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The construction of a low-pressure flash tube and the measurement of some of its properites Robinson, Alexander Maguire 1962

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THE CONSTRUCTION OF A LOW-PRESSURE FLASH TUBE AND THE MEASUREMENT OF SOME OF ITS PROPERTIES by ALEXANDER MAGUIRE ROBINSON B . A . S c , U n i v e r s i t y of B r i t i s h Columbia, 1961  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the Department of PHYSICS We accept t h i s t h e s i s as conforming to the r e q u i r e d standard  THE UNIVERSITY OF BRITISH COLUMBIA December, 1962  In presenting t h i s thesis i n p a r t i a l fulfilment of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t available for reference and study.  freely  I further agree that permission  for extensive copying of t h i s thesis for scholarly purposes may be granted by the Head of my Department or by his  representatives.  It i s understood that copying or publication of this thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission.  Department of The University of B r i t i s h Columbia, Vancouver 8, Canada. Date  %C.  2.0  /?6Z.  ABSTRACT  A low pressure c o a x i a l f l a s h tube of simple design has been constructed which does not suffer from d e p o s i t i o n on the e x i t window.  Some of the e l e c t r i c a l and spectro-  s c o p i c c h a r a c t e r i s t i c s of the tube have been measured. A method of measuring the brightness  temperature  over the v i s i b l e range was used by comparing photoelect r i c a l l y the i n t e n s i t y of the tube w i t h that of a black body.  The temperature was found to be lower than expected.  ACKNOWLEDGEMENT  I would l i k e to thank Dr. R. A. Nodwell for h i s guidance and assistance to me i n t h i s work; and thanks a l s o to the other members of the plasma physics group and the members of the physics workshop. I am a l s o indebted to the N a t i o n a l Research C o u n c i l of Canada for f i n a n c i a l assistance i n the form of a bursary.  TABLE OF CONTENTS Page 1.  Introduction  1  2.  Theory of Low-Pressure Discharges  6  2 . 1 . Introduction  6  2 . 2 . P a r t i c l e Theory of Gases  7  2 . 2 . 1 . D i s t r i b u t i o n Functions  7  2.2.2. Collisions  8  2 . 3 . Discharge Mechanics  10  2 . 3 . 1 . Introduction  10  2 . 3 . 2 . Primary I o n i z a t i o n  11  2 . 3 . 3 . Secondary I o n i z a t i o n  12  2.4. R a d i a t i o n  3.  15  2 . 4 . 1 . R a d i a t i o n Theory  15  2 . 4 . 2 . Recombination  19  2 . 4 . 3 . Continua  23  The F l a s h Tube  24  3 . 1 . The F l a s h Tube Discharge Model  24  3.2. D e s i r a b l e C h a r a c t e r i s t i c s  27  3 . 3 . E a r l y Designs  29  3 . 3 . 1 . The Elbow Tube  29  3 . 3 . 2 . Low Inductance Tube  31  3 . 3 . 3 . Tungsten-electrode Tube  31  3.3.4  32  Covar-electrode Tube  3.4. F i n a l Design  33  3.4.1. Description  33  3 . 4 . 2 . Operation  36  3 . 4 . 3 . Experimental  37  3 . 5 . Apparatus  4.  5.  38  3 . 5 . 1 . Charging Condenser and Charging Unit  38  3 . 5 . 2 . Trigger Unit  38  3 . 5 . 3 . P h o t o m u l t i p l i e r Unit  40  Brightness Temperature Measurement  42  4 . 1 . Introduction  42  4 . 1 . 1 . Black Bodies  42  4 . 1 . 2 . Temperature  43  4 . 2 . Theory  45  4 . 3 . Experimental Procedure  49  4 . 4 . Apparatus  52  Results  53  5 . 1 . F l a s h Tube Spectrum  53  5.2. Current Waveform  54  5.3. L i g h t I n t e n s i t y  57  5.4. D e i o n i z a t i o n  59  5 . 5 . Behaviour of I n t e n s i t y w i t h Number of Shots  65  5.6. Temperature Measurements  66  5.7. Conclusions  70  Appendix  72  References  74  ILLUSTRATIONS  Figure Ka)  The Elbow Tube  f o l l o w i n g page  29  Kb)  Low Inductance Tube  f o l l o w i n g page  29  2(a)  Tungsten-electrode  f o l l o w i n g page  31  2(b)  Covar-electrode Tube  f o l l o w i n g page  31  2(c)  Covar Tube w i t h Dump Chamber  f o l l o w i n g page  31  3  F i n a l Design  f o l l o w i n g page  33  4  Trigger Unit  page  39  5  P h o t o m u l t i p l i e r Unit  page  40  6  Temperature-measuring  f o l l o w i n g page  49  7  Current Waveform  page  54  8  Light  page  57  9  Light I n t e n s i t y of Elbow Tube  page  58  10  Current and I n t e n s i t y of F l a s h Tube  page  58  11  f o l l o w i n g page  59  12  _x j versus Time ?\ Energy Content of Tube  page  64  13  Intensity  f o l l o w i n g page  65  14  I n t e n s i t y V a r i a t i o n w i t h Wavelength f o l l o w i n g page  69  Tube  Apparatus  Intensity  2  Decrease  TABLES Table I  II  Brightness Temperature for V a r i a b l e Wavelength I n t e r v a l  68  Brightness Temperature for Constant Wavelength I n t e r v a l  68  1.  Introduction  Sources of energy for use by man have always played an important part i n the development of c i v i l i z a t i o n .  With each  new and improved source of energy that has been evolved, s i g n i f i c a n t advances i n technology have been made.  Take, f o r  example, the t e c h n o l o g i c a l growth which followed the " u n i v e r s a l " use of c o a l and o i l as f u e l s and marked the beginning of the i n d u s t r i a l r e v o l u t i o n . However, ever i n c r e a s i n g energy requirements  threaten  to deplete the f u e l for the w o r l d ' s present-day energy sources. Fortunately the development of a new source of energy appears p o s s i b l e , and that i s the process of nuclear f u s i o n .  The  f u e l r e q u i r e d for t h i s process can be extracted from sea water at n e g l i g i b l e cost and affords a v i r t u a l l y u n l i m i t e d supply of fuel.  B i s h o p , for example, estimates that at the present 1  Q r a t e of energy consumption (5 x 10  kw) there i s enough sea-  water f u e l i n the world to l a s t f o r 2 x 1 0  1 0  years.  C o n t r o l l e d nuclear fusion i s not without formidable difficulties,  however.  For a p r o f i t a b l e r e a c t i o n to take ft  place extremely high temperatures  (10  K) are needed and hence  the f u e l i s i n gaseous form and h i g h l y i o n i z e d . gases are c a l l e d plasmas.  Such i o n i z e d  -2-  Because of the problems i n v o l v e d i n c o n t a i n i n g a gas  at  such high temperatures, a thorough knowledge of the propert i e s of plasmas under many c o n d i t i o n s i s expedient. Consequently i n the l a s t decade or so, a great d e a l of time, e f f o r t and money has been spent i n the d i a g n o s i s of plasmas merely t o l e a r n more about them, not o n l y w i t h the hope of u l t i m a t e l y a c h i e v i n g c o n t r o l l e d f u s i o n , but a l s o to gather useful information  i n such r e l a t e d f i e l d s as  magnetohydrodynamics, gaseous d i s c h a r g e s ,  and  astrophysics, so  Of the s e v e r a l main d i a g n o s t i c techniques  on. In popular  use  today, s p e c t r o s c o p i c a n a l y s i s g i v e s q u i t e p r e c i s e r e s u l t s and does not s u f f e r from the disadvantage of i n t r o d u c i n g p e r t u r bations i n t o the plasmas. Much can be l e a r n e d from the study of the s p e c t r a of plasmas.  However i t i s u s u a l l y necessary i n subsequent  a n a l y s i s t o know the r e l a t i v e p o p u l a t i o n d e n s i t i e s and t r a n s i t i o n p r o b a b i l i t i e s between two molecules.  Unfortunately  are not known and  the  energy l e v e l s of the  i n many cases these two  so p r e l i m i n a r y experimentation  gas  quantities must be  undertaken t o determine them. Ladenburg  has p o i n t e d out t h a t by measuring  anomalous d i s p e r s i o n and one may  t h d absorption  s p e c t r a of the  determine the r e l a t i v e p o p u l a t i o n  transition  the  d e n s i t i e s and  gas, the  probabilities.  However, t o perform an a b s o r p t i o n  a n a l y s i s upon the  -3-  plasma, a continuum background source i s required whose temperature need be greater than that of the plasma which i t s e l f can be as high as 10® °K.  Also because one would l i k e to  examine the s h o r t - l i v e d plasma r e a c t i o n s , such as a shock front or pinch e f f e c t ,  an extremely short-pulsed background l i g h t i s  r e q u i r e d t o get any time-resolved absorption s p e c t r a . Both the high temperature and short duration r e q u i r e ments are a p t l y met i n the high-voltage f l a s h - d i s c h a r g e tube which i s becoming i n c r e a s i n g l y popular i n spectroscopic work. These f l a s h tubes are a l s o f i n d i n g use i n other f i e l d s such as photography of f a s t events, and f l a s h p h o t o l y s i s . The high i n t e n s i t y continuous spectra of the f l a s h tube i s achieved by the impulsive discharge of high energy through a gas at low pressure; tube of s m a l l diameter.  the gas i s confined w i t h i n an i n s u l a t i n g In e a r l y designs t h i s tube suffered  from f a i r l y r a p i d erosion of w a l l m a t e r i a l because of the high temperatures i n v o l v e d , and subsequent d e p o s i t i o n onto the windows present i n the system.  Also q u a l i t a t i v e i n t e n s i t y  work has been impossible because the i n t e n s i t y has not been r e p r o d u c i b l e to a s u f f i c i e n t l y accurate  degree.  These f l a s h - d i s c h a r g e sources may be roughly d i v i d e d i n t o three main types and although they appear to be quite s i m i l a r i n o p e r a t i o n , a l l employing the discharge through a gas of c a p a c i t i v e i y stored e l e c t r i c a l energy, there i s a fundamental difference i n the mechanism of emission and the  d e t a i l s of o p e r a t i o n . The f i r s t type of discharge tube i s c a l l e d the Lyman f l a s h tube and has been used e x t e n s i v e l y i n the production of  o the Lyman continuum which extends from about 600 A to the v i s i b l e range on the electromagnetic wavelength s c a l e . tube has a narrow bore (  The  .1 cm) and moderate length ('—' 5 cm  This source, however, s u f f e r s from the r a p i d erosion of e l e c t rode and w a l l m a t e r i a l w i t h a consequent r e d u c t i o n of current density . The second type of tube i s a t h i c k - w a l l e d c a p i l l a r y ( ^  .2 cm diameter by 3 cm length) between heavy electrodes  i n s e r i e s w i t h a heavy-current hydrogen t h y r a t r o n and condensor bank. The i n e r t gas i n the tube i s at a r e l a t i v e l y high .:o 4 pressure . The t h i r d general type increases the s i z e of the c a p i l l a r y tube to about 1 cm and uses r a p i d discharge condensors. The c i r c u i t r y i s designed to reduce the inductance as much as p o s s i b l e and hence a c o a x i a l symmetry i s employed. tubes are c a l l e d c o a x i a l f l a s h tubes.  Thus these  They show a marked  improvement i n e l e c t r i c a l and spectroscopic p r o p e r t i e s over the Lyman f l a s h tube, the main advantage being t h e i r r e l i a b l e i n t e n s i t y r e p r o d u c i b i l i t y which allows them to be used for q u a l i t a t i v e work. Budd describes the c o n s t r u c t i o n of a c o a x i a l f l a s h tube 5  i n s p i r e d by the work of G a r t o n and which he used i n some 6  p r e l i m i n a r y experiments i n absorption i n neon gas.-  -5-  D e p o s i t i o n of m a t e r i a l on t h e e x i t window w i t h  consequent  d e c r e a s e of i n t e n s i t y was a g r e a t problem and s i n c e t h e t u b e ' s u s e f u l n e s s depends on an i n t e n s i t y w h i c h does not v a r y w i t h t h e number of f i r i n g s of t h e t u b e , t h e need of a more r e l i a b l y o p e r a t i n g f l a s h tube was  apparent.  This thesis describes  t h e c o n s t r u c t i o n of a c o n t a m i n a t i o n -  f r e e continuum c o a x i a l f l a s h tube h a v i n g good i n t e n s i t y reproducibility.  I t s c o n s t r u c t i o n and maintenance i s  extremely  s i m p l e and i n e x p e n s i v e and t h e complete a p p a r a t u s i s v e r y m o b i l e w h i c h a l l o w s speed and c o n v e n i e n c e i n i t s u s e . brightness  The  temperature of t h e f l a s h tube i s measured i n t h e  v i s i b l e w a v e l e n g t h range by comparison w i t h a t u n g s t e n f i l a m e n t lamp of known t e m p e r a t u r e .  -6-  2.  Theory of Low-Pressure Discharges  2.1  Introduction The main o b j e c t of t h i s experiment  has been t o d e s i g n  and c o n s t r u c t a d e p o s i t i o n - f r e e f l a s h tube w i t h  faithfully  r e p r o d u c i b l e p r o p e r t i e s and simple c o n s t r u c t i o n a l d e t a i l s ; a l s o t o measure i t s temperature.  and  Although some of the e a r l y  d i f f i c u l t i e s were overcome by a s e m i - e m p i r i c a l " c u t - a n d - t r y " method i t i s u s e f u l and i n t e r e s t i n g t o d e l v e i n t o the p h y s i c s of a low-pressure  gaseous d i s c h a r g e .  