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Investigations into the xenon chloride excimer laser Ford, Joseph Earl 1985

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INVESTIGATIONS INTO THE XENON CHLORIDE EXCIMER LASER By JOSEPH EARL FORD B.Sc., The U n i v e r s i t y of C a l i f o r n i a at Los Angeles, 1983 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES DEPARTMENT OF PHYSICS We accept t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA September 1985 © Joseph E a r l Ford, 1985 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 or her representatives. I t 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 P/VZS/cs The University of B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 D a t e i i ABSTRACT A di s c h a r g e pumped LC i n v e r s i o n type XeCl excimer l a s e r was c o n s t r u c t e d , and i t s d i s c h a r g e and output were examined. A maximum output energy of 167 mJ was achieved, with an e f f i c i e n c y of 0.56%, using 60 p s i of a gas composed of 0.56% HC1, 2.48% Xe, 48.48% He, and 48.48% Ne. The 308 nm l a s e r output pulse had a fwhm of 20 ns and a peak power of 8.6 MW. When charged to 35 kV, the v o l t a g e i n v e r s i o n reached a peak of ~45 kV and dropped to zero i n ~35 ns. The fwhm of the di s c h a r g e c u r r e n t was 46 ns, with a peak c u r r e n t of 15.3 kA. The e l e c t r o n d e n s i t y i n the di s c h a r g e was measured u s i n g an i n f r a r e d Michelson i n t e r f e r o m e t e r , and found to have a fwhm of 30 ns and a peak value of 12±5xl0 1 f t cm" 3. i i i TABLE OF CONTENTS ABSTRACT i i TABLE OF CONTENTS • i i i LIST OF FIGURES i v ACKNOWLEDGEMENTS v i CHAPTER 1 INTRODUCTION 1 . 1 O b j e c t i v e 1 1.2 Th e s i s O r g a n i z a t i o n 2 CHAPTER 2 BACKGROUND 2.1 A B r i e f H i s t o r y of the XeCl Laser 3 2.2 Current XeCl Lasers 6 2.3 Why XeCl? 7 CHAPTER 3 THE EXCIMER LASER 3.1 Previous Excimer La s e r s at UBC 8 3.2 The Current Excimer Laser 11 3..3 O p t i m i z a t i o n of XeCl Power Output 17 CHAPTER 4 MEASUREMENTS OF THE XeCl LASER 4.1 Voltage 25 4.2 Laser Output 26 4.3 Current 26 4.4 E l e c t r o n Density During Discharge 30 CHAPTER 5 CONCLUSION 5.1 D i s c u s s i o n of R e s u l t s 44 5.2 Suggestions f o r Future Study 47 REFERENCES 49 i v LIST OF FIGURES Figu r e 3.1: Transverse S e c t i o n of Laser 15 F i g u r e 3.2: Laser C i r c u i t Diagram 16 F i g u r e 3.3: Graph of Output Energy vs Xe:HCl R a t i o 20 Fig u r e 3.4: Graph of Output Energy vs XeCl C o n c e n t r a t i o n 21 Fig u r e 3.5: Graph of Output Energy vs Charging V o l t a g e 22 at S e v e r a l Pressures F i g u r e 3.6: Graph of Output Energy vs T o t a l Pressure at 23 S e v e r a l Charging V o l t a g e s F i g u r e 3.7: Graph of Output Energy vs Percent Helium i n 24 the He/Ne B u f f e r Gas Mix F i g u r e 4.1: O s c i l l o g r a m s of Voltage and Laser Output 27 Measurements Fi g u r e 4.2: O s c i l l o g r a m s of the d i / d t and Current 29 Measurements, and of the Laser Output with the Voltage and Current Traces F i g u r e 4.3: Diagram of the I n f r a r e d Interferometer 36 Fig u r e 4.4: O s c i l l o g r a m s of the Interferometry with a 37 Pure Helium Gas Mix Fig u r e 4.5: Graph of the E l e c t r o n Density vs Time f o r 38 3 Shots with a Pure Helium Gas Mix F i g u r e 4.6: O s c i l l o g r a m s of the Interferometry with a 39 Half C o n c e n t r a t i o n Gas Mix F i g u r e 4.7: Graph of the E l e c t r o n Density vs Time f o r 40 3 Shots with a Half Concentration Gas Mix F i g u r e 4.8: O s c i l l o g r a m s of the Interferometry f o r a 41 F u l l C o n c e n t r a t i o n (Lasing) Gas Mix, and of the XeCl IR Spontaneous Emission F i g u r e 4.9: Graph of the E l e c t r o n Density vs Time f o r 42 3 Shots with a F u l l C oncentration (Lasing) Gas Mix F i g u r e 4.lO:Graph of the 3 Shot Averages of the E l e c t r o n 43 Densi t y vs Time with Pure Helium, Half C o n c e n t r a t i o n and F u l l C o n c e n t r a t i on Gas Mixes V F i g u r e 5.1: Graph Showing the R e l a t i v e P o s i t i o n s of the 45 V o l t a g e , Current, E l e c t r o n D e n s i t y , and Laser Output Traces v i ACKNOWLEDGEMENTS I would l i k e to thank my s u p e r v i s o r , Dr. Jochen Meyer, f o r the o r i g i n a l idea of t h i s t h e s i s , and f o r h i s support and guidance d u r i n g the past two y e a r s . I g r e a t l y a p p r e c i a t e the a s s i s t a n c e rendered by Hubert Houtman, who was a constant source of i n v a l u a b l e p r a c t i c a l l a b o r a t o r y a d v i c e . I a l s o want to thank my f e l l o w graduate students John Bernard, Doug Burbidge, and Grant Mcintosh, f o r t h e i r many h e l p f u l c o n v e r s a t i o n s , and t e c h n i c i a n s Paul B u r r i l l , A l Cheuck, Jack Bosma, and Anton S c h r e i n d e r s , f o r a l l t h e i r h e l p . T h i s work i s d e d i c a t e d to my parents, Harold and L o u i s e Ford. 1 CHAPTER 1: INTRODUCTION S e c t i o n 1 . 1 : O b j e c t i v e Any attempt to model the behavior of a l a s e r t h e o r e t i c a l l y must i n c l u d e estimates of each of the v a r i a b l e s i n v o l v e d i n the r a t e equations d e s c r i b i n g the l a s e r ' s a c t i o n . Among these v a r i a b l e s i s n e , the e l e c t r o n d e n s i t y i n the l a s e r medium dur i n g the pumping d i s c h a r g e . T h i s i s an important parameter, s i n c e the terms i n the r a t e equations which d e s c r i b e the r a i s i n g of the gas molecules to the upper l a s e r l e v e l are d i r e c t l y p r o p o r t i o n a l t o i t . T h e o r e t i c a l models made of excimer l a s e r s have, i n the pa s t , been f o r c e d to use estimates, s i n c e n e has never, to the best of the author's knowledge, been measured f o r any of the excimer l a s e r s . A measurement of n e i s p o s s i b l e , however, using a f a i r l y s t r a i g h t f o r w a r d and a c c u r a t e method. Since the index of r e f r a c t i o n of a plasma i s dependent on the e l e c t r o n d e n s i t y , a graph of the e l e c t r o n d e n s i t y as a f u n c t i o n of time can be made by s e t t i n g up a Michelson i n t e r f e r o m e t e r with one of i t s beams pa s s i n g through the l a s e r medium. By observing the time evolved amplitude of the i n t e r f e r o m e t e r ' s output beam durin g the 2 d i s c h a r g e , a value f o r n £ can be c a l c u l