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An attempt to study ionizing radiation fronts in Cs vapour Zawadzki, Janusz Andrew 1973

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AN ATTEMPT TO STUDY  IONIZING RADIATION FRONTS  IN Cs VAPOUR by JANUSZ A. ZAWADZKI B.Sc., University of B r i t i s h Columbia, 1969 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the Department of PHYSICS We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA July, 1973 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y available for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the Head of my Department or by h i s representatives. It i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of The University of B r i t i s h Columbia Vancouver 8, Canada ABSTRACT C r i t e r i a are developed for the design of an experi-ment to study steady r a d i a t i o n fronts i n Cs vapor. According to these c r i t e r i a , a pulsed l i g h t source of supposedly s u f f i c i e n t i n t e n s i t y (=10 2 2photons/cm 2-sec) and pulse length (=90ys) was b u i l t , and an absorption tube for Cs vapor inside an oven was developed (through several models) which produced a maximum absorber density of ^ l O 1 8 p a r t i c l e s / c m 3 . Light and e l e c t r i c a l probe measurements were ca r r i e d out to detect i o n i z i n g r a d i a -t i o n fronts i n the cesium. While the l i g h t measurements were inconclusive due to stray l i g h t problems, the probe measurements showed that photoionization takes place but at a much lower rate than expected. A subsequent study of the l i g h t source confirmed that indeed the l i g h t source, a constricted c a p i l l a r y arc, driven by a 90ys square current pulse, has a much lower i n t e n s i t y i n the t e s t section of the absorption tube than i s required to drive a front. The low i n t e n s i t y of t h i s l i g h t pulse rendered the i n i t i a l aim of the experiment namely the study of steady i o n i z a t i o n fronts i n cesium vapor, unattainable with the avail a b l e apparatus. i i TABLE OF CONTENTS PAGE ABSTRACT i i LIST OF FIGURES V ACKNOWLEDGEMENTS v i i Chapter 1 INTRODUCTION 1 2 INITIAL DESIGN OF THE Cs EXPERIMENT 4 2.1 An Outline of the Thesis 4 2.2 Absorbing Medium 5 2.3 Some Properties of Radiation Fronts..... 7 2.4 C r i t e r i a f o r Dimensions of the Absorption Experiment 10 2.5 Maximum Absorber Density and Absorber Tube Length 11 2.6 The Light Source . . 12 2.7 Front V e l o c i t y and Pulse Duration 13 2 THE CESIUM OVEN AND ABSORPTION TUBE 14 3.1 Why an Oven i s Required 14 3.2 Design of the Oven 15 3.3 Designs of the Absorption Tube... 19 3.4 Cesium Re c i r c u l a t i o n 26 3.5 Density D i s t r i b u t i o n i n the Absorption Tube 27 i i i Chapter Page 3.6 Maximum Temperature Gradient 32 4 EXPERIMENTAL RESULTS 34 4.1 Apparatus f o r the Optical Measurements. 34 4.2 Search for a Front Signal - Optical Method 34 4.3 E l e c t r i c a l Probes 35 4.4 Analysis of Probe Signals Observed on a Long Time Scale 37 5 THE LIGHT SOURCE 41 5.1 Requirements for the Source 41 5.2 Constricted Arc 41 5.3 The Power Supply 46 5.4 Light Intensity vs Discharge Voltage.... 47 5.5 Relative End-On S p a t i a l Intensity D i s t r i b u t i o n 50 5.6 Absolute Intensity Measurements 52 5.7 Source Temperature 56 6 SUMMARY AND POSSIBLE IMPROVEMENTS 59 6.1 Summary » 59 6.2 Possible Improvements 61 i v LIST OF FIGURES page 1.1 Zuzak's Experiment 2 2.1 Absorption Cross-Section of Cesium 5 2.2 Temperature Dependence of Cs Number Density. 6 2.3 Diagram of Experimental Arrangement 7 2.4 Local Radiation Intensity and Energy Absorption i n the Absorption Tube 8 2.5 D i s t r i b u t i o n of T , p , and Grad p i n a Radiation Front 9 2.6 I n t e r r e l a t i o n of Design Parameters 11 3.1 D e t a i l s of the Oven S h e l l . . 15 3.2 Temperature D i s t r i b u t i o n i n Oven when Heating with the Center Element only 17 3.3 Temperature D i s t r i b u t i o n i n the Oven; Heating with a l l Three Elements 18 3.4 I n i t i a l Design of Absorption Tube 20 3.5 Absorption Tube with Cooling " C o l l a r s " and Helium "Reservoir" 22 3.6 Absorption Tube Showing F i n a l Location of Helium Reservoirs 22 3.7 F i n a l Configuration of Absorption Tube and Light Detecting Arrangement 24 3.8 Cs Vapor D i f f u s s i o n V e l o c i t y vs Distance Along Absorption Tube 30 3.9 Cs Vapor Number Density vs Distance Along Absorption Tube 31 v page 3.10 Maximum Tolerable Temperature Gradient as a Function of Temperature 32 4.1 Location of E l e c t r i c a l Probes 36 4.2 Discharge Current and E l e c t r i c a l Noise Pickup i n Probes (B.V. = 8KV) 37 4.3 E l e c t r i c a l Probe Signals - Fast Sweep Speed (B.V. = 8KV) 37 4.4 E l e c t r i c a l Probe Signals - Slow Sweep Speed. 38 4.5 Signal Value vs Time for the Three Probes... 39 5.1 The Bogen Light Source 42 5.2 Cross-Sectional View of Bogen Source........ 45 5.3 N-Section Lumped Transmission Line 4 6 5.4 Current Pulse from Lumped Transmission Line. 47 5.5 Light Pulse from Bogen Source (taken at 7KV) 47 5.6 Peak Bogen Source Output vs Discharge Voltage 48 5.7 Log Peak Output vs Log Discharge Voltage.... 49 5.8 Arrangement f o r Obtaining the Relative End-On S p a t i a l Intensity D i s t r i b u t i o n of the Bogen Source 50 5.9 S p a t i a l Intensity D i s t r i b u t i o n f o r Two Screen to Source Distances 51 5.10 Equipment Used f o r Absolute Intensity Measurement of Bogen Source 52 5.11 Comparison of Bogen Source and Carbon Arc Outputs 54 5.12 Emissive Power of Bogen Source 55 5.13 Emissivity of Bogen Source and Black Body Emissivity Curves 57 v i ACKNOWLE DGEMENTS I wish e s p e c i a l l y to thank my s u p e r v i s o r , Dr. B. Ah l b o r n , f o r h i s k i n d h e l p and support throughout the experiment and i n the p r e p a r a t i o n o f t h i s t h e s i s . I wish to thank a l s o the o t h e r members of the Plasma P h y s i c s Group w i t h whom I have had many h e l p f u l and s t i m u l a t i n g d i s c u s s i o n s ; i n p a r t i c u l a r , Dr. R. C. Cross, Mr. V. P o t o c n i k and Dr. S. Mi k o s h i b a . Thanks are due a l s o t o the t e c h n i c a l s t a f f o f the group, i n p a r t i c u l a r , Mr. D. S i e b e r g who helped i n the d e s i g n and c o n s t r u c t i o n o f many o f the e l e c t r o n i c d e v i c e s used i n t h i s experiment; Mr. J . Lees who b u i l t the many models o f the a b s o r p t i o n tube; and Mr. D. Haines f o r h i s a s s i s t a n c e i n the machine shop. F i n a l l y , I would l i k e t o thank my w i f e , Lucy, f o r t y p i n g t h i s t h e s i s and f o r her p a t i e n c e i n w a i t i n g f o r me to get out and get a " r e a l j o b " . v i i Chapter 1 INTRODUCTION There are many situ a t i o n s where intense r a d i a t i o n f a l l s onto matter and i s absorbed, thereby heating the material to high temperatures. One may think of s t e l l a r r a d i a t i o n h i t t i n g the surrounding i n t e r s t e l l a r matter or of intense lase r r a d i a t i o n being focussed on small areas of target material as examples of such s i t u a t i o n s . In these cases the r a d i a t i v e heating leads to some motion of the heated gas. The motion can be so v i o l e n t that i t has i n f a c t been proposed as.the d r i v i n g mechanism for a laser compression scheme to reach u l t r a high de n s i t i e s i n fusion experiments. The study of the r a d i a t i o n induced motion i n the two examples mentioned above i s complicated but f o r very d i f f e r e n t reasons. The i n i t i a l conditions f o r the i n t e r s t e l l a r phenomena are only vaguely known, and the process proceeds slowly compared to our own time scales, so that we can hardly hope to witness any changes. Laser sparks, on the other hand are very transient objects of minute spherical or egg-shaped geometry i n which the absorption mechanism apparently changes from multiphoton absorption to inverse bremsstrahlung and possibly to other not yet f u l l y understood mechanisms." - 2 -In order to avoid such complications Zuzak (1) started a d i f f e r e n t l i n e of research with the aim of studying r a d i a t i o n gas dynamics. His apparatus, Figure 1.1 was intended to provide known i n i t i a l conditions f o r the absorber, known r a d i a t i o n i n t e n s i t y , plane geometry, quasisteady motion, and involve an absorption mechanism of single photon i o n i z a t i o n only. L I G H T S O U R C E WINDOW A B S O R P T I O N L E N G T H i i i i Figure 1.1 Zuzak's Experiment Zuzak's experimental goal was unfortunately p a r t i a l l y missed since his l i g h t pulses were very short, and he had to study d i s s o c i a t i o n rather than i o n i z a t i o n phenomena since his gaseous targets: oxygen and iodine cannot be ionized through any known window material. Zuzak was however able to detect r a d i a t i o n induced pressure pulses i n oxygen. In a l a t e r attempt A r d i l a and Cross (2)used a longer l i g h t pulse and measured the p r o f i l e s of the l i g h t - 3 -induced