david b. stahl et al- flyer velocity characteristics of the laser-driven miniflyer system

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Los Alamos National Laboratorv is owrated bv the Universitv of Californ ia or the United States Department of Ene rw under contract W-7405-EN G-36

TITLE: FLYER VELOCITY CHARACTERISTICSOFTHE LASER-DRIVEN MINIFLYER SYSTEM

AUTHOR: David B. Stahl, Russell J. Gehr, Ron W. Harper, Ted D. Rupp, Stephen A. Sheffield,and David L. Robbins

Dynamic Experimentation Division, DX- 1LANL, Los Alamos, NM 87545

SUBMITTEDTO: 1999 A PS Topical Conference on Shock Compression ofCondensed MaterialsSnowbird, UtahJune 27 - July 2,1999

By acceptance of this article, the publisher recognizes that the US . Government retains a nonexclusive, royalty-free license to publish or reproduce the published formof this contribution,or to allow others to do so, for U.S.Government purposes.

The Los Alanios National Laboratorv reuuests that the uublisher identifv this article as work uerformed under the ausoices of the U.S. Departmen t of Energv.

LQS lamos National Laboratory

Los Alamos, New Mexico 87545I



FLYER VELOCITY CHARACTERISTICS OF THELASER-DRIVEN MINIFLYER SYSTEM

I/ / /

David B. Stahl', Russell J. Geh?, Ron W. Harpe2*,Ted D. R&p2, Stephen A.

Shef'field', David L. Robbins'

Dynamic Experimentation Division)Los Alamos National Laboratory, LosAlamos, NM 87545'AlliedSignal FM&T/NM, Los Alamos NM 87544

1

Abstract. The laser-driven MiniFlyer system is used to launch a small, thin flyer plate for impact on atarget. Consequently, it is an indirect drive technique that de-couples the shock from the laser beamprofile. The flyer velocity can be controlled by adjustment of the laser energy. The upper limits on theflyer velocity involve the ability of the substrate window to transmit the laser light without absorbing,reflecting, etc.; Le., a maximum amount of laser energy is directly converted into kinetic energy of theflye r plate. We have investigated the use of sapphire, quartz, and BK-7 lass as substrate windows. Inthe past, a particular type of sapphire has been used for nearly all Min iflyer experiments, Results ofthis study in terms of the performance of these window materials, based on flye r velocity are discussed,

INTRODUCTION

Pulsed lasers have been used extensively togenerate short duration shock waves, includinginvestigations of spal l[l,2 ], detonation ofexplosives[3], high pressure generation[4], and thinfild coating bond strength measurements[5]. Lasergenerated shock techniques offer several advantages,including low cost, high shot throughput, and

simplified experimental apparatuses, However, theseare countered by the requirement for thin samples,complicated laser generated plasma physics andheightened resolution requirements for experimentaldiagnostics,

In this work a laser isused to launch a 0.05mtn by3.0 mm diameter copper flyer plate to terminalvelocities up to 0 6 5 W s . Flyer plates are attachedto substrate windows with several microns ofadsorbed material sandwiched between forabsorption, buffcring, and insulation. Unlike directlaser-generated shock techniques laser-launchedplates detach plate launch &om plate impact.Therefore, the shock generated in the target is de-

coupled from laser deposition and buffered fromdiscontinuities in the laser beam profile.

The Miniplyer has been used to determine spa11strengths of target materials and generate Hugoniotdata when coupled with velocity interferometry[6,71.Our goal is to improve convers ion of the laser energyinto kinetic energy of the flyer plates (achieve higherflyer plate velocities), maintain flat flyer plates (forone-dimensional impact) and understand any changeof state of the flyer plate prior to impact. We reportthe laser energy/flyer velocity relationship as a

function of substrate material (launching windowmaterial). We also include effects which the variouswindow materials have on flyer plate accelerationand ring

EXPERIMENT

Flyer plates are launched with a single shotNd:Glass laser that produces a pulse of - 20 ns(FWHM)uration and 1.054 micrometer wavelength.A maximum of 5 joules was used for all shotsreported here. (At higher energies the laser's Q-switch leaked creating a >0 ,1 ms long pu lse prior tothe Q-switched pulse, which yielded lower flyervelocities and unpredictable results,) The launch

laser beam is directed through a lens and diffractiveoptical element (Mems Optical Inc., Huntsville AL)and finally onto the rear of the substrate window.



Digital images of the beam at the position ofincidence on the substrate were collected andanalyzed.(Beamcode, Coherent Laser Inc,) Twocross-sectional views of the beam profile are shownin figure 1. Just over 80 % of the energy is inside a 3mm diameter.

197

0

283

FIGURE 1. Vertical and horizontal cross-sections through thelaser beam at the position of incidence on the substrate window.Analysis of the profile indicates approximately 80% of the beam iswithin a 3 mn diameter.

