MASS SPECTROMETRIC STUDIES OF IONIZATION PRECURSORS AHEAD OF STRONG SHOCK WAVES
Thesis by
William M. Robinson, IBr,
In Partial Fulfillment of the Requirements
For the Degree sf
Doctor sf Philosophy
California Institute of Technology Pasadena, California
1969
The a u t h o r w i s h e s t o e x p r e s s h i s s i n c e r e a p p r e c i a t i o n t o h i s a d v i s o r , P r o f e s s o r Hans W. Liepmann, whose e n t h u s i - asm and g u i d a n c e s e r v e d a s a c o n s t a n t s o u r c e o f m o t i v a t i o n and i n s p i r a t i o n . The c o n t r i b u t i o n s and a d v i c e s f
P r o f e s s o r Bradford S t u r t e v a n t h e l p e d c o n s i d e r a b l y t o make t h e p r e s e n t work p o s s i b l e , H i s p a t i e n t s u g g e s t i o n s o f how t o t r a n s l a t e t h e m a n u s c r i p t from S o u t h e r n i n t o E n g l i s h a r e g r a t e f u l l y a p p r e c i a t e d .
The a u t h o r w i s h e s t o t h a n k Mr, B r i a n Barce3.o f o r
d e s i g n i n g and c s n s t r u c t i n g t h e p i e z o e P e c t s r i e gauge and f o r h i s a i d i n p r e p a r i n g t h e m a n u s c r i p t . S p e c i a l t h a n k s go t o M r s . J a c q u e l y n Beard f o r t y p i n g t h e m a n u s c r i p t ,
The a u t h o r i s i n d e b t e d t o t h e C a l i f o r n i a I n s t i t u t e s f Technology and t h e N a t i o n a l A e r o n a u t i c s and Space
A d m i n i s t r a t i o n f o r t h e i r f i n a n c i a l a s s i s t a n c e . The work i n t h e GAECIT 6 " shock t u b e was s u p p o r t e d by t h e N a t i o n a l A e r o n a u t i c s and Space A d m i n i s t r a t i o n ,
ABSTRACT
An e x p e r i m e n t a l s t u d y was conducted t o i d e n t i f y t h e n a t u r e and o r i g i n o f p r e c u r s o r i o n s produced by photo- i o n i z a t i o n ahead of s t r o n g shock waves i n xenon. A m a g n e t i c mass s p e c t r o m e t e r i s mounted a t t h e end o f a hydrogen-driven shock t u b e , I o n s produced u p s t r e a m o f a shock wave a r e sampled by t h e s p e c t r o m e t e r and t h e c o l - Bected c u r r e n t p r o v i d e s a c o n t i n u o u s t i m e h i s t o r y o f a p a r t i c u l a r i o n i c s p e c i e s . A mass s p e c t r u m i s o b t a i n e d i n t h e p r e c u r s o r r e g i o n f o r a l l i m p u r i t i e s found i n the shock t u b e . The i n c i d e n t shock Mach number i s v a r i e d from 11.9 t o 21.3, t h e i n i t i a l p r e s s u r e i s v a r i e d from 0.050 t o
6,500 t o r r , and t h e i m p u r i t y l e v e l i s a l t e r e d . I n a l l t h e c o n d i t i o n s s t u d i e d , t h e dominant i o n p r e s e n t i n t h e pre-
+
c u r s o r i s Xe
,
a l t h o u g h i n c e r t a i n i n s t a n c e s , t h e i m p u r i t y i o n c u r r e n t s a r e found t o b e o f t h e same o r d e r o f magnitude a s t h e xenon Eon c u r r e n t , For s m a l l i m p u r i t y l e v e l s ,p h o t o i o n i z a t i o n p r o c e s s e s i n xenon and i m p u r i t i e s a r e a p p a r e n t l y i n d e p e n d e n t . Independent d o u b l e probe measure-
l 2 - 3
ments d e t e r m i n e t h e t o t a l i o n d e n s i t y t o b e a b o u t 1 0 em a t t h e shock f r o n t , t h e o b s e r v a b l e p r e c u r s o r e x t e n d i n g a b o u t 158 em from t h e shock wave,
calculated ion density profiles agree well with expeui-
mental observations at the low pressures, where it appears
that one-step photoionization predominates. Lack of
agreement at high pressures, where, apparently, multi-step
ionization is more efficient than the single-step process,
suggests inadequacies in the treatment of photoexcitation
and multi-step photoionization. Additional areas for
experimental study are suggested,
The mass spectrometric data yield a better under-
standing of the role of radiation in shock structure, of
the kinetics of photoionization processes in rare gases,
and of the influence sf impurities in the experimental
PART
TABLE OF CONTENTS
TITLE
Acknowledgements
Abstract
Table of Contents
List of Figures
9 . Introduction
1.1 Motivation for the Study of Precursors
1.2 Previous Theoretical and Experimental Studies
B,3 Aim of the Present Experimental Technique
11. Description of the Experimental Apparatus
2.
B
Brief Description of the Shock Tube 2.2 The Mass Spectrometer2.3 The kangmuir Double Probe
PIX. Experimental Results
3.1 Measurement of the Total Ion Density of the Double Probe
3.2 Precursor Ionic Products in "Pure" Xenon
3 , 3 Effect of Impurities on Precursors
3.3a Naturally-Occurring Impurities
3.33 Added Empur it ies
3 . 3 ~ Impurities in the Driver Section
3.4 Experiments in Argon
HV. Theoretical Results
4,l One-step Photoionization of Xenon and
Hmpur ities
4.2 MuPt i-Step Photoionization of Xenon
TABLE O F CONTENTS ( c o n t
.
)PART TITLE PAGE
5 . 1 P h o t o i o n i z a t i o n a t Low and High I n i t i a l P r e s s u r e s
5 . 2 P r e c u r s o r Dependence on Shocked Gas Temperature
5.3 D i s c u s s i o n
VI. C o n c l u s i o n s 40
Appendices
A. The GALCIT 6 " Shock Tube 42
24.1 D e s c r i p t i o n o f t h e B a s i c Shock-Tube System 42 A. 2 O p e r a t i n g C o n d i t i o n s and L i m i t a t i o n s 45
A - 3 D e t e r m i n a t i o n o f Shock Wave V e l o c i t y and
P o s i t i o n 46
A.4 Measurement o f I n i t i a l P r e s s u r e and Hmpur i t y Level
A.5 N a t u r a l I m p u r i t i e s P r e s e n t i n t h e 6 "
Shock Tube 50
A , & P r e s s u r e R e l i e f System f o r t h e 6 " Shock Tube 5 1 B. The Mass S p e c t r o m e t e r a s a D i a g n o s t i c Tool 54
B. 1 P h y s i c a l C h a r a c t e r i s t i c s s f t h e S p e c t r o m e t e r 55 B - 2 O p e r a t i n g C o n d i t i o n s and L i m i t a t i o n s o f t h e
Mass S p e c t r o m e t e r 59
B . 3 Mass S p e c t r o m e t e r C a l i b r a t i o n 62 @, The Langmuir Double Probe
@.I The Eangmuir Double Probe a s a D i a g n o s t i c Tool
C . 2 D e s c r i p t i d n o f t h e Probe and i t s A p p l i c a t i o n C.3 Measurement of Number D e n s i t y w i t h a
Mexican Radio S t a t i o n D. T y p i c a l Experimental Procedure E. Theory o f P h o t o i o n i z a t i o n
E.1 I o n i z a t i o n Ahead s f a S t r o n g Shock Wave E. 2 One-Step P h o t o i o n i z a t i o n of Xenon and
PART
TABLE OF CONTENTS ( c o n t . )
TITLE PAGE
E . 3 P h o t o e x c i t a t i o n and M u l t i - S t e p Photo-
i o n i z a t i o n o f Xenon 88
R e f e r e n c e s 98
T a b l e s 102
LIST OF FIGURES
Schematic Diagram of the Apparatus
GALCIT 6 " Shock Tube Test Section
Mass Spectrometer Installation at the End of the Shock Tube
Mass Spectrometer Installation
Schematic Representation of a Mass Spectrum of Impurities in the 6" Shock Tube
Langmuir Double Probe Schematic
Probe Ion Current Prof iles Showing the Effect of Variation of Impurity Levels
Probe Ion Current at a Distance of 5 cm as a Function of Equilibrium Temperature behind the Shock
Comparison of Ion Number Density Determined from Double Probe Currents and Xenon Ionic Currents Obtained from Mass Spectrometric Data for Two Different Initial Pressures
Schematic of Probe-Measured Ion Density and Spec- trometer-Collected Ion Currents as a Function of Distance from the Shock Wave (Low Pressure)
Schematic of Probe Measured Ion Density and Spec- trometer-Collected Ion Currents as a Function of Distance from the Shock Wave (High Pressure)
Comparison of One-Step Photoionization Theory with
Maximum Spectrometer Currents for Injection of Molecular Oxygen and Nitrogen in Pure Xenon
Spectrometer OscilPograms Showing Decrease in Xenon Current Due to Addition of Large Amounts of Impurities
Argon Ionic Current Collected by the Spectromet~r
Simplified Schematic of Xenon Energy Levels and Rates
Shock Tube Radiation Model in Shock-Fixed Coordinates
Comparison between Experimental Spectrometer Measure- ments and One-Step Photoionization Theory for Low
