M. G. Sheppard et al- MC-1 Generator Performance with Higher-Energy Explosives

8/3/2019 M. G. Sheppard et al- MC-1 Generator Performance with Higher-Energy Explosives

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LosAlamosNATIONAL LABORATORY

MC-1 GENERATOR PERFORMANCE WITMHIG-IER--=YEXPLLHVES

MAURICE G. SHEPPARD El AL.

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Los Ala,~os National Labara!o~, an affwmauve aclmnlqual opplurti~ @n@dyer, m operated by the Unwerstly of California Ior lhe U .3 Deparlmanl of Energyunder canlracl W 7405-ENG36 By accaptamcmof Ihmarticle. lhe pubkher rqmzes !hal !he U S Government relams a nonexcluswe, royallr free Itcense 10publish or reproduca the pubhshtxl formd Ihlsconlntwlmn, or 10allow othors 10do so, for U S Government purposes. The Los Alamos Nauonal LataaloWrequests Ibal Ihe pubhsherdentty Itusarucle as worhpdormd undtmlhe auspwes of lhe U S Depamnenl 01Energy Fwm No 83S F15

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MC-1 GENERATOR PERFORMANCE WITHHIGHER-ENERGY EXPLOSIVESM. G. Sheppard,J. H. Brownell,J. M. Christh, C.M. Fowkr,

B. L Freeman.J. D.Goetiee,J. C. King,C.M. Lund,B. J. Papatheofanis,P. J. Rodriguez L R Veeser,W. D. ZrwekhLos AlanmsNationalIAxxatoryLos Alamq N. M. 87545,USAAI. Bykov,M. I. Dolotenko, N. P. Kolokolcbikov,Yu. B. KudasOV,V. V. Platonov,O.M.TatsenkoAll-Russian scientif~cResearchInmituteof ExperimentalPhysicsArzamas-16,Russia 607200

W. LU~B. R,MarshalIEG&GEnergyMeasurements130RobinHill Rd.,Gole& CA 93117,USAThe Russian designed MC-1 ulrra.high magnetic field generator was tested in 5experiments as part of a oint US-Russian collaboration at Los Ahrnos National/aboratory in December o 1993. The stmdard Russian explosive (50/50 RDWINT)was replaced with hi her-energy-density US explosive, either Cornp-B (60/408DIUNf) or PBX-95 1. Generator performance with COMP-Bwas nominally thesame as reported for experiments with rhe slightly lower-energy Russian explosive,The Comp-B experiment produced a measured peak field of 9.4 megagauss. UsingPBX-9501, the measured peak field increased to 10.9megagauss with an appropriateincrease in the time derivative of the field. One-dimensional MHD calculations withtheLagranghn code, RAVENare comparedwith the experimental results.

I. History and BackgroundThe reliable and reproduciblegenerationof multi-megagaus.smagnetic fields usinghigh-explosive flux+oppression techniques has ken of continuing interest to two research groups iorover 30 years - one led byC. M. Fowlerat Los AlamosNationalLaboratory in the U.S., theother led by the lateA. I. Pavlovskii at Arzarnas-16in Russia In 1991,with the reduction ofpolitical andmi.1.kaytensions between our twocountries, these twogroups initiated acollaboration to $enerateand use ultrahighmagnetic fields.The fust sems of expcximentsin this collaborationwas conducted byU.S. and Russianscientists atLos Alamosm Decemberof 1993using the RussianMC-1 flux-compressiongenerator (FCG)l, U.S. high-explosives,and diagnostics fielded by both countries. The fourgoals of the five-shot serieswere accomplished. lle goals were:1. validateMC-1 performance and 10MGpeak field withComp-Bhigh-explosive (shot xl),2. makedirect A-B comparisonwith~higherener y density explosive,fBX-9501,and benchmarkcomputationalmodes (shot #2),3. measure the upper critical field transition, *2(T), of the high temperaturersu rconductorYBa2Cu3@downto 4 K and measure the nonlinear Faradayef ect in CdS (shots #3-5), and4. continuebuilding the foundationfor a joint program to generate 20MGfields.Results of the December 1993experiments relevant toMC-1 performance are re,sented\elow. Results of the high-fieldmeasurementsfor YBazCu$+ andCdS are publis ed

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inner cascades-. -.0.- .0 ----- ------- . . . . . . . 0 ------ ------ %Fig.1. Diagram of theRussian tlme-casc&MC-l FCG.elsewhere,z SectionII describes the MC-1, its operarion,and theexperimental setup. Section IIIdescribes the lD MHDcalculationalmodelsemployed in the WVEN simulations MC-1performanceis comparedwith the RAVENsimulations in section IV; andsectionV reviewsconclusionsof the series.

