TRANSIENT OPTICAL PROPERTIES OF TARGET INDUCED LASER SUPPORTED ABSORPTION WAVES

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TRANSIENT OPTICAL PROPERTIES OF TARGET

INDUCED LASER SUPPORTED ABSORPTION

WAVES

M. Hugenschmidt, W. Baca, J. Wey

To cite this version:

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JOURNAL DE PHYSIQUE CoZZoque C9, suppZ6ment au n022, Tome 42, novembre 2980, page C9-87

TRANSIENT O P T I C A L PROPERTIES OF TARGET INDUCED LASER SUPPORTED ABSORPTION WAVES

M. Hugenschmidt, IJ. Baca and J. Wey.

German-French Research Institute Saint-Louis IISL), 12, m e de Z'Industrie, 68301 Saint-Louis, France.

Abstract.- Efficient energy transfer from short duration high-power laser pulses to metallic as well as dielectric targets causes serious problems as strongly absorbing plasmas are initiated when the power density applied exceeds a threshold of the order of l o 6 to lo7 w/cm2. The purpose of our inves- tigations was to determine temporal relaxation processes of the optic transparency of plasmas produ- ced by powerful C 0 2 laser pulses. The laser system was capable of giving single or double pulses with energies (in both pulses) up to 150 J. The time delay could be varied arbitrarily. The formation and decay of the plasma absorption waves were investigated up to power densities of some 1 0 ~ w/cm2 by means of a low power c w C02 probe laser. The relaxation time the plasma needs for reattaining trans- parency which was thus determined, is a most important parameter which has to be considered if, for example, optical energy has to be transferred to a target by repetitively pulsed lasers.

INTRODUCTION

Strong thermal or mechanical effects are produced both by the interaction of continuous wave (cw) and pulsed high power laser radiation with targets such as' in the field of material processing (' 9 2 y 3 ) . For long range applications cw laser power densities are normally restricted to several

l o 3

5

up to 10 W / d . Considerably higher power densities are achievable with pulsed lasers. If the laser radiation has to be transmitted through air under atmospheric conditions, the maxinun intensities are determined by the threshold for air break-

8 9

down occurring at 10 to 10 w/cm2 for A=10.6 pin (41

This is by about five to six orders of magnitude below the values that are achieved with the high power pulsed glass or C02 laser systems used in the laser fusion program(5'6'7'8 )'9

.

The time &rations necessary for heating, melting or vaporizing a target depend on the type of material used, its optical, mechanical and thermal properties, the surface conditions and the laser power densities applied. To shorten these time dura-

tions high intensities such as attained in pulsed lasers are required. As the pulse shapes and temporal durations depend upon the type of laser (such as the wavelength) and cannot be chosen arbitrarily, the maximum admissible pulse energies that can be transferred to a target are rather limited. They are found to be rmch lower than the energies re- qujred for most applications. Larger amounts of energ\? thu; have to be delivered by trains of pulses

t i i r repetition rates of which have to be determined

or to be optimized theoretically or experimentally for each problem under consideration.

The present paper deals with experiments performed at ISL to determine the time durations that, initially, optically thick C02 laser induced ab- so~ption waves need to reattain transparency. These shall be termed relaxation times in the following. This was done by probing the plasmas generated under the action of different high energy C02 laser pulses with a low power cw CO, laser and a fast IR

detector. As pulsed C02 lasers with energies of several 10 to 100 J per pulse at high repetition rates up to several kHz or even higher are not available commercially, our approach was to conceive and to use a double pulse laser system, which is, however, different from the laser used by Hall and ~aher(lO"~)

.

The laser built up delivered two pulses with total energies up to a maximum of 150 3

(in both pulses), the temporal delay of which can be varied almost arbitrarily.

BASIC CONSIDERATIONS AND EXPERIMENTAL SET-UP Some of the most fundamental processes occurring when high power, high energy laser radiation is striking a target, that is surface heating with subsequent melting or vaporization, formation of

an absorption wave zone behind a rapidly expanding shock-front

,

are schematically shown in Fig. 1

.