Four different windows were used in this study: 1)sapphire (Saphikon Inc. NH USA, part # 15959), 2)fused silica, (Heraeus Amersil, part # 27385), 3) High

grade BK-7, (Rocky Mountain Instrument Products,CO USA part # WI1901K), and 4) Low grade BK-7,(Esco Products, NJ USA part # F907063). Thewindows were coated with the following sequentiallayers: 5,000 8, of carbon, 5,000 8, of aluminumoxide, and fmally with either 5,000 A or 50,000 8, ofaluminum. Copper flyers were purchased in 0.05mm sheets from Goodfellow (OFHC @ 99.95+%)and cut into 3 mm diameter rounds. The flyers werethen glued to the substrate windows with thedeposited material sandwiched between.

Copper

Flyer

VISARLaser

ImpactwindowFused -Silica

Barrel

Substrate BuildupLayer:5,000di Carbon

5,000di Al Oxide5,000di or 50,000 di AI

Launch Laser

'SubstrateWindow:BK-7 GlassFused Silica

spacer - orSapphire

FIGURE 2. Un-scaled schematic of the substratdflyer assemblyprior to firing

The flight path was controlled by placing a spacer(0.125 mm) between the substrate window and the

impact window (figure 2) yielding a total flight pathof 0.075 mm. All shots were fired in air.

Two Velocity interferometers (VISAR), ValynInternational (model VLN V-04), were used to recordthe velocity profiles. A Coherent Verdi laser wasused as the VISAR source and was focused to a spotsize of approximately 0.1 mm with 0.1 to 0.2 W ofincident energy.

RESULTS AND DISCUSSION

A typical measurement of the flyer velocity versustime is shown in figure 3a. (The result from only oneof the two VISARs is shown, since the resultdetermined with the other VISAR agrees with thisplot to within 3%.) In th is case the window m aterialis high grade BK-7 and the aluminum coating is 5000

thick. It is seen that the flyer undergoes a rapidinitial acceleration, followed by a slower accelerationuntil impact occurs. The flyer velocity willeventually reach a limit, if it does not impact a targetfirst. There is a small am plitude oscillation on the

measured velocity, due to a shock wave propagatingin the flyer. To determine the approximate impactvelocity of th e flyer and the properties of theoscillation, this oscillation must be separated fromthe data. To accomplish this, the data is fit to adouble-exponential curve, The value of the fit at thetime of impact is taken as the flyer impact velocity.The difference between the actual data and the fit isthen calculated (figure 3b) to obtain two pieces ofinformation: the period and the m agnitude of theoscillation. The period of oscillation is found to bethe same as the round trip time for a sound wave in a50 micrometer thick copper plate. The amplitude ofoscillation for this shot is found to be about f .4 m /s

at the time of impact. This amplitude determines theminimum uncertainty in the velocity at impact.

Figure 4 shows the velocity versus time curves forfour different window materials: quartz, sapphire,and low and high grade BK-7. The laser pulseenergy is approximately the same for each shot. It isobvious that the initial flyer acceleration isreasonably independent of the window material; thevelocity curves begin in almost identical manners.The flyer from the sapphire window does achieve aslightly higher velocity at first, but its velocity levelsout earlier and so this flyer achieves a slightly lowerimpact velocity. Overall, the accelerations andimpact velocities are too similar to suggest any

significant differences due to window material.



500

400

300

200

I I I I -

400 450 500 550 600

time [ns]

2ot (b) " I I 1 I I I

-20I I I I I I 1

480 520 560 600

time [ns]

FIGURE 3: a,) Typical flyer velocity versus time profile withdouble-exponential fit. b.) Difference between velocity data anddouble-exponential fit, showing oscillation period and amplitude.

However, the amplitude of the oscillations on thevelocities appears to be dependent on the windowmaterial to a small degree. The amplitude of theoscillations is largest for the sapphire window. Thisis due to the higher shock impedance of sapphire.The amplitudes of oscillation for the other windowmaterials are lower by approximately 30%. When

considering the data for all of the high energy shots(5 Joules, -650 m/s) we find the same qualitativeresults. Sapphire has the largest average oscillationamplitude at 10.8 f 1.6 m/s, while low grade andhigh grade BK-7 and quartz have slightly loweraverage amplitudes at 8.8 f 2.5 m/s, 8.7 f 1.6 d s ,

and 8.0 f 2,6 d s , respectively. Thus there is adifference in flyer behavior for the different windowmaterials, However, the difference is sufficientlysmall to be of no concern in choosing a windowmaterial.

The primary goal of our experiments is todetermine the effect of window material and substratethickness on the velocity of the flyer, Figure 5 shows

the flyer impact velocity (determined from thedouble-exponential fits) versus laser pulse energy for

- apphire

-- BK-7LowGrade

-. - BK-7HighGrade

. FussedSilicaJ

4m 450 sa, 550 603

Time [ns]

FIGURE 4 Velocity profiles are shown for each substratewindodw material at a laser energy of 3 .3 Joules .

all of the window materials and thicknesses of thealuminum substrate. It is obvious that the pointscluster together, with only a ten to twenty percentvariation in velocities at a given laser pulse energy,The BK-7 windows tend to yield slightly higher

velocities than do the quartz and sapphire windows,but this advantage is small, In addition, there is nodefinite advantage to using either thick (50000 A) orthin (50008)aluminum coatings. In some shots thethick aluminum yields a higher velocity, in othershots the thin aluminum does. These results aresurprising, and may change at higher laser energies,

