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Thomson scattering from a Z-pinch discharge in helium Hilko, Brian Kent 1976

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THOMSON SCATTERING FROM A Z-PINCH DISCHARGE IN HELIUM by BRIAN KENT HILKO B . S c , University of Waterloo, 1974 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n THE DEPARTMENT OF PHYSICS We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA December, 1976 (3) Brian Kent Hilko, 1976 -In presenting this thesis in par t ia l fulfilment of the requirements for an advanced degree at the University of Br i t i sh Columbia, I agree that the Library shal l make i t freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It i s understood that copying or publication of this thesis for f inancial gain shal l not be allowed without my written permission. The University of Br i t i sh Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 ABSTRACT The plasma of a Z-pinch discharge formed i n helium i s investigated by Thomson sca t t e r i n g of ruby l a s e r l i g h t . Measurements were made at a p o s i t i o n 3 cm off axis where r a d i a l motion of the c o l l a p s i n g plasma s h e l l i s arrested. Preliminary r e s u l t s show that the peak electron density i n the plasma s h e l l i s 16 —3 ^3 x 10 cm with a temperature of 1.7 eV. Both the ion and electron feature of the scattered p r o f i l e were observed. i i i TABLE OF CONTENTS Page ABSTRACT . . . . . . . i TABLE OF CONTENTS . i i i LIST OF TABLES • v LIST OF FIGURES . . v i ACKNOWLEDGEMENTS . . . . . . . . . . . v i i i Chapter 1 Introduction . . . . . . . . . . . . . . 1 Chapter 2 Apparatus . . . . . . . . . . . . . . . . 3 A. Introduction . . . . . . . 3 . B. Z-Pinch Discharge . . . . . . . . 3 C. The Ruby Laser 10 D. Detection . . . . . . . . 12 Chapter 3 Experimental Arrangement . . . . 19 A. Introduction 19 B. Constraints on the Scattering System 20 C. Optical Set-up 23 D. E lec t r ica l Problems 27 Chapter 44 Experimental Results 33 A. Introduction 33 B. Ng Scattering 33 C. Plasma Light 1 1 36 D. Integrated Ion Feature . . 38 E. Electron Feature ' 44 i v Page Chapter 5 Discussions and Conclusions 49 A. Introduction . . . . . . . . 49 B. - Analysis of the Integrated Spectrum 49 C. Comparisons with Model Z-Pinch 53 D. Improvements i n Signal to Noise 54 E. Conclusions .• 55 Bibliography 57 V LIST OF TABLES Table Page I Design details of the Z-Pinch discharge . . 5 II Characteristics and design parameters of the giant pulse ruby laser 13 III Characteristics of the scattered light detection system . . . . . . . . 17 IV Estimated bounds for the plasma parameters 50 V Comparison of the discharges . . . . . . 52 v i LIST OF FIGURES Figure Page 1 Schematic diagram of the discharge c i rcu i t 4 2 Cutaway view of the main spark gap i l lus t ra t ing coaxial construction 8 3 Discharge current waveform 9 4 (A) Component arrangement in laser osc i l la tor 11 (B) Orientation of optical axis in ruby crystal . . . . 11 5 Section through discharge axis locating dynamic features of the plasma 22 6 Cross section through discharge vessel showing arrangement of the viewing ports 24 7 Complete view of the optical arrangement 26 8 Light output from the (A),Z-Pinch and (B) spark gap plasma 29 9 Schematic diagram of the timing electronics 31 10 Relevant timing pulses and associated j i t t e r 32 11 Detection sensit ivity by Rayleigh scattering . 35 12 Plasma emission collected from the scattering volume 37 13 General form of the scattered profi le 1(A) for intermediate alpha . . 39 14 Time resolved variation of electron density in the collapsing plasma shel l . . . 42 15 Signal to noise levels in the ion feature . . 43 16 Detection of the electron feature . . 46 v i i Figure Page 17 Theoretical and experimental in t e n s i t y p r o f i l e . . . . . 47 18 Relative i n t e n s i t i e s i n the electron and ion features. . 51 v i i i ACKNOWLEDGEMENTS My learning experiences at University of Br i t i sh Columbia have been made possible by my supervisor, Dr. Roy Nodwell, who invited me to the Plasma Physics Group. Along with Dr. Nodwell, Dr. Frank Curzon, also my supervisor, has helped greatly towards the progress of my project. Dr. Curzon has been invaluable in the preparation of this thesis. I would l ike to thank my supervisors for their support. The Plasma Physics Group include a number of characters who are both good physicists and fine people. Larry Godfrey has been a very unselfish, continuous source of encouragement. George Albrecht's help and sp i r i t has been received with great pleasure. Many thanks go to Dr. Jochen Meyer. Throughout my project, his comments and suggestions have been very helpful . Dr. Meyer's enthusiasm has been a pleasure to experience. No thesis i s complete u n t i l i t is typed. Thank you Mary Butryn. Financial assistance from the National Research Council is gratefully acknowledged. This work is supported by a grant from the Atomic Energy Control Board of Canada. 1 CHAPTER 1 INTRODUCTION This thesis presents the r e s u l t s of an experiment designed to measure the plasma parameters i n a Z-pinch discharge i n helium. Light s c a t t e r i n g as a plasma diagnostic has been w e l l known for a number of years (Evans, Katzenstein, 1969; Kunze, 1968). Analysis of the s p e c t r a l d i s t r i b u t i o n of scattered l i g h t can y i e l d the plasma parameters n g , T e and T^ with reasonable accuracy. There are several advantages of t h i s technique over other plasma probes. Interaction of the incident l i g h t with the plasma i s very weak so that there i s v i r t u a l l y no plasma perturbations induced. In the v i s i b l e region the necessary high powered l i g h t source i s usually the output of Q-switched lasers (here, a ruby laser with wavelength 6943 A). Intense, short duration l i g h t pulses are obtained which l a s t f o r times of the order of 10-40 ns. Dynamic plasma parameter then may be temporally resolved to t h i s l i m i t . Good s p a t i a l r e s o l u t i o n as well can be obtained with focussed beams. The l i m i t a t i o n here.is due to divergence i n the laser beam though f o c a l volumes ^0.05 mm may be obtained without d i f f i c u l t y . The plasma under i n v e s t i g a t i o n was that of a large dimension Z-pinch studied previously at t h i s lab (Preston, 1974). Using i n t e r -ferometric and spectroscopic techniques, he has mapped the electron density and temperature d i s t r i b u t i o n i n the collapsing plasma and has revealed a quiescent phase l a s t i n g 1 ys and having a s p a t i a l extent of ^1 cm. Conveniently, t h i s stable plasma occurs at a time when the discharge current i s approximately zero so the plasma d i s t r i b u t i o n function should be free of external magnetic f i e l d s . Such- properties make the plasma suited f o r use as a spectroscopic source or f o r wave mixing experiments, and f o r studies of the temporal and s p a t i a l growth of induced m i c r o - i n s t a b i l i t i e s . F u l l assessment of these p o s s i b i l i t i e s w i l l require c a r e f u l time r e s o l u t i o n of the plasma parameters. The scattering experiment being set-up should provide the ultimate 2 required r e s o l u t i o n . Further, measurements of the ion temperature w i l l determine the plasma equilibrium since J . Preston shows that h i s electron temperature measurements depend strongly on the state of LTE. The material of t h i s thesis i s presented i n the following manner. A b r i e f introduction to the experiment has been given above. The next chapter w i l l describe the apparatus used, that i s , the plasma, ruby l a s e r and scattered l i g h t detector. Following t h i s , chapter 3 w i l l expose the complete experimental arrangement and discuss some of the aspects of Thompson sca t t e r i n g that have determined the present design. Experimental r e s u l t s are given i n chapter 4 where the s e n s i t i v i t y of the experiment i s shown and the i n i t i a l s c a t t e r i n g r e s u l t s are presented. The data are very encouraging. At t h i s stage, the r e s u l t s are only preliminary and the discussions of chapter 5 w i l l show that simple improvements w i l l provide very s e n s i t i v e diag-n o s t i c s . Also some comparisons with the r e s u l t s .of J . Preston w i l l be given. A restatement of the projected s e n s i t i v i t y of t h i s experi-ment w i l l conclude chapter 5 and t h i s t h e s i s . 