a t e d at each extremum of the f r i n g e s generated. The o b j e c t i v e of t h i s t h e s i s was to c o n s t r u c t a discharge pumped excimer l a s e r , o ptimize i t s output using xenon c h l o r i d e as the excimer gas, c h a r a c t e r i z e i t s discharge and i t s output, and f i n a l l y make the measurement of the e l e c t r o n d e n s i t y d e s c r i b e d above. S e c t i o n 1.2: Thesis O r g a n i z a t i o n An overview of excimer l a s e r s i n g e n e r a l , and XeCl l a s e r s in p a r t i c u l a r , i s given i n Chapter 2,.which a l s o d i s c u s s e s the c h o i c e of XeCl as the p a r t i c u l a r type of excimer used. Chapter 3 c o n t a i n s d e s c r i p t i o n s of p r e v i o u s excimer l a s e r s c o n s t r u c t e d at UBC's Plasma Physics Laboratory, as w e l l as a d e t a i l e d d e s c r i p t i o n of the l a s e r used f o r t h i s experiment. The procedure followed i n o p t i m i z i n g the output energy i s given i n the l a s t s e c t i o n of Chapter 3. The measurements performed on the l a s e r are d e s c r i b e d , and the r e s u l t s g iven, i n Chapter 4. F i n a l l y , Chapter 5 c o n t a i n s a d i s c u s s i o n of the r e s u l t s , and suggestions f o r f u t u r e s t u d i e s along s i m i l a r l i n e s . 3 CHAPTER 2: BACKGROUND S e c t i o n 2.1: A B r i e f H i s t o r y of the XeCl Laser The p o s s i b i l i t y of s t i m u l a t e d emission i n a t r a n s i t i o n from a bound to a f r e e s t a t e was f i r s t suggested i n 1960 by F.G. Houterrnans 1 , who proposed that vacuum u l t r a v i o l e t r a d i a t i o n c o u l d be produced by s t i m u l a t e d emission from an e x c i t e d dimer (hence 'excimer') system XX*, where X i s any noble gas. The bound-free t r a n s i t i o n seemed an e x c e l l e n t c h o i c e f o r the c o n s t r u c t i o n of a l a s e r s i n c e the unbound lower s t a t e would r a p i d l y d i s a s s o c i a t e , making i t e a s i e r to maintain a p o p u l a t i o n i n v e r s i o n . I t proved d i f f i c u l t , however, t o pump s u f f i c i e n t energy i n t o the excimer gas, and attempts to v e r i f y t h i s e x p e r i m e n t a l l y were u n s u c c e s s f u l u n t i l 1971, when N.G. Basov 2, using l i q u i d xenon e x c i t e d by an e l e c t r o n beam, saw marginal l a s i n g at 176 nm. Low energy l a s i n g i n an e l e c t r o n beam pumped high p r e s s u r e (200-450 p s i a ) xenon gas was found soon t h e r e a f t e r 3 . Late i n 1974, Velazco and S e t s e r " noted that the chemical p r o p e r t i e s of e x c i t e d noble gases were s i m i l a r t o those of the halogens (F,C1,Br,I,At), and suggested that a noble g a s / h a l i d e 4 mix had c o n s i d e r a b l e p o t e n t i a l as an u l t r a v i o l e t l a s e r system. The lower s t a t e s of these systems were bound, but with b i n d i n g e n e r g i e s so low that there was l i t t l e e f f e c t i v e d i f f e r e n c e between them and the true bound-free systems. The f i r s t l a b o r a t o r y demonstration of noble g a s / h a l i d e l a s i n g was made the next year by Brau and Ewing, who a f t e r experimenting b r i e f l y with X e l 5 produced low energy l a s i n g on the 2 Z + 2Z* band of XeF 6 at 353 nm (=-.15 mJ), and on the corresponding bands of XeCl at 308 nm (=-.05 mJ) and KrF at 248 nm (<*10 mJ) 7, while S e a r l e s and H a r t 8 achieved l a s e r a c t i o n from XeBr at 282 nm (===.1 mJ). In each case, an e l e c t r o n beam was used to pump the gas mix, which was composed mostly of a l i g h t w e i g h t i n e r t b u f f e r gas (Ar or He), with a few percent of the l a s i n g i n e r t gas s p e c i e s , and a f r a c t i o n of a percent of the halogen. T o t a l pressure ranged from 10 to 50 p s i a . Up to t h i s p o i n t , a l l of the excimer l a s e r s made had used e l e c t r o n beam pumping. However, while e-beam pumping worked, i t was complicated and, s i n c e the e l e c t r o n source was separated from the gas mix by only a t h i n and e a s i l y damaged metal f o i l , u n r e l i a b l e . A d i r e c t e l e c t r i c d i s c h a r g e would have been an improvement, but i t was d i f f i c u l t to maintain a s t a b l e d i s c harge long enough t o d e p o s i t the necessary e n e r g y — s e v e r a l hundred j o u l e s per l i t e r . At p r e s s u r e s higher than a few hundred m i l l i t o r r , streamers appeared i n the d i s c h a r g e and developed i n t o a r c s w i t h i n a few nanoseconds. I t was found that i f the gas mix was p r e - i o n i z e d by u l t r a v i o l e t or x-ray r a d i a t i o n a few hundred nanoseconds before the main d i s c h a r g e a 5 uniform glow di s c h a r g e c o u l d be maintained, even at pressures i n excess of f i v e atmospheres. In 1977, Ishenko, L i s i t s y n and Razhev 9 a p p l i e d t h i s method to a XeCl l a s e r . Using a discharge e x i t e d He/Xe/BCl 3 mix at p r e s s u r e s of s e v e r a l atmospheres, they were able to produce a 3.4 mJ output, with an e f f i c i e n c y of 0.5% and a f u l l width at h a l f maximum of about 5 ns. By t h i s time, a number of d i f f e r e n t c h l o r i n e donors had been t r i e d , i n c l u d i n g C l 2 , BC1 3, C F 2 C 1 2 , CC1„, and C 2 F 2 C 1 . Much higher output e n e r g i e s — m o r e than one hundred m i l l i j o u l e s — w e r e achieved when HC1 was used as the c h l o r i n e donor, as was shown by Burnham 1 0, and Sze and S c o t t 1 1 , independently. In both cases, e l e c t r i c discharge l a s e r s with He as the b u f f e r gas were used. Ext e n s i v e parametric a n a l y s i s of the power output as a f u n c t i o n of v a r i a t i o n s i n the gas mix and c h a r g i n g v o l t a g e i n a discharge pumped XeCl l a s e r was done by S z e 1 2 i n 1977, who found that u s i n g neon as the b u f f e r gas gave him h i s best r e s u l t s , e s p e c i a l l y at higher charging v o l t a g e s and p r e s s u r e s . A gas mix of 0.2% HC1 and 5% Xe i n 50 p s i a of Ne produced 350 mJ, with an e f f i c i e n c y of about 1%. A XeCl l a s e r u sing a Blumlein type d i s c h a r g e c i r c u i t was c o n s t r u c t e d by Chen, Fu, and L i u 1 3 which produced over 400 mJ with a comparatively high e f f i c i e n c y of 1.7%. 