DEVELOPMENT ON GASDYNAMIC LASERS

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DEVELOPMENT ON GASDYNAMIC LASERS

R. Meinzer

To cite this version:

R. Meinzer. DEVELOPMENT ON GASDYNAMIC LASERS. Journal de Physique Colloques, 1980,

41 (C9), pp.C9-155-C9-166. �10.1051/jphyscol:1980921�. �jpa-00220575�

JOURNAL DE PHYSIQUE CoZZoque C9, suppZ6ment atc n o 11, Tome 41, novembre 1980, page ~ 9 - 1 5 5

R.A. Meinzer

United T e c h o Zogies Research Center, East Hartford, Connecticut 061 08 U. S. A.

Abstract.- Recent developments on gasdynamic lasers include some approaches for increasing the effi- ciency of conventional GDL hardware, methods for obtaining extended

CO

wavelength coverage, and pos- sibilities for some attractive alternate lasing media. These developments are discussed and those that 2 were not reviewed at the Second International Conference on Gas-Flow and Chemical Lasers

(1978)

are emphasized.

INTRO2UCTION

At the Second International Symposium on Gas- Flow and Chemical Lasers, Cassady (1) reviewed the status and advantages of mixing gasdynamic lasers and Anderson

(2)

described the application of compu- tational fluid dynamics to this class of gasdynamic lasers; he also published a book (3)whichsummarizes calculational procedures for premixed and mixing gasdynamic lasers. Since Cassady and Anderson also discussed the early gasdynamic laser history, it will not be repeated herein. This paper will focus on the GDL developments that have occurred since the 1978 Symposium. These developments include some approaches for increasing the efficiency of conven- tional GDL hardware, methods for obtaining enhanced C02 wavelength coverage and possibilities for some' attractive alternate lasing media. The research that led to these developments has been performed by many groups from around the world.

N2-C02 LASER EFFICIENCY ENHANCEMENT

The efficiency of the N2-C02 laser is limited, in part, by the optical efficiency with which

a

quantum of N2 vibrational energy can be converted into a 10.6 pm photon. Based on the energy level spacing in C02, this optical efficiency is 41%.

consequently, the remaining

59%

of the nitrogen vibrational energy is lost in translational energy.

This optical efficiency limitation may be circum- vented by lasing on alternate energy levels within the C02 molecule or by selecting an alternate las- ing species. These possibilities will be discussed in subsequent sections of this paper. In this sec- tion, emphasis will be placed upon methods for in- creasing the laser efficiency of the existing N2- C02 gasdynamic lasers. The discussion will be divided into three parts: conventional GDL, multi- stage GDL, and nonequilibrium pumped GDL.

Conventional GDL Technology

In premixed gasdynamic lasers, N2 and C02 are heated and expanded together through a nozzle. The recent theoretical investigations described in references

4

to

6

are generally intended tosimplify the analytical analysis of the premixed laser and those in references

7

to 12 are concerned with the

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980921

C9- 156 JOURNAL DE PHYSIQUE

interactions between the gain medium and the

50 optical resonator.

A

characteristic saturation

40 parameter was measured and is reported in reference ^C'CO

-

^{x 100% 30}

€0

13. The experimentally measured effect of residual 20 excess CO on laser performance is described in ¹⁰

reference 14. Selection of alternate fuels like ^'0 200 400 600 800 1000 AT. OK

- .

^~

Figure 1 Predicted Performance With Preheated N2- kerosene is discussed in references 15 to 18.

A

Saunders et al. (23)

comprehensive set of cold-flow measurements on a pressure of 30 atm, for example, the calculatedroom gasdynamic laser nozzle wake is described in ref-

erence 19.

An

analytical investigation of the tur- bulent fluctuations in the wakes of gasdynamic laser cusps is discussed in reference 20. An exper- imental method for measuring the vibrational tem- peratures of the C02 asymmetric and symmetric vibra- tional modes under GDL conditions is described in references 21 and 22.

The best performance that can be anticipated for a premixed gasdynamic laser is, summarized by the work of Saunders and Otten (23). They predicted the premixed N2-C02 GDL performance as a function of the temperature of the N2 that entered the combustor and the results are shown in Figure 1. In this figure, the improvement in laser specific energy is plotted against the temperature to which the nitrogen was preheated. These calculations are based on experi- mental measurements in which the nitrogen entering

a premixed GDL laser was preheated to various tem- peratures. The computer model was tested against

temperature specific energy is 37.6 kJ/kg and this value is predicted to increase by 40% to 50 kJ/kg if the nitrogen is preheated to 1000°K. Experi- mentally, Saunders and Otten (23) measured a maxi- mum specific energy of 3 kJ/kg, which is approxi- mately 6% of the maximum predicted theoreticalvalue.

