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A two directional study of scattering of ruby laser light from a plasma jet Godfrey, Lawrence Allan 1973

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A TWO DIRECTIONAL STUDY OF SCATTERING OF RUBY LASER LIGHT FROM A PLASMA JET by  LAWRENCE ALLAN GODFREY B.Sc., U n i v e r s i t y  ofBritish  Columbia,  1971  A THESIS PRESENTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE  in  the  Department of  Phys i cs  We  accept  required  this  as conforming  to  the  standard  THE UNIVERSITY OF BRITISH COLUMBIA September, 1973  In p r e s e n t i n g an the  advanced degree at Library  I further for  this thesis  shall  the  of  this thesis  written  University  of  permission  s c h o l a r l y p u r p o s e s may his  f u l f i l m e n t of  representatives.  be  g r a n t e d by  for f i n a n c i a l gain  of  The U n i v e r s i t y o f B r i t i s h V a n c o u v e r 8, Canada  the  I t i s understood  Columbia  shall  the  requirements  Columbia,  for reference  for extensive  permission.  Department  British  make i t f r e e l y a v a i l a b l e  agree that  by  in partial  that  not  and  copying of Head o f my  be  I agree  that  study.  this  thesis  Department  copying or  for  or  publication  allowed without  my  ii  ABSTRACT  A two channel system i s used to s t u d y , s i m u l taneously i n two d i r e c t i o n s , the s c a t t e r i n g l i g h t from a plasma j e t .  The experimental s c a t t e r i n g  spectrums compare well with t h e o r e t i c a l scattering  from an i n f i n i t e  The f l u c t u a t i o n s directions  plasma.  spectrums f o r the two  are shown to be p o s i t i v e l y c o r r e l a t e d ,  of f l u c t u a t i o n .  as well  to be accounted f o r by known sources  Changes i n the plasma parameters, e l e c t r o n  d e n s i t y and temperature, are r u l e d extra  predictions f o r  homogeneous thermal  i n the i n t e g r a t e d  as to be too l a r g e  the  of ruby l a s e r  fluctuations.  out as the source of  i ii  TABLE OF CONTENTS Paoje ABSTRACT  i i  TABLE OF CONTENTS  i i i  LIST OF FIGURES  v  ACKNOWLEDGEMENTS  Chapter  I  Chpater  II  INTRODUCTION  1  THEORY  3  A.  Summary of S c a t t e r i n g  B.  Determining E l e c t r o n D e n s i t y and Temperature from Experimental Scattering P r o f i l e s  7  Theoretical Calculation of S c a t t e r i n g Signal Dependence on F l u c t u a t i o n s i n E l e c t r o n Density and Temperature.  9  C.  Chapter  Chapter  III  IV  vii  EXPERIMENTAL APPARATUS  Theory  .  3  . . . . . .  14  A.  The Plasma J e t  14  B.  The Ruby Laser  17  C.  The D e t e c t i n g System  . . . . .  20  EXPERIMENTAL RESULTS AND DISCUSSION A.  S c a t t e r i n g P r o f i l e s and Determinat i o n of N and T . . . . . . . . . . . e e  23 .  24  iv  Page B.  Experimental C o r r e l a t i o n and Standard D e v i a t i o n  Chapter  V  C.  E s t i m a t i o n of Known F l u c t u a t i o n s  D.  Discussion  . . . . 33 36  CONCLUSION  43  A.  Conclusions  43  B.  Suggestions f o r F u r t h e r Study  44  BIBLIOGRAPHY APPENDIX A  31  45 Alignment  Procedure  47  I V  LIST OF FIGURES  Figure II -1  11-2  II- 3  Page S c a t t e r i n g of Plane E l e c t r o m a g n e t i c R a d i a t i o n from a Plasma Monochromator T r a n s m i s s i o n F u n c t i o n Wide Passband Theoretical Signal  III — 1  on N g  Dependence of and/or T g  6  12  Scattered  Changing  Experimental Arrangement  13 15  111 - 2  Schematic of the Plasma J e t  16  III- 3  Plasma J e t Power Supply C i r c u i t  18  III- 4  Laser Monitor C i r c u i t .  18  IV- 1  Monochromator T r a n s m i s s i o n Function Narrow Passband  25  IV-2  Typical Oscillograms  26  IV-3  T h e o r e t i c a l F i t to Experimental Data f o r Forward S c a t t e r i n g T h e o r e t i c a l F i t to Experimental Data f o r Backward S c a t t e r i n g . . . . . . . . . . . . . .  29  IV-4 IV-5  Comparison of Experimental V a r i a t i o n s i n S c a t t e r i n g Signal to C a l c u l a t e d V a r i a t i o n s Due to Changing N  30  39  vi  Fi gure IV-6  Page Comparison of Experimental V a r i a t i o n s i n S c a t t e r i n g Signal to C a l c u l a t e d V a r i a t i o n s Due to Changing T  40  vi i  ACKNOWLEDGEMENTS  I would l i k e to thank D r .  R.A.  h i s a d v i c e and encouragement d u r i n g t h i s My thanks a l s o go to Dr.  Nodwell  for  experiment.  H. B a l d i s f o r h i s many h e l p f u l  h i n t s , to Mr. D. S i e b u r g and Mr. J . Aazam-Zanganeh their Mr.  technical  for  a s s i s t a n c e , and to D r . M. C h u r c h l a n d ,  Gary A l b a c h , and o t h e r members of the Plasma P h y s i c s  Group f o r t h e i r d i s c u s s i o n s of t h i s  experiment.  1  Chapter I  INTRODUCTION  The experimental r e s u l t s f o r s c a t t e r i n g of e l e c t r o magnetic r a d i a t i o n from f r e e e l e c t r o n s in general,  i n a plasma  have,  been i n good agreement with t h e o r e t i c a l  dictions.  However, there have been many examples of  deviations  from t h e o r e t i c a l p r e d i c t i o n s , u s u a l l y  pre-  involving  enhancements i n the s c a t t e r i n g spectrum i n the region of the frequency s h i f t corresponding to the plasma f r e q u e n c y , a) , or a t m u l t i p l e s et al.  3  Ringler  o f 1/2 w  (Gerry and Rose,^  Evans  and Nodwel1, 3 ' 4 » 5 C h u r c h l a n d 6 ) .  Another  p o s s i b i l i t y of anomalous s c a t t e r i n g was noted by D. H. B a l d i s while s c a t t e r i n g l a s e r l i g h t from a plasma j e t . The observed standard d e v i a t i o n  of the s c a t t e r e d  signal  appeared to be too l a r g e to be accounted f o r by photon statistics  and other known sources of f l u c t u a t i o n s .  was suggested some non-thermal  It  c h a r a c t e r i s t i c of the plasma  could produce the e x t r a observed f l u c u a t i o n . This  experiment was performed to i n v e s t i g a t e  the f l u c t u a t i o n s  in scattering  s i g n a l , to determine i f  2  these f l u c t u a t i o n s  can be accounted f o r by known s o u r c e s ,  and to t r y to determine another source of f l u c t u a t i o n i f they cannot. adopted. signal  For t h i s purpose, a two  T h i s allowed not only the study of the  s i m u l t a n e o u s l y i n two  shape of the s c a t t e r e d  to determine the plasma and e l e c t r o n scattering  temperature  spectrum was  fluctuations  scattered  spectrum was  signals.  Then the  density  integrated  l i g h t f o r the two  as well as the c o r r e l a t i o n between the two  theory f o r a homogeneous i n f i n i t e  plasma.  