3 CHAPTER 2 APPARATUS (A) Introduction The scattering experiment to be described has three major components: (1) a scattering medium, the plasma of a Z-pinch discharge (2) high power incident r a d i a t i o n , a Q-spoiled ruby l a s e r (3) a dispersive detector of scattered r a d i a t i o n , consisting of a monochromator/ photomultiplier combination. This chapter w i l l treat each component separately, describing c h a r a c t e r i s t i c s and design d e t a i l s that are independent of the coupling required f o r successful s c a t t e r i n g . Much of the information w i l l be presented i n tabular form, and some discussion i n d i c a t i n g techniques w i l l be included where relevant. (B) Z-Pinch Discharge The Z-pinch used i n t h i s experiment i s modelled a f t e r those studied previously i n t h i s laboratory. In p a r t i c u l a r , an attempt was made to reproduce the helium plasma examined by J . Preston. The discharge c i r c u i t i s shown schematically i n f i g u r e 1 with table I giving d e t a i l s of many construction and operating parameters. The discharge was made coaxial by wrapping the plasma v e s s e l with a brass wire mesh to form a c y l i n d r i c a l return conductor. Low inductance current leads were made from f l a t copper s t r i p s clamped between a few sheets of 10 m i l thick polyethylene. A 150 pf capacitor was placed across each capacitor of the main storage bank. Experience has shown that t h i s w i l l increase the l i f e t i m e of the main capacitors. Coaxial, high current switching was accomplished using a standard three electrode spark gap formed ins i d e a brass can. The electrodes were made of 1" diameter brass with 1/4" holes d r i l l e d through along the a x i s . These holes allow insert-ion of a t r i g g e r i n g pin i n one electuode and a f i b e r optics viewing channel i n the other. FIGURE 1 Schematic diagram of the discharge c i r c u i t 0.5 Gil' 51 yf. 1.2 kfi (D—>e 50:1 50 pf (T) main spark gap (g) pyrex discharge vessel (§) SCR trigger u n i t ® solenoid operated switching contacts © emergency dump r e s i s t o r Table I - Design details of the Z-Pinch discharge. Discharge Parameters Energy Storage Capacitor Bank Charging Voltage Voltage Measurement Gas F i l l i n g Pressure Circuit Construction Discharge Vessel Discharge Electrodes Return conductor Current leads Triggering SCR-Trigger Pulse Transformer 366 J . in 51.0 yF NRG Low Inductance capacitors, Type 203 10.2 yF/20kV each 12.0 kV -± .1% Avo Ltd. precision 500 Wl resistor plus yljtAmmeter Helium 99.995% pure 4.0 torr 4 2% Reproducible to ± < 1% Cyl indrica l Pyrex tube Length 76.2 cm 0. D. 17.1 cm 1. D. 15.2 cm 1/16" thick pp:erf orated bass Separation 61.7 cm Brass Gauze Copper sheet 7.5 cm x 0.8 mm Length -1 meter 20 ys duration, -200W Capacitive discharge c i rcui t EEG, Type SER 185 Turns ratio +50:1 Vacuum System Mechanical Pump Diffusion Pump Conductance Pressure Measurements Base Pressure Duo-Seal, Model 1402 Free air displacement, 160£/min 6" diameter Duo-Seal o i l , DC707 (also used in manometer) Limited by <\,150 cm length of 3 cm diameter copper tubing O i l manometer in torr range (travelling microscope monitor) Pirani gauge in mmtorr range < 2 ttmtorr Leak Rate < 15 mmtorr/hr. 7 Figure 2 shows the spark gap arrangement in more deta i l . The l ight pipe merely serves to extract an optical pulse that signals in i t i a t ion of the discharge (discussed more completely in the following chapter). Breakdown of the main gap proceeds when charges are introduced into the f i e ld of the electrodes by means of a spark created between the trigger pin and gap electrode. One important feature to note is that the high voltage electrode in the discharge vessel was arranged to be an electron emitter. I n i t i a l l y , when voltage is transferred from the spai-k gap to the vessel electrode, a breakdown wave must proceed through neutral gas to ground before large currents can flow. Behind this breakdown wave, the gas is par t ia l ly ionized so that high voltage is trans-ferred to the wave front. The electr ic f ields in the front are such that electrons seeking the ground sheath are confined to the walls of the pyrex vessel. Polarity effects were investigated in previous work (Preston, 1974). It was found, in the above configuration, that the plasma formed along the vessel wall was spatial ly well confined compared with that formed when a positive charging voltage is used. The pinching plasma shel l then remains smaller in thickness and in turn, hotter and more dense. Measurements of current I in the discharge were made by inserting a Rogowski c o i l (output voltage a 3l /dt ) between the high current leads and integrating the output with an RC c i rcu i t having a time constant of 100 ys. A typical current waveform is shown in figure 3. Calibration of the current traces is accomplished by equating the total stored charge and the time integrated current. The discharge is underdamped with a period of 21.3 ys and a peak current of 160 kA. FIGURE 2 Cutaway view of the main spark gap i l lus trat ing coaxial construction light pipe Y///\ Plexiglass I 1. Brass j \-vWv\ Copper , 1.0" 0.5" quartz rod high voltage cable to trigger transformer clamping nuts trigger pin encased in pyrex tube high voltage lead to capacitor bank polyethylene sheets ground lead, to discharge vessel and capacitor bank high voltage lead to discharge vessel FIGURE 3 . Discharge .current waveform Time (ys) 10 (C) The Ruby Laser Incident high power radiation necessary for Thompson scattering was obtained from a Q-switched laser osc i l l a tor . An optical ly pumped ruby rod, 6" long and 1/2" in diameter was the active medium, lasing at 6943 The rod ends were cut para l le l at the Brewster angle to minimize reflection losses. Further, the rod was cut to have the crystal optic and geometrical axis separated by an angle of 60°. Thus, the preferred polarization direction was v e r t i c a l , as i l lustrated in figure 4. Two Xenon f i l l e d flashlamps, each capable of dissipating 4 kJ in a single shot, produced intense l ight pulses used to create a population inversion in the chromium ions of the ruby crystal . The flashlamps and ruby were placed co-planar and para l le l with the ruby rod situated midway between. Pumping efficiency is greatly increased by enclosing the flash-lamps and rod in a highly polished aluminum c e l l . The c e l l i s constructed in the shape of two overlapping ell ipses with a single common focal l i n e . Each flashlamp l ies along a separate focal l ine and the crystal placed at the common focus. Much of the flashlamp energy is converted to heat in the ruby and flashlamps making water cooling of these components necessary. A single, short duration, high intensity laser pulse i s obtained by the method of Q-switching. The principle may be described as follows. An optical shutter is placed inside the resonator cavity and held closed while the ruby is pumped. Since feedback from the back mirror i s inhibited, the inversion population builds up being depleted only by slow spontaneous emmission. When inversion (gain) in the rod is suff iciently high, the shutter i s opened. Large feedback is established and a l l the energy in the inverted population is rapidly depleted. Typical giant pulses had peak powers of ^25 MW and were approximately gaussian in shape with a FWHM of ^40 ns (see for example figures 15, 16). FIGURE 4 (A)) Component arrangement i n laser o s c i l l a t o r / / / / / / / / / / / / / / / / / / / / O p t i c a l Bench (B) Orientation of o p t i c a l axis i n ruby c r y s t a l 12 The Q-swltch used in this experiment was a Pickels cell in series with a Brewster angle stacked plate polarizer and arranged for quarter-wave operation. A single crystal KDP cube measuring 25 mm on a side is used. The optic axis is normal to each of two opposing faces of the cube and only those faces are polished to optical .quality. Thin brass electrodes are lightly held against both faces and the electrodes have 1 cm diameter holes drilled through them to allow passage of the laser beam. When a voltage difference is placed between the electrodes, birefringence is induced in the crystal (Baldwin, 1969). At the quarter-wave voltage, radiation