U MC-1 Descrl tion and OperationJdiagram of theRussian three-cascade C-1 FCG is presented inFig. 1. The HEcylinder,which in previous Russian experimentsconsistedof a 50/50RDm mix, is detonatedsimultaneouslyon its outer diametersurface by a ring of 10polystyreneblock initiators. For thiscollaboration, the HEwas replacedwith higher energyexplosives eitherComp-B, a 60/40RDX./TNTmix, or PBX-9501. Inside the HEare 3 concentriccylindrical shellsmadeof auniquecopper-epoxycomposite. 1heseshells,knownas cascades in theRussianLittw!ure,successivelytake on the role of armatureduringimplosion, The shells are made of hundredsof0.25-mmdiameter,enamel-coated,copper heads, arrangedside-by-side in layers, securedin acasting of epoxy. The 500 copper threads of the outer cascade are wound in a 2-turn solenoidand then brought backalong the outside diameter, arallel with the cylindrical axis, to complete?he return cumentpath. The outer diametersof all cascadesare castwitha thicker layer ofepoxy so that they can bemachinedsmoothto inhibit h@odynamic instabilities. An initialmagnetic field of up to 220 kG (160kG for these experiments) is created b discharging arapacitor fhrou~hthe first cascade, The discharge is timed so that peak fie d is achievedjust asthe HEdetonauonwave reaches the fust cascade. The HE shock breaksdown the insulationbetween the solenoid threads andtransforms the fmt cascade into a conductingcylindertrappingand thencompressing tie initial field as the shell begins to move.

he second and thirdcascadesare similarly constructed except that all of the copper threadstue laid parallel to the axis. Beforea cascade is shocked fromoutside, it will only conductcurrent in the axial direction, Hence, it is transparentto the axial field which i beingcompressedby the precedingshell. Oncontact with the outer shell, the next cascade islnnsfonned by the shock into a conductingcylinder,which traps the field inside as the newcascade becomesthenewarmature.


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lle use of multiplecascades serves two imporlant functions. The fmt benefit of multiplecascades is the velocityenhancementwhich is derived from collisions of heavyouter shells withlighter inner shells. The second (andmorecrucial) Ixmefitis related to implosion stability. Asthe outer cascade com~ressesflux,magnetic and hydrodynamicinstabilities tend to disrupt theshell- These instabilitmsaremadeworse by the inherent perturbationsassociatedwith thecopper-epoxycomposite. The inner cas.des are strategically placed tore-collect and smoothout the perturbationsbefore the outer cascade is disrupted. IIMloss of fiuxwhich is incurreddwi.ngthe transition is off set by&chievinga morestile and re roducib!a implosion.!Jn the early systems developedby Fowler,Garnand Caird, initial field coils wwe alsoplaced under the explosive charge. While very large fields were obtained (up to 14MG),performancewas erratic. lle use of additionalPavlovskiicascadeswould presumablyhave ledto better reproducibility, albeit somewhat lowerpeak fields. An alternative approach tocontrolling the instabilities was investigatedby Caird CL al.4~ lhey placed the solenoid ou!sideof the HEandW? a single stainless steel armature. On the tirnescales af the initial capacitordischarge, the stainless steel armature allowedmagnetic flux to diffuse inside tie cylinder but onthe short timescale of tie implosion, the fluxwas essentially trapped and compressed. However,the poorercouplingof the initi coils with the armatureresultsm substantially 10WWnitial, andtherefore, also final compressed fields.The f~st test in this series was a proof test of the 3-cascadeMC-1ge]~eratorusing Comp-BHE instead of the Russian 50/50mix. Thegeneratorwas preloaded to 160kG using thecapacitor bank at Point 88 in AnchoCanyon. Time-dependentfield measurementswere madewithmultiple inductive probes (dB/dtkops) located at different radii and of differentsensitivities; and with Faradaycrystal(s) as close to the axis as possible. The secondexperimentof this serieswas an identical test of the 3-cascadesystemusingPBX-9501, a dramaticallyhigherenergy HE. Results of these tests are com aredwith preshot and posuhot calculationstRescribed in the next section. Benchmarkingof e RAVENcode at these high fields is one steptovtards pursuing the 20MGgoal.In the HTSCexperiments, describedelsewhere,l &e third cascade was removed, and thevolume inside the secondcascade was occupiedby a 0.15 g/ems foamcryostat. The CdS andsuperconductingsampleswereplacednear the center of the cryostatwhere theywere exposed toultrahigh fieldswhile their responseswere measured.