pressure waves, again using oxygen as target material with d i s s o c i a t i n g photons as d r i v i n g energy. In t h i s thesis the attempt i s made to develop the apparatus and design c r i t e r i a for the study of r a d i a t i o n gas dynamics driven by photoionization fronts in a closed container. The only target materials which can be used for such experiments are a l k a l i metals, and i t i s the t o p i c of t h i s thesis to report the d i f f i c u l t i e s encountered i n such an attempt. Chapter 2 INITIAL DESIGN OF THE Cs EXPERIMENT 2.1 An Outline of the Thesis The presentation of t h i s thesis follows the h i s t o r i c a l course of the study. In t h i s chapter, some simple and apparently obvious design c r i t e r i a are derived from the t h e o r e t i c a l model of the r a d i a t i o n f r o n t s , which lead us to choose cs as the working substance and to plan the dimensions and properties of the apparatus. An important part of the equipment and one that proved to be the most d i f f i c u l t to construct, was the container f o r Cs absorber gas of high pressure. The t r i a l s and errors i n the design of t h i s part are discussed i n Chapter 3, where we also present measurements of the i n i t i a l state of the neutral Cs gas and t h e i r i n t e r p r e t a t i o n . Experimental observations of photoionization phenomena i n the cs tube and t h e i r i n t e r p r e t a t i o n are discussed i n Chapter 4. An i n v e s t i g a t i o n of the l i g h t source i s described i n Chapter 5. The main r e s u l t of t h i s t h e s i s , namely more stringent and more r e a l i s t i c design c r i t e r i a f o r such a r a d i a t i o n front study with i o n i z i n g photons, i s given i n the Summary. - 5 -2.2 Absorbing Medium The f i r s t requirement of the experiment i s the finding of an absorbing medium which can be ionized behind a window, hopefully with r a d i a t i o n which i s not absorbed i n a i r , so that r e a d i l y available quartz windows may be used. The a l k a l i metals turn out to be the most promising candidates, with i o n i z a t i o n energies: E c g = 3.89 ev, E ^ = 4.176ev and Efc = 4.339 ev. We selected Cs because i t has the lowest i o n i z a t i o n energy (corresponding to a threshold for photo-i o n i z a t i o n of X = 3185 A) and the highest photoionization cross-section, a(X), (3) (Figure 2.1) of the a l k a l i metals. o 30 Atomic Absorption Coefficient 10 2 °cm2 in 20-10 .32 .28 .24 .20 in u Figure 2.1 Absorption Cross-Section of Cesium - 6 -Since the b o i l i n g point of Cs i s =690oC, i t i s obvious that to obtain a high absorber density i t i s necessary to heat the Cs, Figure 2 .2. T 1 1 1 1 1 1 H 500 eoo 700 800 T in *K Figure 2.2 Temperature Dependence of C s Number Density - 7 -The experimental arrangement involved sealing the Cs metal into a glass tube with windows on the ends and placing the tube inside an oven having openings for the ends of the tube. A diagram of the arrangement i s shown i n Figure 2.3; i t shows a l i g h t source and the absorption tube mounted inside the oven, whose temperature i s v a r i a b l e . Oven Light | I Source I Cs Gas | i fe l -—Cs Metal Oven Figure 2.3 Diagram of Experimental Arrangement 2.3 Some Properties of Radiation Fronts In order to determine the dimensions of the absorption tube one has to ou t l i n e a few basic properties of i d e a l r a d i a t i o n f r o n t s . Suppose the absorption cross-section f o r i o n i z i n g r a d i a t i o n i n Cs i s ctcm 2. We saw i n Figure 2.1 that a varies as function of wavelength, but an approximate average value i s a = 1.5 x 10~ 1 9cm 2. Then over the distance L = 1 , The i o n i z i n g r a d i a t i o n w i l l drop i n i n t e n s i t y by O Quartz Window - 8 -about a factor of e , see Figure 2.4, Absorption Region I Local Intensity - i k Local Energy Absorption 1— u-i \ I A Figure 2.4 Local Radiation Intensity and Energy  Absorption In the Absorption Tube No absorption takes place f a r to the l e f t of the absorption region, since a l l the matter there has already been ionized, and hence i s "transparent". No absorption takes place yet, i n the area f a r to the r i g h t of the absorption f r o n t , but i f the r a d i a t i o n comes i n with constant i n t e n s i t y , the r a d i a t i o n "front", the region where absorption takes place, w i l l eventu-a l l y reach any point i n the neutral region. The r a d i a t i o n front propagates with a v e l o c i t y F _ W where V F " Nl constantx - 9 -F i s the photon f l u x and W i s the r a d i a t i o n energy f l u x . At t h i s point i t i s easy to show how dynamical e f f e c t s can arise i n a ra d i a t i o n front. The l o c a l energy absorption shown i n Figure 2.4 leads to a heating of the absorber, r e s u l t i n g i n a temperature d i s t r i b u t i o n as shown i n Figure 2.5a, For a steady r a d i a t i o n front the temperature i n the hot regime must be high enough to maintain thermal i o n i z a t i o n , so that the matter stays transparent to i o n i z i n g r a d i a t i o n after the front has passed. The temperature increase leads to a pressure increase of quite s i m i l a r p r o f i l e , and a pressure gradient i s created, Figure 2.5b. A ) B) - grad p Figure 2.5 D i s t r i b u t i o n of T,p and Grad  Radiation Front This pressure gradient, by the equation of motion, dv P ^ = -grad p , i s the d r i v i n g force f o r the gasdynamical p i n a - 1 0 -e f f e c t s i n r a d i a t i o n f r o n t s , namely acceleration and compression of the gas. The picture outlined here applies to rad i a t i o n fronts propelled by very intense r a d i a t i o n , for which the front v e l o c i t y , V_ i s faster than the sound speed ahead. For lower i n t e n s i t i e s , the motion of the r a d i a t i o n front r e l a t i v e to the gas ahead i s subsonic, and i n t h i s case i t i s possible that a shock wave w i l l be generated ahead of the r a d i a t i o n front. 2.4 C r i t e r i a for Dimensions of the Absorption Experiment For the design of the absorption tube we use a simple c r i t e r i o n . By assuming that the dynamical e f f e c t s of r a d i a t i o n fronts become noticeable i f the front propagates with a phase v e l o c i t y , V F which i s at l e a s t as f a s t as the Y K T speed of sound i n the absorbing medium, (a = ( _ !^ —) ) which i s i n i t i a l l y estimated as 28 0m/sec, one obtains an upper bound f o r the absorber density W W N (T) < = zuzak a(T) " — 28 0m/sec This upper density should be attainable with the Cs oven. In a second recursive step one can allow for the temperature dependence of the speed of sound i n the neutral absorber and thereby correct f o r the f a c t that and a can't be varied independently. - 11 -Once the absorber density i s determined, the length, 1 of the t o t a l absorption tube i s determined by requiring that i t should be several times the absorption length L , Another design c r i t e r i o n which also follows from the absorber density i s the r a d i a t i o n pulse duration. If one wants to study steady r a d i a t i o n f r o n t s , the d r i v i n g i n t e n s i t y should be avai l a b l e f o r at l e a s t a time t 0 , during which the front propagates through the absorption length L ; see that the parameters are i n t e r r e l a t e d as shown i n Figure 2.6. We now turn to the c a l c u l a t i o n of the parameters of the experiment. 2.5 Maximum Absorber Density and Absorber Tube Length The highest usable oven temperature, dictate d by the softening point of quartz tubing, i s 800°K. This temperature sets the high l i m i t f o r N o i n Cs at 1.5 x 10 1 8/cm 3 (Figure 2.2). L _ | where F i s the photon f l u x . Hence, we Figure 2.6 I n t e r r e l a t i o n of Design Parameters - 12 -Using the above value of N 0 and the average value of o ( l ) i n o _ the region 3185 - 2000A , 1.5 x 10 1 9cm 2, we obtain the minimum r a d i a t i o n front width, L=4.5cm. The minimum length of the absorption tube, according to the c r i t e r i o n 1 = 4L i s then 18 cm. We made 1 = 62 cm to enable us to o look at fronts with L up to 15 cm, or T = 650 K. 2.6 The Light Source To obtain a front which.at 650°K would t r a v e l through the undisturbed Cs vapor at the sonic speed in the vapor (650°K was chosen rather than the 800°K maximum so that a smaller photon f l u x would be required) one would need a ^- J i • • i A i n?9 ohotons • . . source capable of d e l i v e r i n g 1.4 x 10'-^  * i n the cm 2-sec o o range 3185 - 1700A (photons of X > 3185A are no longer able o to ionize Cs and X= 1700A i s the transmission cut-off of the quartz which w i l l be used f o r the absorption tube windows). According to Zuzak,his source, a c a p i l l a r y discharge type commonly c a l l e d a "Bogen source", at a voltage of 3K.V. delivered at a distance of 10 cm from the source, 1.16 x 1 0 2 2 P ^ o n s i n t h e r a n isoOA - 1200A. So cm -sec 3 o o that i n the range 3185A - 1700A the source with the quoted temperature of 60,000°K, would d e l i v e r approximately 3 x 1 0 2 2 P h ° t o n s t Futhermore, Zuzak determined that cm 2-sec by f i r i n g the Bogen source at 5KV he could double the output. Therefore by f i r i n g the source at 5KV i t seems that around - 13 -.