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Development of a hermetically sealed, high energy trigatron for high repetition rate applications

Conference Paper · February 1999 DOI: 10.1109/PPC.1999.825433 · Source: IEEE Xplore

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Jane Lehr W.D. Prather University of New Mexico Air Force Research Laboratory, Kirtland AFB, NM, United States

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The user has requested enhancement of the downloaded file. DEVELOPMENT OF A HERMETICALLY SEALED, HIGH ENERGY TRIGATRON SWITCH FOR HIGH REPETETION RATE APPLICATIONS

J.M. L&r*, M.D. Abdalla+, B. Cockrehan?, F. Gruner*, M.C. Skipper+ and W.D. Prather Air Force ResearchLaboratory, Directed Energy Directorate, Kirtland AFB, New Mexico

Abstract systems. Spark gaps can be relatively Triggeredgas switchesincrease the reliability of pulsed simpleyet exhibit excellentswitching performanceover a power systems. In particular, the performance of high wide pammeterspace. The principal advantagesof spark power, repetitively rated impulse generators is greatly gaps are their fast turn on time, good current handling enhancedby including a triggered switch In order to capability, wide operating voltage range, and acceptable simplify the pulsed power subsystemfor field testing, the pulse repetition rate. In general,high pulse repetition rate Air Force has developeda reliable, fully sealed,trigatron switchesrequire the insulating gas to be flowed switch. A low inductance,70 J capacitorbank chargedto through the discharge regioq cooling the switching 50 kVDC is dischargedthrough the hydrogen insulated channeland sweepingout the heatedgas of the previous trigatron at a pulserepetition rate of 600 Hz. The switch is shot. Dynamic gas schemes,however, can be eliminated designedto be used for up to one year without replacing by careful choiceof gas insulatingmedia, for a given PRR the hydrogengas. [ 11.Other schemesfor increasingthe PRR have also been The hermetically sealed switch is intended as the tested[2,3]. primary switch for a resonanttransformer. In order to test Triggeredswitches are usedto provideprecise timing of the limits of the technology, a high energy, low one or more cascaded components. The objective of inductanceprimary bank was constmcted,and implementinga triggered switch is otten the reduction of switched with the trigatron. The switch construction,the switching jitter since consistentswitching delay can often trigger circuit, and the hermeticseal are fully described. be incorpomtedinto the overall systemdesign. A trend in repetitive pulsed power system design is towards higher I. INTRODUCTION averagepower, larger energy per pulse and faster pulse Many applicatiorq such as space-based,airborne, or repetition rates. The addition of a triggered switch to the mau portable systems, increasingly require compact, pulsed power system increases the pulse-to-pulse lightweight, highly reliable, efficient components and repeatabilityof the overall system.A trigatron was chosen systems.In partictdar,weight and systemcomplexity is of as the switch corrligumtiondue to its simplicity, reliability prime concern and may significantly impact transport and good triggering characteristics.An excellentoveiview platforms,range, and operating costs. In this interest,the of trigatrons and their operationis given by Williams and Peterkin 141. The triaatron de&n snecifications are Air Force ResearchLaboratory has developed a high - A voltage triggered switch that is capable of high pulse sumnmiz&-in Table 1, repetition rates (PRR) and can maintain its insulating gas Voltage 50 kV pressurefor a year without refreshing It is anticipated PRF =-600% that the insulating gas will be recharged at the same Energy per Pulse 70 J maintenance cycle as the electrodes. The immediate AveragePower >40kW applicationof the hermeticallysealed trigatron is for field GasReplenish Period I lyear I tests requiring pulsed power systems.For instance, the testing of high power transientmdiating devicesand their Table 1. The designpammeters of the trigatron. autexmapattern is most effectivelyperformed in a remote, open range. Testing in remote sites can be cumbersome IL SWITCH DESIGN with all of the accompanyingaccomrements necessitated by the measming equipmentand source.To this end, the Air Force wants to facilitate field testing, and the initial A. Electrical Design step was the development of a hermetically scaled The cmsectional view of the trigatron is shown in triggered switch to eliminatethe needfor a gasreservoir. Figure 1. The trigatron is designed as a uniform field Spark gaps are extremely versatile, high voltage electrodegap with the trigger pin imbeddedin a grounded switches that are used extensively in a wide variety of adjacentcathode. The gap spacingbetween the adjacent and oppositeelectrodes, $, is set at 0.43 cm The spacing

