SHOCK COMPRESSION OF CONDENSED MATTER - 1989S.C. SCHMIDT, JN. JOHNSON, L.W. DAVISON (edilors)Elsevier Science Publislpn B.V., 1990

VISAR WAVE PROFILE MEASUREMENTS IN SUPRA{OMPRESSED HE

D.J. ERSKINE, L. GREEN and C. TARVER

Lawrence Livermore National Laboratory, P.O. Box 808, Livermore, CA 94550*

Using a VISAR velocimeter with nanosecond time resolution, wave profiles of the reaction zone were measured inplanar TNT and92.5l7 .5 TATBKEL-F samples impacted by flyer plate s from a two-stage gas gun, at shockpressures exceeding the detonation pressure. The results indicate that for increasing piston-supponed pressures thedifference between inen and reaction product Hugoniots becomes very small. For TNT, the reaction zone is fairlywell defined and does not extend bevond 150 ns.

7 1 7

l. IntroductionIn recent years,it has become possible to supracomprcss

(overdrive) detonating solid explosives to pressuresexceeding their self-sustaining Chapman-Jouguet (C-J)pressures by impacting them with gas-gun accelerated flyerplates at velocities of several kilometers per second.Previous studies of this typel had indicated that it is difficultto fit, with the same JWL equation of state and standardassumptions, both the supracomprcssed and C-J states. Itwas suggested2 ttrat this may be due to the presence of arelatively slow reaction (possibly carbon condensation)following a fast reaction that prevents the conventionallyaccepted C-J state to be in true equilibrium. One of the goalsof the present work was to measure the reaction zone ofvarious rcacting explosives near to the C-J pressure inpiston-supported experiments with sufficient resolution todetermine if such a slow reaction was present. In addition,extending the measurements to pressu€s significantly abovethe C-J state provides new information on the inen andreaction product Hugoniots.

2. ExperimentalPlane shock waves were sent through thin planar TNT

and LX-17 (92.5n.5 TATB/KEL-F) samples using the two-stage light gas gun facility at LLNL and the velocity profilesof the emerging wave at a LiF reference window weremeasured by a VISAR vel@itometer. The t{E samples were

FIGT'RE 1Target Schemat ic. A gold layer between the LiFwindow an HE layer provides specular ref lect ionfor the recording instrunents.

approximately 25 mm in diameter and 2 or 3 mm thick.They were struck by a 3 mm thick I100 Aluminum flyerplate. As shown in Fig. I, a LiF window 5 mm thick wasanached to the rear of the FIE sample. A thin (4000A) goldlayer evaporated onro the LiF window at the HM-iFinterface provided a specularly reflective surface for theVISAR. ln some experimenrs, a 0.5 mm buffer ofMagnesium alloy was placed between the IIE and theLiF to help protect the reflective interface from3-dimensional iregularities in the shock front. Twoshorting pins flush to the impact surface of the tlE across thetarget diameter provided a trigger pulse for the diagnostic

*Work performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratoryunder contract W-7405-Ens-48.

7 1 8 DJ. Erskine et al.

equipment. A 50 mm f1.8 lens mounted at the rear of thetarget assembly focussed light from the VISAR system ontothe reflective surface at the LiFAIE or LiFMg interface andcollected the returning light.

The velocity of the LiFAIE interface was measured usinga VISAR interferometer system of a "push-pull" design byHemsing3 and an Argon ion laser for illumination. TheVISAR system, including the photomultipliers and recordinginstrumentation, had a 1.5 ns net rise time and a 3 m/svelocity resolution limit. The VISAR interferometer wasmounted on an optical table housed in a room separate fromthe gas gun room. Fiber optical cables linked theinterferometer to another set of optics (the gunroom optics)immediately a.ljacent to the gun tank which illuminated andcollected light from the urget. Separate fiben were used forthe illuminating and reflected light. To reduce signaldegradation due to dispersion in electronic cables, theVISAR photomultipliers were mounted adjacent to theelectronic recording instrumentation in the gun control roomand linked to the interferometer via optical fiber.

The gunroom optics shown in Fig. 2 were designed toilluminate the target with light from one fiber, while couplingcoaxially reflected light into the retum frber to theinterferometer. A 3" diameter mirror (M1) wittr a cenrall/8" hole picks offretuming light from the beam path of theilluminating beam. The target lens (Ll) was focussed suchthat the reflected beam diameter at Ml was larger than thishole for all anticipated sample movement during the shot.The coarse focusing of lens MO2 was such that the apertureof Ll was imaged to the aperture of MO3, which coupled thelight into the return-beam fiber. The fine adjustrnent of rhelenses was such to both maximize the amount of retuminglight, and to maximize the depth of field.

