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Laser induced perturbation in a plasma Baldis, Hector Alberto 1971

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LASER  INDUCED PERTURBATION IN A PLASMA by HECTOR A.  Licenciado, M.Sc.,  Universidad  University  A THESIS THE  BALDIS  Nacional  of B r i t i s h  SUBMITTED  IN  de  Cordoba,  Columbia,  PARTIAL  1964  1968  FULFILMENT OF  REQUIREMENTS- FOR THE DEGREE OF DOCTOR in  OF PHILOSOPHY  the  Department of  Physics We  accept  required  THE  this  thesis  as  conforming  to the  standard  UNIVERSITY  OF BRITISH COLUMBIA  . Apri1,  1971  In  presenting  an  advanced  the I  Library  further  for  degree shall  agree  scholarly  by  his  of  this  written  this  thesis  in  at  University  the  make  that  it  purposes  for  freely  permission may  representatives. thesis  partial  be  It  financial  for  is  by  for  the  understood  gain  shall  of  requirements  copying Head  that  not  reference  be  of  agree  and  of my  I  this  or  allowed  without  for that  study. thesis  Department  copying  -  Columbia  the  Britislv Columbia,  extensive  granted  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  of  available  permission.  Department  fulfilment  or  publication my  ABSTRACT The laser  pulse  studied fired  interaction  and a p a r t i a l l y  experimentally.  into  the  both  From measurements  lation  also  of  radiation,  density  densities.  i o n i z e d argon  When t h e  of  in the  The S t a r k to  absolute  excited  the  tent  those  obtained  from the  when the  laser  the  time  plasma  the  Ar  appears This  quickly  asymmetry  density  II  lines after  can  gradient  the  the  II  obtained  in  laser  be e x p l a i n e d  present  in  the  Ar  is  light  pulse in  this  is  this  lines  way  popu-  has  in  are  the  consis-  radiation.  asymmetry  terms  the  electron  incident  has  plasma  emission.  density  continuum  show a s t r o n g the  pulse  of  and o f  electron  plasma  During  and d a t a  been  from t h e  intensities  atoms  ruby  has  have been made o f  transient with  emission  broadening of obtain  laser  c o n t i n u u m and l i n e  estimates  the  been measured  the  plasma  focused  plasma', a t r a n s i e n t  may be o b s e r v e d  transient  between a 2 0 MW Q - s w i t c h e d  on  the  which  dis-  terminated. of  the  electron  expanding perturbed  plasma.  i i i  TABLE  OF CONTENTS Page  ABSTRACT. TABLE LIST  ,  i i  OF CONTENTS OF FIGURES  .  iii .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  v  ACKNOWLEDGMENTS  vi i i 1  CHAPTER  CHAPTER  CHAPTER  I  INTRODUCTION  . . . . .  I-a  Introduction  to  I-b  An o u t l i n e  II  -  -  of  11-b  The l a s e r  II-c  The d e t e c t i o n  111- a  present  experiment  .  1 .  2  thesis  3  APPARATUS The plasma j e t  -  the  the  Il-a  III  . . . . .  .  .  .  5 9  system  1°  OBSERVATIONS AND RESULTS Spectral of  determination the  plasma  111-b  General  features  111-c  Expansion  111-d  The p o p u l a t i o n  111-e  Electron  Ill-f  Line  of  the of  density  shape.of. Ar  jet  of  the  ...14-  of .  the .  .  parameters . . . .  perturbed  perturbed excited  .  .  .  15  plasma  .  22  plasma. states  .  . . . .  of  Ar  .  from c o n t i n u u m e m i s s i o n II  lines.  .  .  . .  .  .  . .  32 .  3  4  . 37 4  5  iv  Page CHAPTER  IV  -  ENERGY  ABSORPTION  IV-a  Introduction  IV-b  Inverse  . . . . . .  IV-c  Photoionization  IV-d  Energy  IV-e  Nonlinear  V -  BIBLIOGRAPHY.  .  .  .  .  .  54  . . .  54  .  absorption c o e f f i c i n e t  a b s o r b e d by the effects  .  .  .  .  .  due t o  .  .  .  .  .  55  .  .  57  plasma. the  DISCUSSION AND CONCLUSIONS .  . . .  bremsstrah-1 ung a b s o r p t i o n  c o e f f i c i en t  CHAPTER  FROM A LASER BEAM.  .  laser  62 beam.  .  68 70  .77  V  LIST  OF  FIGURES  Page Figure  1 - Experimental  arrangement  Figure  2 - The p l a s m a j e t  .  Figure  3 - Syncronization  of the Q-switch  6 7  c h o p p i ng wheel Figure  .  .  12  4 - C o m p o s i t i o n o f t h e plasma j e t pressure  and t h e  as a f u n c t i o n  at  atmospheric  o f the  electron  energ'y Figure  5 - Radial  19  distribution  of e l e c t r o n  number d e n s i t i e s 19 j. L.  Figure  6 - Radial Ar  Figure  „_ mn i  ^ u „ ,. „ uuuvc  distribution I  and Ar II  7 - Oscilloscope  Figure  II  •  •  •  o f some e x c i t e d  •  •  at •  •  states  .  traces  different  traces  of a l a s e r at  pulse,  9 - Intensity along  Figure  10 - T r a n s i e n t  4806 A l i n e  of o b s e r v a t i o n  centre  of continuum e m i s s i o n at axis  emissions  ~  5000 A , and t h e  o f Ar II  locations  the j e t  ~  the  2  3  i n the  ...  region  3  5000 A  from t h e plasma a t  o f the p e r t u r b e d  2  at  plasma Figure  90  21  4806 A l i n e .  8 - Oscilloscope  •  of  .  continuum r a d i a t i o n Ar  neutral  i n the plasma j e t „ j. i, „ J _ ^ U U I I C M ^ .  U I I C  and  .  .  . 26  the 27  vi  Figure  Page 11 - I n t e n s i t y  in  degree Figure  the  in  II  Ar  4806 A l i n e  II  of Figure  16 - P o p u l a t i o n  energy  17 - E l e c t r o n  of  the  18 - E l e c t r o n  of  plasma  as  a  four  Ar  II  function  as  36  at  the  centre  a function  number d e n s i t y  across  .  II  / .  the  .  .  .  .  instrumental width  of II  .  Ar  of  .  .  the  II  4880 A l i n e  4880 A l i n e  at  time, intensities  .  a lorentzian  .  the  perturbed 4  broadening  . . . .  of  43  of  time  levels  levels  widths  Ar  energy  from c o n t i n u u m a b s o l u t e  function of  of  o f the  20 - T h i r d moment o f  21 - Shape  the  33  plasma  observed  Figure  in  31  line  region  Figure  5000 A  .  perturbed  and from Ar  19 - E f f e c t  at  30  increment  number d e n s i t i e s  obtained  Figure  29  p r e - i o n i z a t i on  density  perturbed  Figure  vs.  t i m e . . . . .  vs. Figure  the  of  emission  continuum emission  of  spectrum  15 - D i a m e t e r  vs. 28  4806 A l i n e  continuum e m i s s i o n Figure  emission  density  the  degree  14 - V i s i b l e  Ar  pre-ionization  power  13 - I n c r e m e n t vs.  Figure  of  12 - I n t e n s i t y , i n laser  Figure  the  on  line. shape  . . . .  the . as  .  t=10 n s e c .  4  .. .  4  ^  a  . . . .  .  ^ 5  9  1  Page Figure  22 - Shape  of  Ar  II  4880 A l i n e  at  t=30 n s e c .  .  .  . 52  Figure  23 - Shape  of  Ar  II  4880 A. l i n e  at  t=100 nsec  .  .  . 53  Figure  24 - Power a b s o r b e d  Figure  by  and  by  0.9  joules  laser  absorbed  by  25 - E n e r g y  joules  inverse  the  plasma  by  photoionization  bremsstrahlung  from a  pulse  the  laserpulse  plasma  from a  .  .64  .  .65  0.9 .  .  v.i i i  ACKNOWLEDGEMENTS  I to  Dr.  R.  wish  A.  to  express  Nodwell  during  the  course  thanks  Dr.  J.  during  Dr.  Nodwell's  Dr.  F.  L.  Curzon  tation  of  this I  ance the to  of  Mr.  Meyer  J.  assistance  would  of  Plasma Messrs.  friendship  his  and I  appreciation  encouragement  would  suggestions  Thanks  helpful  to  are  also  also and  due  suggestions  acknowledge  Sie.berg  am a l s o  the  and J .  machine  line  in  A.  like  to  support to the  presen-  a pleasure  being  particular  Standfield,  R.  N.  Board o f  Mr.  Mike  assistwell  as  particular Wu f o r  his  measurements.  in  is  technical  Zang.aneh as  to  g r o u p , and  work  the  shop s t a f f , i n  indebted shape  and s t i m u l a t i n g  Control  guidance  helpful  leave.  has-been  This Energy  experiment.  Physics I  during  L.  the  G.  the  Physics B.  his  like  D.  Bosma.  It  for  for  for  sincere  thesis.  Messrs.  members  of  my most  Morris  associated I  with  am i n d e b t e d  and D.  Camm f o r  the  to their  discussions.  supported  by a g r a n t  Canada.-,  from the  Atomic  1  Chapter  I  INTRODUCTION  This tion  of  plasma laser  the  describes  perturbation  under  the  pulse.  plasma  thesis  of  influence  a partially of  Perturbations  the  in  Lidsky  al . , 1964 , Thompson,  phenomena has  several  reports  theoretical high or  1965, plasma  by  the  experiment  years  Haught,  1969),  and on t h e  et.  we r e p o r t  of  out. the  a plasma  the  There  experimental  and.  is  created  by a  upon a s o l i d ,  subsequent  liquid  and  high  coefficient  of  the  plasma  with  of  the  1 9 6 5 , De  the  1968, 1969, 1970).  Raizer,  heating  beam ( R a i z e r ,  we s t a r t  of been  of  here  Van  have  effect  al.,  1963,  study  1963, Z e l ' d o v i c h  on the of  and  experimental  beam i n c i d e n t  absorption  Generalov  1964 , Nodwell  carried  which  absorption  1969),  beam on the  in  and  De M i c h e l i s ,  been  recent  laser  (Meyerand  Michelis,  1964,  in  (Thompson and F i o c c o ,  a systematic  studies  intensity  gas  literature  not  ruby  reported  the  the  a  have been  in  but  beam o f  from a l a s e r  previously  1968)  focused  argon  from a  a pulse  Kamp,  ionized  radiation  to  der  investiga-  the  due  et.  an e x p e r i m e n t a l  In  intensity (Rand, the  a plasma o f a  2  predetermined changes of  in  a high  degree  the  of  pre-ionization  emission  power  from the  focused  laser  The m o t i v a t i o n fold.  Firstly,  the  beam s c a t t e r e d state  of  the  cessfully ature offers  in  to  plasma  plasmas  the  A detailed  plasma.  perturbation of  effect  scattering  detai1ed  quantitative  plasma  due  the  exceedingly plasma  Introduction In  '  gated  )  a focused pulse  the  is  emission  thus  to  This  presence  to  been  from a  been used  But  only  of  is  the  pulse w i l l  temper-  a  be  light  scattered,  conditions  accurate it  the  technique  investigation  Secondly  laser  suc-  and  diagnostic  the  two-  about  density  modifying  essential  laser  has  resolution.  examination  the  ion  experimental  experiments.  the  duced  is  has  a plasma may not  be a b s o r b e d ,  the  information  and  and t e m p o r a l  but may a l s o  to  radiation  technique  1969).  through  due  experiment  the  electron  (Evans,  good s p a t i a l  this  provides  and the  determine  plasma  the  beam.  spectrum of  by a plasma  wave p r o p a g a t i n g  I-a  for  and we s t u d y  of  in  the  interpretation  felt  that  a  perturbation  help  to  complex phenomena o c c u r r i n g  in  of  clarify laser  pro-  e x p e r i m e n t s .to  the  the  present  experiment  perturbation ruby fired  laser into  from the  of  beam. the  experiment. presented  a plasma  under  When t h e  plasma,  here  high  a change  plasma may be o b s e r v e d .  