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Optics and ¸asers in Engineering 29 (1998) 447—455( 1998 Published by Elsevier Science Ltd. All rights reserved

Printed in Northern Ireland0143—8166/98/$19.00

PII: S0143–8166(97)00119–X

Confining Medium and Absorptive Overlay:Their Effects on a Laser-induced Shock Wave

Xin Hong, Shengbo Wang, Dahao Guo, Hongxing Wu,Jie Wang, Yusheng Dai, Xiaoping Xia & Yanning Xie

Institute of High-power Laser Technology, University of Science and Technology of China,Hefei, Anhui, 230026, People’s Republic of China

(Received 29 April 1997; accepted 17 July 1997)

ABS¹RAC¹

In this paper the characteristics (such as amplitude, width) of a laser-inducedshock wave under confining conditions is studied. For engineering applications,a physical study of this method is useful in order to optimize this technique.¼e have first introduced a new pressure gauge — P»DF (polyvenylidenfluoride) gauge with short rise time and wide linear response range. Exper-imentally, by measuring the generated pressures under different confiningmaterials, the relationship between the pressures and the acoustic impedanceof confining materials, is illustrated, which somewhat agrees with the theoret-ical calculation. ¼e have also found that under confining conditions laser-induced shock waves persist longer than a laser pulse. ¹hen, the effects ofblack paint overlay (absorptive overlay) is studied. ¼e experimentally pointout that a black paint overlay placed before an irradiated target can greatlyincrease the generated pressure under any confining material in our exper-iments for its beneficial effect on the plasma-generating process. ¹o oursurprise, comparing the impulse (P

max) q), which the shock wave induced under

absorptive overlay executes on the target, to that induced under no blackpaint overlay, the increase ratio is approximately equal. ( 1998 Publishedby Elsevier Science ¸td. All rights reserved.

1 INTRODUCTION

When a high-intensity laser pulse irradiates on a small metallic area, thesurface layer instantaneously, vaporizes into a high-temperature (&10 000 K)and high-pressure (&GPa) plasma. In blowing off, this plasma induceda shock wave (laser-induced shock wave). This high-pressure shock waveinduced by a high-intensity laser pulse has been the basis for fundamentalresearches and engineering applications such as inertially confining fusion

(ICF)1 and laser shock processing (LSP).2 For potential industrial applica-tion in the fields of airplane and automobile manufacturing, results on thetreatment of different materials (aeronautical aluminum alloys, steels, weldedparts) with LSP have illustrated the beneficial effects of laser-induced shockwaves on static, cyclic or fretting mechanical resistance and fatigue property ofmetal.3 In order to get a higher amplitude and longer time-durationlaser-induced shock wave, the confining technique was introduced.4 In theconfining technique, a laser irradiates on a target with an intensity ofabout 1 GW/cm2, and the generated plasma is confined by a medium, whichcovers the target, transparent to the laser. This confining configuration hasappeared necessary for metallurgical applications where, for a given laserenergy, enhanced pressures must be realized in order to achieve high-shockpressures. In order to determine the main characteristics of an induced shockwave such as its amplitude, time duration as a function of experimentalparameters (pulse width and laser intensity), an analytical model of theconfining process was built.5 In this model, the amplitude of the inducedshock wave is linear to the square root of the acoustic impedance of theconfining materials when the laser intensity is constant, and the width(FWHM) of shock wave 2—3 times the width (FWHM) of the laser pulse. Inour experiments, polyvenyliden fluoride (PVDF) gauge with short rise time(ns) and wide linear response range (0—20 GPa) is introduced. Using a PVDFgauge, we measured the maximum pressure P

.!9and width q of the shock

wave induced under different media to reveal the relationship between themaximum pressure of shock wave and the acoustic impedance of the confiningmaterial. Under the confining technique, we have studied the effects of theabsorptive overlay and found exciting and interesting results from the ab-sorptive overlay technique.

2 EXPERIMENTAL METHODS

2.1 Laser

The experiments were performed with a Nd : Glass laser of the institute ofhigh-power laser technology (University of Science and Technology of China)operating at 1)06 km wavelength. Typically, output energy is 20 J for aGaussian pulse nearly of 40 ns width (FWHM, full-width at half-maximum).The experimental laser pulse has an approximately flat-top intensity profile.The laser radiation is focused onto the sample with a 130 cm focal-lengthconvergent lens to the desired spot size. In our experimental configuration, theexperiments are performed in air at standard conditions.

