INVESTIGATION OF LASER SUPPORTED DETONATION WAVES AND THERMAL COUPLING USING 2.8µm HF LASER IRRADIATED METAL TARGETS

HAL Id: jpa-00220564

https://hal.archives-ouvertes.fr/jpa-00220564

Submitted on 1 Jan 1980

HAL is a multi-disciplinary open access

archive for the deposit and dissemination of

sci-entific research documents, whether they are

pub-lished or not. The documents may come from

teaching and research institutions in France or

abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est

destinée au dépôt et à la diffusion de documents

scientifiques de niveau recherche, publiés ou non,

émanant des établissements d’enseignement et de

recherche français ou étrangers, des laboratoires

publics ou privés.

INVESTIGATION OF LASER SUPPORTED

DETONATION WAVES AND THERMAL COUPLING

USING 2.8µm HF LASER IRRADIATED METAL

TARGETS

B. Deka, P. Dyer, J. Sayers

To cite this version:

INVESTIGATION OF LASER SUPPORTED DETONATION WAVES AND THERMAL

COUPLING USING 2.8um HF LASER IRRADIATED METAL TARGETS

B.K. Deka, P.E. Dyer and J.A. Sayers

Department of Applied Physios, University of Hull, Hull, HU6 7RX, England.

Abstract.- The formation and propagation of laser supported detonation (LSD) waves and thermal cou-pling for HF laser irradiated solid targets have been investigated as a function of target material, irradiance and ambient pressure. High speed photography has been employed to study the plasma dyna-mics and the thresholds for plasmotron and LSD wave production obtained. For the 300nsec duration

CFWHM) pulses and l-2mm diameter focal spots used, the LSD thresholds for aluminium, stainless steel and platinum targets were measured to be 0,(2-3) x 108 W.cm-2. Above the LSD threshold the initial expansion velocity showed good agreement with the I1'3 irradiance dependence predicted by theory. Additional data on the interaction for platinum were obtained using thermocouples to measure the thermal coupling to the target as a function of spot size, irradiance and ambient pressure. A maxi-mum thermal coupling coefficient of 14-18% was measured at an irradiance level slightly below the LSD wave ignition threshold.

1. Introduction 2. Experimental technique

There is considerable interest in the inter- The pulsed HF chemical laser used in this action of high power, pulsed infrared gas lasers investigation was a pin-bar device and was with metallic targets in gaseous backgrounds. Both operated with SF_ - C.H. gas mixtures at ^ 40-60

b So

C0„ and HF chemical lasers offer high power and torr. With a stable resonator operating multi-energy capabilities, and whilst extensive studies mode, a laser multi-energy of o, 8J was obtained in a of laser supported detonation waves and thermal o. 300 nsec. pulse (FHHM). The laser output was

(1-7)

coupling have been made at 10.6pm there has focused onto various targets using a plano-convex been little published work on experiments at NaCl lens (20 or 50cm foaal length). A direct

2.8um ' . In this paper, we report a study of measurement of the spot size was obtained by the ignition and propagation of laser supported attenuating the beam and irradiating various thin detonation (LSD) waves produced from HF laser metal foils and developed unexposed Polaroid film irradiated metal targets (aluminium, stainless located at the position of the target. A high steel and platinum) and present thermal coupling speed streak camera was employed to study the plas-data for platinum as a function of target irrad- ma dynamics.

iance, ambient pressure and spot size. The experimental arrangement used to measure the thermal coupling coefficient is shown in Fig.l. Résumé.- La formation et la propagation de vagues de détonation supportées par laser (LSD) et

l'ac-couplement thermique pour les cibles solides irradiées par laser HF ont été étudiées en tant que fonction d'un matériau de cible, de l'irradiation et de la pression ambiante. La photographie à grande vitesse a été utilisée pour étudier la dynamique de plasma et les seuils auxquels le plasmo-tron et la production de vagues LSD sont obtenus. Pour les impulsions d'une durée de 300nsec (FWHM) et les points focaux de l-2mm qui ont été utilisés, le seuil LDS pour les cibles d'aluminium, d'a-cier inoxydable et de platine mesurait ^(2-3) x 108 W.cm-2. Au-dessus du seuil LSD, la vélocité

d'ex-pansion initiale semblait conforme à la dépendance d'irradiance prédite par la théorie.

Des données supplémentaires sur l'action réciproque du platine ont été obtenues en utilisant des thermocouples pour mesurer l'accouplement thermique de la cible en tant que fonction de la taille du point, l'irradiance et la pression ambiante. Un coefficient maximum de 14-18% a été mesuré à un niveau d'irradiance légèrement au-dessous du seuil de l'ignition de la vague LSD.

