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A REPETITIVELY PULSED CARBON DIOXIDE
LASER WITH MEAN POWER OUTPUT IN EXCESS
OF 30 kW
J. Wood, P. Pearson
To cite this version:
A REPETITIVELY PULSED CARBON DIOXIDE LASER WITH MEAN POWER OUTPUT IN EXCESS OF
3 0kW
J.D.L.H. Wood and P.R. Pearson
Royal SigrzaZs and Radar Establishment, Ministry o f Defence, BaZdock, Hertfordshire, England.
A b s t r a c t . - A pulsed e l e c t r o n beam s u s t a i n e d atmospheric p r e s s u r e carbon d i o x i d e l a s e r w i t h an a c t i v e volume of 17 l i t r e s has been i n c o r p o r a t e d i n a closed c y c l e g a s r e c i r c u l a t i o n system. Gas flow and e l e c t r i c a l i n p u t can be v a r i e d over a wide range. The problems a s s o c i a t e d w i t h r e p e t i t i v e o p e r a t i o n have been explored and measurements made of multimode and s i n g l e mode o u t p u t q u a l i t y , r e p r o d u c i b i l i t y and r e l i a b i l i t y . P a r t i c u l a r a t t e n t i o n has been given t o e l e c t r o n gun and f o i l window d e s i g n , and t h e mechanical s t a b i l i t y of t h e o p t i c a l c a v i t y . The e f f e c t of g a s p u r i t y , g a s flow r a t e s and damping of a c o u s t i c d i s t u r b a n c e s have been measured. Medium q u a l i t y h a s been a s s e s s e d by pulsed i n t e r f e r o m e t r y and a time l a p s e c i n e f i l m made of t h e decay of p e r t u r b a t i o n s i n t h e i n t e r - p u l s e p e r i o d . Maximum mul- timode o u t p u t a t 10.6 pm i s 600 J p e r 30 microsecond p u l s e with p u l s e r e p e t i t i o n f r e q u e n c i e s up t o
66 Hz. Mean power achieved has been up t o 36 kW f o r 0.5 second o r 22 kW f o r I second d u r a t i o n w i t h t h e c a p a b i l i t y of r e p e a t i n g t h e p u l s e t r a i n a t 30 second i n t e r v a l s . INTRODUCTION Operation of g a s l a s e r s a t h i g h r e p e t i t i o n r a t e s r e q u i r e s t h a t t h e heated g a s b e r e p l a c e d by c o l d g a s i n t h e working r e g i o n p r i o r t o t h e a p p l i c a t i o n of t h e n e x t d r i v e pulse. To achieve t h i s t h e l a s e r i s i n c o r p o r a t e d i n t o a c l o s e d c y c l e g a s r e c i r c u l a - t i o n system. The p r e s e n t paper i s concerned w i t h a pulsed e l e c t r o n beam s u s t a i n e d carbon d i o x i d e l a s e r having an a c t i v e volume of 17 l i t r e s . T h i s h a s been developed using d e s i g n c r i t e r i a e s t a b l i s h e d i n s m a l l e r systems (1,2,)3.
LASER AND GAS CIRCULATION SYSTEM
F i g u r e 1 shows a c r o s s - s e c t i o n of t h e e l e c t r o n gun and l a s e r . The e l e c t r o n beam i s d e r i v e d from a d i s c h a r g e i n helium a t a p r e s s u r e of about 50 mTorr and u s e s a concept devised i n c o n j u n c t i o n with GEC H i r s t Research L a b o r a t o r i e s (4). A continuous c u r r e n t of 100 mA i s maintained through t h e a u x i l i - a r y e l e c t r o d e . Imposed on t h i s i s a pulsed c u r r e n t of s e v e r a l amps during which t h e main gun p u l s e of
150 kV i s a p p l i e d t o t h e gun cathode. I o n s e x t r a c t e d from t h e d i s c h a r g e g e n e r a t e secondary e l e c t r o n s a t t h e cathode. These a r e a c c e l e r a t e d through t h e gun and t h e 25 t h i c k aluminium f o i l
window i n t o t h e l a s e r gas. C a r e f u l e l e c t r o n o p t i c s d e s i g n e n s u r e s a uniform c u r r e n t d i s t r i b u t i o n n e c e s s a r y b o t h f o r good l a s e r medium i o n i s a t i o n and
5
f o i l l i f e exceeding 10 p u l s e s .