III. Computational ModelsSimulationsof the MC-1 have beenconductedwith the lD LagrangianMFiDRAVENcode~ades weremodeled either as a homo eneous composite of the comet average density using!mixed Cu/epoxyequation-of-state(EO ), or as sandwichedlayers of copper and epoxy usingthe same unmixedEOSSemployedin themixingprocedure.b The number of layers used foreach cascadematches the actual numberof layers of copper thread in each cascade and theWlckncssof the layerswas adjusted tomatch the re~rted avtn$e density of each cascade whilefining the total sandwich thickness. This mixedEOSmodel is snnilar to earlier Russiancomputationalmodels,7which used a differentmixing al orithm. As in earlier Russianfalculations,we used a standardcopper resistivity model $ scaled by a factor of 5 for the mixedEOS. To allow the axial magnetic field to pass freely fhrough the cascades until they wereshocked in the calculations, the resistivit~ in eachcascade zonewas multiplied by a step functionwhich remaineduro until the zonedensity f~st exceeded 1%above normaldensit ; the stepc?unction stayedequal to one for the remainder of the simulation. The HEwasmo eledwith aJWLform scaled to give experimentallymeasuredcascodevelocities.b HEdetonation inIWVEN ismodeled as a programmedburn with a specified detonationvelocity.

IV. Corn arlson of Resultsi?ascade radii as functions of time for omp-B simulations using the mixed EOS are shownin Fig. 2a; similar PBX-9501simulationsare shown in Fi . 3a. Calculated turn-around for the1nnercascade is 4.0 mm for Comp-Band 3.5 mmfor PB -9501. Cascade velocities from these


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(a)

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* q.\ \- 1. . . . . . . . . . . . . . . . . . . . . . - - ...-...&

M Slm-uwnm -t w (b) t (d

Fi~. 2. Cascade radii (a) andvelocity (b) forCornp-B simulationsusing themixedEOS.Calculated field isoverl~d ti b).

mm

Lm

mm --------~dc----------

a \. . . . . . . . . . . . . . . . . . . . . . . . . . . .Omcada

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(b) t wFig. 3.. Cascade [email protected] (a) and ve@~y ~) for PBX-?501 s~u~tio~ us~g tie mKed E~S.~ctilatkd fiild (N@)is oretlaidin (b).simulations are shown inFigs. 2b and 3bwithcalculated fields overlaid. Er xgy release in tieHEmodel was adjusted tomatch the experimentalcascadecollision times u closely as possiblefor both ex~ri.ments. The dips in the experimental and theoretical field derivative traces serve asunambiguoussignalsof cascadecollisions, Sincecascadekinetic energy is converted tomagnetic field energy, matching the timing and hence the velocities is an important step insimulatingtheMC-1. Calculationswith the layeredmodel for cascades rovedmore dtificult to

!adjustby scaling only the HEenergy release, lTe main~roblemstems mm shock reflectionsbetween the layers which delay the onset of cascademouon and alter the ultimate casadevelocity followingcollision. From the dynamics, it is clear that a homogeneousmixedEOSofthe right average density is a lxtter lD model of the cascades than a layered representationof thecorrectmass.Figure4a shows themagnetic field trace for theComp-Bexperimentcompared withsimulations using both the layered and the mixedEOSmodels. Measuredr ak field fcr thsComp-Bexpriment was 9,4 MG. Similar cunes are shown for the PBX- 501 version of tie


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(a) t (p) I u n - u -(b) 1 (#4Fig. 4. Calculated field (NICJ)a) and field derivative(MO@)(b) for COIUP-Busing tk mkdEOS nd layeredmdel for cascadescomparedto experimental dam

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Iom

i3aJ @m

OmM ml- M M u(a) t (d

Fig. 5. Calculated field (MG) (a) and field derivative (MC@) (b) for PBX-9501using the mixedEOS and layeredmodel for cascades compared to experimentaldataMC-1 in Fig, 5awith a measured1?ak field of 10.9MG. Conesponding field derivativecurvesare displayed inFigs. 4b and 5b. otice that Aemixed EOSmodel matches the timing for thecascade collisions twtter,e~en thoughthe calculated peak field is ma~chedbetter by the layeredcalculations. M discussed below,we believe that the layered cal~ulationsmatch the peak f~ldbetter for the wrongreason. Also notice that the calculated derivativesare sign~lcantly higherthan experimentjust beforecascade collisions and just before pak field.