„ photons o ° 6 x 1 0 2 2 , i n the range 3185A - 17 00A could be cm/-sec delivered at 10cm from the source, that i s , roughly four times the calculated minimum required photon f l u x . 2.7 Front V e l o c i t y and Pulse Duration If we assume a photon f l u x of 6 x 1 0 2 2 PJ_°__9J1S_/ a n ( j cir/-sec a front width of 12cm, corresponding to T o v e n = 650°K, we obtain f o r the front v e l o c i t y V F = 10 5cm/sec which corresponds to a flow of Mach 4 i n a medium having a speed of sound, a = 2.55 x lO^cm/sec. As was mentioned before, the source must radiate f o r a time t 0 =• L i f a r a d i a t i o n V F front i s to be formed. For L = 12cm and V_ = 10 5cm/sec, r t 0 = 120ps. A c t u a l l y L and V are not independent F since they are both functions of N 0 and t 0 i s only dependent on F and a . Our present bank can produce a 90 Us pulse which i s e s s e n t i a l l y flat-topped. The l i g h t pulse, which roughly seems to follow the current pulse, i s probably not long enough to create a steady front but i t should be long enough to study the formation of an i o n i z a t i o n f r o n t i n Cs, e s p e c i a l l y i f one considers that Zuzak was able to observe d i s s o c i a t i o n front formation i n 0 2 using l i g h t pulses of <10ys. Chapter 3 THE CESIUM OVEN AND ABSORPTION TUBE The three main components needed i n an experiment to observe the formation of a r a d i a t i o n front are: a source of l i g h t ; a container for the absorbing medium; and, i f the absorber density i s co n t r o l l e d by the vapor pressure of Csj, an oven to bring the absorbing medium to the required temperature. In t h i s chapter, the container f o r the absorbing medium and the oven are discussed chronologically, that i s , the d e t a i l s of t h e i r development are described i n the order i n which they occurred. The source of l i g h t w i l l be discussed i n Chapter 5. 3.1 Why an Oven i s Required As was mentioned i n Chapter 2, to obtain a s u f f i c i e n t l y high density of Cs vapor i n the absorption tube, i t i s necessary to heat i t . The oven heating the absorption tube sets the temperature d i s t r i b u t i o n f o r the Cs vessel and i n turn determines the number density N 0 inside the tube. The d i s t r i b u t i o n of N e along the tube determines the rate of flow and d i f f u s i o n v e l o c i t y of Cs and thus sets the length of time that the Cs i n the absorption tube w i l l l a s t . To retard the d i f f u s i o n flow of Cs to the ends of the tube, i n e r t gas (Helium) i s put into the tube at the time that the tube i s f i l l e d with C3. - 14 -- 15 -3 . 2 Design of the Oven The f i r s t oven was constructed from pieces of 1 1/2" thick MARINITE (mainly asbestos fibre) joined together with wood screws (Figure 3.1). Figure 3.1 Deta i l s of the Oven S h e l l The heating elements were c o i l s of nichrome wire anchored at both ends of the oven on s t r i p s of copper which were fastened to the bottom of the oven with wood screws. Three sets of elements (1200W, 600W, 550W) were used. The 1200W element - 16 -was used only i n the warm up period. When the desired temperature was nearly reached, only the 60 0W element cont r o l l e d by a Variac, and the 550W element co n t r o l l e d by a T.C.U. (Temperature Control Unit) were l e f t on. The 600W element was then adjusted with the Variac to reach the desired temperature more c l o s e l y and the 550W element and T.C.U. then automatically provided the required f i n e adjustment. The oven rested on two c y l i n d r i c a l r o l l e r s which allowed the oven (with the absorption tube mounted inside) to be moved forward to place the window of the absorption tube as close as possible to the window of the l i g h t source. This arrangement also allowed the oven to be moved back so that the l i g h t source window could be cleaned (from carbon deposits) af t e r each shot. This oven eventually destroyed i t s e l f because the heating elements, when hot, sagged and burned through the bottom of the oven. The second .oven which has stood up very w e l l , was constructed i n the same way as the f i r s t except that the elements were commercial units which have the resistance wire i n the grooves of a s e m i - c y l i n d r i c a l ceramic base. Three of these heating u n i t s , each with a power capacity of 550W were mounted i n l i n e i n the oven and were connected so that the two outer units were cont r o l l e d by a Variac, while the cen t r a l u n i t was connected to the T.C.U. Two probes were placed i n the oven during normal operation: the sensor of the T.C.U. and - 17 -a chrome1-alumel thermocouple. The two probes were p o s i -tioned at approximately the center of the oven, where the maximum temperature was expected, so that the maximum temperature would be monitored continuously. The fa s t e s t and most convenient way to operate the oven was to turn on a l l three heating elements to t h e i r maximum capacity u n t i l the temperature was near the desired temperature and then to shut o f f the two end elements, leaving only the cent r a l element c o n t r o l l e d by the T.C.U. The temperature would i n i t i a l l y o s c i l l a t e and then s e t t l e down to steady O v e n Wall O v e n Wall Figure 3.2 Temperature D i s t r i b u t i o n i n Oven When Heating With the Center Element Only - 18 -values with a rather uneven temperature d i s t r i b u t i o n as shown i n Figure 3.2. When a more even temperature d i s t r i b u t i o n was desired, the Variac c o n t r o l l i n g the two outside elements was adjusted u n t i l a temperature s l i g h t l y below the desired value was reached. The central element con t r o l l e d by the T.C.U. was then switched on i n addition to provide the f i n e adjustment. Figure 3.3 shows the d i s t r i b u t i o n of the temperature i n the oven using the second procedure. 5 0 0 O v e n W a l l O v e n W j ' l T in °C Figure 3.3 Temperature D i s t r i b u t i o n i n the Oven,Heating With A l l Three Elements - 19 -It has been suggested (4) that one way of obtaining a very even temperature d i s t r i b u t i o n i n the Cs tube i s to convert the tube into a heat pipe. This could be accomplished by i n s e r t i n g s t a i n l e s s s t e e l wire mesh inside the tube so that the walls of the tube and the wire mesh are i n contact. The wire mesh i n contact with the inside wall of the tube would, by c a p i l l a r y action, be able to keep the tube well supplied with molten Cs, s a t i s f y i n g the requirement f o r a heat pipe. However, as a t t r a c t i v e as i s the idea of a heat pipe Cs tube, i t would, i n p r a c t i c e , be very d i f f i c u l t to construct except, perhaps, f o r a p e r f e c t l y c y l i n d r i c a l tube, because of the d i f f i c u l t y of placing the wire mesh i n close contact with the inside wall of the tube. For that reason, the heat pipe Cs tube was not b u i l t at t h i s time. 3.3 Designs of the Absorption Tube The basic requirements of the absorption tube are f i r s t l y to contain Cs vapor of N_=1018 p a r t i c l e s which cm3 requires ambient temperatures of - 800°K. Secondly, a window (quartz) has to be provided so that Cs vapor can be exposed to i o n i z i n g r a d i a t i o n . T h i r d l y , i t was considered advisable to have e l e c t r i c a l wire probes pro-truding into the Cs vapor to carry out conductivity measurements. At the beginning of the work i t was thought - 20 -that these requirements could be met with the design shown i n Figure 3.4. oven oven ,wtill , .wall , ', ' 100 cm 1 r i i I I ' 1 i ' 3 0 c m 25mm | |^ 3mm r 2 2 — u t J L II ; — U 1 Cs IV 2cm 20 'cm i *. „ - 3 0 c m ' t ' reservoir 1 i \ W electrodes Figure 3.4 I n i t i a l Design of Absorption Tube B a s i c a l l y t h i s absorption vessel consisted of a quartz tube with quartz windows fused onto the ends which was evacuated and f i l l e d with Cs metal. The tube was of the same length as the outside dimensions of the f i r s t oven (100 cm) so that the ends of the tube, which were s i t t i n g i n the s l o t s i n the oven were f l u s h with the outside of the oven. No supports were provided for the tube other than at the ends where the tube rested i n the oven s l o t s . The only other s i g n i f i c a n t feature of that f i r s t tube was, three sets of tungsten electrodes (see Figure 3.4) which were to be used to detect e l e c t r i c a l conductivity changes i n the Cs vapor due to photoionization of the vapor. The tube proved to be unsuccessful because Cs vapor condensed on the quartz windows making them opaque to l i g h t from the - 2 1 -Bogen source. In addition, the hot Cs vapor destroyed the tungsten electrodes and seemed to attack the walls of the quartz tube. The second absorption tube was made longer than the outside of the oven (the second