2.2.2 ETICA DOCENTE

The carbon d io xid e la s e r , w hether o f slow a x ia l gas flo w , convective gas flo w o r TEA design (see s e c tio n 1 ,2 ) can a tta ia i e le c t r ic a l to o p tic a l conversion e ffic ie n c ie s approaching 25% fo r s u ita b ly o p tim ised c o n d itio n s . The discussion o f Q ia p te r 1 (s e c tio n 1 ,3 ) has shown th a t the achievem ent

o f th is h ig h e f fic ie n c y depends on b o th the la s e r gas tem perature and e le c t r ic discharge E /N . S e c tio n 1 .4 has i llu s t r a t e d how, in th e o ry , the discharge E/N may be c a lc u la te d from knoivledge o f the plasm a k in e tic s Cie attach m en t, io n is a tio n e t c ) . However, due to lade o f knowledge o f the p re c is e n a tu re o f the ions species p re s e n t, c a lc u la tio n s o f discharge E/N and o th e r dependent param eters (eg la s e r e f f ic ie n c y , s t a b i l i t y ) based on p re s e n tly a v a ila b le d a ta can o n ly be considered s p e c u la tiv e .

An e x p e rim e n ta l system has been designed and co n stru cted to mass- s p e c tro m e tric a lly analyse the p o s itiv e io n species p re s e n t in the p o s itiv e

column o f a ty p ic a l CO^ la s e r d isch arg e. This experim ent is based on

the p r in c ip le s o f p o s itiv e io n sam pling from glow discharge plasmas

discussed in C hapter 2 . The gas discharge source is in te n d e d to s im u la te

e le c t r ic a l co n d itio n s o c c u rrin g in the p o s itiv e column o f a c o n ven tio n al a x ia l flo w CO^ la s e r. V a r ia tio n o f gas flo w r a t e , discharge c u rre n t and pressure has enabled a d e ta ile d u n derstanding o f the p o s itiv e io n k in e tic s o f CO^ la s e r d is c h a rg e s , as w i l l be discussed in C hapter 4. This knowledge enables p ro je c tio n s o f l i k e l y p o s itiv e io n species in EDCL^s and TEA

la s e rs to be made in a more en lig h ten ed -m an n er than has been p o s s ib le in the p a s t.

Th is ch ap ter discusses the d e ta ils o f the p o s itiv e Io n sam pling

experim ent used to determ ine p o s itiv e io n species in CO^-N^-He discharges

and w it h the aid o f the th eo ry o f io n sam pling (c h a p te r 2) the r e l i a b i l i t y o f the re s u lts is .considered.

Tlie e x p e rim e n ta l system is shown in fig u re 3 .1 ,1 and consists o f th ree chambers corresponding to d if f e r e n t pressure regim es. The f i r s t is une discharge tube (p ~ 1—20 to r r ) from w hich ions and n e u tr a l p a r tic le s

e ffu s e to the in te rm e d ia te pumping chaidser (p < 10 t o r r ) through a

prim ary sam pling o r if ic e o f a, 100pm d ia m e te r. P a r tic le s e ffu s e from th is second chaiiher to the mass sp ectro m eter chamber, where the pressure is <10” ^ t o r r , through a 1 ram d iam eter o r if ic e at th e apex o f a s ta in le s s s te e l cone. The quadrupole mass sp ectro m eter (VG-Micromass Q8K) is s itu a te d in th is h ig h vacuum environm ent.

The requirem ents o f an io n m o n ito rin g system such as th a t o f fig u re 3 .1 ,1 have been considered by Mark and Helm (1974) and a r e ; -

1. Adequate re s o lv in g power and h ig h tran sm issio n . The re s o lv in g power is n o rm ally e le c t r o n ic a lly v a ria b le and t y p ic a l tran sm issio n e f fic ie n c y is ^ 90% fo r the quadrupole system.

2. H ig h s e n s it iv it y and la rg e dynamic range. These q u a lit ie s a llo w v e ry weak s ig n a ls to be d ete c te d and compared w ith the m ajor species in th e mass spectrum .

3 , E ith e r minimum en erg y d is c rim in a tio n Cover an energy range o f a t le a s t 10 eV) o r c o n tr o lle d , v a ria b le energy a n a ly s is . The l a t t e r allow s the e f f e c t o f io n energy d is c rim in a tio n to be e v a lu a te d ,

4. H i^ i source pressu re c a p a b iliti.e s , e n a b lin g in v e s tig a tio n o f plasmas over a id ,de range o f p ressu re.

5, Minimum in flu e h c e o f the mass sp ectro m eter a n a ly s in g f i e l d on the plasm a. H ig h m agnetic f ie ld s asso ciated w ith m agnetic s e c to r spectrom eters can d is to r t th e plasm a,

0: I Of z_o Ul Of h* a. O 1™ UJ _j «J 3 LU UJ O O , Df Q < 3 a s u to !

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7, U n c r itic a l io n in je c tio n angle. This s im p lifie s alignm ent in io n beam experim ents.

The ”VG Quadrupoles" Q8K quadrupole mass sp ectro m eter used in the

e x p e rim e n ta l c o n fig u ra tio n described in th is s e c tio n s a t is f ie s a l l o f