C9-76 JOURNAL DE PHYSIQUE

NaCl beam splitter Pt Chrornel-alumel target ThermocouDle - . - . . . . - . . - . - - -

u-1

Telequlpment Screened enclosure JIPS F i g u r e 1 Experimental c o n f i g u r a t i o n f o r thermal coupling s t u d i e s . J - Joulemeter, PS - p u l s e hhape monitor.

The temperature r i s e of t h e t a r g e t was measured u s i n g a s i n g l e thermocouple of chromel-alumel wire s p o t welded o n t o t h e r e a r s u r f a c e of t h e t a r g e t . The time i n t e g r a t e d thermal coupling c o e f f i c i e n t ,

a , corresponding t o a temperature r i s e T, was d e f i n e d a s a = p d ~ c ~ / E , where p i s t h e d e n s i t y of t h e t a r g e t m a t e r i a l , d t h e t a r g e t t h i c k n e s s , C t h e s p e c i f i c h e a t c a p a c i t y , A t h e t a r g e t a r e a and E t h e l a s e r energy i n c i d e n t on t h e t a r g e t . 3 . Experimental r e s u l t s ( a ) Plasma S t u d i e s

F i g s . 2a-b show t y p i c a l s t r e a k photographs

Figure 2 S t r e a k photographs of plasmas-produced from an a l u m i n i m t a r g e t i n atmos2heri.c a i r . I r r a d i a n c e ". 3.5 x 108 ~ . c m - ~ ; ( a ) camera a p e r t u r e f/22, 10 db ND f i l t e r , ( b ) same a s ( a ) , 12 db ND

f i l t e r .

o b t a i n e d , under two c o n d i t i o n s of exposure, u s i n g a p o l i s h e d aluminium t a r g e t . I t can b e seen t h a t f o l l o w i n g i g n i t i o n t h e plasma became r a p i d l y detached from t h e t a r g e t i n a manner c h a r a c t e r i s t i c of LSD wave propagation. The i n i t i a l expansion

-1

v e l o c i t y was e s t i m a t e d t o be a 2 x 106 cm.sec

_,

and was found t o be i n r e a s o n a b l e agreement with t h a t o b t a i n e d from t h e LSD wave modelfl1). I t should be noted t h a t i n c o n t r a s t t o t h e i n t e r a c t i o n w i t h pulsed C02 l a s e r r a d i a t i o n a t s i m i l a r

i r r a d i a n c e ' l )

,

t h e r e was no subsequent r e c o u p l i n g of t h e l a s e r r a d i a t i o n t o t h e t a r g e t . However, when t h e t a r g e t i r r a d i a n c e was reduced t o '1. 9.6 x

7 -2 ( 2 )

1 0 W.cm o n l y a s u r f a c e plasma, a p l a s m o t r o n

,

extending t o t h e e n t i r e d u r a t i o n of t h e l a s e r p u l s e could be seen. The t h r e s h o l d f o r t h e t r a n s - i t i o n from t h e plasmotron t o t h e LSD regime was

-2

e s t i m a t e d t o be ( 1 . 6 - 2.0) x

lo8

W.cm

.

This can be compared with a LSD t h r e s h o l d o f a 2 x 1 0 7

-2

W.cm a t 10.6pm under comparable c o n d i t i o n s . For t a r g e t induced breakdown a t h r e s h o l d o f a 2 x 10'

-

2 N.cm was o b t a i n e d , t h i s a g r e e i n g reasonably w e l l 7 -2 w i t h t h e v a l u e o f 6 x 1 0 W.cm o b t a i n e d from t h e (12) v a p o r i s a t i o n

-

cascade i o n i s a t i o n model

.

Figs. 3 , 4 show s t r e a k photographs o b t a i n e d

when p o l i s h e d s t a i n l e s s s t e e l t a r g e t s were i r r a d i a t e d a t a l e v e l of % 1 x

lo8

and 2.3 x

lo8

-2

W.cm

,

t h e s e r e p r e s e n t i n g t h e approximate t h r e s h - o l d s f o r plasmotron and LSD formation r e s p e c t i v e l y .

F i g u r e 3 S t r e a k photograph of a s t a i n l e s s s t e e l plasmotron i n atmospheric a i r . I r r a d i a n c e 2.

F i g u r e 4 S t r e a k photographs o f plasma production from a s t a i n l e s s s t e e l t a r g e t i n atmospheric a i r . I r r a d i a n c e 2. 2.3 x

lo8

\ l . ~ m - ~ ; camera a p e r t u r e f/22; ( a ) 6 db and ( b ) 1 0 d b f i l t e r s r e s p e c t i v e l y . Comparison w i t h t h e aluminium d a t a i n d i c a t e d t h a t while i n b o t h c a s e s LSD i n i t i a t i o n o c c u r r e d a t s i m i l a r l e v e l s of i r r a d i a n c e , t h e t h r e s h o l d f o r

plasmotron was somewhat higher f o r s t a i n l e s s s t e e l .