The maximum experimental and theoretical specific power is limited by the C02 dissociation tempera- ture, which is approximately 2300°K. This limit is circumvented in the mixing gasdynamic laser by separately heating and expanding the N2 and CO

2;

subsequently, these gases are mixed at low static temperatures and pressures.

A

thorough review of the mixing gasdynamic laser was given by Cassady (1) at the Second 1nter- national Symposium on Gas-Flow and Chemical Lasers.

Since that time several additional articles on mix- ing gasdynamic lasers have been published. An experimental comparison of premixed and nixing gas- dynamic laser efficiency was made by Ostroukhov et these measurements and then subsequently used to .

.

al. (24). Low stagnation temperature character,is- predict the optimum performance that could.be tics of cw N20 and C o p gasdynamic mixing lasers achieved utilizing a totally optimized laser sys- were discussed by Mileswki et al. (25) and the tem. The calculated maximum specific power for measured saturation parameters for the same lasers several stagnation pressures and room temperature are discussed in reference (26). Additional cal- nltrogen is listed in the figure.

At

a stagnation culations for downstream mixing gasdynamic lasers

were performed by Parthasarathy et al. (27) and tained without additional gas usage, the overall Achasov et al. (28). A simplified model for a mix- GDL system efficiency increases. The feasibility ing GDL is discussed by Horioka et al. (29). No of this multistage GDL concept was investigated by significant improvements in perfo~mance, however, Meinzer (30). Since reference 30 is an intern51 over those described by Cassady (1) have been pub- company report and is subsequently not generally lished; he reported that 26 kJ/kg had been extracted available, more of the details of the multistage from a small-scale mixing laser and a saturated GDL work will be discussed than is customary for a specific energy as high as 110 k~/kg, corresponding review paper. The basic testing procedures, probe to an extracted specific power of 70 to 80 kJ/kg, laser and test facility are described in reference was predicted for a large device. These predic- 31. A schematic diagram of the multistage GDL ex- tions assumed a stagnation temperature of 2800'~. perimental hardware is shown in Figure 2. It con- Based on the dissociation temperature limit for N2, sists of two 10 cm wide nozzles labelled A and B.

the stagnation temperature could be increased to Nozzle A behaves like an ordinary GDL. The wedges approximately 4000°K with a corresponding increase

in predicted performance. Unfortunately, no effi- cient chemical gas generators are available that are capable of supplying a low contaminant N2 gas stream.in the 2500-4000°K range.

Based on the 26 kJ/kg specific energy reported for the mixing GDL, mixing gasdynamic lasers are approximately an order of magnitude more efficient than premixed GDL's. An alternate approach to increasing the efficiency of a gasdynamic laser is

downstream of nozzle A form a diffuser which raises the static gas pressure and temperature of the nozzle A exhaust gases. The stagnated gases enter the plenum upstream of nozzle B and are expanded through nozzle B just as in an ordinary GDL.

A

portion of this flow was occasionally bled off ahead of nozzle B through the .butterfly valve located downstream of the diffuser wedges. Tue gases entering nozzle A were generated in a m m i - rocket combustor which was described in reference to make better use of the energy that is stored in 31. Internally, this copper combustion chamber was the supersonic gas stream by using the laser ex- 15.1 cm long, 2.5 cm high, and 10 cm wide. Carbon haust from one GDL to power a second one. Some monoxide, hydrogen and oxygen are mixed in the small-scale tests that were performed on a multi- rear of the combustor. They are ignited by a spark stage GDL configuration will be described.

N o d e A N o a l e B

Multistage GDL

Gasdynamic laser efficiency can be increased in a multistage GDL: gases that are exhausted from one GDL are stagnated via a diffuser aid subsequent- ly reexpanded through a second GDL. Since the optical power produced by the second GDL is ob-

A* = 0.05 cm AIA' = 12.5

J'

BuMr(ly r r h (sealed In closed passion)

.Figure 2 Multistage GDL

C9- 158 JOURNAL DE PHYSIQUE

I n 1

^Beam

channel recorder

Combustor flats

I U I

i n o r d e r t o a c h i e v e r e a s o n a b l e g a i n v a l u e s . The l o c a t i o n of t h e n o z z l e B g a i n measurement was v a r i e d d u r i n g t h e t e s t program. At a p p r o x i m a t e l y 1 cm from t h e t h r o a t , no g a i n c o u l d b e measured. Fur- thermore, e x p e r i m e n t a l and a n a l y t i c a l i n v e s t i g a t i o n s F i g u r e 3 Simultaneous Gain Measurement Configura-

t i o n

p l u g . N i t r o g e n i s p r e h e a t e d i n t h e combustor h o u s i n g and t h e n i n t r o d u c e d i n t o t h e combustor beyond t h e s p a r k p l u g t h r o u g h a s e r i e s o f h o l e s i n t h e t o p and bottom o f t h e h o u s i n g .