comming from the polarizer and passing through the. crystal toward the back mirror (see figure 4) has its plane of polarization rotated by 45°. Upon reflection from the mirror, a second passage through the cell gives a total rotation of 90° and this returning radiation is blocked by the polarizer. When the high voltage is switched off, radiation is transmitted through the crystal unaltered and large feedback to the rod is established. The cell is sealed on each side with glass blanks and fil l e d with index matching liquid to reduce reflection losses. A complete l i s t of component details is given in table II. The ruby laser used here has been carefully designed and extensively studied at this lab (Churchland, 1969; Albach, 1972). Interested readers may consult the cited references for a f u l l discussion of the giant pulse ruby laser. (D) Detection The Thomsonn cross section of electrons for visible light is small. The ratio of total scattered to incident light for a given scattering volume is ^10~^ at a density n e vLO"^ cm~3. Typical collection cones of V5 x 10 -^ steradians reduce the scattered fraction to ^ 5 x 1 0 - ^ so that an incident light pulse of about 1 joule will contribute photons to a detection system. Since information on the plasma parameters is contained in the spectral distribution Table II - Characteristics am giant pulse ruby 1 Laser Light Pulse Wavelength Width: Spectral Temporal Peak Power Pulse Energy Polarization Divergence F Flashtubes Supplier Gas Tube Material Arc Length Maximum Rated Input Typical Energy Input Current Pulse design parameters of the er. 6943 A "< 0.2 A FWHM 40 ns FWHM 25-50 M Watts 'VI Joule Linear (contrast ratio > 10 4:1) < 1.5 mrad. EE & G, Type FX-65B-6.5 Xenon Quartz, 22 mm diameter 6.5 inches 4 kJ per pulse @ > 1 pps 2.2 KJ per pulse @ •'< 1 ppm (960.yf charged to ^2.1 kV) ••II ms duration (approximately square) Pockels C e l l Crystal Quarter Wave Voltage High Voltage F a l l Time Index Matching Liquid Coated Optics Substrates: Material Size Coatings Damage Thresholds Ref lect ivi t ies @ 6943 A Back Mirror Front Mirror Pockels Ce l l Windows KDP, 25 mm cube a-6.5 Kv @ 6943 A < 15 ns Flourinert FC-77 (supplied by 3M Co.) Borosilicate Crown glass 1" diameter x 3/8" thick Multilayer d ie lectr ic > 200 MW/cm~2 R > 99.9 % Intracavity surface R = 40% External surface R <0.1% One surface uncoated (in contact with index matching fluid) Second surface R <0.1% 15 of scattered l i g h t , the number of photons observed i n a s i n g l e wavelength bandpass i s reduced to only a few thousand. These small l i g h t l e v e l s require detectors which are s e n s i t i v e to s i n g l e photons. Here, a high gain RCA photomultiplier was mounted at the e x i t s l i t of a grating monochromator. The anode current pulse from the photomultiplier was fed d i r e c t l y into c o a x i a l s i g n a l cable terminated i n i t s c h a r a c t e r i s t i c impedence (50f i ). An o s c i l l i s c o p e was used to measure the voltage drop and permanent photographic records of the traces were kept. I t was also necessary to monitor quite large shot-to-shot v a r i a t i o n s i n the laser i n t e n s i t y , because the magnitude of the scattered s i g n a l i s d i r e c t l y proportional to the incident l i g h t i n t e n s i t y . A photosensitive, f a s t response PIN diode was used to observe the l a s e r pulse. Ample amounts of l i g h t leaked through the back mirror of the laser c a v i t y and ahneutral density f i l t e r (N.D. ^3 i i0 ) was required i n order to keep the diode i n a region of l i n e a r response. The photodiode was placed as far as possible from the Pockels c e l l because high voltage transient signals from the c e l l were picked up by the diode. These spurious signals were too large to y i e l d r e l i a b l e c a l i b r a t i o n of the diode. A s o l u t i o n s a t i s f a c t o r y fo r the present experiment, was to place the diode approximately 10 m away and transport the l i g h t s i g n a l to i t v i a o p t i c a l f i b e r . This provided good e l e c t r i c a l i s o l a t i o n of the photodiode from the P i c k e l s c e l l , though the d i s p e r s i v e properties of o p t i c a l f i b e r may hamper detection of very r a p i d l y changing l i g h t s i g n a l s . (As an aside, one may consider the path d i f f e r e n c e between a l i g h t ray t r a v e l l i n g s t r a i g h t through fhe f i b e r , and a ray that i s multiply r e f l e c t e d at the c r i t i c a l angle. A simple c a l c u l a t i o n w i l l show that the risetime for monochromatic l i g h t could be of the order of 2 ns per meter of f i b e r ) ) The components described i n t h i s chapter must be arranged to allow both the introduction of laser l i g h t into the plasma and 16 extraction of scattered l i g h t . Though t h i s seems more of a technical problem, some considerations involved i n gaining access to the plasma are basic to a l l l i g h t s c a t t e r i n g experiments. I l l u s t r a t i n g t h i s i s the next chapter's function. As w e l l , some of the problems that were encountered i n detecting such weak scattered signals are discussed. Table I I I - C h a r a c t e r i s t i c s of detection system. Monochromator Supplier Focal Length Aperture Grating Reciprocal Dispersion Phot o m u l t i p l i e r Type Gain Spectral Response Quantum E f f i c i e n c y Anode Pulse Rise Time Overal l Voltage O s c i l l i s c o p e Make Risetimes Main Amplifier Type K plug-in Type L plug-in the scattered l i g h t J a r r e l l - A s h Co. 1/4 meter, s i n g l e mirror f/10.6 maximum 1800 grooves/inch Blazed at 7000 A i n 1st order 16 A /mm i n 1st order RCA 7265 48 x 10^ (approximate) S-20 (Be-0 pphbtocathode) 2.5% @ 6943 A < 3.0 ns' -2.3 kv Tektronics Type 551 Dual Beam 12 ns 66nns (photodiode signal) 10 ns (photomultiplier signal) Photodiode Make S e n s i t i v i t y Rise time Power Supply Load Resistor Monsanto Type MD2 S i l i c o n PIN diode °» 1.6 uA mW-1 cm - 2 @ 7000 A < 0.5 ns Transistor battery, 30V 50 (terminated signal cable) 19 CHAPTER 3 EXPERIMENTAL ARRANGEMENT (A) Introduction The project described i n t h i s thesis was designed to demonstrate the f e a s i b i l i t y of i n v e s t i g a t i n g the Z-pinch plasma with l i g h t s c a t t e r i n g . To t h i s end, the design emphasis has been on s i m p l i c i t y . Some of the considerations involved i n designing the experiment w i l l be described i n t h i s chapter and problems encountered, along with t h e i r solutions w i l l be discussed. The p r o f i l e of scattered l i g h t ( i e : i n t e n s i t y versus wave-length) depends on the angle between incident and scattered beams. Since the p r o f i l e also depends on the plasma parameters, the evaluation of these parameters i s determined i n part by the scattering geometry. Furthermore, the l i g h t i n t e n s i t y received by the detection system containss both the scattered s i g n a l and l i g h t emitted by the plasma i t s e l f . The sca t t e r i n g geometry therefore has to be selected so that plasma r a d i a t i o n i s not e f f i c i e n t l y c o l l e c t e d since i t Represents noise on the desired s i g n a l . The viewing d i r e c t i o n then should intercept as short a length of plasma as possi b l e . In t h i s respect, some attention should be given to the plasma dimensions ( i e : length to width r a t i o ^5-10). Forming the discharge i n a r e l a t i v e l y high p u r i t y gas and at low pressures a f f e c t s the arrangement of o p t i c a l components. F i r s t l y , l i g h t must enter and leave the v e s s e l through vacuum sealed windows or lenses. Any exposure of these components to the plasma w i l l degrade t h e i r o p t i c a l q u a l i t y and introduce unwanted impurities. Secondly, because the r a t i o of scattered to incident l i g h t i s extremely small, great care must be taken to s h i e l d the detection optics from incident high power r a d i a t i o n . In an enclosed v e s s e l t h i s i s p a r t i c u l a r l y important since the laser l i g h t may be scattered from apertures, dust 20 p a r t i c l e s , etc. or d i f f r a c t e d from imperfections (bubbles, s t r i a e ) i n the o p t i c a l components. Careful b a f f l i n g must be provided to insure that t h i s stray l i g h t does