The highest flyer velocities achieved in ourexperiments range from 590 to 680 m /s using laserpulse energies near 5 Joules. The trend of the datasuggests that even higher velocities should beachievable with higher laser energies. We arecurrently in the process of improving our lasersystem to reach 10 Joules and hope in the future to

achieve up to 20 Joules to search for a limit to theflyer velocities.The velocity data fi t well to a curve of the form

V = V,, +Ad& . This is the expected form, since thekinetic ena gy of the flyer is proportional to thesquare of the velocity. The V=O ntercept of the fi toccurs at approximately0.1 Joule, so we predict thatthe flyers can be launched at very low laser energies.

Based on this fit, the efficiency in converting laserenergy into flyer energy may be estimated. The massof the flyer is approximately 3.15 milligrams. Theenergy of the laser pulse is adjusted to reflect severallosses. Only 80% of the pulse energy lies within the3 mm diameter of the flyer. Also, reflections of

around 4% of the energy occur at two surfaces on thewindow between the diffractive optical element andthe flyer window and another4% reflection occurs at



the back surface of the flyer window. Using theseadjustments the conversion efficiency isapproximately 19%.

I I I I

I I I I I

0 1 2 3 4 5

energy IJ1

FIGURE 5.: Impact velocity versus laser pulse energy for all ofthe window materials and both substrate thicknesses.

CONCLUSIONS

From this data we conclude that, for the laserenergies used thus far, the window material and thealuminum coating thickness are not criticalparameters in determining the flyer velocity, which issimilar to the conclusions of Sheffield et. al.[3].Minor differences are apparent: BK-7 windowsproduce slightly higher flyer velocities (with highgrade BK-7 better than low grade BK-7), whilequartz windows produce slightly lower magnitudeoscillations in flyer velocity. However, thesedifferences are too sma ll to be of seriousconsideration in choosing our window and substratematerials. This is very advantageous, since it permits

us to choose the most cost effective window materialand to specify the manufacturing constraints withbroad tolerances, thus reducing cost.

In the future we will continue to increase our laserpulse energies to achieve higher velocities and tosearch for any limitations, One possible concern isthat, at higher energies, absorption by the windowcould have an effect. However, for BK-7 the internaltransmittance of a lmm thick sample at a wavelengthof 1,054 is 99.985% 8]. Hence, a 20 Joule laserpulse would only deposit 3.0 milliJoules in thewindow, which should not damage the window andtherefore should have negligible effect. Based onthese observations, we are optimistic that we can

achieve significantly higher flyer velocities (in the1.O o 1SWsrange for a 0.05mm OFHC flyer) withthis method.

1.

2.

3.

4.

5 .

6.

7 .

8.

ACKNOWLEDGMENTS

The authors appreciate the support and equipmentsupplied by Willard Hemsing (LosAlamos Nat. Lab.)for data acquisitiodana lysis. We also acknowledgeBryan Adams, Bill Price, and Ben Rosenblum ftomAlliedSignaVPM lkT Kansas City Plant for work onsubstrate deposition and flyerhbstrate assembly.

This work was supported by the DOE EnhancedSurveillance Program through contract DE-ACO4-76-DP00613.

REFERENCES

Moshe, E., Dekel, E., Henis, Z., Eliezer, S.,

Dekel, E., Eliezer, S., Henis, Z. Moshe, E.,Ludmirsky, A,, and Goldberg, I. B., J . Appl

Sheffield,S.A,, Rogers, J. W. Jr, and Castaneda,J, N., “Velocity Measurements of Laser-DrivenFlyers Backed by High Impedance Windows,” in

Shock Waves in Condensed Matter-1985, editedby Y. M. Gupta, Plenum Press, New York, 1986,

Romain, J. P, Cottet, F., Hallouin, M., Fabbro,R,, Paral, B., and Pepin, H., Physica B & C 139

Youtsos, A, G., iriakopoulos, M., and Timke,Th., Theoretical and Applied FractureMechanics 31,47-59 (1999).Warnes, . H., Paisley, D. L., and Tonks, D. L.,“Hugoniot and Spa11 Data from the Laser-DrivenMiniflyer,” in Shock Compression of CondensedMatter-1995, edited by S . C. Schmidt and W, C.Tao, AIP Conference Proceedings 370, New

York, 1995, pp. 495-498, 495-498, Seattle, WA,

Paisley, D. L., “Laser-Driven Minature Plates forOne-Dimensional Impacts at 0.5 - 2 6 km/s” inShock- Wave and High-Strin-Rate Phenomena inMaterials, edited by M. A. Meyers, L. E. Murr,and K. P. Staudhammu, Marcel Dekker, Inc.,New York 1992,pp. 1131-1141.Melles Griot, Optics Guide 4, pp. 3-7 (1988)

A@. PhyS. Lett. 69,1379-1381

PhyS, 84,4851-4858 (1998).

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david b. stahl et al- flyer velocity characteristics of the laser-driven miniflyer system

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