we have  the  influence  power in  investi-  the An  laser light  increase  of light  3  occurs  both  f r o m the several  in  the  plasma.  The  transient  microseconds although  50 n s e c .  Further  perturbed  plasma  affecting  a much l a r g e r  original of  the  volume  excited  atoms  electron  the  Ar  quickly  II  the  the in  the  An O u t l i n e  of  in  this  tion  of  the  the  electron  the  also  light  the  the  work  laser  is  the  measurements radiation,  density  of  densities.  The  may be used  to  the Stark  obtain  plasma.  conducted,both-these argon  electron  plasma  During  incident  ceases.  expanding perturbed  only  the  than  transient  asymmetry  pulse  of  is  of  the  plasma  for  lasts  volume  a perturbed  pulse  last  pulse  From  transient  a strong  laser  terms  may  have been o b t a i n d .  on t h e  which This  the plasma  disappears  asymmetry  density  can  gradients  plasma. .  thesis.  The d e s c r i p t i o n used  of  radiation  hence  population  lines  of  show  the  size,  e x p e r i m e n t we have  laser  after  in  the  results  lines  be e x p l a i n e d present  in  have been made i n  and c o n s i s t e n t  laser  of^the  the  line  radiation  f o c u s e d beam.  spectral  the  the  that  volume  intensities  density  measurements  that  the  and o f  of  In  time  expanding  may be made o f  broadening the  is  in  the  one o b s e r v e s  of  absolute  estimates  I-b  c o n t i n u u m and  is  apparatus  of  the  presented has  in  experimental Chapter  been d i v i d e d  apparatus  II.  The  into  three  descripmain  parts:  4  the  production  the  plasma,  of  and  the  the  detection  S i n c e most o f  the  experiment  fairly  only  is  a brief  of  These  some o f  the  from  the  tions mental  of  are  the  laser  of  this  in  to  is  the  of  the  type  the  perturb signals.  course of  of  the  research,  also  observations  detail  included  the  and a  parallel  of  the  included  plasma  the  is  their of  this  energy  presented.  absorbed  electron  in  based  density  and  discussion  to  degree  mechanism o f  energy  of  during  to  prechpater.  absorption Calcula-  on the are  experi-  also  discussed.  Finally, of  conclusions  of  future  are  work  are  IV  beam by  amount  and  some a s p e c t s  in  devoted  plasma  determination  presented  is  used  presented.  presented  Chapter  the  is  laser  and r e c o r d i n g  The d e t e r m i n a t i o n  of In  III  the  used  standard  results  presentation. ionization  equipment  description  Chapter results.  plasma,  the the  Chapter  V presents  results present  presented  that  the  discussion  have been  work as at  the  well  end o f  as the  of  obtained.  The  suggestion  for  chapter.  5  Chapter  II  APPARATUS  The apparatus  purpose of  used  be d i v i d e d  in  in  the  three  parts:  the  laser  unit  11 - a) , t h e  (Section  11 -b) „ and t h e  plasma  axis  of  Figure  argon the  1.  pressure  to  describe  The d e s c r i p t i o n  used  to  the  perturb  system  the  will plasma  the  (Section  plasma  11 - c ) .  jet.  1 and a s c h e m a t i c the  is  p r o d u c t i o n of  detection  The e x p e r i m e n t a l  duce  chapter  experiment.  (Section  11 - a The  this  diagram of  plasma  is  plasma j e t  is  eliminating  w i n d o w s , , and p r o d u c e s  the  shown  The plasma j e t thus  arrangement  is  plasma j e t  in  Figure  perpendicular is  operated  the  at  need o f  a steady  shown  state  used  2. to  in  The the  and good r e p r o d u c i b i l i t y .  tion  details  and o p e r a t i o n  pro-  vertical of  atmospheric  plasma o f  density  to  plane  container  tron  Figure  For  of  a plasma j e t  of  the  walls high  the  or  elec-  construc-  see(Morris,  1968) . The transformer is  used to  power  and o f keep  the  supply  a 36 v o l t jet  jet  battery  running  .at  consist bank. low  of  The  current  a  current  transformer (30 Amp  6  MONOCHROMATOR PHOTOMULTIPLIER  LASER DUMP  GLASS PLATE RUBY LASER  1 — Cu S O4 CELL  —  SLIT AND CHOPPING WHEEL  LASER MONITOR  MONOCHROMATOR CONSTANT DEVIATION PRISM  PHOTOMULTIPLIER  FIGURE Experimental cylindrical is  arrangement.The symmetry  perpendicular • •••• •  1  to  of the  diagram.-  the  axis  plasma  plane  of  of jet  the •• - • ••  7  — 0 4.5 mm  ANODE  FIGURE 2 The  plasma  jet.-  8  typical)  to  remaining  minimize  voltage  transformer, whenever  the  electrode  ripple jet  is  a measurement  on the  performed 300 Amp i s  a current  of  bank  is  in  with  the  arc.  voltage  The  current  across The  tapered are  water  and  the  anode.  beam i s  of  the  mm above  the  in  the  above  the  conditions  can  plasma  also  ring  is  thus  the  bank  plasma.  supply  For  A  to  resistor  stabilize  by m e a s u r i n g  is  the  out  4.7  be m o d i f i e d  The  to  flow  through  laser  plasmas  of  initial  by v a r y i n g  hole  jet The  a distance  electrodes  from a  the  a small  the  tungsten  Both  liter/min.  at  anode,  thorium  anode.  caused  focusing  be s t u d i e d .  the  forminy  plasma  By  the  of  battery  employed.  power  cathode  flow  anode.  positions  can  the  output  on the  between  copper  anode  The gas  focused  passes  the  the  of  shunt.  A r g o n gas  surrounding  centre  the  determined  mSl  current  cooled.  the  is  a 0.25  cathode  chamber  ent  series  with  is  purpose  Because  rectified  powered  this  used  damage.  in  above laser  from 2 to beam a t  differ-  different  conditions the  current  10  initial of  the  in  the  jet. An e l e c t r o n 16000°K with tween 3 x 10 corresponds  temperature  between  corresponding electron 1 to  respecti vely.  ft  cm"  -3 and  a degree  2 x 10" of  11000°K  and  number d e n s i t i e s  1 7 cm"  ionization  is  obtained.  between  beThis  10% and 60%  9  11 -b The  laser. The  duces  ruby  laser  a Q-switched  output  of  is  40 nsec  1 joule.  a TRG model pulse  The o u t p u t  with  is  extended periods  of  laser  fired  intervals  seconds. beam i s  parallel  speed of of  is  of  to  gives is  continual  axis  of  The  an f / 1 0 0.02  for  cm i n  the  than  40  of  the  plasma j e t .  laser  The  prism with a focused with a  combined w i t h convergent  diameter  within  shorter  beam i s  lengthwhich  to  the  a rotating  laser  energy  if  vector  the  pro-  operation  not  electric  performed with  8 cm f o c a l  spot  the  the  30,000 rpm.'  diameter focal  regular  The p l a n e  Q-switching  lens  at  a maximun  reproducible  10% o v e r is  104 A and  the  beam  laser  beam.  and a maximun  power  The  _ 9  density focal  spot  60 GVJ xnv *" i s  obtained.  is  by  determined  foil  placed  at  dent  on t h e  plasma  of  copper  placed  -  of  the  sulphate  between  the  plate  mounted a t  laser  and t h e  light  on to  varied  liner  operation  laser  obtain  light  The power  density  a  metal  inci-  dissolved  in  in  an a b s o r p t i o n  laser  the  plasma.  A reflecting  and  Brewster  water  angle  and r e f l e c t s state  the  a Hewlett-Packard, to  plane.  produced in  the  the  the  of  damage  of  by c h a n g i n g  a solid  volts  The  is  plasma  reproducibility is  focal  the  The d i a m e t e r  a fast up to  diode  laser part rise  is  placed  a small  to  check  output.  2 volts  the  dump c o n s i s t s  of  state  biased  with  glass the the  were a  diode  at  t i m e < 1 nsec  signal  a cell  of  cell  pulse-to-pulse  The s o l i d  A rise  output  between  fraction  number 5 0 8 2 - 4 2 2 0 , time.  concentration  100 and a  obtained.  Brewster  10 angle of  entrance  copper  window  containing  system.  The d e t e c t i o n photomultipiier collecting  incident  focus  plasma  and  the  by  is  placed  beam and  thus  determined  system c o n s i s t s  and o s c i l l o s c o p e . lens  laser  lenses  slit.  the  the  axis  the  spatial  width  to  (lens  the  These d i s p l a c e m e n t s  mi c r o m e t e r s .  scan  of  this  Figure  of  gradients  in  the  plasma and o f  the  of  the  focal  of  spot  image  the  the  wavelength  of  spatial  horizontal  the  narrow  resolution  of  plasma  losing  collected  in  chosen the  The the is  entrance  1)  can  region- in by  the  two  slit  in  in  spatial  matches  plasma.  the  the  by  to  plasma  rotating  90°we  can m a x i m i z e  monochromator and  m a x i m i z e the  the  electron  cylindrical  Then by  resolution  t o o much l i g h t .  to  the  we want  The  with  The a p e r t u r e  aperture  of  in  length  amount o f  monochromator c o n s i s t e n t  resolution optics  laser,  resolution  entrance  good v e r t i c a l  then  the  resolution.  simultaneously without  the  plasma  monitored  configuration  than  (f/6).  the  perturbed  are  both  into  in  the  rather  external  slit  of  good v e r t i c a l  spatial  the  of  -  obtain  is  A in  to  axis  plasma j e t .  Because  geometry  slit  the  height  laterally  density  of  resolution  and the  lens  monochromator,  The o p t i c a l  monochromator e n t r a n c e  . The c o l l e c t i n g  plasma.  of  perpendicular  be d i s p l a c e d  dial  solution  suphate.  11-c The d e t e c t i o n  the  a concentrated  the  obtain the of  the  energy horizontal of  the  monochromator  11  The g r a t i n g laboratory. of  3.8  It  isolate  of  used i n  sixth  A/mm and a t h e o r e t i c a l  A constant  which  is  monochromator was  deviation  the  prism i s  orders.  The r e g i o n  gave  of  lineal  been d e t e r m i n e d density  the  filters.  later  a sweep with  the In  of  the  jet  allowing 300usec at  of  of  of to  each  of  0.65  disk  location  (Tektronix give  to  the  with  two  Kodak  585)  the  with  average direct  and  anode  light  rotating  This  allows  current  thus  a plug of  type measured  current  from  at  us  cable  time  film  division  have  neutral  the  1725 rpm  be e x p o s e d f o r  anode  ratio.  2.6  on P o l a r o i d per  to  of  a combined r i s e  down the  rotation.  