to K e g a l , of determining the e l e c t r o n profiles  directions,  signals.  Chapter II gives a b r i e f summary of  theoretical  (N g )  observed i n order to measure the  i n the s c a t t e r e d  ture from s c a t t e r i n g  the  c a r e f u l l y measured  parameters, e l e c t r o n (Tg).  was  scattered  d i r e c t i o n s , but a l s o  study of the c o r r e l a t i o n of the two The  channel system  scattering  A method, due  d e n s i t y and  tempera-  i s p r e s e n t e d , as well as the  dependence of the s c a t t e r i n g  signal  on  Ng  and T . e Chapter I I I i s devoted to a d e s c r i p t i o n experimental a p p a r a t u s . mental  method and r e s u l t s .  is also given  Chapter IV d e s c r i b e s the A discussion  presented i n Chapter IV.  i n Chapter V, and a d e t a i l e d  of the experi-  of the r e s u l t s  The c o n c l u s i o n i s description  alignment procedure i s presented i n Appendix  of the  A.  3  Chapter II  THEORY  A.  Summary of S c a t t e r i n g The  electrons  Theory  theory of s c a t t e r i n g of l i g h t from f r e e  i n a plasma has been d e r i v e d  ( f o r example see S a l t p e t e r ,  7  8 Rosenbluth and Rostoker ) ,  so only a b r i e f summary i s given The that  by many authors  here.  theory i s based on the c l a s s i c a l  approach;  i s e l e c t r o m a g n e t i c r a d i a t i o n i n c i d e n t on the plasma  causes the f r e e e l e c t r o n s  to be a c c e l e r a t e d , which i n  turn causes the e l e c t r o n s  to r a d i a t e .  the s c a t t e r e d the  r a d i a t i o n from the e l e c t r o n s  ensemble average of the e l e c t r i c  a l a r g e number of e l e c t r o n s . the  electrons will  the e l e c t r i c  The i n t e n s i t y o f  field  d i f f e r e n t frequency  field  The r e l a t i v e  i s equal to vectors  from  positions of  r e s u l t i n d i f f e r e n t phase s h i f t s of vectors; their relative  velocities,  shifts.  Unless there  i s some s p a t i a l  nonuniformity i n  the charge d i s t r i b u t i o n , the r e s u l t a n t s c a t t e r e d  signal  4  will  be  incident the  zero i n a l l d i r e c t i o n s wave v e c t o r .  This nonuniformity can  p a r t i c l e nature of the  fluctuations.  The  except p a r a l l e l to  plasma r e s u l t i n g  density fluctuations  can  a r i s e from  in d e n s i t y be  thought  of as a combination of random thermal waves in the Thus the  laser l i g h t is scattered  waves i n the  plasma, and  distribution will spectrum.  It i s also  distribution of e l e c t r o n  nonuniformity due  shape of the  scattering  to have s p a t i a l to macroscopic  differential  through an  scattering  charge variations  cross s e c t i o n  angle 6 i s given  a |jc-k_ ,u)-w0j = ^f- | l - s i n 2 6 cos2<j> 0  where:  spatial  density.  The scattering  possible  plasma  from random thermal  t h e i r frequency and  determine the  the  for  by:  ^— S |k-ko ,w-w j 0  r0  i s the c l a s s i c a l electron  r a d i u s of  the  4>  i s the angle between the d i r e c t i o n of p o l a r i z a t i o n of the i n c i d e n t r a d i a t i o n and the s c a t t e r i n g plane  n0  i s the  average e l e c t r o n  jc0  i s the  incident  k  i s the  scattering  number d e n s i t y  wave v e c t o r wave v e c t o r  0)  the i n c i d e n t wave frequency  is  the s c a t t e r i n g wave frequency  i s the s p e c t r a l distribution.  k-k 0 ,w-o)  c  See  is  density  F i g . 11-1 f o r the geometry of the l a s e r l i g h t s c a t t e r i n g The  spectral density  distribution  i s given by  the  F o u r i e r transform of the a u t o c o r r e l a t i o n f u n c t i o n of  the  electron  density:  = FT C n (A,x)  k-k0 ,U-CL)0  =  - V  FT f ( r , t )  T/2  J  dt n.  P J  T/2  E J dr dt f ( r , t ) exp  For a Maxwellian plasma t h i s  k-k0,w-w<  r+A ,t+x  27rn« li-JLol  (u-a)o)t - (k-k0) • r  gives:  n  - G  fi  - G.|  2  6  FIG SCATTERING RADIATION  OF  FROM  PLANE A  ELECTROMAGNETIC  PLASMA  7  where:  subscript  e refers  to the e l e c t r o n s  subscript  i refers  to the ions  T  i s temperature  =  is  t h e mass o f t h e p a r t i c l e  is  Boltzman's  constant  K T ) * exp(-  (m/2TT  X ) 2  (m /2KT ) (w-o)o)/|i_k 2  e  G  m  i  m  e  e  T  1  + i  4TT  a.  r  a  B.  e i  T  =-a  Z T  - 2 x exp(-X  n e  function  «  a.  logio(AX),AX  Density  Scattering  general,  o f X. o r X  2  /Iii  Q  Determining Electron  In  exp(-t )dt  )  x exp -X  TT  i  Experimental  c  when  and T e m p e r a t u r e  Profiles S (k_-k_ ,w-u) ) i s p l o t t e d 0  , or preferably  = X - X o , with  from  a e  »  N e  >T > e  0  as a  as a f u n c t i o n o f and T. as p a r a m e t e r s ,  8  there i s an i o n f e a t u r e and an e l e c t r o n feature  i s very c l o s e to the c e n t r a l  is usually d i f f i c u l t  ture.  The e l e c t r o n f e a t u r e  the e l e c t r o n  The shape of the e l e c t r o n  d e n s i t y and tempera-  s a t e l i t e , when  to i t s maximum and p l o t t e d as f u n c t i o n completely determined ^e  a n c  ' ^  e  normalised  of l o g ( A X ) , i s  by the parameter a:  changes i n  t h a t keep the same a merely serve to s h i f t the  spectrum along the log(AX) a x i s . it  The i o n  l a s e r frequency and  to i n v e s t i g a t e .  can be used to determine  feature.  i s p o s s i b l e to determine  Ng  Using  this  principle  and T g from the s c a t t e r i n g 9  spectrum i n the f o l l o w i n g manner, due to K e g a l . A standard s e t of t h e o r e t i c a l  graphs i s drawn  f o r d i f f e r e n t a of S (k-k D , w - u ) 0 ) / S m , v versus — max keeping  Te,T.,6,<j> c o n s t a n t , with T. = T , and changing  N g as r e q u i r e d . scattered and  Then the experimental  s i g n a l , normalised  the maximum s c a t t e r i n g  graph i s found  A.  to the i n c i d e n t  plots.  radiation  logio(AA) i s  Once the t h e o r e t i c a l  t h a t has the same shape as the experimental  ( i . e . the experimental  temperature  graph o f the  s i g n a l , versus  compared to the t h e o r e t i c a l  graph  logio(AX),  a i s determined) the e l e c t r o n  and d e n s i t y can be c a l c u l a t e d  A i s the amount that the t h e o r e t i c a l  u s i n g the s h i f t graph must be  moved along the l o g i o ( A X ) a x i s of the experimental so that the two graphs c o i n c i d e . and  temperature  are then given by:  graph  The e l e c t r o n d e n s i t y  9  log10Tex  Np  + 2A  sin (6ex/2)  logj  log10  + 1ogx0[Vin2(8st/2) j  = logi0Tst  2  = log10  N st  + 2A  e  ex  where the s u b s c r i p t s and standard values  ex and s t r e f e r  to the experimental  respectively.  