De- pending upon the material parameters and the power densities applied laser induced combustion or det- onation waves are initiated, expanding with sub- sonic or supersonic velocities D, respectively(' ')

.

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JOURNAL DE PHYSIQUE pllsed C02 laser radiation \

_*

-

/ - cw C02 laser probe beam

-*

Fig. 1: Technique for determining the absorptivity of target induced laser sup- ~orted ~lasmas

Under special conditions these plasmas may ensure enhanced thermal coupling such as reported in recent papers by G. WEYL et a1 with respect to Al-targets. In most cases, however, shielding of these waves due to inverse bremsstrahlungsabsorp- tion is largely decoupling the target surface from the incident radiation field which, for special material processing applications, can be avoided by suitably chosing the focussing geometry(1 ' Fig. 2 shows the experimental set up used to de-

double- pulse C02 laser

@

,

photwdrag detector

-

\

discharge ' dis&arge

,/

attenuator

laser cawrty

I I

pmbe laser Fig. 2: Experimental set up

termine the temporal variations of the plasma absorption. The high energy pulses are delivered by a double-pulse C02 laser using two different types of high pressure (TEA) discharges inside the laser cavity. The temporal shapes as well as the energies of these pulses were measured by means of a photon drag detector and a calorimeter. The plasma transmission was probed with a continuous wave C02 laser (3 to 5 W) which had to be suitably atten- uated to avoid saturation of the fast PbSnTe detector. As already shown in Fig. I the probing beam is transmitted through a small central hole of 1.5 mm of diameter drilled in the targets. A schematic drawing of the electric circuit diagram

of the double pulse laser is given in more detail in Fig. 3. The system includes an electron beam

3 slap Marx wpp$ 3 stam Marx wstaimr

Fig. 3: Electric circuit diagram of the double pulse C02 laser

preionized and a W preionized (commercially available Lumonics, model 600) discharge section. Both components can be ignited separately within an arbitrary variable delay. The energies of the two main discharges are stored in three-stage Marx

generators (3 times 0.32 pF / 50 kV for the W preionized discharge and 3 times 1.4 pF / 24 kV for the electron beam preionized discharge). Both discharge sections have a length of about 40 an. The electrode spacing was 8 and 10 an respectively. The two laser chambers were separated by a KC1 window allowing the gas mixtures to be optimized inde- pendently. The W preionization was provided by an array of sliding sparks whereas the electron beam preionization was given by a cold cathode electron beam gun fed by a 25-stage Farx generator (25 times 8 nF / 25 kv).

The cavity consists of a totally reflecting mirror MI and a 60% partially reflecting ZnSe mirror M, both with five inches in diameter. Max-

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gering the two discharges. Similar curves are ob- breakdown builds up which is spatially separated

tained for any delay time chosen. from the absorption wave near the surface. The set-

up for measuring the transmission through these A

~ ( t ) _{plasmas is shown in more detail in Fig. 6 which also}

relat~w units lock-in 2 UV preioniad laser /

Fig. 4: Temporal shape of the high power ~b Sn ~e

50% detector

laser pulses

EXPERIEENUL RESULTS

The high energy laser pulses are focussed using a 54 an focal length ZnSe lens thus illumi- nating, on the target, an area of 1 . I 2 an2 centered around the small hole which covered only a small sized area of 1 .76-

lo-'

an2. The energy or power densities are varied by using attenuating foils placed between the laser output mirror and the KC1 prism used to split a small part of the pulsed

laser beam for measuring purposes. Without changing the geometry peak power densities could be varied from several Wl/cm2 to 100 MW/cm2, Higher values have not been considered because of the additional gas breakdown which then occurs in front of the target surface. These are lowering the energy fed to the laser induced target absorption wave zone. The time integrated photographs recording the plasma luminosities at various energy densities are illustrating this effect (Fig. 5). At the highest

UV preionized laser energy density 7 13 2 5 [~/cm2] power density 25 47 9 0 ( M W / C ~ ~ electron beam preionized laser energy density 14.3 20 4 0 b/cm2] pawar density 15 22 4 5 [ ~ w t c m q

Fig. 5: Photographic recordings of laser absorption waves on alun~inum alloy

(2024) targets

power densities of 90 MW/m2 attained in these experiments with the UV preionized laser, a gas

Fig. 6: Probing of the plasma induced chaxges of the 1R transparency at 10.6 pm

includes a typical transmission curve as observed for most types of material if a single pulse I (t)

P hits the target. The transmission curve can be seen to drop to zero nearly instantaneously if the laser power density required for absorption wave ignition is attained. Depending upon the material parameters and power or energy densities the plasma relaxation times -rRthus defined are usually much longer than the corresponding laser pulse durations.