the  an RCA 7265  50ft to match  cut  cm and 0.3  of  is  magnifier.  only to  work  increasing  The c h o p p i n g wheel  consists  circular  with  apertures  cm r e s p e c t i v e l y  7 cm and 8 cm r e s p e c t i v e l y (the  is  to  photomu1tipiier  calibrated  a  due  time  the  a c h o p p i n g wheel  signal-to-noise  eters  of  50 nsec  instantaneous  a rotating  it  load  photomu1tipiier  during  a higher  the  help  order  the  of  were r e c o r d e d  speed o f  we use  front  in  nsec.  a set  photomultipiier  plasma  placed  rise  amplifier.(#82)  410 a t  180,000.  nominal  The anode  Signals  of  voltage  in  nsec.  power  anode-to-cathode  The o s c i l l o s c o p e  4.4  a dispersion  a total  impedance. vertical  our  resolving  operation  with  in  order with  The p h o t o m u l t i p i i e r  when b i a s e d w i t h  3000 v o l t s  built  from the  second h o l e  is  located  centre  of  the  irrelevant).  diam-  at disk The  12  reference light pulse  photo  diode  amplifier  magnetic head pulse V  trigger  scr  gate  voltage  circuit  trigger transform.  FIGURE  S y n c r o n i z a t i on o f  3  the  thechopping  Q-swi'tch  wheel.-  and  13  first into  aperture the  produce laser the  a reference the  plasma  would  fire  the  (see  the  when  1).  the  aperture  laser  timing  (see of  interval  of  the  signal  diagram of the  used  to  trigger  aligned  supply  3).  the  switchs which  Figure  motor  the  with  signal  the  Q-switch  the  enter  from  d u r i n g which  monochromator,  spite  to  is  power  to  is  The r e f e r e n c e  the  the  rotation  time  in  first  of  arrives  enters  perfectly  the  circuit  of  plasma j e t  from a p h o t o d i o d e  Figure  laser  period  than  plasma  works  jet  from the  The s e c o n d a p e r t u r e  pulse  that  Q-switch  the  shorter the  time  triggering  rotating Since  ligth  monochromator.  at  on the  allows  is  much  light  from  synchronization  random phase  of  the  two  rotations. A second d e t e c t i o n length  band pass  ticularly  in  the  line  total  chromator purpose.  the  is  system with  used to m o n i t o r  experiments  intensity.  on the  A half  o f . f / 1 1 and a band pass  the line  meter of  a wider plasma  waveand,  shapes, Jarre!'Ash  5 A is  par-  to  monitor mono-  used f o r  this  14  Chapter  III  OBSERVATIONS AND RESULTS  In and r e s u l t s deals  with  this  chapter  are p r e s e n t e d . the s t a t e  laser  the  plasma when t h e . l a s e r where  perturbed c* o  nnnnv  w ^ i . i ^  obtain of  A general  region.are 1^ w r l ^  » ^- *  I _»  is-fired  t  also A v* (It  some i n f o r m a t i o n plasma  The enhancement  been d e t e r m i n e d ments  ^ -F  \J  using  TT • J. A  in s e c t i o n s  sented  in.section  in  Ill-e.  of  section  U -> w o IIU  V V,  L. y-\ o v> n ri - — n ••> /* A U  C  C  II  of  IIIUU  J  U  in  section  d e n s i t y has methods.  Measure-  o f the c o n t i n u u m  b r o a d e n i n g o f Ar II  i n t h e A r II  —  WV>  equilibrium  are presented  intensity  t  lines  111 -d and 111-e r e s p e c t i v e l y .  observed  of  The p o p u l a t i o n o f  two i n d e p e n d e n t  presented  111 - a  t o the f i r i n g  presented  o f the e l e c t r o n  and on the S t a r k  shape  is  the s t a t e  and t h e y  radiation  section  on the e x p a n s i o n o f the  r\ <n r~ * *J M J  about  observations  of the p e r t u r b a t i o n  included.  b a s e d on t h e a b s o l u t e  asymmetric  prior  picture  the o b s e r v a t i o n s  the t r a n s i e n t  111 -c .  The f o l l o w i n g  o f the plasma  the  111-b,  pulse.  the experimental  lines  is  also  are The pre......  15  111 -a S p e c t r o s c o p i c plasma  determination  before  the  firing  ionization)  is  has  absolute  have  local  plasma which  is  of  line  the  laser  intensity  by  to  were v e r y optical  near  absorption plays  at  of  the  plasma was  the  plasma  jet  truly  role  using  measurements  that  (LTE)  the  plasma  and t h a t  determined  in  but  measurements  LTE,  in  an a t m o s p h e r i c  the  conditions  argon  in  is  also  the  than  emission the  it  at  the  more  varies  atmos-  The  important  absorption for  showed  continuum  wavelength.  is  in  although  determination  self  general  in  6500 A ,  longer  the  (1963)  a plasma j e t thin  jet  plasma  reasonable  by T o u r i n  the  to  The a s s u m p t i o n a b o u t  The amount o f and i n  pre-  not  at  in  call  We have  shorter  line  transition  assumptions  optically  of  will  These  performed  thick  a significant  I  I.  produced in  wavelength  atomic  Ar  plasma  optically  coefficients. on the  that  Measurements  pressure  was  (1968)  showed  the  (which  of  equilibrium.  argon  radiation  thin.  Freeman  of  equilibrium  plasma.is  thickness  the  plasma  our  ours  plasma.  pheric  of  spectroscopically  the  thermodynamic  extent  similar  that  parameters  ionization  been o b t a i n e d  optically  performed  our  of  been p e r f o r m e d under  in  the  jet. The d e g r e e  the  of  of  the self and  emission  depends different  16  lines. that the  The work  although line  tion  at  Ar  I  detection  measurements emissivity The  of  absolute  This  the  emission  upper  levels  assumed  to  I  to  system with  is  of  the  be i n  the  number d e n s i t y  n  where n ( p )  is  will  =  n  the  line  of  9(P)  COT  absorbed, self  of  pre-ionization intensity  lamp u s i n g  Ar  profiles  relate  for  by DeVos  II  were  the  (1954).  Abel  un-  obtained. of  plasma  the is  equilibrium, this  the  6965 A was  population If  absorp-  plasma.  numerical  the  shows  absolute  given  and by  then  our  for  thermodynamic  equation  "(P)  to  arc  of  degree  transition.  Boltzmann total  the  yields  local  free  ribbon  emission  profiles  the  values  plasma  radial  strongly  calibrated  the  intensity  an a r g o n  similar  a tungsten  the  are  measure  tungsten  in  practically  densities  order  of  (1963) lines  6965 A i s  measured a c r o s s folding,  Olsen  some Ar  electron In  the  of  the  population  atoms:  PXD  (1)  kT  P  number d e n s i t y  of  atoms  in  the  upper  • ('  level of  the  of  the  level,  transition, n  Q  the  g(p).  total  to  the  statistical  number d e n s i t y  of  weight atoms,  17  and U  the  Q  partition  function  for  the  atom.  The  Saha  equation  n n i  2 U (T) (2ir m kT.)  e  "12  I  i  exp  U^TT  -AI O  0  kT  (2) 2U.(T)  -07TT7 S ( T )  coupled with  the  equations  for  the  total  pressure  n = kT(n . + n + n ) i e o'  (3)  %  and w i t h  the  condition  e(n  can  t h e n be used  ferent and  components  U.j(T)  function the  to  are of  atoms,  the  charge  - n )  i  =  e  obtain  of  the  of  the  the  0  and AI  is  I  plasma.  the  a small  (4)  number d e n s i t y  number d e n s i t y  ions,  conservation  In  these  and t h e  ionization correction  of  the  dif-  equations  n^  partition potential of  the  for ionization  0  potential  due to  electric  equations  (2)  (4)  to  the  microfields presence  of  in  the  plasma.  a second stage  In of  18 ionization S(T) of  has  have been  been  calculated  T and d i f f e r e n t  (1965), values  of  the  the  in  of  jet  are  The  g  at  degree  as  and n  levels  between 5 are  also  given  they  (t  the  the  Felenbok  for  different  we have  of  the  I  atoms  electron  of  these  II  of  the  and anode  temperature  uncertainty 1954),  at  of  in self  the  the  the  in  in  9  mm.  the this  Figure  with a  The  may be  results as  present  work.  error  are:the  tungsten  the  of  plasma  emissivity  of  is  atmospheric  given  absorption  1963) , r e p r o d u c i h i 1 i t y  The  population  and e r r o r s to  popula-  ions.  plasma j e t ,  for  electron  variables  are  of  contribute  the  temperature  5 c o r r e s p o n d to  accurate  that  and  5 and the  and A r  anode  calculated  and the  plasma j e t  are- s u f f i c i e n t  2%)(DeVos,  (Olsen,  (4)  pre-ionization  Figure  Ar  very  of  (< 5%)  values  and  and n e u t r a l s ,  argon  cathode  determination 5%),.  in  the  sources  tungsten  different  J  to  on t h e  Q  Figure  not  2 5 % , but  (f  o  dependence  of  of  of  the  presented  The d i f f e r e n t  lamp  are  (1)  states  3 mm from  separation Figure  q  by Drawin  electrons  4 for  profiles  located  high  of  The r a d i a l  some e x c i t e d 6.  n  Figure  pressure.  of  equations  some e x c i t e d  dependence shown  A I  AI  function  e  temperature, of  of  for  Saha  n .  number d e n s i t y  tion  of  The r e d u c e d  and t a b u l a t e d  values  where v a l u e s  Using the  neglected.  in  ribbon of the  plasma  plasma jet  19  ELECTRON  ENERGY ( eV )  FIGURE 4 Composition  of  the  plasma  jet  at  1 atm.-  20  —i  0.15  r~  r — — i  r  i  i  i  0.10  j  i__  0  0.05  0.05  RADIUS'. ( cm )  i  i  0.10  0.15  .  FIGURE 5 Radial  distribution  densities  in  the  of  electron  plasma j e t  at  and n e u t r a l 12 mm above  number the  cathode.-  21  RADIUS ( cm ) FIGURE 6 Radial  distribution of  Ar  I  of  some e x c i t e d  and Ar  II.-  states  22  (+ 3%),  and e r r o r s  procedure 111-b  General  (<10%)  features  of  most o f  of  some Ar  Although 60%)  the  most o f  Figure  the the  II  light  are  few  II  ions  Ar of  are  in  the  some e x c i t e d  Ar  II  laser  observed  shown  the  in  of  so s m a l l plasma  this due  transit  after  the consists  also  weakly. high  ground ions  (up  to  state.  are  the  the  the  Typical  shown  the to  between  the  the  of  few  in  the  a laser  the  plasma  I  pulse  laser  pulse  radiation  of  emission the  intensity  region,  for  laser, of  e m i s s i o n from t h e  perturbed  a  the  photomultipiier.  Ar  firing the  into  photomu1tipiier  7 compared to  of  in  fired  c o n t i n u u m and  transient  time  is  with duration  The d e l a y  the Targe  surrounding  pulse  plasma.  decrease  decrease to  in  Figure  d i o d e and  a small  light  radiation  from the  microseconds  ratio  We can  the  the  observe  radiation.  into plasma  is  given  to  line  by the  pre-ionization  photodiode.  due  emitted  fired  of  f r o m the  is  is  degree  is  emission  by  pulse  plasma.  very  a transient  signals  I  1961).  