HAL Id: jpa-00219472
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Submitted on 1 Jan 1979
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ROLE OF O2 (a’ ∆) IN THE OXYGEN PLASMA
J. Dettmer, A. Garscadden
To cite this version:
J. Dettmer, A. Garscadden. ROLE OF O2 (a’ ∆) IN THE OXYGEN PLASMA. Journal de Physique
Colloques, 1979, 40 (C7), pp.C7-135-C7-136. �10.1051/jphyscol:1979766�. �jpa-00219472�
JOURNAL DE PHYSIQUE CoZZoque C7, suppZ6rnent au n07, Tome 40, JuiZZet 1979, Page C7- 135
J.W. Dettmer and A. Garscadden
KirtZand Air Force Base, Albuquerque, New Mexico, U.S.A. 87110 and Wright-Patterson Air Force Base Ohio, U.S.A. 45433.
In t h i s study, comparisons between experiments and mathematical models i n d i c a t e t h a t 02*(a1A) a f f e c t s t h e discharge i n two d i f f e r e n t ways: f i r s t , two-step i o n i z a t i o n i s r e q u i r e d t o achieve t h e observed e l e c t r o n d e n s i t i e s , and secondly, detach- ment by o2*(a1A) on t h e negative i o n , 0-, i s r e q u i r e d t o match t h e t r a n s i t i o n p o i n t s between t h e s t a b l e and u n s t a b l e forms o f t h e d i s c h a r g e .
Our experiments encompassed 1-10 t o r r p r e s s u r e and a c u r r e n t d e n s i t y range o f -35 t o 35 ma/cm2 i n a discharge tube o f 19mm diameter and 50cm l e n g t h . Observations were t h e impedance c h a r a c t e r i s t i c s ; e l e c t r i c f i e l d , p r e s s u r e and temperature ( f o r determination o f E/N and N) ; ~ l e c t r o n d e n s i t y ; and
t h e t r a n s i t i o n n o i n t between t h e s t a b l e acd u ~ s t z b l e Figure 1. E f f e c t o f 02*(atA) on E l e c t r o n Density
I
n e u t r a l and charged s p e c i e s and u t i l i z e d 69 chemi-
1
121forms o f t h e d i s c h a r g e .
Our models encompassed 1 t o 100 Townsends f o r E/N and 3.2 x 1016 t o 5.6 x 1017 molecules/cc f o r N.
A Boltzmann code with momentum t r a n s f e r ; r o t a t i o n a l ,
. v i b r a t i o n a l and e l e c t r o n i c e x c i t a t i o n ; d i s s o c i a t i v e
attachment; i o n i z a t i o n ; d i s o c i a t i v e i o n i z a t i o n ; and
c a l and p h y s i c a l r e a c t i o n s , i n c l u d i n g 3-species ambipolar d i f f u s i o n , was used t o c a l c u l a t e t h e s p e c i e s number d e n s i t i e s and t r a n s i t i o n between t h e s t a b l e and u n s t a b l e forms o f t h e d i s c h a r g e .
d i s L o c i a t i o n c r o s s s e c t i o n s was used t o determine
-CHEMCO
0 E X P E R I M E N T S
t h e e l e c t r o n energy d i s t r i b u t i o n and forward pump- u C O N T I N U I T Y CALC.
i n g r a t e s . A chemistry code which tracked 10
8e
/
10,.
-
10%
',-
>t
Lo z w n
P = 5 TORR
4,
0 6
i 1 i
P = 4 T O R R ,
i !
I & ' : : : : ! : :
20 40 60 80
C U R R E N T ( m a )
The c r o s s s e c t i o n f o r i o n i z a t i o n from t h e Figure 2 . E l e c t r o n Density Comparisons
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979766
02*(a1A) state was assumed to be the same shape and magnitude as the process from the grouqd level, with the onset reduced by the difference in energy
between the two states. Calculations were perform- ed by the chemistry code, with and without
17 cm-3 two-step ionization. Results for n = 10 are shown in Fig. 1. Figure 2 compares the calculated electron densities with experimental values determined from toroidal cavity resonance and values calculated from current continuity equations. In Fig. 2 the calculated values are the ones which include two-step ionization. For the experiments the calculated ionization densities without 02'A ionization would be approximately an order of magnitude lower.
The oxygen discharge exists in two forms: a high-electric-field, stable form and a low-electric- field, unstable form. The latter can be described as an ionization-attachment-detachment instability.
The transition between stable and unstable forms has been postulated to occur when two criteria are
satisfied: first, a necessary condition is that the logarithmic derivative of the attachment coefficient with respect to E/N must dominate that of the ionization coefficient; and second, a sufficient condition is that >I. Figure 3
Ne
depicts code results of the first criterion for the two limiting conditions of all ground state and all metastable molecules. Figure 4 depicts the chemistry code calculations of the second criterion both with and without 02*(a1A). The primary contribution of 02*(a1A) in increasing the electron to ion ratio is as a detacher of electrons from 0-. Comparisons of the code-calculated transition points show that 02*(a1*) must be included to match the experimental data.
I I
Figure 3 . . Stability Criterion #I
Figure 4. Electron-to-Negative Ion Ratio
18
id.
z' 1 .
>
Z
l&
I$
1b3-
*Work performed in partial fulfillment of PhD requirements at the Air Force Institute of Technology.
EIN (Td)
N (cmn3) 3.16 ~10'~
STABLE-- --
--,- -- -- -- --
UNSTABLE
-WITH o2[a'A) ---WITHOUT oZ(a1A)
I
; 40 60 80 100