LIST OF FIGURES (cont 'd. )
18. Comparison between Experimental Spectrometer Measure- ments and Theory for High Initial Pressure
19. Profile Comparison between One-Step Photoionization Theory and Double Probe Data as a Function of Tempera- ture behind the Shock
20. Schematic of Piezoelectric Detector
21. Typical Piezoelectric Crystal and Thin-Film ~esistance Gauge Oscillograms
22. Typical Spectrometer Oscillograms Showing the Method of Data Reduction
23. Typical Double Probe Response Showing Localized Perturbation
24. Double Probe Response Showing EocaPized Perturbation with Fast-Rise-Time Circuitry
25. Shock Tube and Associated Equipment Ground Reference
26. Detail of Probe Perturbation Showing ModuLations and High Frequency Components
2 7 , Comparison of Spectrometer Current Using Semi-Empirical
I. INTRODUCTION
The p r e s e n c e o f e l e c t r i c a l l y - c h a r g e d p a r t i c l e s i n t h e u n d i s t u r b e d g a s ahead of s t r o n g shock waves h a s r e c e n t l y been the s u b j e c t o f i n t e n s e s t u d y ; t h e i s m s and e l e c t r o n s
a r e c a l l e d " i o n i z a t i o n p r e c u r s o r s " , Such p r e c u r s o r s a r e observed i n e x p e r i m e n t a l f a c i l i t i e s g e n e r a t i n g s t r o n g shock waves, There e x i s t s s t r o n g e x p e r i m e n t a l e v i d e n c e t h a t t h e i o n i z a t i o n i s due t o t h e p r o p a g a t i o n o f r a d i a n t e n e r g y i n t o t h e r e g i o n ahead o f t h e shock, a l t h o u g h d i f f i c u l t i e s a r e e n c o u n t e r e d i n t h e o r e t i c a l l y p r e d i c t i n g t h e p r e c u r s o r ion- i z a t i o n Bevels due t o p h o t o i o n i z a t i o n ,
M o t i v a t i o n f o r t h e Study o f P r e c u r s o r s
There i s no d o u b t t h a t a s shock-wave s t r e n g t h s in- c r e a s e , t h e e f f e c t o f r a d i a t i o n from t h e shocked g a s must b e c o n s i d e r e d . Large r a d i a n t f l u x e s may p r o p a g a t e ahead of
t h e shock wave and p r e h e a t sr i o n i z e t h e u n d i s t u r b e d g a s . Under t h e s e c o n d i t i o n s t h e g a s b e h i n d the shock f r o n t l o s e s energy and i s t h e r e b y r a d i a t i v e l y c o o l e d , w h i l e t h e shock wave i s p r o p a g a t i n g i n t o an i o n i z e d plasma i n s t e a d of a
c o l d g a s , The r e s u l t i s a m o d i f i c a t i o n of t h e f l o w con- d i t i o n s i n t h e shocked g a s .
Thus, f o r s t r o n g shock waves t h e precursor becomes a p a r t o f t h e t o t a l shock s t r u c t u r e , and a knowledge o f p r e c u r s o r p r o p e r t i e s i s mandatory. A s t u d y of p r e c u r s o r s
r a d i a t i o n k i n e t i c s on r e a c t i o n s i n shock h e a t e d f l o w s i n e x p e r i m e n t a l f a c i l i t i e s , b u t b e c a u s e c l o s e l y r e l a t e d
e f f e c t s may o c c u r i n p r a c t i c a l h y p e r v e l o c i t y problems such a s s p a c e c r a f t r e - e n t r y ( s e e f o r example, r e f e r e n c e 1 ) . The p r o p a g a t i o n o f e l e c t r o m a g n e t i c r a d i a t i o n i n t h e
v i c i n i t y o f a r e - e n t r y body would c e r t a i n l y b e a f f e c t e d by t h e i o n i z a t i o n l e v e l s u r r o u n d i n g it.
1.2 P r e v i o u s E x p e r i m e n t a l and T h e o r e t i c a l S t u d i e s
The f i r s t manned o r b i t a l r e - e n t r y by t h e MA-6 Mercury c a p s u l e i n 1962 o f f e r e d a p o s s i b l e o b s e r v a t i o n sf p r e c u r - s o r s i n t h e l e a d i n g edge r a d a r e c h o e s , A p p a r e n t l y t h e u l t r a v i o l e t r a d i a t i o n from t h e h i g h t e m p e r a t u r e r e g i o n behind t h e bow shock wave o f t h e b l u n t - n o s e d body i o n i z e d t h e c o l d g a s ahead s u f f i c i e n t l y t o r e f l e c t r a d a r s i g n a l s from t h e ground, The p r e c u r s o r i o n i z a t i o n Bevel was s u f - f i c i e n t t o r e t u r n r a d a r e c h o e s a t a l t i t u d e s f a r i n e x c e s s sf where t h e p h y s i c a l dimensions o f t h e c a p s u l e would have r e t u r n e d them. L i n , e t . a l . ( R e f , 21, c a l c u l a t e d t h e
e l e c t r o n number d e n s i t y from p h o t o i o n i z a t i o n t o b e
-
710 emw3 a t a mean r a d i a l d i s t a n c e o f 1 8 m e t e r s from t h e bow shock. T h i s phenomena was observed a t an a l t i t u d e o f
220,000 f e e t ( p r e s s u r e
-
8.100 tsrr) w i t h a c a p s u l e v e l o c i t y o f 7 xl o 5
em/sec (Ms--
2 5 ) .p o s s i b l e mechanisms: 1) p h o t o e m i s s i o n from t h e shock t u b e w a l l s and i n s t r u m e n t a t i o n 2 ) d i f f u s i o n o f charged p a r t i c l e s
from t h e h o t r e g i o n b e h i n d t h e shock 3 ) p h o t o i o n i z a t i o n by s t r o n g r a d i a t i o n from b e h i n d t h e shock wave, S e v e r a l
e x p e r i m e n t a l s t u d i e s i n argon and xenon a t h i g h Mach numbers were conducted i n v a r i o u s shock t u b e f a c i l i t i e s and d i f f e r -
i n g r e s u l t s were o b t a i n e d . H o l l y e r (Ref. 3 ) i n v e s t i g a t e d t h e e f f e c t w i t h b i a s e d p r o b e s and concluded t h a t photo- e m i s s i o n and i m p u r i t y l e v e l were v e r y i m p o r t a n t , In some o f t h e e x p e r i m e n t s , h e found b o t h i o n s and e l e c t r o n s p r e s e n t ahead o f t h e shock, Groenig (Ref. 41, u s i n g a s p a r k d i s - c h a r g e p r o b e , and Weymann ( R e f s . 5 , 6 ) u s i n g p o t e n t i a l p r o b e s , e l e c t r o s t a t i c p r o b e s , and i n d u c t i o n c o i l s , found t h a t e l - e c t r o n d f f f u s i o n from b e h i n d t h e shock was t h e dominant mechanism. G l o e r s e n (Ref. 7 ) , u s i n g p o t e n t i a l p r o b e s , found a 1 1 t h r e e mechanisms p r e s e n t and d e t e r m i n e d t h a t t h e i n t r o d u c t i o n of i m p u r i t i e s caused i n c o n s i s t a n t r e s u l t s ,
Lederman and Wilson ( R e f s . 9 ,
l o )
conducted h i g h Mach number e x p e r i m e n t s i n argon i n a s m a l l d i a m e t e r shock t u b e , u s i n g tuned microwave c a v i t i e s . F o r s i m i l a r c o n d i t i o n s , t h e i r measured p r e c u r s o r i o n i z a t i o n l e v e l s were lower t h a n Holmes' ,
implying t h a t shock t u b e geometry may b e i m p o r t a n t . Using f i r s t a b i a s e d e l e c t r o s t a t i c g r i d t o c o n t a i n d i f f u - s i n g e l e c t r o n s and t h e n an opaque r a d i a t i o n b l o c k w i t h o b l i q u e l y d r i l l e d h o l e s t o a l l o w d i f f u s i o n , t h e y concluded t h a t a t l e a s t f o r t h e i r c o n d i t i o n s , p h o t o i o n i z a t i o n was t h e major c a u s e o f p r e c u r s o r i o n i z a t i o n . They found t h a t t h ei n j e c t i o n o f r a t h e r l a r g e amounts o f hydrogen a s an i m p u r i t y a p p r e c i a b l y changed t h e p r e c u r s o r c h a r a c t e r i s t i c s ,
P r e c u r s o r s i n e l e c t r o m a g n e t i c shock t u b e s have been observed and r e p o r t e d i n t h e l i t e r a t u r e . These shock t u b e s have l a r g e amounts o f r a d i a t i o n and induced e l e c t r o m a g n e t i c e f f e c t s emanating from t h e i n i t i a l e l e c t r i c a l d i s c h a r g e . A s t u d y u s i n g a combustion-driven shock t u b e (Ref, 11) r e p o r t e d s p u r i o u s s i g n a l s d u r i n g i g n i t i o n sf t h e d r i v e r . The r e s u l t s o f t h e s e e x p e r i m e n t s were e x c l u d e d from con- s i d e r a t i o n h e r e due t o t h e shock t u b e d r i v e r t e c h n i q u e f u r t h e r c o m p l i c a t i n g measurements o f t h e p r e c u r s o r phenomena.