perturbed  laser  unfolding  6.  microseconds line  numerical  e m i s s i o n but  When the plasma  the  line  The p o p u l a t i o n s in  the  c o n t i n u u m and Ar  observe  to  (Bockasten,  Before plasma,  due  the  We a  but line  the is  ambient  that  it  makes  23  CO  o ^ d E  +  u  a)  —  A  Q O +  . t, ,i iiiym  •  J  CO  CO  P  — il—  tftffu  O  IA  i  b)  j  A • f t  1 c)  0  U  V  CO  f— Q rr O  A.  lu >  M 4«+  <  i  O  it**  d  250 5 0 0 TIME ( nsec )  A. ir  0  FIGURE 7 Oscilloscope pulse  (a),  radiation Ar  1 1  at  laser  continuum  5000 A ( b ) ,  4806 A l i n e  rt  T I M E ( psec) FIGURE 8  traces, of a  the  V  '  CD  S  »  4  Oscilloscope Ar  and t h e  (c ) . -  II  4806 A l i n e  different  The f o c a 1  eter  is  of  at  locations  observation  laser  traces  i n the spot  of plasma.  of  the  0 . 0 2 cm i n diam- .  and i s  centered  at  0.  24  quantitative time  measurements  observed  within  the  any  of  imprecise.  the  frequency  Ar  pass  III  We have not  1ines' avai1able  band o f  our  optical  at  to  aay  us  detecting  system. If plasma  jet  different  we now s c a n  we o b s e r v e time  further can  we g e t  to  in  traces  a different  suggests  affects spot. moves the  a fixed  that  other  away f r o m the  volume  of  the  Measuring  times  we can g e t  a function variation  of  axis the  laser  of of  of  the  laser. pulse  the  for  in  pi ace  spot  the  of  Each  focal  is  spot  in  0 in  The g r a p h  in  Figure  plasma The t i m e our  at  of  the  the  is  results.  laser  the  of  plasma  the  focal  front  that  observation  at  predeterminate  perturbed  region  9 illustrates  continuum r a d i a t i o n  different  origin  the  radiation this  phcto-  considerably  during  of  the  Figure 8 ) .  volume  section  the  correspond  a perturbation  region  increase  typical  keeping  increasing  has a  This  produced in  the  the  and  laser.  of  (position  of  increases  trace  plasma,  than  there  intensity  all  that  the  region  cross  positions,  8 where a s e t  perturbed  time. the  focal  axis  radiation  a delay  perturbation  words,  time.  the  the  a much l a r g e r In  transient  displayed.  position  beam f o c u s e d a t This  are  vertical  different  with  Figure  the  the  at  occurs  from- the  be o b s e r v e d  multiplier  that  behavior  maximum i n t e n s i t y  along  times  taken  at  after the  the peak  as the  along  firing of  the  25  • The o b s e r v e d plasma  as  emission  shown  profiles  accomplished across  in  the  spot at  are  any  shown  II  in  positions  that  the  This  perturbed  at  10.  the  the  the  and  obtained  centre  e m i s s i o n at  of  for  the focal  observing  region  the  is  not  the  and  centre  plasma was  intensity  radial  unfolding  perturbed  perturbed  an a b s o l u t e  into  The p r o c e d u r e o f  across  obtain  the  perturbed  region  The r e s u l t s  emissions  p o s i t i o n of  time  unfolding. the  Figure  across  9 can be c o n v e r t e d  plasma.  u n f o l d i n g to  other  each  across  entire  different  later  Figure by A b e l  only  c o n t i n u u m and Ar  intensities  or  followed  measurement  was  performed. The d e p e n d e n c e o f of  the  Ar  11 4806 A l i n e  and on t h e  laser  power  12 r e s p e c t i v e l y .  upon the  density  e m i s s i o n on the  in  13 f o r  One s e e s on t h e  that  dependent  on the  ionization density.  it Only  three  the  transient  is  1968)  of  e m i s s i o n of degree  of  pre-ionization  shown  in  Figures  this  case  previous it  was  of  increment  pre-ionization  laser  the  the  the  energy  laser  continuum is  function of  low  results  found that  of  the  11 and  of is  the  shown  density  strongly  At  low  laser  preenergy  pre-ionization is (Nodwell  the  densities.  energy  p re-ion i zati on.  a nonlinear  in  where  different  influence  ment o b t a i n e d w i t h Kamp,  of  are  degree  intensity  degree  The d e p e n d e n c e o f  continuum Figure  t h e maximum t o t a l  and Van  perturbation  agreeder in  the  26  2.0 JET  FLOW IX  1.5 ®  I \ ' \ / \ / \ / \  ®  I i  "  1.0 i  ;  /  I  /  i \  /  ,  t  /  @ \ X  m  i  /  v  \  •7 r  /  I  i  -0.8  v \  »  i f  0|  \  » i  * x  :  -0.4  \  •  i  •  •  0  \ \  V  \ V  i  0.4  0.8  POSITION IN T H E P L A S M A ( m m ) FIGURE 9 Intensity the t  jet  of  axis  = 30 nsec  continuum at  t  =-10  ( V ) , and  t  emission nsec  at  (0),  = 100 nsec  5000 A a l o n g t  = 10 nsec (+).-  (©),  27  T I M E ( nsec ) FIGURE Transient of  the  emissions  perturbed  10 from  region.-  the  plasma  at  the  centre  28  FIGURE Intensity  in vs.  the  Ar  degree  11  II  4806 A l i n e  emission  of  pre-ionization.-  29  10  20  30  LASER POWER DENSITY CGW cm~2 ) FIGURE 12 Intens i ty vs .  in  the Ar  laser  II  power  4806 A density.  line  emission  40  30  2.0  o 4 0 GW c r r r V 2 0 GW cm-2 © 10 GW c m " 2  I  -»-»  2  9  A  1.5  E  V  ro i  E o  1.0 X7  CO  u i-  0.5  0  V  4 1 0  10  20  DEGREE O F  30  vs.  in  the  degree  50  P R E - I O NIZATIO N  FIGURE Increment  40  13  continuum of  (%)  emission  at  pre-ionization.-  5000 A  60  31  4.5 o  t = 5 nsec  ®  t = 5 0 nsec  v  CO I  u 3.0 T  O <  <  or  2 1-5 Z> Z h-  z O  o  0 4000  5000 WAVELENGTH ( A ) FIGURE  Visible  6000  spectrum  of  the  14 increment  emission.-  in  the  continuum  32 continuum e m i s s i o n from the increase  up t o  increased  rapidly  ionizations, able  in  a laser  the  power.  observed  over  of  showed  20 MW,  power.  At  little  that  high  pre-  density  obtainin  the  not  depend a p p r e c i a b l y  on  the  The t r a n s i e n t  continuum emission  has  the  entire  visible  the  it  change  does  pressure,  very  but  maximum e l e c t r o n  atmospheric  continuum emission laser  power  beyond t h i s  near  Ar a t  plasma  spectrum  (see  been  Figure  14). 111-c  Expansion  of  the  The focus  with  speed o f the as  perturbed  plasma.  perturbation  expands  a velocity  sound i n  velocities observed  of  in  approximately  the  up-flowing  laboratory  twice  same  perturbed  Figure  in  of  p l a s m a as  the  1  plasma front  that  of  frame  of  of  between  This front  stream  assumes relative  a function  fronts  reference,  the  flow  that to  gives  the  the  stream  The d i a m e t e r  of  time  the  is  of  plotted  15.  t=20  nsec,  velocity  sec" .  laser  and d o w n - f l o w i n g  both d i r e c t i o n s .  The v e l o c i t y and a t  the  times  The d i f f e r e n c e  velocity 1  flow the  the  700 m s e c " ) .  velocity  the  several  medium.  propagation is  is  the  the  (approximately  which  away from t h e  of  Since is  near  the  the  the  the  speed o f  front  end o f  radially  approximately  moves  of  varies  the  with  laser  pulse,  expanding f r o n t  sound i n  the  1700 m s e c " ,  w i t h a Mach number o f  1  3.7.  time  is  6300 m  pre-ionized  this As  expanding the  in  33  20  40  60  100  TIME (nsec ) FIGURE Diameter  of  the  15  perturbed  plasma as  a function  of  time.  _2  The  incident  vertical  bars • of  laser  power  represent the  mean o f  density  is  a typical eight  40 GW cm  standard  readings.-  . The  deviation  of  34  perturbation strongly  expands  attenuated  maximum d i s t a n c e 111-d The p o p u l a t i o n . The of  Ar  of  the  the  II  ions  0.08  some e x c i t e d  intensity of  these  an  idea  of  total  plasma  I'(t) v  is  =  ;  the  of  the  the  states  is  P-  A p,q  level  known  equilibrium  per  p to  to dis-  temperature  unit  level  solid q in  an  1966)  n (x,t) p  If  temperature.  radiated  (Cooper,  line.  are  excitation  electron  states-  measurements  emitting  from l e v e l  4TT  upper  the  II.  from  a Boltzmann  energy  a transition thin  of  excited  near  could  n  spot.  of  an e s t i m a t e  where  to a  population density  tribution,  optically  focal  is  Ar  have been d e t e r m i n e d  The  it  observations  cm from the  excited  have a d i s t r i b u t i o n  in  the  plasma  of  populations  angle  ambient  states  absolute  give  the  limiting  of  of  into  dx  "  population  (5)  density  and A „  P is  the  p,q  transition  determination yield p,  the  „  of  total  integrated  probability I(t)  in  for  absolute  number d e n s i t y  along  the  line  of  of  the  transition.  units ions  sight.  will  in  excited Using  A principle  in  state  equation  (5),  the  the  upper  as  populations levels  a function  The A b e l  of  of  at four  time  unfolding  the Ar  centre  11 l i n e s  from t h e  has  of  ing  is  performed  under  the  of  the  perturbed  plasma  is  a reasonable  laser  in  the  To measure was to for  opened t o the  admit  absolute  by DeVos  for  (1954).  are  level  to  is  kT = 1.7  e V but  clearly electron  not  very  in  measurements of  close the  by t h e  of  the  the  the  line  uniform-  of  the  monochromator  is  calibrated  a tungsten the  statistical  weight  have plot of  line  at  t =  with  times of  would  among  distribution  Any d e t e r m i n a t i o n  of  the  line  obtained  earlier  given  Ap  equilibrium distribution  at  ribbon  values  The l o g a r i t h m i c  a Boltzmann  5).  intensity  system  with  This  Figure  probabilities  distribution  using  expansion  firing (see  The d i s t r i b u t i o n to  the  1 6 , where a s t r a i g h t  equilibrium.  temperature  I(t).  symmetry.  tungsten  (1963).  a Boltzmann  100 n s e c  the  slit  of  This  view o f  The d e t e c t i o n  Figure  populations.  to  exit  The t r a n s i t i o n  in  in  perturbation  emissivity  divided  shown  prior  (> 95%)  from O l s e n  populations  correspond the  the  make  the  most  intensity  been o b t a i n e d  levels  I(t)  that  a cylindrical to  the  photomultipiier.  lamp u s i n g  the  has  density of  intensities  (1961).  assumption  assumption  region  obtained  unfold-  by B o c k a s t e n  electron  of  using  given  the  focus  been p e r f o r m e d n u m e r i c a l l y  coefficients  of  laser  have been  observed  the  ity  the  is the  e m i s s i o n d u r i n g the  time  1012  o t = 3 0 nsec t = 10 nsec  i  E u Q.  °  10 1 1  t= 0 nsec t =100 nsec  cn \ QL  2  -  10 10 19  <  <  <  CO O 00  00 CO  ID CO  <  <  <  t = -10 nsec 20  E  FIGURE Population energy the  of  density the  statistical  of  levels.. weight  (eV)  p  21  16  four  Ar  II  energy  The p o p u l a t i o n s of  22  the  level.