FIGURE 1 LASER CROSS SECTION
The e l e c t r o n beam l e a v e s t h e window w i t h consider- a b l e divergence due t o s c a t t e r i n t h e f o i l . The e f f e c t s of t h i s i11 spreading t h e main discliarge o u t s i d e t h e o p t i c a l c a v i t y a r e l i m i t e d by i n s u l a t - ing s l a t s which a c t a s e l e c t r o n beam s t o p p e r s b u t
C9-352 JOURNAL DE PHYSIQUE
a l l o w adequate gas flow. The l a s e r anode i s s i t u - a t e d 12 cm above t h e l a s e r cathode. The main i n p u t energy i s f e d through t h i s a t a v o l t a g e (50-60 kV) which i s l e s s t h a n t h e breakdown p o t e n t i a l and which i s oprimised f o r e x c i t a t i o n of t h e upper
l a s e r energy l e v e l . The l a s e r modulator was b u i l t by Marconi Research L a b o r a t o r i e s and i s designed t o have f l e x i b i l i t y t o enable a range of p u l s e energy, p u l s e l e n g t h and p r f o p t i o n s t o be explored (5). Maximum energy s t o r e d i s 5 k J p e r p u l s e and mean power consumption 400 kVA d u r i n g a p u l s e t r a i n . The 4-gap hydrogen r h y r a t r o n s used a s s e r i e s switch, charge and t a i l - b i t e r tubes were s p e c i f i c a l l y made f o r t h i s system by E n g l i s h E l e c t r i c Valve CO Ltd (6). F i g u r e 2 o u t l i n e s t h e l a s e r i n t h e gas r e c i r c u l a t i o n
-
1system. Gas speeds up t o 30 ms can be achieved through t h e l a s e r c a v i t y by a v a r i a b l e speed fan.
FIGURE 2 GAS CIRCULATION SYSTEM The f l o w d i s t r i b u t i o n and p e r t u r b a t i o n s due t o t h e d i s c h a r g e l i m i t e r s a r e shown i n F i g u r e 3.
The duct work i s r o b u s t enough t o withstand being evacuated t o a pressure of 100 mTorr p r i o r t o f i l l i n g with l a s e r g a s mixture which i s a t atmos- p h e r i c p r e s s u r e having a m i x r a t i o of He 5: N2 5: CO2 2. A purge r a t e of 1.5 changes per hour removes d e l e t e r i o u s d e c m p o s t i o n products. Oxygen contamin- a t i o n i s held a t l e s s than 0.02%. If t h i s i s allowed
to r i s e t o 0.2% t h e d i s c h a r g e impedance doubles and i n c i p i e n t a r c i n g o c c u r s a t t h e anode. 1 5 . 0 ~ I !
901
1
FAN W E E 0 1.000 R P Mi
14 0 13 0 MEAN 1 2 0 -.
E 1 1 0 - D W L0 a 1 0 0 - UAeOUSTlC DAMPING REDUCES ! M E A N FLOW TO 1 0 9 m / * l 8 . 0
I I
I
- - -FIGURE 3 VERTICAL FLOW DISTRIPJUTION GAS WTING AND ACOUSTIC EFFECTS
0 l0 ms 20rns 0 lOms 20rns
( a ) INITIAL CONDITIONS ( b ) W I T H ABSORBERS
FIGURE 4 CAVITY PRESSURE WAVEFORMS For s i n g l e mode o p e r a t i o n phase f r o n t e r r o r s i n t h e l a s e r o u t p u t should n o t exceed XI20 correspond- i n g t o an average d e n s i t y e r r o r Aplp of l e s s than 1.5 x 1
o - ~ .