6 S e c t i o n 2.2: Current XeCl Lasers The t r e n d towards higher pressures and e f f i c i e n c i e s i n XeCl o s c i l l a t o r s has continued in the l a s t few y e a r s . In 1983, Long, Plummer and S t a p p a e r t s 1 4 r e p o r t e d on a d i s c h a r g e pumped l a s e r which uses a h i g h voltage (=50 kV) p r e p u l s e f o l l o w i n g uniform p r e i o n i z a t i o n to provide s t a b i l i t y and n e a r l y p e r f e c t impedance matching f o r the d u r a t i o n of the e l e c t r i c p u l s e , which i s of lower v o l t a g e (=17 kV) and longer d u r a t i o n (=200 ns) than i n most d i s c h a r g e pumped excimer l a s e r s . Using t h i s method they were able to produce a 4.2 J output energy at a 4.2% e f f i c i e n c y , with a f u l l width at h a l f maximum of 120 ns. The gas mix used was 0.1% HCl and 1 % Xe i n 4 atm of neon. I t seems reasonable to assume that much higher e n e r g i e s may be produced by such a design simply by extending the l e n g t h of the lower v o l t a g e main d i s c h a r g e , although t h i s energy would be spread over an output p u l s e of g r e a t e r d u r a t i o n . Even more r e c e n t l y , M i y a z a k i , Toda, Hasama and S a t o 1 5 have b u i l t a simple and comparatively e f f i c i e n t XeCl l a s e r which uses a d i s c h a r g e c i r c u i t which sends the main d i s c h a r g e c u r r e n t through an a r r a y of p i n spark gaps l o c a t e d near the cathode s u r f a c e . T h i s provides automatic UV p r e i o n i z a t i o n with a f i x e d t i m i n g d e l a y r e l a t i v e to the main d i s c h a r g e , without i n t r o d u c i n g a secondary p r e i o n i z a t i o n c i r c u i t . I t uses a c a p a c i t o r t r a n s f e r type charging c i r c u i t , with a primary (storage) capacitance of 59.4 nF and a secondary (discharge) c a p a c i t a n c e of 54.0 nF. The e l e c t r o d e s were 4 by 54 cm, with a 7 s e p a r a t i o n of 1.8 cm. The gas mix used was 0.07% HC1 and 1.3% Xe i n 4 atm of neon. A peak e f f i c i e n c y of 2.9% was achieved with an output energy of 280 mJ, while the maximum output energy was 680 mJ, with an 1.8% e f f i c i e n c y and a fwhm of 20 ns. S e c t i o n 2.3: Why XeCl? Excimers are s i m i l a r enough that the same l a s e r d e v i c e can be used f o r s e v e r a l d i f f e r e n t gas mixes, so that a KrF l a s e r becomes a K r C l , XeCl, XeBr, ArF, e t c . l a s e r with only a change of gas f i l l . There was not enough time, however, to optimise the power output and make the e l e c t r o n d e n s i t y measurement on more than a s i n g l e type. Xenon c h l o r i d e was chosen over the others because of i t s reasonably high energy, power, and e f f i c i e n c y and because i t s 308 nm wavelength allowed f o r the f u t u r e p o s s i b i l i t y of i n j e c t i o n mode l o c k i n g u sing a frequency doubled rhodamine 6G p e r c h l o r a t e dye l a s e r to produce s h o r t , high i n t e n s i t y UV l i g h t p u l s e s from the XeCl l a s e r . I t c o u l d then be used to pump a v a r i e t y of short pulse dye l a s e r s . Shorter wavelength r a d i a t i o n , such as produced by a KrF l a s e r (243 nm), i s more l i k e l y to d i s a s s o c i a t e the dye l a s e r molecules, reducing the dye's usable l i f e t i m e , while longer wavelength l i g h t i s not as w e l l absorbed. Xenon c h l o r i d e ' s i ntermediate l a s i n g frequency makes i t a good c h o i c e f o r a dye pump l a s e r . 8 CHAPTER 3: THE EXCIMER LASER S e c t i o n 3.1: Previous Excimer Las e r s at UBC The f i r s t excimer l a s e r to be b u i l t at the U n i v e r s i t y of B r i t i s h Columbia's Plasma Physics Department was designed i n 1978 by Dr. J . Meyer. I t was intended to be a powerful and f a i r l y h i g h e f f i c i e n c y UV l a s e r , f o r use as a general purpose d i a g n o s t i c t o o l f o r studying plasmas. The l a s e r was c o n s t r u c t e d by H. Houtman and A. Bhanji and was to be used by P. P i l o n f o r h i s Ph.D. P r o j e c t . I t used a Marx bank to p u l s e charge a double a r r a y of doorknob c a p a c i t o r s to approximately 70 kV. In order to prevent high v o l t a g e a r c i n g , the c a p a c i t o r s were submerged in an o i l tank l o c a t e d beneath the gas chamber. Current was conducted to a p a i r of l a r g e (12 by 60 cm) e l e c t r o d e s v i a low inductance copper sheets. P r e i o n i z a t i o n was powered by a second Marx bank, which sent a high c u r r e n t through a s e r i e s of small spark gaps (the p r e i o n i z e r rods) running a l o n g both s i d e s of the e l e c t r o d e s . The e l e c t r o d e s e p a r a t i o n was 5 cm, making an a c t i v e volume of 0.75 l i t e r s , which was l a r g e compared to contemporary excimer l a s e r s . The o p t i c a l c a v i t y used was an unstable resonator. A number of d i f f e r e n t excimer gas mixes 9 were used, i n c l u d i n g XeF, XeCl, and KrF. I t s performance was, however, d i s a p p o i n t i n g . While i t s peak energy f o r XeCl was good (=0.4 J) the e f f i c i e n c y , at about 0.2%, was much lower than expected. XeF and KrF were even worse, with output e n e r g i e s and e f f i c i e n c i e s of 0.2 J and 0.1%, and 0.15 J and 0.07%, r e s p e c t i v e l y . Power d i d not s c a l e with s i z e as w e l l as expected, and the energy per u n i t a c t i v e volume was very low compared to other excimer l a s e r s . Attempts to i n j e c t i o n mode lock the XeCl l a s e r with a frequency doubled Rhodamine 6G dye l a s e r were not s u c c e s s f u l , and an attempt to measure the e l e c t r o n d e n s i t y of the d i s c h a r g e using C0 2 i n t e r f e r o m e t r y produced h i g h l y ambiguous r e s u l t s , probably due to the l a c k of f a s t enough e l e c t r o n i c s . The l a s e r was shelved for some time, and when a new graduate student (myself) began working on the excimer p r o j e c t i t was decided not to continue working on t h i s d e s i g n . The l a r g e s e p a r a t i o n between the c a p a c i t o r s and the e l e c t r o d e s made i t impossible to have a very f a s t f i r i n g c i r c u i t , and the charging and p r e i o n i z a t i o n Marx banks were bulky and c o m p l i c a t e d . A new l a s e r was to be designed and c o n s t r u c t e d 'from s c r a t c h ' . In the new l a s e r , the f i r i n g c i r c u i t inductance was to be reduced. One way to do t h i s i s to put the c a p a c i