The r e s u l t s of doing  have e x p l a i n e d some of the shortcomings  so  of the f l a s h tube i n  i t s present c o n f i g u r a t i o n and w i l l d o u b t l e s s be e q u a l l y r e warding i n the f u t u r e . The theory of low-pressure  d i s c h a r g e : and a l l i t s  r a m i f i c a t i o n s covers an enormous f i e l d  and much work has  been done both t h e o r e t i c a l l y and e x p e r i m e n t a l l y over l a s t few decades.  Much t h e o r y i s s t i l l  incomplete,  the however,  and consequently  i n t e r p r e t a t i o n of data i s sometimes somewhat  unsatisfactory.  I t i s obvious t h a t the d i s c u s s i o n t o f o l l o w  must be a b b r e v i a t e d perhaps to the p o i n t of b e i n g  incomplete;  the main a s p e c t s of d i s c h a r g e s which are of i n t e r e s t t o t h i s work have been b r i e f l y touched upon and i t i s hoped the above mentioned f a u l t i s not  apparent.  -7  2.2  :  P a r t i c l e Theory of Gases 2.2.1  D i s t r i b u t i o n Functions  To d e s c r i b e e x a c t l y the a c t i o n s of two i n t e r a c t i n g p a r t i c l e s i s u s u a l l y p o s s i b l e but when the number i s t h r e e more the mathematics becomes f o r m i d a b l e m a t i o n s must be made. p a r t i c l e d e n s i t y may  or  and a n a l y t i c a p p r o x i -  In d e a l i n g w i t h plasmas, where t h e v a r y between 1 0  ] 0  cm  —3  to 10  25  cm  —3  , a  m a t h e m a t i c a l d e s c r i p t i o n of t h e i n d i v i d u a l p a r t i c l e motions i s obviously  impossible.  Many p r o p e r t i e s of gases and plasmas may  be  predicted  by c o n s i d e r i n g t h e gas as composed of a l a r g e number of p a r t i c l e s and t r e a t i n g them s t a t i s t i c a l l y .  T h i s i s done by  the i n t r o d u c t i o n of the d i s t r i b u t i o n f u n c t i o n d e f i n e d such t h a t represents  f  which i s  f ( x , y , z , u , v , w , t ) dx dy dz du dv  dw  the average number of p a r t i c l e s between x  and  x + dx, y and y + dy, z and z + dz w i t h v e l o c i t i e s between u and u + du, v and v + dv, w and w + dw at t i m e t , where x, y and z are t h e r e c t a n g u l a r  cartesian coordinate  and u, v, and w a r e t h e c o r r e s p o n d i n g v e l o c i t i e s . p a r t i c l e density i n position-velocity Many u s e f u l r e l a t i o n s may of the d i s t r i b u t i o n f u n c t i o n . d e n s i t y I n space i s g i v e n  n(x,y,z,t)  -  positions f  i s the  6-space.  be deduced from a knowledge  For example, t h e  particle  by  \ \ \ f ( x , y , z , u , v , w , t ) du dv  dw  ~8  and  the average v e l o c i t y by  where  v„„  i s the average v e l o c i t y and  av  volume du dv dw i n v e l o c i t y space.  from t h i s  position-  distribution.  For  precise results  i n the k i n e t i c s of plasmas,  Boltzmann's equation must be a p p l i e d . d i f f e r e n t i a l equation i n v o l v i n g behaviour as a r e s u l t and  r e p r e s e n t s the  Many other macroscopic  p r o p e r t i e s of a gas may be c a l c u l a t e d velocity  dv  f  T h i s i s an i n t e g r o -  and d e s c r i b e s i t s  of e x t e r n a l f o r c e s on the p a r t i c l e s  a l s o encounters among the p a r t i c l e s .  I t i s d e r i v e d by  c o n s i d e r i n g the c o n s e r v a t i o n of p a r t i c l e s i n a s m a l l volume element i n 6-space.  In the absence of c o l l i s i o n s , t h i s  equation reduces t o L i o u v i l l e ' s e q u a t i o n .  2.2.2.  Collisions  C o l l i s i o n s between p a r t i c l e s i s an event which, at first  sight,  seems simple enough, but when a d e t a i l e d  investi  g a t i o n i s made, any apparent s i m p l i c i t y d i s a p p e a r s .  Even a  c o n s i s t e n t d e f i n i t i o n of a c o l l i s i o n  thing.  i s not an easy  A dictionary  w i l l d e f i n e i t as an a c t o f h i t t i n g or coming  into violent  c o n t a c t , which, from an atomic p o i n t of view,  i s wholly unsatisfactory.  For i t i s a w e l l accepted  fact  t h a t an atom i s not s o l i d and t h e word " c o n t a c t " has no r e a l meaning on t h i s  scale.  A c o l l i s i o n c o u l d be d e f i n e d , however, as an event between two or more p a r t i c l e s t h a t approach each o t h e r r e l a t i v e l y c l o s e l y and whose s t a t e s of m o t i o n a n d / o r energy  are  changed s i g n i f i c a n t l y by v i r t u e of t h e i r p r o x i m i t y . C o l l i s i o n s between p a r t i c l e s may be r o u g h l y d i v i d e d i n t o two main c a t e g o r i e s - t h o s e i n which t h e n a t u r e of one o r more o f t h e c o l l i d i n g p a r t i c l e s i s a l t e r e d , and t h o s e i n w h i c h t h e n a t u r e of t h e p a r t i c l e s r e m a i n u n a l t e r e d .  Those i n  the f i r s t - m e n t i o n e d category are c a l l e d i n e l a s t i c c o l l i s i o n s and those i n t h e second e l a s t i c  collisions.  I n b o t h e l a s t i c and i n e l a s t i c c o l l i s i o n s , a u s e f u l concept i s t h e c o l l i s i o n c r o s s - s e c t i o n w h i c h i s by  represented  G~ and d e f i n e d f o r b i n a r y c o l l i s i o n s by t h e e q u a t i o n  do  cr n . n z U .  dt  where  n^  and  ng  c o l l i d i n g species,  are t h e p a r t i c l e d e n s i t i e s u  between t h e p a r t i c l e s , A l a r g e v a l u e of and c o n v e r s e l y .  i s the average r e l a t i v e v e l o c i t y and  i s t h e r a t e of  collision.  CT i m p l i e s a h i g h p r o b a b i l i t y f o r The c r o s s - s e c t i o n  n a t u r e of t h e i n t e r p a r t i c l e f o r c e s of t h e p a r t i c l e s .  of t h e two  collision,  i s dependent upon t h e and t h e r e l a t i v e v e l o c i t y  -10-  In e l a s t i c c o l l i s i o n s t h e t o t a l k i n e t i c energy of the p a r t i c l e s i s conserved; t h i s type of c o l l i s i o n i s not of much i n t e r e s t i n the study of t h e d i s c h a r g e  of t h e gases and  w i l l not be f u r t h e r mentioned. On t h e other  hand, i n e l a s t i c c o l l i s i o n s p l a y a c a r d i n a l  p a r t i n i o n i z a t i o n and e x c i t a t i o n of gases. s i o n p r o c e s s e s may. be s u b d i v i d e d  Inelastic c o l l i -  i n t o two c l a s s e s , which a r e  c a l l e d c o l l i s i o n s of the f i r s t k i n d and c o l l i s i o n s of the second k i n d .  In the f i r s t , t h e t o t a l k i n e t i c energy of the  c o l l i d i n g system i s reduced by the c o l l i s i o n ; and i n the second, t h e p o t e n t i a l energy o f a t l e a s t one member i s reduced. The  product i n both these c o l l i s i o n s i s such t h i n g s as i o n i z a -  t i o n , e x c i t a t i o n , d i s s o c i a t i o n and so f o r t h . Some of the more frequent  participants for binary  i n e l a s t i c c o l l i s i o n s a r e e l e c t r o n s and n e u t r a l atoms e i t h e r e x c i t e d or i n t h e ground s t a t e , e l e c t r o n s and i o n i z e d atoms, n e u t r a l s and i o n s , ions and i o n s .  2.3.  Discharge Mechanics  2.3.1.  Introduction  When a gas i s l o c a t e d i n a r e g i o n of an e l e c t r i c a c u r r e n t w i l l flow  field  i n t h e d i r e c t i o n of the f i e l d ; however i n  s m a l l e l e c t r i c f i e l d s gases show l i t t l e c o n d u c t i v i t y and the current  i s of low value.  Cosmic r a y s , ^ - r a d i a t i o n and r a d i o -  a c t i v e t r a c e s i n c o n t a i n e r w a l l s produce t h i s s m a l l amount o f  -11-  conductivity.  A p p l i c a t i o n of any agency which causes l i b e r a -  t i o n of e l e c t r o n s and ions from surfaces or l i b e r a t i o n of e l e c t r o n s from atoms i n the gas w i l l augment these currents and  breakdown of the gas w i t h i t s corresponding increase i n  c o n d u c t i v i t y may occur.  2.3.2. The  Primary I o n i z a t i o n  '  processes c o n t r o l l i n g conduction i n the gas may be  d i v i d e d i n t o two main c a t e g o r i e s , primary and secondary. In the primary process each e l e c t r o n l i b e r a t e d creates an avalanche of new e l e c t r o n s and ions by c o l l i s i o n w i t h the gas molecules by t r a n s f e r r i n g part of the k i n e t i c energy of the impacting e l e c t r o n to the gas molecule which subsequently ionizes.  The observed current growth i s given by the  relation  or more e x a c t l y nX  I~I £xp)ad*  (1)  o  where  I  Q  i s the i n i t i a l current at  avalanche current at  x .  a  x = 0  and  I  i s the  i s c a l l e d the f i r s t Townsend  c o e f f i c i e n t and represents the number of new i o n p a i r s created per u n i t distance i n the f i e l d d i r e c t i o n per e l e c t r o n .  -12-  Th i s can be seen from the f o l l o w i n g c o n s i d e r a t i o n s . Let  n  e l e c t r o n s be present at  Q  number has been increased to These  n  I  n  x  the  by i o n i z i n g c o l l i s i o n s .  e l e c t r o n s i n moving through a lamina of thickness dx  i n the d i r e c t i o n collision  x = 0 ; at the point  x  andx  of the e l e c t r i c f i e l d w i l l generate by  new e l e c t r o n s so that  n , equation (1) f o l l o w s .  dn = andx.  Since  I t may a l s o be w r i t t e n i n the  form  I = I (H+ I.) where \  ". xC  M= exp^Joc dx )-1 o  and i s c a l l e d the a m p l i f i c a t i o n f a c t o r .  2.3.3.  Secondary I o n i z a t i o n  In the process of c r e a t i n g  I  Q  M avalanche e l e c t r o n s  and i o n s , there may be secondary agencies which can regenerat i v e l y create  new e l e c t r o n s to s t a r t new avalanches.  yI M 0  The current can then be given under many circumstances by  T -  T  ° *P e  I* * d  ...  (2)  7  A d e r i v a t i o n of t h i s equation i s given by von Engel . c a l l e d the second Townsend c o e f f i c i e n t and f o r  j  <yM l e s s  is  -13-  than u n i t y , I represents the current of increased magnitude over that caused by the primary 7  I (M+1) Q  process alone.  i s i n general r e l a t i v e l y s m a l l . Some of the p o s s i b l e secondary processes are: 1 ) Secondary emission of e l e c t r o n s due to p o s i t i v e  ions i n c i d e n t on the cathode, 2 ) Cathode emission of e l e c t r o n s due to incidence of photons from e x c i t e d gas molecules. 3 ) I o n i z a t i o n by p o s i t i v e ions and metastable atoms on impact w i t h molecules of the gas. 4) The a c t i o n of p h o t o - i o n i z a t i o n i n the gas. The f i r s t two processes are influenced by the pressure of the gas, for the number of e l e c t r o n s released to a c t i v e l y take part i n the subsequent conduction and i o n i z a t i o n processes i s reduced by b a c k - d i f f u s i o n to the cathode. I t should be pointed out that these mechanisms play a t r u l y secondary role.because they are r e l a t i v e l y s m a l l w i t h respect to the primary f a c t o r  M , and t h e i r most important  function i s t o s u s t a i n primary a c t i o n . Returning now to equation ( 2 ) , i t i s seen that as long as yM  1 , the current flowing i n the gas has a d e f i n i t e  v a l u e , and i s l a r g e r than i t would be represented by  a  if  the primary effect  were a c t i n g alone.  Ignoring equation ( 2 ) f o r the moment, note that when 7 M = 1 the c o n d i t i o n i s such that f o r each avalanche of M  -14-  e l e c t r o n s there i s produced a new e l e c t r o n capable of y i e l d i n g a new avalanche.  This i s what i s c a l l e d the threshold c o n d i -  t i o n of a s e l f - s u s t a i n i n g discharge.  An avalanche s t a r t e d by  a s i n g l e e l e c t r o n becomes s e l f - p e r p e t u a t i n g and the discharge i s capable of maintaining i t s e l f regardless of the value of I . 0  The gain i n e l e c t r o n d e n s i t y r e s u l t i n g from the i o n i z a t i o n of the gas becomes equal to the l o s s of e l e c t r o n s by d i f f u s i o n , recombination or attachment. For <yM > 1  ...  (3)  the succession of avalanches creates more current c a r r i e r s than i s necessary for self-maintenance and the current w i l l become e x c e s s i v e l y l a r g e , the c o n d u c t i v i t y nominally going to infinity.  The gas has undergone an e l e c t r i c a l breakdown to  a conducting s t a t e . This breakdown c o n d i t i o n may be brought about i n s e v e r a l ways.  One i s to increase the voltage across the  electrodes i n the gap.  This voltage r e q u i r e d to accomplish  breakdown i s c a l l e d the breakdown voltage and e f f e c t i v e l y increases the f a c t o r  M so that c o n d i t i o n (3) i s met.  A second method i s to use a mechanism which increases the effect of the above-mentioned secondary processes such as an  u l t r a v i o l e t l i g h t pulse t o increase secondary emission 8 and i o n i z a t i o n or a spark introduced i n the gas across a  -15-  spark gap (the production of the spark i s accomplished, of course, by the breakdown voltage method j u s t mentioned). c o n d i t i o n ( 3 ) i s met by an e f f e c t i v e increase i n 2.4.  