have r e c e i v e d c o n s i d e r a b l e t h e o r e t i c a l a t t e n t i o n . S e v e r a l a n a l y s e s ( R e f s . 1 2 , 1 3 , 1 4 ) o f t h e e l e c t r o n d i f f u s i o n model have b e e n made, u s i n g d i f f e r e n t e l e c t r i c f i e l d d i s t r i b u t i o n s
i n t h e shock wave (due t o c h a r g e s e p a r a t i o n ) and v a r i o u s models f o r e l e c t r o n p r o d u c t i o n . A 1 1 i n v e s t i g a t i o n s men- t i o n e d t h e s t r o n g i n h i b i t i n g e l e c t r i c f i e l d a r i s i n g from t h e f a s t d i f f u s i o n o f e l e c t r o n s away from t h e l e s s mobile i o n s ( r a t i o o f d i f f u s i o n c o e f f i c i e n t s f o r e l e c t r o n s and i o n s i s i n v e r s e l y p r o p o r t i o n a l t o t h e s q u a r e r o o t o f t h e i r mass r a t i o ) . The g e n e r a l c o n c l u s i o n was t h a t e l e c t r o n d i f f u s i o n may b e n e g l e c t e d f o r d i s t a n c e s g r e a t e r t h a n one Debye l e n g t h ahead s f t h e shock wave, an i n s i g n i f i c a n t d i s t a n c e i n comparison w i t h e x p e r i m e n t a l o b s e r v a t i o n s .
F e r r a r i and C l a r k e (Ref, 1 5 ) t h e o r e t i c a l P y t r e a t e d t h e p h o t o i o n i z a t i o n problem. Assuming o n e - s t e p p h o t o i o n i z a t i o n a s t h e dominant mechanism, t h e i r r e s u l t s showed a l m o s t
n e g l i g i b l e i o n i z a t i o n i n comparison t o t h e e x p e r i m e n t a l o b s e r v a t i o n s i n argon.
Wetzel (Ref. 1 3 ) found t h a t r a d i a t i o n o f s u f f i c i e n t energy t o p h o t o i o n i z e a r g o n from t h e a t o m i c ground s t a t e decayed much t o o r a p i d l y t o e x p l a i n t h e observed i o n i z a - t i o n p r o f i l e s . HoSmes (Ref. 8 ) h a s s u g g e s t e d t h a t r a d i a t i o n p r o c e s s e s i n v o l v i n g one o r more a b s o r p t i o n s t e p s t o
and most recently Dobbins (Ref. 18) have studied the role
of line radiation and "radiation trapping" in ionization
processes ahead of strong shock waves. Their results also
failed to predict the observed ionization levels.
Dobbins gave specific consideration to shock tube
geometry and the physical characteristics of the problem
(e.g., length of the radiating volume of shocked gas,
reflection of the shock tube walls, ete.) This aspect
of his work was especially important in view of the fact
that most of the previous theoretical investigations
assumed a one-dimensional model. In all
sf
the experimentalwork reported, the shock tube diameter was less than two
inches. Correlating a theoretical radiation model with a
small geometry was very difficult. Dobbinss axially-
symmetric treatment is relevant in the present experiment
since the shock tube radius is much larger than those used
in previous studies and in the present work a discontin-
uous change of diameter occurs in the shock tube (see
Appendix B. l)
.
Appleton (Ref, 14) and Wilson and Lin (Ref, 19) found
that photoionization of small amounts of impurities present
in a shock tube might explain the magnitude of ionization
experimentally observed, The lower number densities and
smaller photoionization cross sections of various impurity
s o t h e decay r a t e o f t h e e l e c t r o n and i o n p r o f i l e s f o r one- s t e p p h o t o i o n i z a t i o n i s n o t l a r g e i n some c a s e s .
The d i a g n o s t i c s used i n a l l s f t h e p r e v i o u s s t u d i e s were such t h a t o n l y t h e t o t a l e l e c t r o n d e n s i t y o r c u r r e n t
and t h e t o t a l i o n d e n s i t y c o u l d b e o b s e r v e d . I n s i g h t i n t o t h e b a s i c e x p e r i m e n t a l problem c o u l d n o t b e g a i n e d u n t i l t h e r e l a t i v e magnitudes o f t h e independent p a r a m e t e r s , s u c h a s i m p u r i t y l e v e l , c o u l d b e i s o l a t e d and s t u d i e d s e p a r a t e l y . The main c o n c l u s i o n s t h a t may b e drawn from t h e l i t e r a t u r e were t h a t f o r t h e c a s e of s t r o n g ( i . e . ,
Ms > 1 0 ) shock waves, t h e p r e c u r s o r was dominated by photo- i o n i z a t i o n , and t h a t t h e i m p u r i t y l e v e l was o f extreme importance. Of t h e t h e o r e t i c a l a n a l y s e s t h a t have been made, a l l p r e d i c t e d i o n i z a t i o n a l l e v e l s o r d e r s o f magnitude
lower t h a n t h e p e r t i n e n t e x p e r i m e n t a l r e s u l t s .
1 . 3 A i m and Methods o f t h e P r e s e n t Experiments
I n e a r l i e r s t u d i e s , c o n v e n t i o n a l probe and microwave d i a g n o s t i c s produced d i f f e r i n g r e s u l t s depending on t h e e x p e r i m e n t a l f a c i l i t y u s e d , implying t h a t t h e o b s e r v a t i o n s may b e i m p u r i t y dominated. These s t u d i e s emphasized t h e complexity of o b s e r v i n g t h e p r e c u r s o r phenomena, and some- t i m e s made it d o u b t f u l a s t o what was b e i n g s t u d i e d ,
o f i m p u r i t i e s a r e a consequence o f t h e i n t e r f e r e n c e s o f t h e e x p e r i m e n t a l f a c i l i t y .
I t i s w e l l known t h a t i m p u r i t i e s can a p p r e c i a b l y a l t e r and sometimes dominate t h e shock-wave c h a r a c t e r ist ies
,
s u c h a s r e l a x a t i o n t i m e , i n a shock t u b e ( f o r example s e e r e f e r e n c e s 20 and 2 1 ) . The c o n f l i c t i n g e x p e r i m e n t a l d a t a and t h e f a c t t h a t Some i m p u r i t y g a s e s have l o n g e r r a d i a t i o n mean f r e e p a t h s t h a n t h e n o b l e g a s e s emphasizes t h e impor- t a n c e o f s t u d y i n g t h e r o l e o f shock-tube i m p u r i t i e s i n t h e p r e c u r s o r . S i n c e p h o t o i o n i z a t i o n h a s t h e p r o p e r t y of
p r o d u c i n g f r e e i o n s a t l a r g e d i s t a n c e s from t h e shock wave, and i d e n t i f i c a t i o n of t h e s e i o n s would g i v e i n s i g h t a s t o t h e n a t u r e and o r i g i n o f t h e p r e c u r s o r i o n i z a t i o n , a mass s p e c t r o m e t e r i s used t o examine t h e p r e c u r s o r r e g i o n , T h i s d i a g n o s t i c t o o l a l l o w s an uncoupPing or s e p a r a t i o n of t h e mechanisms i n f l u e n c i n g shock s t r u c t u r e i n t h e " p u r e " t e s t g a s from t h e e f f e c t o f v a r i o u s f o r e i g n g a s e s i n t h e , t e s t f a c i l i t y i t s e l f . I n d i v i d u a l e f f e c t s , s p e c i e s by s p e c i e s a r e , examined and i n t h i s manner t h e t o t a l e f f e c t i s deduced.