Apart from the use of fiber optics, the design of theinterferometer followed that of Hemsing and others and thereader is referred to Reference 2 for details ofthe opticalarrangement of the interferometer and data analysis schemCfor a "push-pull" VISAR. The gist of the technique is thatdue to aDoppler shift, a shift in fringes (sinusoidal variationof light output from the interferometer) is obsewedproportional to the change in target velocity, with theproportionality constant dependent on the length of an etaloninside the interferometer. (For our experiment,the constant

MO

FIGT'RE 2Schemat ic of gunroom opt ics, which couplesl ight between f ibers and target . MO-10xmicroscope object ive. L2 and Ll are 50mrn 1.8camera l enses .

was 231.4 m/s per fringe for a free surface target' For a

target windowed with LiF a correction must tre supplied due

to the shock induced index of refraction change. This has

been calib'rated by other laboratories5, and in our experimentimplied a l8l m/s fringe constant.

3 . R e s u l t sFigures 3 and 4 show the measured velocity profiles for

TNT and for LX-17 respectively. The indicated pressures

are the calculated piston supported Pressures based on the

measured flyer velocity and estimated equation of states(EOS) of the two explosives. Due to the finite response timeof the VISAR photomultipliers, the instrumentation couldnot rccord the nearly instantaneous passage of fringes at themoment of shock when the velocity jumps discontinuously.We allowed 3 ns for the photomultipliers to settle (twice the

response time). As a consequence, we must consider anunknown, but integer number of fringes to have skippedpast uffecorded at the moment of shock. By comparing thedata wittr the expected velocityrwe determine the integernumber of fringes to add as an offset to the record, and aftermultiplying by l8l m/s per fringe we obtain the velocity

returning lighttointerferometer

100 pm coreslep index fiber

M03

Ml ( r i8 '

ocentrol hole)

<-' ( ( \ \\/ii5#'frgl'

Visar wave profile measurements

records shown in the Figure. Since the difference betweenmeasured and calculated velocities behind the reaction zone(>70ns) are significantly less than l8l rnls we havecorrectly identified the integer number of fringes shpped.

The calculated curves for TNT were made by an ignitionand growth reactive flow hydrodynamic calculation assuminga JWL EOS with the parameters: PCI=190 kB, p61.645,Vosr=0.693 cn/ps, E0=0.07, R 1 =5.6, R2=2. 1, 1p=Q.1,A=8.797808, 8=0.322774, C=0.01914. This JWL EOSwas obtained by fiaing cylinder test expansion data and

3 . 1

V(km/s)3.0

-'' o 10 20 30 40 50 60 70 80 90 r00

2 . 1V(kmis)

2

1 .91 .9

1 . 8

1 .7V(km/s)

1 . 6

1 .5

1 .4

in supra-compressed HE 7 t 9

Kineke and West6 supra-comprcssive data on TNT. A moredetaileddescription of this calculation is in Reference 7.4. Discussion

4.I TNTT\e 20.2 GPa shot is close to the C-J state and shows a

reaction zone which ends at about t=100 ns, but seems totail--out possibly to 200 ns. There is no apparent long timeconstant component of the decay since the data is fairly levelfrom t=200 to t=6(X) (only the first 350 ns are shown), whenthe release wave from the rear of the flyer arrives. For the30.9 GPa shot, the change in velocity (Av) across thercaction zone has decreased from about 0.4 km/s to 0.1 km/s

5 . 2

2.2

3.3

3.2V(km/s)

3 .1

J

2.92

1.9

V(km/s)

1 .8

1 .7

t'6 o'

LX:17 60:GPa. . . . . . . . . . . . . + . . - . - . . . . . . . i . . . . . . . . . . . . . . . - .

. . . .8U. - . . . . . . . , : . .Y , , . . . . . , , , , r , , , , , . , . , ;

)

I\ t x-tz1 i.-\---.--: --

l n il l \ .

t i..... -\

l \,\:i

I

' :t l

3l GPa It i :+ t +l t lt i

, i

- i . . . . . , . . . . . , . r . . . . . - . ' . - . . + . . . -

50 100 lso 2w 2s0 300 350Time after shock (ns)

FIGURE 4

1 .30 50 100 150 zffi 250 300 350Time after shock (ns)

FIGURE 3VISAR measured velocity profiles of LiF/HE inter-face for TNT at three different (calculated) piston sup-ported pressures. Note the diferent time scale of thetop plot. The dashed curves are calculated results us-ing a JWL equation of state.