l email: [email protected] + ASR Corporation,Albuquerque NM YKinetech, Inc., Edgewood,NM 14 o-7803~54952@9!$10.OCQ1999 IEEE. betweenthe trigger pin and the adjacentcathode is 0.217 insulator,IEL-F, is corrugatedparallel to the discharge cm. The trigatron is designed to withstand 50 kWC path in the well known techniqueto increaseits resistance chargevoltage, and fixed gap space,$. and the self break to surfaceflashover. Since the switch is sealedand usedin pressureis 150 psi of hydrogen. The electrodefaces are high PRP application the corrugations also serve to made of for its superior resistanceto collect any debris. Although extensive electrostatic erosion and is backed by aircraft aluminum (7075-T6). modeling was not performed,a field plot confirmed that Aim&t alumimnu is lightweight and retains the excessiveincrease in the electric field did not occur in the conductivity of common aluminmu but is siguificantly CUllU&itiOnS. stronger. The plates provide the mass necessary for dissipation of the heat generatedunder the high pulse B . Mechanical Design repetition frequency as well as necessary mechauical The mechanical design consists of two intermingled support tasks: a pressmevessel design with a sufficient factor of safety for mamtedoperation, and the incorporation of a semi-permanentseal for a variety of high pressuregas. The ASTM skmdardlists the requiredfactor of safetyas four for the safe operation in the vicinity of humans [6]. Careful selectionof materialswas the main method used to handle the mechanicalstresses. For instance, Aircmft aluminum (707ST6) is lightweight yet considerably strongerthan standardaluminum. The aircmft aluminum is both the mechanicalstructure and a sink for the heat generated during high PRF bursts. Threaded glass reinforced plastic (G-10) gave the switch structural integrity and provided access to the interior of the trigatron. Bulk breakdown is a well-known difliculty in exploiting the superior strength of GlO, and lining the interior of the G-10 pressurevessel with KEL-P mitigated this problem. The trigatron is designedto remain sealed with high Figure 1. Crossectionof the hermeticallysealed trigatron. pressure hydrogen gas for a minimmn 1 year cycle. Hydrogen, being a small molecule, is notorious for The threat of faihue by surface breakdown becomes migmting into materialsand its difficulty for contaimnent. more likely as the size of the component decreases. KEL-F is a unique thermoplasticthat, among many other Moreover, there is evidencethat the likelihood of both desirableproperties, has a particularly low permeabilityto bulk and surface breakdown increases with increased hydrogen [7]. Combined with its resistanceto surface pulse repetition rate [5]. Fortunately, the applied DC flashover,a listed breakdown strengthof 200 kV/cm, and voltage is not very high, however, the total length of the its ability to withstand UV make KJZL-F an exciting trigatron switch housing is a mere 2.4 inches. The material for compactpulsed power switches.Other seals Hz V;olume Hydrogen supply _ Copper tungsten oin) GlO

electrode holder Polysulphide O-ring electrode holder O-ring Figure 2. Exploded view of trigatron showing details of the mechanical design. 147 used polysulphide, which is also resistant to hydrogen, the primary of the coil producing a negative50 kV on the with a gas permeability rate of 1.2 cm/cm2-set-barfor secondary with a 15 us rise time. Note that the coil hydrogen and for comparison,V&on* has a rate of 160. primary current during the charge phase is opposite in The junction between the ahminum electrodeplates and direction to the current during the discharge phase. the KEL-F insert is sealedwith polysulflde o-rings and Therefore, the charge cummt resets the iron core Sylgard is used to fill the air gaps externalto the o-rings. immediately prior to the main trigger pulse to prevent Insuring a long term contaimuentfor hydrogen,however, core saturationat high rep-rate(up to 1 kB2) operation. also required special design and manufacmr& The electrical isolation of the trigger circuit from the techniques. In particular, the use of CAD 3-D solid trigger pin is important since, upon switch closure, the modeling allowed the creation of electronic data to allow triggerpincanrisetothepotentialoftheswitchcharge the accurateproduction of the switch as visual&d. This voltage for a short time. Additionally, accessto the gap direct inter&e between data and machine allows closer can also present difficulty [4]. The connection between tolerancesto be achieved,which were required for the the ignition coil and the trigger pin is madeusing ferrite- long-termcontainment of hydrogengas. loaded,spiral wound ignition originally designedfor automotive applications. The inductive/resistive wire C. Triggering System greatly reduces the high frequency signals that couple The trigger pin is an extension of a commercially back to the solid-statecoil driver. Access to the switch available spark plug and either a Champion C65YC or gap is providedby a modified automotivespark plug. Accel 117 worked equally well, These particular spark plugs were chosen for the length of the ceramic path IIL EXPERIMENTAL SETUP between the trigger pin and the alhum support plate. The trigatron switch was testedat voltagesof up to 50 The 0.22 cm trigger pin is a copper tungsten extension kVDC and pulse repetition rates of 600 Hz. One of the that is threadedonto the spark plug centerpin. The spark targeted applicationsfor the trigatron is to dischargethe plug is sealed to the trigatron housing with a :z capacitanceof an air core trausformer.Because manuf&umr-provided copper gasket that was tightened xstmmtswhich may be imposedby the low primary to the recommended25 fi-lbs. inductanceof an air core ,the inductanceof The trigger voltage is generatedusing an off-the-shelf, the 55 nF capacitorbank was kept small by enclosingthe Ford automotive ignition coil driven by a solid-state in a rectangular coaxial geometry. To avoid circuit. The solid-state trigger generator, shown interior to the capacitor bank, the schematicallyin Figure 3, is cons&u&d as a push-pull capacitors are encapsulatedin the rectangular coaxial network using 10 highpower MOSFET’s. The storage geometry with Sylgard Dielectric cornpod [8]. In capacitoris chargedto 500 V by a 1000 V, 1 kW power addition to a good bulk breakdown strength E BR= 175 supply. The “push” FETs charge the decoupling kV/cm, Sylgard is used as a protection from mechanical capacitorthrough the primary of the ignition coil in 160 stress and atmosphericcomammams, such as moisture, us. A ceramicresistor (2 Ohm, 50 Watt) in serieswith the that may be introduced during field tests. Moreover, “push” FET’s critically damps the charging current. A Sylgard is applied in liquid form which then cures, reset prevents voltage from being applied to the primerless, to form a cushioning, self-healing gel-like trigger pin during the charge phase. The “pull” FET’s mass.Thus,thegelretainsmanyofthebestfeaturesofa subsequentlydischarge the de-couplingcapacitor through liquid, while also providing non-flow stability. The Cathode “Push- n 1 Pull” Coil Power Pin SUDDh

