E-I CHARACTERISTICS OF AN OPTICALLY PUMPED MERCURY POSITIVE COLUMN

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E-I CHARACTERISTICS OF AN OPTICALLY PUMPED MERCURY POSITIVE COLUMN

H. Saelee, M. Cooke, J. Allen

To cite this version:

H. Saelee, M. Cooke, J. Allen. E-I CHARACTERISTICS OF AN OPTICALLY PUMPED MER- CURY POSITIVE COLUMN. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-179-C7-180.

�10.1051/jphyscol:1979788�. �jpa-00219494�

JOURNAL DE PHYSIQUE CoZZoque C7, suppZ6ment au n07, Tome 40, JuiZZet 1979, VagQ C7- 179

€-I CHARACTERISTICS ,$F AN OPTICALLY PUMPED MERCURY POSITIVE COLUMN

H.T. Saelee, M.J. Cooke and J.E. Allen.

Department o f Engineering Science, University o f Oxford, Parks Road, Osford, U.K.

INTRODUCTION l e v e l s a r e shown(Isc= 0 and 10A). The d e n s i t i e s of

Optically pumped d i s c h a r g e s ( 0 ~ ~ ) have been widely t h e 6 3 ~ and 6 3 ~ metastable s t a t e s do not change

0 2

much with o r without i r r a d i a t i o n . However, t h e used i n gas l a s e r s f o r population inversion. The

d e n s i t y of t h e 6 3 PI s t a t e i n c r e a s e s by more than present i n t e r e s t i n OPD i s i n t h e context of iso-

tope s e p a r a t i o n where a c a t a p h o r e t i c method has an order a t low Icell and l e s s t h a n twice a t high been proposed t o c o l l e c t t h e s e l e c t i v e l y excited Icell(> 100 d)*

atoms'. However, t h e k i n e t i c s of an OPD has t o be Fig. 3 shows t h e v a r i a t i o n s of t h e a x i a l f i e l d with well known i n order t o operate t h e cataphoresis a t vapour pressure (reduced t o O'C) f o r two i r r a d i a t i o n an optimal condition. l e v e l s on two discharge c u r r e n t s ( I ~ 10 and 100 ~ ~ =

EXPERIMENT ^mA). The e f f e c t of t h e i r r a d i a t i o n i s g r e a t e r as

t h e pressure i n c r e a s e s . The experimental apparatus has been described pre-

viously 2

.

It i s e s s e n t i a l l y two c o a x i a l Hg dis- DISCUSSION charges with t h e o u t e r source discharge(1 _{- .}

S C I n a normal ~ o s i t i v e column, t h e charged-pai-titles i r r a d i a t i n g t h e inner c e l l d i s ~ h a r ~ e ( 1 ~ ~ ~ ~ ) . The gain energy from t h e a x i a l e l e c t r i c f i e l d and d i s - i n n e r c e l l i s made of quartz g l a s s allowing t h e s i p a t e it through i n e l a s t i c c o l l i s i o n s . The a x i a l main r a d i a t i 0 n ( 6 ~ ~ ~ - 6 1 s o ) t o be t r a n s m i t t e d i n t o f i e l d a l s o maintains t h e current c o n t i n u i t y of t h e t h e inner discharge. The e x c i t e d s t a t e populations column. The l o s s of charged p a r t i c l e s by recombinat- have been measured by a resonant absorption method3 ion, both a t t h e w a l l and t h e plasma volume i s a n a t h e a x i a l e l e c t r i c f i e l d i n t h e p o s i t i v e column balanced by i o n i z a t i o n processes i n t h e plasma vol- by two v a r i a b l e probes. ume. The main i o n i z a t i o n processes a r e

1

+

RESULTS ~ g ( 6 s o ) + e-Hg + 2e

- ₊

The E-I c h a r a c t e r i s t i c s of t h e p o s i t i v e column a t a H ~ ( ~ ~ P , ) _v

+

e

-

^Hg

⁺

^2e

reduced pressure of 26.3 mtorr a r e shown i n Fig. 1

~ g ( 6 ~ P ~ ) + e

-

^{~ g + + 2e}

f o r various i r r a d i a t i o n l e v e l s (I =0,5,10 and 2 0 ~ ) . s C

+

The a x i a l e l e c t r i c f i e l d s were measured over a ~ g ( 6 3 ~ 2 )

