HAL Id: jpa-00219569
https://hal.archives-ouvertes.fr/jpa-00219569
Submitted on 1 Jan 1979
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 AND MANAGEMENT OF ACOUSTICAL WAVES IN CLOSED CYCLE HIGH
ENERGY PULSED LASERS
G. Karr, C. Shih, Charles Cason, A. Werkheiser
To cite this version:
G. Karr, C. Shih, Charles Cason, A. Werkheiser. INVESTIGATION AND MANAGEMENT OF
ACOUSTICAL WAVES IN CLOSED CYCLE HIGH ENERGY PULSED LASERS. Journal de
Physique Colloques, 1979, 40 (C8), pp.C8-353-C8-355. �10.1051/jphyscol:1979863�. �jpa-00219569�
JOURNAL DE PHYSIQUE CoZZoque C8, suppZ6ment au n o 11, tome 40, novembre 1979, Page C8-353
INVESTIGATION AND MANAGEMENT OF ACOUSTICAL WAVES I N CLOSED CYCLE HIGH ENERGY PULSED LASERS,
G . R. K a r r and C.C. Shih
The University of Alabama i n HuntsviZZe, HuntsviZZe, AZabama 35807.
and
Charles Cason and A.H. Werkheiser
U.S Army Missile Research and DeveZopment Cornand, Redstone Arsenal, Alabama 35809.
Resume.- En vue d ' e t u d i e r l e s problemes acoutiques dans l e s l a s e r s puls6s
a
haute energie operant en~ c l e fern@, un systeme 1 c y c l e ferme a @ t S c o n s t r u i t e t r a t t a c h e 1 un l a s e r operant en impulsion, contrbli! p a r f a i s c e a u electronique. L 1 6 n e r g i e e l e c t r i q u e a
e t e
appliqueea
un f l u x de melange gazeux t y p i q u e de c e l u i e x i g e 3 r l e s l a s e r s C02 e t excimer. Le s p e c t r e de puissance de 1 ' i m p u l s i o n acoustique p r o d u i t e aete
mesure e t compare avec l e s p e c t r e de puissance de 1 ' e n e r g i e acoustique transmise e t r e f l e c h i e p a r l e d e f l e c t e u r . Les r e s u l t a t s montrent que l e d e f l e c t e u r attenue 1 'onde d ' e n t r e e de pres de 40 db autour de 500 Hz e t d ' e n v i r o n de 12 db autour de 5000 Hz.Abstract.- To i n v e s t i g a t e t h e a c o u s t i c problems i n h i g h energy, closed cycle, pulsed l a s e r s , a c l o s e d c y c l e system was f a b r i c a t e d and attached t o an e x i s t i n g E-beam c o n t r o l l e d s i n g l e p u l s e l a s e r system.
E l e c t r i c a l energy was i n p u t t o one atm. f l o w i n g gas m i x t u r e t y p i c a l o f t h a t r e q u i r e d by CO and excimer l a s e r s . A frequency power spectrum o f t h e produced a c o u s t i c p u l s e was measured and compare$ w i t h t h e frequency power spectrum o f a c o u s t i c energy t r a n s m i t t e d and r e f l e c t e d by t h e m u f f l e r . The r e s u l t s showed t h e m u f f l e r attenuated the i n p u t wave n e a r l y 40 db a t near 500 Hz and 12 db a t 5000 Hz.
I n t r o d u c t i o n . - The energy i n p u t i n t o t h e c a v i t y o f a pulsed h i g h energy gas l a s e r causes shock waves and expansion waves which propagate away from t h e c a v i t y and i n t e r a c t w i t h system compo- nents 1y2s3. T h i s a c o u s t i c a l energy, i f unmanaged, w i l l i n t e r a c t w i t h gas i n t h e c a v i t y , thereby s e r i o u s l y degrading t h e o u t p u t l a s e r beam qua1 i t y . The work r e p o r t e d here describes experiments which
i d e n t i f y t h e nature o f t h e a c o u s t i c environment and t h e e f f e c t i v e n e s s o f a c o u s t i c c o n t r o l devices i n managing a c o u s t i c waves.
The sequence o f o p e r a t i o n o f a pulsed h i g h energy gas l a s e r c o n s i s t s o f (1) t h e l a s e r gas media f l o w s i n t o t h e c a v i t y r e g i o n ; (2) energy
i s deposited i n t h e gas a t a magnitude o f
lo2-lo3
J/R i n a t o t a l time o f magnitude 10 s 6 ; (3) shock waves and expansion waves t r a v e l throughout t h e system a t speeds of magnitude 10 m/s 3 ; (4) t h e process i s repeated as soon as new gas l a s i n g media f l o w s i n t o t h e c a v i t y . An experimental program was i n i t i a t e d t o i n v e s t i g a t e t h e nature o f the a c o u s t i c energy $t i n t o t h e l a s e r gas and devices were t e s t e d which were designed t o remove these disturbances from t h e system.
D e s c r i p t i o n o f Experiment.- Experiments wave p e r - formed employing a h i g h energy l a s e r o f t h e ebeam e l e c t r i c discharge t y p e having t h e c a p a b i l i t y o f d e p o s i t i n g energy o f t h e o r d e r 100-1000 J/R i n a s i n g l e p u l s e o f o r d e r s d u r a t i o n . No a t t e m p t was made t o e x t r a c t l a s e r power from t h e system and n e t power i n p u t t o t h e gas was assumed t o equal t h e e l e c t r i c a l i n p u t t o t h e c a v i t y .
