FIG 2 2 0 STEP THREE: DECAY OF IONISATION

U N !

rs

0

8

0*6

0*Z

J . Phys. D ; Appl. P h y s . , 1 1 , 3 1 3 - 3 2 3 ,1 9 7 8 .

BREl/ER, A.K. and VJESTHAVEH, J.U .

J . Appl. P h y s . ,8 ,7 7 9 - 7 8 2 , 1 9 3 7 .

BROtm, s . c .

" B a s ic Data o f P l a s u a P h y s i c s " , Wiley (Mew York) and Chapoan and H a l l (London), 1961.

FRANCIS, G.

Handbuch d e r P h y s i k , 2 2 ,5 4 - 2 0 8 ,1 9 5 6 .

FÜ JII, K . , TAKÂHASHI, T. and ASAKI, Y.

IEEE J . Quant. E l e c t r o n . . OE-11 .1 1 1 - 1 1 4 .1 9 7 5 .

FU JII, K. , OSHIIIA, T ., OTAKA, M. , NAGASHIÎ1A, S . , NIYAZ AW A, S. and OIKAWA, T.

- 64

GRACE, J .R ,

"Hollow Cathode î i e t a l Vapour L a s e r s " , ILSc. T h e s i s , U n i v e r s i t y o f New England, New South Wales,

A u s t r a l i a , 1978.

KIM-HUNG, U. and GREY NORGAM, C.

J . Phys. D : Appl. Phys. ,Ji_ ,L I-L 4 , 1983•

LEHMAN, J . F .

Proc. Roy. S o o . ,1 1 5 ,6 2 4 - 6 3 9 ,1 9 2 7 .

MCDANIEL, E.U.

" C o l l i s i o n Phenomena i n I o n i s e d G a s e s " , Wiley (New Y ork), 1964.

MIZERACZYK, J . and NEIGER, H.

XVI I n t e r n a t i o n a l C o n feren ce on Phenomena i n I o n i s e d Gases, 2 10-211, 1983,

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PIPER, J.A . and WEBB, G.E.

, J . Phys. D : Appl. Phys. ,£ , 4 0 0 - 4 0 7 , 1 9 7 3 .

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P r o c e e d i n g s o f t h e S o c i e t y o f P h o t o - O p t i c a l I n s t r u m e n t a t i o n E n g i n e e r s , 2 3 2 , 4 2 - 4 6 , 1 9 8 0 ,

CHAPTER THREE

THERMAL DESIGN CRITERIA FOR HOLLOW CATHODE He-Cd LASERS.

3.1 I n t r o d u c t i o n .

A l l o f t h e hollow cath o d e d e v i c e s u sed a s e x c i t a t i o n media f o r He-Cd l a s e r s depend on therm al e v a p o r a t i o n o f tlia m eta l

t o m a i n t a i n t h e m e t a l atom d e n s i t y w i t h i n t h e r an g e over v/hich optimum l a s e r o u t p u t o c c u r s . S in ce t h e number d e n s i t y / v a p o u r p r e s s u r e of cadmium i s s t r o n g l y t e m p e r a t u r e d e p e n d e n t, th erm al s t a b i l i t y i s an i m p o r t a n t f e a t u r e of a l l hollow c a th o d e ile-Cd l a s e r s . The a p p ro x im ate e m p i r i c a l law r e l a t i n g cadzium vapour p r e s s u r e and t e m p e r a t u r e i s g i v e n by

l o g / ( T o r r ) - 4 - 0 2 - ^ (Mesmeyanov, 1963).

66 -

v a r i e t y (eg F u j i i e t a l , 1975), t h e whole tube s t r u c t u r e needs t o be a t a uniform t e m p e r a t u r e o t h e r w i s e l o c a l v a r i a t i o n s i n th e cadmium vapour p a r t i a l p r e s s u r e may o c c u r. On t h e o t h e r hand, th o s e hollow c a th o d e l a s e r s which have an oven s o u r c e of cadmium ( P i p e r and Webb, 1973) g e n e r a l l y depend on d i s c h a r g e h e a t i n g t o m a i n t a i n t h e e n t i r e c a th o d e s t r u c t u r e a t a t e m p e r a t u r e above t h a t

o f t h e oven s o u r c e .

For both of t h e s e c a t e g o r i e s o f l a s e r , a x i a l t e m p e r a t u r e g r a d i e n t s produce l o c a l v a r i a t i o n s i n g a s d e n s i t y and cadmium vapour d e n s i t y and t h u s an a x i a l l y u n ifo r m t e m p e r a t u r e w i t h i n t h e cath o d e i s a d e s i r a b l e d e s i g n f e a t u r e .