Now  y .  Radiation  2.4.1.  Radiation Theory  Two processes must be considered when r a d i a t i o n from gaseous discharges i s discussed; the r e l e a s e of radiant  energy  from the gas molecules, and the d e l i v e r y of energy to the gas molecules. In r e l e a s i n g radiant energy, the energy s t a t e of the molecule proceeds from a higher to a lower l e v e l ; the  frequency  of the released r a d i a t i o n i s given by the Planck r e l a t i o n  K9. = E where and  h  i s P l a n c k ' s constant,  y  nrn  the frequency, and  n  Em are the higher and lower s t a t e s of the molecule.  p r o b a b i l i t y per u n i t time that a t r a n s i t i o n from the n the m* energy s t a t e w i l l occur i s given by n  where  E  t h  The to  -16-  [jJ  n  and  ijj  n  and  states  m  are the wave functions p e r t a i n i n g t o the energy m , and  dV  i s an element of volume.  If the s t a t e s are bound, that i s , negative then the  energies,  \jj - f u n c t i o n s are normalized to u n i t y , and i f e i t h e r  s t a t e i s unbound, that i s , the molecule i s i o n i z e d , then the corresponding wave function has the asymptotic form of a plane-wave. When both s t a t e s are unbound, c l a s s i c a l l y the  free  e l e c t r o n passes c l o s e to a nucleus and i s accelerated by the nuclear charge; r a d i a t i o n takes place by v i r t u e of the electron's acceleration.  This process i s c a l l e d bremsstrahlung,  I f the f i n a l energy s t a t e of the molecule l i e s higher than the i n i t i a l , the reverse process to emission fakes p l a c e , absorption.  This process i s governed by the p r o b a b i l i t y  B  m n  that absorption w i l l occur per u n i t time per u n i t density of monochromatic energy i n the r e g i o n of the absorber,  EL g  n  where  Af  stands f o r the s t a t i s t i c a l weight of s t a t e  n .  In a d d i t i o n to spontaneous emission, governed by A  n m  ,  there i s the process of induced emission which can occur i n the presence of r a d i a t i o n . Bnm  For t h i s process the c o e f f i c i e n t  i s the p r o b a b i l i t y f o r forced emission to occur per u n i t  -17.  time, per u n i t density of monochromatic r a d i a n t energy i n the v i c i n i t y of the e m i t t e r .  Theory shows that  An energy balance equation can be w r i t t e n i f a s t a t e of thermal e q u i l i b r i u m e x i s t s between r a d i a t i o n and r a d i a t o r s ,  (/L  B_pX=  +  ^nm)  where  *  s  t  h  e  m  B_pN  m  ° o o c h r o m a t i c radiant energy d e n s i t y ,  and N and N are the number of molecules i n s t a t e s n  m  n  and m  respectively. However i n most p r a c t i c a l instances, the r a d i a t o r s i n a low-pressure gas discharge are not i n a s t a t e of thermal e q u i l i b r i u m with the r a d i a t i o n .  Spontaneous emission g r e a t l y  exceeds absorption, which i n turn occurs much more frequently than forced emission. Before the r a d i a t i v e process can occur, the molecule must be i n an elevated energy l e v e l .  This brings up the  second process that must be considered, the d e l i v e r y of energy to the gas molecules.  This i s achieved by i n e l a s t i c c o l l i s i o n s .  The prime source for energy e l e v a t i o n i s e l a t a t i o n which i s the name for e x c i t a t i o n and i o n i z a t i o n by i n e l a s t i c c o l l i s i o n s of the f i r s t k i n d .  Radiators i n any s t a t e may  -18-  absorb the k i n e t i c energy of impinging p a r t i c l e s and transform t o a higher energy l e v e l .  The c o n d i t i o n s to be f u l f i l l e d  for  t h i s t o happen are that energy and angular momentum must be conserved.  The angular momentum requirement p r o h i b i t s c e r t a i n  bound t r a n s i t i o n s . Free e l e c t r o n s are the most e f f i c i e n t impacting p a r t i c l e s because they possess the most favourable mass r a t i o for energy transfer.  Von Engel  shows that the maximum energy, A  , that  can be t r a n s f e r r e d from the impinging p a r t i c l e to the atom i s given by  where E  Q  m = mass of atom,  mj[ = mass of impinging p a r t i c l e , and  = i n i t i a l k i n e t i c energy of the impinging p a r t i c l e .  an impinging e l e c t r o n , m ^ ^ ^  m and therefore  for any heavier p a r t i c l e , A < C E  Q  •  / \  For  == E  Q  ;  P o s i t r o n s are presumably  j u s t as e f f e c t i v e . Another process, mentioned p r e v i o u s l y , which e x c i t e s or i o n i z e s r a d i a t o r s i s photon absorption.  Rigorous s e l e c t i o n  r u l e s govern the e x c i t a t i o n process and for i o n i z a t i o n there are probably three processes which occur. F i r s t , the d i r e c t absorption of photons w i t h energy exceeding the i o n i z a t i o n energy and subsequent e j e c t i o n of an electron.  'i  Second, the absorption of d i s c r e t e l i n e r a d i a t i o n s  -19-  f o l l o w e d by i o n i z a t i o n by c o l l i s i o n w i t h other p a r t i c l e s w h i l e the e x c i t e d s t a t e p e r s i s t s .  T h i r d , non-monatomlc molecules can  be e x c i t e d t o h i g h molecular s t a t e s and then d i s s o c i a t e i n t o molecular  spontaneously  ions.  However, f o r gases at low p r e s s u r e a b s o r p t i o n of photons does not p l a y as important a p a r t i n e x c i t i n g or i o n i z i n g as do c o l l i s i o n s .  2.4.2.  Recombination  Once the gas molecules have been i o n i z e d and energy i s no longer b e i n g i n j e c t e d i n t o the system, d e - i o n i z a t i o n takes p l a c e . recombined  the process of  The e l e c t r o n s and the i o n s a r e  and the gas r e t u r n s t o a n e u t r a l s t a t e .  The two  types of recombination a r e volume and s u r f a c e recombination; volume recombination takes p l a c e i n the volume of the gas and s u r f a c e recombination takes p l a c e on the s u r f a c e of the container walls. Volume recombination depends on random c o l l i s i o n s o c c u r r i n g i n the volume of the gas.  For b i n a r y c o l l i s i o n s ,  the number of recombinations per u n i t time i s g i v e n by  where  <J~  T  i s the recombination c r o s s - s e c t i o n ,  r e l a t i v e speed,  and  n,  and  n_  u  t h e d e n s i t y of the  i s the  -20-  recombining p a r t i c l e s . three-particle  where  The product CT (X r  Is  averaged.  For  c o l l i s i o n s , the recombination r a t e i s given by  i s the e f f e c t i v e volume of the t h i r d p a r t i c l e and n  i t s density.  Q  In e i t h e r case these f a c t o r s are combined i n t o a  single coefficient  a  , c a l l e d the recombination  so the r a t e i s expressed  coefficient,  as  (4)  The p r o b a b i l i t y of recombination depends on the r e l a t i v e speed of the p a r t i c l e s and decreases the f a s t e r p a r t i c l e s move w i t h respect to one another.  the  This i s because  the time i n t e r v a l during which they are i n c l o s e p r o x i m i t y and which i s a v a i l a b l e for recombination v a r i e s i n v e r s e l y w i t h the speed.  It should a l s o vary w i t h the e f f e c t i v e diameter of  the i o n s , being larger for e x c i t e d i o n s . Volume recombination takes place by f i v e p r i n c i p a l methods. 1) R a d i a t i v e e l e c t r o n recombination.  A free e l e c -  tron combines w i t h a p o s i t i v e i o n i n t o an e x c i t e d atom and a photon possessing the surplus energy required to s a t i s f y  the  -21-  conservation of energy.  The p r o b a b i l i t y for e l e c t r o n capture  i n t o the lower s t a t e s i s greater the lower the s t a t e . a d d i t i o n to s a t i s f y i n g conservation of energy, the  In  liberated  photon must s a t i s f y conservation of angular momentum. 2) D i e l e c t r o n i c recombination.  In t h i s instance a  free e l e c t r o n combines w i t h a p o s i t i v e ion and the excess energy of the e l e c t r o n i s given to a second e l e c t r o n i n the atom so that a doubly e x c i t e d ion i s formed.  This s i t u a t i o n i s unstable  and may lead to a u t o - i o n i z a t i o n or be s t a b i l i z e d by c o l l i s s i o n or by r a d i a t i n g . 3) Three-body e l e c t r o n recombination.  If a t h i r d  p a r t i c l e i s i n the v i c i n i t y of an e l e c t r o n - i o n recombination, the surplus energy from the act i s taken up by the t h i r d body i n the form of k i n e t i c energy. 4) D i s s o c i a t i v e recombination.  When a p o s i t i v e  molecular i o n combines w i t h an e l e c t r o n , the excess energy may be u t i l i z e d i n d i s s o c i a t i n g the molecule i n t o i t s c o n s t i tuent atoms of which one or a l l may be e x c i t e d . 5) Mutual n e u t r a l i z a t i o n .  This process i s simply the  recombination of a p o s i t i v e and a negative i o n .  Preliminary  processes must i n v o l v e the preparation of negative ions which limits  t h i s process to electronegative  gases.  In a high density mixture of approximately equal conc e n t r a t i o n s of ions and e l e c t r o n s ,  the electrons tend to  d i f f u s e r a p i d l y away towards regions of lower density such as  -22-  the w a l l s of the containers thus c r e a t i n g a p o s i t i v e space change.  An e l e c t r i c f i e l d i s set up which r e t a r d s the e l e c t -  rons and accelerates the ions so that both d i f f u s e at the same rate.  This process i s known as ambipolar d i f f u s i o n and i s  governed by the ambipolar d i f f u s i o n constant  D . a  The  equation for p a r t i c l e concentration under ambipolar d i f f u s i o n e f f e c t s only i s  ^  dt  -  'C^V n  ... (5)  2  Under ambipolar d i f f u s i o n ,  the p a r t i c l e s d i f f u s e u n t i l  they s t r i k e the w a l l s of the enclosure and surface recombinat i o n takes p l a c e .  I t may r e s u l t i n luminescence but  i s known about t h i s process.  little  Probably most of the surface  recombination r e s u l t s merely i n heating the surface.  Surface  recombination i s an extreme case of three-body e l e c t r o n recombination. I f both ambipolar d i f f u s i o n and volume recombination e f f e c t s are present,  then the equation for e l e c t r o n concentra-  t i o n becomes, from (4) and (5)  I t i s assumed that the e l e c t r i c a l discharge maintains a high degree of l o c a l n e u t r a l i t y so that  n  e  = ni  for s i n g l y  -23-  i o n i z e d gases. E x p l i c i t s o l u t i o n s of t h i s equation are p o s s i b l e but have not been u t i l i z e d .  L i m i t i n g s o l u t i o n s are o b t a i n a b l e ,  however, both for when recombination i s n e g l i g i b l e , and when diffusion i s negligible.  In the former case, the s o l u t i o n i s  i n the form of an exponential decay, w i t h the time constant dependent on the d i f f u s i o n constant and the geometry of the discharge. When d i f f u s i o n i s n e g l i g i b l e , the s o l u t i o n of (6) i s  ~  ~  He  where  (n ) e  i  s  t  h  e  0  2.4,3.  ^  ... (7)  =<*t  e l e c t r o n d e n s i t y at  t =0  .  Continua  The continuous spectrum observed i n a gas discharge can o r i g i n a t e i n s e v e r a l ways. For the case of e l e c t r o n - i o n recombination, the e l e c t r o n can approach and be caught by the i o n , w h i l e the energy of recombination may be emitted as a quantum of radiation.  In general the e l e c t r o n has a k i n e t i c energy  and so emits r a d i a t i o n of frequency  =  gy +  E  E  -24-  where  V j ^ i s the i o n i z a t i o n p o t e n t i a l of the i o n .  I f the  e l e c t r o n s have an energy d i s t r i b u t i o n , then the emitted spectrum i s a continuum beginning at the s e r i e s and extending to i n f i n i t y . for  limit  However, s i n c e the p r o b a b i l i t y  recombination i s l a r g e r for a s m a l l r e l a t i v e v e l o c i t y  between the e l e c t r o n and the i o n , the continuum w i l l be most intense at the s e r i e s l i m i t and w i l l f a l l r a p i d l y beyong i t . C e r t a i n l y the most important c o n t r i b u t i o n to the continuum comes from the s o - c a l l e d f r e e - f r e e the e l e c t r o n , or bremsstrahlung.  t r a n s i t i o n of  This i s , from a quantum  point of view, the displacement of the e l e c t r o n from one free energy l e v e l to another w i t h release of energy frequency  -pn  .  E of  Thus i n a plasma where e l e c t r o n s move  at random, energy i s r a d i a t e d continuously over a wide band of frequencies.  This r a d i a t i o n i s a l s o c a l l e d r e t a r d a t i o n  radiation. Some other types of cbntinua of l e s s e r importance are molecular d i s s o c i a t i o n continua, pressure continua from c o l l i s i o n i n t e r f e r e n c e w i t h l i n e r a d i a t i o n , and attachment continua.  -24-  3.  The F l a s h Tube  3.1.  The Flash Tube Discharge Model B r i e f l y , the f l a s h tube operates i n the f o l l o w i n g  manner.  A gas at low pressure i s confined i n an i n s u l a t i n g  tube between two e l e c t r o d e s .  This tube i s placed across the  t e r m i n a l s of a low inductance condenser which i s charged t o a high v o l t a g e .  The condenser i s discharged through the gas  which emits copious amounts of h i g h - i n t e n s i t y l i g h t . An equivalent c i r c u i t for the f l a s h tube and discharge condenser i s d e s i r a b l e f o r a n a l y s i s .  The c i r c u i t i s shown  below.  L  C  where  C = capacitance of the discharge condenser,  L = con-  denser and tube inductance,  R = tube lead r e s i s t a n c e , and  Rrj. =» f l a s h tube impedance.  To make a n a l y s i s s i m p l e r , i t has  L  been assumed that the discharge impedance i s p u r e l y r e s i s t i v e .  -25-  The d i f f e r e n t i a l equation governing t h i s c i r c u i t when the s w i t c h i s closed at time  t = 0 , corresponding to the  f i r i n g of the f l a s h tube, i s  [ ^ where  R  Die  + = R  dt  + R.  Three d i f f e r e n t s o l u t i o n s are p o s s i b l e . 1)  The s o l u t i o n i s then given by  where  7  I2L  b = -/ L  LC.  7  \/o = i n i t i a l voltage on the condensor. This i s an exponential current form which for  0^  r i s e s w i t h time constant approximately equal to - ^  t((  ~ ~k  since  -26-  ]>b , and f o r  t  ^  —  constant  LC  2L  The  , decays w i t h time  solution i s  at L(t)  L  R  where  2.L  T h i s i s a c r i t i c a l l y - d a m p e d wave. 3)  (R  )z  <  \2.LJ  i LC  ^  T h i s case y i e l d s a s o l u t i o n which i s g i v e n by  c(t) -  _ R  V± £  2 L  T  sinwt  where  T h i s i s a damped s i n e wave. I f the l o g a r i t h m i c decrement i s d e f i n e d by  -27  then  L  and  L=  2 C  R may be c a l c u l a t e d from  (8)  4VM-*,  r where  3.2.  i s the period of o s c i l l a t i o n .  Desirable Characteristics Before a d i s c u s s i o n of the c o n s t r u c t i o n of the f l a s h  tube and measurement of some of i t s p r o p e r t i e s i s g i v e n , some of the d e s i r a b l e p r o p e r t i e s of a f l a s h tube w i l l now be mentioned.  They are given i n the approximate order of t h e i r  importance. The f i r s t requirement i s that the emitted r a d i a t i o n be of a continuous nature. for  The need f o r continuum r a d i a t i o n  absorption spectroscopy i s obvious and any emission or  absorption l i n e s introduced by the background source can only r e s u l t i n a further c o m p l i c a t i o n i n the a n a l y s i s .  A few  i s o l a t e d l i n e s may perhaps be t o l e r a t e d provided they dp not l i e c l o s e to the wavelengths involved i n any experiment, but any e f f o r t to remove them should be encouraged. As was pointed out i n A n e r s o n ' s ^ i n v e s t i g a t i o n of 1  -28-  the spectrum of f l a s h tubes, current d e n s i t i e s i n the discharge must be of the order of 20,000 amp/cm for the production of 2  a strong continuum.  I t can be seen from the equations derived  i n the preceding s e c t i o n that high current may be achieved by using a high charging voltage and having a low c i r c u i t inductance. The f i r s t r e q u i s i t e i s easy to accomplish.  A low c i r c u i t induc-  tance demands the use of a low inductance c a p a c i t o r and a c o a x i a l design of electrode leads.  A low inductance w i l l a l s o  reduce the discharge time which may or may not be an advantage depending on the d e s i r e d use of the tube. For absorption to take place i n a gas, the background source must have a greater i n t e n s i t y than the absorber.  This  r e q u i r e s a high brightness temperature from the f l a s h tube 5  and as pointed out by Budd , both a continuous spectrum and high temperatures are products of the same c o n d i t i o n ; both may be achieved w i t h high c u r r e n t s . I f the i n t e n s i t y of the tube i s high enough, a s i n g l e f l a s h through the absorber may be s u f f i c i e n t to get a useful photographic r e c o r d , otherwise a s u p e r p o s i t i o n of more than one f l a s h may be necessary.  In any case, the method t h a t , i s  hoped to be used for future work i n absorption a n a l y s i s w i l l r e q u i r e the exposure of at l e a s t two d i f f e r e n t photographic plates.  A prime n e c e s s i t y for the success of t h i s procedure  i s that the i n t e n s i t y not vary from one shot to the next. i  Without t h i s c h a r a c t e r i s t i c , q u a n t i t a t i v e a n a l y s i s i s not  -29-  possible. I t i s hoped i n the future to use the f l a s h tube for a n a l y s i s of s h o r t - l i v e d phenomena such as shock fronts and pinched plasmas.  This w i l l r e q u i r e f a s t i d i o u s timing i n the  t r i g g e r i n g of the f l a s h tube and whether t h i s i s p o s s i b l e depends on the j i t t e r time of the t r i g g e r i n g process.  By  j i t t e r time i s meant the v a r i a t i o n i n time from the a p p l i c a t i o n of the t r i g g e r i n g mechanism to the i n i t i a t i o n of the discharge through the tube.  Since shock fronts move at a speed of the  order of 1 cm/u.sec a j i t t e r time of no greater than 1 u.sec i s r e q u i r e d for any t i m e - r e s o l u t i o n  studies.  And f i n a l l y there are two mechanical p r o p e r t i e s to be desired.  The f i r s t i s easy alignment w i t h other u n i t s ,  the absorber and the spectroscope,  notably  and the second i s simple  c o n s t r u c t i o n d e t a i l s for easy replacement or r e p a i r s . 3.3.  E a r l y Designs A d e s c r i p t i o n of some of the e a r l i e r f l a s h tube designs  w i l l now be given before the f i n a l design i s discussed. 3.3.1.  The Elbow Tube  In an e f f o r t to e l i m i n a t e the d e p o s i t i o n of m a t e r i a l s on the e x i t windows, i t was decided to t r y to use the scouring effect of the discharge c u r r e n t .  This could most simply  be done by using a r i g h t - a n g l e d tube, as shown s c h e m a t i c a l l y i n f i g u r e 1(a).  The current flowing around the corner would  1(a) scale  Kb)  ;  elbow  tube  full  size  low-inductance scale:  tube  half size  -30-  scour the top s e c t i o n of the tube at the curve and prevent any e b u l l i t i o n from being deposited there. The scouring process worked quite w e l l ; i n i t i a l l y a transparent yellow substance was deposited a l l over the i n s i d e of the tube, but after the f i r s t 10 shots or so, d i d not get appreciably worse.  The main drawback was that the spectrum  contained too many emission l i n e s o v e r l y i n g the continuum; because of the bend, the p o r t i o n of the discharge which was scouring was cooled by the w a l l s of the tube and the l i g h t from t h i s cooler gas was entering the spectroscope, as w e l l as the l i g h t from the h o t t e r , continuum-forming gas i n the center of the tube.  A minor mechanical disadvantage was the  d i f f i c u l t y encountered i n a l i g n i n g the tube a c c u r a t e l y . The tube was attached to the aluminum electrodes w i t h black wax and the discharge t r i g g e r e d by a spark across a p a i r of tungsten wires waxed through a hole i n the grounded electrode.  The electrodes were clamped i n copper c o l l a r s by  r a d i a l screws which were i n t u r n attached to the condenser electrodes by copper p l a t e s . I n i t i a l l y a g l a s s tube was used, but the heat from the discharge crazed the g l a s s , e v e n t u a l l y causing i t to break after about 50 shots.  With a quartz tube, the c r a z i n g d i d not  occur so the tube d i d not break. The waveform of the current was an e x c e l l e n t reproduct i o n of a damped s i n e wave, case 3) of s e c t i o n 3 . 1 .  The  31-  r e s i s t a n c e and inductance computed from equations (8) and (9) were  .034 ohms and 84.4 mu. henries r e s p e c t i v e l y .  3.3.2.  Low Inductance Tube  In an e f f o r t to achieve a b e t t e r continuum, while s t i l l employing the scouring e f f e c t , a low inductance c i r c u i t was designed, as shown i n f i g u r e 1(b). The low inductance was achieved by use of a more c o a x i a l c i r c u i t about the f l a s h tube, and by having the connecting leads to the condenser c l o s e together.  The grounded electrode  of the tube was connected to the ground p l a t e by a metal gauze collar. The discharge was viewed s i d e - o n instead of end-on as before, by looking through a hole i n the wire gauze. spectrum d i d not seem to be much improved, i f at a l l .  The Some  of the emission l i n e s were more d i f f u s e but others were sharper than before.  Also the i n t e n s i t y was much l e s s so  t h i s c o n f i g u r a t i o n was abandoned. The current was a damped s i n e wave w i t h found to be  3.3.3.  . 0 3 6 . \ - & -  and  R and L  31.3 mp.h .  Tungsten-electrode Tube  In an attempt to keep the i n t r o d u c t i o n of f o r e i g n m a t e r i a l s i n t o the discharge t o a minimum, a r a d i c a l type of f l a s h tube was t r i e d next.  The electrodes at both ends of  tungsten  electodes  window pump  tungsten-electrode  2(a)  covar-to-glass  seal  covar  trigger  pin  tube  -window  pump  2(b)  z 2(c)  covar  tube  window brass  covar  dump-chamber  tube  with  dump  chamber  -32-  the tube consisted of 4 r a d i a l tungsten wires set through the g l a s s tube, as shown i n f i g u r e 2 ( a ) .  These wires were then  joined together e x t e r n a l l y by a copper s t r i p and thence attached to the condenser.  At one end of the tube, beyond the  e l e c t r o d e s , was attached the vacuum pump w h i l e at the other end, a g l a s s window was cemented on w i t h de Kotinsky wax. T r i g g e r i n g was effected by a spark between one of the grounded tungsten electrodes and a f i f t h tungsten w i r e i n s e r t e d through the g l a s s c l o s e to the grounded e l e c t r o d e . The window stayed clean for the f i r s t ten shots but then a d e p o s i t i o n r a p i d l y formed.  Also the spectrum, while  possessing a good background continuum, had many o v e r l y i n g emission l i n e s .  For t h i s reason^ and a l s o because of the  awkward geometry, work on t h i s tube was h a l t e d .  3.3.4.  Covar-electrode Tube  S t i l l keeping i n mind the n e c e s s i t y f o r e l i m i n a t i o n of f o r e i g n m a t e r i a l s from the discharge, the next attempt used covar electrodes w i t h a c o v a r - t o - g l a s s s e a l to the tube. This was to circumvent the use of wax as a s e a l which could cause contamination i f any of i t was immersed i n the discharge. The arrangement of the c a p a c i t o r leads was the same as was used i n the f i n a l design and hence w i l l be described below. The tube was t r i g g e r e d by a covar wire sealed through the g l a s s near the grounded electrode and bent back so i t was  -33-  c l o s e to the e l e c t r o d e .  I n i t i a l l y the window was Cemented  onto the covar (see f i g u r e 2(b)) but t h i s introduced wax i n t o the discharge so then a brass chamber was soldered onto the covar and the window waxed onto the brass (figure 2 ( c ) ) .  It  was hoped that l i t t l e of the discharge would reach the wax around the window.  Deposit s t i l l formed on the window however,  and a l s o the covar t r i g g e r p i n was affected by the discharge so that a spark between i t and the electrode would not occur after only 5 discharges.  F i n a l l y , after c l e a n i n g the t r i g g e r  p i n , a c i r c u l a r g l a s s b a f f l e was placed i n the brass chamber c l o s e to the covar e l e c t r o d e .  The hole i n the center of the  b a f f l e was of such a s i z e that the covar electrode could not "see" the window.  This l e d t o the f i n a l design of the tube and  i s now to be described.  3.4.  F i n a l Design  3.4.1.  Description  The f l a s h tube design f i n a l l y a r r i v e d and i s shown i n f i g u r e 3.  The operation of the tube i s as f o l l o w s .  The  E^ cemented to the quartz discharge tube  aluminum electrode  i s connected to the 4.8 cm long, 2.7 cm inner diameter copper collar  C i by means of 8 r a d i a l screws  S .  This copper c o l l a r  i s connected to the ground terminals of the condenser through the copper lead  L  1  .  The other aluminum electrode  attached to the copper c o l l a r  C„  E£  is  by a wire gauze which i s  fig. 3  final design scale full size :  -34-  soldered to the c o l l a r and secured to the electrode by a metal strap.  Collar  C 2 i s connected to the other t e r m i n a l of the  condenser by the copper lead  L£ .  