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A POTENTIAL ATOMIC IODINE LASER PUMPED
BY ELECTRICALLY GENERATED 1∆ OXYGEN
G. Fournier, J. Bonnet, D. Pigache
To cite this version:
JOURNAL DE
PHYSIQUEColZoque
C9,suppZe'ment
au
n O 1 l , Tome
41,novembre 1980, page
C9-449A
POTENTIAL ATOMIC IODINE LASER PUMPED BY ELECTRICALLY GENERATED '
A
OXYGEN
G.
Fournier, J. Bonnet andD.
Pigache.O f f i c e National dlEtudes e t de Recherches Ae'rospatiales,
92320Cha^tiZZon, France.
RSsum6.- Une d6charge con~rS16e par faisceau d16lectrons pourrait produire le singulet
b
de l'oxy- gSne avec une concentration assez grande pour obtenir un effet laser aprbs mslange avec de l'iode. Diff6rents processus de collision entre les 6lectrons et les mol6cules d'oxygsne dans 1'Etat I A li- mitent la densits de celle-ci. I1 n'est pas certain que la stabilit6 de la d6charge permette d'at- teindre la valeur choisie de E/N (rapport du champ Qlectrique 1 la densit6 du gaz). I1 pourrait Btre ngcessaire de faire le m6lange 1 une tempgrature 16gSrement infgrieure1
l'ambiante pour obtenir un effet laser.Abstract.- Singlet l~oxygen could be generated in an electron-beam controlled discharge with a con- centration large enough to permit a laser effect after mixing with iodine. Various collision proces- ses of electrons with molecules limit their density. It is not sure that the selected value of
E/N
(electric field over gas density) is permitted by discharge stability. The laser operation could require mixing at a temperature slightly lower than room temperature.The usual atomic iodine laser However, that efficiency in ordinary dis-
I
2 ~ 1 2+ I 2 ~ 3 / 2 at 1.315urn)
is charges (glow or microwaves) is only a photolytically pumped [I] and this results few percent and the ratio E/N (electric in a very short pulse and a rather poor field/gas density) for optimum metastable efficiency. On the other hand, it has been pumping is an order of magnitude lower demonstrated that an atomic iodine laser than the value for self-sustained oper- can be continuously pumped with a chemical ation. This paper shows that an electron generator of 1~ oxygen [ 2 1.
A
condition beam controlled discharge could be an for lasing is a concentration ratio['A]
/
efficient oxygen generator to lase withC3z]
> 0.17 at 295K
131.
More pre- atomic iodine. The concept of this potell- cisely, the gain per cm in argon diluent tial Laser is given in Fig. 1. Upstream can be deduced from the stimulated emission the discharge, oxygen is dilutedin
a rare cross-section (41 and from the excitation gas in order to provide a high density balance between oxygen and atomic iodinemedium for efficient ionization by the electron beam and satisfactory energy loading. Iodine is mixed with this excited where T is the gas temperature and
I
means flow downstream the discharge and exci-2
I
2P3,2 only iPlI2 will b e labelledI
) . tation transfer occurs in the laser cavity.In order to produce high energy pulses, it The major fraction of this paper deals is attractive to generate the I A oxygen with a theoretical model but a few exper- with a discharge since the energy conver- imental results a-re also discussed. sion efficiency to singlet states can The properties o f electrical energy reach about 70% in pure oxygen [5]
.
transfer to the metastable states of oxy-~ 9 - 4 5 0 JOURNAL DE PHYSIQUE g e n a r e a n a l y z e d w i t h a B o l t z m a n n c o d e [51. The c r o s s s e c t i o n s f o r e l e c t r o n c o l l i s i o n s w i t h m o l e c u l a r o x y g e n i n t h e g r o u n d s t a t e a r e known 161 a n d t h o s e f o r s u p e r e l a s t i c c o l l i s i o n s c a n b e d e d u c e d f r o m t h e m b y m e a n s o f t h e d e t a i l e d b a l a n c e p r i n c i p l e .