For s l o t t e d hoi loi; c a th o d e s (eg, Sohuebcl 1970), an a x i a l l y uniform t e m p e r a t u r e i s f a c i l i t a t e d by t h e use o f a d i s t r i b u t e d anode, and t h u s t h e i n p u t power p e r u n i t l e n g t h i s c o n s t a n t . However, f o r f l u t e - t y p e g e o m e t r i e s w i t h d i s c r e e t anodes, and c o n c e n t r i c c y l i n d e r g e o m e t r i e s w i t h p e r f o r a t e d c a t h o d e s , th e i n p u t power p e r u n i t l e n g t h i s not c o n s t a n t and t h u s a x i a l t e m p e r a t u r e g r a d i e n t s w i l l o c c u r. The c h o ic e o f i n t o r - a n o c o s p a c i n g f o r f l u t e - t y p e g e o m e t r i e s and t h e s p a c i n g o f the p e r f o r a t i o n s i n c o n c e n t r i c c y l i n d e r g e o m e t r i e s i s t h u s not s o l e l y d ep en d en t on t h e n e c e s s a r y c o n d i t i o n o f o v e r l a p p i n g a x i a l n e g a t i v e glows. The a x i a l t e m p e r a t u r e g r a d i e n t s which r e s u l t must a l s o be ta k e n i n t o a c c o u n t .

I n t h i s c h a p t e r , a tw in t h i n - w a l l e d c a th o d e s t r u c t u r e w i t h d i s c r e t e anodes i s d e s c r i b e d . The th en .ial o p e r a t i n g c h a r a c t e r i s t i c s o f t h i s tu b e, t o g e t h e r w i t h t h o se o f t u b e s Hos 1, 2 and 3, show th e n e c e s s i t y f o r a d e t a i l e d a n a l y s i s upon which th erm al d e s ig n c r i t e r i a f o r hollow cath o d e l a s e r s may be b ased.

Tube No.4 i s o f s i l i c a and s t a i n l e s s s t e e l c o n s t r u c t i o n , w ith two 8 cm c a th o d e s e c t i o n s and t h r e e t u n g s t e n p i n anodes a r r a n g e d a s i l l u s t r a t e d i n f i g 3» I . The c a th o d e s e c t i o n s were o f t h i n w a l l e d ( 0 . 9 ma) s t a i n l e s s s t e e l p i p e , each w i t h a 4 .5 om bo re. Threaded n i c k e l t i p p e d t u n g s t e n rods screw ed i n t o t h e

c a th o d e w a l l s e r v e d a s e l e c t r i c a l " f e e d t h r o u g h s " f o r t h e c a t h o d e s , A c e n t r a l l y s i t u a t e d s i d e a r n c o n t a i n e d c a d n iu n o f n a t u r a l i s o t o p i c abundance, and t h e tu b e was e q u ip p e d w i t h B r e w s t e r angled windows. A u x i l i a r y ovens s i t u a t e d a t each end o f t h e d i s c h a r g e r e g i o n c l o s e t o t h e end anodes w e re i n c l u d e d i n t h e d e s i g n . The o u t e r s i l i c a e n v e lo p e f i t t e d c l o s e l y o v er t h e e x t e r n a l c a th o d e s u r f a c e s t o p r e v e n t t h e d i s c h a r g e from s t r i k i n g to thorn and g e n e r a t i n g unwanted h e a t . The oven h e a te r elem en t c o n s i s t e d o f a 50 cm l e n g t h of 36 swg Eureka w i r e , wound on a f o rm er, and p l a c e d o v er t h e s id earm , t h e t e m p e r a t u r e o f which was m o n ito re d w i t h a thcrmocoup].c. An a d j a c e n t t h e r m i s t o r provided t h e n e c e s s a r y f e e d b a c k f o r t h e " e l e c t r o n i c t h e r m o s t a t " d e s c r i b e d i n Appendix One, c a p a b l e of m a i n t a i n i n g a s te a d y sid e a r m t e m p e r a t u r e t o w i t h i n ± 2 °C i n the

68

3.3

The tu b e was i n i t i a l l y o p e r a t e d w ith t h e c i r c u i t i l l u s t r a t e d i n f i g 3 . 2 ( a ) , each of t h e t h r e e anodes b ein g i n d i v i d u a l l y b a l l a s t e d such t h a t t h e c e n t r a l anode c a r r ie d tw ice as much c u r r e n t a s e i t h e r of t h e a u x i l i a r y anodes.

With t h e c u r r en ts s e t such t h a t 1^ - = 120-15 i.iA, t h e i n t r o d u c t i o n o f cadmiun in to t h e d i s c h a r g e produced l o s i n g on t h e g r e e n l i n e s o f Cd I I w i t h th e same o p t i c a l c a v i t y as was d e s c r i b e d i n C h a p te r Two, However, l a s i n g c e a s e d when t h e c u r r e n t d i s t r i b u t i o n became uneven, w i t h I^ = 175 mA and. := 45 mA. A c l o s e r i n v e s t i g a t i o n o f t h i s w ith t h e c i r c u i t i l l u s t r a t e d i n f i g 3 . 2 ( b ) showed t h a t t h e uneven current d i s t r i b u t i o n o c c u r r e d i n a random fa sh io n . A ttem p ts to overcome t h i s by i n t r o d u c i n g a v a r i a b l e b a l l a s t r e s i s t o r R , _{e x t r a} i n t o t h e c i r c u i t , a s i l l u s t r a t e d