance. The spectrometeris a Jarrel–Ash 0.5-m scanning Ebert with a 35-mm cameraattachment and a 200-nm viewing window. The containmentvessel was evacuated to 10−3 torr before backfilling withindustrial-grade (dry) nitrogen and oxygen in tests that requiredan atmosphere other than atmospheric air. Relative humidity isnot controlled for these tests, but measurements made duringthe course of the investigation show it typically in the 20% to30% range. The relative humidity is low and relatively constantpresumably due to a combination of laboratory air conditioningsystems and dry local weather.

ENGEL AND KRISTIANSEN: MECHANISMS AND PREDICTORS OF INSULATOR DEGRADATION AND EROSION 1867

TABLE IOPERATING PARAMETERS OF THE SDS EXPERIMENT

Table I lists the maximum and typical SDS operating pa-rameters. Peak discharge current is controlled by varying thebreakdown voltage via the gap length of the surface discharge.The length of the discharge channel is typically 2.54 cm whichyields approximately 30-kV breakdown voltage, a 330-kA peakdischarge current, and approximately a 1-Hz pulse dischargerate. Discharge rates up to 3 Hz are possible with the experi-mental arrangement but only with a lower breakdown voltageand peak discharge current.

A wide variety of polymeric, elastomeric, and ceramic insu-lator materials are tested in the SDS experimental arrangement.Molybdenum and graphite electrode materials are used in thetesting since they have low erosion rates and are single-elementmaterials with a large molecular weight difference makingspectral analysis of the arc channel simpler. Insulator materialsurfaces are wiped clean with an ethanol-moistened cotton clothprior to testing. Whenever possible, the materials are testedwith “as manufactured” surfaces, the exceptions to this are thesteatite and cordierite ceramics which are sawn to size with aslow-speed diamond saw.

IV. EXPERIMENTAL RESULTS AND DISCUSSION

Table II lists the measured lifetimes of all multishot insulatormaterials tested in this investigation. Table III lists the single-shot insulator materials tested in this investigation along withtheir respective failure mechanisms. The tests in Tables IIand III use molybdenum electrodes and atmospheric air. Themajority of single-shot insulator materials fail by conductive (orsemiconductive) tracking. The ceramic materials have a metal-lic track, whereas the polymeric and elastomeric materials havea carbon track. There are only two insulator materials that failby shattering, boron nitride and yttria, both are mechanicallybrittle materials.

Table IV lists the HDR figure of merit and the experimentallymeasured insulator lifetime. From this table, it can be seenthat there is a reasonably good correlation between the HDRvalue and the lifetime and that insulators with an HDR valueless than ∼5 has a single-shot lifetime (excluding insulatorsthat shattered during testing). Materials that have HDR valueshigher than ∼5 (e.g., zirconia) but have single-shot lifetimes arematerials that experience solid-state transitions which destroythe lattice rendering the insulator conductive. The HDR ofcomposite materials is calculated according to the percentagecomposition of each component. For example, the HDR of the

TABLE IIMEASURED LIFETIME OF MULTISHOT INSULATOR MATERIALS

composite silicon nitride +30% mullite ceramic is found from0.7(HDRsiliconnitride) + 0.3(HDRmullite).

Composite materials can have properties that exceed thoseof its constituent materials. Silicon nitride and zirconia aregood ceramics, from a mechanical strength viewpoint, but bothhave single-shot lifetimes when tested in the SDS. Siliconnitride composited with 30% mullite, however, has an 89 shotlifetime. This material is engineered to have the high thermalconduction and mechanical strength of silicon nitride with thehigh decomposition resistance of mullite. Its design is based onthe results of this investigation [41]. Zirconia composited withalumina (CZA 500 in Table II) has a 125 shot lifetime. CZA

1868 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 37, NO. 9, SEPTEMBER 2009

TABLE IIISINGLE-SHOT INSULATOR MATERIALS AND FAILURE MECHANISMS

500 is not included in Table IV since its specific formulationis proprietary. The exact mechanism of this beneficial action isnot known at the present time. Other examples of compositematerials that display these types of properties are the silicon-bonded mica-based ceramics Macor and Mycalex (silicon is asolid-state transitional insulator).