t o r s i n s i d e the gas chamber, sandwiched between the e l e c t r o d e s , as was done by Arm a n d i l l o , Bonanni, and G r a s s o 1 6 . Another way i s to use low inductance metal sheet c a p a c i t o r s and a Blumlein type d i s c h a r g e 10 c i r c u i t , a s w a s d o n e b y B u r n h a m 1 7 . T h e r e a r e d i f f i c u l t i e s a s s o c i a t e d w i t h b o t h m e t h o d s , o f c o u r s e . T h e p r o b l e m w i t h t h e f o r m e r w a s t h a t t h e m o r e t h i n g s o n e p u t s i n t h e g a s c h a m b e r t h e h a r d e r i t b e c o m e s t o k e e p t h e g a s m i x p u r e , a v e r y i m p o r t a n t c o n s i d e r a t i o n , e s p e c i a l l y i n v i e w o f t h e h i g h r e a c t i v i t y o f t h e h a l o g e n g a s e s . I n t h e l a t t e r c a s e , t h e p r o b l e m w a s t h a t m e t a l s h e e t c a p a c i t o r s w i t h t h e d e s i r e d c a p a c i t a n c e w o u l d c o v e r a n a r e a o f s e v e r a l s q u a r e m e t e r s , m a k i n g t h e l a s e r t o o c u m b e r s o m e . I t w a s d e c i d e d t o u s e l u m p e d ( a s o p p o s e d t o m e t a l s h e e t ) c a p a c i t o r s a n d k e e p t h e m o u t s i d e o f t h e g a s c h a m b e r , b u t a s n e a r a s p o s s i b l e t o ' t h e e l e c t r o d e s . I n o r d e r t o d o t h i s , a r e c t a n g u l a r r a t h e r t h a n c y l i n d r i c a l g a s c h a m b e r w a s u s e d . A v o l t a g e d o u b l i n g L C i n v e r s i o n c i r c u i t w a s d e s i g n e d , s o t h a t t h e c h a r g i n g v o l t a g e c o u l d b e l o w e r e d , a n d s t i l l h a v e a p p r o x i m a t e l y t h e s a m e d i s c h a r g e v o l t a g e . T h e v o l t a g e i n v e r s i o n c u r r e n t w o u l d b e r o u t e d t h r o u g h t h e p r e i o n i z e r r o d s b e f o r e g o i n g t o t h e e l e c t r o d e s , s o a s t o p r o v i d e a u t o m a t i c p r e i o n i z a t i o n w i t h a f i x e d t i m i n g d e l a y r e l a t i v e t o t h e m a i n d i s c h a r g e , w i t h o u t u s i n g a s e p a r a t e p r e i o n i z a t i o n c h a r g i n g a n d f i r i n g c i r c u i t . T h e a c t i v e v o l u m e w a s s m a l l e r , a b o u t o n e q u a r t e r t h a t o f t h e p r e v i o u s l a s e r , b u t i t w a s h o p e d t h a t a s u f f i c i e n t l y h i g h e r e f f i c i e n c y w o u l d b e a c h i e v e d t o r e s u l t i n s i m i l a r o u t p u t e n e r g i e s . P l a n e - p l a n e o p t i c s w o u l d b e u s e d , a t l e a s t i n i t i a l l y . T h e n e w l a s e r a n d a n e w g a s h a n d l i n g s y s t e m w a s c o n s t r u c t e d . I t l a s e d , b u t o n l y w e a k l y , p r o d u c i n g t w o t h i n s t r i p e s o f l o w e n e r g y (=*2 m J ) r a d i a t i o n , a n d o n l y w i t h a g a s 11 mix of extremely high Xe and HC1 concentration--more than ten times normal. A f t e r a month or so of t e s t i n g , i t was c l e a r that there were other problems as w e l l . The maximum v o l t a g e i t could be charged to was only 25 kV, at which p o i n t a i r sparks would occur between the c a p a c i t o r charging p l a t e s . The l u c i t e body was glued together with a compound s u s c e p t i b l e to c o r r o s i o n in the XeCl gas, and was too t h i n i n p l a c e s to prevent a r c i n g from the e l e c t r o d e s through to the e x t e r n a l e l e c t r i c a l components. Leaks s t a r t e d to develop, making i t hard to keep the gas mix pure and making the l a s e r unsafe to operate. F i n a l l y , the discharge o f t e n arced from the p o s i t i v e e l e c t r o d e to the p r e i o n i z e r rods, wasting most of the di s c h a r g e energy. The ba s i c design was s t i l l good, but i t was obvious that the l u c i t e body would have to be redesigned, as w e l l as the way the c a p a c i t o r s were mounted. T h i s was done, and the f i n a l - - o r at l e a s t c u r r e n t - - l a s e r was c o n s t r u c t e d . S e c t i o n 3.2: The Current XeCl Laser In order to stop the gas leakage from the seams of the l u c i t e body, the new body was machined out of a s i n g l e 40 by 13 by 7 cm block of l u c i t e , so that there would be no pressure bearing glue seams. The w a l l s were made t h i c k e r i n the pla c e s where a r c i n g through had occured i n the previous l a s e r . The same 2.5 by 28 cm e l e c t r o d e s were used, but they were reshaped to a smoother p r o f i l e to make the di s c h a r g e more uniform, so as 1 2 to a v o i d the uneven output beam p a t t e r n produced by i t s predecessor. The e l e c t r o d e s e p a r a t i o n was 2.5 cm. The same p r e i o n i z e r rods were reused. Twenty-four 2.7 nF doorknob c a p a c i t o r s were arranged i n t o two rows of s i x along each side of the l a s e r body, and were connected d i r e c t l y to the o u t s i d e s u r f a c e of the e l e c t r o d e s with low inductance copper sheet. Copper p l a t e s connected the top and bottom rows of c a p a c i t o r s . See f i g u r e 3.1 f o r a diagram showing a t r a n s v e r s e s e c t i o n of the l a s e r . Plane-plane o p t i c s were used. The rear r e f l e c t o r was a fused quartz f l a t with aluminum evaporated on the rear surface (so that i t c o u l d not be corroded by the gas mix, as had happened in e a r l i e r attempts) having 84% r e f l e c t a n c e and 0% t r a n s m i t t a n c e . The r e f l e c t i v i t y of the output coupler was v a r i e d by c o a t i n g the o u t s i d e s u r f a c e of another fused quartz f l a t with d i f f e r e n t t h i c k n e s s e s of aluminum, but the best output energy was achieved u s i n g an uncoated quartz f l a t , h aving 6% r e f l e c t a n c e and =75% t r a n s m i t t a n c e . The o p t i c a l parameters were measured at 308 nm with a Beckmann Spectrophotometer to a p r e c i s i o n of ±2%. E s s e n t i a l l y the same e l e c t r i c a l f i r i n g c i r c u i t was used as i n the p r e v i o u s l a s e r , the only major d i f f e r e n c e being i n the p r e i o n i z e r c i r c u i t . An attempt was made to stop the a r c i n g between the p o s i t i v e e l e c t r o d e and the p r e i o n i z e r rods by i n t r o d u c i n g a high inductance i n the p r e i o n i z e r c i r c u i t . T h i s 1 3 worked, but only when t h i s inductance was made so l a r g e as to i n t e r f e r e s i g n i f