SINGLE PULSE JEASUREMENTS

In a first series of experiments single pulse measurements were performed by separately using the laser 1 or 2 of the double pulse system. Fig. 7 shows some oscillograms as recorded when metallic

UV preionized

laser

electron heam

preionized

laser

29 J/cm2 32 MW/cm2

aluminum alloy

2024

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C9-90 JOURNAL DE PHYSIQUE

superposed. In some other experiments even more pronounced variations from shot to shot were observed. The results illustrating the relaxation time versus peak power density or energy density for an aluminum alloy (2024) target are given in Fig. 8. Strong absorption is only observed above a threshold of about 10 MkV/an2. In the power density range considered, the relaxation times for the more en- ergetic pulses of the electron beam laser are in- creasing up to 50 ys. They are thus approximately

aluminum alloy 2024 Fig. 8 10 20 targets ~ 7 . t eleclron beam preionized laser

twice as long as the times required for the plasmas produced by the shorter pulses of the W preionized laser, in spite of their higher peak power densi: ties. The target surfaces were heated and cleaned in the same way and laser pulse energies were checked in each shot. In spite of this, the temporal shapes of the absorption curves partially exhibit severe fluctuations corresponding to the statis- tical nature of the processes involved. In order to determine the maximum scattering (as given by the length of the error bars in Fig. 81, about 8 to 10 successive shots have been superposed for each energy density chosen. The same holds for the measurements given in figures 9 and 10. These figures show the results obtained with dielectric

target materials such as.glass or plexiglass. It is interesting to note that the relaxation times observed are longer than those of metallic targets. Above threshold the relaxation times of glass, for example, are rapidly growing to approximately 200 us or above. As compared to the aluminum alloy target, no marked difference was found for the two types of laser pulses. A slightly different behaviour was observed in the case of the plexiglass target which can be explained by the low transformation temper-

g l a s s

d

UV preimizad Fig. 9 10 20 30 [J/cm21 Relaxation times of C02 laser induced absorption waves on glass targets electmn beam prnicdued lasnr plexiglass Fig. 10 I 10 20 _%'cmq _{Relaxation times of} GO2 laser induced absorption waves on plexiglass targets

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intervals varying between 200 and 350 us.

I t should be mentioned here t h a t i n some experiments performed with other target nlaterials including aluminum alloy, even f o r both types of l a s e r pulses absorption curves have been recorded similar t o those described above f o r plexiglass. The long t a i l absorptivity, however, is much l e s s pronounced and often only reaches 10 or 15%. Within the range of experimental accuracy these values a r e then rather d i f f i c u l t t o be interpreted correctly. DOUBLE PULSE MEASWIENTS

I n the single pulse experiments described i n the previous section, the relaxation times were determined a s a function of the power densities f o r d i f f e r e n t t a r g e t materials and various l a s e r pulse shapes. In addition, by the use o f the double pulse l a s e r f a c i l i t y , it was possible t o investigate the mwtual influence of two subsequent pulses as a function of the delay time. Some r e s u l t s of the double pulse interaction with an aluminum alloy (2024) target are put together i n Fig. 11. Total energies i n both pulses are 78 J , about 65% of t h i s

of 25 J/un2, respectively, these plasmas now need about 40 t o 50 us t o become transparent. Measurements a t even shorter pulse delay times down t o 50 u s

show t h a t these second pulse relaxation times do not change noticeably.

