Mass spectrometric studies of ionization precursors ahead of strong shock waves

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 Spectrometer

2.3 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

-

7

10 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 x

l 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 e

i 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 experimental

work 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 n

d 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 mass

p 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 the

11. 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 equilibrium

temperature increase, so does the impurity ionization,

Below Mach numbers Ms -- 13

,

all impurity currents are

below 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 no

9

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 y

currents, 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 y

p 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 the

presence 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 end

wall, 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 shock

wave

are

where 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 at

a 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 R

v = frequency of radiation

= number density of neutral atoms N~

in region

7

-

-

Qoi ( V ) NA

CD

qR = spectral optical depth

V

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 ies

of 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 above

equations. 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 calibration

constant 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 as

measured 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-step

photoionization 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 xenon

atoms 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 the

treatment 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

calculations

of 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 y

r 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 .