Above t h e LSD t h r e s h o l d t h e i n i t i a l expansion

v e l o c i t y showed a n approximate dependence on

i r r a d i a n c e ( F i g . 5 ) , t h e r e being no s i g n i f i c a n t

d i f f e r e n c e between t h e d a t a f o r aluminium and

s t a i n l e s s s t e e l .

ed a s a f u n c t i o n of a i r p r e s s u r e i n t h e range 20-

760 t o r r . This showed t h a t t h e r e was some reduc-

t i o n i n t h e plasmotron t h r e s h o l d a s t h e p r e s s u r e

was d e c r e a s e d , while t h e LSD t h r e s h o l d remained

e s s e n t i a l l y unchanged. Above 100 t o r r t h e i n i t i a l

expansion v e l o c i t y e x h i b i t e d a p -0.32 p r e s s u r e

dependence ( F i g . 6 ) , t h i s being i n v e r y good agree-

ment with theory('')

10'1 , , * , I J

10 10' lo2 to3

Pressure (tor?)

Figure 6 I n i t i a l expansion v e l o c i t y a s a f u n c t i o n of p r e s s u r e a t c o n s t a n t i r r a d i a n c e .

Fig. 7 shcws a s t r e a k photograph of t h e plasma produced when an unpolished platinum t a r g e t was

8

i r r a d i a t e d a t 2. 1 . 3 x 1 0 ~ . c r n - ~ ; a LSD wave can

be s e e n , t o g e t h e r w i t h a weak plasmotron. From

s t r e a k photographs o b t a i n e d under v a r i o u s condit-

-

0 2 4

6

8 1 0 ( x 1 0 0 n s ) F i g u r e 5 I n i t i a l plasma expansion v e l o c i t y a s a f u n c t i o n of t a r g e t i r r a d i a n c e . 2 x - Aluminium ( s p o t a r e a 2. 0.036 cm )

Figure 7 S t r e a k photograph of plasma produced from

0 ₂

A - S t a i n l e s s s t e e l ( s p o t a r e a 2. 0.036 cm ) a platinum t a r g e t i n a i r a t atmospheric p r e s s u r e .

C9-78

JOURNAL DE PHYSIQUE

i o n s , t h e plasmotron threshold was estimated t o be 8 -2

% 1.2 x 10 W-cm

.

The i n i t i a l expansion velocity

showed an ' i r r a d i a n c e dependence s i m i l a r t o t h a t

obtained with aluminium and s t a i n l e s s s t e e l (Fig.5).

(b) Thermal coupling

I n Fig. 8 t h e thermal coupling c o e f f i c i e n t , a,

f o r platinum i s shown a s a f u n c t i o n of l a s e r -2

f l u e n c e (J.cm ) f o r t h r e e t a r g e t diameters, t h e

r a t i o o f t h e f o c a l s p o t t o t a r g e t diameter i n each

case being held approximately constant. I t was

found t h a t t h e coupling c o e f f i c i e n t i n i t i a l l y

increased a s t h e l a s e r f l u e n c e was increased,

reaching a peak of 2. 14

-

18% a t o r near LSD t h r e s h o l d fluence. Thereafter, t h e coupling

decreased with i n c r e a s i n g f l u e n c e due t o t h e more

r a p i d movement of t h e plasma away from t h e t a r g e t

i n a LSD mode. Measurements performed using

v a r i o u s f o c a l s p o t s i z e s and t a r g e t diameters

i n d i c a t e d no s i g n i f i c a n t dependence of t h e therma

coupling on t h e s e parameters. I t i s worth noting

t h a t coupling measurements a t 1 0 . 6 ~ 1 ~ have shown

t h a t i n one c a s e an i n c r e a s e i n t h e s p o t s i z e

Figure 8 Thermal coupling c o e f f i c i e n t a s a funct- i o n of l a s e r fluence.

Target diameter Spot diameter

(mm) (mm)

m

3

.o

1.1

4.8 2.1

A

8.1 3.3

r e s u l t e d i n a marginal enhancement i n t h e peak

coupling(4) whereas i n another i n s t a n c e ( 6 ) a s l i g h t

r e d u c t i o n occurred.

I n Fig. 9 t h e l a s e r i r r a d i a n c e corresponding t o

peak coupling i s shown a s a function o f t h e s p o t

s i z e , t o g e t h e r with t h e o r e t i c a l l y p r e d i c t e d r e s u l t s

( s o l i d curves) f o r t h e LSD maintenance threshold(13!