A closed c y c l e r e c i r c u l a t i n g system was a t t a - ched t o t h e l a s e r t o move gas through t h e c a v i t y r e g i o n a t a speed o f about 3m/s. Pressure transdu- cers capable o f f o l l o w i n g t h e pressure r i s e asso- c i a t e d w i t h shock waves were placed a t f o u r l o c a - t i o n s i n t h e r e c i r c u l a t i o n system. The simultane- ous o u t p u t o f a l l f o u r transducers were recorded on photographs o f t h e f o u r t r a c e s made on an oscilloscope. 1
The r e c i r c u l a t i n g system had p r o v i s i o n f o r i n t r o d u c i n g a c o u s t i c management devices i n t o t h e r e c i r c u l a t o r f l o w . Devices employed i n c l u d e d honey- comb s t r u c t u r e s , screens, and a m u f f l e r designed t o e x t r a c t t h e a c o u s t i c energy w i t h o u t i n t r o d u c i n c l a pressure drop i n t h e flow. Since o n l y t h e mufl- 1 e r proved e f f e c t i v e i n t h e t e s t s perfiortned, t h e m u f f l e r design and t e s t r e s u l t s w i l l be discussed i n t h e f o l l o w i n g section.
Article published online by EDP Sciences and available at
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979863
JOURNAL DE PHYSIQUE
0.20.
, ,
I,
I I 1 1 I 1 1 1L.l
M u f f l e r design.- The a c o u s t i c wave produced by a \ N 'd
t h e l a s e r i n p u t i s represented approximately as * O
0 ) *
.c
,
2 0.15a square wave pressure pulse. Since t h e square ..1 a a 0 )
a c e
wave can be approximated by an i n f i n i t e s e r i e s o f ;
,. :
>s i n e and cosine waves, a m u f f l e r must be a b l e t o a Y 0 0.10
o m
a t t e n u a t e waves over a wide band i n frequency i n a
,.
0 ) norder t o be e f f e c t i v e . An a d d i t i o n a l requirement .A Y U r.* a V?
f o r t h e m u f f l e r i s t h a t i t present a low pressure
2
0.0s 0< INr.
l o s s t o the f l o w o f gas s i n c e energy must be a s
expended t o counter any pressure drop. I n o r d e r t o
'i
3s a t i s f y t h e two major requirements o f t h e m u f f l e r , a" O.OO
I I I 1 1 1 1 1 1
100 1000
a horn coupled resonator was designed and b u i l t . Frequency, Hertz
The resonator volume i s placed o u t s i d e t h e f l o w f i e l d t o t h a t i n t e r f e r e n c e w i t h the main flow i s avoided. An exponential horn i s used t o couple t h e resonator volume t o t h e f l o w since t h i s horn design has t h e p o t e n t i a l t o a c t as a h i g h pass f i l t e r . The m u f f l e r design i s shown i n F i g u r e 1.
FIGURE 1 : M u f f l e r design.
R@sults.- The pressure records were analyzed t o f i n d t h e nature o f the a c o u s t i c energy produced by t h e l a s e r p u l s e and t o determine t h e e f f e c t i v e - ness o f t h e m u f f l e r i n removing t h e a c o u s t i c disturbance. By analyzing t h e r e c o r d from two probes mounted a known d i s t a n c e apart, t h e t r a v e - l i n g acoustic wave could be a c c u r a t e l y determined.
This wave was examined f o r i t s frequency c o n t e n t and t h e r e s u l t s o f t h e power s p e c t r a l a n a l y s i s o f t h e l a s e r produced a c o u s t i c wave i n shown i n F i g u F i g u r e 2. The r e s u l t s show t h a t t h e energy peaks i n t h e 5000 Hz range w i t h a sharp drop o f f on b o t h sides.
FIGURE 2 : Acoustic pressure spectrum r e s u l t i n g from l a s e r f i r i n g s .
The pressure records were examined t o f i n d t h e e f f e c t i v e n e s s o f t h e m u f f l e r by examining t h e power spectrum o f t h e wave l e a v i n g t h e m u f f l e r . The r e s u l t s o f t h e m u f f l e r e f f e c t i v e n e s s i s g i v e n i n F i g u r e 3 which shows t h a t the m u f f l e r attenua- t e d t h e i n p u t wave n e a r l y 40 db a t near 500 Hz and 12 db a t 5000 Hz. The m u f f l e r i s found t o be e f f e c t i v e and t o present no measurable pressure drop f o r t h e main f l o w due t o t h e s t r a i g h t p i p e design.
C.R. Karr and C.C. Shih
--
F"~w€Y" (HZ)FIGURE 3 : Results of muffler attenuation showing input wave a t 40db near 500 Hz and 12db a t 5000 Hz.
References
/1/ !ornelius C. Shih and Gerald R. Karr,
Investigation of Transient Flow and Heating Problems Characteristic of High Energy Laser Gas Circulation Systems, UAH Research Report No. 199, March 1977.
/2/ C. Cason and T.E. Horton, I' Thermal-Acousti- cal Phanomena i n Pulsed High Energy Lasers,"
AIAA/ASME (78-HT-61), May 1978.
/3/ Cornelius C. Shih and Charles M. Cason, "Gas Dynamic and Acoustic Management f o r Visible Wavelength Lasers," SPIE Technical Symposium East 78, Washington, D.C., March 1978,