A s c h e m a t i c of t h e a p p a r a t u s i s shown i n f i g u r e 1- The GALCIT* 6 " shock t u b e i s used t o r e p r o d u c i b l y g e n e r a t e s t r o n g normal shock waves. A m a g n e t i c mass s p e c t r o m e t e r i s mounted on t h e shock t u b e end w a l l , sampling i o n s from a r e g i o n on t h e a x i s of t h e shock t u b e , I o n s produced
u p s t r e a m of t h e a p p r o a c h i n g shock wave a r e sampled by t h e s p e c t r o m e t e r and t h e c o l l e c t e d c u r r e n t p r o v i d e s a c o n t i n - uous t i m e h i s t o r y of a p a r t i c u l a r i o n i c s p e c i e s . Assuming a c o n s t a n t shock v e l o c i t y
Us
i n t h e o b s e r v a t i o n t i m e t,
t h e c u r r e n t h i s t o r y becomes a s p a t i a l p r o f i l e o f i o nd e n s i t y ahead of t h e shock wave t h r o u g h t h e t r a n s f o r m a t i o n x = U s t
.
A complete mass spectrum i s o b t a i n e d by d u p l i - c a t i n g r u n s w i t h t h e s p e c t r o m e t e r s e t a t d i f f e r e n t massp e a k s , Dominant i m p u r i t i e s i n t h e shock t u b e a r e d e t e r m i n e d and t h e r e l a t i v e abundance o f t h e i n d i v i d u a l i o n s a r e s u r - veyed under a v a r i e t y s f t e s t c o n d i t i o n s , The i n c i d e n t shock Mach number i s v a r i e d from 82 t o 21, t h e i n i t i a l p r e s s u r e i s v a r i e d from 0.058 t o 0,508 t o r r , and t h e
i m p u r i t y l e v e l is a l t e r e d . To o b t a i n a q u a n t i t a t i v e
e s t i m a t e o f t h e i o n i z a t i o n Bevels and t o check t h e o v e r a l l r e p e a t a b i l i t y o f t h e shock c o n d i t i o n s , an e l e c t r o s t a t i c
i o n c o l l e c t i n g probe i s u s e d ,
A t h e o r e t i c a l model o f t h e r a d i a t i o n p r o c e s s e s i s compared w i t h t h e e x p e r i m e n t a l r e s u l t s , The geometry o f t h e l a r g e r a d i u s shock t u b e used i n t h e p r e s e n t i n v e s t i - g a t i o n s h o u l d b e more a p p l i c a b l e t o t r e a t m e n t by t h e o r e t i c a l c o n s i d e r a t i o n s s i n c e it i s d i f f i c u l t t o e s t a b l i s h a c o r r e c t r a d i a t i o n model f o r t h e shocked g a s i n a c o n s t r i c t e d
geometry s u c h a s a s m a l l r a d i u s shock tube. Gaining
most fundamental property of the precursor, i.e., the
relative abundance of its constituents, the experimental
conclusions may be used to locate aspects of the theory
that are incomplete and to determine the accuracy sf
theoretical assumptions. Identification of the correlation
between theory and experiment leads to the prediction of
the relative importance sf precursors in
the
rest of the11. DESCRIPTION OF THE EXPERIMENTAL APPARATUS
2 . 1 B r i e f D e s c r i p t i o n o f t h e Shock Tube
The e x p e r i m e n t s were performed i n t h e GAECIT 6'" d i a m e t e r shock t u b e ( s e e f i g u r e s 1, 2 ) . A more complete d e s c r i p t i o n o f which i s g i v e n i n Appendix A and r e f e r e n c e 22. The e s s e n t i a l a d v a n t a g e s o f t h i s shock t u b e f o r t h e p r e s e n t i n v e s t i g a t i o n a r e : a ) i t s low l e a k r a t e and e v a c u a t i o n c a p a b i l i t i e s which p e r m i t t h e i m p u r i t y Bevels
i n t h e t e s t g a s t o b e c o n t r o l l e d , b ) i t s u n i q u e diaphragm- opening mechanism, t h e u s e o f which r e s u l t s i n r e p r o d u c t i o n o f shock s t r e n g t h t o a n a c c u r a c y o f +l%, and c) i t s
p r e v i o u s l y d e t e r m i n e d c h a r a c t e r i s t i c s ( e . g . , i o n i z a t i o n a l e q u i l i b r i u m r e l a x a t i o n t i m e s ) f o r u s e i n t h e o r e t i c a l
c a l c u l a t i o n s ( s e e r e f e r e n c e 2 3 ) .
The s t a i n l e s s s t e e l t u b e c o n s i s t s of a 6 " P.D., l o w p r e s s u r e t e s t s e c t i o n , 37"ong and honed i n t e r n a l l y to a m i r r o r f i n i s h , and a 6 - 5 " I . D , d r i v e r s e c t i o n , 6 . 5 ' l o n g . In t h e p r e s e n t i n v e s t i g a t i o n , room t e m p e r a t u r e hydrogen o r helium i s used a s t h e d r i v e r g a s t o produce i n c i d e n t shock Mach numbers r a n g i n g from 11.9 t o 21.3 i n xenon ( a l i m i t e d number of e x p e r i m e n t s were a l s o conducted i n a r g o n ) . The i n i t i a l p r e s s u r e o f xenon i s v a r i e d from 8.85 t o 8.5 t o r r .
The i n c i d e n t shock v e l o c i t y and p o s i t i o n a r e d e t e r - mined t o
21%
by o b s e r v i n g t h e r e s p o n s e s o f two p l a t i n u m' I
t h i n f i l m h e a t t r a n s f e r gauges and an e l e c t r o s t a t i c a l l y - s h i e l d e d p i e z o e l e c t r i c c r y s t a l mounted a d j a c e n t t o t h e mass s p e c t r o m e t e r s
With t h e mass s p e c t r o m e t e r i o n s o u r c e connected
d i r e c t l y t o t h e shock t u b e t e s t s e c t i o n , t h e abundance o f " n a t u r a l l y o c c u r r i n g " (due t o l e a k s and o u t - g a s s i n g )
i m p u r i t i e s i s d e t e r m i n e d , The predominant i m p u r i t i e s a r e N 2 # 5 0 , O Z Y and s e v e r a l g r o u p s sf o r g a n i c compounds, i n d e c r e a s i n g abundanee.
2 . 2 The Mass S p e c t r o m e t e r
A m o d i f i e d N i e r - t y p e magnetic mass s p e c t r o m e t e r * i s mounted on t h e end w a l l o f t h e shock t u b e ( s e e f i g u r e s 3 , 4 ) - I o n s produced by p h o t o i o n i z a t i o n i n f r o n t o f an
a p p r o a c h i n g shock wave a r e sampled t h r o u g h s m a l l o r i f i c e s i n t h e end w a l l and t h e measured c u r r e n t s p r o v i d e a
c o n t i n u o u s t i m e h i s t o r y o f a p a r t i c u l a r i o n with micro- second r e s o l u t i o n , i . e . , t h e i o n abundance a s a f u n c t i o n sf d i s t a n c e from t h e shock wave, High mass r e s o l u t i o n and e x t r e m e l y f a s t time-response a r e o b t a i n e d by o b s e r v i n g t h e t i m e h i s t o r y f o r o n l y one mass peak d u r i n g each r u n .
On t h e o t h e r hand, s e v e r a l r u n s a r e r e q u i r e d t o o b t a i n t h e complete spectrum, making d u p l i c a t i o n o f e x p e r i m e n t a l
c o n d i t i o n s v e r y i m p o r t a n t . The mass r e s o l v i n g power o f t h e s p e c t r o m e t e r i s v a r i a b l e from l i n 1 0 t o l i n 200, and i o n d e n s i t i e s g r e a t e r t h a n
lo9
cm-3 i n t h e shock t u b e can b e d e t e c t e d .I n t h e p r e s e n t shock t u b e e x p e r i m e n t s , t h e i m p u r i t y l e v e l s , t h e i n i t i a l p r e s s u r e and t h e Mach nunibers a r e
s y s t e m a t i c a l l y a l t e r e d . The r e l a t i v e abundance o f i n d i v i d - u a l i o n s i n t h e p r e c u r s o r r e g i o n o f t h e shock wave a r e
surveyed under a v a r i e t y o f t e s t c o n d i t i o n s . By d e t e r m i n i n g t h e e f f i c i e n c y of t h e sampling p r o c e s s , a c a l i b r a t i o n
c o n s t a n t i s o b t a i n e d f o r t h e s p e c t r o m e t e r , r e l a t i n g t h e c o l l e c t e d i o n c u r r e n t w i t h a p a r t i c u l a r i o n number d e n s i t y i n t h e shock t u b e d u r i n g an e x p e r i m e n t ,
A n e x t e r n a l i o n s o u r c e which i s used t o set t h e
p a r t i c u l a r mass number peak b e f o r e e a c h r u n ( s e e Appendix ID) a l s o d e t e r m i n e s t h e dominant i m p u r i t i e s i n t h e shock t u b e .