The e x p e r i m e n t a l c o n f i g u r a t i o n f o r making s i m u l t a n e o u s g a i n measurements i n b o t h s t a g e s o f t h e tandum GDL is shown i n F i g u r e 3. The o u t p u t of t h e p r o b e l a s e r p a s s e s t h r 0 u g h . a p i e c e o f germanium which a c t s a s a beam s p l i t t e r . P a r t o f t h e p r o b e l a s e r beam goes s t r a i g h t t h r o u g h t h e beam s p l i t t e r and n o z z l e B. The r e f l e c t e d p a r t of t h e beam i s t u r n e d w i t h a m i r r o r and p a s s e s t h r o u g h n o z z l e A.

The p r o b e l a s e r beams t h a t p a s s t h r o u g h t h e n o z z l e A a n d B r e g i o n s are measured o n s e p a r a t e d e t e c t o r s whose o u t p u t i s r e c o r d e d o n a d u a l c h a n n e l r e c o r d e r .

R e p r e s e n t a t i v e performance o f t h e m u l t i s t a g e GDL i s shown i n T a b l e 1. Gain d a t a a r e shown f o r combustor s t a g n a t i o n p r e s s u r e s of 28 and 1 2 atm, b u t s l i g h t l y d i f f e r e n g g a s c o m p o s i t i o n s were u s e d

T a b l e I E l u l t i s t a g e GDL Performance

&s'CGmitt&

Mot. % N.,

12 atm r1!30eK

7295 22.10 4.95 oms nn

25 1.6 cm M atrn -1000-K

0.05 cm I 2 5

2 6 cm 0.07% em-1

0.52 cm-'

i n d i c a t e d t h a t n o z z l e B g a i n i n c r e a s e d a s t h e a r e a r a t i o was d e c r e a s e d . I n i t i a l measurements were made a t a n a r e a r a t i o o f 30, which was found t o b e f a r from t h e optimum v a l u e . The a r e a r a t i o was g r a d u a l l y d e c r e a s e d t o 1 2 . 5 , which was much c l o s e r t o t h e optimum a r e a r a t i o .

The g a i n v a r i a t i o n w i t h plenum p r e s s u r e t h a t was measured 3 cm downstream of n o z z l e B i s shown i n F i g u r e 4 . When t h e plenum p r e s s u r e was i n - c r e a s e d above 0 . 3 atm, by c l o s i n g t h e b u t t e r f l y v a l v e , t h e f l o w from n o z z l e A was no l o n g e r s u p e r - s o n i c . Examination of t h e p l o t t e d s t a g n a t i o n tem- p e r a t u r e , which was measured v i a a thermocouple, shows t h a t i t r e a c h e s a maximum a t a plenum s t a g - n a t i o n p r e s s u r e o f a p p r o x i m a t e l y 0 . 3 a t m and de- c r e a s e s a t h i g h e r v a l u e s . The measured s t a g n a t i o n t e m p e r a t u r e and p r e s s u r e were used t o c a l c u l a t e t h e t h e o r e t i c a l n o z z l e B g a i n t h a t i s l i s t e d i n F i g u r e 4. The o b s e r v e d d i f f e r e n c e s between t h e c a l c u l a t e d and e x p e r i m e n t a l l y measured g a i n s may b e a t t r i b u t a b l e t o a t e m p e r a t u r e g r a d i e n t a c r o s s t h e

Area retloa Stagnation

0.25C

!-./-

temperature Cslculatlon 10.0

Experiment 12.5

Stagnation 900 temperature,

\ deg K

F i g u r e 4 Nozzle B Gain a t 3 cm L o c a t i o n

width of the plenum. Since the butterfly valve was not completely symmetric, the asymmetry in the flow around the butterfly valve may have caused a tem- perature gradient within the plenum.

The efficiency of a multistage GDL may be in- creased by using a bow shock wave instead of a dif- fuser to stagnate the gases flowing from nozzle

A.

Numerical estimates of the performance of a gasdy- namic laser that operates via a bow shock wave is discussed by Kasuya et al. (32).

A

similar discus- sion by Kasuya was presented at this Symposium and is published in this same journal (33). With bow shocks, stagnation gas temperatures are predicted which are higher than those at which the gases would ordinarily dissociate. These higher temperatures are predicted because the gases remain at them for periods of time which are shorter than those required for gas dissociation to occur: a thermodynamic- kinetic rate optimization is made. Similar non- equilibrium effects have been noticed in a number of other experiments that will now be discussed.