Measured velocity profiles of LiF/HE interface for LX-17 at two different (calculated) piston supported pres-sures. The expected velocities after the reaction zonefor the top and bottom records are 3.1 and 1.8 km/s,respectively. In the top record, the feature at t:270 nsis related to the arrival of a second shock from thealuminum/LX-l7 interface. The fluctuations att:25 ns iri the bottom record are spurious, not relatedto a velocity feature, judging from the radii of thefringe signala. (This may be due to 3-dimensionalityof the shock front.)

TI {Ts. 7 ( Pa

mcs

calcu

,red

i a a :lated

720 DJ. Erskine et al.

and has narrowed to 60 or 70 ns.At the highest pressure,Av has reduced even further. But

the most significant feature of this record is that for3<t<15 ns the velocity increases, whereas for the otherrecords the velocity monotonically decreased from >3 ns.We are confident the initial rise is not an artifact of therisetime (1.5 ns) of the photomultiplien since it is too slow,and since the horizontal and vertical fringe sigrals ofthequadrature coding scheme of the push-pull VISAR areconsistent with each other4 from r>3 ns.

An additional TNT shot at 36.5 GPa (not shown) had aAv of zero; the velocity record was flat (t5 rds) from>3 ns. We interpret the decreasing magnitude of Av versuspiston pressure to suggest that the product and reactantHugoniots may cross at elevated pressures.

4.2 LX-17The LX-17 data shows a similar decrease of Av in the

reaction zone versus piston pressure, except that it occurs ata higher pressure than the TNT. For the 31 GPa case,which is close to ttre C-J prcssure, the knee of the reactionzone is more rounded than in the corresponding TNT record,and there may be a very slight long time constant componento the decay. Otherwise the reaction zone is similar to that inTNT.

The D(-17 records show more fluctuations than theTNI records. This may be due to a 3dimensionality of theshock front since a shot (not shown) identical to dre botomshot ofFig. 4 except using a Mg buffer layer in benreen theLiF and the tIE showed significandy less fluctuations.

AcknowledgementsThanks to Gary Otani for his assistance in the

operation of the VISAR and to Leona Meegan for urgetpreparation.

References

L. Green, E. l,ee, A. Mirchell and C. Tarver, 8th Symp.(Intematnl.) on Detonation, Naval Surface WeaponCenter MP86-194 (1985) 587; L.Green, N. Holmes andJ. Kury, Proceedings of Intmatl. Symp. on Pyrotechnicsand Expl., Beijing (1987) 518 (LLNI- Rprt. *195461).

C. Tarver, "On the Difference Between Self-sustainingConverging Detonation Waves and Piston-SupportedOverdriven Detonation Waves", l,awr. Livnn. NarLab.Rprt. 90714 (1984).

W. F. Hemsing, Rev. Sci. Instr. 50 (1979) 73.

The push-pull VISAR consists of two independentinterferometers, out of phase by 90o so that the directionof the fringe movenrent can be deternined by notingwhich way the loci rotates about the origin when thefringes are ploued in X-Y fashion. The angularmovement of ttre loci is proponional o the change intarget velocity. The radii of the locus grves the intensityof light retuming ftom the target and typically variesslowly as the loci revolves. During the shock jump((kt<3 ns) the average loci moves into ttre origin untilthe photomultiplien recover. A relatively seady radiithereforc indicates the data is real and not an anifact ofthe photomultiplier response time or a noise fluctuation.

J. Wise and L. Chhabildas, Laser interferometermeasurements of refractive index in shock-compressedmat€rials, in: Shock Waves in Condensed Matter, ed.Y.M. Gupta (Plenum Press, NY 1986) p. ,t41.

I. Kineke and C. West, 5th Symp. (Internatnl.)on Detonation, U.S. Naval Ord. Lab., Pasadena (1970)ACR-184, ONR 0852-0057 and private communication.

L. Green, C. Tarver, and D. Erskine, "Reaction ZoneStructure in Supracompressed Detonating Explosive", tobe published in proceedings of 9th DeonationSymposium to be held in Portland, Ore. Aug. 1989.

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