The medium homogeneity has been a s s e s s e d by i n c o r p o r a t i n g t h e l a s e r d i s c h a r g e r e g i o n i n one arm of a Mach-Zehnder I n t e r f e r o m e t e r i l l u m i n a t e d by a 113 u s pulsed xenon l a s e r . I n t e r - ferograms taken a t v a r i o u s times w i t h i n and a f t e r t h e l a s e r d i s c h a r g e a r e i l l u s t r a t e d i n F i g u r e 5 taken w i t h g a s flow of 16 ms'l.
to) ? P S (b) 31ps (c) 7 m s (d) (Oms (e) l 4 m s (delay from pulse start)
FIGURE 5 MACH-ZEHNDER INTERFEROGRAMS I n 5 ( b ) , taken a t t h e end of t h e d i s c h a r g e p u l s e t h e beginnings of a c o u s t i c d i s t u r b a n c e a r e a p p a r e n t p a r t i c u l a r l y n e a r t h e cathode, w h i l s t i n 5 (c) t h e medium i s h i g h l y d i s o r d e r e d 7 ms a f t e r t h e pulse. I n 5 ( d ) t h e h o t / c o l d i n t e r f a c e i s seen midway a c r o s s t h e c a v i t y a t a d e l a y of 10 ms. By 14 ms t h e d i s t u r b a n c e s a r e almost over w i t h f r i n g e e r r o r s of l i t t l e more than X i n t h e v i s i b l e i.e. XI20 a t t h e 10.6 pm CO2 l a s e r wavelength and h i g h q u a l i t y performance should b e p o s s i b l e f o r f r e q u e n c i e s of 66 Hz o r l e s s . A time l a p s e c i n e f i l m has been made o f t h e i n t e r f e r o g r a m s which provides s t r i k i n g v i s u a l d i s p l a y of t h e medium p e r t u r b a t i o n s .
Multimode performance has been a s s e s s e d a t 30 p s i n p u t p u l s e l e n g t h f o r r e p e t i t i o n r a t e s up t o 66 Hz, Four r e s o n a t o r s have been used comprising a 65% r e f l e c t i n g germanium o u t p u t window and diamond turned copper m i r r o r s of 20, 50, 80 and 400 m R of
C. These a r e mounted i n a s t r o n g frame supported independently of t h e l a s e r ductwork. T y p i c a l l a s e r
aatput and discharge c u r r e n t waveforms a r e shown
i n F i g u r e 6 ( a ) . By c s e c £ t h e t a i l - b i t e r i n t h e modulator t h e s e p u l s e s can b e c u r t a i l e d a s i n d i c a -
t e d i n F i g u r e 6(b). Shot t o s h o t r e p e a t a b i l i t y h a s been a s s e s s e d a s 2 5% i n energy o v e r a wide range of p u l s e r e p e t i t i o n r a t e s .