, Nonequilibrium Pumped GDL

In conventional GDL technology, the N2 vibra- tional temperature is directly related to the stag- nation temperature and consequently, it is always lower than the stagnation temperature. Vibrational temperatures that are higher than the stagnation temperature can only be achieved via nonequilibrium effects. Some recent experiments on reacting-non- reacting gas mixtures and arc heated N2 suggest the possibility of using nonequilibrium effects to pump a GDL.

The reacting-nonreacting gas mixture experi-

T~ ( 0 ~ ) - l o - 3

Figure 5 Gain Dependence on Stagnation Tempera- ture

-

Kudriavtsev et al. (34)

These investigators compared the gain and vibration- al temperatures for a reacting 0.05 N20

+

^0.25

^CO +

0.7 He mixture with its equivalent nonreacting 0.05 N2

+

^{0.05 C02}

⁺

^{0.2 CO}

+

0.7 He mixture, which con- sists of the final products produced by the reacting mixture. Gas mixtures corresponding to these two gas compositions were heated in a shock tube to tem- peratures between 1500 and 2700°K. Measurements were performed in approximately 0.5 m sec at a stag- nation pressure of 5.2 atm. The gain that was measured as a function of the stagnation temperature is plotted in Figure 5. At low stagnation tempera- tures, the reacting gas mixture has a much higher gain than that measured with the equivalent nonreacting gas mixture; at high stagnation tem- peratures, the gains are comparable. The large difference in gain at low stagnation temperatures

(1500 to 2100'~) is attributed to over-equilibrium, pumping of the (001) level in C02. The vibrationai temperature of the (001) level in C02 was also measured and its variation with stagnation tempera-

ture is shown in Figure

6.

For the nonreacting gas mixture, it increases monotonically with the stag- nation temperature. The vibrational temperature for the reacting mixture, however, is much higher than

. ments were performed by Kudriavtsev et al. ( 3 4 ) . that for the nonreacting mixture at

low

stagnation

C9- 160 ^JOURNAL DE PHYSIQUE

Figure 6 C02 (001) Tv Dependence on S t a g n a t i o n Temperature

-

Kudriavtsev e t a l . (34)

temperatures; some of t h e measured v i b r a t i o n a l tem- p e r a t u r e s exceed t h e c a l c u l a t e d n o z z l e t h r o a t gas temperature. A t high s t a g n a t i o n t e m p e r a t u r e s , t h e measured CO asymmetric s t r e t c h i n g (001) mode v i -

2

b r a t i o n a l temperature i s e s s e n t i a l l y e q u a l f o r t h e r e a c t i n g and n o n r e a c t i n g g a s m i x t u r e s . Although t h e s e experiments s t r o n g l y s u g g e s t t h a t t h e N20-CO r e a c t i o n could b e used t o produce nonequilibrium pumping of t h e (001) C02 l e v e l i n a N2-C02 GDL, a d d i t i o n a l experiments a r e s t i l l needed t o charac- t e r i z e t h e magnitude of t h i s e f f e c t and t o e l i m i n a t e o t h e r p o s s i b l e i n t e r p r e t a t i o n s of t h e measurements.

A s i m i l a r t y p e of nonequilibrium e f f e c t h a s been observed i n a r c heated N2 experimentsperformed by,Sakumov e t a l . (35). I n t h e s e experiments, the w e n t was v a r i e d from 100 t o 300 A and t h e

.

p l 9 - p r e s s u r e ranged from 1 t o 2 atm. A schema- t i c diagram of t h e a r c c o n f i g u r a t i o n i s shown i n F i g u r e 7. The plenum chamber h a s a n i n s i d e diam- e t e r of 2 cm and was 5 cm i n l e n g t h . High p r e s s u r e n i t r o g e n was i n j e c t e d along t h e w a l l s v i a t h e porous c e r a m i c + l i n e r a s shown i n t h e f i g u r e . Quartz windows were a t t a c h e d t o t h e middle of t h e chamber and were used f o r making s p e c t r o s c o p i c measurements on t h e heated n i t r o g e n gas.