oven), so that cooling " c o l l a r s " could be placed around the ends of the absorption tube which now protruded from the oven. The cooling " c o l l a r s " (made from s t r i p s of copper braid, one end of which was wound around the absorption tube and fastened i n place and the other end placed i n a dewar of l i q u i d Nitrogen) were meant to cool the ends of the absorption tube so that the Cs vapor could condense on the walls of the tube before reaching the quartz windows. To re t a r d the flow of Cs vapor to the cold ends, the tube was f i l l e d with Helium gas (p=160 t o r r ) . This Helium gas was drawn from a "rese r v o i r " , connected as i n Figure 3.5, to keep the t o t a l pressure at any point i n the absorption tube roughly the same. As a further change, no wire electrodes were fused into the tube, but instead small depressions were provided i n the quartz wall of the tube so that small capacitive probes could be attached there. The pyrex tubing connection between the absorption tube and the He re s e r v o i r proved to be quite f r a g i l e , and a f t e r i t broke, the tube was modified by changing the lo c a t i o n of the r e s e r v o i r , as i n Figure 3.6, to make i t more sturdy. - 22 -collar dewars with liquid J for i icapacitive probes i i i oven walls Figure 3.5 Absorption Tube With Cooling " C o l l a r s " and Helium "Reservoir" * 35 cm • Figure 3.6 Absorption Tube Showing F i n a l Location of Helium "Reservoir" The basic operation of the tube was found to be s a t i s f a c t o r y since no deposit of Cs could be noticed on the windows of the tube and transfer of Cs from the c e n t r a l r e s e r v o i r to the ends of the tube was s u f f i c i e n t l y slow that i t was possible to run the tube for a couple of hours before - 23 -the one gram of Cs i n the rese r v o i r was transferred to the ends. Eventually the cooling " c o l l a r s " were discarded when i t was found that even without themr a l l the Cs seemed to condense i n the f i r s t 4 cm past the oven w a l l . Since i n t e r p r e t a t i o n of e l e c t r i c a l probe signals to obtain quantitative data i s , i n most cases, quite d i f f i c u l t , the p r i n c i p a l means of detecting the photoionization front was to be the observation of Csl l i n e s which would appear when the Cs, ionized by the front, recombined. The i n i t i a l attempt to observe Csl l i n e s by looking at the side of the absorption tube through a hole i n the oven wall (using a monochromator (M.C.) with a photomulti-p l i e r (P.M.) mounted on the e x i t s l i t ) revealed that the walls of the absorption tube were opaque, probably due to a f i l m of l i q u i d Cs coating the inside wall of the tube. To allow l i g h t from insi d e the absorption tube to be detected, the tube was changed by the addition of four side tubes with quartz windows fused on the ends (Figure 3.7). The side tubes were surrounded by a u x i l i a r y heating c o i l s which made the side tubes "hot spots" where Cs would not condense. Although one pair of side tubes on one side would seem superfluous, the tubes are necessary to prevent r e f l e c t i o n s from the tube wall (of the l i g h t from the Bogen source) from entering the monochromator. The purpose of having two places at which the absorption tube could be observed was to allow measurement of the front v e l o c i t y from the Figure 3.7 F i n a l Configuration of Absorption Tube and Light Detecting Arrangement - 25 -delay i n the a r r i v a l of the signal between the two ports. In addition, with the two port system, the development of the front could be deduced from the shape of the signals at the two ports. In order that the signals from the two ports could be studied simultaneously, using the one avai l a b l e monochromator, and to prevent s p o i l i n g the alignment between the monochromator and the side ports each time the oven was moved (to allow the window of the l i g h t source to be cleaned) each side tube was o p t i c a l l y coupled to the entrance s l i t of the M.C. ( J a r e l l Ash l/2m) by a f i b e r o p t i c s cable, as shown i n Figure 3.7. At the oven, a holder, screwed onto the oven wa l l , positioned each f i b e r o p t i c s cable exactly opposite a side port of the absorption tube. A s u i t a b l e clamp held both f i b e r o p t i c s cables i n place i n front of the M.C. entrance s l i t . This arrangement eliminated o p t i c a l adjustment problems. The sum of the l i g h t signals passing through the M.C. e x i t s l i t was transformed into an e l e c t r i c a l s i g n a l by an R.C.A. IP21 photomultiplier and displayed with a 555 Tektronix o s c i l l o s c o p e . Since no s i g n a l could be obtained, i t was obvious that i n s pite of the a u x i l i a r y heaters, Cs had coated the side tube windows. Therefore, i n the f i n a l design, as shown i n Figure 3.7, the side tubes were extended so as to protrude from the oven i n order that Cs would condense on the walls of the side tubes before reaching the windows. - 26 -3 . 4 Cesium Recirculation The l a s t two tubes were used f o r longer periods of time than e a r l i e r models, so that the problem arose as to how to return the Cs that had condensed at the ends back to the middle of the tube, once the r e s e r v o i r i n the middle of the tube was depleted of Cs. One way of returning the Cs to the middle of the tube would be to heat the ends of the tube (using heating tapes, for example) and cool the middle, reversing the temperature gradient which had brought the Cs to the ends. This method was discarded because i t was slow and would endanger the end windows which would be exposed to the hot Cs vapor. We then resorted to using gravity to return the Cs to the middle of the tube. The procedure was to allow enough Cs to accumulate at the ends, take the tube out of the oven, cool one end of the tube to s o l i d i f y the Cs accumu-lat e d there, then t i p the cold end down, allowing drops of Cs at the uncooled end to s l i d e down into the rese r v o i r i n the middle. S i m i l a r l y , the Cs was coaxed from the other end of the tube back into the middle. The above method allowed some Cs from the ends to be brought back to the middle so that the tube d i d not have to be r e f i l l e d with Cs each time that the re s e r v o i r was empty. However, t h i s method i s not e n t i r e l y successful because much of the Cs adhered to the tube wall (some reacted with the quartz) and could not be coaxed to the - 27 -r e s e r v o i r . As a consequence, a f t e r the Cs had been returned to the middle,two or three times, there wasn't enough Cs at the ends to form a drop that could be returned to the middle. 3.5 Density D i s t r i b u t i o n i n the Absorption Tube For absorption measurements, one has to know the density d i s t r i b u t i o n of Cs atoms inside the tube. The d i s t r i b u t i o n may be obtained as follows (5). We s t a r t with the tube cold and the i n e r t gas present at a ce r t a i n pressure P^. Let us assume that the tube i s connected to a r e s e r v o i r of large volume so that subsequent changes i n the temperature of the absorption tube do not a l t e r the pressure i n the re s e r v o i r very much. Then the t o t a l pressure at any point i n the absorption tube w i l l always be P t. As the Cs i s heated, the p a r t i a l pressure of the vapor, P c, at any point i n the absorption tube w i l l increase and that of the i n e r t gas, P^, w i l l decrease so as to keep t h e i r sum equal to P t. Thus the vapor drives out part of the i n e r t gas, and when a steady temperature i s reached, the vapor-gas mixture i s i n a state of dynamic equilibrium. There i s no net movement of the i n e r t gas i n any d i r e c t i o n , but there i s a slow transfer of the Cs by d i f f u s i o n of the vapor (driven by the gradient of the cesium pressure, grad P c) through the i n e r t gas. This type of equilibrium remains only so long as the vapor pressure i s less than P.. I f the vapor pressure exceeds P^» - 28 -convection of the vapor (as opposed to diffusion) takes place and the metal i s very r a p i d l y transferred to the ends of the tube. I f a vapor pressure of the order of 0.8 Pfc i s used (5), the rate of d i f f u s i o n of the vapor i s s t i l l slow and the p a r t i a l pressure of the i n e r t gas i n the main portion of the absorption tube i s considerably less that that of the vapor. The usual theory (6) f o r the one-dimensional problem of i n t e r - d i f f u s i o n of two gases i n a tube of uniform bore leads to the r e l a t i o n s f o r mass f l u x r , (3.1) r - - n d n l v - - n d n 2 r - r dx dx where n j a n d n 2 are the concentrations of the vapor and gas respectively at the point x ; Y\ , r 2 are the numbers crossing a unit area from l e f t to r i g h t , at the point x, per second and D, i s the d i f f u s i o n c o e f f i c i e n t . In the usual problem to which equations (3.1) apply, both gases move along t h e i r own concentration gradients and nj. and n 2 are functions