A THEORETICAL AND EXPERIMENTAL STUDY OF A C.W.HC1 CHEMICAL LASER

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A THEORETICAL AND EXPERIMENTAL STUDY OF A C.W.HC1 CHEMICAL LASER

H. Brunet, M. Mabru, P. Chauvet, J. Rocca Serra, P. Vincent

To cite this version:

H. Brunet, M. Mabru, P. Chauvet, J. Rocca Serra, P. Vincent. A THEORETICAL AND EXPER-

IMENTAL STUDY OF A C.W.HC1 CHEMICAL LASER. Journal de Physique Colloques, 1980, 41

(C9), pp.C9-25-C9-29. �10.1051/jphyscol:1980904�. �jpa-00220558�

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JOURNAL DE PHYSIQUE Colloque C9, supplément au n°ll, Tome 41, novembre 1980, page C9-25

H. Brunet, M. Mabru, P. Chauvet, J. Rocca Serra and P. Vincent.

Laboratoires de Marcoussis, Centre de Recherches de la Compagnie Générale D'Electricité, Division Lasers, Route de Nozay, 91460 Marcoussis, France.

Résumé - Un modèle de calcul pour un laser chimique HC1 continu à initiation électrique est présenté. Ce modèle utilise les divers processus chimiques et moléculaires nécessaires à une description complète des performances laser suivant la direction de l'écoulement. Les valeurs approchées des degrés de dissociation et d'excitation vibrationnelle de H„ à la sortie des tubes à décharge sont obtenues à partir d'une analyse numérique bidimensionnelle de la décharge auto-entretenue. L'importance de la réaction Cl + H?(V - 1) -* HC1(V = 1) + H sur le coefficient d'amplification de la bande 1-0 est soulignée. La distribution spectrale et la puissance laser totale ont été mesurées. Un accord satisfaisant entre les valeurs expéri- mentales et celles prédites par le calcul est observé.

A THEORETICAL AND EXPERIMENTAL STUDY OF A C.W.HC1 CHEMICAL LASER

Abstract - A computer model for an electrically initiated cw HC1 chemical laser is presented.

This model employs finite kinetics for the various pumping and molecular energy transfer processes required for a complete description of the laser performance along the flow direction.

Reasonable estimates for the degrees of dissociation and vibrationnal excitation of H„ at the exit of the discharge tubes are obtained from a two-dimensional computer analysis of the self-sustained discharge. Emphasis is made on the influence of the reaction

CI + H„(v = 1)—» HCl(v = 1) + H on the gain coefficients for the 1-0 vibrational band. The output spectrum and the total output laser power was measured. Good agreement between experimental measurements and the computer simulation is observed.

INTRODUCTION - This work presents laser performance 1. Elzctxlc diAChaAQe. - The flow composition at the predicted by a computer code developped as a part of exit of the discharge tubes is obtained from a two- package of programs designed to characterize elec- dimensional computer analysis of the self-sustained trically initiated chemical lasers from the dischar- discharge (1). A program has been established to pre- ge tubes through the laser cavity. diet the discharge voltage, the H-atom and HL(v = 0

The model predicts the performance of a subso- to 3) molecule flow rates as a function of pressure, nic HC1 chemical laser where excited HC1 molecules flow velocity, length and diameter of the discharge are produced by the reaction of H atoms with Cl„ tube and current. Fig.l shows for example the results molecules. The H atoms are produced by the dissoci- obtained for pure H„ at a pressure of 15 Torr.

ation of H„ molecules in an electric discharge. Ins- tantaneous mixing of the reactants is assumed.

MODELING - The one dimensional coupled-fluid-chemis- try model requires the following inputs before the small-signal gain or the laser output power may be computed : 1/ composition, temperature and mass flow rate at the exit of the discharge tubes. 2/ de- finition of the entrance conditions to the laser cavity, i.e, composition, pressure, temperature and Mach number, 3/ cavity geometry, 4/ chemical kinetics and 5/ mirror reflectivities.