B o t h t h e t r a n s l a t i o n a l a n d v i b r a t i o n a l n i t r o g e n g a s t e m p e r a t u r e s w e r e ~ p e c t r o ~ ~ ~ p i ~ a l l y m e a ~ ~ r e d . The t r a n s l a t i o n a l g a s temperature was determined from t h e allowed r o t a t i o n a l l i n e s , J=12-56, of t h e 0-0 band (391.4 nm) of t h e f i r s t n e g a t i v e system of n i t r o g e n . The v i b r a t i o n a l temperature of t h e ground

1

+

s t a t e N2, TV(X E ), was o b t a i n e d i n d i r e c t l y because g

a d i r e c t measurement r e q u i r e s t h e u s e of s p e c i a l i n f r a r e d d i a g n o s t i c methods which a r e v e r y d i f f i c u l t t o apply under high p r e s s u r e d i s c h a r g e c o n d i t i o n s . I n t h e s e experiments, t h e ground s t a t e molecular N2 v i b r a t i o n a l temperature was assumed t o be e q u a l

2

+

t o t h a t of t h e B C s t a t e of t h e molecular N i o n ,

+

2

N2

.

T h i s i o n s t a t e i s populated d i r e c t l y by e l e c t r o n c o l l i s i o n s and d e e x c i t e d i n t h e process of quenching by t h e e l e c t r o n s . Furthermore, t h e c h a r a c t e r of t h e energy d i s t r i b u t i o n over t h e v i b r a t i o n a l l e v e l s is n o t s i g n i f i c a n t l y p e r t u r b e d s i n c e t h e a n g u l a r momentum does n o t change when t h e

2

+

B C s t a t e is populated by e l e c t r o n c o l l i s i o n s . Moreover, t h e p o t e n t i a l energy curve f o r t h e

+

^{2 +}

N (B C ) i s o n l y s l i g h t l y d i s p l a c e d r e l a t i v e t o

2 u

t h a t of t h e e l e c t r o n i c ground s t a t e of t h e n i t r o g e n

+

2 +

i o n , N2 (X C ), s o t h a t t h e energy d i s t r i b u t i o n s g

o v e r t h e v i b r a t i o n a l l e v e l s i n t h e s e s t a t e s a r e s i m i l a r . Consequently, t h e v i b r a t i o n a l temperature,

r Quartz windows

Porous ceramic

Cathode

F i g u r e 7 NonequiZik.rium Arc C o n f i g u r a t i o n

-

Abakumov e t a l . (35)

2

+

T ( B Z ) , measured in the experiment should equal v U

the vibrational temperature of the ground state N 2 2

+

ion, TV(X Zg). Since the exchange of vibrational quanta between N molecules and N2 ions occurs

+

2

via a fast charge-exchange reaction with charac- teristic times that are 2 to 3 orders of magnitude faster than the vibrational-translational relaxa- tion time, the measured ion vibrational temperature was assumed to approximate the ground state N2 vi- brational temperature.

The experimentally measured variation of the vibrational and translational temperatures with position are shown in Figure 8. Near the center line of the plenum, radius = 0 mm, the translational and vibrational temperatures are approximately equal. At the edges of the plenum where the cold nitrogen gases are injected, the vibrational

temperature exceeds the translational temperature, Tt, by as much as 3000°K. If the N2 injection rate is increased at constant discharge current, the difference between Tv and Tt also increases. In- creasing the current at constant gas injection rate, causes the central equilibrium discharge region to expand but the temperature difference between the T and T in the outer region is maintained at

v t

Temp

Figure

8

Tv and Tt Dependence on Position

-

Abakumov et al. (35)

Optical resonator

Cathode

COrHe injection

-I ^Q

Figure 9 Proposed Nonequilibrium Arc Heated GDL

-

Abakumov et al. (35)

higher absolute values. This arc configuration was proposed for use with a gasdynamic laser and the configuration proposed by the above authors (35) is shown in Figure

9.

In the proposed configura- tion, the throat of the GDL nozzle is placed close to the arc so that the excess N2 vibrational energy is not lost upstream of the throat. For the assumed operating pressure of 10 atm, this distance has to be less than

2.8

cm. The proposed geometry also provides the possibility of freezing-in the nonequilibrium electron density via the supersonic expansion. The authors speculated that the arc plenum could act as a preionizer for a nonself- sustained discharge, in the resonator region, which could provide additional N vibrational eneqgy down-

2

stream of the nozzle throat and thereby inckease the overall laser .efficiency.