of time. In our problem, which i s s i m i l a r to that of Ditchburn (5.) , the vapor d i f f u s e s through the i n e r t gas and there i s no net motion of the l a t t e r . To take into account t h i s e f f e c t , we assume that at any point there i s a movement of the whole system considered above, s u f f i c i e n t to transfer r 2 molecules of the i n e r t gas from - 29 -ri g h t to l e f t . This w i l l also transfer — r 2 molecules y n 2 z of the vapor from r i g h t to l e f t or - r_ , from l e f t to n 2 r i g h t , so that the t o t a l movement of the vapor component i s : (3.2) r* = -( J_ + i)r_ = ( N I + I ) D D N I n 2 n 2 dx After dynamic equilibrium i s reached, the density of Cs vapor at each point i n the tube i s nearly the saturated vapor density at that point, and the flow of Cs vapor i s determined, at each point, by the l o c a l p a r t i c l e density. Since d i f f u s i o n of Cs molecules c e r t a i n l y takes place i n the Cs tube heated up i n the oven, i t i s important to know whether t h i s motion i s s i g n i f i c a n t when studying the dynamics of r a d i a t i o n fronts i n Cs vapor. For instance, i f the bulk v e l o c i t y of Cs, due to d i f f u s i o n , were equal and opposite to the propagation v e l o c i t y of a r a d i a t i o n front, such a ra d i a t i o n front would not move at a l l i n the lab frame of reference. The d i f f u s i o n v e l o c i t y can be obtained from the r* d i f f u s i o n flux r* : V_ = where the d i f f u s i o n f l u x n_ can be obtained approximately assuming that the density d i s t r i b u t i o n (x) i s s o l e l y determined by the temperature d i s t r i b u t i o n (Figure 3.2) and the vapor pressure curve of Cs (Figure 2.2). By t h i s method the l o c a l d i f f u s i o n v e l o c i t y - 30 -r was calculated. The numerical r e s u l t i s displayed i n Figure 3.8. 0 10 20 30 4 0 5 0 6 0 X in cm Figure 3.8 Cs Vapor D i f f u s i o n V e l o c i t y vs Distance  Along Absorption Tube I t i s seen that V_. i s very small indeed, and small compared to the speed of sound i n the Cs gas, a. Therefore, we f i r s t l y conclude that the density d i s t r i b u t i o n i s not s i g n i f i c a n t l y changed from the " s t a t i c " value, which we used to c a l c u l a t e Vp . This density d i s t r i b u t i o n i s shown i n Figure 3.9. Secondly, we see that t h i s small bulk motion - 31 -i s completely n e g l i g i b l e when in t e r p r e t i n g any r a d i a t i o n gasdynamical v e l o c i t y measurements, since v e l o c i t i e s of the order of 0.1 m/sec can c e r t a i n l y not be measured with any availab l e method. N in \ U cm X in cm Figure 3.9 Cs Vapor Number Density vs Distance Along  Absorption Tube - 32 -3.6 Maximum Temperature Gradients I t i s , at t h i s stage, informative to know how large a temperature gradient, and hence r e s u l t i n g density gradient, could be sustained before the d i f f u s i o n v e l o c i t y would become so large that i t changed the density d i s t r i b u t i o n appreciably and would a f f e c t the i n t e r p r e t a t i o n of front v e l o c i t i e s i n Cs. dT The siz e of the temperature gradient (^) which max w i l l lead to .a non-negligible flow v e l o c i t y , can be calculated i f one chooses a maximum to l e r a b l e v e l o c i t y . dX 1 0 3 V c m 1 T i n ' k Figure 3.10 Maximum Tolerable Temperature Gradient as a  Function of Temperature - 33 -For the present c a l c u l a t i o n s , tha maximum toler a b l e v e l o c i t y , ( V p ) m a x , was chosen equal to — , ( where a i s the speed of sound) for which the p l o t i s shown i n Figure 3.10. The r e s u l t s of Figure 3.10 can, for instance be used to estimate how f a s t the temperature may drop o f f without having to worry about d i f f u s i o n . As an example take two points A and B , point A being at a temperature T A = 720°K and B at T f i = 400°K. Using the minimum temperature gradient on Figure 3.10 between A and B , 3 °K 6 x 10 — , i t may be seen that the two points can be cm i x . dx A m 320°K nt-as close together as: Ax = AT = ^— - .05 cm. dT r ,„3 6 x 10 0 R cm This distance i s so small that one can safel y assume that i n any oven configuration i n which the Cs i s heated from outside, d i f f u s i o n e f f e c t s can be neglected when studying macroscopic gasdynamical phenomena. I t appeared then that the absorbing medium f u l f i l l e d a l l requirements f o r r a d i a t i o n front experiments. We therefore proceeded with the study assuming that the l i g h t source behaved as described by Zuzak. Chapter 4 EXPERIMENTAL RESULTS With most of the problems of making a Cs absorption tube overcome, detection of the de-exitation s i g n a l from Cs, i n d i c a t i n g the presence of a r a d i a t i o n front, was attempted. A des c r i p t i o n of the experimental arrangement, which w i l l be outlined b r i e f l y here, was given i n d e t a i l i n Chapter 3 (diagram on Figure 3.5). 4.1 Apparatus f o r the O p t i c a l Measurements The apparatus used to measure the photoionization i n the Cs absorption tube consisted of two f i b e r o p t i c cables attached to observation ports on the Cs tube; the cables were connected to the entrance s l i t of a single photo-m u l t i p l i e r whose output was fed into an o s c i l l o s c o p e . When the oven was heated to the required temperature (650°K), the capacitor bank (charged to 8-10KV) was triggered and the oscillogram of the sum of the two l i g h t pulses (from the two f i b e r o p t i c cables) was taken. 4.2 Search f o r a Front S i g n a l - O p t i c a l Method The sum of the two observation port signals unfortunately d i d not d i f f e r i n shape from the s i g n a l coming from either one of the two ports alone, i n d i c a t i n g that either there was no r a d i a t i o n front produced i n the Cs vapor or that the front v e l o c i t y was very large r e s u l t i n g i n the almost simultaneous a r r i v a l of the front at the two observation - 34 -- 35 -ports or that most of the detected l i g h t was not coming from an i o n i z a t i o n front at a l l , but rather was stray l i g h t from r e f l e c t i o n s of the Bogen l i g h t pulse from the absorption tube and from the i n t e r n a l surfaces of the oven. Unfortunately, however, we were unable to determine the o r i g i n of the o p t i c a l s i g n a l s , so that these observations were abandoned without conclusions. In order to avoid problems r e l a t i n g to the detection of l i g h t signals buried i n a background of stray l i g h t , an attempt was made to detect the front or any net photoionization i n the Cs vapor by means of e l e c t r i c a l probes. 4.3 E l e c t r i c a l Probes With the e l e c t r i c a l probes, the contention was that i f the photoionization proceeds i n a r a d i a t i o n f r o n t , there should be a delay i n s i g n a l between the probes positioned along the absorption tube. The probes were capacitive probes of the type investigated by Whelan (7), and consisted of single turns of bare 2 8 gauge nichrome wire wrapped around the absorption tube at three d i f f e r e n t points, as shown i n Figure 4.1. The probes, terminated with 50ft to ensure a fas t r i s e time were connected to a dual beam Tektronix 555 scope with two 1A1 dual trace plug-ins so that four signals could be recorded simultaneously, the three from the probes and the bank current. In addition, the signals - 36 -from any two probes could be combined to give the sum or difference s i g n a l . \6J5 cm 5.6 cm 13.6 cm Electrical Probes 1 2 3 Figure 4.1 Location of E l e c t r i c a l Probes At the s t a r t , the oven was heated and a few shots were taken with the l i g h t from the Bogen source (using discharge voltages of 8-10KV) blocked o f f from the absorption tube to check on the t o t a l e l e c t r i c a l noise. The oscillograms from those f i r s t shots (Figure 4.2) show that the e l e c t r i c a l noise induced i n the probes i s smaller than 10 mV peak to peak except f o r the f i r s t 3ys where the noise can r i s e to approximately 20 mV P«P-Next, the obstacle between the Bogen source and the Cs tube was removed to allow the l i g h t pulses to reach the Cs vapor. Oscillograms taken under those conditions (and using o s c i l l o s c o p e sweep speeds of 2-10ys/cm) showed signals from a l l three probes (Figure 4.3) which are c l e a r l y caused by the l i g h t pulse. - 37 -.05v/cm probe1 2 " 3 10 us/cm 5 us/cm F i g u r e 4.2 Discharge C u r r e n t  and E l e c t r i c a l Noise Pickup i n Probes (B.V. = 8KV) F i g u r e 4.3 E l e c t r i c a l Probe  S i g n a l s - F a s t Sweep Speed (B.V. = 8KV) U n f o r t u n a t e l y , the probe s i g n a l s were not r e p r o d u c i b l e and r e s i s t e d many attempts to make them r e p r o d u c i b l e . Furthermore, t h e r e was no i n d i c a t i o n t h a t any d e l a y i n the r i s e o f the probe s i g n a l s 1 , 2 , 3 c o u l d be d e t e c t e d . 