I
OUTPUT 8.7 MWIDIV.CURRENT
500 A / DIV.
(a) FULL PULSE LENGTH ( b ) EFFECT O F TAILBITER
( 8 s h o t s ) FIGURE 6 LASER CURRENT LLND LASER OUTPUT
(Photon drag)
Output energy i s p l o t t e d i n F i g u r e 7. For t h e 400 m
m i r r o r u s a b l e energy i s reduced by t h e o p t i c a l s t o p s needed t o i n h i b i t e l e c t r o d e r e f l e c t i o n s . MIRROR RADIUS ,Or 0
1
I I I I 1 0 l000 2 0 0 0 3 0 0 0 4 0 0 0 5 0 0 0 INPUT ENERGY I J O U L E S IC9-354 JOURNAL DE PHYSIQUE
W*,, WIRE METE* OlFFIIICIIOU
,m FOCAL LENOT*
FIGURE 8 MULTIMODE DIAGNOSTICS The o p t i c a l d i a g n o s t i c s a r e shown i n F i g u r e 8. A
beam s p l i t t e r and a w i r e d i f f r a c t i o n g r a t i n g of 20 mR d i s p e r s i o n reduce energy t o a l e v e l s u i t a b l e f o r each d e t e c t o r . S p a t i a l energy d i s t r i b u t i o n s i n F i g u r e 9 a r e obtained from a 50 element p y r o e l e c t r i c a r r a y (7) which r e c e i v e s 1om3 of p u l s e energy a t t h e f o c u s of t h e 1 m l e n s . Each f i g u r e has 8 s h o t s superimposed a t 1 Hz, and g i v e s a good i n d i c a t i o n of energy p r o f i l e , r e p r o d u c a b i l i t y and s p a t i a l s t a b i l i t y . Divergence measured by h e a t s e n s i t i v e paper i s shown i n F i g u r e 10. T h e o r e t i c a l divergence
(8) i s l i s t e d and compared w i t h t h a t measured from l u c i t e burns and t h e d e t e c t o r a r r a y a t t h e 1 m focus.
TOP LEFT
,
X MIRROR 2 0 m R of C
FIGURE 1 0 MULTIMODE DIVERGENCE
With t h i s information e x t e r n a l be- h a a d l i n g o p t i c s a r e r e a d i l y designed. Beam p r o f i l e s a f t e r 24 m
propagation a r e shown i n F i g u r e I I (compare Fig. 9). Energy l o s s was l e s s than 10%.
Burns i n l u c i t e show l i t t l e spread a s p u l s e r e p e t i t - i o n r a t e i s increased from 1 t o 66 Hz f o r t h e c a v i t i e s using 20, 50, and 80 m r a d i i m i r r o r s . Power measurements have been c o r r e l a t e d w i t h watt- meter v a l u e s and photon drag power d e n s i t y wave- forms remain unchanged a s t h e pulse r a t e i s varied. V i b r a t i o n i n s t a b i l i t y makes t h e 400 m r a d i u s m i r r o r u n s u i t a b l e f o r h i g h p u l s e r e p e t i t i o n r a t e s .
TO P LEF' TO P L E F l
.
VERTICAL HORIZONTAL
FIGURE 9 MULTIMODE ENERGY DISTRIBUTION (Element s e p a r a t i o n 0.5 m)
TOP LEFT
VERTICAL HORIZONTAL
Input energy 5 kJ max PRF 66 Hz max Pulse l e n g t h E f f i c i e n c y Output 1 Hz 600 J max Output 62 Hz f o r 0.5 s 600 J (36 kW) Output 66 Hz f o r 1.0 S 330 J (22 kW) Shot t o s h o t v a r i a t i o n
+
5% Output beam c 12 X 12 c m Divergence (20 m m i r r o r )+
l 0 mR Burst r e p e t i t i o n 30 s i n t e r v a l s SINGLE MODE RESONATORSExploratory measurments have been made w i t h two s i n g l e mode r e s o n a t o r s exemplified i n F i g u r e 12.
CONFOCAL UNSTABLE
I
I
CONTINUOUSLY COUPLED UNSTABLEGe
FIGURE 12 RESONATOR TYPES
Typical o p t i c a l d i a g n o s t i c s a r e shown i n Figure 13.