An

electric discharge gasdynamic laser has been operated by Manuccia et al. (36) on the 16- and 14-um transitions

of

Cog. A

N

CO energy

2-

2

level diagram which characterizes the transitions 'is presented in Figure 10; the Einstein A values

were taken from reference 36. Laser operation at

14

pm occurs when a 10.6 pm saturating pulse equi- libriates the

(001)

level with the (100) level and creates a transient inversion on the (100)-(01 1 0)

C9- 162 JOURNAL DE PHYSIQUE

Symmetrtc Banding Asymmetric stretch (001)

A-

^N2

01

t

t

F i g u r e 1 0 C02-N2 Energy L e v e l Diagram t r a n s i t i o n a t 1 4 pm. S i m i l a r l y , l a s e r o p e r a t i o n a t 1 6 pm o c c u r s when a 9.4 pm s a t u r a t i n g p u l s e e q u i - l i b r i a t e s t h e (001) l e v e l w i t h t h e (02'0) l e v e l and c r e a t e s a t r a n s i e n t i n v e r s i o n o n t h e (02O0)-

(0l10) t r a n s i t i o n a t 1 6 pm. T h i s 1 6 pm r a d i a t i o n t h a t i s

produces

w i t h t h e e l e c t r i c d i s c h a r g e gas- dynamic l a s e r may b e u s e f u l f o r uranium i s o t o p e s e p a r a t i o n and c o n s e q u e n t l y t h e a u t h o r s c o n t i n u e d t o d e v e l o p t h e d e v i c e .

S i n c e t h e o r i g i n a l p u b l i c a t i o n , s i g n i f i c a n t improvements i n t h e 1 6 pm performance of t h e d e v i c e have b e e n made a n d t h e s e a r e d i s c u s s e d i n r e f e r - e n c e s 37 and 38. A s c h e m a t i c diagram of t h e de- v e l o p e d e l e c t r i c d i s c h a r g e gasdynamic laser, which was d i s c u s s e d by Wexler ( 3 8 ) , i s shown i n F i g u r e 11. The s u p e r s o n i c c h a n n e l m e a s u r e s 30 cm i n l e n g t h and i s 25 cm wade. I t s h e i g h t i n c r e a s e s

0 switch

F i g u r e 11 9- and 16- pm EDGDL

-

Wexler e t a l . (38)

from 1.9 cm a t t h e n o z z l e e x i t p l a n e t o 2.8 cm a t t h e end of t h e c h a n n e l . During l a s e r o p e r a t i o n , He and N2 f l o w through t h e DC d i s c h a r g e and t h e n i n t o t h e s u p e r s o n i c n o z z l e a r r a y where C02 i s added. As t h e CO mixes w i t h t h e p r i m a r y f l o w , t h e

2

(001) l e v e l i s p o p u l a t e d by v i b r a t i o n a l e n e r g y t r a n s f e r from t h e e x c i t e d N2. A r o t a t i n g m i r r o r Q-switches t h e l a s e r o n t h e (001)-(02 0) t r a n s i t i o n 0 a t 9.4

urn

and p o p u l a t e s t h e ( 0 2 ~ 0 ) l e v e l . A f t e r t h e 9.4 pm p u l s e , a n i n v e r s i o n and l a s i n g o c c u r o n t h e ( 0 2 ~ 0 ) - ( 0 1 0) t r a n s i t i o n a t 1 6 1 um. An i n t r a - c a v i t y c e l l f i l l e d w i t h BC13 s u p p r e s s e s o s c i l l a - t i o n a t 10.6 Dm and f o r c e s l a s i n g t o o c c u r a t t h e 9.4 and 1 6 um t r a n s i t i o n s . The ( 0 1 0) l e v e l i s 1 r a p i d l y e q u i l i b r i a t e d w i t h t h e t r a n s l a t i o n a l tem- p e r a t u r e o f t h e s u p e r s o n i c f l o w by a d d i n g H2 t o t h e d i s c h a r g e o r n o z z l e s .

The measured l a s e r o u t p u t p e r p u l s e a t 9.4 and 1 6 pm v e r s u s t h e C02 mass f l o w f o r t h e m u l t i p a s s s e t u p and t h e sum of s i n g l e - p a s s o u t p u t s a t 5 c a v i t y p o s i t i o n s a r e shown by t h e d a t a p r e s e n t e d i n F i g u r e 1 2 . These d a t a were o b t a i n e d w i t h a p u l s e r e p e t i t i o n r a t e o f 3600 Hz. Examination o f t h e d a t a i n d i c a t e s t h a t b o t h t h e 9.4 and 1 6 pm o u t p u t s peak a t much l o w e r CO f l o w r a t e s w i t h t h e m u l t i -

2

p a s s c a v i t y . An e x p l a n a t i o n of t h i s e f f e c t may e,m Sum of 5 single passes

0.0 5 pass cavity He: N2: H2

67: 24: 0.03 moleslsec

1

.P 9 p n energy!

pulse (mJ)

0 -

0 0.08 0.1 0.15 0.20 0.25 C02 flow, (mdes/m)

F i g u r e 1 2 S i n g l e and M u l t i p a s s Laser Output

-

Wexler e t a l . (38)

provide t h e i n f o r m a t i o n t h a t i s needed t o make s i g n i f i c a n t improvements i n l a s e r performance.