A s l i g h t change was made from to f a c i l i t a t e  fitting  the t h e o r e t i c a l  actual  Kegal's method  experimental  results.  In  p r o f i l e s , the maximum value i s well  However, i n experimental r e s u l t s seldom o b s e r v e d .  T h i s can r e s u l t  t i o n of the experimental the v e r t i c a l a x i s  results.  defined.  the t r u e maximum i s in incorrect  normalisa-  To c o u n t e r a c t t h i s ,  was changed to a l o g i o  s c a l e , which  allows small up and down s h i f t s when comparing the e x p e r i mental and t h e o r e t i c a l  C.  Theoretical Electron  graphs.  Calculation  of Signal  D e n s i t y and Temperature  Since i t i s to be determined fluctuations  Dependence on  are too l a r g e  i f the experimental  to be accounted  f o r by known  sources of f l u c t u a t i o n , a c a l c u l a t i o n must be performed  10  to  determine the changes in the two  if  there are small changes in N g  for  shot-to-shot v a r i a t i o n  in N  scattering  and/or T .  The  and T  be d i s c u s s e d  will  e in  the l a s t s e c t i o n of Chapter The  plier will and  total  signal  signals mechanisms  e  IV.  detected by the  photomulti-  be a c o n v o l u t i o n of the s c a t t e r i n g  spectrum  the t r a n s m i s s i o n f u n c t i o n of the monochromator.  instrument  p r o f i l e can be determined e x p e r i m e n t a l l y , and  depends on the wavelength s e t t i n g entrance  The  and  exit s l i t s .  Let T(w)  and  the width  of the  be the t r a n s m i s s i o n  f u n c t i o n on the monochromator f o r a given  frequency  setting  experimental  and  e x i t and  entrance  s i g n a l s are normalised <j> = 9 0 ° , so we  P  P  5cat  intensity.  and  have  Scat  ,OJ-O)<  k-k 0  a  S(k-k0,w-too)  i s the s c a t t e r e d power, and  I 0 i s the i n c i d e n t  Since we  in a b s o l u t e  constants may multiplier  The  to the i n c i d e n t r a d i a t i o n ,  I,  where  slits.  are not  interested  be n e g l e c t e d , and  i s then given  by:  the s i g n a l  values,  at the photo-  11 + 00  PM  for  T(OJ)S  Scat  t h e g i v e n monochromator This  calculation  k-k  0  ,u3-0)  dw  o  settings. was  done f o r N  = 2.07  x  10  1 6  e cm" ,  T  3  130°  = 19,000  e  32'.  First  °K, and s c a t t e r i n g N  g  was  quoted  above, w h i l e J  varied  by ± 5 0 % w h i l e  ^  a n c e  ' ^  same a. shown  e  were  was  Q  varied  varied  N  g  by ± 5 0 % from  kept  was  The m o n o c h r o m a t o r  constant.  kept  i n such  constant.  a manner  49° 22' and the value Then T  direction.  g  was  Finally  both  as t o keep t h e  had t h e t r a n s m i s s i o n  i n F i g . 11-2, c e n t r e d o v e r  peak f o r t h e f o r w a r d  angles  the e l e c t r o n  function  satellite  The w a v e l e n g t h  setting  o  was  6972 A,  400u w i d e .  the e x i t F i g . 11-3  slit  2 mm  shows  wide and t h e e n t r a n c e  the r e s u l t s  of this  slit  calculation  !  12  10 -  (A) FIG 11-2 MONOCHROMATOR FUNCTION - WIDE  TRANSMISSION PASSBAND  a.«i  0.6  1  1  1  1  1  1  o.e  0.7  0.8  ae  1.0  1.1  IN  PARAMETER(S)  RELATIVE FIG  J-2  11-3  THEORETICAL ON  CHANGE  1  N  E  DEPENDENCE OF  AND^R  T  E  SCATTERED  CHANGING  SIGNAL  1  1.3  r  L4  1.8  14  Chapter I I I  EXPERIMENTAL APPARATUS  F i g . 111-1 i s an i l l u s t r a t i o n arrangement. the  L i g h t from the ruby l a s e r  plasma j e t .  of the experimental i s focussed  Scattered l i g h t i s c o l l e c t e d  into  at 49° 28'  and 130° 38' to the a x i s of the ruby l a s e r and d e t e c t e d by a monochromator and p h o t o m u l t i p ! i e r .  A.  The Plasma J e t The plasma j e t used i n t h i s experiment i s s i m i l a r  to t h a t used by C h a n 1 0 and B a l d i s 1 1 For  a complete d e s c r i p t i o n  (see F i g . 111-2).  of the c o n s t r u c t i o n of a plasma  12 j e t see M o r r i s .  The j e t i s run with helium r a t h e r  than argon s i n c e helium has c o n s i d e r a b l y l e s s radiation  than argon.  continuum  Also the helium j e t i s much  less 13  l i k e l y to be perturbed by the l a s e r  pulse  (van der Kamp  ).  The j e t i s i d l e d at low c u r r e n t , about 70 amps, with a M i l l e r model SRH 333s welding s u p p l y . laser  When the  i s to be f i r e d , the c u r r e n t source i s changed  to a  MONOCHROMATOF  RUBY  LASER  FIG 111-1 EXPERIMENTAL  ARRANGEMENT  16  FIG  IU-2  SCHEMATIC ALL  OF THE  MATERIAL  PLASMA JET,  BRASS.  - 3/ 4  EXCEPT WHERE  SCALE NOTED  17  48 v o l t , 240  amp-hour b a t t e r y , and  s i x steps  to 230  ohm,  watt r e s i s t o r s with  1000  111-3).  The  amps by p a r a l l e l i n g  anode and  reduces the e r o s i o n of  The  c u r r e n t i s then  e x a c t l y to 230±3 amps using a c u r r e n t and  r e g u l a t e d power supply The  j e t i s allowed  and  then  (Trygon  a few  one  cathode, thereby i n c r e a s i n g  the r e p r o d u c i b i l i t y of the j e t . adjusted  combinations of  the b a l l a s t r e s i s t o r ( F i g .  l a r g e number of steps  the water-cooled  the c u r r e n t r a i s e d i n  voltage  E l e c t r o n i c s Model M36-30a).  seconds to come to e q u i l i b r i u m ,  the l a s e r i s f i r e d . The  j e t c u r r e n t i s monitored by measuring  v o l t a g e drop across a 10 _lf voltmeter  ohm  shunt with  the  a digital  (Dana 3800) which reads to the nearest  tenth  of a m i l l i v o l t .  This g i v e s a d i r e c t reading of the c u r r e n t :  1 mv  The  = 10 amps.  meter to 0.1  mv.,  shunt i s a c c u r a t e  to 1%  and  the  volt-  so that the c u r r e n t could be s e t to  o  230±3 A.,  but could be set to the same c u r r e n t from  to-shot to w i t h i n 1 The  amp.  a x i s of the j e t i s v e r t i c a l , and  to the monochromator and  B.  The  shot-  laser  perpendicular  axis.  Ruby Laser A m o d i f i e d TRG  t h i s experiment, capable  Model 104  ruby l a s e r i s used i n  of a 20 megawatt pulse with a 35  WELDING S U P P L Y  l — — r 10  1  <\AA.—I Wv10  a -A/W  JL  1  1Q  Vo 2  V n 2  AA/V ia J E T  FIG  111-3  P L A S M A  L1 I  J E T  PHOTO  100  DHL  V m  POWER  SUPPLY  CIRCUIT  DIODE •TO  o.oi pf  J>  50  OSSILOSCOPE  n  CERAMIC  FIG  T  111-4  LASER  MONITOR  CIRCUIT  19  nsec width at h a l f  intensity.  with a c o r n e r r e f l e c t i n g  The  laser  i s Q-switched  prism, rotation  at 30,000  The f r o n t m i r r o r of the l a s e r c a v i t y has 40%  rpm.  reflectivity  o  at 6943 A. slightly  The g l a s s ruby rod holders have been m o d i f i e d  to accept a 3/8  the o r i g i n a l  10 mm.  rod and one f l a s h c o n t a i n s 1000 The  inch diameter rod r a t h e r than  diameter r o d .  