-  levels are  vs.  divided  by  37  that  the  laser  erroneous Ill.-e  Electron  pulse  is  present  would  obviously  resuIts. density  from c o n t i n u u m e m i s s i o n .  The c o n t i n u u m e m i s s i o n a r i s e s free  and f r e e - b o u n d  electron in  the  transitions  number d e n s i t y  visible  dependence  is  region rather  and  is  and e l e c t r o n  of. the weak,  the  product  of  the  line  sight;  rt  of  f \ •f•^*r ^ v "t" ^ -j \ A j l. y I I ^ ^ /\ j U / r  the  ^\i IllWJf  ,  and  ion  spatial  of  temperature.  Since  temperature  measurement  be used to  densities  of  determine  integrated  dependence of  /-> rl ^ + A » " v. A Kw t-/V- 14 C. U U I III I H C U U J  4- l-> o v> UIICII  free-  a function  an a b s o l u t e  c o n t i n u u m e m i s s i o n may t h e r e f o r e electron  from both  s p e c t r u m the  the  1 1  give  the  along  product  c"1 o p u u i u i  « i *-> UI I ~  f o l d i ng . Free-free the  radiation  transitions,  emitted  from a f r e e  an e x t e r n a l  field.  In  part  energy  of  its  kinetic  section  for  this  electrodynamics motion  of  or  the  transition by s o l v i n g  process  bremsstrahlung,  electron the  and slows is the  electron  down.  obtained  in  a hyperbolic  As a r e s u l t  of  free-free  The  from  mechanical  an. e l e c t r o n  moving  is  in  loses cross  classical  problem of  orbit  around  an  ion. i .• energy volume dfi i s  emitted per given  in  unit by  the time  frequency into  (Finkelnburg  the  transitions  interval element  and P e t e r s ,  dv  of  per  solid  1957)  the unit angle  38  ff j ' '  16TT  dvdft =  :  3 /3" m  v  c  2TT  3  exp  rn kT e e  hv kT  (6) x  where Z i s perature  and e l e c t r o n s  of  the e m i t t e d In  along  give  the t o t a l  j^ (t) f  n^ and n  g  respectively,  to a p p l y  the tem-  g  t h e number and v t h e  this  equation  densities  frequency  to a  p l a s m a , we have to i n c l u d e  dependence  tion  o f the i o n , T  radiation.  order  non-homogeneous  dvdft  charge  of the e l e c t r o n s ,  ions  We t h e n  n e  i  the e l e c t r i c  of  spatial  n.  o f n^ , n  the l i n e  g  of s i g h t  contribution  and T . g  transient  t h e time and  Then,  an  integra-  s h o u l d be p e r f o r m e d to the e m i t t e d  to  radiation.  obtain  dvdfi =  1 6-rre  1  3/3" rn c e  3  exp  2iTm.  hv k  T (x,t) e  X (7)  k  T (x,t) e  n.j(x,t)  n (x,t)  dx dv dfl  39  Let. us c o n s i d e r tinuum r a d i a t i o n culated  in  consider  but  transitions lower  of  But  free-bound  consi der  well  given  electron  contribution arises  is  the  final  in  then  on the  or  interval.  number o f  density  To o b t a i n  of the  a complex a t o m i c  performed  two  stages.  for  modified  to  this £,  take  into  purpose S c h l u t e r  into  the'expresion  hydrogen. in  an h y d r o g e n  atomic  The f a c t o r structure  frequency  and Norman v,  and  in  for y/U.  cal-  free-free  or  with  atom. electron one a t  provided is  electron levels.  is  like  a more complex two  section  into  Ar,  are is  first  equation  is  structure. factors,  account  the  a  the  emission c o e f f i c i e n t  takes  into  within  coefficient the  has  captured  encountered  and then  the  positive.  The c r o s s  system,  a  that  r e c o m b i n a t i o n , one  introduces  the  is  For  ^j— and  for difference  between .the., compl ex atom and h y d r o g e n .  The s e c o n d c o r r e c t i n g Biberman  system,  (1965)  con-  one must  another  state  The e m i s s i o n  account  from  ion  possible  the  emission c o e f f i c i e n t  for  obtained  to  levels  calculations in  the  to This  because  one d e a l  state  transitions,  finite  of  orbit  t r a n s i.ti oris where the  defineaand  depends  the  collisions  Any f i n a l  energy  to  as  from one h y p e r b o l i c  the  contribution  complex, s t r u c t u r e  free-free  energy.  in  way  some c o m p l e x i t y  now the In  the  from f r e e - b o u n d t r a n s i t i o n s .  a similar  transitions  next  factor  (1960),  £(v,T), depends  g e n e r a l , on the  first on' the  introduced element,  temperature  T  by  the  . • The  40  radiation atomic  j  T  emitted  structure  by f r e e - b o u n d t r a n s i t i o n s s u c h as A r i s  dvdft =  D  3/3" m  v  c  then  g i v e n by  hv kT  1 - exp  2ir rn k T e e  3  i n a complex  (8)  X  where  IK (T)  factor in  [1  is  of  ?(v,T)  )  the p a r t i t i o n  caused  Ar are given interest,  small  T  - exp(-hv/kT)]  absorption  for  yJ  n.  n  e  dv d^  function  accounts  by i n d u c e d  emission. In  dependence  n^ , n  t h e time and T  g  j^ (t) b  g  and s p a t i a l  into  dv d'R =  equation  dependence (8)  3  o f £ and y region  of £ ( v , T )  very  is  Taken  o f the  into  variables  we g e t :  16 e' 3/3" m c e  Values  decrease  the s p e c t r a l  and £ can be assumed t o be c o n s t a n t .  account  and t h e  f o r the e f f e c t i v e  by Schiu.fjzr (1 968) .  the t e m p e r a t u r e  f o r the i o n s  hv  1 -exp [ k T  2irm.  (x,t)J (9)  k  T (x,t) e  U (T  (x , t ) 7 5 ( v , T ( x , t ) ) e  n..(x,t)  n (x,t) e  dx dv do,  41  The  total  continuum emission  equations  (7)  is.then  -jj (t) f  v  [k  2Trm,  which  gives  absolute is n  very  a  p  relation  intensity j  .  :  small  i* e n  n  e^"  b  Y U(T)  T (x,t).  X n..(x,t)  jj (t)  +  16 e" 3  by t h e sum o f  and ( 9 ) :  j (t)  3/3m c  given  n (x,t)  between Since  hv' f kT  1 -exp  {»'  n  n  e  »  ^ "'and t h e  t h e number d e n s i t y  since  equation  use  to determine  density  that  we have  n o t measured T „ .  of Ar  (10) i s  temperature,  the e l e c t r o n  total III  the product  d e p e n d e n t on t h e e l e c t r o n  fact  J  (10)  weakly it  Y?  dx dv dfi  i n o u r p l a s m a we c a n r e p l a c e furthermore,  U(T))  1  we c a n  in spite  Because  only still  of the  of this  weak  e dependence is  rather  As  a lower  firing  on T  g  small, limit  the u n c e r t a i n t y even  if  for T  o f the l a s e r .  1  Q  i n the e v a l u a t i o n  is estimated  we t a k e  only  i t s value  As an u p p e r  limit  of n  very  prior  we, t a k e  g  roughly  to the the  value  42  that by  T  would  g  the  plasma  electrons. to  the  j^(t) as  have  been  had gone  into  Abel  a function  error  unfolded of  emission of  the  have  spectrum.  ment a t  the  a function the  laser  of  The  12,000 ° K ( k T  = 1.1  at  two  across  the  different  pulse  is  (see  its  frequencies..  perturbed  time  even  nonlinear electron mum o f  sharp  The  faster  density.  the, l a s e r  of  is  the comparabl  intensities the  the  emission  continuum  different obtained  plotted  correspond  to  and  the  regions  from  in  measure  Figure the  c o r r e s p o n d to  near  This  reason than  This pulse  the  of the  this  rise  the  is  an  17 as  centre  of  electron  lower- t r a c e  of  electron  shown  decay  feature  for  the of  the  feature  peak  of  region  during  7).  dependence  density  distribution  times  Figure  value  several  trace  of  eV).  An i n t e r e s t i n g emission  obtain  absorbed  plasma.  °.K(kT = 1.8eV)  The s p a t i a l density  the  values  upper  21 ,000  to  5000 A a r e  These  energy  The o b s e r v e d  absolute  of  the  so i n t r o d u c e d  g  inside  The e l e c t r o n  focus. of  n  performed, in  time.  all  numerically  in  wavelength  temperature to  been  in  (10%).  position  Measurements  if  trans1 ationa1 energy  The u n c e r t a i n t y  experimental are  reached  the  of  in  the  transient  spike  laser  at  with  pulse  c o n t i n u u m e m i s s i o n on  sharp made i t  peak  e m i s s i o n near  impossible  to  18  continuum  the  observed  sharp the  Figure plasma.  maximum o f is  number  use  laser all rise is  the  the the  maxi-  laser  43  ol  i  _ i  0  -50  50 TIME  :  perturbed  number d e n s i t y  plasma  as  a function intensities  Ar  (0).-  line  widths  17  at  continuum a b s o l u t e II  J 150  :  100  I 200  ( nsec )  FIGURE Electron  _ i  '  the of  centre time,  (solid  of  the  obtained  line)  and  from from  44  10  LU  —  —  j  t = 10 nsec t = 30 nsec  0 -0.6  -0.3  0  POSITION IN  THE FIGURE  Electron  number d e n s i t y  determined  wavelength  of  0.6  PLASMA (mm) 18  across  from c o n t i n u u m  the  0.3  the  intensity 5000 A . -  perturbed  region  measurements  at  45  scattering transient laser III-fLine  as  a diagnostic  electron  p u l s e was shape  of  density  Ar  II  jet  at  action  is  due  trons  and  emitted is  the  line  atoms, and with  fields  electric which  spectral dominant  by  cause  line. Stark have  If  slowly By  varying  measuring  obtain atoms  the are  function  the  In  strongest  Stark  the  broadening with  of  line  interelec-  the  electrons  shape w i t h  emitted  half-half  1966)  of  width  n  e  parameter the of  Q, i s  electron these  number d e n s i t y  pulse.  shape  several  To o b t a i n  are  the  generally  temperature  lines  one  where t h e  of  several  times  Ar  during  II  T .  emitting  lines  and a f t e r  good s t a t i s t i c s  a  can  submerged.  been measured a t laser  line  electron  The  of  the  plasma  p r o d u c e d by  a Lorentzian  Broadening  the  the  b r o a d e n i n g m e c h a n i s m , the  (Cooper,  Stark  in  interaction  e  the  that  particles.  to  w =.n(T )  where  other  pressure,  ions,  given  time  ions  atmospheric  s h a p e would  width  the  measurements  o r  lines.  by c o l l i s i o n s  plasma  during  f  present.  The e m i t t i n g perturbed  technique  no l e s s  have the than  46  five  measurements  lengths in  taken  Figure  across  21 where  4880 A i s  plotted  line  shape  with  the  line  each  for  using  line.  the  of  instrumental  actual  optical  tion  for  because the of  the  width  only  the  the  been  is  II  actual  deconvoluted determined  In  Figure  19  for  an  shape o f  the  instrument  the  spectral  triangular  not  observed  lines  shown  Ar  the  very  lines  narrow  observed  line.  in  monochromator.  