+

e

-

^Hg

⁺

^2e

range of 20.0 cm and found t o be very uniform..For ~ g ( 6 ~ ~ ~ )

+

H g ( 6 3 ~ 1 )

-

^Hg2

+ ⁺

^e. ( 5 ) Isc= 0 we have t h e normal E-I c h a r a c t e r i s t i c of a

c o lwhere t h e a x i a l f i e l d f a l l s with i n c r e a s i n g ~ current (Icell). When t h e column i s i r r a d i a t e d t h e E-I c h a r a c t e r i s t i c changes completely and f o r ISc= 20A t h e a x i a l f i e l d increases monotonically with Icell,but a t a lower absolute value. The i r r a d i a t i o n has l e s s e f f e c t on t h e column a t high c u r r e n t ( I c e l ~ 2 0 0 mA). With Isc=20A and a t low current!Icell<50 MA) t h e a x i a l f i e l d is re- duced t o a n e g l i g i b l e value.

Fig. 2 shows t h e corresponding number d e n s i t y of t h e t r i p l e t s t a t e s f o r t h e same vapour pressure a s t h e r e s u l t s presented i n Fig. 1. Two i r r a d i a t i o n

-

Other i o n i z a t i o n processes a r e e l e c t r o n impact of t h e higher excited s t a t e s 4 . The r e l a t i v e importance of e l e c t r o n impact i o n i z a t i o n (1 t o 4 ) and associa- t i v e i o n i z a t i o n ( 5 ) v a r i e s with t h e discharge current and t h e vapour pressure.

I n t h e o p t i c a l l y pumped C0lUIOn processes ( 3 ) and ( 5 ) become more impm.tant due t o t h e increase i n t h e d e n s i t y of t h e 6 P I s t a t e . TO maintain t h e 3 charged p a r t i c l e balance, t h e a x i a l f i e l d lowers

(corresponding t o a lower e l e c t r o n tempera7ure) and i o n i z a t i o n by e l e c t r o n impact, e s p e c i a l l y t h e ion- i z a t i o n from t h e ground s t a t e becomes l e s s important.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979788

I n a normal column t h e a x i a l f i e l d increases with t h e pressure i n order t o maintain a c e r t a i n f i x e d E/P ( e l e c t r i c f i e l d / p r e s s u r e ) r a t i o such t h a t t h e

electron temperature remains constant. However, ⁿ

t h i s does not apply when a s s o c a i t i v e ionization

Y

becomes dominant which i s ' t h e case i n t h e o p t i c a l l y pumped column.

A q u a n t i t a t i v e explanation can bct obtained by an ion f l u i d model c a l c u l a t i o n since we a r e i n t h e t r a n -

s i t i o n regime of ambipolardiffusion and f r e e f a l l ;

a

t h i s was used by Johnson e t d2. We are a t present extending t h e model t o explain t h e E-I c h a r a c t i s t i c s .

This would require solving t h e Boltzmann equation

2

f o r t h e e l e c t r o n energy d i s t r i b u t i o n i n order t o

I I I I I I 1 I 1

calculate t h e e l e c t r o n d r i f t v e l o c i t y and t h e r a t e

foe

of electron impact ionization.

REFERENCES

1. Stangeby, P.C., Allen, J.E. and F r a s e r , D.A.,

xth

I .C.P.I .G. Proc., 1971, Oxford.

2. Johnson, P. C,

,

Cooke, M. J. and Allen, J.E.

,

J. Phys. D: Appl. Phys., Yol. 11, 1978, 1877- 92.

3. Johnson, P.C., Cooke, M.J. and Allen, J.E., I.E.E. Gas Discharges Conf. Proc., 1978, Liverpool.

4. Vriens, L., K e i j s e r , R.A.J. and L i g t h a r t , F.A.S., J. Appl. phys., 49(7), 1978, 3807-13.

Figure 2. Excited s t a t e number d e n s i t i e s . 3 ~ 0 ( * ) a 3~2(1)9 IsC = 0 A(----) I = 10A (-).

S C

1

^{I I}

2

10 100

REDUCED PRESSURE (m torr)

Figure 3. A x i d e l e c t r i c f i e l d v a r i a t i o n with pressure. Icell= 1 OmA ⁽⁰ ,o)

,

Icell= 1 OOmA (m ,R)

.

Figure 1. E-I C h a r a c t e r i s t i c s of t h e p o s i t i v e column.