The 3 inch long  tube are a i r c o o l e d .  j o u l e s at about 900 laser light  ruby  The c a p a c i t o r bank  volts.  i s f o c u s s e d i n t o the  plasma  and then absorbed i n a copper s u l p h a t e s o l u t i o n  l i g h t dump  with an entrance window at the Brewster a n g l e .  The  v e c t o r of the l a s e r l i g h t i s p e r p e n d i c u l a r to the plane (<j> =  electric  scattering  90°). The  laser  i s monitored by r e f l e c t i n g  some of  the l a s e r l i g h t from a t h i n g l a s s p l a t e , placed at the Brewster angle between the l a s e r and the plasma, on to a solid  s t a t e photodiode.  The d i o d e , a Hewlett-Packard  p a r t #5082-4220, i s biased at 100 v o l t s risetime  ( l e s s than 1 n s e c ) , and  S u f f i c i e n t neutral  l i n e a r i t y up to 10  density f i l t e r s  (actual  s i g n a l s about 0.5  volts.  are placed i n f r o n t  of the photodiode to keep i t o p e r a t i n g region  to g i v e a f a s t  in i t s l i n e a r  volts).  shows the c i r c u i t f o r the l a s e r m o n i t o r .  F i g . 111-4  21  the  detecting  system i s very c r i t i c a l , . a  detailed  descrip-  t i o n of the alignment procedure i s presented i n Appendix There are s e v e r a l optical  F i r s t , of c o u r s e , i s the convenience  having only one monochromator and p h o t o m u l t i p i i e r as  well of  obvious advantages of t h i s  delay system over a two monochromator, two photo-  m u l t i p l i e r system. of  A.  as the reduced expense.  calibrating  analyzing  A l s o , there i s no problem  the two monochromators so that they are  the same wavelength, or have the same t r a n s m i s s i o n  function.  There i s no problem of d i f f e r e n t s p e c t r a l  sitivities  of two d i f f e r e n t p h o t o m u l t i p i i e r s .  advantages are the very c r i t i c a l l o s s i n the delay s e c t i o n  (35%).  sen-  The d i s -  alignment and the high The 35% l o s s i s not  unreasonable c o n s i d e r i n g 4% l o s s each time the s c a t t e r e d l i g h t enters or leaves lens C, and p o s s i b l e 10-20% on r e f l e c t i o n  from the alluminum  loss  coated m i r r o r .  The g r a t i n g monochromator, b u i l t i n our l a b o r a t o r y , i s used i n 5th order with l i n e a r d i s p e r s i o n of about o  4.5 A/mm,  and a t h e o r e t i c a l  resolving  A Kodak f i l t e r #29 i s used to i s o l a t e collecting  power of about the o r d e r s .  30,000.  The  lenses are stopped to match the monochromator  speed f / 6 .  For a complete d e s c r i p t i o n of the monochromator 14  see VahAndel . Two p h o t o m u l t i p i i e r s were used.  A high  gain  tube, RCA 7265, was mainly used which had a S-20 response  22 o  and nominal  quantum e f f i c i e n c y of 3% at 6943 A.  31034 was a l s o used, which  A RCA  had an extended response i n  the red due to i t s GaAs photocathode s u r f a c e .  I t s quantum  e f f i c i e n c y of 16% should g i v e an improvement i n s i g n a l to noise of about a f a c t o r of 3 over a p h o t o m u l t i p i i e r with a S-20 r e s p o n s e .  The RCA 7265 was operated at 2200 v o l t s ,  and the RCA 31034 at 1900 v o l t s . A dual beam o s c i l l o s c o p e , T e k t r o n i x  551, with  type L plug i n u n i t s i s used to r e c o r d the s i g n a l s the p h o t o m u l t i p i i e r and l a s e r m o n i t o r . recorded on P o l a r o i d type 410 f i l m . time of the o s c i l l o s c o p e and plug  the s i g n a l  The t r a c e s are  The combined  in units  wich serves to i n t e g r a t e the s i g n a l  from  rise-  i s about 15 n s e c ,  s l i g h t l y and improve  to n o i s e . To help to reduce the s t r a y l i g h t , the d e t e c t i o n  system from lens B to the monochromator i s encased i n a l i g h t proof t u b e , and the l a s e r i s covered except f o r an e x i t hole f o r the l a s e r  light.  23  Chapter IV  EXPERIMENTAL RESULTS AND DISCUSSION  The purpose of t h i s chapter i s to present and d i s c u s s the experimental profiles  results.  F i r s t , the s c a t t e r i n g  and the d e t e r m i n a t i o n of the e l e c t r o n  and temperature correlation  are g i v e n .  density  Then the r e s u l t s f o r the  and standard d e v i a t i o n of the forward and  backward s c a t t e r i n g  s i g n a l s are p r e s e n t e d .  of  due to d i f f e r e n t sources i s made  the f l u c t u a t i o n s  An estimate  and then the r e s u l t s are d i s c u s s e d . For a l l the experimental work, the f o l l o w i n g conditions apply. focal  The j e t was p o s i t i o n e d so t h a t the  volume was c e n t r e d 14.8 mm above the t i p of the  c a t h o d e , 1.8 mm above the anode.  A l s o the s c a t t e r i n g  angles were 49° 22' i n the forward d i r e c t i o n s , 130° 38' in  the backward d i r e c t i o n .  The j e t c u r r e n t was 230 amps  and the helium flow r a t e was 32 c u b i c f e e t per hour. Except where noted, the RCA 7265 p h o t o m u l t i p i i e r was used.  24  A.  S c a t t e r i n g2 P r o f i l e: s and Determination of N e  and Te  So that work could be done with a plasma of known parameters, the s c a t t e r i n g d e n s i t y and temperature  profiles  were determined.  the monochromator e x i t s l i t the entrance s l i t , r e s u l t i n g  and e l e c t r o n For t h i s  purpose,  was s e t to 400u wide to match in a t r i a n g u l a r  instrument  o  p r o f i l e , 1.8 A h a l f - w i d t h , as i l l u s t r a t e d A run c o n s i s t e d of scanning the spectrum  i n F i g . IV-1 . at l e a s t four  times,  o  with one scan composed of a shot every 1.8 A from  about  6955 A to 7000 A. The o s c i l l o g r a m s were then analyzed i n a manner such that the e l e c t r o n d e n s i t y and temperature obtained as d e s c r i b e d i n Chapter a typical  oscillogram.  II.  could be  F i g . IV-2a shows  The maximum heights of the  s c a t t e r e d s i g n a l s were measured and normalised to the height of the l a s e r monitor for  signal.  The measurements  each wavelength were then averaged  and standard  d e v i a t i o n s about the mean c l a c u l a t e d .  The means were  then normalized to the maximum average  s i g n a l , and a graph  of  versus l o g i o ( A X )  the l o g i o  was made.  of t h i s normalised s i g n a l  T h i s experimental  a s e t of t h e o r e t i c a l  p l o t was then compared to  graphs f o r 1  Q  = 16,000 ° K , e  t  =  1 3 5 ° , a = 0.8 to 1.2 and 1.8 to 2.2 i n steps of 0.05, so that a , N . T could be determined as d e s c r i b e d i n e e Section  II B.  -  4  -  O  2  AX  2  (Al  FIG IV-1 MONOCHROMATOR TRANSMISSION FUNCTION - NARROW  PASSBAND  a. NARROW EXIT  SLIT  RCA 7265 PM TUBE  "1  t 1  b. WIDE EXIT SLIT RCA  7265  PM TUBE  20mV/  J  L_l  I l-PM I I I4 I C. WIDE EXIT RCA  div  100 mV/. 'div  SLIT  31034 PM TUBE  FIG IV-2 TYPICAL OSCILLOGRAMS X = 6972. ICOnsec^jy  27  For s c a t t e r i n g found  to be 2.0.  The  i n the forward  direction, a  e l e c t r o n d e n s i t y and  was  temperature  were: T N The  e  e  = 19,000 °K  = 2.07  x 1016  cm"3  sources of e r r o r f o r determining  the plasma parameters  i n t h i s f a s h i o n are the u n c e r t a i n t y i n determining in determining  the s h i f t A.  An estimate was  a and  made of  the e r r o r by measuring the s h i f t s f o r a o b v i o u s l y too l a r g e and  too small  calculating  the e l e c t r o n  these s h i f t s .  = (2.07 Te  The has  a $ 1. fits  and  1.95  respectively)  temperature and  and  d e n s i t y from  T h i s gave the average d e v i a t i o n s : Ne  tion  (2.05  ± 0.13)  x 1016  cm-3  = (19,000 + 200) ° K .  scattering  p r o f i l e f o r the backward  direc-  a broad, G a u s s i a n - l i k e shape, c h a r a c t e r i s t i c of For low a , a wide range of a's  to the experimental  r e s u l t s , so t h a t i t i s d i f f i c u l t  to o b t a i n accurate values f o r a , N e , profiles.  The  r e s u l t s obtained a = 0.95 Te Ne  give acceptable  Tg  from the  are:  ± 0.15  = 15,700 ± 3,500 °K  = 2.0  ± 0.4  x 1016  cm- 3 .  scattering  28  The  errors  quoted  above, determined  as f o r the forward of  the spectrum  electron and  in the same manner  direction, reflect  to N g  d e n s i t y and  and T .  the  insensitivity  However the values f o r  temperature  f o r the two  directions  self-consistent. Fig.  results  IV-3  and  F i g . IV-4  f o r the s c a t t e r i n g  theoretical  0  from  experimental  spectrums, along with  graphs of S(k-k ,w-a)0 ) /  as determined forward  show the  the s c a t t e r i n g  s m a x  the  using N g  profile  and  Tg  f o r the  direction. It should be noted  t h a t the s c a t t e r i n g  angles,  g e o m e t r i c a l l y measured using the entrance a p e r t u r e at the monochromator, the hole i n the j e t anode, and  the  middle  refer-  of the f r o n t m i r r o r of the l a s e r c a v i t y  ence p o i n t s , were 4 6 ° 25' and presence  of the f o c u s s i n g  angle of the l a s e r beam.  133° 35'.  measure with lens D a f f e c t i n g  the i n c i d e n t  determined  gave good t h e o r e t i c a l  fits  both forward  and  a n g l e s , 4 9 ° 22' and mental  r u n s , also  angles.  to  radiation,  to be t h a t angle t h a t  to the experimental  backward s c a t t e r i n g  data  profiles.  130° 38', when used f o r other  gave good t h e o r e t i c a l  p r o f i l e s , indicating  incident  angles are d i f f i c u l t  so t h a t the angle was  for  However the  lens D can change the The  as  fits  These experi-  to the  t h a t these were the proper  two  scattering  29  FIG IV-3 T H E O R E T I C A L FIT TO E X P E R I M E N T A L DATA FOR FORWARD S C A T T E R I N G  30 1.21-  I  1.0  0.8  >V) Z'  - 0.6  a  EXPERIMENTAL THEORETICAL  <  POINTS FIT  s  6 = 130° 38"  O z  N = 2J07 x i o  CC  e  o cf 0.4  FOR  cm'  l a  3  T = 19.000 °K E  0.2  0.0  1.0  1.2  1.4  1.6 L O G (ax(A))  FIG  IV-4  THEORETICAL DATA FOR  FIT  TO  BACKWARD  EXPERIMENTAL SCATTERING  U8  2.0  31 B.  Experimental C o r r e l a t i o n  and  Standard  Once the plasma parameters the c o r r e l a t i o n  were  between the backward and  Deviation determined, forward  signals  was  s t u d i e d , as well as the standard d e v i a t i o n of the  two  signals. For  was  t h i s purpose, the monochromator e x i t  widened to 2.0 mm.  This resulted  p r o f i l e shown i n F i g . 11-2. the t o t a l  The  in an  slit  instrument  larger s l i t  increased  number of photons i n c i d e n t on the p h o t o m u l t i p i i e r ,  and  t h e r e f o r e reduced  the shot n o i s e .  was  s e t so that the t r a n s m i s s i o n f u n c t i o n was  the e l e c t r o n s a t e l l i t e found  The monochromator  i n the forward  centred on  scattering  o  spectrum.  Because of the 10 A wide instrument  e s s e n t i a l l y a l l of the s a t e l l i t e was  observed.  A s e r i e s of at l e a s t 20 shots were with a one-minute wait between s h o t s . was  profile,  fired,  The one minute  necessary f o r the c o o l i n g of the l a s e r , as well  making the power output more s t a b l e . a typical  F i g . IV-2b shows  oscillogram. The  height of the maximum of the forward  backward s i g n a l s were measured and  d e v i a t i o n s were then c a l c u l a t e d . between two  and  normalised to the  height of the l a s e r monitor s i g n a l .  relation  as  The  v a r i a b l e s X and  The means and  standard  c o e f f i c i e n t of c o r Y i s defined  as:  32 KX-X.MY-Y^  i  P  " [I(X-X.)2(Y-Yi)2]i i  where J means the average value o f X, and X^ i s the i t h measurement of are  X.  Because only the q u a n t i t i e s  (X-X..)  u s e d , i t i s unnecessary to s u b t r a c t the s t r a y  light,  approximately c o n s t a n t , from the observed s i g n a l s . presence of s t r a y  The  l i g h t w i l l , however, c o n t r i b u t e to the  standard d e v i a t i o n s of the s i g n a l s . to the p h o t o m u l t i p i i e r s i g n a l  The  contribution  made by the s t r a y  was c o r r e c t e d f o r when c a l c u l a t i n g  the mean  light  scattering  signal . A t y p i c a l experimental of c o r r e l a t i o n  of +0.85 f o r 25 s h o t s .  were not c o r r e l a t e d , of g e t t i n g larger  r e s u l t gave a c o e f f i c i e n t I f the two s i g n a l s  there would be l e s s  a c o e f f i c i e n t of c o r r e l a t i o n  than t h i s .  than 1/2% chance equal  to or  For a l l the r u n s , the c o e f f i c i e n t of  c o r r e l a t i o n was p o s i t i v e , with l e s s than 10% chance of obtaining  p equal to or l a r g e r  than that o b s e r v e d .  standard d e v i a t i o n s of the forward and backward were u s u a l l y  about 12-16% and 16-20%  signals  respectively.  Great care was taken to make the plasma tions  the same from s h o t - t o - s h o t .  The  The helium flow  condirate  was s e t at the beginning o f a run and never changed.  