F-ig. f - H-atom and H„[v)-mole.culz llow KoJbu,

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980904

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C9-26 JOURNAL DE PHYSlQUE

2 . Chemicd k i n e ; t i c ~

-

Chemical r e a c t i o n s b o t h r e d i s t r i b u t e t h e v i b r a t i o n a l e x c i t a t i o n which i s g e n e r a t e and d e s t r o y v i b r a t i o n a l l y e x c i t e d H C 1 mole- formed i n i t i a l l y by chemical r e a c t i o n s . The most c u l e s on comparable time s c a l e s w h i l e v a r i o u s energy i m p o r t a n t of t h e s e p r o c e s s e s a r e g i v e n i n Table I t r a n s f e r p r o c e s s e s (V-T, R , V-V) t e n d t o d e g r a d e and a l o n g w i t h t h e r e l e v a n t r a t e s ( 2 - 1 4 ) .

Table 1 : k i n e t i c h paoce~neo and aateo w e d i n M e cdcu&ztLon

I

^PROCESSES RATE COEFFICIENT (CM~MOLECULES-1 5 ~ C - l )

I

^RENCES^REFE-

k ( v )

=

1.2 x 10-lo g ( v ) exp(- 600/T) ; g ( v ) = 0.12 0.42,0.39,0.04, 0 . 0 0 2 f o r ( v )

=

l t o 6 , r e s p e c t i v e l y k = 3.2 x 10-l1 exp(- 600/T).

k = 1 . 6 x 10-l1 g ( v ) ( ~ / 3 0 0 ) ~ ; g ( v ) = 0 , 1 , 2 , 2 , 2 , 2 f o r v = 1 t o 6 , r e s p e c t i v e l y .

k ( v ) = 1.0 x

lo-''

g ( v ) ( ~ / r o o ) 2 ; ? ( v ) = 0,1,4.7,4.

5.0,S.O f o r v = l t o 6 , r e s p e c t i v e l y . k ( v ) : 1 . 3 x v(T/300

+

300/T).

k ( v ) = 2.3 x 10- 11 g ( v ) e x p ( - 325/T) ; g ( v ) = 1 . 0 , 1 . 2 , 1 . 4 , 1 . 4 , 1 . 4 , 1 . 4 f o r v = 1 t o 6 , r e s p e c t i v e l y k ( v ) = 2.8 x 10-l1 exp( - 325/T) ; g ( v ) = 1 . 0 , 3.3,8.3,9.0,9.0,9.0 f o r v = 1 t o 6 , r e s p e c t i v e l y k ( v ) = 2.8 x 10-11 g ( v ) exp(- 1360/T),g(v) = 1 . 0 , 5.7, 29,110,110 f o r v = l t o 6 , r e s p e c t i v e l y . k(2,O) = 2 . 3 x 1 0 - ~ ~ ( 3 0 0 / ~ ) .

k = 4.4 x 1 0 - 1 4 ( 3 0 0 / ~ ) .

O t h e r minor p r o c e s s e s such a s H and C1-atom recom- b i n a t i o n , V-V exchange i n Hz and slow V-T,R proces- s e s a r e a l s o i n c l u d e d i n t h e c a l c u l a t i o n and w i l l b e d e t a i l e d elsewhere.

3 . La&% emhnian - The magnitude o f t h e i n t r a c a v i t y f l u x i s e v a l u a t e d under t h e g a i n e q u a l s - l o s s condi- t i o n w i t h o u t c o n s i d e r a t i o n o f any c a v i t y modes.

R o t a t i o n a l e q u i l i b r i u m is assumed. Consequently l a s e r e m i s s i o n o n each v i b r a t i o n a l bands i s allowed t o o c c u r o n l y on t h e P-branch l i n e e x h i b i t i n g maximum g a i n . E x p e r i m e n t a 1 d a t a a r e used f o r t h e s p e c t r o s c o - p i c c o n s t a n t s , l i n e w i d t h s and o s c i l l a t o r s t r e n g t h s ( 1 6 ) ( 1 7 ) ( 1 8 ) .