( s e e f i g u r e 5 ) . The spectrum s c a n i s o b t a i n e d by c o n t i n - u o u s l y a l t e r i n g t h e s p e c t r o m e t e r m a g n e t i c f i e l d s t r e n g t h w h i l e o b s e r v i n g t h e c o l l e c t e d i o n i c c u r r e n t .
2.3 The kanqmuir Double Probe
To o b t a i n a q u a n t i t a t i v e e s t i m a t e o f t h e i o n i z a t i o n l e v e l s , an e l e c t r o s t a t i c d o u b l e probe ( s e e Appendix @)
d i f f e r e n c e which a s s u r e s p o s i t i v e - i o n - c u r r e n t s a t u r a t i o n ( s e e f i g u r e 6 ) . The probe s y s t e m c o n s i s t s o f two w i r e s e x t e n d i n g i n t o t h e t e s t s e c t i o n i n a p l a n e p e r p e n d i c u l a r t o t h e a x i s of t h e shock t u b e . Assuming t h a t t h e i o n i c c u r r e n t f a l l i n g on t h e probe i s c h a r a c t e r i s t i c o f t h e random t h e r m a l motion i n t h e g a s ahead o f t h e shock wave, t h e c u r r e n t i n t h e n e g a t i v e l y b i a s e d probe i s d i r e c t l y r e l a t e d t o t h e i o n d e n s i t y i n t h e t e s t s e c t i o n . The
i n i t i a l t e s t p r e s s u r e s i n t h e p r e s e n t e x p e r i m e n t s a r e such t h a t t h e c r i t e r i o n f o r t h e f r e e m o l e c u l a r f l o w model o f t h e probe c u r r e n t co8l.ection s h o u l d b e r e a s o n a b l y w e l l s a t i s f i e d , The mass of t h e predominant i o n c o l P e c t e d by t h e probe i s d e t e r m i n e d by t h e mass s p e c t r o m e t e r ,
O b s e r v a t i o n of t h e t o t a l i o n c u r r e n t c o l l e c t e d d u r i n g e a c h r u n a s s u r e s r e p e a t a b i l i t y o f p r e c u r s o r i o n i z a t i o n l e v e l s w h i l e t h e i n d i v i d u a l c u r r e n t s a r e c o l l e c t e d by t h e s p e c t r o m e t e r . The d o u b l e probe i s used t o monitor t h e
p r e c u r s o r i o n i z a t i o n i n d i f f e r e n t r e g i o n s o f t h e shock t u b e a s w e l l a s i n t h e s p e c t r o m e t e r sampling p o s i t i o n , Probe o r i e n t a t i o n i s v a r i e d t o d e t e r m i n e t h e e f f e c t o f photo- e m i s s i o n from t h e probe s u r f a c e ( s e e Appendix C.2). Under p a r t i c u l a r o p e r a t i n g c o n d i t i o n s , a n i n t e r e s t i n g r e s o n a n c e phenomenon i s o b s e r v e d , which l e a d s t o a f u r t h e r independent d e t e r m i n a t i o n o f number d e n s i t y . T h i s e f f e c t i s d e s c r i b e d
i .
111. EXPERIMENTAL RESULTS
The p r e c u r s o r i o n i z a t i o n l e v e l s a r e s u f f i c i e n t l y h i g h t o p r o v i d e measurable s p e c t r o m e t e r i o n c u r r e n t s i n " p u r e " xenon. The r e l a t i v e abundance i n t h e p r e c u r s o r o f t h e i n d i v i d u a l shock-tube i m p u r i t i e s under a v a r i e t y o f t e s t c o n d i t i o n s i s d e t e r m i n e d ,
Examination o f t h e e x p e r i m e n t a l d a t a shows t h a t i n s i g h t i n t o t h e fundamental problem may b e g a i n e d by
comparing two p r e c u r s o r p r o f i l e s t h a t e x e m p l i f y c o n t r a s t i n g f e a t u r e s . The p r e c u r s o r d a t a may b e grouped i n t o Bow and h i g h - p r e s s u r e r e g i m e s .
The g r o s s f e a t u r e s o f t h e p r e c u r s o r w i l l b e q u a l i t a - t i v e l y d e s c r i b e d f i r s t , Then t h e mass s p e c t r o m e t r i c r e s u l t s f o r the two p r e s s u r e d i v i s i o n s w i l l b e g i v e n , f o l l o w e d by a d i s c u s s i o n sf t h e r o l e of i m p u r i t i e s on p r e c u r s o r s . The abundance o f i m p u r i t i e s i n t h e p r e s e n t s e c t i o n a r e d e t e r m i n e d by %he p a r t i a l p r e s s u r e o f
" n a t u r a l l y - o c c u r r i n g " f o r e i g n g a s e s , u n l e s s o t h e r w i s e n o t e d .
3.3. Measurement o f t h e T o t a l Ion D e n s i t y w i t h t h e Double Probe
During e a c h r u n , a measurement o f t h e t o t a l i o n
s p e c t r o m e t e r samples s e p a r a t e mass peaks. I n g e n e r a l , if a s t r i c t p r o c e d u r e i s Eollowed f o r a s e r i e s o f r u n s , t h e t o t a l i o n d e n s i t y measurements a r e q u i t e r e p r o d u c i b l e . The d o u b l e probe i s u s e f u l f o r e x h i b i t i n g t h e g r o s s be- h a v i o r o f t h e p r e c u r s o r b e c a u s e w i t h t h e s p e c t r o m e t e r t h i s would r e q u i r e s e v e r a l measurements.
T y p i c a l probe i o n c u r r e n t s observed d u r i n g e x p e r i m e n t s t o d e m o n s t r a t e t h e e f f e c t o f t h e i n j e c t i o n of i m p u r i t i e s a r e shown i n f i g u r e 7 . Changing t h e i m p u r i t y l e v e l s r e s u l t s i n s m a l l v a r i a t i o n s a s compared t o t h e t o t a l i o n c u r r e n t s c o l l e c t e d . A t i m p u r i t y PevePs g r e a t e r t h a n a b o u t 1%, t h e p r e c u r s o r c h a r a c t e r i s t i c s show d i s t i n c t a l t e r a t i o n s , s u c h a s a complete Pack o f p r e c u r s o r ; t h u s i m p u r i t y con- c e n t r a t i o n s above t h i s Bevel a r e n o t c o n s i d e r e d ( s e e p a r t 3,3).
The probe i s a l s o used t o d e t e r m i n e t h e dependence on p a r t i c u l a r p a r a m e t e r s i n t h e e x p e r i m e n t s , I t i s n o t
a p p a r e n t what c h o i c e of independent p a r a m e t e r s b e s t e x h i b i t s t h e n a t u r e s f t h e p r e c u r s o r . The two major
Holding t h e Mach number c o n s t a n t and v a r y i n g t h e i n i t i a l p r e s s u r e w i l l n o t merely change t h e r a d i a t i o n mean f r e e p a t h s i n c e v a r i a t i o n o f p r e s s u r e a l s o changes t h e e q u i l i b - r i u m t e m p e r a t u r e . Keeping t h e i n i t i a l p r e s s u r e c o n s t a n t and v a r y i n g t h e Mach number, on t h e o t h e r hand, s h o u l d show t h e dependence o f t h e p r e c u r s o r on t h e e q u i l i b r i u m t e m p e r a t u r e o f t h e shocked g a s . F i g u r e 8 shows a
s e m i l o g a r i t h m i c p l o t o f t h e probe i o n c u r r e n t , a t a
d i s t a n c e o f 5 c m from t h e shock wave, v e r s u s t h e e q u i f i b - rium t e m p e r a t u r e , A comparison w i f f b e made i n P a r t 5.2 between t h i s p l o t and a t h e o r e t i c a l c a l c u l a t i o n o f one-
s t e p p h o t o i o n i z a t i o n .
3.2 P r e c u r s o r I o n i c P r o d u c t s i n "Pure" Xenon
The v a r i o u s i o n i c p r o d u c t s produced by p h o t o i o n i z a t i o n ahead of s t r o n g shock waves i n " p u r e " xenon, i n c l u d i n g
i m p u r i t i e s i n t r o d u c e d by t h e e x p e r i m e n t a l f a c i l i t y , a r e measured w i t h t h e mass s p e c t r o m e t e r . A l a r g e nuniber o f d i f f e r e n t i o n s a r e observed. w i t h ~ e ' i o n s predominating under a l l c o n d i t i o n s .
For comparison, the ion density profiles (dashed lines)
determined from the double probe currents are shown for
the same conditions. The probe current tends to follow
the xenon ion current for both cases. The impurity levels
refer to the abundance of "naturally-occurring" impurities,
i.e., impurities resulting from test section leak and out-
gassing.