V. ConclusionsBoth simulations andexperiments verify tiat the higherenergyexplosive prtiuces a higherpeak field. However,the higher field comes at tie expenseG;a smaller turn-around radius.Anoiherway to compare the data is to lookat field vs. mdius of the active cascade.Unfortunately, these curvesonly deviate at very high fields after the ti]ird cascade becomesactive. Experimentaldata is neither temporallynor spatially resolvedwell enou~hto help at this


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time, even if the inside radius of the cascadewerewellenough clef@ tomakesuch ameasurementmeaningful. In the absenceof dam we can onlycompam the fie!dsfromsimulations for the two different ex losives. At a radius of4.0 mm the CompB simulationgivesf0.0MG and the PBX-9501gives 0.9 MG. Since bothsimulations startw with the same160ffi initial field,we conclude that the MC-1drivenby the higherenergy explosive allows lessflux loss because the compression time is shorter. Itd.shap~ns even though the highershockstrength heats the cascadesmore rtxmltingin a higherreshvity. Extrapolating this obsenationto real [email protected] however,is only conjecture.Disagreementbetweenmeasured and simulated time derivativesof the field, just beforecascade collisions andjust before~ field point to deficienciesin the one-dirnemakmalnatureof the simulations. Yearsof Rusmanexperimentscontributed to the empirical choice of innercascade radii such that an unstable and spent outer cascade would be re-collectedjust before itdisintegrated and lost too much of its compressedfield. In the lD simulations, cascadesdo notgo unstable, electric-alconductivities of the composite cascadematerials are uncertain; and themodelingof the,col.lisionalturn-on of electricalumduction in thecascades is ad hoc. Giventhese caveats, themagnitudeof tie disagreement is surprisinglysmall.As notedearlier, the better agreement Ixmmen ?heexperimental peak fields and the layeredsimulationsis fortuitous. Because of the caveats explainedabove, the onedimensional modelunderestimatesflux loss duringcascade collisions and at turn-around. Since tie lay~redsimulations d~ipate too muchenergy inmscade collisions, they have an anomalously lowkinetic energywhich is available to be ccm:erted intomagnetic field energy. This under-estimation is offset by an anomalously low VT10SS,herefore introducing a cancellation oferrors hit gives the right snswer for the wroitgreason,Finally, one-dimensionalMHDsirnulatiom~of theMC-1 with Comp-B andwith PBX-9501are in good agreementwith the data collected fmm joint experiments performed byUS andRussian scientists in Decemberof 1993. Information, gained in this collaborationprovides afoundationuponwhichadditional ultrahighfield experimentscan be conducted in the future.The detailedmeasurcmem.sand simulationsassociatedwilh the collaboration alsoprovideexcellent benchmarksfor investigationsat even higher fields.

References1. Pavlovskii,A. I., UltrahighMagneticFieldsCumulation,in Megagauss Fields and PdredPower System, ed. byTitov, V.M, and Shvetsov,G. A., (NovaSciencePublishers, Inc.,NewYork, 1990),p. 1.2. Goettee,J. D. et, ef,, ComplexMicrowaveConductivity ofYBa2Cu307 inMagneticFieldsUp to 500T,Physics C 235-240,2090 (1994).3. Fowler,C M., Garn, W.B. and Caird, R. S., ProductionofVe~ HighFields by Implosion,1./@@.Fhys.,31,588 (1960).4. Caird R. S., Gun, W,B., Thomson,D, B. and Fowler,C. M., AnExplosive-DrivenHigh-Field System for PhysicsApplications, J. Appl. Phys., 35,781 (1964).5. caird. R. S,, Ga.rn,W.B., Thomson,D. B. and Fowler,C.M., A CylindricalExplosiv>Flux-CompressionSystem,in proceedingsof theConferenceon MegagaussMagneticFieldGeneration by Explosives andRelatedExperiments,(Frascati, Itsly, Septembx 21-23, 1965),p. 101.6. Sheppard,M.G. and Lund, C. M., Modeisfor CompositeMaterials Applied to theMC-1Generator,Los AlamosNational IAoratory memo,LACP-94-248(Oc~ 1994).7. l%vlovskii,A, I., Karpilmv,A. A,, Dolotenko,M. I., and Mamyshev,V. L, DynamicCharacteristicsAnalysisof a Ultra-HighMagneticField CascadeMagnetocumulativeGenerator (MC-l), inMc?agauss Fields and Puked Power Syst ems, ed. byTltov, V.M. andShvetsov,(3.A., (NovaSciencePublishers, Inc., NewYork, 1990),p. 2i.8. More, R.M., LawrenceLivennore National Laboratory,Livermore,CA, UCRL-84991(March 1981).9. IAW,Y.T. andMore,R.M., Phys. Fluids, 27,1273 (1984).

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M. G. Sheppard et al- MC-1 Generator Performance with Higher-Energy Explosives