The maximum o u t p u t power f o r a PRF of 4800 Hz was 5.4 w a t t s a t 16 um and 19.7 w a t t s a t 9.4 vm.

T h i s corresponds t o an o u t p u t energy per p u l s e of 1.1 m J a t 1 6 um and i s 70% of t h e a v a i l a b l e energy based on t h e magnitude of t h e 9.4

urn

p u l s e . This performance was achieved a t a measured plenum pres- s u r e of 80 Torr and a c a v i t y p r e s s u r e of 4.8 Torr.

I

S i n c e t h e power expended i n t h e DC d i s c h a r g e was 4.8 kW, t h e observed e f f i c i e n c y was g r e a t e r t h a n 0.1%.

A t h e o r e t i c a l a n a l y s i s o f a c o n v e n t i o n a l N ₂

-

CO gasdynamic l a s e r t h a t o p e r a t e s a t 16 um was 2

p r e s e n t e d by S a i t o e t a l . (39) a t t h i s Symposium and i s included i n t h i s p u b l i c a t i o n . I f t h e 16 vm r a d i a t i o n produced by e i t h e r a n e l e c t r i c d i s c h a r g e GDL o r a c o n v e n t i o n a l GDL i s u s e f u l f o r uranium i s o t o p e s e p a r a t i o n , t h e n a s i g n i f i c a n t new GDL a p p l i c a t i o n may develop.

Examination of t h e energy l e v e l diagram l i s t e d i n F i g u r e 1 0 a l s o s u g g e s t s t h e p o s s i b i l i t y of ob- t a i n i n g CO GDL r a d i a t i o n a t o t h e r t r a n s i t i o n s . 2 One t r a n s i t i o n t h a t h a s been considered i s t h e

(03 0)-(100) t r a n s i t i o n a t 18.4 1 um. An a n a l y s i s is d i s c u s s e d by Konyukhrv e t a l . (40) and by Brunne (41). The a n a l y s i s by Brunne was p r e s e n t e d a t t h i s Symposium and i s included i n t h i s pub- l i c a t i o n . Another method of extending t h e wave- l e n g t h range of gasdynamic l a s e r s i s t o s e l e c t an a l t e r n a t e l a s e r media and v a r i o u s candidates a r e d i s c u s s e d below.

ALTERNATE GDL MEDIA

The a l t e r n a t e GDL a c t i v e media combinations

Table I1 Demonstrated Gut, VMeclj?

Pump b u n 1 Cablye1 Excitation D* Rotemma

N2 C02 Hfl.He All modes 1989

-

02 C02 He E h h k dluhmrp 1975 Slmgackstal. (42) D2 N20 He E h t ~ I ~ d l u h m r p 1975 Slfogocketal. (423 CO C S ~ HO shock tub. 1978 aaw~kovetal. (a)

CO CO Ar Shock lube 1971 Wall (W

N2 CO Ar Shock tub. 1972 McKoculael sl. f45)

CO CO

-

^{Rf I980} Hugel el 01. (a)

CO CO

-

Mkmwave 1980 Schall ot al. (47)

t h a t have a l r e a d y l a s e d a r e l i s t e d i n Table 11.

T h i s t a b l e l i s t s t h e pump, l a s a n t and c a t a l y t i c s p e c i e s t h a t were used i n t h e v a r i o u s GDL t e s t s The demonstrated pump s p e c i e s i n c l u d e N 2 9 D2, CO.

The p r i n c i p l e l a s a n t s p e c i e s a r e C02, N O , CS2 and 2

CO; i s o t o p i c d e r i v a t i v e s of t h e s e molecules have been omitted from t h e p r e s e n t d i s c u s s i o n . The p r i n c i p a l c a t a l y s t s t h a t have been used t o d a t e a r e H 0, _{He and} A r . The range of e x c i t a t i o n t e c h n i q u e s

2

which have been employed range from thermal and shock h e a t i n g t o a v a r i e t y of e l e c t r i c d i s c h a r g e techniques.