The HDR figures of merit for the thermosetting polymersof Table II cannot be calculated since their free energies offonnation, ΔGf , are unknown as are the free energies of forma-tion for the majority of the thermoplastic polymers (excludingpolytetrafluoroethylene and low-density polyethylene) and allthe elastomers. It is to be noted from the multishot insulator dataof Table II that although the viton and chloroprene elastomersdisplayed lifetimes comparable to (and sometimes less than) thelifetimes of some of the thermoplastic polymers (e.g., poly-tetrafluoroethylene and polyvinylchloride), these elastomericinsulators are considered superior to the thermoplastics in someapplications since they display lower degrees of melting anderode cleanly. Another characteristic making these elastomerssuperior in certain applications is their ability to survive theharsh mechanical shock of high-current breakdown.

Table V lists the MVC figure of merit versus the measurederosion rate. There is a reasonably good agreement betweenthe MVC figure of merit and the measured erosion rates ofTable V. As in the case of calculating the HDR, the MVC ofmerit cannot be calculated for the majority of the thermosetting

TABLE IVCOMPARISON OF THE HDR FIGURE OF MERIT AND MEASURED LIFETIME

TABLE VCOMPARISON OF THE MVC FIGURE OF MERIT VERSUS MEASURED

EROSION RATE FOR MULTISHOT INSULATOR MATERIALS

TABLE VICOMPARISON OF THE HVC FIGURE OF MERIT VERSUS MEASURED

CONDITIONABILITY

polymers (excluding polytetrafluoroethylene and low-densitypolyethylene) and all the elastomers. The erosion rates of thesingle-shot materials are not included in Table V and weretypically on the order of 1 to 2 μg/J.

Table VI lists the HVC figure of merit and the measuredconditionability. The water absorption coefficient is also shownin that table. From the Table VI data, there is reasonably good

ENGEL AND KRISTIANSEN: MECHANISMS AND PREDICTORS OF INSULATOR DEGRADATION AND EROSION 1869

TABLE VIICOMPARISON OF G-SERIES INSULATOR LIFETIME, EROSION, AND CONDITIONABILITY

AS AFFECTED BY GAS, ELECTRODE MATERIAL, AND STABILIZATION

agreement between the HVC figure of merit and the measuredconditionability of an insulator material.

The lifetime, erosion rate, and conditionability of the G-9,G-10, and G-11 insulators as a function of electrode material,ambient gas, and the use of a UV stabilizer are shown inTable VII. The G-series insulators are used in this type testingsince these materials have a relatively large lifetime and con-ditionability in the virgin (untreated) state. The UV stabilizerbenzophenone is applied by immersing the insulator samples ina heated saturated solution of benzophenone and methanol. Thesamples are removed after 1 h and air dried with compressedair. The possibility of increased lifetime for these insulatorsdue to vapor shielding was eliminated by performing placebotests with coatings of anthracene, napthalene, and paraffin (i.e.,compounds chemically similar to benzophenone but possessingpoor UV absorbing properties). The placebo tests producedno experimentally significant changes in insulator lifetimes ascompared to untreated samples. From Table VII, it is clear thatall the G-series insulators had significantly greater lifetimeswhen tested with graphite electrodes as compared to tests withmolybdenum electrodes due to the significantly lower UV emis-sion of the arc channel [27]. Decreased UV emission from thechannel also results in lower erosion rates and conditionability,as expected. The pure nitrogen tests show that oxygen in theambient atmosphere is seen to increase the lifetime of the G-series polymers by reacting with any decomposition-producedcarbon to form a stable gas as opposed to remaining on theinsulator surface [26]. Erosion and conditionability are lessaffected by the presence of oxygen. The use of a UV stabilizer(benzophenone) also significantly increases the lifetime of theG-series insulators by absorbing the high-energy UV arc energyand reemitting it at as heat [27]. UV stabilizers have a smallaffect on erosion rate and conditionability.