In
general, at low initial pressures (P,--
0.100 torr) as the Mach number and shocked gas equilibriumtemperature increase, so does the impurity ionization,
Below Mach numbers Ms -- 13
,
all impurity currents arebelow the sensitivity limit sf the spectrometer. The
higher the initial pressure of xenon, the greater the ratio
of xenon to impurity ionic currents, implying that the
contributions to the precursor of the individual species
are uncoupled and proportional to their partial pressures.
<
The use sf the mass spectrometer in the present application
then allows measurements to be made of impurity effects and
-
of photoionization in the xenon.
The results of a series of runs at a particular
impurity level are shown in figure PO. The solid lines are
the ion currents collected by the mass spectrometer. The
dashed line is again the total ion numb& density as
measured by the double probe. The conditions considered
i o n i z a t i o n i s of t h e same o r d e r o f magnitude a s t h e xenon i o n i z a t i o n . The g r o u p o f i o n s w i t h mass numbers 68-71 amu i s t h e dominant i m p u r i t y i n t h e p r e c u r s o r , a l t h o u g h it i s n o t t h e most abundant i m p u r i t y i n t h e shock t u b e b e f o r e e a c h r u n ( s e e f i g u r e 5 and Appendix A.5). Some s m a l l amounts o f i o n i z a t i o n p r o d u c t s w i t h mass numbers o f 1 (H*)
,
16 (0'),
65-67 ( ~ e + * ) and 57 amu.
b u t no9
d i s c e r n i b l e amounts of 2 ( H ~ + ) and 1 8 amu (H20 ) a r e o b s e r v e d . Consequences of v a r y i n g t h e i m p u r i t y l e v e l s w i l l b e d e s c r i b e d i n t h e f o l l o w i n g s e c t i o n *
Eons w i t h mass 65-67 amu
,
p o s s i b l y a t t r i b u t a b l e t o d o u b l y - i o n i z e d xenon, a r e d e t e c t e d a t t h e h i g h e r Mach numbers (Ms > 1 7 ) The abundance o f t h e predominant i s o t o p e s o f xenon a r e g i v e n i n t h e f o l l o w i n g t a b l e(Ref. 2 4 ) .
.
I s o t o p e % N a t u r a l Abundance
Xe 12 9 26.24
Xe 1 3 1 2 1 - 2 4
Xe 132 26.93
Xe 134 10.52
Xe 136 8.93
Doubly-ionized X e 132
,
X e 134,
x e 136 have mass numbers ( 6 6 , 67, 68 amu) l o c a t e d i n c l o s e p r o x i m i t ycurrents, the test section is evacuated to a pressure
below which negligible impurity currents are collected by
the spectrometer. The measured currents at these mass
numbers do not change with impurity level, implying they
++
are related only to the xenon atoms present, i.e., Xe
The effect of increasing the initial pressure is
shown in figure 11. At this high pressure, xenon ioniza-
tion completely prevails over impurity ionization far
ahead of the shock wave as well as near the wave. Xenon
ions are detected up to 120 crn from the shock wave.
Typical impurity ion profiles are shown for two different
impurity levels, Pn comparison with figure 10, the
magnitudes of the impurity ionization Bevels are about the
same as at lower initial pressures, showing the radiation
mechanism in% the impurity photoionization process has not
basically changed; only the xenon ionization level shows
a behavioral change, e.g., a change in slope at distances
far from the wave. Sampling limitations imposed by the
mass spectrometer preclude the study of pressures
Pl > 0.500 torr (see Appendix B.2); however, no major
changes in the basic precursor structure are anticipated
for higher pressures. Xenon ionization should be much
3 . 3 E f f e c t o f I m p u r i t i e s on P r e c u r s o r s 3.3a N a t u r a l l y - O c c u r r i n q I m p u r i t i e s
A t low p r e s s u r e s (PI -- 0.100 t o r r xenon) t h e
p r e s e n c e o f s m a l l amounts o f i m p u r i t y g a s e s i n t h e shock t u b e t e s t s e c t i o n r e s u l t s i n i m p u r i t y i o n i z a t i o n Bevels o f t h e same o r d e r of magnitude a s t h e xenon l e v e l s i n t h e p r e c u r s o r . A s t h e i n i t i a l p r e s s u r e o f xenon i n c r e a s e s , t h e r o l e o f i m p u r i t i e s i n t h e p r e c u r s o r becomes n e g l i g i b l e i n comparison t o t h a t of xenon ( F i g . 1 % ) . The e f f e c t o f v a r y i n g t h e i m p u r i t y l e v e l s i n P o w - i n i t i a l - p r e s s u r e xenon e x p e r i m e n t s w i l l now b e q u a l i t a t i v e l y d e s c r i b e d .
A s d i s c u s s e d i n Appendix 24.4 t h e " n a t u r a l l y - o c c u r r i n g " i m p u r i t y Bevel f o r e a c h r u n i s c a l c u l a t e d from t h e degas- s i n g r a t e and t h e t i m e e l a p s e d between c l o s i n g t h e
d i f f u s i o n pump v a l v e and b u r s t i n g t h e shock t u b e diaphram. Allowing l o n g e r e l a p s e d t i m e s between c l o s i n g t h e pump and
f i r i n g c a u s e s t h e p a r t i a l p r e s s u r e s o f t h e main c o n s t i t - u e n t s o f a i r t o i n c r e a s e w h i l e t h e o t h e r i m p u r i t y l e v e l s remain f a i r l y c o n s t a n t , a s would b e e x p e c t e d from l e a k a g e i n t h e shock t u b e t e s t s e c t i o n ( s e e Appendix A . 5 ) , The b e h a v i o r sf t h e O2
'
and N2 + i o n c u r r e n t s i n t h e pre-c u r s o r f o r a v a r i a t i o n i n n a t u r a l l y - o c c u r r i n g i m p u r i t y l e v e l s w i l l b e d i s c u s s e d i n P a r t 3.3b.
Lowering t h e i m p u r i t y l e v e l from 0 . 1 t o 0.05% r e s u l t s i n a d e c r e a s e i n t h e c u r r e n t o f t h i s g r o u p o f i m p u r i t i e s , u n t i l t h e ion d e n s i t y l e v e l i s below t h e s e n s i t i v i t y of
t h e mass s p e c t r o m e t e r . T h i s group o f o r g a n i c i o n s i n t h e p r e c u r s o r i s p r o b a b l y a t t r i b u t a b l e t o o i l s i n t h e shock- t u b e d i f f u s i o n pump. The p r e s e n c e o f t h e s e same heavy m o l e c u l e s i n t h e background s p e c t r u m s f t h e mass s p e c t r o - m e t e r , a l s o d i f f u s i o n pumped, s u p p o r t s t h i s i m p l i c a t i o n
( s e e f i g u r e 5 ) . The abundance o f t h e 68-71 amu i o n i c group i n t h e p r e c u r s o r , i n comparison t o i t s r e l a t i v e abundance i n t h e shock t u b e b e f o r e e a c h r u n , i m p l i e s p h o t o i o n i z a t i o n c h a r a c t e r i s t i c s t h a t a r e d i f f e r e n c e from
3.3b Added Hmpur it i e s
Small amounts s f a s p e c i f i c g a s a r e i n t r o d u c e d w i t h t h e xenon i n t o t h e t e s t s e c t i o n t o s t u d y t h e e f f e c t o f a p a r t i c u l a r i m p u r i t y s p e c i e s . The e f f e c t s o f a d d i n g q u a n t i - t i e s of p u r e Q2 o r N2 a r e shown i n f i g u r e 12, The
b e h a v i o r o f t h e N2
+-
and O2 Q i o n s i n t h e e a s e o f natu- r a l l y o c c u r r i n g i m p u r i t i e s i n P a r t 3.3a, i s t h e same a s i n t h i s p a r t i f t h e same p a r t i a l p r e s s u r e s a r e i n i t i a l l yp r e s e n t i n t h e t e s t s e c t i o n . The maximum s p e c t r o m e t e r c u r r e n t s ( a t t h e shock f r o n t ) f o r t h e s e r i e s o f r u n s a r e shown a s a f u n c t i o n of t h e p e r c e n t abundance o f t h e p a r t i c - u l a r g a s s p e c i e s i n p u r e xenon. 4-
+
c u r r e n t s a r e below t h e mass s p e c t r o m e t e r s e n s i t i v i t y f o r i m p u r i t y l e v e l s below 0.1%. For e q u a l amounts o f O2 and N2 i m p u r i t i e s , t h e o b s e r v e d abundance o f O2 i s h i g h e r t h a n f o r N2. C u r r e n t s f o r b o t h s p e c i e s e x h i b i t a maximum a s t h e i m p u r i t y l e v e l i s i n c r e a s e d . For i m p u r i t y l e v e l s above a b o u t 1%, t h e xenon i o n i z a t i o n l e v e l s d e c r e a s e r a p i d l y , implying a b a s i c change i n t h e r a d i a t i o n chem- i s t r y . ~ i g u r e 1 3 shows t h e s p e c t r o m e t e r o u t p u t f o r t h e ~ e * i o n i c c u r r e n t c o l l e c t e d a s t h e shock wave a p p r o a c h e s ,
3 . 3 ~ I m p u r i t i e s i n t h e D r i v e r S e c t i o n
The r o l e o f i m p u r i t i e s i n t h e d r i v e r g a s i s a l s o examined. V a r i o u s amounts o f a i r a r e l e f t i n t h e d r i v e r s e c t i o n b e f o r e f i l l i n g w i t h d r i v e r g a s . S i n c e t h e h o t e q u i l i b r i u m r e g i o n o f t h e shocked t e s t g a s i s i n c o n t a c t w i t h t h e d r i v e r g a s , i t i s p o s s i b l e t h a t r a d i a t i o n from t h e shocked g a s i s absorbed by t h e i m p u r i t i e s i n t h e d r i v e r
I /
g a s and t h e n r e - e m i t t e d back t h r o u g h t h e h o t g a s ( t r a n s - p a r e n t i n c e r t a i n s p e c t r a l i n t e r v a l s ) t o t d e r e g i o n i n f r o n t
I I
o f t h e shock wave. The r a d i a t i o n s o u r c e couPd t h u s b e
I I
changed, a l t e r i n g t h e p h o t o i o n i z a t i o n mechanisms ahead o f t h e shock. For a l l i m p u r i t y l e v e l s ( u p t o t h e l e v e l s u f - f i c i e n t t o change t h e shock wave v e l o c i t y ) no such e f f e c t i s n o t i c e d i n csmpar i s o n w i t h t h e pure-dr i v e r - g a s
c o n d i t i o n s ; b o t h t h e d o u b l e probe and t h e mass s p e c t r o m e t e r s i g n a l s remain u n a l t e r e d , implying t h a t d r i v e r i m p u r i t i e s d o n o t i n f l u e n c e t h e p r e c u r s o r phenomena,
3.4 Experiments i n Arqon
photoionization level in argon should then fall off less
rapidly than for xenon.