i c a n t l y with the LC i n v e r s i o n e f f e c t i v e n e s s . I t was f i n a l l y decided to remove the p r e i o n i z e r s from the i n v e r s i o n c i r c u i t a l t o g e t h e r and use a second, smaller set of c a p a c i t o r s (charged by the same power supply as the main banks), d i s c h a r g e d through a second spark gap to p r o v i d e energy for the p r e i o n i z a t i o n . T h i s way, the r e l a t i v e t i m i n g of the two d i s c h a r g e s c o u l d be v a r i e d , and although the l a s e r was made more complicated, the problem of a r c i n g was at l a s t s o l v e d . The l a s e r c i r c u i t i s shown in f i g u r e 3.2. Four small (0.5 nF) c a p a c i t o r s (not shown i n the diagram) were used to. c a p a c i t i v e l y couple the p r e i o n i z e r rods to the top and bottom e l e c t r o d e s , e nsuring that the rods stayed at the midplane p o t e n t i a l . They d i d not measurably a f f e c t the power output, but s i n c e they seemed to make the d i s c h a r g e a l i t t l e more uniform they were l e f t on. The impedence, L 2 , and r e s i s t a n c e , R 2, of the i n v e r s i o n c i r c u i t were measured i n d i r e c t l y by f i r i n g the l a s e r with too low a c h a r g i n g v o l t a g e to break down the d i s c h a r g e gap, r e s u l t i n g i n a r i n g i n g s e r i e s LRC c i r c u i t . Photographing the v o l t a g e t r a c e allowed the frequency of the o s c i l l a t i o n s (co) and the decay time (r) to be measured. The c i r c u i t can be a n a l y z e d 1 8 to get: i ( t ) = (V/uL 2)e sin(cot) eq. 3.2.1 L 2 = 1 / ( C ( O ) 2 + 1 / T 2 ) ) eq. 3.2.2 1 4 R 2 = 2 L 2 / T eq. 3.2.3 Where C=half of the t o t a l capacitance=32.4 nF. T h i s g i v e s L a=l90 nH and R_=0.27 fl. The impedance of the d i s c h a r g e c i r c u i t , L,, was determined by the p h y s i c a l design of the l a s e r , and c o u l d a l s o not be measured d i r e c t l y . I t was p o s s i b l e to estimate i t , however, by t r e a t i n g the discharge c i r c u i t as a s i n g l e t u r n c o i l type i n d u c t o r , so t h a t : L, = N 2 = = 24 nH eq. 3.2.4 1 1 Where N=the number of turns=1, A=the c r o s s s e c t i o n a l area of the coil=46 cm 2, l=the l e n g t h of the c o i l = t h e l e n g t h of the electrodes=28 cm, and M 0=the p e r m e a b i l i t y of f r e e space. While an e l e c t r i c a l d i s c h a r g e does not behave as an ohmic r e s i s t o r , i t can s t i l l be t r e a t e d as such, and an approximate value f o r i t s r e s i s t a n c e can be c a l c u l a t e d by d i v i d i n g the average p o t e n t i a l between the e l e c t r o d e s by the average c u r r e n t in the d i s c h a r g e (see Chapter 4 f o r the v o l t a g e and c u r r e n t measurements) which r e s u l t e d i n a value of about 2 Ji. T h i s allows the s i m p l i f i e d d i s c h a r g e c i r c u i t a f t e r v o l t a g e i n v e r s i o n (see f i g u r e 3.2, i n s e t ) to be drawn, so that t h i s l a s e r can e a s i l y be compared to ot h e r s which have very d i f f e r e n t kinds of discharge c i r c u i t s . 16 PULSE GENERATOR j.—WV\—|l> -AVA—• +V, C= 6A-8 nF R2= 0 - 2 711 L2= 190 nH L t S 2 4 nH RG= 1MO F^ = 50 MO C'= 10-8nF Re= 15 MO \%= 30 MO SIMPLIFIED D ISCHARGE CIRCUIT (AFTER V O L T A G E INVERSION) J=L = 12nH 2 1/4 C = 16-2nF- V=1-3VC v r " R " ~ y i s 2n F i g u r e 3.2: C i r c u i t diagram f o r the excimer l a s e r . 1 7 Se c t i o n 3.3: O p t i m i z a t i o n of XeCl Output Energy For the reasons given i n Chapter 2, XeCl was the excimer chosen f o r t h i s experiment. In order f o r the e l e c t r o n d e n s i t y measurement to be meaningful, the o p e r a t i o n a l parameters of the l a s e r had to be at l e a s t c l o s e to the values at which i t would normally f u n c t i o n . T h i s meant that a systematic search had to be conducted f o r a set of values that would g i v e a high energy and reasonable e f f i c i e n c y . The v a r i a b l e s were: the ch a r g i n g v o l t a g e , V ; the p r e i o n i z a t i o n t i m i n g delay, At; the t o t a l gas pressur e , P t; the percentage of the gas of each of the two l a s i n g components, Xe and HC1; and the percentages of each of three p o s s i b l e b u f f e r gasses, He, Ne, and Ar. While these seven v a r i a b l e s gave too many p o s s i b l e combinations f o r a completely c o n c l u s i v e search to be made w i t h i n a reasonable l e n g t h of time, i t was s t i l l p o s s i b l e to get a good idea of the dependence of the performance on each f a c t o r . Output energy was measured with a Gentec ED200 Joulemeter. The f i r s t s t e p was to f i n d the best Xe:HCl r a t i o and XeCl c o n c e n t r a t i o n . F i g u r e 3.3 shows energy vs Xe:HCl r a t i o f o r four d i f f e r e n t c o n c e n t r a t i o n s , with V c and P t near t h e i r maximum p o s s i b l e v a l u e s , at 35 kV and 60 p s i a , r e s p e c t i v e l y , a p r e i o n i z a t i o n delay time of 200 ns, and with pure He used as the b u f f e r gas. From t h i s a Xe:HCl r a t i o of 4.5:1 was chosen. Next, energy as a f u n c t i o n of XeCl c o n c e n t r a t i o n was examined more c a r e f u l l y ( f i g u r e 3.4), with P t, V cand At as befo r e . The 18 energy seemed to l e v e l o f f when the percentage of HCl was 0.5 or more, and so a HCl percentage of 0.56 was s e l e c t e d , to be on the safe s i d e . T h i s meant that the Xe percentage would be 2.48. When the p r e i o n i z a t i o n delay was v a r i e d , i t turned out that the dependence of energy on i t was so weak as to be n e g l i g i b l e over a broad range, from 100 to 1500 ns. An intermediate value of At=600 ns was chosen. At t h i s p o i n t , the dependance of energy on v o l t a g e and pressure was examined. F i g u r e s 3.5 and 3.6 p r e s e n t the same data two d i f f e r e n t ways, both of which make i t c l e a r t h a t the higher the pressure and v o l t a g e , the higher the output energy, at l e a s t w i t h i n the l i m i t s of t h i s system. In order to prevent damage to the c a p a c i t o r s , a maximum volt a g e of 35 kV would be used. A maximum pressure of 60 p s i a was chosen, so t h a t there would be no danger of the gas chamber b u r s t i n g , but i t seems probable that l i m i t was set too low, and that the l a s e r c o u l d be operated at pressures of 70 to 80 p s