4.4 A n a l y s i s o f Probe S i g n a l s Observed on a Long Time S c a l e F o l l o w i n g the attempts d e s c r i b e d above, d e t e c t i o n o f s i g n a l s from the probes a t a long o s c i l l o s c o p e sweep speed (lms/cm) was t r i e d . As b e f o r e , o s c i l l o g r a m s were obt a i n e d w i t h the l i g h t from the source o b s t r u c t e d t o check on the e l e c t r i c a l n o i s e p i c k e d up by the probes. Except f o r a v e r y s m a l l d i s t u r b a n c e r i g h t a t the o r i g i n , the t r a c e s were completely f l a t i n d i c a t i n g t h a t n o i s e p i c k u p would not be a problem. When the l i g h t from the source was a l l o w e d to r e a c h the Cs vapor, s i g n a l s were o b t a i n e d from a l l t h r e e probes (Figure 4.4). These s i g n a l s showed l i t t l e , i f any, v a r i a t i o n from shot to shot. - 3 8 -1 ms/cm Figure 4.4 E l e c t r i c a l Probe S i g n a l s - Slow Sweep Speed The i n t e r p r e t a t i o n of the s i g n a l s from the probes i s th a t they are due to p h o t o i o n i z a t i o n , and represent the charge on the absorption tube w a l l a t the l o c a t i o n of the probe. The charge on the tube w a l l , which according to the i n t e r p r e t a t i o n of the s i g n a l s , r i s e s q u i c k l y (<5ps) and decays r a t h e r s l o w l y (.43-3.55 ms) i s due to s e p a r a t i o n of e l e c t r o n s and Cs ions which r e s u l t s from t h e i r g r e a t l y d i f f e r e n t m o b i l i t i e s . The s i t u a t i o n seems to be t h a t a f t e r the Cs has been i o n i z e d by the r a d i a t i o n , e l e c t r o n s a r r i v e very q u i c k l y t o the r e l a t i v e l y c o l d w a l l s of the tube, fo l l o w e d by the much slower Cs ions which then n e u t r a l i z e the charge. E s s e n t i a l l y , two sets of measurements may be obtained from the t r a c e s of Figure 4.4 : the maximal s i g n a l values and the s i g n a l decay times f o r the three probes. In a d d i t i o n , i t i s p o s s i b l e to get an i n d i c a t i o n of s i g n a l r i s e times, but these are more e a s i l y obtained from o s c i l l o g r a m s taken at s h o r t e r sweep speeds. Comparison of the r i s e and decay times of the s i g n a l s shows t h e i r r a t i o to be approximately the same as the r a t i o of the m o b i l i t i e s o f an e l e c t r o n - 39 -and a Cs ion, which gives weight to the i n t e r p r e t a t i o n that there i s charge separation. Furthermore, plots of the signal value vs time on semilogarithmic paper, Figure 4 . 5 , show that the decay i s exponential, i n d i c a t i n g that the decay process i s d i f f u s i o n and/or electron attachement ( 8 ) . 2 4 6 8 Time in ms Figure 4 . 5 Signal Value vs Time f o r the Three Probes Electron attachment i s strongest to those atoms which have t h e i r outer e l e c t r o n i c s h e l l s nearly f i l l e d ; there i s no attachment to the Helium buffer gas and the attachment to Cs atoms, to form Cs , i s very weak(9), so that ambipolar d i f f u s i o n becomes the most probable process i n the probe measurements. Attempts to r e l a t e the probe sig n a l decay times and maximum values to the electron density were not f r u i t f u l , due mainly to the complex geometry of the absorption tube. - 40 -T h e p r o b e m e a s u r e m e n t s s h o w e d t h a t a l t h o u g h p h o t o i o n i z a t i o n o f C s v a p o r w a s t a k i n g p l a c e , t h e r a t e o f i o n i z a t i o n a n d h e n c e t h e i n t e n s i t y o f t h e l i g h t s o u r c e was n o t s u f f i c i e n t t o c r e a t e a f r o n t . F r o m t h e c a l c u l a t i o n s o f C h a p t e r 2, w h e r e c e r t a i n a s s u m p t i o n s a b o u t t h e l i g h t s o u r c e w e r e m a d e , i t a p p e a r e d t h a t t h e i n t e n s i t y a n d d u r a t i o n o f t h e l i g h t p u l s e f r o m t h e B o g e n s o u r c e w o u l d b e s u f f i c i e n t . T h e r e f o r e , a t t h i s t i m e , i t w a s d e c i d e d t o i n v e s t i g a t e t h e s o u r c e t o c h e c k w h e t h e r i t s c h a r a c t e r i s t i c s m a t c h e d t h e a s s u m p t i o n s t h a t w e r e made a b o u t i t o n t h e b a s i s o f Z u z a k ' s a n d C r o s s ' s w o r k . T h e i n v e s t i g a t i o n o f t h e l i g h t s o u r c e i s t h e s u b j e c t o f C h a p t e r 5. Chapter 5 THE LIGHT SOURCE 5.1. Requirements for the Source In Chapter 2 i t was mentioned that to obtain a r a d i a t i o n front which at 650°K would t r a v e l through Cs vapor at the 2 sound speed, a source was needed which could supply 1.4 x 10 2 ° ° photons/cm -sec i n the range 1700A - 3185A, 10 cm from the source, f o r 120ys. Since i t would be desirable to drive the front f a s t e r than at the sound speed, the figure 1.4 x 1 0 2 2 photons/cm 2-sec (1700A <X< 3185A)for 120ys, i s r e a l l y the minimum that the required source should d e l i v e r to the Cs vapor contained i n the absorption tube. The i d e a l source would be able to supply much more 22 2 than 1.4 x 10 photons/cm -sec continuously. Such a source i s not avail a b l e and even i t i t were, i t would require an im p r a c t i c a l l y large power supply (- 100 MegaWatt). Thus the only possible sources for the experiment are pulsed sources. Probably the most common pulsed sources are arc discharges. There, a charged capacitor or bank of capacitors discharges through an i n s u l a t o r (usually a gas) i o n i z i n g i t and creating a luminous plasma. 5.2 Constricted Arc One type of arc discharge, the design of which has been optimized by P. Bogen et a l (10), for the production of l i g h t - 41 -i s a c a p i l l a r y arc. Our version of the c a p i l l a r y arc i s shown i n Figure 5.1. Bogen Source window holder pyrex tube I Dump Chamber pyrex tube T I r to vacuum pump 5 0 UF D.C. Power Supply 0-10 kv Figure 5.1 The Bogen Light Source B a s i c a l l y the "Bogen" source produces a high current discharge i n a narrow channel d r i l l e d through a polyethylene, or s i m i l a r i n s u l a t i n g material rod. The discharge, squeezed through the hole, vaporizes the polyethylene at the walls and produces an extremely hot high density plasma which radiates along the axis of the hole. Unfortunately, much of the polyethylene plasma consists of vaporized carbon which squeezes out of the ends and tends to coat the window of the source with an opaque layer, r e q u i r i n g that the window be cleaned a f t e r each shot. The usual f i r i n g procedure was as follows. The system was pumped down to below 20uHg, which was s u f f i c i e n t l y - 43 -low to ensure that breakdown did not occur. The condenser bank, described below, was charged to the desired value (usually from 3 to 10KV). The l i g h t source was f i r e d by allowing Argon into the system by way of a selenoid actuated valve. This raised the pressure i n the system u n t i l , f o r the applied voltage, a point on the Paschen curve was reached where breakdown occurred. Afte r the breakdown, the system was brought to atmospheric pressure so that the window holder (designed f o r easy removal) could be taken o f f and the window cleaned of the coating carbon. The window holder having been returned to i t s place, the system was once again pumped down i n preparation f o r another shot. Before each shot the Cs absorption tube, i n i t s moveable oven, was placed as close as possible to the l i g h t source, to obtain the highest possible i n t e n s i t y . A f t e r the shot the oven was pulled back so that the Bogen source window could be removed, cleaned and replaced. The e n t i r e f i r i n g cycle took from 2 - 1 0 minutes depending on the condition of the vacuum pump and the a g i l i t y of the operator on that day. I n i t i a l l y , the l i g h t source was triggered e l e c t r i c a l l y , by applying a pulse of =16KV which started breakdown i n an external spark gap i n series with the source. However, i t was found that the large amount of e l e c t r i c a l noise generated i n connection with the t r i g g e r pulse was unacceptable for some of the investigations which were performed, and so t h i s method was eventually abandoned. - 44 -The design of the electrodes and the spark channel which i s i l l u s t r a t e d i n Figure 5.2 was based on the source described by Zuzak (1). I t consists of two brass electrodes cast i n epoxy (strengthened with f i b e r g l a s s tape) which also separates the electrodes. A 3/4" diameter threaded poly-ethylene rod i s screwed into the epoxy such that the 3 - 4 mm diameter hole i n the rod serves as the axis of the c y l i n -d r i c a l l y symmetric apparatus. The dump chambers (Figure 5.1) consisting of 2" I.D. Pyrex tube (3" long i n the front, 12" i n the back) were sealed to the l i g h t source with rubber gaskets. The windows were 1/8" t h i c k , 1" diameter o p t i c a l f l a t s of quartz, U.V. saphire or L i F . L i F windows have the best transmission o c h a r a c t e r i s t i c s , passing =50% of the l i g h t at 1200A as o compared to 