width  of  are  A.  is  shape  The  because  correc-  significant and  during  affects the  decay  plasma.  during  the  Figure  21).  appears plasma  time  that  quickly  of  the  width  test of  the the  observed  the  and the  decays.  line  line  Immediately  mination  metry  of  when  Every  to  the  instrumental  the  results  in  shape  has  15 wave-  width  for not  the  result  these  0.36  line  is  of  To o b t a i n  line  shape  shape  aberrations  the of  line  of  expected  and a s s u m i n g a L o r e n t z i a n The  line  which  a width  each  A typical  values,  profile,  the  at  = 10 n s e c .  observed  and has  calculated  taken  experimental t  instrumental  width,  been  the  from the  experimentally we have  have  Since  the  a basic  line line  that for  of  lines  symmetry.  we c a l c u l a t e  the  the line  f r o m the are  (see  asymmetry reduces  disas  the  the deter-  measurements  symmetric,  As a measure the  asymmetry  present  a s s u m p t i o n made i n  density  the  a strong  p u l s e was  afterwards, width  electron is  laser  showed  skewness  of of  we the the  of  need asym-  47  0  0.3  0.6 ACTUAL  0.9 LINE  WIDTH ( A )  FIGURE Effect the  of  the  observed  19  instrumental width  of  1.2  broadening  a Lorentzian  on  line.-  1.5  48  distribution the  third  of  experimental  moment of  the  skewness  as  a function  is  in  Figure  shown  the  value  of  n  the  width  of  the  symmetric  determined  densities Ar  II  the  and  are  Stark  to  of  Figure is the  also two  are  the  in  time  of  lines  Figure  in  gradients  present  only  Stark  shift  of  in  4880 A disappeared  time  from  the  Ar  can  the  Stark  The  II  electron  4806 A and  The l a r g e r  from the  uncertainty  uncertainty  deviations  size of  n  of g  line be  error in  in  in  the  these  (Roberts,  the  error  1968)  bars.in  from c o n t i n u u m  emission  17 and a good a g r e e m e n t  pulse  inside the  line.  has  the time  agreement  the  using  the  between  obtained.  during  was  II  density  14% r e s p e c t i v e l y  Figure  laser  density  asymmetry  this  17.  The s t a n d a r d  50% o f  is  the  using  of  times  (1968).  The asymmetry, o b s e r v e d the  line  has  of  later  line  and from the  18% and  presented  at  Roberts  The d e t e r m i n a t i o n  methods  Ar  asymmetry  electron  from t h e  lines  represent  17.  line  a function  the  contributions  coefficients.  they  the  that  of  by  plotted  the  coefficients and  width  determined  due  A plot  of.the  as  20 shows  so o b t a i n e d  width  After  therefore  4880 A a r e  bars  time  the  line.  from the  coefficient  of  obtained  Figure is  across  distribution.  20.  is  g  points  been  in  the  attributed  perturbed  region  of  the  laser  with  the  sign  By  lines  a s s u m i n g the  to  the  line  electron  which  pulse. of  during  are  This known  shape  for  FIGURE Third as  moment o f  a function  of  Ar  II  20 4880 A l i n e  time.-  shape  50  the  Ar  with for  II  line  appropriate each  have  generated  line  density the  of  the  the  of  The  the  of  point  shape of  laser  transition  calculations.  22 the  compared to  subsequent  times  deviations  found f o r  lines In  4880 A i s  plasma.  The  taken  of with  at line  laser  the  pulse.  either  100 n s e c  blue  line  narrower  and  in  from  Figure  the  18.  upper account  shapes line  shapes  are shapes.  for at  plasma,  red  shape  later,  electron  Ar  II  two immediate!  bars  A good  or  the  the  into  Error  we  of  points  the  mean v a l u e .  23 the  is  line  line  of  region,  along  of  experimental  decay  shift  plasma  shown  experimental  the  the  Figure  plotted  the  during  the.maximun of  standard  as  been  calculated  and  be o b t a i n e d  population  also  the  the  can  The so g e n e r a t e d  and compared to  F i g u r e s . 2 1 and  after  has  Lorentzian  by summing  distribution  the  be  perturbed  regions  the  of  to  of w i d t h  the  line  spatial  time  4880 A a r e  the  inside  expected  distribution  the  normalized  is  coefficients  from d i f f e r e n t  during  given  continuum e m i s s i o n measurements,  level  In  any  density  sight.  The s p a t i a l  in  at  Stark  electron  contributions the  emitted  denote  agreement  shift.  for  the  same Ar  during  the  decay  symmetric.  of  II  51  4876.5  4873.0  4883.5  4880.0  WAVELENGTH ( A ) FIGURE Shape o f predicted the  Ar  II  4880 A l i n e  shape  (solid  electron  density  21 a  t  t  = 10 n s e c .  1ine)-is  distribution  continuum  emission.  computed obtained /  The based from  on  52  4873.0  1  4876.5 WAVELENGTH  of- Ar  1  4880.0 (A )  FIGURE Shape  I  1 1.4880 A l i n e  4883.5  22 at  t•= 30  nsec-  53  4873.0  4876.5 48800 WAVELENGTH ( A ) FIGURE  Shape  o f Ar  II  23  4880 A l i n e  at  t= 100 nsec  4883.5  54  Chapter  IV  ENERGY ABSORPTION FROM A LASER BEAM IV-a  Introduction In absorption beam i s of  beam i n  Although the  is  order  we do not  of  of  plasma  !  of  its  the  last  is  one  light the  light  energy  may s c a t t e r ,  tion s o f  is  refract, beam.  one which is  to  at  energy  absorbed.  determination  compare w i t h ,  (inverse  bremsstrahlung)and  the of  beam i s  fired  into  lost  f r o m the  and a b s o r b  achieved in  in  the  thesis.  a plasma The frac-  processes, state  free-free  to  process  beam.  the  bound-free  A b s o r p t i o n due  an  different  three  perturb  of  transmitted  point  will  Absorption  measure  this  Ofthese  plasma.  (photoionization) .  the  the  section  and a d i s c u s s i o n o f  interest  When a l a s e r a fraction  and  the  fraction  (see  have an e x p e r i m e n t a l  energy  is  the  a reliable  incident  determine  discuss  p u l s e , when  small  get  absorbed energy  this  absorption  to  the  to  to  Because  relatively  been a b l e  like  laser  a plasma.  between  amount o f  estimate of  not  I would  from the  into  absorbed  difference  laser  chapter  energy  we have  11-c)  of  of  focused  energy  the  this  of  the the  transitions  transitions  resonance  or  line  55  absorption place is  in  in  a p l a s m a when a l a s e r  the  the  processes  tion  are  effects the  sity  is  to  laser  cerned  the  any  not  take  beam from a  laser  beam i s  and  and an  ion.  fulfil  the  ruby  the  cross  1962)  we may  process  presence  f f  (t)  in  energy  nonlinear  electric  photon  beam t h a t  field  number we a r e  discussion  IV-e  dencon-  would  property  section  absorption  of  a  where  of  for the  = a  f  f  coefficient.  refers  to  collision  laws  section  v  total  IV-e  The c o h e r e n t only  chapter  discussed.  calculate  K  The  this  and p h o t o i o n i z a -  high  following  beam.  conservation  Using  the  of.the  the  are  of  the  section  of  a three-body  The  IV-d  discussed.  atomic  in  in  presence  bremsstrahlung  a photon  section  considered  effects  sections  bremsstrahlung  property  light  This  1  In  although  to  nonlinear  the  only  apply  of  inverse  beam i s  with,  Inverse  of  following  calculated  due  is  two  discussed.  absorbed  IV-b  light  does  used. In  of  bound-bound t r a n s i t i o n s  the of this  ion  with is  energy  n,(t) i  absorption an  electron  required  to  and momentum.  collision  absorption  n (t) v e  the  a^  f  (Spitzer,  coefficient  :  (1)  56  where  a  ff v  3  e k T ( t )  h c m  3  / 2 v  3  9ff  e  and where g . ^ of  the  is  electron  electron  the and  during  the  of  energy  produce tion  length  time  of  factor.  have been the  that  we can  f r o m the  taken  electrons  the  laser  ionization calculate 1aser  The t i m e  number d e n s i t i e s ,  is  a b s o r b e d from trie  further  (1)  ion  temperature  number d e n s i t i e s  the  Gaunt  of the  1ight  into  and  ions  will  beam  is  AZ v  K  per  pulse:  ff V  (t)  L(t)  n (t)  n.(t)  dt  J o  e  L(t)  part  used  Using  absorbed  The  vary  t  ff f A E> )  as  because  plasma.  energy  well  account.  present  light  the  as  dependence  dt  to  equaunit  57  = - 3.6-9 x 10  n (t)  8  n.(t)  e  v  3  is  duration  intensity  L(t),  under  effect  the  the  not  is  also  space  very  lens  only  a region  of  the  order  plasma  can  be  absorption  larger  than  its  free.  This  process  and t h e 15.68 or  ions  1.78  in  Ar  I  an e x c i t e d  near of  If  focusing  of  plasma  the  plasma  n  since  and n..  g  L(t) lens  the  focus  1 mm),  the  transition  of  is and the  inhomo-  ionization  the  a result an  laser  are  whose  is  only  much l a r g e r Ar  II)  energy  potential  only lays  would  of  energy  electron  photoionization.  2 7 . 6 2 eV f o r state  as  acquires  and  a ruby  potentials and  beam.  But  the  electron  called  a photon of  eV from the  ionized.  is  length  coefficient.  b i n d i n g energy  ionization  eV f o r  the  the  time  neglected.  a bound-free  a photon a b s o r p t i o n ,  of  of  a region  pulse with  dependence o f  account.  the  During  energy  spatial  because  in  laser  light  of  Photo i o n i z a t i o n  the  the  and A Z i s  into  dependent  large  (in  genity  be t a k e n  of  laser  h o m o g e n e o u s , the  should also  of  (10)  e  dependent  is  IV-c  the  dt  [T (t)3  o  where  L(t)  escape Since 1,78  eV  (e.g. atoms within  become p h o t o -  58  Atoms photoionized or  more  with  lower the  photons.  atom a b s o r b i n g field  at  simultaneous  However,  corresponding  The d e p e n d e n c e atoms  small.  in  For  to  ruby  laser  on the  the Ar  probability  the  (Gold  ~ F  the  and  = 1.1  of  P(n)  F  be  two of  when i n  density  is  this  an  the is  of  very  sharp,  probabi1ity  ground  Bebb,  also  .  absorption  P(9)  flux  n  state  can  absorption  n photons  photon f l u x  in  states  probability  a photon  ground  atoms  for is  the  simultaneously  P(n)  for  excitated  state  9 photons  F  3 3  is  very  n=9, and  1965, Morton,  x TO"  and  from  the the  1967):  9  -2 where  F is  fluxes  in  would  and we can  GW cm this  tion  tron)  state,  moves  nucleus  dimensions  of  by  the  the  at  extremely  have to the  In  orbit  the  remaining  system  of  into  process  electron the  of  atom (or  account  field  photoioniza  in  a  strongly  valence  elec-  produced  electrons.  