100% Double pulse l a s e r

50% plasma induced ab-

0% absorption sorption changes _{on glass} 32 J/div

glass

Some f u r t h e r r e s u l t s obtained under t h e same experimental conditions f o r glass and plexiglass t a r g e t s a r e given i n Fig. 12 and Fig. 13, respectively. Within the experimental limits of accuracy the relaxation times of both materials correspond t o those values obtained from the s i n g l e elcposure measurements. The relaxation times of the second energy thereby being provided by the electron beam

preionized l a s e r discharge which was triggered f i r s t .

plexiglass

aluminum alloy

2024

Fig. 1 1 : Double pulse measurements,absorption changes produced on Al-alloy 2024 t a r g e t s

Temporal delays between the two pulses were chosen t o be 200 us and 500 u s . A s compared t o the relax- a t i o n times observed with metals, these times a r e r a t h e r long. I n both cases the o p t i c a l disturbances produced by the f i r s t pulse seem t o have dropped t o a negligible value when the second pulse i s applied. It can c l e a r l y be seen, however, t h a t the second plasma relaxation occurs i n a somewhat slower way as should be expected from Fig. 7. A t incident power d e n s i t i e s of 90 MW/an2 o r energy d e n s i t i e s

absorption 32 ~ / d i v absorption 32 J/div Fig. 13 Double pulse l a s e r plasma induced absorption changes on plexiglass

plasmas a r e only s l i g h t l y longer. Experimental v e r i f i c a t i o n i s r a t h e r d i f f i c u l t as the PbSnTe- detector element i s very small (sensitive area 3 x a?), so t h a t p a r t of the strong fluctuations observed on the transmitted cw probe l a s e r signals especially a t l a t e times night originate from beam deflections due t o r e f r a c t i v e index changes.

SUPERPOSITION OF CONTINUOUS WAVE AND PULSED LASER RADIATION

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C9-92 JOURNAL DE PHYSIQUE

pulses whose power densities are about 4 to 5 orders paint. This can be seen in comparing the oscillo- of magnitude higher. By using a Set-Up as shown in grams of Fig. 16. The additional cw radiation in- Fig. 14, the influence of such an additional con- creases the relaxation times up to several hundreds tinuous wave radiation on plasma absorption induced _{of us, thereby once more a longer lasting, only}

slowly decreasing low absorption tail being found in some of the shots.

double pulse laser system

Fig. 14: Set-up for combined cw-pulsed laser beam target interaction

by the laser pulses has therefore been investigated. By means of a 200 W laser, power densities up to a little more than 2 kW/an2 could be achieved on small areas of about 3 mm in diameter. cw power

could be switc&ed on by means of an electro-mechanical shutter inside the laser cavity. The continuous wave preheating times Atl and the pulse separa- tions At2 were varied and checked electronically. Fig. 15 gives a comparison of the double pulse induced 'plasma absorption with and without superposition of continuous wave radiation. Preheating times were chosen to be 2 s, at power densities

?

,

=2.103 W/cm2. Some of the oscillograms show also the above mentioned weak tendency for a longer lasting lower value absorption tail. But the dif-

double pulse double pulse + c w

-

IOOps/d~v

-

100ps/d~v

aluminum alloy 2024

Fig. 15: Comparison of double pulse plasma induced absorption on Al-alloy with and without superposition of 2 kW/an2 cw radiation ,

ferences between the measurements where cw radiation is applied or not, are negligible small and can be considered to be within the experimental uncertain- ty. A marked difference, however, is observed if the aluminum alloy surface is covered with black

double pulse double pulse + c w

aluminum alloy 2024 black painted

Fig. 16: Double pulse plasma induced absorp- tlon on Al-alloy black painted targets with and without superposition of 2 kW/cm2 cw radiation

A similar behaviour is observed with dielectric target materials. Fig. 17 gives some oscillograms as obtained with glass. These curves have to

double pulse + c w

glass

be compared with those given in Fig. 12 which, however, were recorded with a time scale of

50 psfdiv. The cw preheating time had to be reduced to about 0.3 s to avoid destruction of the glass targets by thermally induced mechanical shocks before the pulsed laser radiation is superimposed. The relaxation times of both plasmas produced by

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delays of 500 ps between the two pulses a r e considered. For the shorter delay time of 200 ys i n the lower oscillogram it takes about 800 u s f o r the plasma t o a t t a i n f u l l transparency. These r e s u l t s a r e comparable with those obtained with other d i e l e c t r i c materials such a s , f o r example, plex- i g l a s s , whereby the cw preheating time had t o be reduced t o 0.1 s t o avoid complete burnthrough of the 2 mm thick t a r g e t p l a t e s .