For initial pressure P1 = 0.100 torr argon,
-
Ms-
12.2
,
and impurity level 0.4%, the mass spectrum shows thepresence of only argon ions. Figure 14 shows an indication of
+
the Ar ionic current collected by the mass spectrometer
as the shock wave approaches. The maximum sensitivity of
the spectrometer must be used at these shock conditions.
8 s with xenon at this Pow Mach number, no impurity ions are
found in the precursor; the impurity ionization levels are
probably below the spectrometer sensitivity.
Increasing the Mach number in argon experiments is
precluded by the pressure limitations on the spectrometer
sampling orifice diaphragm (see Appendix 8.6 and B.2); the driver pressures reqaired for higher Mach nuders result
in high shock tube final pressures, Replacing the mass
spectrometer sampling fixture with
a
solid shock tube endwall, the effect of higher initial pressures and Mach
numbers were studied. Using the double probe as a diag-
nostic tool, the slopes of the ion density profiles at
large distances from the shock wave are found to change in
the same manner as with xenon, as the initial pressure
IV. THEORETICAL RESULTS
A theoretical model accounting for one-step and multi-
step photoionization of xenon and impurities is developed
in Appendix E. The analysis is similar to Dobbins
'
(Ref. 18) for pure argon.
In the present investigation, the existence of ions
at large distances upstream of the shock wave are attri-
buted to photoionization caused by the intense radiation
from the region downstream of the shock- Using the
notation discussed in Appendix E, the conservation equa- tions for ions and excited state atoms produced by radiation
ahead of
a
strong shockwave
arewhere Us = shock velocity
N, = number density of excited-state atoms N+ = number density of ions
NA = number density of neutral atoms R = photochemical rates (see figure 15)
x = distance upstream of shock wave
in
shock-f ixed coordinates.4.1 One-Step Photoionization of Xenon and Impurities
Ignoring contributions due to ionization of excited
state atoms, only the first term on the right hand side of
equation (3.1) need be considered. Evaluating the rate of
one-step photoionization. Roi
.
the ion number density ata point on the axis of a shock tube a distance q radii
upstream of the shocked gas is found (Equation E.6) to be
where T =
-
X Rv = frequency of radiation
= number density of neutral atoms N~
in region
7
-
-
Qoi ( V ) NACD
qR = spectral optical depthV
Qo i = one-step photoionization cross section
4
z = sece = (1
+
q-2)4
@
= equilibrium temperature in region @e,k,h= physical constants
R = shock tube radius
The formuPation is general with regard to composition
p h o t o i o n i z a t i o n c r o s s s e c t i o n s o f xenon and many i m p u r i t i e s , S u b s t i t u t i o n o f t h e a p p r o p r i a t e v a l u e s g i v e s an e s t i m a t e of t h e i o n number d e n s i t y i n t h e p r e c u r s o r f o r o n e - s t e p p h o t o i o n i z a t i o n , Provided t h e s p e c t r a l i n t e r v a l s f o r t h e p h o t o i o n i z a t i o n c r o s s s e c t i o n o f i m p u r i t y g a s and xenon do n o t o v e r l a p ( i . e . , a l l p h o t o n s a r e n o t a b s o r b e d by e i t h e r xenon o r i m p u r i t y ) , t h e f o r m u l a t i o n g i v e s indepen- d e n t p r e c u r s o r i o n d e n s i t i e s f o r xenon o r f o r i m p u r i t i e s .
The i n v e r s e o f t h e o p t i c a l d e p t h T~ i s t h e r a d i a t i o n
mean f r e e p a t h , a measure o f t h e d i s t a n c e t r a v e l e d by a photon o f f r e q u e n c y v b e f o r e p h o t o i o n i z i n g a ground s t a t e atom. I f b o t h t h e p h o t s i o n i z a t i o n c r o s s s e c t i o n and t h e n e u t r a l atom n u d e r d e n s i t y a r e l a r g e , t h e r a d i - a t i o n i s a t t e n u a t e d v e r y r a p i d l y i n t h e c o l d g a s , a s
shown b y t h e t e r m s i n t h e s q u a r e b r a c k e t s in e q u a t i o n ( 3 . 3 ) . On t h e o t h e r hand, i f e i t h e r t h e c r o s s s e c t i o n o r t h e
number d e n s i t y i s s m a l l , photons o f s u f f i c i e n t e n e r g y f o r p h o t o i o n i z a t i o n w i l l p r o p a g a t e f u r t h e r from t h e wave.
T y p i c a l v a l u e s o f c r o s s s e c t i o n and number d e n s i t y i n t h e p r e s e n t e x p e r i m e n t s a r e g i v e n i n Table
B.
The ground s t a t e p h o t o i o n i z a t i o n c r o s s s e c t i o n f o r xenon is v e r y l a r g e .Hence, i f t h e n e u t r a l atom number d e n s i t y ( i n i t i a l p r e s - s u r e PI i n a n e x p e r i m e n t ) i s low t h e o n e - s t e p
i m p u r i t y atoms such a s oxygen are s m a l l and t h e number d e n s i t i e s o f such i m p u r i t i e s a r e s m a l l i n t y p i c a l
e x p e r i m e n t s . T h e r e f o r e , it i s p o s s i b l e t h a t photon mean f r e e p a t h s a r e long enough f o r i m p u r i t i e s t o become im- p o r t a n t i n p r e c u r s o r s .
4.2 M u l t i - S t e p P h o t o i o n i z a t i o n s f Xenon
I f t h e i n i t i a l p r e s s u r e i s h i g h , o n e - s t e p photoion- i z a t i o n o f xenon i s n e g l i g i b l e a t l a r g e d i s t a n c e s from t h e wave b e c a u s e o f t h e s h o r t r a d i a t i o n mean f r e e p a t h .
S o l u t i o n s of e q u a t i o n s ( 3 . 1 ) and ( 3 . 2 ) f o r i o n number d e n s i t i e s due t o m u l t i - s t e p a s w e l l a s one-step photoion-
P z a t i o n must b e examined. From Appendix E , 3 , t h e
photochemical r a t e s Roi R,i Rol a r e g i v e n by e q u a t i o n s (E4) ( E l . 7 ) (ElO) ( E l l )
,
r e s p e c t i v e l y ., I I (
Long-range t r a n s m i s s i o n s f photon e n e r g y i s i m p o r t a n t i n t h e wings o f t h e broadened r e s o n a n c e P i n e s i n t h e shocked-
I
g a s e m i s s i o n spectrum. The e f f e c t i v e " t r a p p i n g t ' o f r e s -
1 1
onance r a d i a t i o n i n t h e c o l d , u n d i s t r i b u t e d g a s ahead o f a shock wave i s a l s o found t o b e i m p o r t a n t . The r e f l e c t i o n and a b s o r p t i o n o f r a d i a t i o n a t t h e shock t u b e w a l l s i s
n e c e s s a r i l y n e g l e c t e d .
test pressure increases, ionization of photoexcited atoms
becomes the dominate mechanism. Ionization levels due to
one-step photoionization decay exponentially with distance,
while multi-step ionization levels decrease with some power
of the distance from the shock wave. The importance of
initial pressure is further noted in that the contribution
to ionization due to multi-step processes depends on the
square of the number density while single-step photoion-
ization is determined by a linear relation with the number
density
.