33  The c u r r e n t was r e s e t to the same v a l u e to w i t h i n  less  than 1/2%. Two methods were used to o p e r a t e the j e t a run.  during  The j e t was u s u a l l y i d l e d at about 70 amps as  b e f o r e , and then the c u r r e n t r a i s e d to 230 amps.  However  runs were a l s o made w i t h the c u r r e n t m a i n t a i n e d at 230 amps.  Both methods produced the l a r g e p o s i t i v e p . The RCA 31034 p h o t o m u l t i p i i e r was a l s o  Fig.  tried.  I V - 2 c shows the improvement i n s i g n a l to n o i s e ,  but  I the s t a n d a r d d e v i a t i o n s of the s i g n a l s a c t u a l l y were o n l y reduced by one or two per c e n t .  T h i s i s because  t h e r e are o t h e r l a r g e sources of e r r o r which the improvement i n S/N.  The c o e f f i c i e n t of  remained a t i t s high p o s i t i v e  C.  override correlation  value.  E s t i m a t i o n of Known F l u c t u a t i o n s To see i f  the s c a t t e r i n g s i g n a l s as measured i n  S e c t i o n IV B. have more f l u c t u a t i o n s than can be e x p e c t e d , e s t i m a t e s of the s i z e of f l u c t u a t i o n s from sources were made.  different  The sources c o n s i d e r e d were:  n o i s e due to the s m a l l number of p h o t o e l e c t r o n s ,  shot fluctua-  t i o n s due to the continuum r a d i a t i o n , f l u c t u a t i o n s due to v a r i a t i o n s the s i g n a l  in stray  heights.  light,  and e r r o r i n measurement of  I  34  To of  estimate  photoelectrons  the curve  had  of a s i g n a l  the shot n o i s e , the t o t a l to be determined.  to one  one  photoel ectron can  There are two  signal  can  be  main d i f f i c u l t i e s  from one  the  photoelectron.  in deterFirst,  most d i r e c t way of  i t was  observed i s due  of the dynodes.  to a  to an  I n s p i t e of  f e l t t h a t t h i s method was  of g e t t i n g a good estimate  photoelectrons.  the  i n height at the  s i n g l e p h o t o e l e c t r o n , to more than one, or due  these d i f f i c u l t i e s  total  Secondly, i t i s d i f f i c u l t  know whether or not the s i g n a l  from one  signal  estimated.  i s very s m a l l , l e s s than 5 mv  e l e c t r o n emitted  from  Thus i f the s i z e of the  photomul t i p i i e r v o l t a g e used. to  emitted  be determined, then  number of p h o t o e l e c t r o n s  mining the s i g n a l  under  p h o t o e l e c t r o n , where  number of p h o t o e l e c t r o n s  the photocathode s u r f a c e . of  area  from the p h o t o m u l t i p i i e r i s n  times the area a t t r i b u t e d n i s the t o t a l  The  number  The manufacturer's  the  of the number  specifications  for  the gain of the p h o t o m u l t i p i i e r tube cannot be used  for  a good e s t i m a t e , s i n c e there are such l a r g e v a r i a t i o n s  in gain from tube to t u b e . The in  signal  from one  the f o l l o w i n g manner.  The  p h o t o e l e c t r o n was  determined  monochromator was  set to  o  100 A from the ruby l a s e r wavelength so t h a t the of  more than one  photon being d e t e c t e d  by  the  probability  photomultip!ier  35  was  small.  When the l a s e r was f i r e d , the p h o t o m u l t i p i i e r  s i g n a l was u s u a l l y f l a t , but o c c a s i o n a l l y there a small  signal.  The p o s i t i o n of the s i g n a l along the  time a x i s v a r i e d , but the area constant.  appeared  of the s i g n a l remained  The t r i a n g u l a r pulse was approximately 20  nsec wide at i t s base, and 2 mv h i g h .  Using t h i s  area  i t was then p o s s i b l e to estimate the number of photoe l e c t r o n s i n any p h o t o m u l t i p i i e r s i g n a l as s t a t e d  above.  o  For the case of a 10 A bandpass s i t u a t e d on the e l e c t r o n s a t e l l i t e as d e s c r i b e d  in Section  IV B,  i t was estimated that there were 400 p h o t o e l e c t r o n s i n the forward d i r e c t i o n  s i g n a l , and 300 i n the backward.  This would give a c o n t r i b u t i o n of 5% and 6% to the standard d e v i a t i o n s of the two s i g n a l s r e s p e c t i v e l y . The fluctuations  existence  of continuum r a d i a t i o n causes  i n the b a s e l i n e of the s i g n a l from the  p h o t o m u l t i p i i e r , and t h e r e f o r e c o n t r i b u t e d tuations  i n the s c a t t e r i n g s i g n a l s .  to the f l u c -  An e s t i m a t i o n of  t h i s c o n t r i b u t i o n was made by c o n s i d e r i n g  that  part  of an o s c i l l o g r a m which has no s c a t t e r i n g s i g n a l . F i f t e e n measurements were t a k e n , one every 20 nsec over a 300 nsec range, of the p o s i t i o n of the b a s e l i n e from a horizontal  graticule line.  The standard d e v i a t i o n of  t h i s measurement was then compared to the average  height  36  of  the  scattering  signals.  the  baseline  was  and  backward  scattering  The  found  stray  to  The be  standard  about  signals  light  was  5%  deviations  and  8%  of  of  the  forward  respectively.  assumed  to  originate  from  o  the  laser  Thus  the  light stray  radiation. to  the  of  about  After  20%.  light  of  entering  stray  light  the  forward 3%  and  the  incident  signal  was  normalised  of  the  a standard  and  5%  about  forward  monochromator. to  s i g n a l , i t had  contribute  deviations  the  proportional  about  would  backward  stray 1/2% and  deviation signals  light,  the  and  to  1%  backward  the  scatter-  signals.  were m e a s u r e d estimated  heights using  error The  11%  of  total  scattering  vidual and  the  Since  The  the  was  l a s e r monitor  standard ing  6943 A  light  were c o m p r i s e d stray  at  the  the  a xlO  signals  on  magnifying  the  glass  oscillograms with  an  2%. estimated  signals,  estimates for  of  and  adding  taking  forward  standard  and  the  the  deviation  square  of  of  the  indi-  square-root,  are  8%  backward  scattering  signals  respectively.  D.  Discussion The  ment w i t h  scattering  theoretical  profiles  profiles  are  for T  g  in excellent =  19,000  °K,  agree-  37 N g = 2.07 x 1 0 1 6 cm" 3 , and e = 49° 22', 130° 38'. The temperature i s s l i g h t l y (van  higher than r e p o r t e d  der Kamp,13 C h a n , 1 0 S t a n s f i e l d , 1 5  previously,  Baldis,11  12 Morris van  ) , but i s w i t h i n  der Kamp and M o r r i s .  parable to that reported The  total  the  two s c a t t e r i n g  the  actual  The e l e c t r o n  i s com-  earlier.  signals  are both l e s s than 3/4 of  standard d e v i a t i o n s .  