I t can be seen t h a t t h e experimental p o i n t s a r e i n

c l o s e proximity of t h e t h e o r e t i c a l curve, t h i s

i n d i c a t i n g t h a t t h e optimum i r r a d i a n c e f o r peak

thermal coupling i s approximately t h e minimum

i r r a d i a n c e required t o maintain a LSD wave. This

i s i n broad agreement with previous r e s u l t s obtain- ed using 1.06 (14) and 10. 6 ~ m ' ~ ) pulsed l a s e r s . A

c l o s e examination of t h e s t r e a k photographs showed

t h a t t h e fluence corresponding t o peak coupling was

l o c a t e d i n t h e very narrow regime bounded by t h e

plasmotron and LSD thresholds.

- Theory

Figure 9 I r r a d i a n c e a t peak thermal coupling a s a function of f o c a l s p o t radius.

I n Fig.10 t h e v a r i a t i o n of thermal coupling

with fluence ( f o r a 3mm diameter t a r g e t and -I,l m m s p o t ) i s shown f o r p r e s s u r e s of 760 and 100 t o r r .

There was no s i g n i f i c a n t d i f f e r e n c e between t h e two

cases, t h e peak coupling being 'l. 15 - 16%. The

increased LSD wave velocity (Fig. 6). In Fig. 11 the thermal coupling coefficient is shown as a function of pressure at a fixed laser fluence of

-

2

% 100 J.cm

.

It can be seen that the coupling

coefficient showed little variation in the range 760

-

40 torr, but increased slightly as the pressure was reduced to 10 torr. These results are in general agreement with previous results reported for 1.06 (I4) and 10.6um (4 6, laser radiation.

.

70" t o m a,? loo torr aqr

The formation and propagation of laser supported detonation waves produced when the radiation from a pulsed, 'L 8J HF laser was focused onto metal

targets, have been investigated as a function of various parameters such as target irradiance and ambient pressure. For 300 nsec (FWHM) duration pulses and 1

-

2 mm diameter spots employed in

8

these experiments LSD thresholds of Q (2-3) x 10

-

2

W.cm were obtained for aluminium, platinum and stainless steel. When the target was irradiated at or above the LSD threshold irradiance, a rapid decoupling of the laser radiation from the target occurred and in contrast to the case of target interaction using C02 laser radiation there was no subsequent recoupling of the laser to the target.

The thermal coupling of 2.8pm laser radiation

Figure 10 Thermal coupling coefficient as a function of fluence; 3mm diameter platinum target in air at pressures of 760 and 100 torr.

o I n I n I . ~ ~ I ~to platinum was investigated as a function of I ~ ,

0 40 80 120 (60 200 240 280 Fluence u cm.9

spot size, target irradiance and ambient pressure. The maximum thermal coupling coefficient was measured to be 'L 14

-

18%, and the irradiance at

peak coupling found to be approximately the same as, or slightly less than, the LSD formation threshold. It should be noted that the 14

-

18% coupling coefficient obtained in this study is considerably lower than that reported for various metals at 10.6pm (4p6)

.

Unfortunately, the lack of 10.6wm coupling data for platinum at this stage preclude a direct comparison.

References

0 100 200 300 400 500 600 700

AIC pressure (to?r) 1. W.E. Maher,R.B. Hall and R.R. Johnson, J. ~ppl.

Phys. 45, 2138, 1974.

2. A.I. Barchukov, F.V. Bunkin, V.I. Konov and A.A. Lyubin, Sov. Phys. JETP, 39, 469, 1974. Figure 11 Variation of thermal coupling coeffic-

ient with air pressure. 3mm diamter platinum _{3. W.E. Maher and R. B. Hall, J. Appl. Phys., 47,} target and l.lm diameter focal spot. Laser 2486, 1976.

JOURNAL DE PHYSIQUE

4. S. Marcus, J.E. Lowder and D.L. Mooney,

J.

Appl. Phys. 47, 2966, 1976.

5. R.E. Beverly 111 and C.T. Walters, J. Appl. Phys., 3485, 1976.

6. W.E. Maher and R.B. H a l l , J. Appl. Phys., 49, 2254. 1978.

7. C.T. W a l t e r s , R.H. Barnes and R.E. Beverly 111,

J. Appl. Phys., 49, 2937, 1978.

8. D.B. Nichols and R.B. H a l l , J. Appl. Phys. 49, 5155, 1978.

9. W.E. Maher and R.B. H a l l , J: Appl. Phys. 51, 1980

10. B.K. Deka and P.E. Dyer, Gas Flow and Chemical L a s e r s , J.F. Wendt, e d . , 465, Hemisphere, New York, 1979.

11. S.A. Ramsden and P. S a v i c , Nature, 203, 1217, 1964.

12. D.C. Smith, J. Appl. Phys., 48, 2217, 1977. 13. C.O. Allingham and H.V.H. Bishop, AWRE Report

LDPN/12/77, 1977, (unpublished)

.