50% at 1900A f o r saphire (A1 20 3) and 50% at o o from 1700A - 2200A f o r d i f f e r e n t grades of quartz. However, quartz windows seemed to be the best i n withstanding the combined thermal and mechanical shocks r e s u l t i n g from the f i r i n g of the source, with L i F and saphire windows being roughly the same and poorer than quartz. At a discharge voltage of 9KV or greater none of the windows lasted f o r more than two shots before they became "frosted", at which point t h e i r transmission dropped to a f r a c t i o n of t h e i r "undamaged value". Although damage to the windows could be avoided, either by keeping the discharge voltage s u f f i c i e n t l y low or by moving the window away from the source, i t was S C A L E 1:1 Figure 5.2 Cross-Sectional View of Bogen Source - 46 -found t h a t a good t r a d e o f f was ac h i e v e d i f the window was kept as c l o s e as p o s s i b l e to the d i s c h a r g e and the v o l t a g e kept below 8KV. 5.3 The Power Supply To s i m u l a t e a d i r e c t c u r r e n t o f l a r g e magnitude (thus a v o i d i n g time-dependent phenomena a s s o c i a t e d w i t h a time v a r y i n g c u r r e n t ) a power supply i s r e q u i r e d which can p r o v i d e a square c u r r e n t p u l s e . To o b t a i n square c u r r e n t p u l s e s i n the kiloamp range, a lumped t r a n s m i s s i o n l i n e , . F i g u r e 5 . 3 , was used. bogen source n=5 C ^ s r l O u F R=1/\ L v__ 4=4 uH F i g u r e 5 .3 N - S e c t i o n Lumped.Transmission L i n e The l e n g t h of the square c u r r e n t p u l s e i s g i v e n by T = 2N/LC and the c u r r e n t i i n the p u l s e i s r o u g h l y - 47 -given by - — . The power supply used for the Bogen source consisted of f i v e lOyF capacitors connected with inductances of =4uH and gave a pulse which was 90usec long with a top that was e s s e n t i a l l y f l a t f o r =50us (see Figure 5.4). 10 us/cm P.M. signal .05 v/cm 10 us/cm Figure 5.4 Current Pulse  from Lumped Transmission Line Figure 5.5 Light Pulse from Bogen Source TTaken at 7KV) Figure 5.5 shows a t y p i c a l trace of photomultiplier output vs time f o r a l i g h t pulse from the Bogen source, using the power supply described above. As may be seen by comparing Figure 5.4 and Figure 5.5 , the general shape of the l i g h t pulse follows c l o s e l y that of the current pulse. 5.4 Light Intensity vs Discharge Voltage Traces l i k e the one i n Figure 5.5 were obtained f o r d i f f e r e n t values of the discharge voltage. From these traces, a graph was constructed of the peak photomultiplier output vs discharge voltage. Since the duration of the l i g h t pulse i s constant, and the i n t e n s i t y i s p r a c t i c a l l y constant f o r - 48 -a l l v o l t a g e s , the graph of peak l i g h t i n t e n s i t y vs d i s c h a r g e v o l t a g e i n the i n v e s t i g a t e d range 1-10KV, F i g u r e 5.6 w i l l be almost i d e n t i c a l to a graph of i n t e g r a t e d p u l s e i n t e n s i t y v s . d i s c h a r g e v o l t a g e . 1 1 1 1 1 1 1 i 1 r-2 4 6 8 10 Discharge Voltage in kv F i g u r e 5.6 Peak Bogen Source Output vs Discharge  V o l t a g e A graph o f l o g i n t e n s i t y vs l o g d i s c h a r g e v o l t a g e i s shown i n F i g u r e 5 . 7 . On the same axes i s p l o t e d l o g (dis c h a r g e 2 v o l t a g e ) [ p r o p o r t i o n a l to the energy s t o r e d i n the c a p a c i t o r bank] vs l o g d i s c h a r g e v o l t a g e . - 49 -log (peak R M . output) Zuzak's Results (for short / light pulses) / log(D.V.) 1 1—I I I I I I I 2 3 5 7 10 log (Discharge Vol tage) Figure 5.7 Log Peak Output vs Log Discharge Voltage From Figure 5.7, i t i s apparent that the i n t e n s i t y from the Bogen source increases as the energy i n the discharge except f o r a "kink" between 2 and 3KV, most l i k e l y due to " f r o s t i n g " of the source window at voltages greater than =2KV. This " f r o s t i n g " appears to be due to a combination of thermal shock which creates a surface fracture pattern and p i t t i n g of the surface from the impact of carbon p a r t i c l e s emitted from the source. The curve of i n t e n s i t y vs discharge voltage shown i n Figure 5.6 i s e s s e n t i a l l y s i m i l a r to that obtained by Zuzak, the only dif f e r e n c e being that, i n Zuzak's - 50 -case, the kink due to window damage occurs at =5KV rather than at 2KV. The shorter l i g h t pulse (=6us) used by Zuzak i s apparently not as damaging to the window as the longer (=90ys) pulse used for t h i s experiment. 5.5 Relative End-On S p a t i a l Intensity D i s t r i b u t i o n To obtain the s p a t i a l d i s t r i b u t i o n of i n t e n s i t y f o r the Bogen source, photographs were obtained using the arrangement shown i n Figure 5.8. S bogen source frosted screen Figure 5.8 Arrangement fo r Obtaining the Relative End-On Sp a t i a l Intensity D i s t r i b u t i o n of the Bogen Source Photographs were taken at two screen to source distances, s , the longer distance corresponding to the p o s i t i o n i n the absorption tube at which the Cs vapor density became s i g n i f i c a n t . In addition, photographs were taken of an evenly l i g h t e d step-wedge and were used to construct the H & D curve for the - 51 -f i l m (Panatomic x). From densitometer measurements of the photographs and the H & D curve, i n t e n s i t y plots were obtained for the two distances s , as shown i n Figure 5.9. x ^ D is tance f r o m Ax is in cm Figure 5.9 S p a t i a l Intensity D i s t r i b u t i o n for Two Screen to Source Distances These pl o t s yielded the spreading of the beam with distance, which was used to determine that the cons t r i c t e d arc radiates l i k e a point source located inside the c a p i l l a r y - 52 -channel of the Bogen, 2.5 cm from the end of the c a p i l l a r y . The r a t i o of the i n t e n s i t i e s at the two screen positions d i f f e r s by a factor of two from what i s expected by considering the r a t i o of the source to screen distances. The discrepancy, however, can be accounted f o r by observed shot to shot differences and s l i g h t differences i n the discharge voltages. 5.6 Absolute Intensity Measurements The absolute i n t e n s i t y of the Bogen discharge was measured at the source window by comparison with the i n t e n s i t y of a standard carbon arc. Care was taken that the o p t i c a l arrangements were the same f o r the two sources. The experimental set up i s shown i n Figure 5.10. Figure 5.10 Equipment Used for Absolute Intensity Measurement of Bogen Source - 53 -The quartz lens was used to magnify the image of the source so that the brightest central 2 mm of the source covered the entire length of the monochromator s l i t . F i r s t , the output of the carbon arc (which was run as prescribed by N u l l and Lozier (1962) (11) was measured o o as a function of wavelength i n the range 3150A - 2000A (see Figure 5.11). A chopping wheel was used between the carbon arc and the monochromator to avoid the d i f f i c u l t i e s associated with D.C. measurements. Next, using the i d e n t i c a l o p t i c a l arrangement, but without the chopping wheel, oscillograms were obtained f o r the Bogen source at o o d i f f e r e n t wavelengths between 3100A and 2000A, using a discharge voltage of 3KV. Neutral density f i l t e r s (aluminized quartz o p t i c a l f l a t s ) of known transmission c h a r a c t e r i s t i c s i n the U.V. were used between the Bogen source and the monochromator to prevent saturation of the photomultiplier and to keep the P.M. output i n the same range as the output f o r the carbon arc. From the oscillograms, the curve of peak P.M. output (adjusted by the attenuation fa c t o r of the neutral density f i l t e r s ) vs wavelength was constructed; i t i s shown i n Figure 5.11 which, i n addition shows the r a t i o of adjusted Bogen source P.M. output over carbon arc P.M. output for the range 3100A - 2000A. Wavelength in u Figure 5.11 Comparison of Bogen Source and Carbon Arc Outputs - 55 -The emissive power of the standard carbon arc at d i f f e r e n t wavelengths may be calculated from Planck's Radiation formula ( E X = X ~ 5 [ e x p ( - C 2 O T " " 1 ) - ! ] ) ; then B.S. using the C . A . r a t i o curve (Figure 5.11), the emissive power of the Bogen source window i s obtained (Figure 5.12). 10-Ex in 10 w/cm2-u X in u Figure 5.12 Emissive Power of Bogen Source - 56 -Integrating the curve of Figure 5.12, the value of 367 £^2~ i s obtained for the peak i n t e n s i t y i n the range o o 3150A - 2200A. Since the l i g h t emitting area of the 2 Bogen source window i s approximately 2.4 cm , then through the window the peak power (Bank Voltage = 3K.V.; o o 3150A > X > 2200A ) i s 882 watts and the energy per pulse i s 56 m i l l i j o u l e s . From Figure 5.12 one can also e s t a b l i s h the peak photon f l u x through the source window 20 under the same conditions; that turns out to be 5.4 x 10 2 photons/cm -sec. This value i s unfortunately a factor of 25 below the minimum photon f l u x wanted fo r the r a d i a t i o n f r o n t experiment. 