i a without s i g n i f i c a n t hazard, e s p e c i a l l y s i n c e the l a s e r was contained i n a l u c i t e box. F i n a l l y , the other b u f f e r gases were t r i e d , non s y s t e m a t i c a l l y , with a v a r i e t y of p r e s s u r e s , c o n c e n t r a t i o n s , and Xe:HCl r a t i o s . Under no circumstances d i d argon perform as w e l l as helium, but when neon was mixed with helium a s l i g h t improvement in output energy was seen ( f i g u r e 3.7), with the 19 maximum o c c u r i n g with a b u f f e r gas mix composed of 50% helium and 50% neon. The f i n a l parameter values were: Charging V o l t a g e : 35 kV T o t a l P r e s s u r e : 60 p s i P r e i o n i z a t i o n Delay: 600 ns Gas Composition: 0.56% HCl 2.48% Xe 48.48% He 48.48% Ne 2 0 (pui) R6jeu_ Figure 3.3: Graph of output energy vs Xe:HCl r a t i o . Vc=35 kV, Pt= 60 psia, At=200 ns, Helium buffer gas. Energy vs XeCl C o n c e n t r a t i o n F i g u r e 3.4: Graph of output energy vs XeCl c o n c e n t r a t i o n . V c=35 kV, P t= 60 p s i a , At=200 ns, Helium b u f f e r gas. 22 Energy vs V o l t a g e @ s e v e r a l p r e s s u r e s o-i 1 \ — i 1 —I 15.0 20.0 25.0 30.0 35.0 40.0 Voltage (kV) F i g u r e 3.5: Graph of output energy vs c h a r g i n g v o l t a g e f o r 4 p r e s s u r e s . Gas mix c o n t a i n e d 0.56% HCl and 2.48% Xe. At=600 ns. 23 Energy vs P r e s s u r e @ s e v e r a l v o l t a g e s o _ o o-oo o-- 5 CO c. <D C o ^ d-<- 25 kV o-CNJ o d. 20 kV 15 kV 20.0 30.0 40.0 50.0 Pressure (psL) 60.0 70.0 F i g u r e 3.6: Graph of output energy vs t o t a l p r e ssure f o r 6 c h a r g i n g v o l t a g e s . Gas mix contained 0.56% HCl and 2.48% Xe. At=600 ns. 24 Energy vs "/.Helium In He/Ne B u f f e r Gas Mix o o ° 4 , , , , 0.0 20.0 40.0 60.0 80.0 100.0 Percent Helium F i g u r e 3.7: Graph of output energy vs the percentage He in the He/Ne b u f f e r gas mix. V c =35 kV, Pt =60 p s i a , At = 600 ns. The gas mix c o n t a i n e d 0.56% H c l and 2.48% Xe. 25 CHAPTER 4: MEASUREMENTS OF THE XeCl LASER Se c t i o n 4.1: Voltage In order t o measure the v o l t a g e at v a r i o u s p o i n t s i n the l a s e r ' s c i r c u i t , two i d e n t i c a l high v o l t a g e a t t e n u a t o r s were c o n s t r u c t e d , so that the v o l t a g e s at two p o s i t i o n s c o u l d be r e l i a b l y compared. T h e i r a t t e n u a t i o n f a c t o r s were measured to be 90±2:1, wi t h s i g n a l r i s e times of about 10 ns. They used r e s i s t o r s wired i n s e r i e s , and used the 50 impedence of the o s c i l l o s c o p e as pa r t of t h e i r c i r c u i t . They were used to measure the v o l t a g e on the p r e i o n i z a t i o n rods and the top main e l e c t r o d e , so that the r e l a t i v e t i m i n g At of the p r e i o n i z a t i o n to the main d i s c h a r g e c o u l d be measured ( f i g u r e 4.1 A). When they were used to look at the r i n g i n g LC i n v e r s i o n ( f i g u r e 4.1 B), they worked only f o r a few shots before breaking down. They were r e b u i l t , and s e v e r a l more r e s i s t o r s were added, b r i n g i n g the a t t e n u a t i o n f a c t o r to 123±3:1 and the s i g n a l r i s e time to 18 ns. They were then used to measure the p o t e n t i a l between the top and bottom e l e c t r o d e s . The bottom e l e c t r o d e was grounded, but i t was p o s s i b l e that the small inductance i n the ground wire would a l l o w i t to a c q u i r e a b r i e f h i g h v o l t a g e d u r i n g d i s c h a r g e . T h i s turned out not to be the case, and only a 26 s i n g l e a t t e n u a t o r was needed to measure a c c u r a t e l y the p o t e n t i a l between the e l e c t r o d e s ( f i g u r e s 4.1 C and 4.2 D). S e c t i o n 4.2: Laser Output The t o t a l output energy was measured with a Gentec ED200 Joulemeter, which produced 8.52 V / J . The s i g n a l was d i s p l a y e d on a c o n v e n t i o n a l o s c i l l o s c o p e with the beam i n t e n s i t y turned up high enough that the s i n g l e sweep t r a c e c o u l d be read f o r s e v e r a l seconds a f t e r i t t r i g g e r e d . The l a s e r p u l s e shape ( f i g u r e 4.1 D) was determined by sending a h e a v i l y a t t e n u a t e d (=1000X) s i g n a l i n t o a Hamamatsu R1193U-03 phototube, which showed a smoothly shaped pulse with a 8 ns r i s e time and a f u l l width at h a l f maximum of 19 ns. The output was c a l i b r a t e d by t a k i n g the average output energy and d i v i d i n g by the average area under the t r a c e , which gave a peak power of 8.58 MW. O s c i l l o g r a m s showing the r e l a t i v e t i m i n g of the l a s e r l i g h t and the c u r r e n t s i g n a l s , and the l i g h t and the v o l t a g e s i g n a l s , are shown i n f i g u r e s 4.2 C and 4.2 D, r e s p e c t i v e l y . The l a s e r output beam p a t t e r n was a one i n c h by one i n c h square. There were two broad bands of somewhat high e r i n t e n s i t y along the s i d e s of the beam p a t t e r n . T h i s e f f e c t had been more prominent i n the p r e v i o u s l a s e r (see s e c t i o n 3.1), and was reduced by smoothing the e l e c t r o d e p r o f i l e . Reshaping the e l e c t r o d e s once again c o u l d have evened the output s t i l l f u r t h e r , but s i n c e i t would have been easy to remove too much 27 F i g u r e 4.1: A: Vo l t a g e t r a c e s from the main e l e c t r o d e s (top) and the p r e i o n i z a t i o n rods (bottom). V e r t i c a l s c a l e was ~9.5 kV/div, h o r i z o n t a l was 500 n s / d i v . Vc =25 kV, gas mix 2.5% HC1 and 10% Xe i n 50 p s i a Ne. B: V o l t a g e t r a c e of the p r e i o n i z e r rods (top) and main e l e c t r o d e s (bottom), showing a r i n g i n g LC i n v e r s i o n c i r c u i t . V e r t i c a l s c a l e was ""9.5 kV/div, h o r i z o n t a l was 2 u s / d i v . V c=25 kV, Gas mix was a i r at 24 p s i a . P r e i o n i z e r s d i d not f i r e . C: V o l t a g e t r a c e f o r f i n a l gas mix. A t t e n u a t i o n f a c t o r was 6100. V c=35 kV, At=600 ns, Gas mix was 0.56% HC1, 2.48% Xe, 48.48% He, and 48.48% Ne. Pt =60 p s i a . D: Laser output, measured with a Hamamatsu photodiode and an unknown a t t e n u a t i o n (500 mV/div). Same o p e r a t i n g parameters as i n C, above. 