not enter the plasma chamber to bounce around at w i l l . This experiment shows promisingly small stray l i g h t l e v e l s . Noise i n a l l forms i s of prime importance i n detecting weak scattered s i g n a l s . Stray l i g h t may swamp the s i g n a l . S t a t i s t i c a l f l u c t u a t i o n s i n the plasma emission have severely limited the si g n a l to noise r a t i o . E l e c t r i c a l noise from high voltage, high current switching c i r c u i t s may be picked-up i n the detection e l e c t r o n i c s v i a ground loops or electromagnetic r a d i a t i o n . Ground loops commonly occur when an e l e c t r i c a l connection i s made to the main discharge c i r c u i t , so as to obtain a tr i g g e r i n g s i g n a l f o r the recording o s c i l l i s c o p e . To eliminate t h i s p o s s i b i l i t y , recording systems are triggered by l i g h t emitted from the spark gap of the main discharge. This l i g h t i s conveyed by an o p t i c a l cable to a conveniently (Located photodiode which provides the required s i g n a l . Although ground loop noise i s thus eliminated, some radiated noise s t i l l a f f e c t s the signals recorded by the o s c i l l i s c o p e . (B) Constraints on the Scattering System The shape of the scattered spectrum i s determined by the parameter a .^-(kXp) -^, where k i s the s i z e of the wavevector k = ko~ k g k D and k s define the wavevectors f o r the respective incident and scattered s i g n a l s , and X n i s the plasma Debye length proportional to J* e. (T e/n e) . A l l scattered l i g h t i s l i m i t e d to a region M.00 A wide and o centered about X0= 6943 A (k = 2T T /X 0) . To a very good approximation, k Q = Ks.. so that k = 2k Q sin6/2 i s determined only by the angle 0 between incident and scattered beam d i r e c t i o n s . Once 0 i s given, measurement of the p r o f i l e shape w i l l y i e l d information on a and hence on T e and/or n e . The plasma parameters one can ca l c u l a t e depend very much 21 on the value of a. Three regions for a should be considered: (1) a <_ 1 (2) 1 <_ a <_ 2 (3) a >_ 2. For a < < the scattered spectrum is gaussian in shape with a width proportional to the spread in electron ve loc i t ies . Only T^, the electron temperature may be measured from the shape: to determine n , the total amount of radiation scattered e in a given direction must be measured. On the other extreme, a >_ 2 the scattered light is concentrated i n two sharp spikes which appear centered about the laser wavelength and shifted by a frequency equal to the electron plasma frequency, t o ^ ^ ^ ) 2 . Locating these spikes w i l l give n^, the electron density, direct ly but information of T^ is lost . The most useful profiles for general plasma analysis occur i f a has some intermediate value, 1 <_ a <_ 2. Here the spectrum is a complicated function of n^, T g and a but f a i r ly simple analysis, introduced by Kegel (1965) w i l l y ie ld reasonably accurate values of both T and n , as well as the value of a. e e My interest was in setting, upui aaaThbmson t^' scattering experiment for measuring plasma properties in a Z-pinch discharge. Therefore I decided to examine a piece of plasma whose properties had been fa i r ly well established by previous investigators in our lab. Clearly, the safest choice was to select a position and time such that spatial and temporal gradients in the plasma are small. Then, sl ight fluctuations in the location of the laser focus or in the time when the laser i s f ired w i l l not lead to serious errors. The region of plasma investigated was the electron density peak that trai led behind the collapsing current shel l (see figure 5). Further, the time of interest was between 10 and 12 ys after i n i t i a t i o n of the discharge current. It w i l l be remembered that the Z-pinch used here was modelled after the one studied by J . Preston. His measurements provide good knowledge of the plasma parameters and dynamics. It was found that collapse of the plasma is arrested at a time when the total discharge current f i r s t becomes zero (^10.5 ys) . Beyond this time, a shock wave peels away from the plasma shel l and converges onto the 22 FIGURE 5 Section through the discharge axis locating dynamic features of the plasma a l l . s h e l l s tend to converge to the discharge axis 23 axis. During formation of an axial plasma, the plasma shel l remains stationary at a radius of ^3 cm and maintains approximately stable values of electron temperature and density. The interesting aspects of the shel l are: (1) i t appears quiescent for a re lat ively long time (1-2 ys) , (2) i t may be formed at a time when the discharge current is zero so that the plasma w i l l be free of e lectr ic and magnetic f ields (3) the electron temperature and density (^ 4 x 10^ ° K , 5 - 10 x 10 1 6 cm"3) are reasonably large. These properties make the plasma a good candidate for wave mixing experiments or for use as a spectroscopic source, but careful, time resolved measurements are needed to assess these pos s ib i l i t i e s . Using the above temperature and densities, an intermediate value of a may be obtained when the scattering angle i s near 9 0 ° . The smallest plasma volume intercepted by the detection optics w i l l be realized when the viewing direction is perpendicular to the cylinder axis. This , of course, excludes the poss ib i l i ty of inserting hollow viewing tubes into the vessel since such exposure to the plasma is undesirable. (C) Optical Set-up Arrangement of the optical components both in and on the vacuum vessel i s shown in the diagrams of figures 6 and 7. Vir tua l ly a l l of the incident l ight i s transmitted through the plasma and must be absorbed as e f f ic ient ly as possible. Spurious laser l i ght , and plasma l ight as wel l , can be multiply reflected inside the vessel into a region and angle that i s accepted by the monochromator. The viewing dump shown in figure 6 fu l ly encloses the detector acceptance cone. Thus, the viewing optics "looks" at a non-reflecting surface. The beam dumps, known as Rayleigh horns, are made from blue tinted glass that i s strongly absorbing for red l ight . Radiation entering the mouth of the horn w i l l be reflected deeper and deeper, FIGURE 6 Cross section through discharge vessel showing arrangement of the viewing ports nylon extension glued to vessel with s i l i c o n e sealant to detection system viewing window (crown glass) region of undisturbed plasma that enters the s c a t t e r i n g volume Rayleigh horn viewing dump extent of perturbed plasma pyrex discharge vessel return conductor (brass mesh) 25 eventually getting completely absorbed. The outside of the glass i s painted f l a t black so that no l i g h t may be transmitted. These dumps proved to be very e f f e c t i v e absorbers. It was necessary to use glass ^4 mm thick i n order to absorb the laser energy completely while only a thickness of M mm was needed f o r the viewing dump. Light s c a t t e r i n g as a plasma diagnostic has the advantage over many other techniques of being a non-perturbing probe. Any holes d r i l l e d i n the v e s s e l or electrodes w i l l produce disturbances that should not be allowed to perturb the plasma i n the s c a t t e r i n g volume. In t h i s respect, the large pinch dimensions have made l i f e easy. Rather large holes (1-2" i n diameter) may be d r i l l e d i n the electrodes but e f f e c t s on the plasma extend only a few centimeters from the electrode surface. ,In order to mount the viewing window and dump horn, holes were cut i n both the outer conductor and pyrex v e s s e l (1" and 3/4" diameter holes r e s p e c t i v e l y ) . The plasma i n the i n i t i a l discharge stages forms near the w a l l and severe disturbances are set-up around the holes. However, these disturbances do not propagate s i g n i f i c a n t l y around the circumference of the c o l l a p s i n g discharge column, so that only unperturbed plasma enters the s c a t t e r i n g volume. Figure 6 serves to i l l u s t r a t e t h i s more c l e a r l y . The l a s e r beam was focussed into the plasma through a long evacuated extension tube mounted behind the ground electrode (see f i g u r e 7). Two, 3" long sections of the channel were made of f l e x i b l e brass bellows. One section was placed between the extension tube and discharge