Electrode
p u r e o x y g e n a n d a l l m o l e c u l e s i n t h e g r o u n d s t a t e . H o w e v e r , a m o r e c o r r e c t a p p r a i s a l o f A i s g i v e n b y t h e r a t i o : r 2=
k l / ( k 2+
k,+
k4
+
k 5+
k 6+
k 7+
k g ) w h i c h i s a l s o g i v e n i n F i g . 2 ( i n d i c e s r e f e r t o p r o c e s s n u m b e r i n T a b l e 1 ) .Laser cavity
Electron
Beam
8 d r a s t i c a l l y l i m i t t h e I A p o p u l a t i o n . A l s o g i v e n i n F i g . 2 a r e t h e v a l u e s o f 1-2 f o r a v e r y s m a l l f r a c t i o n o f 02 d i l u t e d i n t h e v a r i o u s r a r e g a s e s . A r a n d K r a r e t h e b e s t c a n d i d a t e s b u t t h e d i f f e r e n c e s a r e m i n o r . A r i s c h o s e n i n t h e f o l l o w i n g s i n c e i t i s c h e a p e r f o r e x p e r i m e n t s . F i g . 3 g i v e s t h e v a r i a t i o n s o f t h e r a t i o
r 2
i n a m i x t u r e A r : 02=
995 : 5 u n d e r v a r i o u s c o n d i t i o n s :-
f o r a l l m o l e c u l e s i n t h e g r o u n d s t a t e .-
f o r t h e m e t a s t a b l e f r a c t i o n s a t 1 2u s
i n F i g . 4 w h i c h a r e o b t a i n e d f r o m a k i n e t i c s c o d e w h i c h i s d i s c u s s e d l a t e r ( c u r v ei s
u n c h a n g e d ) .-
when e l e c t r o n - e l e c t r o n c o l l i s i o n s a r e t a k e n i n t o a c c o u n t ( t h e e l e c t r o n d e n s i t y i s a l s o t h a t a t 1 2 p s o n F i g . 4 ) . F i g u r e 3-
R a t i o r p i n A r : 02=
995 : 5-
A l l s p e c i e s i n t h e g r o ~ n d s t a t e o r with some m e t a s t a b l e population ( s e e t e x t ) ----. With an e l e c t r o n f r a c t i o n of 0 . 3 7 1 0 d t a k e n i n t o account. F i n a l l y , s i n g l e t m e t a s t a b l e o x y g e n p u m p i n g d o e s n o t d e p e n d v e r y much o n t h e v a r i o u s p h y s i c a l p r o p e r t i e s f o r a g i v e n e l e c t r o n t e m p e r a t u r e . E f f i c i e n c y f o r ' A p u m p i n g i s a b o u t 2 0 % i n t h e e x c i t e d A r : 02=
9 9 5 : 5 m i x t u r e a t Te=
1 . 9 eV. Figure 4 - Species k i n e t i c s .Table 2
-
Dominant r e a c t i o n s a f t e r mixing( t
;Rate c o e f f i c i e n t s
1
through the discharge chamber at a pressure
=
1.18 bar and a temperature=
300K.
The electron beam current den- sity is 10 mA/cm2 and E/N=
2 x 10-l7 vcm2 (corresponding T=
1.9 eV). Electron collision rates are given by the Boltzmann code. From 0 to 12 us the results computed with all molecules in the ground state are used. After 12 us the results used are those recomputed with the Boltzmann code for the various densities at 12 us. Beam ionization is cut off between 90 and loops. The flow relaxes between 100 and 140 us; this time corresponds to an injection distance of 4 mm at a flow velocity of 100 m/s. At that time, it is assumed in the model that a uniform iodine density of 1015 ~ mhas been mixed to the - ~ flow and, further, dissociation and excitation of iodine occur.
Fig. 4 shows that thepopulation inversion (
[
I
*
]
/
[
I
]
^I 0.5) is hardly reached for that case. This prediction is not very optimistic but it has been noted that the calculation of the density is based on crude estimates for some cross-sections. If the pro- posed laser cannot be actually operated, Eq. (1) suggests that two main parameters can be adjusted:and
T.
[I]
depends on [ l ~ l and Fig. 4 shows clearly that iodine dissociation could be improved by a shorter mixing time. Iodine could also be mixed before the discharge but basic data are not available to predict excitation properties of a discharge with iodine.T appears in an exponential in Eq. (1). A moderate decrease of T with respect to room temperathre results in a much less severe requirement about
C14
.
There is, however, a conflict between a large [I] and a lowT
because the equilibrium iodine pressure at lower temperature cannot provide the required iodine atom concentration. The solution could be a supersonic expansion of preheated mixture including iodine, or mixing of hot iodine in a cold supersonic rare-gas-oxygen flow.
The results of the kinetics code show that there is no accumulation of charged species even if ionization of Ar and 02 excited states is taken into account. This situation is favourable from the point of view of discharge stability. However preliminary experiments performed in various 02
-
Ar and 02-
Nemixtures exhibit discharge current instabilities as shown in Figure 5 and breakdown occurs at E/N values lower than those of interest.
These oscillations are probably due to the attachment instability [lo] but they occur at E/N values lower (by a factor of 2 to 3) than the theoretical threshold well verified experimentally according to [lo] in other gas mixtures. No satisfactory explanation has been found up to now.Figure 5 - Typical discharge current trace showing oscillations
-
Time scale 100 us/div. - Current scale 5 A/div.-
Gas mixture=
Ne : O2=
0.96 :0.04 - Pressure 1.15 bar
-
Discharge voltage 800 volts-
Discharge gap 2 cm-
Discharge cross- section 75 cm2-
Electron-beam current density 1.6 mA/cm2 - Under these conditions the voltage threshold for occurrence of the oscillation is 650 volts and the breakdown voltage is 1650 volts.As a conclusion, these preliminary theoretical and experimental results predict that the
require a carefull operation of the discharge
It can also be attempted to replace iodine by an
to obtain the selected E/N value, and a sophis-
iodine donor molecule.
ticated mixing at a temperature lower than usual
This research was supported by DRET and
room temperature to improve population inversion.
DGRST
.
References
1
K
.,
HOHLA, K.L., KOMPA, "The Photochemical
Iodine Laser", in Handbook of Chemical
laser, R.W.F. GROSS and J.F. BOTT, eds.,
Wiley-Interscience, New York (1976).
[2]