they are s t i l l  density  estimated standard d e v i a t i o n of  rough, and the t o t a l but  e r r o r of the values quoted by  deviation  The estimates are very could be 2 or 3% low,  much l e s s than the observed  deviations. It was found that the f l u c t u a t i o n s two  scattering  fluctuations  signals  originated  r a d i a t i o n , and s t r a y the  fluctuations  fluctuations  are h i g h l y  i n the  correlated.  I f the  from shot n o i s e , continuum  l i g h t a l o n e , one would expect  to be independent.  are too l a r g e  The f a c t that the  and are c o r r e l a t e d  indicates  that there must be at l e a s t one other source of f l u c t u a t i o n , about 12% i n magnitude and p o s i t i v e l y  correlated  f o r the two d i r e c t i o n s . One might propose that i f the e l e c t r o n  density  and/or temperature changed from s h o t - t o - s h o t , then t h i s might produce the r e q u i r e d  variation in signal.  38  Mechanisms  that  might  change  in  t h e j e t c u r r e n t o r gas  in  position  of  the cathod  or  decrease  more t h a n  of the  N  unreasonable:  flow  The  and  e o f f the  1 mm  and  g  T  rate,  plasma j e t f l a m e  o r anode.  both  N  this  T  later  a t the  e centre of  would  are  g  or  variations  perhaps  due  to e r o s i o n  would  tend  to i n c r e a s e  same t i m e .  A  the j e t would  produce  changes  a change  shift  of  seem  in N  and  g  T  fi  12 of  17%  II-C  (Morris  ).  show t h a t  However, c a l c u l a t i o n s  i f both  N  and  T  e  of S e c t i o n  decrease  by ,  e  20%  then  .  J  i  the  signal  H%,  and  8i%.  These  i n the  i n the  negative  forward  backward  changes  are  correlation It  T  jet  current.  any  o t h e r mechanism  leaves then  resulting  one  this  should  i n c r e a s e by  direction,  should  decrease  too  small  rather  i s not obvious  and  g  direction  from  than  exactly  changes  of T  that  or N  g  Q  extra  i n F i g . IV-5  and  theoretical  graph  signal  assuming  c o n s t a n t at the  °K,  reproduced  plotted  on  this  o f the  from  the  c o n s t a n t and  illustrated  g  the  IV-6. as  flow that  plasma changes  The  were d e t e r m i n e d  N  these  or  parameters the  other, as  is a  of  N  g  value of  19,000  experimental i n the  g  or  F i g . IV-5 function  in  rate  fluctuations  experimental  S e c t i o n II C.  graph  having  changes  i n t h e gas  changes  p r o v i d e the  as  positive.  However, i f i t i s assumed  cannot  T  as w e l l  by  points  following  39  24  r  EXPERIMENT THEORY  0.8  0.8  IP N  FIG  1.2  e/2.07x i o  1 6  1.4  cm"  3  IV-5  COMPARISON  OF  EXPERIMENTAL  VARIATIONS IN SCATTERING SIGNAL TO  C A L C U L A T E D VARIATIONS  TO CHANGING  N  p  DUE  24  22  20  BACKWARD  18 If)  >-  cr < or i— cn or < co z OI tz  16  14  12  FORWARD  1 i  EXPERIMENT THEORY  10  0.8  0.8  1.0 T  FIG  1.2  1.4  e / 1 9 , 0 0 0 °K  IV-6  COMPARISON OF  EXPERIMENTAL  IN  SIGNALS  SCATTERING  VARIATIONS  DUE TO  TO  CHANGING  VARIATIONS CALCULATED T, e  1.6  41  manner.  Assume that an i n c r e a s e i n the s i g n a l  backward d i r e c t i o n from Section  III B i s due  new  Ng  Ng,  the new  is plotted  theoretical  i n the forward  actual  experimental  x 1016  forward  average  signal for  then  the  as d e s c r i b e d should l i e about  curve f o r the forward d i r e c t i o n .  be seen from the graph, t h i s i s not the c a s e . e r r o r bars drawn come from  signal  I f the changes  to changes i n N g ,  points plotted  the t h e o r e t i c a l  cm - 3 .  new  direction  normalised so that the  = 2.07  This  Using t h i s  to the t h e o r e t i c a l  are r e a l l y due  experimental  IV-5.  signal  Ng,  i s equal  = 19,000 ° K , N e  in s i g n a l  The  at the new  forward s i g n a l  value as measured i n  e n t i r e l y to a change i n N g .  can be read from the graph  can be determined.  Te  i t s average  i n the  As  can  The  the estimate made i n S e c t i o n  IV C f o r a l l known sources of e r r o r . A s i m i l a r procedure was  used  variations  are due  the s i g n a l  i n the forward d i r e c t i o n was  new  then the experiment  Tg  and  backward d i r e c t i o n curve and  and T . e  i s probably not due  T h i s time  used  to f i n d the  Again the t h e o r e t i c a l  p o i n t s do not match  above c a l c u l a t i o n s  I t might still 3  alone.  p o i n t s were drawn f o r the  ( F i g . IV-6).  experimental The  deviation  to changes i n T g  to check i f the  indicate  up. that the  to f l u c t u a t i o n s  be r p o s s i b l e  for N e  in  and T e  extra  Ng to be  42  changing  i n some s p e c i f i c way to give the r e q u i r e d  devia-  t i o n s , but t h i s does not seem reasonable because the variations very  i n the c a l c u l a t e d s i g n a l are too small  l a r g e changes  unless  (about 50%) are made i n N g and T .  It would seem then that the plasma j e t has some nonthermal  c h a r a c t e r i s t i c s which c o n t r i b u t e to the standard  d e v i a t i o n of the s c a t t e r i n g s i g n a l , but are not so b i g as to cause l a r g e d e v i a t i o n s from normal ing  profiles.  thermal s c a t t e r -  43  Chapter V  CONCLUSIONS  A.  Conclusions Scattering  profiles for scattering  of  ruby  l a s e r l i g h t from a plasma j e t were measured s i m u l t a n e o u s l y in two delay.  d i r e c t i o n s using a two The  experimental  theoretical  channel system with o p t i c a l  data agrees very well  profiles for scattering  homogeneous thermal plasma with N g Tg  = 19,000 ° K .  The  integrated  from and  with infinite  x 1016  = 2.07  scattering  cm - 3 ,  s i g n a l , measured  o  with 10 A wide monochromator t r a s m i s s i o n to have a standard d e v i a t i o n f o r by  too  large  known sources of f l u c t u a t i o n s .  grated s c a t t e r i n g scattering  signals  angles had  correlation.  f o r the  a large  Theoretical  fluctuations  changes i n N g  T .  plasma j e t has  and  to be  proved  accounted  Also the  forward and  inte-  backward  p o s i t i v e c o e f f i c i e n t of  calculations  extra, correlated  function,  show that  cannot o r i g i n a t e  Thus i t i s suggested that  non-thermal p r o p e r t i e s  the from the  that have only a  small e f f e c t on normal  thermal s c a t t e r i n g  of l a s e r  light  from a plasma j e t .  B.  Suggestions f o r Further Study The dependence of the e x t r a f l u c t u a t i o n s  p o s i t i v e c o r r e l a t i o n on the s c a t t e r i n g be s t u d i e d .  