When the tube i s f i r e d by  a p p l i c a t i o n of the t r i g g e r pulse on the t r i g g e r p i n , the current flows from the condenser up through E2  L g , along  C 2 to  by way of the wire gauze, through the i n s i d e of the quartz  tube to  and thence to the other condenser t e r m i n a l v i a S ,  C]_, and  Li .  perspex sleeve  and  L 2 are i n s u l a t e d from each other by a  P , 7.5 cm long and 2.0 cm inner diameter, g  i n a perspex p l a t e  set  P^, 1.5 mm t h i c k .  The aluminum electrodes  Ej  and E 2  are j o i n e d to the  quartz tube w i t h de Kotinsky wax, but because of the snug f i t of the tube i n t o the inner shoulder of the e l e c t r o d e , the wax remains i n the outer shoulder and i s not immersed i n the discharge i n any way.  Aluminum was chosen as electrode  m a t e r i a l because of i t s low s p u t t e r i n g r a t e . The tube i t s e l f was made of quartz 10 cm long and .9 cm inner diameter, 1.2 cm outer diameter.  Quartz was chosen f o r  i t s better a b i l i t y to withstand the heat generated by the discharge.  Quartz a l s o does not show strong absorption l i n e s  caused by absorption i n the vapour scoured from the w a l l s of the tube.  Glass shows the sodium  D lines strongly in  absorption. The t r i g g e r i n g device i s a s m a l l covar tube i n glass set i n t o the e l e c t r o d e  Ej  and a tungsten wire jplaced far  -35-  enough through i t so the end of the wire i s c l o s e to the opposite inner s i d e of the e l e c t r o d e . manently sealed to  Ej  The covar p i n i s per-  w i t h epoxy r e s i n , and the wire w i t h  de Kotinsky wax for easy replacement i f necessary. When a high voltage pulse i s a p p l i e d to the t r i g g e r w i r e , a spark jumps from the end of the w i r e to the electrode and discharge of the condenser begins. A 1/16 inch c i r c u l a r g l a s s b a f f l e i s set i n t o the electrode  E 2 as shown, and stops the discharge d e b r i s from  reaching the window W . 2  enough  so  that  any p a r t i c l e s t r a v e l l i n g i n a s t r a i g h t  from the discharge s i d e of than the window.  The hole i n the b a f f l e i s small  E  2  line  w i l l h i t the b a f f l e rather  The b a f f l e i s held i n place by a t h i n - w a l l e d  aluminum tube which f i t s c l o s e l y i n t o the chamber of i s braced against the window W . 2  Both  E and 2  W^ and Wg are  glued to t h e i r r e s p e c t i v e electrodes w i t h de Kotinsky wax of which a minimum amount i s used to allow as l i t t l e as p o s s i b l e to enter and contaminate the discharge. The c o a x i a l symmetry of the leads were chosen for the low inductance and a l s o for the low r a d i a t i n g e f f i c i e n c y . Because of the high current flowing i n opposite d i r e c t i o n s i n the copper leads, a r e p u l s i v e force i s set up and the leads tend to be forced apart*  The purpose of 'the  wire gauze connecting the electrode to the copper c o l l a r i s to absorb t h i s shock.  A perspex clamp i s a l s o used to help  -36-  hold the p l a t e s together.  O r i g i n a l l y both electrodes were  clamped by r a d i a l screws, but the wax or even the tube would crack after not too many discharges. 3.4.2.  Operation  The voltage impressed across the electrodes was from 15 to 20 kV.  Below 15 kV the spectrum showed many emission  l i n e s o v e r l y i n g the background continuum and at 15 kV these l i n e s were few and d i f f u s e .  An increase i n charging voltage  tended to broaden the l i n e s and make the continuum b e t t e r . Twenty kV was chosen as an upper l i m i t for the charging voltage because t h i s i s c l o s e to the maximum voltage rated by the condenser manufacturer and a l s o above 20 kV the danger of the tube s h a t t e r i n g upon f i r i n g was increased. The pressure i n the tube was about .1 u. or lower. The a c t u a l value of the pressure was not measured a c c u r a t e l y s i n c e the pressure required was j u s t t h a t which would, " h o l d " the a p p l i e d voltage without spontaneous breakdown. would e l i m i n a t e the need for any s e r i e s t r i g g e r  This  spark-gaps  which would only complicate the c i r c u i t r y and make a l e s s e f f i c i e n t discharge. this f i e l d  1 1  I t has been found by other workers i n  that the i n t e n s i t y of the r a d i a t i o n i s indepen-  dent of i n i t i a l pressure i n the tube over a wide pressure range.  Thus the low pressure was r e q u i r e d o n l y because of  the high v o l t a g e .  -37-  3.4.3.  Experimental  The experimental procedure i n designing the f l a s h tube was f a i r l y s t r a i g h t f o r w a r d .  The two important r e q u i s i t e s that  had to be met were a high i n t e n s i t y continuum and a constant peak intensity.  The l i g h t output of the d i f f e r e n t tube c o n f i g u r a t i o n s  was examined w i t h a H i l g e r medium quartz spectroscope and a H i l g e r automatic quartz the continuum.  spectroscope to observe the q u a l i t y of  A p h o t o m u l t i p l i e r - e m i t t e r - f o l l o w e r u n i t , des-  c r i b e d below, was used to observe the i n t e n s i t y of the r a d i a t i o n from the f l a s h tube.  This was done by s e t t i n g the p h o t o m u l t i p l i e r  about 10 feet from the tube and observing the i n t e n s i t y trace on a Tektronix o s c i l l o s c o p e type 533.  The decrease i n peak i n t e n -  s i t y w i t h number of shots was recorded w i t h a P o l a r o i d o s c i l l o scope camera. To observe the discharge current a small p i c k - u p c o i l was placed c l o s e to the discharge.  The change of magnetic f l u x i n ; t h e  c o i l induced an e.m.f. given by  V  where  =  M  di dt  M i s the mutual inductance between the c o i l and the  discharge c i r c u i t and  — dt  i s the d e r i v a t i v e of the c u r r e n t . ••  _  This s i g n a l was fed through an analog i n t e g r a t o r to the o s c i l l o scope, g i v i n g a s i g n a l p r o p o r t i o n to the c u r r e n t .  -38-  3.5.  Apparatus  3.5.1.  Charging  Condenser and Charging  As mentioned i n S e c t i o n 3.2.  a low inductance  d e s i r a b l e t o achieve high c u r r e n t d e n s i t i e s . the condenser used was  a 1,6  [if  Unit circuit  For t h i s  low inductance,  The and  c h a r g i n g u n i t i s capable  network.  A c u r r e n t - s e n s i t i v e r e l a y i n s e r i e s with  power to the high v o l t a g e transformer Thus the v o l t a g e may  denser up u n t i l  kV  323.  filtered  c u r r e n t meter c l o s e s a s e t of c o n t a c t s which opens  switch.  25  of d e l i v e r i n g 27 kV at 50  i s a standard f u l l - w a v e voltage-doubler c i r c u i t  an L-C  reason  25 mu,h,  condenser manufactured by C o r n e l 1 - D u b i l i e r , model NRC  is  ma  by  the  primary  and drops a s h o r t i n g  be a p p l i e d to the c h a r g i n g con-  the i n s t a n t of f i r i n g ; when the f l a s h tube i s  d i s c h a r g e d the c h a r g i n g u n i t i s prevented  from r e c h a r g i n g  the  condenser by the c u r r e n t t r i p and s h o r t i n g s w i t c h .  3.5.2. The  Trigger Unit  t r i g g e r u n i t was  designed  a f t e r the work of  12 Theophanis pin  and d e l i v e r s a 40 mu.sec, 32 kV p u l s e to the  to i n i t i a t e  the f l a s h tube d i s c h a r g e .  See f i g u r e 4.  trigger A  three-meter l o n g , type RG58u c o a x i a l c a b l e i s charged to 16 The  f a r end of the c a b l e i s terminated  condenser p a r a l l e l e d w i t h a 100 KX1 e s s e n t i a l l y an i n f i n i t e t e r m i n a t i o n .  w i t h a 500  p f , 20  kV  r e s i s t o r ; t h i s gives The  sheath of  c o a x i a l c a b l e i s grounded w h i l e the inner conductor  the at 16  kV  kV.  -39-  F i g . 4.  Trigger Unit  i s c o n n e c t e d t o t h e anode of a 5C22 h y d r o g e n - f i l l e d t h y r a t r o n which i s i n i t s non-conducting s t a t e .  When a p o s i t i v e p u l s e  i s a p p l i e d t o t h e g r i d of t h e t h y r a t r o n c a u s i n g i t t o conduct h e a v i l y , i t e s s e n t i a l l y s h o r t s t h e end o f t h e a t t a c h e d c o a x i a l c a b l e , s e n d i n g a n e g a t i v e 16 kV p u l s e down t h e c a b l e .  This  p u l s e i s r e f l e c t e d a t t h e f a r end w i t h a r e f l e c t i o n f a c t o r of +1 and i n o r d e r t h a t t h e v o l t a g e a c r o s s t h e condenser remains " i n s t a n t a n e o u s l y " c o n s t a n t , t h e f a r s i d e o f t h e condenser must f a l l t o minus 32 kV. resistor.  This pulse i s taken o f f across the  40-  The provided  p o s i t i v e p u l s e r e q u i r e d t o t r i g g e r the t h y r a t r o n i s  by a 2D21 t e t r o d e which i s normally  i n a s t a t e of  c u t - o f f u n t i l i t s g r i d i s shorted by a manual micro-switch  or  by a p p l i c a t i o n of a p o s i t i v e p u l s e .  3.5.3. The  Photomultiplier  Unit  p h o t o m u l t i p l i e r u n i t was used t o determine the  v a r i a t i o n of the wavelength-integrated f l a s h tube w i t h the number of s h o t s .  l i g h t output of the T h i s u n i t c o n s i s t e d of  a 931A p h o t o m u l t i p l i e r and a 2N1177 t r a n s i s t o r i n a common c o l l e c t o r or e m i t t e r - f o l l o w e r c o n f i g u r a t i o n .  F i g . 5.  See f i g u r e 5.  Photomultiplier  Unit  -41-  The u n i t was completely enclosed w i t h i t s power s u p p l i e s i n a brass box to avoid e l e c t r i c a l pickup from the discharge.  Even  then i t was found that the u n i t had to be placed about 10 feet from the discharge before pickup was n e g l i g i b l e . I t was found that shot noise i n the p h o t o m u l t i p l i e r g r e a t l y masked the output s i g n a l at low output v o l t a g e s ; t h i s shot noise o r i g i n a t e s e s s e n t i a l l y i n the cathode-dianode-numberone r e g i o n and can be reduced by i n c r e a s i n g the current between these stages.  Non-linear and s a t u r a t i o n e f f e c t s were found to  occur i n the l a t e r stages of the p h o t o m u l t i p l i e r when a l l dianodes were connected, however, and so dianode No. 7 was used as the anode, and the anode and dianodes No. 8 and 9 were strapped to dianode No. 6.  This almost completely e l i m i n a t e d  the noise i n the output voltage range that was encountered. The common c o l l e c t o r t r a n s i t o r  c i r c u i t was used to  match the high output impedance of the p h o t o m u l t i p l i e r to the 52 ohm c h a r a c t e r i s t i c impedance of the c o a x i a l cable connecting the u n i t to the o s c i l l o s c o p e .  With a 52 ohm impedance match  at (the 'scope end of the c a b l e , r i n g i n g i n the c i r c u i t was eliminated.  The u n i t was tested f o r l i n e a r i t y using a .5 p.sec  l i g h t pulse and was found to be l i n e a r for an output s i g n a l up to .4 v o l t s .  The r i s e - t i m e was l e s s than .1 p.sec.  -42-  4.  Brightness Temperature  4.1.  Measurement  Introduction  4.1.1.  Black Bodies  L i g h t r a d i a t i o n i s the transfer  of energy t r a v e l l i n g i n  the form of electromagnetic waves, and the standard for a l l r a d i a t o r s i s the black body. A black body may be simply defined as a body that absorbs a l l the r a d i a t i o n which f a l l s upon i t .  I t can be shown theore-  t i c a l l y and has been v e r i f i e d experimentally that such a body w i l l r a d i a t e energy of an amount which depends only on the temperature of the body. This leads to one of the fundamental laws which determines the manner i n which a black body r a d i a t e s i t s energy, the c e l e brated Planck r a d i a t i o n law which t e l l s how the r a d i a n t  energy  v a r i e s w i t h the wavelength of the r a d i a t i o n and w i t h the temperature of the r a d i a t i n g body.  This i s given by  (10)  where  = the s p e c t r a l r a d i a n t i n t e n s i t y per u n i t wavelength  at a temperature  T  i n the wavelength i n t e r v a l (A, \ + dX),  C i and C2 are constants connected to the fundamental constants  -43-  h, c  and  k  and  C2 =• 1.438 cm-deg.  P l a n c k ' s law has two s i m p l i f y i n g forms for the opposite end of the spectrum.  If  AT i s l a r g e w i t h respect to ' Q2 ,  then the Rayliegh-Jeans formula i s obtained  On the other hand i f  AT i s s m a l l , Wien's formula i s  a r r i v e d at  4;1.2.  Temperature In the measurement of r a d i a t i o n from bodies, one  u s u a l l y needs to know, or i s i n t e r e s t e d i n , i t s  temperature.  In pyrometry and c o l o r i m e t r y the important q u a n t i t i e s are brightness temperature and colour temperature. i n t e r e s t here i s the e l e c t r o n temperature.  Also of  They are defined  as f o l l o w s : 1) Brightness temperature.  The brightness  temperature  of a body i s that temperature of a b l a c k body which has the same r a d i a n t i n t e n s i t y of the surface of the body i n question * at a f i x e d wavelength.  