28 m a t e r i a l ( r e s u l t i n g i n a t h i n c e n t r a l s t r i p e ) , i t was deci d e d to leave the e l e c t r o d e s as they were. S e c t i o n 4.3: Current The c u r r e n t was measured i n d i r e c t l y , using a Rogowski c o i l , which c o n s i s t e d of ten turns of wire wrapped i n t o a h e l i x 10 cm long and connected to le n g t h of c o a x i a l c a b l e , the whole being h e a v i l y i n s u l a t e d to prevent a r c i n g through to the c o i l . When i n s e r t e d i n t o the space j u s t o u t s i d e the gas chamber, between the discharge c a p a c i t o r s , the c o i l picked up the magnetic f i e l d generated by the main di s c h a r g e c u r r e n t , so that the s i g n a l generated by the c o i l ( f i g u r e 4.2 A) was p r o p o r t i o n a l to the d e r i v a t i v e of the c u r r e n t , with a c e r t a i n amount of n o i s e due to c u r r e n t s f l o w i n g through other p a r t s of the l a s e r . T h i s s i g n a l c o u l d be e l e c t r i c a l l y i n t e g r a t e d to get the c u r r e n t t r a c e ( f i g u r e 4.2 B and C). T h i s s i g n a l c o u l d be c a l i b r a t e d by c a l c u l a t i n g the t o t a l charge i n the c a p a c i t o r s and comparing i t to the i n t e g r a t e d c u r r e n t s i g n a l , so that i f the f i n a l charge l e f t on the c a p a c i t o r s i s assumed to be n e g l i g i b l e , then; A K = eq. 4.3.1 CV Where A=the area under the c u r r e n t s i g n a l , K=the constant of p r o p o r t i o n a l i t y between the s i g n a l and the discharge c u r r e n t , Figure 4.2: A: Output of a Rogowski c o i l pickup placed near the main discharge, attenuated by a factor of 18.5 (5V/d iv ) . Vc=35 kV, At = 600 ns, Gas mix was 0,56% H C l , 2.48% Xe, 48.48% He, and 48.48% Ne. P t=60 p s i a . B: E l e c t r i c a l l y integrated Rogowski c o i l s i g n a l , showing discharge current (2V/d iv ) . Same gas mix as in A, above. C: Relat ive timing of current s ignal (top, 2V/div) and laser output (bottom, 1V/d iv ) . Same gas mix as in A, above. D: Relat ive timing of electrode po tent ia l (top, 5V/div) and laser output (bottom, 1V/div) . Same gas mix as in A, above. 30 i=the c u r r e n t , Q=the t o t a l charge c a r r i e d by the d i s c h a r g e c a p a c i t o r s , C=the t o t a l c a p a c i t a n c e of the d i s c h a r g e c a p a c i t o r s , V=V c=the charg i n g v o l t a g e . Using t h i s method, the maximum c u r r e n t was determined to be 1 5 . 3 ± . 8 kA, with a f u l l width at h a l f maximum of 46.1 ns. Se c t i o n 4.4: E l e c t r o n D e n s i t y During Discharge The measurement of the e l e c t r o n d e n s i t y d u r i n g the discharge was made by c o n s i d e r i n g the e f f e c t of f r e e e l e c t r o n s on the index of r e f r a c t i o n of the gas mix. If Tj=the index of r e f r a c t i o n , X=the wavelength of l i g h t i n the gas, n=the f r e e e l e c t r o n d e n s i t y i n the gas, and n c=the c r i t i c a l e l e c t r o n d e n s i t y f o r r a d i a t i o n of wavelength X i n a plasma, then: T?(X) = J l - ( n / n C ( X ) ) ' eq. 4.4.1 The longer the wavelength of the probe beam, the l a r g e r the dependence of the index of r e f r a c t i o n on the e l e c t r o n d e n s i t y , and so an i n f r a r e d C 0 2 l a s e r was chosen as the source of the probe beam. One was assembled, and the wavelength of the p a r t i c u l a r band i t operated on was measured to be 10.74 /xm. At that wavelength, ire 2m e n ( X ) = = 0.969X10 1 9 cm"3 eq. 4.4.2 e 2 X 2 31 Where e=the e l e c t r o n charge, m e=the e l e c t r o n mass, and c=the speed of l i g h t . A four pass Michelson i n t e r f e r o m e t e r (see f i g u r e 4.3) was set up. I t s beam path c r o s s e d the d i s c h a r g e r e g i o n at such an angle as to perform a u t o m a t i c a l l y a h o r i z o n t a l average of the d i s c h a r g e . T h i s was important because although l a s i n g occurred over the e n t i r e one inch by one inch output f i e l d , the output was more intense along two broad v e r t i c a l bands at the s i d e s of the square beam p a t t e r n , r e f l e c t i n g a somewhat lower energy d e n s i t y , and thus e l e c t r o n d e n s i t y , along the cente r of the e l e c t r o d e s . If the i n t e r f e r o m e t e r output i s examined duri n g the d i s c h a r g e , f r i n g e s should be seen which r e f l e c t any changes i n the index of r e f r a c t i o n of the gas. Since the gas mix c o n t a i n s s t r o n g e l e c t r o n scavengers, the f r e e e l e c t r o n s l e f t a f t e r the c u r r e n t pulse should be q u i c k l y a t t a c h e d to the ion s , and so the d u r a t i o n of the f r i n g e t r a i n should be approximately the same as the c u r r e n t p u l s e . A l s o , f o r the same reason, there should be the same number of f r i n g e s which r e f l e c t i n c r e a s i n g d e n s i t y as the number of f r i n g e s which show the decrease i n d e n s i t y as the gas r e t u r n s to i t s o r i g i n a l s t a t e . I f we c a l l m the number of f r i n g e s up, i . e . h a l f of the t o t a l number of f r i n g e s , and A l the change i n path l e n g t h seen by the probe beam, then: 32 2AL 2LATJ m = A 7? = mX 2L eq. 4.4.3 Where L i s the t o t a l d i s c h a r g e path (twice the e l e c t r o d e l e n g t h , s i n c e the i n t e r f e r o m e t e r arm c r o s s e s i t twice) and A77 i s the change in the index of r e f r a c t i o n . The f a c t o r of two comes from the r e t r a c i n g of the path done in a Michelson i n t e r f e r o m e t e r . Combining t h i s with equation 4.4.1, mX A17 = 1 - / l - ( n / n c ) = eq. 4.4.4 2L Which can be rearranged a l g e b r a i c a l l y to get: n mX m 2X 2 + n c L 4L' eq. 4.4.5 Since X i s much l e s s than L, the second order term can be n e g l e c t e d to get the simple r e s u l t : mXn - 1 It „ m - 3 n = = m x 1.8x10 1" cm"3 eq. 4.4.6 A l i q u i d helium c o o l e d Santa Barbara Research Center copper doped germanium i n f r a r e d d e t e c t e r was used to measure the i n t e r f e r o m e t e r output. The s i g n a l was sent i n t o a g i g a h e r t z bandwidth o s c i l l o s c o p e . The combination should have been capable of r e s o l v i n g f e a t u r e s with d u r a t i o n s as low as 400 ps. By t r i g g e r i n g the o s c i l l o s c o p e on the v o l t a g e t r a c e , an 33 estimate was made of the timing of the f e a t u r e s of the i n t e r f e r o m e t e r output r e l a t i v e to v o