448 X. Hong et al.

Fig. 1. Configuration of target system. (a) With black paint overlay (b) Without black paint overlay.

2.2 Target

The target samples were 2024T62 aeronautical aluminum alloys. In one case,on areas that desired impacting, we placed a layer of black paint. Then theconfining medium is placed on this layer and firmly clamped against thealuminum target with a holder [see Fig. 1(a)]. In the other case, the confiningmedium is clamped directly against the target without black paint overlay [seeFig. 1(b)]. Five confining media: perspex, silicon rubber, K9 glass, quartz glassand Pb glass with different acoustic impedances were used.

2.3 Pressure measurements

The pressure of the laser-induced shock waves were measured with a poly-venyliden fluoride (PVDF) gauge, 50 km thick, 10 mm in diameter and platedwith 0)5 km-thick gold electrodes. The PVDF gauge with electric signal risetime of about 1 ns and a wide linear response range (0—20 Gpa) is an idealgauge6 for pressure and measurement of the laser-induced shock wave.

In the linear response range of PVDF gauge, the pressures obtained fromthe measurements of the voltage output »(t) of PVDF gauge is given by therelation

P(t)"KA P

t

0

»(t)R

dt ,

where P is the pressure, A is the area of collecting electrodes, » is the voltage,R is the resistance, t is the transmit time of pressure pulse, and k is thepiezoelectric coefficient.

Confining medium and absorptive overlay 449

TABLE 1Parameters of the Laser-induced Shock Wave Under Black Paint Overlay

Confining Z2

I0

q %9P.!9

45P.!9

#!P.!9

%9Immedium (106 g/cm2 s) (109 W/cm2) (ns) (108 Pa) (108 Pa) (108 Pa) (Pa s)

Perspex 0)32 0)74 53 11)3 11)1 11)0 58)8Silicon rubber 0)47 0)74 54 13)8 13)5 12)9 72)9K9 glass 1)14 0)68 160 15)9 16)6 17)6 265)6Quartz glass 1)31 0)76 131 17)2 16)6 18)3 217)5Pb glass 1)54 0)90 126 22)8 19)3 19)2 240)2

3 EXPERIMENTAL RESULTS AND DISCUSSION

3.1 Effects of the confining medium on the laser-induced shock wave

The confining process is described in following steps.(1) During the laser pulse duration, the pressure generated by the plasma

induces a shock wave which propagates into the target and the confiningmaterial.

When supposing this pressure to be strong enough for generating two shockwaves in two media, we can obtain the monodimensional solution of thisproblem.

We consider here the simple situation of a rectangle-shape laser with a pulseduration q and obtain the following:

P.!9

(108Pa)"0)10 Aa

2a#3B1@2

Z1@2 (g/cm2 s)I1@20

(109 W/cm2),

where P.!9

is the maximum pressure of the laser-induced shock wave, a is theratio of the thermal to the internal energy of the black point, I

0is the laser

intensity, Z is given by the relation:

2Z"

1Z

1

#

1Z

2

,

where Z1

and Z2

are the acoustic impedances of the target and the confiningmedium, respectively. Thus, we can get the linear relationship betweenP.!9

and Z1@2. Typically, a"0)1. If the laser intensity is constant(0)72]109 W/cm2), we can calculate the maximum pressure #!P

.!9of the laser

shock wave (see Table 1).(2) When the laser is switched off, the plasma still maintains a pressure

which decreases during its adiabatic cooling.

450 X. Hong et al.

Fig. 2. (a) Voltage—time and (b) pressure—time evolution of the induced shock wave.

The pressure can be described by the relation:

P(t)"P(q) A¸(q)¸(t)B

c.