\ Reset / Da&ping Storage De-c\oupling Caoacitor CatJacitor

Figure 3. Trigger system diagram. 148 inductance of the capacitor bank was measured by The true reliability of the trigaton is indicated in Figure 5 discharging the capacitor bank into a short circuit and that showsthe first two pulsesin at a a seriesfor the 600 recordingthe waveformand found to be 100 nH. Hz. Note the slight “flattop” which was experimentahy The capacitor batik is charged to 50 kV in de&m&d to result in a reduced pulse to pulse jitter. approximately 900 :s with 8 Maxwell CCDS, 8 W/s operation at 600 Hz is shown in Figure 4. The slight power supplieswhich providessufficient current to obtain inconsistencyis attributedto an ins&lcient samplingrate a pulse repetition rate of 600 Hz. For repetitiveoperation, on the oscilloscope.It is notable, that neither nitrogen or the hydrogen pressurein the trigatron is increasedto 300 dry air was able to sustainthe 600 Hz PRF at 70 J/pulse. psi, which results in the switch being operated at Somedependency on the hydrogen purity was observed, approximately 70% of the self-breakingvoltage, and is and the above data is taken with UHP bottled triggered 500:slater. Resultsof repetitiveoperation at 600 (99.99999%)hydrogen. HzareshowninFigure4.

Cqwitor Bank Volwe ACKNOWLEDGEMENTS (I = 600 Hz) The authorswould like to thank KennethR Prestwich for manyvaluable discussions as well as supportand encouragementduring the developmenteffort. We are also grateful to Ken for his commentsduring the g 50 preparationof this mauuscript, B

i 2s V. REFERENCES [l] S.L. Moran and L.W. Ha&sty, ‘High RepetitionRate .* ii (lllllrlllllllfll Illll~lllll~l~llllllllllllf~l’l Illl~l~~lllllll~llllllll~ll~l~l~lllllI\ Hydrogen Spark Gap,” IEEE Trans. ElectDev, Vol.

B O t 38, No.4, pp.726-730,April, 1991.

-25 ' ' I 1 ' ' ' ' ' a ' ' [2] J.A. Harrower, S.J. McGregor, S.M Tumball, J.M. 0 0.05 0.10 0.15 Houtsoubis, F.A. Tuema, and A.J. McPhee, ccDesign

Time (r) Considerationsin Corona StabilisedHigh Repetition Rate Switches,” IEEE Pulsed Power Conference, Figure 4: Capacitorbank and trigatron voltage at 600 Hz 1998. PRR. The irregularity is caused by an insufficient oscilloscopesampling rate. [3] S.J. McGregor, S.M Tumball, F.A. Tuema, and 0. Farish, “Factors A&cting and Methods of ImprovQ the Pulse Repetition Frequencyof Pulsecharged and DC charged High Pressum Gas Switches,” IEEE Trans.Plas.Sci,Vol25, No.2, 1997,pp 110. [4] P.F. Williams and F.E. Peterkin, ‘cTrigatron Spark Gaps” in Advancesin PulsedPower Technology,Vol. II - Gas DischargeClosing Switches,” AH. Guenther, M. Kristiansen, Series Fds, G. Schaefer,Vol. Ed., PlenumPress, 1990. [5] Gerald J. Rohwein, Albuquerque,NM, private

COmmunication. [6] ASME SectionVIII, Division I Type Vessels“Untired pressurevessels,” 1995. -25- [7] KEL-F, Furon, Anaheim,CA 92806. 0 0.001 0.032 0.003

Tii (8) [8] Sylgarde,Dow Coming, Midlaud, MI 48686.

Figure 5. The first two pulses in a 600 Hz burst of capacitor bank voltage as it is discharged by the hermetically sealed IrigiUron. It was experimentally determinedthat the trigatron had lessjitter when a sight ‘Yhttop” was permitted on the capacitor bank voltage prior to triggering. 149

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