RESULTS

-

The r e s u l t s o f c a l c u l a t i o n f o r a t y p i c a l c a s e a r e summarized i n Fig. 2-6. I n p u t c o n d i t i o n s a r e t h e f o l l o w i n g s :

H-atom c o n c e n t r a t i o n : H/H2

=

0.56

X,

H ₂v i b r a t i o n a l t e m p e r a t u r e : Tv = 2600 K , composition : Hz-H-C12 = 0.885 - 0.005 - 0.110, c a v i t y p r e s s u r e : 7 T o r r ,

c a v i t y t e m p e r a t u r e : 400 K . Mach number : 0.76

Fig.2 r e p r e s e n t s t h e e v o l u t i o n o f t h e s m a l l - s i g n a l g a i n a s t h e flow p r o g r e s s e s downstream.Note t h a t t h e g a i n f o r t h e 1-0 band is p o s i t i v e up t o 3 cm down- s t r e a m o f t h e C12 i n j e c t i o n h o l e s and peaks a t

5

x = l cm. The g a i n f o r t h e 2-1 and 3-2 bands i s l o w e r

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t h a n t h a t f o r t h e 1-0 band and l i m i t e d t o a r e g i o n

c l o s e t o t h e C12 i n j e c t i o n h o l e s . H C L LASER

-

^{G A I N}

ALPHA (CM-1)

"

1

H C L LASER - G A I N

Fig.2

- Sm&-nignd g a i u doh Tv = 2600 K F i g . 3 r e p r e s e n t s t h e l a s e r o u t p u t i n t e n s i t i e s a l o n g t h e f l o w d i r e c t i o n . Most o f t h e l a s e r power ( a b o u t 65 L) a r i s e s from t h e 1-0 band.

HCL LASER - LASER I N T E N S I T Y 1 - L-8 TRANSITION

P - 2-1 TRANSITION 3 - 3-2 TRANSITION

I n o r d e r t o d e t e r m i n e t h e i n f l u e n c e o f r e a c t i o n 2 i n v o l v i n g e x c i t e d H2(v = 1 ) m o l e c u l e s and C1 a t o m s , a c a l c u l a t i o n was p e r f o r m e d assuming Tv = 400 K , Hz (l)%!O.Results a r e i n d i c a t e d by F i g . 4 and F i g . 5 . N o t e t h a t i n c o n t r a s t t o t h e c a s e where T v = 2600 K ( H 2 ( 1 ) / H 2 ( 0 ) = 0 . 1 ) , i t i s now t h e 2-1 band which e x h i b i t s t h e maximum g a i n and l a s e r i n t e n s i t y . Note a l s o t h a t t h e l a s i n g r e g i o n is much s h o r t e r t h a n t h a t f o r Tv = 2600 K.

H C L LASER - LASER I h T E H S I T Y 1 - 1-8 TRANSITION

2

-

2-1 7RANSITION 3 - 3-2 TRANSITION

X <CH>

- Lanu

in;tenn.itia doh Tv = 400 K COMPARISON WITH EXPERIMENT

-

The t r a n s v e r s e f l o w l a s e r c o n s i s t s o f an a r r a y o f d i s c h a r g e t u b e s , two C12 i n j e c t o r s l o c a t e d o n e a c h s i d e o f t h e hydrogen s t r e a m and an o p t i c a l c a v i t y . The C l i s i n j e c t e d

2

t h r o u g h small h o l e s o r i e n t e d s o t h a t t h e i n j e c t i o n a n g l e i s l a r g e w i t h r e s p e c t t o t h e hydrogen f l o w d i r e c t i o n . T h i s t y p e o f i n j e c t i o n a l l o w s a r a p i d mixing o f t h e r e a c t a n t s s o t h a t t h e a s s u m p t i o n o f i n s t a n t a n e o u s m i x i n g seems r e a s o n a b l e .