And the thickness of the plasma ¸(t) is evoluted as follows:

¸(t)"¸(q) A1#c#1

q(t!q)B

1@(c`1)

where q (also the duration of laser pulse) is the time taken to reach maximumpressure, c is the adiabatic coefficient. Typically, for the plasma c"1)35. Wesee that the pressure decreases to P(q)/2 at the time 2q. As the experimentallaser profile has a Gaussian shape [see Fig. 2(a)], we have q"2d, d is thewidth (FWHM) of the Gaussian laser pulse at that duration. Supposing thatthe pressure-increase process is smooth enough, the pressure increases fromzero to P(q)/2 at the time d. So the width (FWHM) of the laser-induced shockwave can be calculated as: 2q!d"3d. We see here the interesting advantageof the confining technique: the generated pressure persists 2 times longer thanthe duration of the laser pulse [see Fig. 2(b)].

We select five confinement media with different acoustic impedances: per-spex, silicon rubber, K9 glass, quartz glass and Pb glass. Using a PVDF gauge,we measured the maximum pressures and widths of the laser-induced shockwaves under different confinement media.

According to Phipps et al.7 in Los Alamos national laboratory, we convertthe experimental maximum pressures %9P

.!9into standard maximum pres-

sures 45P.!9

. Results from experiments are described in Table 1.

Confining medium and absorptive overlay 451

Fig. 3. (a) Voltage—time and (b) pressure—time evolution (confining medium: Pb glass; absorptive overlay:black paint).

Table 1 shows that the greater the acoustic impedance, the higher is themaximum pressure of the laser-induced shock wave, and the standardmaximum pressure 45P

.!9is somewhat in agreement with the theoretical

calculation. However, the increase ratio of the pressure is limited to only21@2"1)41 comparing the confining medium with a very high acoustic impe-dance (high Z

2) to the one with a common acoustic impedance (the same order

of magnitude with that of aluminum).There is also the anticipated increase of the width of the shock wave at

a higher acoustic impedance. But at a low acoustic impedance, the widths ofthe laser-induced shock waves are nearly equal to the widths of the laser pulse.

The disagreement between the experimental results and the theoreticalexpectation shows the imperfectness of the analytical model. We shouldimprove the model in order to explain more phenomena in the laser-inducedshock wave.

3.2 Effect of absorptive overlay on the laser-induced shock wave

Using Pb glass as the confining material, comparing the maximum pressureand the width of the induced shock wave under black paint overlay to thoseunder no black paint overlay, we find that the characteristics aforementionedhave increased greatly. The voltage—time and pressure—time evolutions of theshock wave induced under absorptive overlay and no absorptive overlay areshown in Figs 3 and 4, respectively. In our experiments, we have observed thatthe effects of different confining materials on the increase of amplitude andtime duration of the shock wave is different. In order to uncover a moreuniversal effect of the absorptive overlay on the induced shock wave, wemeasured the maximum pressure and width of the shock wave induced under

452 X. Hong et al.

Fig. 4. (a) Voltage—time and (b) pressure—time evolution (confining medium: Pb glass; absorptiveoverlay: no).

TABLE 2Parameters of the Laser-induced Shock Wave Under No Black Paint Overlay

Confining Z2

I0

q %9P.!9

45P.!9

%9Immedium (106 g/cm2 s) (109 W/cm2) (ns) (108 Pa) (108 Pa) (Pa s)

Perspex 0)32 0)84 75 4)3 3)8 28)5Silicon rubber 0)47 0)80 62 6)4 6)1 37)8K9 glass 1)14 0)72 99 15)9 15)9 133)7Quartz glass 1)31 0)72 81 13)9 13)9 112)6Pb glass 1)54 0)75 89 14)2 13)8 122)8

black paint overlay (see Table 1) and no black paint overlay (see Table 2) fordifferent confining materials.

The experiments show that with black paint overlay, both maximumpressure P

.!9and impulse (P

.!9) q) of the laser-induced shock wave have

obviously increased.In our experiments for any confining materials, we find the maximum

pressure of the shock wave induced on target with black paint overlay to havebeen improved to some degree as compared to that without an overlay. It isrelatively easy to explain the increasing amplitude effect of the absorptiveoverlay. When a high-intensity laser irradiates the target, the impacted areavaporizes into high-temperature, high-pressure plasma. In our experiments, inthe case of the absorptive overlay, a thin layer of the absorptive mediumvaporizes into plasma, whereas in the case of no overlay, a layer of Al does.Black paint and Al defer greatly in their absorption ability to laser. In our

Confining medium and absorptive overlay 453

experimental condition, black paint can absorb almost all the laser energy,while Al can absorb only 80%. That means when a laser with intensityirradiates an Al target, only 80% of the laser intensity is used to generateplasma. As we know, with a higher intensity, the laser can generate a denserand higher-temperature plasma, and thus induce a higher pressure and longertime-duration shock wave.