chamber. At?, the other end of the tube, a second bellows was soldered to a vacuum sealed lens holder. The lens holder was f i t t e d with micrometer adjustable feet and held i n p o s i t i o n by the bellows tension. The bellows allowed independent alignment of the discharge vessel and l a s e r . The extension tube ^as.uinstal'ledsiricorderitb minimize .stray , - l i g h t s A long focusi-rigig. channel provides good baffilling rof'the l a s e r l i g h t r e f l e c t e d and d i f f r a c t e d from the lens. F i n a l l y , the Monochromator e u cn CN 23.3 cm P.M. tube ^150 cm cn CN FIGURE Complete view of the optical arrangement. Dotted lines show extent of l ight tight enclosure. Focal plane stop diameter is 0.3 mm. f = 50 cm focal plane stop 50 cm laser cavity I I 27 figures show that a l l windows and dumps are recessed to prevent them from being touched by the plasma. This feature reduces contamination of the plasma by impurities, and also prevents possible damage to the o p t i c a l components. A complete view of the o p t i c a l arrangement i s shown i n f i g u r e Ui. The two viewing lenses give a 1:1 imaging of the monochromator ex i t s l i t i n the s c a t t e r i n g volume. Scattered l i g h t i s rendered p a r a l l e l by the f i r s t lens and may be transported any convenient distance to the monochromator. When the laser i s focussed into an enclosed v e s s e l , i t i s generally necessary to co n t r o l the beam divergence. From the point of view of stray l i g h t , the beam i n t e n s i t y i s s t i l l quite s i g -n i f i c a n t out to large angles from the. forward d i r e c t i o n . To l i m i t divergence, the beam passes through a f o c a l plane stop and t h i s aperture i s then imaged into the plasma. The e n t i r e resonant cavity and output beam was enclosed i n a l i g h t t i g h t chamber that blocked off a l l l a s e r r a d i a t i o n that could bounce around the room. When the chamber was l e f t open, p r o h i b i t i v e l y large stray l i g h t signals were observed to leak into the monochromator. (D) E l e c t r i c a l Problems T There are two basic problems encountered i n recording the scattered s i g n a l s . The f i r s t problem occurs bsU-flc'ge the output signals from the photomultiplier are of the order of m i l l i v o l t s . These must be observed against a possible noise background produced by switching transients from the Pockels c e l l and discharge c i r c u i t . Secondly, i f good time r e s o l u t i o n of the plasma i s to be r e a l i z e d , then any j i t t e r i n the r e l a t i v e timing of the discharge and laser pulse must be minimized. The i d e a l r e s o l u t i o n l i m i t would be the temporal width of . the incident l i g h t pulse, where a f i g u r e of ^40 ns could e a s i l y be considered an upper bound. The signals recorded were the photodiode pulse that monitors the laser i n t e n s i t y , and the photomultiplier current r e g i s t e r i n g 28 scattered l ight . The detection electronics consisted of an osci l l i scope, battery powered photodiode, and photomultiplier with power supply. Optical i solat ion of the photodiode has been described. The osci l l i scope was triggered from the photodiode pulse so that there was no direct e lec t r ica l connection (except through the a.c. supply mains) between the discharge/timing c ircuits and detection electronics. The detection system was located only a few feet away from the Z-pinch capacitor bank and discharge vessel. Some problems were experienced with switching transients radiatively transmitted to the osci l l i scope. High frequency noise signals appearing after the i n i t i a l breakdown of the main spark gap were associated with signals generated by the combination of SCR trigger unit and high voltage transformer. This noise i s only observed when a triggering spark is produced in the main spark gap, independent of whether the capacitor bank i s charged or not. The noise levels , ^10 mV peak to peak, were a factor of 5 smaller than the plasma l ight levels , thus the e lec t r ica l noise was considered suff iciently low for the present investigation. An important time interval that must be carefully controlled is the delay between i n i t i a t i o n of the discharge current and f i r ing of the laser. A well defined timing pulse that signals the beginning of the discharge was obtained from the l ight emission of the main spark gap. Since direct , noise free e lec t r ica l signals (such as dl/dt from a Rogowski co i l ) are d i f f i cu l t to obtain, a second photodiode was used to investigate l ight signals from both the discharge and main spark gap. The observeddtraces are shown in figure 8f. The. second photodiode was placed near the vessel wall and viewed the discharge side-on through the gauze return conductor. Although creation of the on-axis plasma appears as an intense well defined spike, the beginning of the discharge i s rather obscure. Figure 2 showed how the spark channel in the main gap was observed. The quartz rod is exposed to the plasma and provides a self cleaning viewing port for the fiber optics bundle. The onset of discharge current i s very evident. By increasing 29 FIGURE 8 Light output from the (A) Z-pinch and (B) spark gap plasmas. Time scale 2 us/div. 30 the photodiode load r e s i s t o r and using a 30 v o l t t r a n s i s t o r battery for power, the photodiode was r a p i d l y saturated by the spark gap l i g h t . In t h i s way, a r e l i a b l e t r i g g e r pulse was obtained. The leading edge, r i s i n g from 0 to 30 v o l t s i n less than 250 ns, was used to tr i g g e r the Pockels c e l l high voltage switch. The laser pulse then i s timed r e l a t i v e to the discharge current (see f i g u r e 9). Relevant timing pulses and associated j i t t e r are shown i n the diagrams of f i g u r e 10. In order to achieve the temporal r e s o l u t i o n l i m i t (^40 ns) these j i t t e r s w i l l have to be s i g n i f i c a n t l y reduced, but the ultimate l i m i t w i l l come from the r e p r o d u c i b i l i t y of the plasma dynamics. y. Manual Trigger (Switch Pulse Generator Delay 0-1.5 ms s rigger Unit I Flashlamps of (fuby (Laser SCR Trigger Unit Main Spark Gap FIGURE \9 Schematic diagram of the timing e l e c t r o n i c s Optical 'Fiber"^" PIN Diodfe r Pockels C e l l Driver Delay 6-15 ys H.V. Swit ch Pockels C e l l FIGURE 10 Relevant timing pulses and associated j i t t e r ^1.0 ms Flashlamp Current Photodiode Trigger Pulse Discharge Current Pockels C e l l Voltage Laser Output 30 V + 25 ys 150 ns delay, <30 ns j i t t e r 8.5 kV v a r i a b l e delay, (<100 ns j i t t e r I I ! I i ! I ^100 ns delay, <50 ns j i t t e r 33 CHAPTER 4 EXPERIMENTAL RESULTS (A) Introduction This chapter presents the r e s u l t s of four i n v e s t i g a t i o n s and constitutes the present experiment: •preMmlnary.tinYesti'gation of scat t e r i n g from the Z-pinch plasma. The o p t i c a l system was c a l i -brated by Rayleigh s c a t t e r i n g o f f N| molecules, determining the si g n a l l e v e l s to expect as well as showing the stray l i g h t l e v e l s . Both the i o n and el e c t r o n feature of the scattered spectrum were measured. A l l measurements were performed using the o p t i c a l arrange-ment described i n the preceeding chapter. Throughout the follow-ing i n v e s t i g a t i o n s , the entrance s l i t s of the monochromator were 1.4 mm liO.2 mm, where the smaller dimension i s the width. Also, the e x i t s l i t s were .2 mm wide so that the monochromator trans-mission function was tria n g u l a r and had a FWHM of 3 Al In the plasma, the focussed laser beam had a diameter of 1.4 mm and the s l i t s were masked to t h i s height so that no unnecessary plasma was observed. (B) N| Scat ter i n g The s e n s i t i v i t y of a detection system may be e a s i l y c a l i b r a t e d by Rayleigh s c a t t e r i n g from neutral gasess^ Here, N? gas was used. The t o t a l i n t e n s i t y , I , of Rayleigh scattered l i g h t (normalized to the incident i n t e n s i t y ) i s equal to the pro-duct MQO" , where Ino , i s the number density of molecules and a the Rayleigh cross s e c t i o n . S i m i l a r l y , the t o t a l i n t e n s i t y of Thomson scattered l i g h t , I =n a S(k) , wheretfrio and a apply to the electrons T e e e ee ' 34 