A final note is made regarding statistical variation in theexperimental data of this investigation. This investigation isintended to be a survey of material performance over the largestpossible group of insulators in an effort to discover physicalprocesses and general performance tendencies. While exper-iments are repeated three times to ensure repeatable results,this small sample population yields insignificant statistical data.The experimental results of this investigation are presented astypical results only.

V. CONCLUSION

Insulator erosion and degradation produced by high-currentsurface discharges is a complex process involving many fac-tors. The complex thermochemical interactions between the arc

channel, the ambient gas and the insulator make predictingan insulator’s behavior quite difficult. The HDR, MVC, andHVC figures of merit presented in this investigation are rea-sonably good predictors of an insulator’s response in a high-current SDS. This investigation has shown that degradation,erosion, and conditionability are related to each other usingthe conservation of mass flux model. The HDR factor can beused to develop better performing materials by compositingpoor performing materials with another material that has betterperformance. The HDR, MVC, and HVC figures of merit arelimited by the assumption that the reacting material remains onthe insulator surface during decomposition and that mass fluxis congruent.

Controlling the arc channel spectrum, the ambient gas, andthe use of UV stabilizers have significant effects on a material’sbehavior in this type of environment. A suitable choice ofelectrode material can limit the amount of UV photons availableto decompose the insulator into conductive carbon or metalconstituents. Stabilizers can be used to absorb the high-energyUV photons and prevent decomposition. The availability ofoxygen in the ambient gas will allow decomposed carbon andmetal to form stable gases and oxides instead of remaining onthe insulator surface and affecting surface flashover.

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Thomas G. Engel (M’89–SM’02) received thePh.D. degree in electrical engineering from TexasTech University, Lubbock, in 1990.

After graduation, he joined the R&D firm En-fitek, Inc., Lubbock, later becoming Vice-Presidentof Research and Development. In 1993, he becamea Research Assistant Professor with Pulsed PowerLaboratory, Texas Tech University. He moved toColumbia, MO, in 1995 and became an AssistantProfessor with the University of Missouri, Columbia.In 2001, he was promoted to Associate Professor. He

is currently James C. Dowell Associate Professor in electrical and computerengineering with the University of Missouri, teaching both graduate and under-graduate courses in circuits, electronics, systems, electrical machines, energyconversion, pulsed power engineering, high-voltage engineering, engineeringmathematics, and direct energy conversion. He has consulted for academia,industry, and federal agencies. His publication record includes 82 journal,conference, and book publications in his fields of specialization. He has threepatents and one registered copyright. His research interests include pulsedpower, energy conversion, electromechanical systems, supercomputing, andcomputer-generated imaging.

Dr. Engel is a member of Tau Beta Pi, Eta Kappa Nu, and Sigma Xi.

Magne Kristiansen (S’59–M’66–SM’72–F’78–LF’96) was born in Elverum, Norway. He receivedthe B.S.E.E. and Ph.D. degrees from the Universityof Texas, Austin, in 1961 and 1967, respectively.

Since 1966, he has been with the faculty of TexasTech University, Lubbock, where he is currently theC. B. Thornton/P. W. Horn Professor of electrical andcomputer engineering/physics and the Director ofthe Center for Pulsed Power and Power Electronics,Electrical and Computer Engineering Department.He has published over 450 journal articles and con-

ference proceeding papers, has coauthored two books, and is the coeditor of aseries of books on advances in pulsed-power technology. His current researchinterests and specialties include high-power microwaves, pulsed-power tech-nology, and explosive power generation.

Dr. Kristiansen is a member of the American Society for EngineeringEducation and is a Fellow of the American Physical Society. He is a foreignmember of the Ural Division of the Russian Academy of Sciences and arecipient of the USAF Meritorious Civilian Service Award in 1985, the IEEENPSS Merit Award in 1991, the IEEE International Pulsed Power ConferencePeter Haas Award in 1987, and the IEEE Millennium Medal in 2000.