charges  photon  discussion.  a complex  in  large  be t a k e n  present  consider  "optical"  a large  and  in  photon.  the  in  it  then  by a s i n g l e  excited  the  us  Only  process  neglect Let  .  forming  If the  by  the atomic  59  remainder orbit can  of  are the  not  very  optical  be r e p r e s e n t e d  Coulomb  field.  results  obtained  these  to  The  tive  capture,  by  Once  that  cross  known,  (see  e.g.  (or  the  16  TT  e  I  is  tion of  (3),the  the  and all  4  for  and  an  v  (p)  of  the  1966)  '  exp  3  process,  v.  been  kT  atom  in  be  after  of a level  calculated integration  hv' kT  -1  (3)  J  and  order  to  n  Q  the  obtain  among t h e  assumed  different  balance.  can  exp  potential, In  radia-  levels:  r  distribution  has  using  extending  absorption  possible  atoms.  by  hydrogen-  detailed  coefficient  Tn  c  the  of  of  a  the  k  2  levels  n(p)  atoms  reverse  Raizer,  ionization of  producing a  v by an h y d r o g e n - 1 i k e  Boltzmann  atomic  populations  h  the  number d e n s i t y  Z  6  3 /3~  where  from the  absorption  system  simplified  correction  section  over  2  is  whole  the  atoms.  principle  Zel'dovich  summation)  bf  the  frequency  p is  problem  the  hydrogen-1ike  obtained  the  of  charge  absorption  atom  photon  a point  more complex  like  comparison with then  as  for  in  electron,  Then the  results  is  large  to  levels  total equa-  population  relate  the  to  number  the  60  density ibrium  of  atoms  the  equality plasma  Boltzman  (Wilson,  is  then  energy  replaced  the  by  W i i c r i c  II  l o  (.tie  can  under  thin  is  the  "  u u o u i  thermal  the  free  by an  in-  that  the  electrons  The B o l t z m a n n  E  p  U (T) o '  e  x  p k T  v  I I . U I I I U ^ I  e  u . c i i o i . u y  o n e  ut  IM o  .  .  electron  for  the  temperature,  the  an u p p e r  tained.  system  value  of  Furthermore,  structure  of  by  been  introduced  with  the is  is  U  the to  not  Q  ( T ) the  the  take  partition  thermal  function  in  equilibrium,  a b s o r b e d would  into  account  atoms,  factors  previously  in  energy  non-hydrogenic  multiplied  obtain  and  atom. If  U.j(T)  • ~>  e  the  is  have  equation  o  only  equil  inequality:  9  o  in  assumptions  distribution. the  not  be r e p l a c e d  and t h a t  n(p) n  system  equation  1962)  optically  a Maxwellian is  n . If  the  the  section  more  absortion  and £ ( v , T )  Y/U.(T)  be o b -  111 -d i n  complex  coefficient which  have  connection  continuum e m i s s i o n from f r e e - b o u n d  transitions.  the  ions.  for  equ i p a r t i t i on f u n c t i on f o r  the  emission  coefficient  the  the  inequality  We  then  61  16-rr  e  2  K  b f  (t)  <  6  3 / 1  k T (t) n (t) £ 5 h c v 4  exp  :  kT (t)  exp  e  hv kT (t)  -1  e  (4)  Y  where  the  time  dependence  of n  Q  and T  g  is  taken  into  account  2 The  factor  Z  have  been  s e t equal  tion  from  the atoms  ions  give  a contribution  because  of  the  have  larger  to 1 s i n c e  t o be t a k e n two o r d e r s  ionization  a b s o r b e d by p h o t o i o n i z a t i o n  plasma  is  given  AE AZ  16TT  2  3 V3  e  6  4  3  bf  K  U(T(t))  X  of magnitude  per u n i t  The  smaller  The maximum length  in the  by:  k  h cv  contribu-  account..  potential.  energy  then  into  only  T(t)  b  f  ( t )  £(v,T(t))  n (t) o  L(t) dt  exp  L(t)  dt  kT(t)  exp  hv kT(t)  (5)  62  In  this  equation,  the  of  time  n (t)-can  be o b t a i n e d  electron time  density  that  diffusion of  the  pulse,  the of  we  n (t)  laser  number o f  which  e  atoms  laser  total  beam i s and  from the  as  time  a  function  dependent  have measured d u r i n g present.  ions  beam d u r i n g  atoms  f r o m or  a time  of  If  we n e g l e c t  into the  the  the  order  the  focal of  region  the  laser  have n  o  (t)  + n.(t) i l  = N  = N  n (t)  T  = constant  or n (t) 0  -  T  e  since "  -d E n e r g y  a b s o r b e d by  amount o f  into  V ^;  " j l M  ~  the  plasma.  now compare  the  contributions  energy  a b s o r b e d by  the  processes  the  and p h o t o i o n i z a t i o n .  effective  account,  the  absorption  equations  by  1 - exp(-hv>/kT) . The  (5)  have been p e r f o r m e d  ent  electron  The e l e c t r o n the  laser  •  We can  bremsstrahlung reduces  e  density  and  numerically  obtained  Because  is of  Induced  coefficient;  integration  temperature  pulse.  (2)  of  (5) of  must  inverse emission to be  take  the  this  multiplied (2)  time  and  depend-  continuum e m i s s i o n .  considered a constant the  the  equations  using  from the  to  during  weak d e p e n d e n c e on  this  63  paramet  r,  results  for  have  been  argon.  the  assumption  the  obtained  Y /U^  is  For the  focal  (2)  and  purpose  of  our  not  from S c h l u t e r  a 0.9  joules  of  0.06  affect  appreciably  discussion.  approximately  volume  (5)  does  (1968)  the  The v a l u e  and  is  ^=  of ^  2.8  for  one.  laser  pulse  cm we o b t a i n  and a l e n g t h from  of  equations  respectively:  AE  f f  =  0.24  10"  3  joules  (6)  AE  f b  =  0.17  10"  5  joules  (7)  and  K n u t ,  Of  i  «N ^  the  ~ , „ „ +. C J C H I,  two  a  laser  oo<" , u u »  a n u  mechanisms,  contribution absorbed  n u  by  the  total  the  two  mechanisms  of  0.9  in  Figure  during the  the  25.  absorption  of  first Ar  I  due  half  atoms  is  of is  done to  energy is  dependence inside  that  second h a l f  contribution the  plasma Note  J.u _ L u c  i . _ l a i  ^ _ . . - ! . d | j u I b e  gives  a  absorbed. shown  I C) j ( ,  negligible  The  in  e i i c  power  Figure  24  for  joules.  The t i m e the  „ r u I  photoionization  to  pulse  a b s o r b e d by  r> o n r>./>/!<" > j . u u u t - T / o  the  most o f  of  focal the  of  the  laser  by  the  new  still  laser high.  pulse  total  energy pulse  is  energy  volume  is  is  most  The  absorbed  when the  plotted  absorbed  since  electrons.  photoionization  the  the  of  small  during  number  density  64  0.24 j — X 10  ~r  5  -60  -30  0  30  60  TIME ( nsec ) FIGURE Power a b s o r b e d  by  and  bremsstrahlung  by  inverse  the  24  laser  plasma  pulse .-  ' by  photo i o n i z a t i o n  from a 0.9  joules  65  0.30 X 10~  3  0.24  0.18  0.12h  R  0.06  0 -60  30  0  TIME ( nsec ) FIGURE  Energy  absorbed  by  25  the,plasma  1aser  pulse.-  from a 0 . 9  joules  66  Results into  account  creased laser  from  pulse  absorbed density  its  initial  in  electron  laser  If  density  during  the  which  time  we c a l c u l a t e  the  electron  taking  density  that  the  into  during  has  in-  the  energy  beam c o n s i d e r i n g o n l y and not  taking  electron  account  the  laser  pulse,  the  obtain  &E  we compare  fore,  (6)  energy  the  f  • = 0.97  f  b  = 0.61  these  and  of  (7),  erroneous  joules,  6  joules.  5  with that  the  a large  a c c o m p l i s h e d by  perturbation of  values  of  the  the  obtained  fraction  of  bethe  new e l e c t r o n s .  plasma  An  based o n l y  p r e - i o n i z a t i o n would  give  on  obviously  results.  temperature  us  in  consider  the  shorter  Since than  random m o t i o n o f  the the  the  from t h e  now the  hypothetical  a b s o r b e d by  electrons.  ized  x 10"  results  density  Let  absorbed  x 10"  we f i n d  is  the  electron  energy  f  absorbed  estimate  is  present.  the  have been o b t a i n e d  value  pre-ionization  A E  If  (7)  transient  from the of  and  the  was  enhancement we  (6)  plasma  laser the  by a t e m p e r a t u r e  situation  remains  relaxation  duration  of  beam i s  electrons T  If  increase  time the  laser  converted which  the  can  energy  electron  where  only for  in  the  among the  electrons  pulse, quickly then  the  be  the  energy  into character-  a b s o r b e d by  the  67  electrons the  is  time  of  not the  temperature  laser  would  A(kT ) V m  transferee! pulse,  then  = i 3  K  = 1.44  to  the  the  be g i v e n  n  e  AF. A  x 10  kT  section  is  g  of  1  2 5  9  section  equation Kunze of  (1965)  a plasma  assure the  of  that  to  of  to in  laser  the  time  plasma  t  L <  jet,  L(t)  the  watts  the  (8)  laser  in  used  between  is  than  assumption and i o n s the  inequa1ity  assumption  that  still  the  i n cm  of  using  before  the In  by  perturbation order  contained  to  only  in  that  the  equi-  t.  (as  given  of  the  laser  valid  in  the  absorbed energy  is  con-  L  is  g  III  the  calculated  duration t <t.  cross  Chapter  from  experiments.  electrons  larger the  the  in  density  limit  the  the.plasma  been o b t a i n e d  upper  scattering  (8)  , and A i s  electron  have  the  Kunze makes  is  dt  temperature  absorbed energy  Although the  of  i  continuum emission,was  estimate  by S p i t z e r ,1962) pulse  obtain  the  in  electron  Equation  electrons,  partition  the  is  spot  electron  (kTj^  eV,  (8).  n  3  in  111 - e ,  measurements  V  ff  e  focal  in  during  by  (l-exp(-hV>/kT )) \  the  The e s t i m a t e  increase  X  L(t)  and i o n s  f f  "A  where  atoms  68  tained  only  in  necesarily present, to  the  electron  During  In  intensity  field.  The  by  (1964) field  and Rand  (1965).  begin  to  be i m p o r t a n t  which  a free or  exceeds  independently electron emit  the  acquires  of  peak  oscillates  the  of  when the can  not  pulse  important and to  is role  increase  amount o f  the  fields  energy  is  the.  of. the  radiation  have been  calculated  absorbed from  of  nonlinear  in the  the  energy  velocity  radiation  such  that  field  Then,  it  the may  field.  an a l t e r n a t i n g  and f r e q u e n c y . v ,  Albini  energy  velocity,  radiation  of  by  an  the  deviations  photons.  electron  inverse  no l o n g e r a  maximum k i n e t i c  obtain  energy  presence  peak  an  is  beam.  energy  that  initial  E  ,  laser  have been s t u d i e d  sufficient  value  with  a function  show  photons, modifying In  field  of  the  laser  coefficient  by a plasma  electron  the  play  radiation  rate  They  t.  L  electrons  cross-section  and t h e  radiation  equals  the  absorption  nonlinear  t <  pulse.  