and S(k) i s a function characterizing the scattering geometry and the thermodynamic properties of the plasma. The scattering power of single free electrons (as measured by a ) is modified by S(k) to account for the collective behaviour of an ensemble of interact-ing electrons. The fraction of scattered l ight observed depends in great detai l on the detector efficiency and collection optics, but for a given system, the measured signals, P, w i l l have a ratio depending only on the relat ive cross--sections and densities: p P a Pf lA = R _T n a S(k) n o" e e F i l l i n g the vacuum vessel with and measuring the scattered signal w i l l then show the sensit ivity of a particular system to Thompson scattered photons. The cross sections §"R and a e are well known and a simple calculation w i l l show that 110 torr of N„ w i l l give the same scattered signal as an electron density of 16 ——3 M x 10 cm""' , for typical values of S(k). The above correspondence in signals does not account for the distr ibution of Thomsonn scattered l ight and represents the wavelength integrated s ignal . Rayleigh scattered l ight though-has a bandwidth which is very small since i t i s determined by Doppler broading produced by molecules in a gas at room tempera-ture. Therefore, the Rayleigh scattered signal must be measured at the laser frequency. Even at zero gas pressures, the laser l ight can be accidentally scattered into the detector and constant levels of stray l ight w i l l be added to the Rayleigh signal . By examining the variation of the scattered intensity with gas pressure, the true Rayleigh signal can be distinguished. Extrapolation to zero pressure w i l l reveal the stray l ight ,lev,el..r-'Observed Rayleigh FIGURE 11 D e t e c t i o n s e n s i t i v i t y by R a y l e i g h s c a t t e r i n g 36 sc a t t e r i n g signals are shown i n f i g u r e 11. Each point represents the average of 4-5 shots with error bars given by the standard deviation of the mean. As expected, the plot i s l i n e a r i n pressure. The l i n e a r i t y i n the p l o t may also be used i n low pressure systems to check that alignment does not change with gas pressure. Small motions of the o p t i c a l components w i l l d r a s t i c a l l y a f f e c t the c o l -l e c t i o n e f f i c i e n c y of the optics and therefore the l i n e a r i t y of fig u r e 11. The presence of suspended dust or other small r e f l e c t -ing p a r t i c l e s would show up i n the data as wild shot-to-shot f l u c t u a -t i o n i n the scattered s i g n a l at f i x e d pressure. Iddid not observe such behaviour i n t h i s experiment. The scattered i n t e n s i t y has been pl o t t e d i n terms of the o s c i l l i s c o p e recorded voltages and normalized to an incident l i g h t i n t e n s i t y corresponding to 50 Mwatts. According to the photomultiplier s p e c i f i c a t i o n s an observed scattered pulse of 10 mv (measured across 50 fi) corresponds to roughly 40 photons incident on the photocathode. The stray l i g h t l e v e l s observed at zero nitrogen pressure are the same strength as would 16 be produced by a plasma with an electron density, n of M5.02 x 10 cm 16 —3 ^ i i = 1 x 10 cm would y i e l d a s p e c t r a l l y integrated s i g n a l of e ^46 mv/MW. (C) Plasma Light The l i g h t detected from the f o c a l volume w i l l be the sum of both plasma and scattered r a d i a t i o n . S t a t i s t i c a l f l u c t u a -tions i n the photomultiplier s i g n a l due to the background plasma l i g h t l e v e l s have l i m i t e d the s i g n a l to noise ratio^tinnthe p r o f i l e measurements to N^' The plasma l i g h t l e v e l s were considered adequately low f o r the present though great improvements can e a s i l y be made. A discussion of improved S/N w i l l be held for the following chapter. Figure 12. shows the time evolution of plasma l i g h t c o l l e c t e d from the f o c a l volume. This p i c t u r e was taken at a wavelength 37 FIGURE, 12 Plasma emission c o l l e c t e d from the sca t t e r i n g volume 2 ys/div 50 mV/diy X=6943 A 38 of 6943 A i n 1st order. The detected l e v e l s do not change s i g n i f -a i c a n t l y throughout a region + 60 A from the laser l i n e , i n d i c a t i n g a pure bremsstrahlung continuum with no l i n e r a d i a t i o n . Second o order wavelengths (^3500 A) contributed i n t e n s i t i e s about 4 times greater then those shown and i t was necessary to use a blocking f i l t e r (#29 wratten) at the monochromator•entrance s l i t . The large peak appearing 7uus a f t e r the discharge begins represents passage of the current s h e l l through the sca t t e r i n g volume. T r a i l i n g behind i s the electron density peak which appears here at ^ 12.9 ys as a small hump i n the plateau. The shock wave colla p s i n g on axis (see fi g u r e 8) cannot be seen here because the detection optics accepts l i g h t only from the off axis region ( i e : the viewing dump horn works well as a non-reflecting "surface"). Break-up of the on axis plasma and i t s r a d i a l d i f f u s i o n i s seen at l a t e r times as the l i g h t l e v e l s begin to r i s e sharply. The scattered s i g n a l w i l l be largest when n e i s at i t s maximum value, thus i t i s desi r a b l e to know the time when the peak electron density reaches the sca t t e r i n g volume. In order to deter-mine t h i s time, the ion feature of the scattered spectrum was used be-cause the s i g n a l to noise was considerably greater than that expected when attempting to resolve the much broader electron feature of the scattered p r o f i l e . (D) Integrated Ion Feature The objective of the i n i t i a l run of experiments was to examine the s e n s i t i v i t y of the o p t i c a l system f o r the sca t t e r i n g measurements. Figure 13 shows the form of the scattered i n t e n s i t y 1(A) as a function of wavelength observable i n t y p i c a l plasmas at intermediate a's. The c e n t r a l feature (appearing near the wavelength XQ of incident l i g h t ) i s due to sca t t e r i n g from electrons which follow low frequency ion acoustic o s c i l l a t i o n s . The electron s a t e l l i t e i s a r e s u l t 39 FIGURE 13 General form of the scattered p r o f i l e 1 ( A ) f o r intermediate alpha. The scattered i n t e n s i t y ( i n a r b i t r a r y u n i t s ) i s indicated at the peaks of the electron and ion features. ion acoustic feature M000 electron s a t e l l i t e ^4 A 40 of scattering from charge density waves. The width of the ion=acoustic peak is determined by the thermal motion of the ions. This width is smaller than the bandpass of the detection-system and therefore the structure of this feature cannot be resolved. On the other hand the width of the electron feature is determined by Landau damping of the Langmuir waves and is therefore a function of the temperature. For a plasma in thermodynamic equilibrium, the intensity of the scattered spectrum" contained in the ion and electron feature are comparable. Hence the power per unit band width scales inversely as the widths of the respective f e a t u r e s „ where Ip is the power per unit bandwidth in the electron sa te l l i te and IA is the corresponding power in the ion=acoustic feature, v e and v^ are the average thermal velocit ies of the electrons and ions. Assuming thermal equilibrium between the ions and electrons, Since the acoustic feature is clearly the most intense characteristic of the scattered intensity prof i le i t i s sensible to see f i r s t whether this particular feature of the scattered signal can be detected. Secondly, i f the ion-acoustic peak can be observed, i t can be used to determine how n e (ie: electron density) varies with time. The scattered power 1^  depends on n g , and T g , and as J . Preston has shown, T e is f a i r ly constant for times exceeding 9 ys after the i n i t i a t i o n of the discharge. Hence with the geometry 41 fixed, a w i l l ' depend only on n . Thus I is essentially a function of n^ i n the discharge, for t > 9 ys. Measuring 1^ as a function of time allows me to find the time at which n has the least varia-e t ion. At this time, i t can be assumed that the shot-to-shot re-producibil i ty of the plasma conditions are good enough to permit the wavelength dependence of the scattered signal to be measured. By f i t t ing a theoretical curve