The r e l a t i o n between brightness  -44temperature  . S and true temperature  Jjs) where  (S)  T  i s given by  =r£^,T)j,(T)  ( 1 3 )  represents the r a d i a n t i n t e n s i t y of a black body  w i t h a temperature S  }  J^(T)  represents the r a d i a n t i n t e n s i t y  of the body under c o n s i d e r a t i o n w i t h a temperature £(A,T)  i s the e m i s s i v i t y of the body.  T, and  i s the t r a n s -  mission f a c t o r of any absorbers i n the system between the source and the observer;  f o r example the window of a tungsten  filament lamp whose brightness temperature i s to be measured. 2) Colour temperature.  The colour temperature of a  l i g h t source may be defined as the temperature of a black body which has the same c o l o u r .  Thus colour temperature has meaning  only i n the v i s i b l e range. 3) E l e c t r o n temperature.  I f a c o l l e c t i o n of e l e c t r o n s  are i n thermal e q u i l i b r i u m w i t h themselves a constant mean energy of the c o l l e c t i o n may be defined.  With the e q u i -  p a r t i t i o n theorem f o r energies i n mind, i t i s p o s s i b l e to define the e l e c t r o n temperature as  E where  £  =|H =  i s the mean energy of the e l e c t r o n gas,  e l e c t r o n mass,  k  i s Boltzmann's constant,  v  r  m the  the random  -45-  v e l o c i t y and  T  e  the e l e c t r o n temperature.  A local electron  temperature may be defined i f the e l e c t r o n s are i n l o c a l e q u i l i b r i u m even though not n e c e s s a r i l y i n o v e r a l l e q u i librium.  A temperature for other types of p a r t i c l e s can  s i m i l a r l y be defined.  I f a gas c o n s i s t s of two types of  p a r t i c l e s , t h e i r temperatures need not be the same.  4.2.  Theory To c a l c u l a t e the brightness temperature o f . t h e f l a s h  tube the r a d i a n t i n t e n s i t y per u n i t wavelength of the tube i s compared to the r a d i a n t i n t e n s i t y of a  tungsten-filament  lamp by measuring the i n t e n s i t i e s at various wavelengths w i t h a photomultiplier. I f the i n t e n s i t y of the r a d i a t i o n emitted by the f l a s h  wavelength i n t e r v a l measured and i s a function of wavelength, the i n t e n s i t i e s observed on the o s c i l l o s c o p e w i l l be determined by 1) The v a r i a t i o n i n s e n s i t i v i t y  of the  p h o t o m u l t i p l i e r w i t h wavelength. sSfhjvtlll be a s i n g l e - v a l u e d function whose value l i e s between  0 and 1, i f 1 i s i t s value  at peak s e n s i t i v i t y . 2) Any f i l t e r s which need to be placed i n the path between the lamps and the p h o t o m u l t i p l i e r i n order to keep  -46-  the  amplitude  the  photomultiplier.  should  for  of  be  used.  3)  Any  lamp.  different  intensity For  geometric  measuring  sten  the  the  This s o l i d  angles,  Therefore  differences  be  the  the  s i m p l i c i t y ,  r a d i a t i o n w i l l  below  from  such  signals  seen  the  flash  things  different  neutral  i n  the  non-linear  as  size on  the  density  of  f i l t e r s  experimental  setup  tube  tung-  and  the  subtending  sources,  the  level  'scope  of  and  so  on.  can  be  written  V(A)= K ^ ) ^ G y d A n tl  T L  where  "  amplification  t3(?\)  amplifier, transmission and  the  for  neutral Gy  and such  factors  f l a s h  tube  the  effects  system  may  -r-Tt J .  T  the  also  f i l t e r s  not  chromatic  s e n s i t i v i t y ,  for  not  a  the  and  V  JL  tungsten  function  of  = the  A/\  TL *  U /  factors a  for  function  aberration  spectral  tungsten °*  VT  =  lamp wavelength  of  the  of  tungsten  wavelength  any  lenses  lamp provided  i n  the  FT  of  'scope  ,^  neglected.  J  wavelength c\\  be  of  geometric  tube; as  emitter-follower  f i l t e r s .  "  flash  of  photomultiplier  respectively;  density  ^ ^jrr  L  =  factor  L  the  radiant  lamp  and  wavelength  intensity  the  f l a s h  range  over  per  unit  tube. which  the  -47-  i n t e n s i t y was measured f o r t h e t u n g s t e n lamp and t h e f l a s h tube. For  t h e t u n g s t e n lamp Wien's l a w can be used i n t h e  v i s i b l e region so that  whereas f o r t h e f l a s h tube P l a n c k ' s law g i v e s  so t h a t i f t h e maximum i n t e n s i t i e s o f t h e lamp and t h e f l a s h tube a r e measured a t one wavelength and t h e r a t i o o f t h e i n t e n s i t i e s t a k e n , one g e t s  I f t h e w a v e l e n g t h i n t e r v a l f o r t h e two s o u r c e s a r e t h e same —r-TL  then  M  R  rn G„ J?  I f t h e geometry i s d i f f e r e n t  f o r t h e two lamps,  w i l l be d i f f i c u l t t o c a l c u l a t e ;  i f t h e geometry  Q , ^TL/ L7  F T  i s made  i d e n t i c a l , then H  (  A  ' " ^ F T J  K  r  T  r  F  T  [ e X  P  ^ - | ]  (14)  -48-  X ^p " may be measured on a microphotometer or densitometer, 1  RfX)»^ ^ anC  a  STL  r  e  * wn, cnG  a n  d hence  SfT  m  a  v  ^  e  c a l c u l a t e d from equation (14). 13 This i s the method used by Parkinson and Reeves  , in  t h e i r experiment of measuring the brightness temperature of. various types of f l a s h tubes. I f , however, the i n t e n s i t i e s are measured at two d i f A and  ferent wavelengths  A',  say, and the r a t i o of the  i n t e n s i t y r a t i o s i s taken, then the f i l t e r f a c t o r s and the geometric f a c t o r s cancel and  where  Sp-p  wavelengths  a n d  5  3  X  FT  A and  Anderson  6  brightness temperatures  at  A ' respectively.  found that the r e l a t i v e i n t e n s i t y d i s t r i -  10  b u t i o n of the f l a s h tube behaves i n a fashion s i m i l a r to that of a black body and i f t h i s i s the case then one should be able to equate  5  p  with  Sp-y- *  *  n  a n  y case, looking at  the r e s u l t s of Parkinson and Reeves "^, i t can be seen that 1  the temperature there c a l c u l a t e d i s a s l o w l y varying f u n c t i o n of wavelength so i f the i n t e r v a l  J A - A ;J ,!  i s not too l a r g e ,  49one can put  S^^.  FT  RfX) If  ^>  =  v  B =  L  FT  without too much e r r o r .  Then  S l X A TL  r—  B« = - - J  A=  then  (15)  This equation can be solved for as i s done here by successive  4.3.  N  by i t e r a t i o n or  substitution.  Experimental Procedure A general schematic view of the  temperature-measuring  apparatus i s shown i n f i g u r e 6. ; Because of the e m i t t e r - f o l l o w e r c i r c u i t f o l l o w i n g the  rotating  mirror  mirrors  photomultiplier  constant-deviation spectroscope  fig.  6  temperature-measuring  apparatus  -50  p h o t o m u l t i p l i e r u n i t a d . c . measurement of the tungstenfilament lamp i n t e n s i t y was not p o s s i b l e and so a pulsed tungsten source was necessary.  Also to achieve more i d e n t i c a l  c o n d i t i o n s w i t h respect to t r a n s i e n t s i n the p h o t o m u l t i p l i e r eraitter-follower c i r c u i t s , a tungsten l i g h t pulse of duration of the same order as that of the f l a s h tube was d e s i r e d .  This  meant a pulse of the order of 10 p.sec and necessitated the use of a shutter of some s o r t .  An o p t i c a l shutter using a r o t a t i n g  mirror seemed i d e a l for t h i s purpose. Unfortunately, the only appropriate r o t a t i n g mirror shutter was being used i n another experiment and hence could not be r e a d i l y removed for use i n t h i s one.  See reference 5  wherein i s described p r e l i m i n a r y work i n the study of absorpt i o n s p e c t r a of plasmas, which employs the use of a f l a s h tube.  This work i s being continued and the experimental  setup i s almost e x a c t l y the same as described except f o r the s u b s t i t u t i o n of the o p t i c a l shutter for the mechanical one. In order to save the time and expense necessary to b u i l d a similar shutter,  i t was decided to mount the f l a s h  tube i n place of the one being used i n the above-mentioned experiment and d e f l e c t the l i g h t from the tube i n t o the c o n s t a n t - d e v i a t i o n spectroscope being used f o r i n t e n s i t y measurements.  The plasma absorption tube shown i n the  diagram i s used i n the the r e s u l t s of t h i s one.  other experiment and does not affect  -51For measuring the i n t e n s i t y of the tungsten lamp, the lamp was placed d i r e c t l y behind the adjustable s l i t and as c l o s e to i t as p o s s i b l e to ensure maximum i l l u m i n a t i o n .  The  s l i t was focused by lens A onto the s l i t of the constant d e v i a t i o n spectroscope when the r o t a t i n g mirror was l i n e d up. The length of the pulse could be adjusted e i t h e r by v a r y i n g the speed of the motor d r i v i n g the mirror or by changing the width of the adjustable s l i t .  This second method was  employed to procure a pulse of about 10 p.sec d u r a t i o n .  The  i n t e n s i t i e s were measured w i t h the p h o t o m u l t i p l i e r and o s c i l l o s c o p e and 'scope camera.  Measurements were made  every 100 angstroms on the drum s c a l e of the spectroscope (which was approximately every 120 angstroms upon c a l i b r a t i o n of the drum) from .4440 A to 6555 1. For the f l a s h tube measurements, lens B and C focussed the l i g h t from the discharge onto the slit.  adjustable  The r o t a t i n g mirror was stopped and p o s i t i o n e d so the  s l i t was focused on the spectroscope and s i m i l a r i n t e n s i t y measurements were made. I t was found necessary to use a f i l t e r w i t h the f l a s h tube and three sheets of mylar were used.  The mylar was a  good n e u t r a l - d e n s i t y f i l t e r and had a transmission f a c t o r of about 45%.  -52-  4.4.  Apparatus The black body which was used as the standard l i g h t  source was a 120 v o l t G . E . p r o j e c t i o n lamp which was operated on 125 v o l t s d . c .  The colour temperature of the lamp was  measured with a Hartmann and Braun filament o p t i c a l pyrometer i n the 1500 to 3500°C range.  The pyrometer c a l i b r a t i o n was  checked using a G . E . T-24, 86-P-50 standard lamp. The brightness temperature of the lamp was then found by using a nomogram c a l c u l a t e d by Rutgers and de V o s ^ which 1  gives the r e l a t i o n between the brightness temperature,  true  temperature and colour temperature of a tungsten f i l a m e n t . The pyrometer was averaged over 10 readings and gave a colour temperature of 2940°K which corresponds to a brightness temperature of 2560°/K. The spectroscope used was a H i l g e r  quartz-prism  constant d e v i a t i o n spectroscope w i t h an adjustable s l i t mounted i n place of the p l a t e - h o l d e r .  The p h o t o m u l t i p l i e r was placed  behind the s l i t and was enclosed i n a brass container attached to the s l i t to reduce pickup. using the  The spectroscope was c a l i b r a t e d  A 5791, 5770, 5461, and 4358 angstrom l i n e s of  mercury and the  A 6563 and 4861 l i n e s of hydrogen.  The p h o t o m u l t i p l i e r c i r c u i t was as was described before except a 900 v o l t supply was used i n s t e a d of 600 v .  This was  necessary to achieve s u f f i c i e n t s e n s i t i v i t y when measuring the l i g h t i n t e n s i t y of the tungsten lamp and the f l a s h tube at both ends of the v i s i b l e spectrum.  -53-  5.  Results  5.1.  F l a s h Tube Spectrum Using the H i l g e r medium quartz spectroscope, o  the spec-  o  trum was observed from 6500 A down to about  2300 A .  At low  condenser voltages (11 kV) quite a few l i n e s and bands were observed on a background continuum throughout the range and as the voltage was increased most of these l i n e s spread out i n t o the background.  Several absorption l i n e s were present o  at a l l v o l t a g e s ; a group of s i x around 2500 A but none i n the v i s i b l e range which i s the region of i n t e r e s t for the work the tube i s hoped to be a p p l i e d .  Some l i n e s seen i n emission  at low voltages were seen i n absorption at higher voltages indicating a faster  increase i n temperature i n the centre of  the gas than i n the e x t r e m i t i e s .  An improvement i n the con-  tinuum was noted when the capacitance was increased from 1.6 j i f to 3.2 j i f . A t i m e - r e s o l u t i o n of the f l a s h was attempted using the r o t a t i n g mirror shutter and a delay u n i t to see at what stage of the discharge the emission l i n e s occurred.  Anderson*^  found that the emission l i n e s d i d not appear u n t i l the l a t e r part of the discharge when the current d e n s i t y was low. This same e f f e c t was found here but because the t i m i n g of the  -54-  shutter was not known very a c c u r a t e l y , the i n s t a n t of i n i t i a l occurrence of the emission l i n e s was unknown.  In a p p r o x i -  mately the f i r s t 20 i&sec of the discharge, however, hardly any emission l i n e s were seen.  