We have also discovered that under different confining media, the absor-ptive overlay’s effect of increasing the amplitude of the shock wave variesgreatly. For example, the maximum pressure of the shock wave induced underthe perspex confining medium has been improved by about 192%; undersilicon rubber: 121%; under K9 glass: 24%; under quartz glass: 20%;under Pb glass: 40%. As for the increase in the widths of the induced shockwaves, it appears randomly, and even with a negative increase. These phe-nomena are caused by different effects of the different confining materials onthe laser-generated plasma processes. From the above data, we can concludethat when comparing two cases (having absorptive overlay and no overlay)the maximum pressure and width of an induced shock wave are not the maincharacteristics. After further study the data, much to our surprise, whenselecting impulse (P

.!9) q) as a main characteristic of induced shock wave, we

see an interesting benefit of absorptive overlay technique: the increase ofimpulse of shock wave is approximately equal under any confining materialused in our experiments. For detail, perspex: 106%; silicon rubber: 93%; K9glass: 99%; quartz glass: 93%; Pb glass: 96%. The average increase of impulseof laser-induced shock wave is about 97%, with a mean-square deviation of10% in our experiments, the total error of data is about 10% considering bothmeasurement error and error arising by converting experimental maximumpressure to standard maximum pressure. Thus, in our experiments we canconsider the increase in the impulse to be more or less equal when comparingthe case with absorptive overlay to the case without overlay under anyconfining material. This is an exciting result and there is no analytical modelin the field of high-intensity laser and material interaction that can explain theresult exhaustively. We expect to reveal the internal physical mechanism ofthis phenomenon after more detailed experimental and theoretical researches.

4 CONCLUSION

We have experimentally studied the effects of confining material and ab-sorptive overlay. For a given laser intensity, higher pressure is obtained whenthe shock wave is induced under the confining medium with higher acousticimpedance, and the width of the shock wave is much wider than that of a laserpulse. As expected, black paint overlay on target can improve the amplitude of

454 X. Hong et al.

induced shock wave. But it is more interesting that the increase ratio ofimpulse of shock wave is approximately equal under any confining material.In order to interpret the experimental results exhaustively, more detailedexperimental and analytical research is required. It is clear that to optimizeconfining and absorptive overlay techniques is a promising method to get anideal induced shock wave for fundamental researches and industrial appli-cations.

ACKNOWLEDGEMENTS

We thank associate Professor Jingyi Cheng for providing us with the oppor-tunity of using the PVDF gauge for pressure-measurements and acknowledgethe helpful direction provided by Professor Jilin Yu in the measurementtechnology of a laser shock.

REFERENCES

1. Charatis, G. et al. Plasma physics and controlled nuclear fusion research. 5th Conf.Proc. Tokyo, 1974, 317—25.

2. Fairand, B. P., Wilcox, B. A. & Gallagher W. J. et al. Laser shock inducedmicrostructural and mechanical property changes in 7075 aluminum. J. Appl. Phys.43(9) (1972) 3893—5.

3. Peyre, P. & Fabbro, R. Laser shock processing: a review of the physics andapplications. Opt. Quantum Electron 27 (1995) 1213—9.

4. Okeefe, J. D. & Skeen, C. H., Laser-induced deformation modes in thin metaltargets. J. Appl. Phys. 44(10) (1973) 4785—93.

5. Fabbro, R., Fournier, J. & Ballard, P. et al. Physical study of laser-producedplasma in confined geometry J. Appl. Phys. 68(2) (1990) 775—84.

6. Romain, J. P. et al. Measurements of laser induced shock pressure using PVDFgauges. In S. C. Schmidt, editor, High-pressure science and technology, 1993.American Institute of Physics, (1994) 1915—9.

7. Phipps, C. P. et al. Impulse coupling to targets in vacuum by KrF, HF and CO2single-pulse lasers. J. Appl. Phys. 64(3) (1988) 1083—96.

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