to the feleetar.onpf ea1iur-e jh and w ' e T can then be determined, e The experiment performed was as follows. The mono-chromator bandpass was centered on the laser wavelength and a l l l ight scattered into the ion feature was-observed as a function of the time delay of the laser pulse. Figure Vk presents the re-sults and shows very clearly the passage of the plasma shel l through the scattering volume. Each point is the average of 3-5 shots with error bars showing the standard deviation. Again, the intensity is normalized to an incident power of 50 Mwatts. A sample of the high S/N ratio may be seen in the photograph of figure 16 where a de-lay of 12.9 yis was used. There are two aspects of the .plot that should be noted. The dip in intensity appearing at a delay of 9.4 yis i s within the sensi t ivi ty of the detection system. In the observations of J. 3 Preston, a sharp r i se in electron temperature from 40 x 10 " K to 50 x 10 ° K w i l l pass through the scattering volume at approximately this time. Secondly, extremely large fluctuations in the scattered signal can be seen between delays of 11.5 ys and 12.5 ys. Such deviations cannot be accounted for with simple photon s tat i s t ics and must be attributed to a combination, of electronic j i t t e r and the large value of diJ /dt . The fluctuation in n (An ) i s given e e e by Ane^-(dne/dt)At where At is the j i t t e r . Beyond 12.5 ys the plasma appears re lat ively quiescent with a slow decay. If the ion feature does indeed represent a good indicator of electron density, then a peak intensity in the electron feature w i l l be obtained at a delay of; ^13 ys with abnormal fluctuations Time from I n i t i a t i o n of Discharge Current (ys) FIGURE 15 Signal to noise levels in the ion feature 0.1 V/div 0.2 V/div Photomultiplier signal (plasma l ight baseline plus scattered pulse) Photodiode signal (incident laser pulses are ^25 MW peak) 200 ys/div 44 being avoided. The following section shows a measurement of the scattered p r o f i l e at a delay of 12 .9 ys. (E) Electron Feature The electron feature was s p e c t r a l l y resolved at a f i x e d delay (12.9i.ips) that corresponded to a maximum electron density i n the sc a t t e r i n g volume. At t h i s time, the scattered s i g n a l would be l a r g e s t . The data was c o l l e c t e d shot-by-shot i n the following manner. A rhythm was established where the pinch was f i r e d every 5 minutes, evacuated a f t e r :each shot and r e f i l l e d (to 4 t o r r . ) with fresh Helium. A l l the steps necessary to prepare each shot were taken at fi x e d time i n t e r v a l s i n an e f f o r t to maintain the constancy of v a r i a b l e s that a f f e c t the discharge r e p r o d u c i b i l i t y . For ex-ample, a f t e r each pressure measurement, the manometer o i l would return to i t s equilibrium l e v e l with a time constant ( i n i t s f i n a l r e l a x a t i o n stages) of about 10 minutes. P r o h i b i t i v e l y long waiting periods were avoided by regulating the o i l s e t t l i n g time. Although the o i l never s e t t l e d to a true equilibrium l e v e l , a reproducible "zero" l e v e l was established and a f i l l i n g pressure r e p r o d u c i b i l i t y of ^1% could be maintained. . A f t e r i n i t i a l evacuation of the pinch vessel the rhythm was established while monitoring the plasma l i g h t l e v e l s that were shown i n f i g u r e 12'. Five shots were required to clean the discharge chamber to a point where no changes i n the l i g h t signals from successive shots could be observed. At t h i s point the scan was begun. Approximately 150 shots were taken and the simultaneous o s c i l l i s c o p e traces of scattered s i g n a l and laser monitor were photographed. The wavelength region X Q = 6943 A. to A D - 50 A was scanned three times, beginning at the laser wavelength and incrementing 45 i n 3 A steps. Three successive shots were made each time before moving to the next wavelength. A sample of the observed traces are shown in figure 16. The scattered signals are large though severely masked by fluctuations in the detected plasma l ight . In order to extract the scattered signal each photograph trace was carefully digit ized with a time reference taken at the laser pulse peak. The delay between laser and scattered pulse should be fixed and correspondstto the difference between the travel time (both optical and electr ical) of the two signals. A l l the traces for each wavelength were then added together. S ta t i s t ica l fluctuations in the baseline should average out while the scattered l ight w i l l be additive. Since ^9 shots at each wavelength were taken, the signal to noise is improved by a factor of ^3. The scattered signal was then taken relat ive to the average baseline and normalized to the laser intensity. Estimates of a, T and n were obtained using the method e e of Kegel (1965) where the data is compared with a set of theoretical curves. A f i t to the theoretical prof i le was then performed by an i terat ive procedure where the scattering and plasma parameters are varied u n t i l a.minimum least squares deviation between the theoretical and experimental prof i le is obtained. A plot of the data and the best-f it theoretical spectrum is given in figure 17'. The error bars shown represent the standard deviations of data points from the predicted mean. The plasma parameters that gave the best f i t are as follows: a = 1.34/ T = 1.93 x 104 K e n = 2.72 x 10^ cm ^ e FIGURE 16 Detection of the electron feature 200 ns/div Wavelength Shift from Laser Line (A) 48 Although f a i r l y large f l u c t u a t i o n s i n the data are evident, i t i s believed that the above values are r e l i a b l e to within ± 20%. The experimental r e s u l t s presented i n t h i s chapter con-s t i t u t e the observation of both the ion and electron feature of scattered l i g h t from a Z-pinch plasma. The r e s u l t s i n many respects are very encouraging. Further analysis of the data and projected improvements i n the experiment w i l l be considered i n the following chapter. 49 CHAPTER 5 DISCUSSIONS AND CONCLUSIONS (A) Introduction The main concern of this chapter i s to provide some estimates of the sensit ivi ty obtainable from the present experiment. Great improvements on S/N in the electron feature are easily obtained. Present stray l ight levels w i l l allow spectral resolution of the ion feature and hence some knowledge of T_ ,^ the ion temperature. Observations of both electron and ion features of the scattered spectrum can yie ld information on the degree of ionization of the plasma or two estimates of alpha may be obtained. This is i l lustrated through an analysis of the present data. Also, some comparisons of the discharge characteristics observed here and observed by J . Preston are in order since the Z-pinch constructed for this scattering experiment was intended to be identical to h i s . (B) Analysis of the Integrated Spectrum The shape of the scattered prof i le is given by the function S(k,co) which is defined through the di f ferent ia l cross-section per unit volume d2a , J r , = n a S(k,co) dcodfi: = ne e Here, a is the Thomsonn cross-section for an electron, n the electron e e density; k i s the wavevector for scattering defined in chapter 3, and co is the frequency difference between incident and scattered l ight . The shape factor, S, may be written as the sum of two parts, S(k,to) = S_^ (k,co) + Se(k,co), where the subscripts refer to the electron and ion features of the prof i le . The detailed dependence of either S g or S. on co is rather complicated (Salpeter, 1963) but the wavelength 50 integrated shape factors take on the following simple forms: oo ? -1 S (k) = fS (k,u))du) = (1+a ) e —<*> e Z a 4 s:: (k) = - ^ a (1+a ){l+a +Z(T /T.)a } e l where Z i s the degree of i o n i z a t i o n of the atoms. These factors represent the f r a c t i o n of t o t a l scattered l i g h t that w i l l appear i n either the electron or ion feature. Since both features were measured then a comparison of the experimental and t h e o r e t i c a l r a t i o S.