CONCLUDING ' lEM4FX.S

The experiments performed a t ISL, some r e s u l t s of which a r e described i n the present paper, give some insight i n t o the o p t i c a l properties of l a s e r induced absorption waves on d i f f e r e n t t a r g e t mate- r i a l s . A s compared t o our previous single pulse measurements where mainly high speed photographic techniques have been used t o determine the thermo- dynamic properties of l a s e r absorption waves such as the s p e c i f i c i n t e r n a l energy o r the mechanical pressures(16) the IR e l e c t r o o p t i c a l measurements y i e l d information on the rapidly time-varying plas-

ma absorptivities.

The relaxation times of metals such as f o r the aluminum a l l o y 2024 a r e rather short, they are of the order of 20 t o about 50 ps a t power d e n s i t i e s up t o 100 hIw/an2. An additional lower paver density of 2 kw/an2 has an influence which is nearly negligible. A d i f f e r e n t behaviour is observed with die- l e c t r i c t a r g e t s i n which the relaxation times over the power density range considered grow up t o a t t a i n several hundreds of ys. For these materials t h e continuous wave power considerably a l t e r s the temporal behaviour of the plasma opacities. For glass t a r - g e t s , f o r example, values up t o 800 ~s were found.

To our knowledge such relaxation times have not been reported so f a r i n the l i t e r a t u r e . They are most important parameters as they may s e t an

upper limit t o maximum pulse r e p e t i t i o n r a t e s i f t r a i n s of l a s e r pulses s h a l l be applied t o improve l a s e r beam target interaction.

REFERENCES 1) J.F. READY

Effects of High-Power Laser Radiation Acad. Press, New York, London (1971)

3) T.P. HUGHES

Plasmas and Laser Light Adam Hilger, Bristol (1975) 4) D. DUFRESNE

ThSse present6e 2 11universit6 d'Aix-Marseille (pour obtenir l e grade de Docteur 5s-Sciences) (1 978) 5) H. G. AHLSTROM Entropie N r . 89-90 p. 19 (1979) 6) A. BEKIARIAN, M. DECROISETTE Entropie N r . 89-90 p. 36 (1979) 7) J. VALENSI, G. INGLESAKIS Entropie N r , 89-90 p. 105 (1979) 8) C. FENSTERMACKER Entropie N r . 89-90 p. 53 (1979) 9) V.A. ADAFIOVICH e t a l . Applied Optics

19,

p. 918 (1980) 10) R.B. HALL, W.E. biWER, D . J . NELSON

AFWL-TR-73-296, Report N r . AD 72 4868 (1974) 11) R.B. HALL, W.E. E@WER, P.S. VEX

AFWL-TR-75-342 I1 Report N r . AD A032454 (1976) 12) P. BOURNOT, P.A. PINCOSY, G. INGLESAKIS,

M. AUTRIC, D. DUFRESNE, J.P. CAQESSA Acta Astronautics

5,

p. 257 (1979) 13) G. hEYL, A. PIPAI, R. ROOT

Physical Sciences Inc. Report SR-69 (1980) 14) Ven H. SWI, B. KIVEL, G.M. VJEYL

J. Quant. Spectrosc. Radiation Transfer 20, p. 627 (1978)

-

1 5) E

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S-R, M. von ALLMEN

I n s t . f . angew. Physik Universitat Bern, Forschungsbericht TA-2, FOP-3 (1977) 16) M. HU(;ENSCHMIIYT, K . VOLLRATH, W. BACA

Deutsch-Franzijsisches Forschungsinstitut St-Louis

-

Bericht R 118/78 (1978).

2)' S.S. CHARSCHAN Lasers i n Industry