L a s i n g i s o b s e r v e d o n t h e 1-0 and 2-1 v i b r a - t i o n a l b a n d s up t o 2 . 5 cm downstream o f t h e C12 i n - j e c t i o n h o l e s . F i g . 6 shows t h e s p e c t r a l d i s t r i b u t i o n o f t h e l a s e r o u t p u t . The o p t i c a l a x i s i s p o s i t i o n e d a t t h e maximum power l o c a t i o n 0.9 cm downstream o f t h e C12 h o l e s . The m u l t i l i n e o u t p u t power i s 8 W .

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JOURNAL DE PHYSIQUE

1

-

⁰^band

2

-

1 band P8

P9

FLJ& -

Lmen

outpu-t hpec&urn

Note that most of the laser power (about 80 k ) co- mes from the 1-0 vibrational band in good agreement with the value of 6 5 76 predicted by the model. This comparison shows that reasonable agreement between theory and experiment can only be obtained if reaction 2 is allowed for in the chemical kinetics.

However, the model overestimates the total output power by 50-70 ?A in part because of its Limitations

(i.e rotational equilibrium, no mode approach, ins- tantaneous mixing) and also because the H-atom flow rate and the deactivation rates of the HCl(v ), 2) molecules are estimated values only.

Note also that lasing is observed on several lines in a same vibrational band (J

=

6 to 10 for the 1-0 band). The model which assumes rotational equilibrium and any mode approach predicts one J value only which lies in the range of the experimental values and varies along the flow direction.

CONCLUSIONS

-

A one-dimensional coupled-fluid-che- mistry model for predicting the performance of a subsonic HC1 chemical laser has been briefly descri- bed. Despite its limitations (i.e, rotational equilibrium, gain equals-loss condition only, instanta- neous mixing) the predicted performance agrees reasonably well with experimental performance pro- vided reaction of excited H2(v = 1) molecules with C1 atoms is allowed for in the chemical kinetics.

REFERENCES

(1) H. Brunet, M. Mabru and Rocca Serra, to be published.

(2) J.V.Michae1 and J.H Lee

Chem. Phys. Letters,51, 303(1977).

(3) H.G.Wagner,U.Welzbacher and

R.

Zellner Ber. Bunsenges Physik Chem.80, 902 (1976).

(4) P.D.Pacey and 3.C Polanyi J. Appl. Opt.10, 1725 (1971).

(5) R.L.Wi1ki.n~

J. Chem. Phys.63, 2963 (1975).

(6) A. Persky

Private communication, june 1978. k(H2(v = 1) ) / k(H2(v=O)) = 50 at 500

K

and 400 at 300

K.

(7) C. Bradley-Moore

Private communication, march 1978. Estimated values for v

>

2.

(8) 1.W.M.Smith

Chem. Phys. Letters,47, 219 (1977).

(9) Hao-Lin Shen and C. Bradley-Moore

J. Chem. Phys.54, 4072 (1971)-harmonic oscilla- tor values for v

>

^1.

(10) J.F. Bott and R.F. Heidner

J. Chem. Phys.64, 1544 (1976)-estimated values for v

>

^1.

(11) R.G. Mac Donald, C. Bradley-Moore, I.W. M. Smith and F. J. Wodarczyk

J. Chem. Phys.62, 2934 (1975).

(12) B.A. Ridley and I.W.M. Smith Chem. Phys. Letters.9, 457 (1971)

(13)

I.

Burak, Y.Noter, A.M. Ronn and A. Szoke Chem. Phys. Letters.17, 345 (1972).

(14)

H.

Brunet, M. Mabru and P. Vincent

Second Gas-Flow and Chemical Lasers Symposium, Bruxelles 1978, p.225.

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(15) R.G. Miller and J.K. Handcock J. Chern. Phys. 66 5150,(1977).

( 1 6 ) D.H. Rank

J. Opt. Soc. Arn.52, 1 (1962).

(17) H. Babrov, G. Arneer

J. Chern. Phys.33, 145 (1960).

(18) R.A. Toth, R.H. Hunt and E.K. P l y l e r J. Mol.Spectry, 35, 110 (1970).