At the present time work i s  being done to c o r r e c t the u n c e r t a i n t y i n timing of the s h u t t e r .  5.2.  Current Waveform The observed current waveshape i s shown i n f i g u r e 7.  1*1111.  if  \  \ ,ij F i g . 7.  Current Waveform  I t i s seen to be a damped s i n e wave as discussed i n 3 . 1 . showing that the model chosen was quite c o r r e c t , and i s given by -6.1xl0 t 5  i = 81,400 e  s i n 3.19x10  6  t amps  C a l c u l a t i o n of R and L u s i n g (8) and (9) gives R = 0.07X1L = 58 mu,h.  -55The maximum o f t h e c u r r e n t o c c u r s when t a n t = p and g i v e s a v a l u e o f t = .43 p.sec.  The f i r s t z e r o o f  di was observed a t t = .44 fisec which agrees w e l l and g i v e s  — i  x ".97,000 amp/cm which i s w e l l above t h e 20,000 amp/cm 2  m a  2  found n e c e s s a r y by Anderson f o r t h e p r o d u c t i o n o f a continuum. To g e t a s i m p l e e s t i m a t e o f t h e time a t which t h e c u r r e n t d e n s i t y ceases t o be above t h e "continuum d e n s i t y " (20,000 amps/cm ), t h e s i n u s o i d a l c u r r e n t v a r i a t i o n i s put 2  e q u a l t o 1 and one can s a y 5t  81,400 e " * TTYO.45) 6  1 X l  °  c  <^  20,000  2  where TT(0'^5) Thus t  c  Z  i s t h e c r o s s - s e c t i o n a l a r e a of t h e tube.  ^ 2 . 2 p.sec, whereas  t  c  about 20 p.sec as mentioned above.  was found t o be g r e a t e r than Although the i n e q u a l i t y i s  c o r r e c t , a p o s s i b l e e x p l a n a t i o n o f t h e order of magnitude d i f f e r e n c e i s t h a t even though t h e c u r r e n t d e n s i t y f l o w i n g between t h e e l e c t r o d e s i s z e r o i n about 3 {isecs, t h e energy i n t h e gas i s s t i l l h i g h enough t o ensure t h a t much i o n i z a t i o n i s present, so t h a t the r a d i a t i o n emitted i s of a c o n t i n uous n a t u r e . Of i n t e r e s t i s t h e time l a g between t h e a p p l i c a t i o n of t h e s p a r k p u l s e and t h e i n i t i a t i o n o f t h e c u r r e n t d i s c h a r g e . T h i s i s shown i n f i g u r e 7 where t h e t r i g g e r p u l s e and subsequent n o i s e o c c u r s a t t h e b e g i n n i n g o f t h e t r a c e , and t h e c u r r e n t b e g i n s about 1 u.sec l a t e r , a t which time i t b e g i n s  -56-  abruptly.  T h i s r a i s e s t h e q u e s t i o n o f t h e mechanism f o r  i n i t i a l breakdown o f t h e d i s c h a r g e and t h e r e a s o n f o r t h e delay. The mean f r e e p a t h between a i r m o l e c u l e s a t . 1 p. i s about 10 m e t e r s , and i f k i n e t i c gas t h e o r y a p p l i e s , t h e mean f r e e p a t h between an e l e c t r o n and t h e a i r m o l e c u l e s w i l l be about f o u r times as g r e a t .  When t h e t r i g g e r i n g v o l t a g e i s  a p p l i e d and breakdown o c c u r s between t h e t r i g g e r p i n and t h e grounded e l e c t r o d e , t h e p o s i t i v e i o n s w i l l be a t t r a c t e d t o wards t h i s e l e c t r o d e w h i l e t h e e l e c t r o n s w h i c h escape recomb i n a t i o n w i t h t h e i o n s w i l l be a t t r a c t e d and move towards t h e p o s i t i v e e l e c t r o d e a t t h e f a r end o f t h e tube.  Because o f  the l o n g mean f r e e p a t h , i t i s u n l i k e l y t h a t t h e e l e c t r o n s w i l l c o l l i d e w i t h any o f t h e a i r m o l e c u l e s and t h u s most o f the e l e c t r o n s w i l l s t r i k e the f a r e l e c t r o d e .  Here some s o r t  of s u r f a c e p r o c e s s must t a k e p l a c e s u c h as t h e e j e c t i o n o f e l e c t r o n s and atoms and p o s i t i v e i o n s .  The p o s i t i v e i o n s  w i l l move towards t h e f a r (grounded) e l e c t r o d e , s t r i k e i t , and a l s o e j e c t e l e c t r o n s and i o n s , s o t h e p r o c e s s  i s repeated.  Once t h e p a r t i c l e d e n s i t y i s i n c r e a s e d enough by t h i s e l e c t r o d e s p u t t e r i n g and a l s o s p u t t e r i n g from t h e q u a r t z  tube,  i o n i z a t i o n t h r o u g h c o l l i s i o n c a n t a k e p l a c e and a "normal" breakdown c a n o c c u r .  One would e x p e c t from t h i s e x p l a n a t i o n ,  however, t h a t t h e i n i t i a l c u r r e n t would r i s e g r a d u a l l y and not a b r u p t l y as observed.  A reasonable  explanation i s  -57.  l a c k i n g and a more thorough i n v e s t i g a t i o n w i l l be n e c e s s a r y . The d e l a y o f t h e i n i t i a t i o n o f t h e c u r r e n t a f t e r t h e a p p l i c a t i o n o f t h e t r i g g e r p u l s e was found t o be almost o f the same d u r a t i o n i n e v e r y c a s e .  The j i t t e r - t i m e was about  .2 u.sec and a l l o w s t h e tube t o be s y n c h r o n i z e d a c c u r a t e l y w i t h s h o r t - l i v e d events.  5.3..  Light  Intensity  The t i m e v a r i a t i o n o f t h e l i g h t o u t p u t o f t h e f l a s h tube i s shown i n f i g u r e 8 w h i c h g i v e s t h e i n t e n s i t y e m i t t e d  I  Fig.  8.  Light  Intensity  o  a t a w a v e l e n g t h o f 5100  A .  s i m i l a r a t other wavelengths.  The shape o f t h e wave i s v e r y I n i t i a l l y the l i g h t r i s e s  r a p i d l y t o a maximum i n about i5 p.sec, t h e n b e g i n s t o drop off.  At about 1 u.sec a f t e r t h e peak, a break i n t h e r a t e o f  f a l l sometimes o c c u r s , i n d i c a t i n g t h e p r e s e n c e o f a s m a l l e r second peak i n t h e i n t e n s i t y .  I n some o f t h e tube c o n f i g u r a -  t i o n s , such as t h e elbow t u b e , t h e second peak was v e r y  -58-  prominent, being almost as large as the f i r s t , as shown i n f i g u r e 9.  The degree of prominence of the second peak not  M\ A  F i g . 9.  Light I n t e n s i t y of Elbow Tube  only v a r i e d w i t h the tube used but a l s o w i t h the wavelength of the r a d i a t i o n , tending to be s l i g h t l y more prominent at higher wavelengths. The current and the i n t e n s i t y were examined s i m u l t a n eously using a Tektronix type 551 dual beam o s c i l l o s c o p e . The trace i s shown i n f i g u r e 10.  Notice that the l i g h t  1  A  A  lyusec \  • \  / •  F i g . 10.  Current and I n t e n s i t y of F l a s h Tube  -59-  i n t e n s i t y does not appear u n t i l about the f i r s t maxima of the current and the peak i n t e n s i t y occurs at the f i r s t zero of the c u r r e n t .  The second i n t e n s i t y peak occurs s l i g h t l y  before the second zero of the c u r r e n t .  5.4.  Deionization Once the current between the electrodes ceased to flow,  the process of d e i o n i z a t i o n takes p l a c e .  I f volume recom-  b i n a t i o n i s considered, and the emission of l i g h t i s assumed to be caused by recombination only, then i t seems reasonable that the i n t e n s i t y of l i g h t i n a small wavelength range w i l l be p r o p o r t i o n a l to the number of e l e c t r o n s present and thus a l s o to the number of i o n s .  and = ^  n««rV=h where  I f l o c a l n e u t r a l i t y holds, then  i s the constant of p r o p o r t i o n a l i t y .  j\  (16) Thus  equation (7) becomes  -r-i  and a p l o t of of slope  J  •  2versus  t  should y i e l d a s t r a i g h t  line  This was done f o r three d i f f e r e n t wavelengths  and i s shown i n f i g u r e 11.  Care was taken to ensure the  i n t e n s i t y was measured after the current had stopped f l o w i n g .  -60-  To get an estimate of the value of ot , the f o l l o w i n g procedure was used.  The r i s e i n pressure i n the f l a s h tube  after f i r i n g and after the temperature was back to room temperature was about 35 p.. 1.1 x l O ^ c m 1  - 3  This gives a molecular density of  using the perfect gas r e l a t i o n  If none of the m a t e r i a l sputtered from the electrodes and the quartz tube i s redeposited  (highly questionable) and each  atom of m a t e r i a l i s s i n g l y i o n i z e d , then an approximation for I  C  the e l e c t r o n d e n s i t y i s 10  O  cm"" .  I f i t i s assumed that t h i s  i s the e l e c t r o n density at peak i n t e n s i t y of the f l a s h , a value for r\\  can be c a l c u l a t e d from (16) and hence  measured from the slope of the graph.  In the three cases shown — 11  i n f i g u r e 16 CX was found to be  can be  2 x 10"  °i  cm / s e c .  It  i s d i f f i c u l t to compare t h i s r e s u l t w i t h others that have been measured because 0( perature,  pressure,  doubtless v a r i e s with the gas, i t s temetc.  Values of  CX found under various  c o n d i t i o n s given by Loeb ^ show that the value found here 1  could be p o s s i b l e .  Massey  16  found  C<  10~  13  cm /sec for 3  a temperature of 8,000°K i n oxygen. I t should be pointed out that the value for h i g h l y doubtful f o r two reasons.  n  is  One i s the assumption that  no sputtered m a t e r i a l i s redeposited after the discharge, and the second i s that t h i s p a r t i c l e density occurs during the discharge and not at a l a t e r stage.  Some r e s u l t s explained  below i n d i c a t e t h i s second assumption may be f a l s e .  A lower  -61-  value of  n  would give a higher value of  The s t r a i g h t - l i n e p l o t of  c<\ . versus time would  2  i n d i c a t e that the primary process of d e i o n i z a t i o n i s volume recombination.  However caution should be used because  recombination i s a process which i s far from being completely understood and the apparent r e s u l t s may be m i s l e a d i n g . at the d i f f u s i o n - s u r f a c e 15 Loeb  A look  recombination process i s necessary.  s t a t e s that i f the Debye length of the plasma i s  much l e s s than the dimensions of the c o n t a i n i n g v e s s e l ,  the  d i f f u s i o n w i l l be ambipolar, w h i l e i f the Debye length i s much g r e a t e r , the e l e c t r o n s w i l l d i f f u s e independently of the i o n s , and the d i f f u s i o n w i l l be c a l l e d f r e e . Debye length A  D  i s given by  To get an order of magnitude of A  , assume a value  D  of  The e l e c t r o n  Te = 10,000°K which i s of the same order as that c a l c u l a -  ted by Budd for a s i m i l a r f l a s h tube. 5  He  i s as c a l c u l a t e d  before I  ~,  so that  ?\  ^  rsJ t>  (|0-*f  ^  IO  L-^rx-  16  = \0 B ro 8  _2  2  10~^ cm which i s much smaller than the  dimensions of the tube, and thus i t appears that ambipolar  -62-  diffusion will  occur r a t h e r than f r e e d i f f u s i o n .  states that i f it  f  h i t s the wall  where  length.  1 7  i s t h e mean l i f e t i m e o f an e l e c t r o n b e f o r e then  i s the ambipolar d i f f u s i o n  characteristic  Francis  c o n s t a n t a n d 1—  is a  l e n g t h o f t h e d i s c h a r g e known a s t h e d i f f u s i o n  For a cylinder  of length  d  and r a d i u s  r  B = (?R?)' which g i v e s  L = .18 cm f o r t h e f l a s h  V o n Engel ® 1  where  D  and  positive  T  gives the ambipolar d i f f u s i o n  are the free d i f f u s i o n  and n e g a t i v e i o n s , and k  tive mobilities. Te y y  D  i  tube.  +  constants f o r the  a n d k~ a r e t h e i r  F o r t h e u s u a l c a s e where  , the i o n temperature,  c o n s t a n t by  k~ ^>)> k  respec+  i t c a n b e shown t h a t  and  -63-  If k <^ +  10  3  c m / v o l t s e c , w h i c h s h o u l d be about 2  the  r i g h t order, then  and f r o m (18)  ?  ^  =  ^  S  e  C  which i s t h e c o r r e c t o r d e r of magnitude f o r the time of t h e discharge.  Thus from t h e s e rough c a l c u l a t i o n s i t seems t h a t  t h i s i s a b o r d e r l i n e c a s e - a m b i p o l a r d i f f u s i o n may be possible.  just  A c c o r d i n g t o Mohler- -® i f s u r f a c e r e c o m b i n a t i o n 1  t a k e s p l a c e because of d i f f u s i o n , the c u r v e s of J " ^  4  t  will  i n g e n e r a l be s t e e p e r and concave upwards. T h i s upward c o n c a v i t y i s apparent i n t h e graphs shown. I f the prominant r a d i a t i o n p r o c e s s i s a c c e p t e d as b e i n g a p r o d u c t of volume r e c o m b i n a t i o n , t h e n a q u a l i t a t i v e e x p l a n a t i o n of t h e shape o f t h e i n t e n s i t y w i t h t i m e c u r v e can be given.  The energy put i n t o t h e tube d u r i n g t h e d i s c h a r g e as  a f u n c t i o n of time i s g i v e n by  E(t)-cVdt = R$Vdt T  i f the r e s i s t a n c e of t h e tube i s assumed c o n s t a n t . 2 2 i,  i , and  i dt  a r e shown i n f i g u r e 12.  I f t h e energy  decay due t o r a d i a t i v e r e c o m b i n a t i o n i s t a k e n i n t o  account  -64-  Fig.  12.  Energy Content  the d o t t e d l i n e shows the energy of the d i s c h a r g e as f u n c t i o n of t i m e .  I f the o u t p u t of i n t e n s i t y i s  t o the energy c o n t e n t o f the d i s c h a r g e , o u t p u t s h o u l d be s i m i l a r t o t h i s c u r v e .  