(k)/S (k) can be made, l e In f i g u r e 17s (chapter 4), the observed p r o f i l e , S e ( k , o j ) , i s shown along with the best f i t t h e o r e t i c a l curve. The four points « a between 31 A and 40 A show very large deviations from the t h e o r e t i c a l curve and the dependence of i h e calculated plasma parameters on these points was determined i n the following manner. F i r s t , the two highest points were rejected from the data set and a f i t to the remaining data was obtained giving a = 1.18. Next, the two lower points were excluded and an exceptionally good f i t to the data was found at a = 1.64. The plasma parameters a, n and T obtained from the above e e analysis (see table TV) were compared with those obtained from the unbiased f i t of f i g u r e 17i. Hence the r e l i a b i l i t y of ±120% as quoted i n chapter 4. Table IV - Estimated bounds for the plasma parameters T e (xlO 3 " K) ( x l O 1 6 cm - 3) a 1.18 21.9 2.38 1.34 19.3 2.72 1.64 14.8 3.10 51 52 Table V - Comparison of Discharges. Present J . Preston Peak-Discharge Current 160 175 (kA) Period of Discharge Current 21.3 22.4 (ys) 1st Current Zero 11.1 10.4 (ys) On axis collapse of shock wave 8.9 9.0 (ys) Current density peak enters ~ 7.5 ~ 8 ••(ys) sc a t t e r i n g volume Electro-density peak enters ~ 13 ~ 13 (ys) sc a t t e r i n g volume Electron density * 2.7 < 6 ( x l O 1 6 cm"3) Electron temperature * .119 40 (xlO 3 k) * at p o s i t i o n (.g = 3 cm) and time (12.9 ys) that the scat t e r i n g measurements were made. 53 The va l id i ty of the above procedure may of course be questioned, though such analysis does give an indication of the upper and lower bounds that may be placed on the data. The ion feature now may be used to further discriminate between these bounds. For each of the three f i t s mentioned above, the theoretical prof i le was integrated to give S^Ck) and each prof i le was converted to the experimental units of intensity. The normalization imposed on the three profiles was that they each must have the same intensities in the region around 10 A from the laser l ine , where the experimental intensity seems well defined. The ion spectrum S^(k,co) was integrated in a single bandpass of the monochromator so the observed scattered intensity was S^(k). Three values of the ratio S^(k)/Se(k) were obtained and are plotted in figure 18,. At the measured temperatures (^ 2 eV) there should be no doubly ionized helium and the theoretical ra t io , with Z = 1 i s : S.(k) g,4 = a-x{-l+ci'f+(-T-/-TT-)aZ} " . e The theoretical ratio for an LTE plasma and one where the ions are somewhat cooler i s shown also in figure 18. The confidence estimate of ±20% indicated earl ier does appear to be overly pessimistic. (C) Comparisons with ••^P^Lele'ZfBinph The present Z-pinch was intended to reproduce the discharge studied by J . Preston. Table V l i s t s the observed dynamic aspects of the plasma used here along with the corresponding figures indicated by J . Preston. The parameters indicated in the table do not appear to dif fer s igni f icant ly . Since only one measurement of the electron density was obtained, i t i s not known where in the plasma shel l the observation was made. In the data of J . Preston, the plasma shel l had a peak 16 —3 electron density of ^6 x .10 cm with a FWHM density of M..5 cn 3 His measured temperature though, i s approximately uniform at 40 x 10 ° K ' 54 whereas the scattering results indicate ^20 x 10 *K . This i s a substantial difference that may only show the relative cleanliness of the vacuum and gas handling systems. Although the two discharges do appear to be very s imilar , c r i t i c a l comparison of the plasma parameters cannot be made at this time. (D) Improvements in Signal to Noise At present, the signal to noise ratio in the electron feature is limited by the plasma radiation :reacHing-the detector. In situations where the laser l ight pulse lasts for only a fraction of the plasma pulse, only fluctuations in the plasma l ight are important and not the average levels on top of which the scattered l ight appears. If I represents the number of plasma photons and I the number of scattered photons reaching the photomultiplier then the noise figure w i l l be given by S/N = 1^//l and any reduction of the plasma levels w i l l improve the signal to noise. A simple polarizer in the viewing system w i l l reduce the detected plasma l ight by a factor of two. The scattered l ight w i l l remain unchanged since i t is l inearly polarized while the plasma l ight has no preferred polarization. A second significant reduction in the noise levels may be realized by reducing the volume of plasma from which l ight i s collected. The monochromator accepts l ight from a length of plasma which is fixed by the viewing direction. The observed plasma volume is then determined by the area of the imaged monochromator entrance s l i t s . At present, the focussed laser beam has a diameter of 1.4 mm but this could be reduced to a spot diameter of M).5 nun without much d i f f i c u l t y . Then the entrance s l i t could be reduced in height without any loss in the scattered s ignal , though the plasma l ight detected would be diminished by a factor of three (ie: the ratio of s l i t heights). In a l l then, I could be made M./6 smaller. The laser power used in the experiments was V30 MW and could be increased to 150 MW by means of a laser amplifier. With these powers, 55 the scattered s i g n a l I would be increased f i v e - f o l d . A t o t a l improvement i n s i g n a l to noise of 5^6 ==12 could be made and the electron feature 15 -3 could be resolved down to a density n =1 xlO cm without serious J e noise problems. T]je improvements i n plasma r a d i a t i o n l e v e l s w i l l apply equally as w e l l f o r detecting the ion feature, though stray l i g h t w i l l increase i n proportion to the laser i n t e n s i t y . Some experimentation with improved beam dumps and b a f f l i n g systems w i l l be necessary and . considerable reduction i n stray l i g h t l e v e l s i s ant i c i p a t e d . The ion temperature may be measured by s p e c t r a l l y r e s o l v i n g the ion feature. As" mentioned e a r l i e r , the doppler width of the ion feature i s a d i r e c t measure of T.. Since the wavelength spread i s only of the order of 1-2 A, 'the linewidth of the laser output must be made correspondingly smaller. At present, the laser cavity i s approximately 75 cm long with only a 40% front r e f l e c t o r . AJlarge number of lo n g i t u d i n a l modes are possible. If the front mirror i s replaced by a Fabry-Perot etelon of small spacing, only the modes common to both c a v i t i e s w i l l be allowed to lase. In t h i s manner the laser linewidth may be reduced from 0.5 A to ^ 0.05 A and good r e s o l u t i o n of the ion feature should be obtainable even with the present stray l i g h t l e v e l s . (E) Conclusions The experiment described i n t h i s thesis has examined the p o s s i b i l i t y of probing the Z-pinch plasma by means of Thomson sc a t t e r i n g Simple improvements i n the o p t i c a l system can greatly increase the si g n a l to noise r a t i o and allow r e s o l u t i o n of the electron feature 15 -3 down to d e n s i t i e s , n =1x10 cm e Stray l i g h t l e v e l s at the laser frequency correspond to the 14 -3 t o t a l s i g n a l that would be recieved from a plasma with n e=2xl0 cm The frequency integrated ion feature can thus detect changes i n a 56 plasma with electron d e n s i t i e s A.10 cm . A t h e o r e t i c a l study of the ion spectrum w i l l be necessary i n order to determine how v a r i a t i o n s i n the observed s i g n a l can be r e l a t e d to changes i n the plasma parameters. 57 BIBLIOGRAPHY 1. Albach, G. G. 1972. M.Sc. Thesis, University of Br i t i sh Columbia. 2. Baldwin, G. C. 1969. "An Introduction to Nonlinear Optics". Plenum Publishing Corp., New York. 3. Churchland, M. T. 1969. M.Sc. Thesis, University of Br i t i sh Columbia. 4. Evans, D. E. and Katzenstein, J . 1969. Rep. Prog. Phys. 32, 207. 5. Kegel, W. H. 1965. Internal Report. Institut fur Plasma Physik IPP 6/34/ 6. Kunze, H. J . 1968. Jr'Plasma Diagnostics", Editors W. Lochte-Holtgreven, North-Holland Publ. Co. , Amsterdam; Ch. 9. 7. Preston, J . M. 1974. Ph.D. Thesis, University of Br i t i sh Columbia. 8. Preston, J . M. and Curzon, F. L. 1973. "The Performance of a High Frequency Interferometer for Direct Recording Densities". J . Phys. E. Sci . Instr. 6_, 604-607. 9. Salpeter, E. E. 1963. J . Geophys. Res. 68, 1291. 

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