With the appropriate spectral characteristics of xenon
and oxygen impurity (Refs
.
25-29),
thermodynamic prspert iesof high temperature xenon (Ref, 30) and the experimentally
determined test-conditions from reference 23, the CPTRAN*
system is used to obtain a numerical solution
to
the aboveequations. Table
H
gives selected values for test condi-tions and parameters for typical. experiments in xenon
with oxygen impurity.
Theoretical ion number densities for xenon and oxygen
are compared directly with the individual ionic currents
collected by the spectrometer in Part V. Re-examination of
the theoretical considerat ions after comparison with.
experimental results will then yield insight to the validity
of the theoretical treatment.
V. DISCUSSION AND COMPARISON OF EXPERIMENT AND THEORY
5.1 Photoionization at Low and Hiqh Initial Pressures
Figure 17 shows the mass spectrometric measurements
-
of xenon and oxygen ions for low initial pressure (PI
-
0.120 torr) and Ms = 18.5
.
where impurity photoionization is found to be significant. The spectrometer calibrationconstant is used to relate the ion density in the shock
tube to the ionic current coklected by the spectrometer
(see Appendix B. 3) , For comparison the one-step photo-
ionization calculations (using equation 3.3) are shown
for xenon and oxygen at the experimental. conditions. At this Pow initial pressure, the contribution due to multi-
step photoionization is negligible.
+
Figure 1% shows the number density of
Xe
ions asmeasured by the mass spectrometer at a high initial
pressure (PI = 0.500 torr) and Ms = 15.2
.
Theoretical caPcufation of ion number density according to one-stepphotoionization of xenon is seen to decrease to a level
several orders of magnitude below the experimentally
observed nurriber density. The numerical solution of
equation (3.2)
,
which gives the number density of xenonatoms excited to the first state,. is also shown in this
figure. HncEusion of this profile
for
N, (q) in a numerical solution of equation (3.1) results in a con-magnitude t h a n even t h e o n e - s t e p p h o t o i o n i z a t i o n l e v e l . F i g u r e 1 2 shows t h e maximum i o n d e n s i t y , o c c u r r i n g a t ' t h e shock f r o n t , c o l l e c t e d by t h e s p e c t r o m e t e r f o r v a r i o u s l e v e l s o f oxygen and n i t r o g e n i m p u r i t i e s ( s e e P a r t 3 . 3 b ) . F o r comparison t h e o n e - s t e p p h o t o i o n i z a t i o n l e v e l s of oxygen and n i t r o g e n a r e shown by t h e dashed l i n e s . A t l e v e l s above 0.4% f o r oxygen and -- 1.2% f o r n i t r o g e n , t h e observed number d e n s i t i e s d e p a r t s i g n i f - i c a n t l y from t h e v a l u e s p r e d i c t e d by t h e o r y .
5 . 2 P r e c u r s o r Dependence on Shocked Gas Temperature
F i g u r e 8 ( P a r t 3.1) shows t h e dependence o f photo- i o n i z a t i o n on e q u i l i b r i u m t e m p e r a t u r e b e h i n d , t h e shock
wave a s measured by t h e d o u b l e probe. Comparison o f t h e o r y and e x p e r i m e n t a t low i n i t i a l p r e s s u r e s shows t h a t one- s t e p p h o t o i o n i z a t i o n p r e d o m i n a t e s t h e p r e c u r s o r , e s p e c i a l l y c l o s e t o t h e shock wave. Examination o f t h e t h e o r e t i c a l f o r m u l a t i o n ( e q u a t i o n 3 , 3 ) shows t h a t f o r a f i x e d
p r e s s u r e t h e d e g r e e of i o n i z a t i o n ahead o f t h e shock
depends o n l y on t h e e q u i l i b r i u m t e m p e r a t u r e b e h i n d t h e wave. Performing t h e n u m e r i c a l c a l c u l a t i o n s a t an i n i t i a l
of slopes.
At the lower temperatures (low Mach numbers) it is
doubtful that equilibrium is attained in the shocked gas
(even at the given impurity level) and hence the theory,
which assumes blackbody radiation at the equilibrium
temperature, suggests higher precursor levels of ionization
than would be observed. The sharp change in slope of the
measured ion current at the lower temperatures substan-
tiates this conclusion.
5.3 Discussion
Correlation between experimental evidence and theoret-
ical understanding of the precursor phenomena is good at
Pow initial pkessures, if only small quantities of
impurities are present, The small changes in the double probe measurements with variations in the impurity level
(Fig, 7 ) substantiate the spectrometric data. Impurity
domination of the precursor at extreme distances from the
shock wave is implied by the slopes of both spectrornetrical-
By determined and theoretically evaluated ion densities
(see figure 171, but the region in which this might occur
is below the range of sensitivity of the spectrometer.
At higher pressures, the experimental results show a
negligible impurity contribution to the precursor ion-
ization level, due to a different behavior of xenon at
pressures, xenon ions are detected at much larger distances
from the shock wave than predicted by the one-step ion-
ization theory. The theoretical formulation for multi-step
photoionization predicts a high number density of excited-
state atoms but ionization from these photoexcited atoms is
much lower than the observed values, at the pressures used
in the present investigation. Eventual domination by the
multi-step ionization process at high pressures is implied
by theory (see Part 4 - 2 )
.
In view of the present experi- mental results, it appears that inadequacies exist in thetreatment of multi-step photoionizakion processes. Experi-
mental implications and theoretical shortcomings in the
multi-step photoionization theory will now be discussed.
The experiments indicate the areas which are still
not understood thesreticalPy. With the recent work
sf
Schldter (Ref. 272, the emission spectrum sf xenon is
I I
fairly well known, but the mechanism of absorption in the
cold gas is still not clear, The accuracy
sf
calculationsof the edges of the resonance line profiles, which are
shown to be important in multi-step photoionization
(Appendix E.31, are subject to doubt. The cross sections for excited state photoionization should also be studied
further.
Using a somewhat different theoretical approach,
Holrnes (Ref. 8) found' that the main contribution to photo-
t h e wings of t h e r e s o n a n c e l i n e a t f r e q u e n c i e s g r e a t e r t h a n 500 l i n e b r e a d t h s away from t h e l i n e c e n t e r . I n t h i s r e g i o n o f r e s o n a n c e r a d i a t i o n on t h e edge o f t h e P i n e , t h e blackbody assumption ( ~ p p e n d i x E.3) f o r t h e e m i s s i o n
spectrum and t h e a c c u r a c y of t h e a b s o r p t i o n c o e f f i c i e n t a r e c e r t a i n l y q u e s t i o n a b l e . E x p e r i m e n t a l s t u d i e s of t h i s s p e c t r a l regime a r e n o n - e x i s t e n t .
The r a d i a t i o n model a s p r e s e n t e d , a c c o u n t s f o r o n l y t h e f i r s t e x c i t e d s t a t e i n t h e c o l d g a s , a l t h o u g h t h e
c o r r e c t i o n f a c t o r s (Appendix E.3) f o r t h e e m i s s i o n s p e c t r a o f t h e h o t g a s t a k e i n t o a c c o u n t c o n t r i b u t i o n s due t o
P
r e c o m b i n a t i o n o f a l l e x c i t e d s t a t e s . I n c l u s i o n o f t h e Pl e x c i t e d s t a t e o f xenon i n t h e c o l d g a s s h o u l d r o u g h l yr a i s e t h e d e g r e e o f i o n i z a t i o n by a f a c t o r o f two, s t i l l an i n s i g n i f i c a n t amount i n view o f t h e p r e s e n t e x p e r i m e n t a l r e s u l t s . I t i s p o s s i b l e t h a t e x c i t a t i o n t o Bevels h i g h e r t h a n t h e f i r s t s t a t e and s u b s e q u e n t d e - e x c i t a t i o n t o t h e r e s o n a n c e s t a t e ( w i t h e m i s s i o n s f r a d i a t i o n ) may s i g n i f i - c a n t l y i n c r e a s e t h e e x c i t e d atom n u d e r d e n s i t y , and
consequent Py
,
t h e d e g r e e o f i o n i z a t i o n .