then the  a  proportional observed  The r e l a t i v e  -65-  prominence of the second peak would then depend on the value of G ( , a high value of 0{ allowing a s m a l l or no second peak, w h i l e a s m a l l value of  0\ gives a large second peak.  decreases w i t h i n c r e a s i n g temperature,  C?(  so the hotter  the  gas, the more prominent the second peak.  5.5.  Behaviour of I n t e n s i t y w i t h Number of Shots  The peak i n t e n s i t y of the f i n a l design of the f l a s h tube was found to decrease by l e s s than about 1% w i t h each shot.  The r a t e of decrease of i n t e n s i t y tended to vary  s l i g h t l y w i t h each t r i a l i f some part of the tube had been replaced such as a new window glued on, so the above f i g u r e represents an approximate average.  For an experiment r e q u i r -  ing no decrease i n the peak i n t e n s i t y and not r e q u i r i n g too many shots, t h i s i s probably a t o l e r a b l e decrease.  Figure 13  i l l u s t r a t e s t h i s decrease g r a p h i c a l l y for the f i r s t hundred shots.  For comparison a curve for a " b a f f l e - l e s s " f l a s h  tube i s a l s o drawn. For t h i s tube the i n t e n s i t y dropped d r a s t i c a l l y i n the f i r s t 10 shots, and then f e l l o f f more s l o w l y i n a a p p r o x i mately l i n e a r manner.  To be noted i s the d i p and then the  s l i g h t r i s e beginning at shot No. 7.  This would seem to  i n d i c a t e that i n i t i a l l y m a t e r i a l i s only being deposited on the window so the i n t e n s i t y f a l l s o f f r a p i d l y .  Eventually  ( i n t h i s case, shot No. 7) some of the m a t e r i a l already  number  of  shots fig. 13  intensity  decrease  -66-  deposited i s removed, e i t h e r by the heat generated by the discharge, or by mechanical impact from other e b u l l i t i o n j u s t produced by the discharge, and the i n t e n s i t y r i s e s somewhat. The two opposing processes of d e p o s i t i o n and removal then occur c o n c u r r e n t l y , w i t h the d e p o s i t i o n process n e c e s s a r i l y dominating.  In some cases the d i p occurred at the second shot  and the r i s e at the t h i r d .  Hence the e b u l l i t i o n i s apparently  deposited after the l i g h t has been emitted and thus the c a l c u l a t i o n above the e l e c t r o n density during the discharge i s even more p r e c a r i o u s . The i n t e n s i t y of the f i n a l design tube, on the other hand, shows none of these c h a r a c t e r i s t i c s but merely a slow decrease.  5.6.  Temperature Measurements The s o l u t i o n of equation (15) gives the brightness tem-  perature of the f l a s h tube; the r e s u l t s were not very s a t i s f a c t o r y however.  When (  I  was s m a l l ( = 120 A ) , the  c a l c u l a t e d temperatures, as the v i s i b l e range was "scanned", v a r i e d between 1,000 and 60,000°K w i t h no obvious r e g u l a r i t y or pattern w i t h respect to the wavelength.  In some instances  the equation had no r e a l s o l u t i o n . Some of the p o s s i b l e causes of these unstable  results  were not hard to f i n d . One of these i s an inherent i n s t a b i l i t y i n equation  (15).  -67If the d e r i v a t i v e of ( 1 5 ) i s taken  -exp(|)(f)f  = d A [ e x p ( | ) - , ] - e x B\/B| ( | ) ( f j AdS  s  P  or  B'exp^l'j-lexp^  S  | A - A*|  now i f  "  5  i s small,  " *  1  the percentage change i n which i t w i l l be i f I f £>_  A" cannot equal 1 for f i n i t e  Since exp ^-  given.  ^1 =• exp j |  B ' = B and exp  S can be large i f A i s c l o s e to 1 ,  A—>-A'»  1  and  S ,  A t y p i c a l example can be  A = .95, then  so any e r r o r i n A i s magnified 20 times i n  -  4^ S  .  20^ Almost  a l l error associated with A would come from measuring the i n t e n s i t i e s which has a p o s s i b l e e r r o r of about 5%. | A - A'|  If  i s made l a r g e r , then A tends to move away from 1  and the s o l u t i o n may become more s t a b l e . o  This was t r i e d using A =• 4440 A which was the lowest v/avelength i n the range, and A ' ranged from 4700 A o  o  to 6555 A i n increments of about 250 A . one t r i a l are tabulated below.  The r e s u l t s from  -68-  TABLE I B r i g h t n e s s Temperature f o r V a r i a b l e Wavelength A;,  Interval A' 3943° K 5666 6891 6268 5624 5850 5277 4490  4680 A 4923 5177 5448 5720 5997 6276 7555  4440 A  T h i s method does n o t a l l o w a temperature t o be a s s o c i a t e d w i t h a c e r t a i n w a v e l e n g t h , o r s m a l l w a v e l e n g t h range as i t does i f  | A - A'|  i s small.  S  has an average  v a l u e o f 5500°K and a s t a n d a r d d e v i a t i o n of 880°K.  I  i s shown below.  TABLE I I B r i g h t n e s s Temperature f o r C o n s t a n t Wavelength  Interval AV  4440 A 4680 4923 5177 5448 5720 5997 6276  4680 A 4923 5177 5448 5720 5997 6276 6555  3943°K 11549 14669 4826 3787 5850 2994 1848  »  o  == 250 A  -69-  The v a r i a t i o n i n the v a l u e s of t h e temperature was not as g r e a t as when When  |x  j A - "K 'J Tp  120 A.  i s not s m a l l , the q u e s t i o n i s  - V|  as t o how much e r r o r i s i n t r o d u c e d by p u t t i n g eqn ( 1 5 ) .  That i s ,  S  - S'  relation  T  ^ JL_ T  &  in  does the f l a s h tube behave as a b l a c k  To c a l c u l a t e the r e l a t i v e i n t e n s i t y of the f l a s h tube F T  raised  T L  R(K) ^  c a l c u l a t e d from W i e n ' s l a w .  W a S  u s e <  *'  w n  ere  f  the 1  1  was  T h i s was done u s i n g the v a l u e s  R(A) observed from t h e t r i a l of t a b l e s I and I I and t h e i s shown i n f i g u r e 14.  result  C l o s e r spaced p o i n t s  and a w i d e r t o t a l w a v e l e n g t h range would t e l l more of  the  Because of t h e n o n - b l a c k body b e h a v i o u r ,  i t i s perhaps s u r p r i s i n g the temperatures as c l o s e as t h e y  of  I t appears t h a t the i n t e n s i t y d i s t r i -  b u t i o n i s f a r from t h a t of a b l a c k body.  i n t e n s i t y behaviour.  body?  given i n Table I  are  are.  I f 5 5 0 0 ° K i s a r e a s o n a b l e measure of the temperature of the f l a s h t u b e ,  brightness  i t i s obvious that t h i s i s  hot enough f o r a b s o r p t i o n a n a l y s i s o f hot p l a s m a s .  In  not  fact  at p r e s e n t work i s b e i n g done s i m i l a r t o t h a t done by B u d d " u s i n g the f l a s h tube d i s c u s s e d h e r e , and i t has been found t h a t v e r y s l i g h t a b s o r p t i o n t a k e s p l a c e but not enough f o r any accurate c a l c u l a t i o n .  Budd was a b l e t o a c h i e v e b e t t e r  t i o n i n d i c a t i n g h i s f l a s h tube had a h o t t e r Significantly, peak.  absorp-  temperature.  t h e i n t e n s i t y waveform had a prominent second  ©  444 0  4680  4920  5180  5450 5720 wavelength  fig, 14  intensity  6000  variation  with  6280  wavelength  6550  -70-  5.7.  Conclusions A low pressure f l a s h tube of simple design has been  constructed which does not suffer from d e p o s i t i o n on the e x i t window.  The spectrum i n the v i s i b l e range i s a good continuum  probably o r i g i n a t i n g from e l e c t r o n r e t a r d a t i o n .  A few emission  and absorption l i n e s are present but are b e l i e v e d to appear after  the f i r s t 20 jisec of the discharge. The d e i o n i z a t i o n process i n the discharge appears to be  volume e l e c t r o n - i o n recombination and the recombination coefficient  0(  was estimated to be 2 x 1 0 "  1 1  cm /sec. 3  A method of measuring the brightness temperature of the f l a s h tube over the v i s i b l e range was used by comparing photoe l e c t r i c a l l y the i n t e n s i t y of the tube w i t h that of a b l a c k body.  The temperatures c a l c u l a t e d i n d i c a t e that the i n t e n -  s i t i e s need to be measured extremely a c c u r a t e l y and the temperature of the present tube ( expected.  rsJ  5500°K) i s lower than  The r e l a t i v e i n t e n s i t y d i s t r i b u t i o n of the tube  shows that i t does not r a d i a t e as a black body. I t i s not c e r t a i n tube i s so low.  why the temperature of the f l a s h  The t o t a l energy put i n t o the tube i s ISO J  and g r e a t e r ; other workers have constructed tubes which have achieved much higher temperatures at about the same energy, so other e n e r g y - d i s s i p a t i n g processes must be looked for to e x p l a i n t h i s low temperature.  Perhaps d i f f u s i o n and surface  recombination plays a more important r o l e than i t i s thought  -71-  to,  or the low p r e s s u r e r e q u i r e s t o o much energy t o be  expended i n s p u t t e r i n g t o a c h i e v e breakdown. At any r a t e i t seems t h a t one problem has been s o l v e d at the expense of c r e a t i n g a n o t h e r .  The f i n a l d e s i g n of  the  tube no l o n g e r s u f f e r s from d e p o s i t ! on of m a t e r i a l on the window as i t d i d b e f o r e .  B u t on t h e o t h e r hand, e a r l i e r  d e s i g n s were hot enough f o r use i n c e r t a i n a b s o r p t i o n ments.  What had seemed a t f i r s t g l a n c e t o be a  exit tube  experi-  reasonably  s i m p l e m e c h a n i c a l problem o f p r e v e n t i n g d e p o s i t i o n has  turned  out t o be an e x t r e m e l y complex s i t u a t i o n of w h i c h l i t t l e  is  known. O b v i o u s l y t o s o l v e t h i s p r o b l e m , a more thorough i n v e s t i g a t i o n o f the mechanisms o c c u r r i n g i n the f l a s h  tube  w i l l be n e c e s s a r y and w i l l d o u b t l e s s be u n d e r t a k e n i n the near  future.  -72-  APPENDIX  Computer Program f o r S o l v i n g Temperature E q u a t i o n  Equation F o r t r a n 1A. stitution.  (15) was s o l v e d on an IBM 1620 computer u s i n g  The method of s o l u t i o n was that of d i r e c t subI f equation  (15) I s w r i t t e n  then the s o l u t i o n i s that v a l u e of S  Eq =» 0 .  S was then i n c r e a s e d i n s t e p s  100 s t a r t i n g at the p r e v i o u s 1000 s t e p .  was repeated down t o u n i t s . the  f o r which  was f i r s t put equal t o 1000, then i n c r e a s e d i n s t e p s of  1000 u n t i l Eq changed s i g n . of  S  range  1000  —  S  ^  T h i s procedure  The s o l u t i o n was sought o n l y i n 101,000.  The program i s shown below.  1 17 18 19  DIMENSION WL1(100).WL2(100),VTL(100),VTL2(100),VFT1(100) DIMENSION VFT2(100) FORMAT(6F8.2) FORMAT(13,F8.0) FORMAT(13) READ 18, N DO 19 1=1,N READ 1,WL1(I),WL2(I),VTL1(I),VTL2(I),VFT1(I),VFT2(I) DO 101 I-1,N Bl= (1.438E8/WL1(I)) B2=(1.438E8/WL2(I)) C-EXP ((1.438E8/2560.)*(1./WL1(I)-1./WL2(I)))  73-  2 20 5 8 6 9 7 10 4 22 11 12 13 14 15 16 3 101  A- ( (VTL1 (I) *VFT2 (I) )/(VTL2 (I) *VFT1 (I) ) ) *C T-1000. EQ=EXP(B1/T)-(A*EXP(B2/T)+1.-A) IF(EQ)2,3,4 T-T+1000. IF(T-1.E5)20,20,3 EQ-EXP(B1/T)-(A*EXP(B2/T)+1.-A) IF(EQ)2,3,5 T-T-1000. T-T+l00. EQ-EXP(B1/T)-(A *EXP(B2/T)+1.-A) IF(EQ)8,3,6 T-T-100. T-T+10. EQ-EXP(B1/T)- (A*EXP(B2/T)+1.-A) IF(EQ)9,3,7 T-T-10. T-T+l. EQ-EXP(B1/T)-(A*EXP(B2/T)+1.-A) IF(EQ)1C,3,3 T-T+1000. IF(T-1.E5)22,22,3 EQ-EXP(B1/T)-(A*EXP(B2/T)+1.-A) IF (EQ) 11,3,4 T-T-1000. T-T+100. EQ-EXP (Bl/T)-(A*EXP(B2/T)+1.-A) IF(EQ)13,3,12 T-T-100. T-T+10. EQ-EXP(Bl/T)-(A*EXP(B2/T)+1. - A) IF (EQ) 15,3,14 T-T-10. T-T+l. EQ-EXP (Bl A ) - (A*EXP(B2/T)+1.-A) IF(EQ)3,3,16 PRINT 17,1,T CONTINUE STOP END  RELOCATABLE EXP  SUBROUTINES CAT •LED  OBJECT PROGRAM DATA TABLE 06550 STORAGE POSITIONS.  -74-  REFERENCES  1.  Bishop, A.S.,  P r o j e c t Sherwood, New  York, Doubleday,  1960,  p.6. 2.  Ladenburg,  R.,  3.  Lyman, T.,  S c i e n c e , 64, 89  4.  C h r i s t i e , M.T. 389  5.  Rev,  Mod.  Phys., j5, 234  (1933).  (1926).  and P o r t e r , G.,  P r o c . Roy.  Soc. A,  212,  (1952).  Budd, S.E.,  M.Sc.  T h e s i s , U n i v e r s i t y of B r i t i s h  Columbia,  1961. 6.  Garton, W.R.S., J . S c i . I n s t . , 30, 119 J . S c i . I n s t . , 36, 11  (1953). (1959).  Garton, e t a l , Proc. of the F o u r t h I n t e r n a t i o n a l Conference on I o n i z e d Phenomena i n Gases, 7.  Von E n g e l , A.,  8.  Curzon, F.L., and Smy, 756  9.  I o n i z e d Gases, P.R.,  p.518.  London, 1953, Rev.  S c i . Inst.,  p.152. 32,  (1961).  Von E n g e l , A., o p . c i t . , p.  10.  Anderson,  11.  C h r i s t i e , M.T., 398  1961,  J.A.,  49.  Astrophy. J . , 75, 394 and P o r t e r , G.,  (1932).  Proc. Roy.  Soc. A.  212,  (1952).  12.  Theophanis, G.A.,  13.  P a r k i n s o n , W.H., 262, 409  Rev.  S c i . I n s t . , 31, 4  and Reeves, E.M.,  (1961).  (1960).  P r o c . Roy.  Soc. A,  -75-  14.  Rutgers, G.A.W., and de Vos, J.C., P h y s i c a , 20, 715  (1954).  15.  Loeb, L.B.,  16.  Massey, H.S.W., Adv.  17.  F r a n c i s , G., I o n i z a t i o n Phenomena i n Gases, 1960,  Handbuck der Physik, XXI, p.490. i n Phys.,  London,  p.112.  18.  Von E n g e l , op. c i t . , p.122.  19.  Mohler, F.L., 447  1, 395 (1952).  (1937).  Jour, o f Research, Nat. Bur. Stand., 19  

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