due to  i s . instead  that  can  the  of  m o d i f i n g the  effects  but  time  of  thus  laser  high  because  collissions  density,  bremsstrahlung constant  the  temperature  from the  Nonlinear  Rand  electrons  inelastic  absorbed  IV-e  true.  reduce  the  the  electric electron  69 e E rn  Nonlinear  effects  would  In  the  m  e  radiation  <  2TTV  become i m p o r t a n t  e  \  e  2  2  E  = r r - f — 8TT vz m  -  h  in  cm o f  wri t t e n  the  the  laser  focal  irr  = /?  o  laser  case  )  E  4TT  W  c  irr •  power  in watts  volumn.  Then,  V o l t , cm"  irr'  and r  is  condition  2  8TT  hv  :  3  m  (26.6)  = 0.112  4  due to  8  the (8)  radius can  be  as :  W  At  (  e  = 26.6  W is  •  v  field  E  where  when  energy  the  this  laser is  d e n s i t i e s < 10 beam can  completely  hv  3  m  -  - 2  GW cm  be n e g l e c t e d .  justified.  nonlinear In  our  effects  particular  70  Chapter DISCUSSION AND  In studied  the  the  the  beam i s  electron  and a b e t t e r been  thesis  focused  density  this  with  has  by the  of  final  this  light  thesis  emitted  been  satisfactorily  the  have  by a plasma  it.  of  The  we  into  this  us  plasma.  p l a s m a we  know t h a t  energy  absorbed w i l l  of  the  and  enhancement  transient  since  laser  determined  plasma  .has  upset  the  the  our  T .  balance  excitation,  and d e - e x c i t a t i o n .  its  kinetic  quickly  between  such as  discuss  c o n c 1 < • d <? the  of  energy  fired  energy  into  is  absorbed  bremsstrahlung.  The  energy  of  thermalize  (-j  we may c o n s i d e r  The change  collisions  and t o  beam i s  of  plasma)  on e l e c t r o n  to  absorption  light  the  like  work.  inverse  electrons in  plasma  a fraction  increase  picoseconds  I would  future  first  mechanism o f  have a t e m p e r a t u r e will  for  When the  by  order  perturbed  consider  mainly  electrons  chapter  suggestions  Let  ture  the  in  obtained.  some f e a t u r e s  to  in  understanding  In  the  CONCLUSIONS  presented  perturbation  when a l a s e r of  work  V  in  electron  processes  recombination, The r a t e  of  that  the n  the them  temperadepend  ionization,  these  processes  71  would  be m o d i f i e d  and  excitation.  the  degree  will  the  photons  of  The  the  that  laser  on the by  we  ionization are  still  collisions  have o b t a i n e d  Ar  pulse,  plasma.  to  or  ions  their  are  higher  Ar  I  At  high  electron  present  in  the  smaller  and c o n s e q u e n t l y  density  is  obtained.as  that  absorb  electron near  the  are  ionization  being  the  plus to  produced but  be  further  potential.  the the  number d e n s i t y increase  in  remaining  to  obtainable  from the  maximum o f  first  are  the  density  ionization,  ionization  a smaller  seen  time  laser  unlikely  plasma, p r i o r the  also  complete  number d e n s i t y  pre-ionizations  inFigure!3.  the  electrons  density  the  beam.  a time  very  the  to  New e l e c t r o n s  for  of  and c o n t r i b u t e  by c o l l i s i o n a l  source II  at  an a l m o s t  maximum e l e c t r o n . number  of  so t h e y  excitation  collisions.  produced d u r i n g  present  increase  levels  by e l e c t r o n  in  atoms.  the  upper  obtained  the  sum o f  the  increase  light  due  then  will  collisional  f r o m the  case  ionized  the of  ionization  ionization  absorbed  photoionization  I  and  favor  energy  perturbed  either Ar  is  to  collisional  free-free  amount  shows  the  pulse  in  a way  population  electrons  laser  total  The  favoring  New f r e e  such  ionization,  increase  atoms,  the  of  in  is  The  then  number  the density  laser  pulse.  of  I  in  continuum  Ar  the  is electron  radiation  72  the  laser  sity  After  the  injection  pulse  the  strong  disappears  decreases  quickly  and compared  ments  jet and  density higher tion to  the  laser  Ar  errors  and Van d e r  similar  than  the  ours,  from  lines. in  to  the the  electron  density  was  in  using  II  line  coefficients  line  from  electron  den-  electron  of  discrepancies obtained. this  (1968)  density  Although  in  that  absolute  by 25% at  the  have  laser  measureelectron  systematically  discrepancy  easily  argon  the  were  s p e c t r o s c o p i c m e t h o d , the could  an  spectroscopic  from t h e the  have m e a s u r e d . .  profiles  widths Ar  shown  II  of in  the  line  was  radia-  attributed •  perturbation  increased  power  output  4&06 A and Ar  and a r e several  well  the that  known.  other  Ar  II  of  the  lines  values  Since  the  c o n d i t i o n s : are that  the  II  been  from  performed  4880 A whose (see  for  Stark example  We have measured  the  suggest  density  17 have  others  in  would  electron  Figure  have been measured by  1968)  width  lines,  plasma  experiment.  the-lines  Roberts,  the  scattering  The d e t e r m i n a t i o n s Ar  into  They f o u n d  laser  plasma  the  Kamp  using  one o b t a i n e d  in  the  in  peak  density  scattering.  produced  used  gradient  and the  electron  obtained  of  energy  rapidly. Nodwell  plasma  of  and we have  electron  the found  densities  identical  for  discrepancies  all  so the  obtained  73  are  due  to  suggests  that  perturbed could  discrepancies  determination  of  width the  of  outside  the  transient  the  present  the of  the  of  this  situation  emitted  from t h e  the  only  perturbation  need o f  the  Abel  region  is  of  of  beam, the  by m e a s u r i n g From the electron  temperature absorption  laser,  is  similar  conditions  the  laser  one would  obtain  the  In  this  the  Ar In  ions order  using  has  in  can  of  been done  the  coefficient transmitted  be o b t a i n e d  the  determination  of  the  a perturbed  in  this  a transmition  Because  electron plasma  thesis, volumn o f  of  if  10.6um w a v e l e n g t h  In  same f o c a l  to  coefficients,  absorption  at  way  entire  II  known a p p r o x i m a t e l y .  presented  beam i n t o  region  independently  variation  density  be a c c o m p l i s h e d . as  it  reliable measure  made.  Stark  the  coefficient  a sensitive  could  work  small  small.  be measured  c o n t i n u u m e m i s s i o n as  beam.  we  excited  very  determination could  is  a  unfolding across  number d e n s i t y  or  in a  this  for  lines  density  laser,  in  the  work,  large  presented  because  density  electron  coefficients  coefficients  a CC>2 l a s e r laser  that  a favorable  absolute  electron  Stark  This  results.  perturbed  the  of  coefficients.  Stark  the  this  to  Stark  of  because  p e r f o r m an  of  in  p l a s m a where  we e l i m i n a t e plasma  similar  consistent We a r e  the  the  a determination  plasma  give  in  by  focusing  the  99% b e f o r e  of  ruby the  74  plasma  has  been  maximum o f  the  perturbed  perturbation.  used f o r  this  with  sensitivity  the  and a t r a n s m i t i o n  purpose, with of  the  doped germanium d e t e c t o r  of  only  A small  laser  unit  an o u t p u t  power  in  detector  could  at  could  be  accordance  employed.  be s u i t a b l e  6%  A mercury-  for  this  pur-  pose. For  any  further  aC02 p u l s e d  use  of  the  ruby  laser  work  laser  employed  on p e r t u r b e d  should  in  this  be c o n s i d e r e d work  turbation.  The r e a s o n  is  twofold.  absorption  coefficient  of  the  A ,.at  wavelength  the  3  would  be .3560 t i m e s coefficient  for  only  a 6KW l a s e r  to  in  obtainable data  in  this in  of  the  work.  in  will  interval  the  of  perthe  laser,  C0^  to it  will  then  require  perturbation repetition to  time  record  thus  as  rate  a lot  assuring  improving  the  of the  statistic  results .  interest  predict on the  allow  plasma and  The p r o p a g a t i o n is  It  the  of  since  of  corresponding absorp-  laser.  Secondly,  laser  short  same c o n d i t i o n s  the  the  proportional  the  produce a s i m i l a r  a CO2  a very  is  the  instead  produce  Firstly,  ym o f  than  a ruby  to  plasma  10.6  larger  tion  reported  of  plasmas,  the  since  energy  expansion  Chapman-Jouguet  the  of  the  kinematic  a b s o r b e d by  should  perturbation  the  be d o n e .  detonation  of  theory  the  in  the  expansion  plasma.  Further  .The a p p l i c a b i l i t y and o f  the  Taylor  plasma could study of  the  blast  75  wave t h e o r y laser the  in  a plasma  produced plasmas  expansion f r o n t  experiment  the  should also these  moves  theories  into  perturbation  be i n v e s t i g a t e d . are  a gas,  front  valid  whereas  moves  In  because  in  our  into  ah  ionized  presented  in  this"  medium.  As a resume I  will  that  mention  to  briefly  we have made  in  the  the  this  work  major  electron  density  been d e t e r m i n e d obtained the of  in  caused  the  and s p a t i a l  by a. f o c u s e d  from measurements  of  the  II  lines.  an a l m o s t density  It  complete  gradients  has  are  from the  ionization,  set  They  absolute  been f o u n d t h a t  first  distribution  laser  t r a n s i e n t 'plasma .•  c o n t i n u u m e m i s s i o n and a l s o Ar  contributions  experiment:  1 - The enhancement the  original  thesis  up i n s i d e  the  have  been of  broadening  plasma  and t h a t the  beam have  intensity  Stark  in'  reaches  strong  perturbed  electron  region  of  the  .plasma; 2 - The been d e t e r m i n e d assymmetries spatial  l i n e , shape  in  have  the been  distribution  Ar  II  line  the  laser  presence  of  found  the. l i n e s .  the  measurements  of  metries  been e x p l a i n e d ' ,  have  the  of  of  in  electron  density  c o n t i n u u m e m i s s i o n , the  radiation pulse  have  and  Based on  the  determined observed  strong  from  assym-  76  3 - The p e r t u r b a t i o n incident of  laser  ionization  present  work.  to  the  in  laser  power of  the  This  limitations scattering  of  densities plasma  and f o r  have  information on the  use  the  plasma  for  different  been d e t e r m i n e d could  of  experiments.-  high  lead power  to  a  laser  different degrees in  the  criterio outputs  77  BIBLIOGRAPHY  Albini,  I.and  Biberman,  Rand,  S.  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