-CAVE "mm
FIGURE 13 SINGLE MODE DIAGNOSTICS
A r u l e d p l a n e r e f l e c t i v e g r a t i n g w i t h 2 mR d i s p e r s - i o n i s used w i t h beam s p l i t t e r s t o reduce energy
i s measured w i t h t h e arrangements shown i n Figure 8. CONFOCAL UNSTABLE RESONATOR
The u n s t a b l e r e s o n a t o r u s e s c i r c u l a r near confocal m i r r o r s of m a g n i f i c a t i o n 2.1 and t h e o u t p u t i s annu-
l a r % 10 cm diameter. This o n l y p a r t l y couples t o
the a c t i v e medium and energy o u t p u t was about 113 of t h a t from a multimode c a v i t y f o r t h e same i n p u t energy. Energy d i s t r i b u t i o n s i n t h e f o c a l p l a n e a r e shown i n F i g u r e 14. Each f i g u r e r e p r e s e n t s 20 s h o t s a t 1 Hz. The half-height width i s seen t o approximate t o t h e 1 s t Airey Minimum, b u t i t i s c l e a r t h a t most of t h e energy i n t h e c e n t r a l maxi- mum occurs e a r l y i n t h e p u l s e s i n c e the upper
f i g u r e used t h e modulator t a i l - b i t e r to s h o r t e n t h e d r i v e p u l s e t o 15
v s ,
t h e remainder employed t h e f u l l 30 v s l e n g t h . As t h e f a n speed i sincreased from 400 t o t h e 1500 rpm (16 ms-I through t h e c a v i t y ) needed f o r high r a t e o p e r a t i o n t h e p o s i t i o n a l s t a b i l i t y d e t e r i o r a t e s .
C9-356 JOURNAL DE PHYSIQUE
(a) NEAR FIELD
(d) FAR FIELD ( b and c
superimposed )
U
40 ps
FIGURE 15 UNSTABLE RESONATOR PHOTON DRAG WAVEFORMS The temporal p r o f i l e s i n F i g u r e 15 r e v e a l r a p i d l o s s of an-axis i n t e n s i t y a f t e r s 6 p s i f t h e d e t e c t o r i s a p e r t u r e d t o 5.6 mm diameter ( s 4 AID) a s compared with 20 mm diameter a p e r t u r e ( s 14 AID). This power c o l l a p s e i s a s c r i b e d t o l a s e r induced medium p e r t u r b a t i o n (LIMP) where s e v e r e phase
e r r o r s develop due t o d i f f e r e n t i a l h e a t i n g of l a s i n g and non-lasing zones. The o u t p u t a l s o shows r a p i d f l u c t u a t i o n s a t t r i b u t e d t o complex r e l a x a t i o n o s c i l l a t i o n s i n an overcoupled o s c i l l a t o r . Both e f f e c t s a r e d e s c r i b e d by Lamberton and Roper (9)
who d i s c u s s dependence on r e s o n a t o r m a g n i f i c a t i o n , power, time and l a s e r k i n e t i c s .
CONTINUOUSLY COUPLED UNSTABLE RESONATOR
T h i s comprises a 200 m r a d i u s convex m i r r o r a p e r t u r - ed t o 10 cm and 65% r e f l e c t i v i t y Ge window. Equivr l e n t m a g n i f i c a t i o n i s 1.2. The o u t p u t i s c i r c u l a r s 10 cm diameter and d i v e r g e s by s 5 mR. E f f e c t - i v e - o u t p u t i s % 113 of t h e corresponding m u l t i - mode c a v i t y w i t h a p r o p o r t i o n absorbed i n t h e m i r r o r a p e r t u r e . T h i s c l a s s of r e s o n a t o r i s d e s c r i b e d i n a g e n e r a l review by Siegman (10). Eqergy d i s t r i b u t i o n s i n t h e f o c a l p l a n e a r e shown i n F i g u r e 16. ShORT P J - S E 4 0 0 r o m TOP L E F T TOP L E F T TOP L E F T VERTICAL HORIZONTAL 1 s t A i r e y rninlrnum H 2 4 m m (5elemmts)
FIGURE 16 CONTINUOUSLY COUPLED RESONATOR ENERGY DISTRIBUTION (Element s e p a r a t i o n 0.5 mm) Each f i g u r e r e p r e s e n t s 20 s h o t s a t 1
Hz.