Some p o t e n t i a l new GDL l a s a n t s a r e t h e hydro- gen h a l i d e s . Lasing on t h e s e s p e c i e s may b e a c h i e v a b l e because t h e y can l a s e on a p a r t i a l Bn- v e r s i o n . This p a r t i a l i n v e r s i o n c r i t e r i o n ca$ be

converted i n t o a r a t i o of a r o t a t i o n a l t o v i b r a - t i o n a l temperature. A good d e r i v a t i o n of t h i s c r i t e r i o n , which is l i s t e d i n Table 111, is given

by U l t e e (48). He a l s o lists t h e v i b r a t i o n a l and r o t a t i o n a l c o n s t a n t s w h i c h a r e r e q u i r e d t o c a l c u l a t e

Table I11 P o t e n t i a l GDL L a s a n t s

HF DF HCI DCI HBr DBr CO OH

2 (Blm) x 103 10.6

7.6 7.3 W 6.6 4.7 1.8 10.6

C9- 164

^JOURNAL DE PHYSIQUE

the mixiimum values of the necessary temperature ratios. These values are listed in Table I11 to- gether with the computed values for 2B/w. Assuming a vibrational temperature of 2000°K and a lower level rotational quantum number, J=5, the values in the last column should be multiplied by lo4 to give the minimum rotational temperature required for lasing. For these assumptions, HC1 lasing would require the rotational temperature to be less than 73OK. Alternatively, the vibrational temperature could be increased.

An increase in vibrational temperature can be

. achieved via anharmonic pumping. By selecting the pumping species to have a larger vibrational con- stant than the lasing species, the lasing species will be anharmonically pumped as described. for

example, by McKenzie (45). Anderson

(3)

has referred to this type of pumping as the "Teare effect". Selected potential pump and lasant species are listed in Table IV. Some of these species were listed by Oraevskii et al. (49). They also described how the partial inversion condition listed in Table I11 can be satisfied and predicted the D2-HC1 GDL gain shown in Figure 13. Their calculations were performed for a stagnation

temperature of 2800°1f and a stagnation pressure of 50 atm. The

J

value of the upper level of the Table IV Potential

Pump (cm

-

¹⁾

N2 (2331) H2 (4161)

02 (2gW

CO (21 43)

GDL Pump-Lasant Combinations

LASANT (cm- 1) AE (cm- 1)

CO (2143)

+

189 DCI (2091) 240

OH (%TO) 591

HCI (2888) 1275

HF (3958) 203

HCI (2886) 104 DCI (2091) 899 HBr (2559) 431

DCI (2091) 52

DBr (1841) 302

. .

Operating condtlions: T o = 2 8 0 0 0 ~ Po = SO aim

1

t

^{He: D2: tic1 = 3:l:O.l}

Figure 13 Predicted D2-HCL GDL Gain

-

Oraevskii et al. (49) pertinent vibrational-rotational transition is listed on each gain line. The gain at the low ro- tational transitions is high because the low static temperatures enhance the population in the lower J states. In addition, these authors also predict that the power for the D2-HC1 GDL will be in the range of 50 to 150 kj/kg. Similar results were predicted for the H -HC1 GDL laser by Taylor et al.

2

(50). These results are shown in Figure 14. As the HC1 concentration increases, the potential powel that can be extracted at low stagnation pressures decreases. Nevertheless, the HC1 gasdynamic laser appears to be a viable candidate for a future high- powered gasdynamic laser system.

Specific power (kJllb)

Stagnation temperature, (OK)

Figure 14 Predicted H2-HCL Premixed GDL Perfor- mance - Taylor et al. (50)

CONCLUSIONS

Improvements in the efficiency of conventional gasdynamic lasers will continue to be made. The basic limitations to-achieving significant improve-

. .

ments r e s u l t from t h e d i s s o c i a t i o n o f C02 i n pre- J o n e s , A. T., J. Phys. D: Appl. Phys.

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- mixed gasdynamic l a s e r s and by N2 d i s s o c i a t i o n i n

S i x i o n g , C., S c i e n t i a S i n i c a 22, No. 5, 589 (1979).

mixing gasdynamic lasers. An a d d i t i o n a l problem

i n mixing l a s e r s i s t h e achievement of t h e v e r y Bakanov, D. G . , A. I. Odintsov, A. I.

Fedoseev, and V. F. Sharkov, Sov. J . Quantum E l e c t r o n

2,

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t e c h n o l o g y . A m u l t i s t a g e GDL improves t h e o v e r a l l Biryukov, A. S . , R. I. S e r i k o v , and A.

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n o l o g y b u t a d d i t i o n a l measurements a r e s t i l l r e -

q u i r e d t o c h a r a c t e r i z e t h e magnitude o f t h e s e Aleksandrov, B. S., Yu. A. Anan'ev, A. V..

Lavrov, and V. P. Trusov, Sov. J. Quantum E l e c t r o n

I ,

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2 2

l a s e a t t h e 9-,

lo-,

14- and 16-urn bands l a s i n g a t Hara, H., J. Appl. Phys.

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Ktalkherman, M. G., V. A. Levin, V. M.

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Ktalkherman, M. G., V. M. Mal'kov, A. V.

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Evtyukhin, N. V.. A. P. Genich, A. A.

M a n e l i s , F i z i k a Goreniya i Vzryva

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