5.7 Source Temperature Referring the integrated peak r a d i a t i o n i n t e n s i t y 2 2 (367 W/cm ) to the c a p i l l a r y e x i t (.15cm , 10 cm from the window) of the Bogen source, one obtains the value 2 1.5 Megawatt/cm for the r a d i a t i o n i n t e n s i t y . This fig u r e f o r the peak i n t e n s i t y i s an e f f e c t i v e value along the axis of the Bogen source; i t implies that, on axis, the Bogen source i s equivalent to a black body source which radiates 2 1.5 M.watt/cm into a half-sphere. Figure 5.13 shows an attempt to assign a black body temperature to the source. - 57 -Figure 5.13 Emi s s i v i t y of Bogen Source and Black Body  Emiss i v i t y Curves Since the Bogen emissivity curve cuts across the emi s s i v i t y curves of sources from 10,000°K to 170,000°K, i t i s apparent that a c h a r a c t e r i s t i c temperature cannot be ascribed to the source. However, the range, 10,000°K to 170,000°K includes the temperature of 60,000°K assigned - 58 -by Zuzak to the Bogen source with a short ( = 6 y s ) discharge. The peak r a d i a t i o n i n t e n s i t y i s dependent not so much on the length of the current pulse as on i t s risetime: the f a s t e r the risetime the greater the peak i n t e n s i t y . The lower r a d i a t i o n i n t e n s i t y produced by a slowly r i s i n g current pulse may be explained i f one examines conditions i n the source channel. A slowly r i s i n g current pulse allows a considerable amount of the hot plasma i n the channel to escape during the current pulse, reducing the pressure i n the channel. If the r a d i a t i o n i s not black, a lower.pressure i n the channel w i l l produce a lower peak r a d i a t i o n i n t e n s i t y than i f the current pulse i s f a s t where i n e r t i a prevents the gas from escaping out of the ends. In some cases, the length of the current pulse may be important. For a long pulse, the ejected plasma cools and becomes a r a d i a t i o n absorbing cloud which attenuates the r a d i a t i o n . This contention i s i n agreement with Cross's observations of the discharge plasma hydrogen and carbon escaping out of the ends of the c a p i l l a r y . Chapter 6 SUMMARY AND POSSIBLE IMPROVEMENTS 6.1 Summary For the study of steady i o n i z a t i o n fronts one needs an intense l i g h t source and a s u f f i c i e n t l y high absorber number density. In t h i s study, design c r i t e r i a for such an experiment were developed and some of the ideas experi-mentally tested. The absorbing medium, Cs gas, can be heated up to a maximum of about T=800°K so that an 18 3 absorber density of N 0 = 1.5 x 10 /cm can be reached. This leads to an absorption length, for i o n i z i n g r a d i a t i o n , of T _ ! „ , c The length of the absorption N a ~ " o tube was chosen to be 62 cm., which i s large compared with the absorption length L. The i n t e n s i t y W = hvF of the i o n i z i n g r a d i a t i o n should be so high that the r a d i a t i o n F front v e l o c i t y Vp = ^ — i s comparable with the speed of sound i n the absorber gas. This c r i t e r i o n leads to a 22 -2 -1 minimum photon f l u x , F m i n = x ^ phot-cm sec , which should l a s t for the time t Q = —~— - 120us, within F which the front would sweep through the absorption tube. An absorption tube and an oven f o r the tube were designed according to the above c r i t e r i a and were sub-sequently b u i l t and tested . A constricted arc l i g h t source was constructed which was believed capable of d e l i v e r i n g the required i n t e n s i t y (1) and time duration. - 59 -- 60 -The number density of Cs vapour, N. i n the absorption tube was determined from measurements of the temperature along the tube,and from the d i s t r i b u t i o n of N„ i t was established that the convection flow of Cs i n the absorption tube was small and that the Cs flow v e l o c i t y , V_, was n e g l i g i b l e compared to the expected r a d i a t i o n front v e l o c i t i e s . Attempts to measure the r a d i a t i o n front v e l o c i t y from the delay i n the o p t i c a l signal at two observation ports on the absorption tube proved to be inconclusive due to stray l i g h t problems. E l e c t r i c a l probe measurements taken at three points on the absorption tube showed that r a d i a t i o n induced i o n i z a t i o n of the Cs vapour was taking place, but that the rate of i o n i z a t i o n was not s u f f i c i e n t l y high to produce a front. This r a i s e d the suspicion that the l i g h t source was less intense than was expected. A subsequent i n v e s t i g a t i o n of the c h a r a c t e r i s t i c s of the Bogen l i g h t source used for t h i s experiment showed that the output of r a d i a t i o n capable of i o n i z i n g Cs was, at the p o s i t i o n of the dense Cs absorber, a factor of 25 too low to produce a front under the present arrangement of the experiment. Since the l i g h t i n t e n s i t y from the source drops quickly as one moves farther from i t , i t appears that r a d i a t i o n fronts cannot be produced with the present source unless the t e s t section of the absorption tube and the source can be located closer together without - 61 -exposing the window of the absorption tube to the corrossive action of the hot Cs vapor. 6.2 Possible Improvements In the experiment to study i o n i z a t i o n fronts i n Cs vapor, several areas can be discerned where improvements possibly could be made. The greatest shortcoming of the experiment proved to be an inadequate source of l i g h t . The shortage of i o n i z i n g photons could, at le a s t p a r t i a l l y , be overcome i f one were to place the source much closer to the active (high N 0) region of the absorption tube. That can be accomplished i n one of two ways: eithe r one cools strongly the absorption tube at the p o s i t i o n where i t emerges from the oven so that a long length of tube becomes unnecessary (to prevent damage to the windows) and the tube's active region can be placed very close to the l i g h t source, or another way would be to enclose i n an oven the absorption tube and part or a l l of the l i g h t source, again minimizing the distance between source and absorption tube. A t h i r d method of increasing the number of photons reaching the absorption tube i s to produce more i n the f i r s t place. This could be accomplished by upgrading the power supply of the l i g h t source. If the capacitance of the discharge bank i s increased, then the value of the required - 62 -load r e s i s t o r would drop and the peak discharge current as well as the l i g h t i n t e n s i t y (which depends on the discharge current) would increase. To obtain a measurable delay between the signals at the e l e c t r i c a l probes or observation ports on the absorption tube, a r a d i a t i o n front can be slowed down by increasing the absorber density, N o. Raising N 0 then has the added advantage that the absorption length L decreases so that the length, 1, of the absorption tube can be made smaller, making the apparatus more compact. Using the quartz absorption tube, the maximum oven temperature i s 8 00°K which i s the softening point of quartz. To obtain the higher Cs d e n s i t i e s that are possible only i f the absorption tube i s heated to temperatures i n excess of 800°K, a material other than quartz must be employed. Stainless s t e e l of a type that could withstand the corrosive properties of Cs vapor, would be a s u i t a b l e material f o r the body of the absorption tube while U.V. grade saphire could be used f o r the windows i f these were subjected to high temperatures. An area of the experiment where improvements would be welcome i s the window of the source. As matters stand, the source window must be cleaned a f t e r every shot and must be replaced p e r i o d i c a l l y when cumulative damage to the window (from the c a p i l l a r y discharge) becomes severe. A method i s required which w i l l remove the need to clean the window a f t e r each shot as well as prevent the destruction - 63 -of expensive o p t i c a l f l a t s . Such a method has not, to t h i s time been found. BIBLIOGRAPHY (1) W.W. Zuzak, Ph.D. Thesis, U.B.C., (1968). (2) R. C. Cross and R. A r d i l a , Can. J . Phys., 48, 2640, (1970). (3) H. J. J . Braddick and R. W. Ditchburn, P. Roy. Soc. London, A143, 472, (1934). (4) J . Cooper, Private Communication. (5) R. W. Ditchburn, J . Tunstead and J . G. Yates, P. Roy. S o c , A181, 386, (1943). (6) F. W. Sears, Thermodynamics , p. 269, Addison-Wesley Publishing, (1959). (7) P. J . A. Whelan, Ph.D. Thesis, U.B.C, (1964). (8) J. B. Hasted, Physics of Atomic C o l l i s i o n s , p. 432, American E l s e v i e r Publishing, (1972). (9) S. C. Brown, Basic Data of Plasma Physics, p. 164, M.I.T. Press, (1961). (10) P. Bogen, H. Conrads and D. Rusbuldt, Z. Physik, 186, 240, (1965). (11) M. N u l l and W. Louzier, J . Opt. Soc. Am., 52, 1156, (1962). - 64 -

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