I t may  and  v e c t o r jc could  be p o s s i b l e f o r the non-thermal  e f f e c t s to have a sharp dependence on k_ such as R i n g l e r and Nodwell  found f o r the t o t a l  integrated  i n t e n s i t y of  l a s e r l i g h t s c a t t e r e d from a m a g n e t i c a l l y  stabilised,  low pressure hydrogen arc where the t o t a l  integrated  i n t e n s i t y was retical  found to be approximately twice the t h e o -  value at one p a r t i c u l a r jc v e c t o r , then dropped  q u i c k l y to that p r e d i c t e d by theory as k_ was A multichannel  varied.  system might be adopted where  the e n t i r e s c a t t e r i n g  spectrum can be found with  one  f i r i n g3  T h i s would allow N  to be  of the l a s e r .  e  and T  e  determined from shot to s h o t , and t h e r e f o r e make the experiment more independent from changes parameters.  Such a multichannel  on the market.  i n experimental  system i s only  recently  1  45  BIBLIOGRAPHY  1.  G e r r y , E.T. and Rose, D.J. 37: 2715-2724.  2.  Evans, D.E. et al.  3.  R i n g l e r , H. and Nodwell, R.A. 29A: 151.  1969.  Phys. L e t t .  4.  R i n g l e r , H. and Nodwell, R.A. 30A: 126.  1969.  Phys. L e t t .  5.  R i n g l e r , H. and Nodwell, R.A. 1969. Third Europ. Conf. on C o n t r . Fusion and Plasma Physics, Utrecht.  6.  C h u r c h l a n d , M.T. 1972. of B r i t i s h Columbia.  7.  S a l t p e t e r , E.E.  1960.  8.  R o s e n b l u t h , M.N. F l u i d s 5: 776.  and R o s t o k e r , N.  9.  K e g a l , W.H. 1965. I n t e r n a l Report. I n s t i t u t f u r Plasma P h y s i k , IPP 6/34.  1 966.  Ph.D.  1 966.  J. App1 . Phys.  Nature 2jJ_: 23-24.  Ph.D.  Thesis,  University  Phys. Rev. 1_20: 1962.  1528-1 535, Phys.  10.  Chan, P.W. 1966. B r i t i s h Columbia.  T h e s i s , U n i v e r s i t y of  11.  B a l d i s . H.A. 1971. B r i t i s h Columbia.  Ph.D.  T h e s i s , U n i v e r s i t y of  12.  M o r r i s . R.N. 1968. B r i t i s h Columbia.  Ph.D.  T h e s i s , U n i v e r s i t y of  46 13.  van der Kamp, G.S.J.P. 1968. M.Sc. U n i v e r s i t y of B r i t i s h Columbia.  Thesis,  14.  VanAndel , H.W.H. 1 966. of B r i t i s h Columbia.  Ph.D.  Thesis, University  15.  S t a n s f i e l d , B.L. 1971. of B r i t i s h Columbia.  Ph.D.  Thesis, University  47  Appendix A  ALIGNMENT  Because is  critical,  PROCEDURE  the alignment  of the d e t e c t i n g  a d e t a i l e d procedure  First  a HeNe l a s e r  i s given  o f t h e m o n o c h r o m a t o r , t h e beam g o i n g  slit  which  with  t h e monochromator  two  has been  straight  edges  wide e n t r a n c e centred the  over  slit  s e t to the r e q u i r e d s e t to t r a n s m i t  are placed so t h a t  the l a s e r  Next away so t h a t centre.  The beam forms  wide  at the m i r r o r .  just  i n f r o n t of the m i r r o r  tric  circles  over  line,  t h e 400u  formed i s to d e f i n e  i s placed  approximately  a diverging  i t leaves  helps  mirror  beam h i t s  when  Then,  plasma.  a front surface  the l a s e r  i n the e x i t  the l a s e r  500u a p a r t  This  the o p t i c a l  width.  the r e c t a n g l e  beam.  volume o f t h e o b s e r v e d  here.  i s a l i g n e d with  axis  system  50 m e t e r s i n the  diffraction  pattern  t h e m o n o c h r o m a t o r , so i t i s 4 t o 5 cm. Because o f t h i s , that  drawn on i t s f a c e ,  a stop  was  placed  has a s e r i e s o f c o n c e n centred  around  a 1/8  48  inch diameter h o l e .  This stop  i s placed so that the  hole i s i n the centre of the d i f f r a c t i o n using the c i r c l e s  as a g u i d e .  c i r c l e of l i g h t with  The r e s u l t i s a small  some d i f f r a c t i o n  r e f l e c t e d from the m i r r o r . so t h a t the r e f l e c t e d  pattern,  rings  being  The m i r r o r i s then  circle  adjusted  i s centred on the entrance  r e c t a n g l e of the monochromator. The position  along  lenses are now placed i n t h e i r  proper  the a x i s of the d e t e c t i n g system with  the a i d of a t e l e s c o p e focussed  to i n f i n i t y .  The  t e l e s c o p e i s placed between the m i r r o r and the expected p o s i t i o n of lens C A i s approximately adjusted  slit  i s in  as the cross h a i r s of the t e l e s c o p e .  Thus the entrance  slit-lens  d i s t a n c e i s equal  to the  l e n g t h of lens A. Next lens B and C are p o s i t i o n e d .  placed so t h a t i t i s about one f o c a l p o s i t i o n of the plasma j e t . critical  Lens B i s  length from the  The placement i s not too  because of the adjustments on the j e t .  the t e l e s c o p e a g a i n , lens C i s placed focal  Lens  centred on the l a s e r beam and then  so that the image of the entrance  the same plane  focal  centred on the l a s e r beam.  Using  so t h a t the  p o i n t s of B and C c o i n c i d e , which occurs when  the image of the entrance  slit  i s again  i n the same  I  49  plane  as the cross h a i r s .  optical  Stops are placed on the  benches so that the lenses can be removed and  r e p l a c e d without changing t h e i r p o s i t i o n  along the  a x i s of the d e t e c t i o n system. Next i s the adjustment of the h o r i z o n t a l and  vertical  p o s i t i o n s of the l e n s e s .  g r e a t l y d i v e r g i n g beam a f t e r passing  Because of the through a l e n s ,  there i s no good r e f e r e n c e f o r p o s i t i o n i n g C by themselves.  lenses B and  T o g e t h e r , however, there i s a good j  r e f e r e n c e beam s i n c e the only e f f e c t they have i s to enlarge  the beam by the r a t i o of t h e i r f o c a l  They are then a l i g n e d , with  lengths.  lens A removed, so t h a t  the l a s e r beam i s centred on both the aperture at the m i r r o r and at the monochromator.  L a s t l y lens A i s  p o s i t i o n e d so that the beam i s centred on the stop at the m i r r o r , with The  both B and C i n p l a c e .  l a s t step i s to stop down the l e n s e s so  that they match the monochromator speed f / 6 .  With the  stop at the m i r r o r removed, and f/3 l e n s e s , f o c a l  lengths  12, 12, 30 cm. f o r A, B, and C r e s p e c t i v e l y , the r e f l e c t e d l a s e r beam should  cover a l l the area of the lenses and  be centred on each. Because of the d i s t a n c e r e q u i r e d , the f i r s t attempt at t h i s experiment used three m i r r o r s and f i v e  reflections l i g h t path.  i n the o p t i c a l  delay s e c t i o n  However, the extreme  l o s s e s of t h i s  ment made i t i m p r a c t i c a l , so that i t was adopt a one m i r r o r system.  to f o l d the arrange-  necessary to  

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