The h a l f - h e i g h t width approximates t o t h e I s t Airey Minimum. No i n c r e a s e i n energy i n t h e c e n t r a l maximum o c c u r s a s t h e d r i v e p u l s e i n c r e a s e s from 15 p s t o 30 p s , b u t more s e n s i t i v e measurements show s i g n i f i c a n ti n c r e a s e i n
off-
a x i s i n t e n s i t y f o r t h e Longer p u l s e . S t a b i l i t y i s adequate a t 400 rpm b u t n o t a t 1500 rpm f a n speed. Temporal p r o f i l e s i n F i g u r e 17 show l o s s of on-axis i n t e n s i t y a f t e r % 3 p s w i t h t h e d e t e c t o ra p e r t u r e d t o 5.6 mm diameter ( a 5.6 AID) compared w i t h 20 mm diameter ( s 20 AID). The r a p i d temporal f l u c t u a t i o n s a r e n o t p r e s e n t s i n c e t h e m a g n i f i c a t i o n i s lower and t h e r e s o n a t o r i s n o t over-coupled. The i n t e r f e r o g r a m s showed t h a t t h e l a s e r medium homo- g e n e i t y should b e adequate f o r good q u a l i t y s i n g l e mode performance and f a s t recovery from d i s c h a r g e
( a ) N E A R FIELD
Id) FAR FIELD l b and c
superimposed )
FIGURE 17 CONTINUOUSLY COUPLED RESONATOR PHOTON
DRAG WAVEFORMS
However t h e p r e l i m i n a r y r e s u l t s from b o t h s i n g l e mode r e s o n a t o r s showed s e r i o u s l o s s of beam q u a l i t y w i t h i n t h e p u l s e due t o LIMP, and much work remains on r e s o n a t o r d e s i g n i f t h i s l a s e r is t o g e n e r a t e n e a r d i f f i c t i o n l i m i t e d o u t p u t a t p u l s e l e n g t h s up t o 30 ps.
P o s i t i o n a l i n s t a b i l i t y h a s been i d e n t i f i e d with a low frequency s p e e d d e p e n d e n t f an-induced v i b r a t i o n i n t h e o u t p u t window/mirror mount of up t o
+
120 PR. L u c i t e burns from t h e s i n g l e mode r e s o n a t o r s showedl i t t l e i n c r e a s e i n s c a t t e r a s r e p e t i t i o n r a t e i n c r e a s e d up t o 66 Hz. The c a v i t y o p t i c s seem t o b e s t a b l e a g a i n s t t h e impulse l o a d i n g from t h e d i s c h a r g e energy. CONCLUSIONS Many a s p e c t s of r e p e t i t i v e l y pulsed o p e r a t i o n of an e l e c t r o n beam s u s t a i n e d atmospheric p r e s s u r e carbon d i o x i d e l a s e r have been explored experiment- a l l y . The system i s w e l l c h a r a c t e r i s e d f o r m u l t i - mode o u t p u t s up t o 30 kWmean power. R e l i a b l e performance has been e s t a b l i s h e d o v e r a wide range of p u l s e r e p e t i t i o n f r e q u e n c i e s up t o 66 Hz. Mean
o r 22 kW f o r 1 second d u r a t i o n . P u l s e t r a i n s can be r e p e a t e d a t 30 second i n t e r v a l s . F x p l o r a t o r y work h a s confirmed t h e d i f f i c u l t i e s t o be expected i n a c h i e v i n g high q u a l i t y s i n g l e mode o u t p u t of long d u r a t i o n due t o l a s e r induced medium p e r t u r b a t i o n s .
ACKNOWLEDGEMENTS
We a r e indebted t o C W Glanford, D L Jordan, A Thomas, M F P a r i s , and D A Buckley who c o n t r i - buted t o t h e d e s i g n and o p e r a t i o n of t h e l a s e r and a s s o c i a t e d equipment.
REFERENCES
1 . Crocker A, Lamberton H M , F o s t e r , H, E l e c t r o n i c s L e t t e r s
8,
No 18, Sept 1972.2. Crocker A, Lamberton H M , Probyn H C , P a r c e l 1
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