l t a g e drop, and so to a l l of the other measurements. T h i s allowed the f r i n g e s produced by changes in the e l e c t r o n d e n s i t y to be s i n g l e d out from other f e a t u r e s , which c o u l d have been produced by n o i s e , or by f l u c t u a t i o n s in the index of r e f r a c t i o n caused by shock waves in the gas caused by the d i s c h a r g e . F r i n g e s due to the e l e c t r o n s should be much more r a p i d than those caused by macroscopic f l u c t u a t i o n s i n gas d e n s i t y , so the two types should have been e a s i l y d i s t i n g u i s h a b l e . In p r a c t i c e , no pressure f r i n g e s were i d e n t i f i e d . The data was d i g i t i z e d by measuring the p o s i t i o n of each maximum p o i n t of the f r i n g e s with r e s p e c t to a r e f e r e n c e time determined more p r e c i s e l y by the f i r s t r e c o g n i z a b l e f e a t u r e of the i n t e r f e r o m e t e r output, o which should have c o i n c i d e d with the onset of c u r r e n t . Each f r a c t i o n of a f r i n g e s i g n i f i e d a change i n the e l e c t r o n d e n s i t y of the same f r a c t i o n of 1.80±.02x10 1 0 cm - 3. Whether i t meant an i n c r e a s e or a decrease depended on i t s p o s i t i o n , on the l e f t or r i g h t , r e s p e c t i v e l y , of the center of the f r i n g e t r a i n . Assuming that the f e a t u r e s were c o r r e c t l y i d e n t i f i e d , the e l e c t r o n d e n s i t y c o u l d be measured to w i t h i n a one or two percent of true values at each time p o i n t where there was a d i s t i n g u i s h a b l e f e a t u r e . The i n t e r f e r o m e t e r was set up to make four passes through the a c t i v e volume. T h i s would produce readable r e s u l t s f o r peak e l e c t r o n d e n s i t i e s of 10 1 a-10 1 5 cm - 3. I f the e l e c t r o n d e n s i t y had been too low, the measurement c o u l d not have been made with 34 t h i s apparatus, s i n c e only a f r a c t i o n of a s i n g l e f r i n g e would have been produced, and t h e r e f o r e i t would have been impossible to i d e n t i f y the f e a t u r e . On the other hand, i f the e l e c t r o n d e n s i t y peaked at a value too l a r g e , i t would have been impossible to analyze the data, s i n c e i f there were hundreds or thousands of f r i n g e s , they c o u l d not have been i n d i v i d u a l l y d i s t i n g u i s h e d . F o r t u n a t e l y the apparatus d i d not have to be m o d i f i e d , and the r e s u l t s from the f i r s t v e r s i o n were a n a l y z a b l e . The a c t u a l o s c i l l o g r a m s generated by the apparatus are shown in f i g u r e s 4.4 ( f o r a pure He g a s ) , 4.6 ( f o r a gas mix with h a l f of the optimum c o n c e n t r a t i o n of the l a s i n g s p e c i e s i n a He b u f f e r ) , and 4.8 ( f o r a f u l l c o n c e n t r a t i o n mix i n a He b u f f e r ) . There were a number of d i f f e r e n t problems that arose. The s i g n a l s t r e n g t h of the i n t e r f e r o m e t e r output was low, r e s u l t i n g i n a very low v o l t a g e (=20 mV) output from the d e t e c t e r . When neon was used in the gas mix, i t proved to be impossible to get a good s i g n a l , probably due to the presence of an a b s o r p t i o n band at the probe l a s e r ' s wavelength. During the e l e c t r i c d i s c h a r g e , the probe beam was s t r o n g l y a t t e n u a t e d . T h i s can most c l e a r l y be seen in f i g u r e s 4.4 A and B, where the amplitude of the f r i n g e s drops s h a r p l y i n the middle of the d i s c h a r g e . T h i s problem became more pronounced when the c o n c e n t r a t i o n of the l a s i n g s p e c i e s was i n c r e a s e d , although i t was s t i l l p o s s i b l e to i d e n t i f y a l l of the f r i n g e s u n t i l , with a f u l l s t r e n g t h mix, the c e n t r a l p o r t i o n of the f r i n g e t r a i n was obscured, and an e x t r a p o l a t i o n of the e l e c t r o n d e n s i t y data had 35 to be made. F i n a l l y , s trong spontaneous emission i n the i n f r a r e d was observed (see f i g u r e 4.8 D). I t passed through a germanium f l a t without s i g n i f i c a n t a t t e n u a t i o n , i n d i c a t i n g a wavelength longer than 1.8 Mm, which i s c o n s i s t e n t with the 2.026 Mm emissions r e p o r t e d Dyer and T a i t 1 9 i n 1984. A 10.6 Mm band pass i n t e r f e r e n c e f i l t e r was f i n a l l y used to remove most of t h i s r a d i a t i o n , and although i t was s t i l l present, i t s amplitude was reduced to such a l e v e l that i t d i d not i n t e r f e r e s i g n i f i c a n t l y with the d e n s i t y measurements. Three shots of each of the three gas mixes used were analys e d . The r e s u l t s are graphed i n f i g u r e s 4.5, 4.7, and 4.9 fo r 60 p s i a of pure helium, f o r a gas mix with one h a l f of the optimum c o n c e n t r a t i o n of XeCl i n 60 p s i a of helium, and f o r a mix with the f u l l (optimum l a s e r output power) c o n c e n t r a t i o n of XeCl i n 60 p s i a of helium, r e s p e c t i v e l y . The curves f o r the f u l l c o n c e n t r a t i o n mix are broken i n the middle because the amplitude of the f r i n g e s grew too low to be d i s t i n g u i s h e d r e l i a b l y when the e l e c t r o n d e n s i t y was near i t s peak. F i g u r e 4.10 shows the three shot averages of each of the three gas mixes. The peak e l e c t r o n d e n s i t y f o r 60 p s i a of pure helium was 6.2±1.0X10 1 4 cm - 3, and f o r the h a l f c o n c e n t r a t i o n mix (0.28% HCl and 1.24% Xe i n 60 p s i a of He) the maximum d e n s i t y was 8.3±1.0xl0 1 4 cm - 3. An e x t r a p o l a t i o n was made f o r the f u l l c o n c e n t r a t i o n (0.56% HCl and 2.48% Xe i n 60 p s i a He) curve, which i n d i c a t e s that the l a s i n g mix had a peak e l e c t r o n d e n s i t y of 1 2 ± 5 x 1 0 1 4 cm" 3. 36 F i g u r e 4.3: Diagram of i n f r a r e d i n t e r f e r o m e t e r used to measure discharge e l e c t r o n d e n s i t y . gure 4.4: O s c i l l o g r a m s of the i n t e r f e r o m e t r y with a pure He gas. V c=35 kV, P t=60 p s i a . Each shot was taken on a d i f f e r e n t day. The two peaks on the end of the upper l e f t t r a c e were caused by a lower energy secondary d i s c h a r g e which r e s u l t e d from a poor impedence match in the main dis c h a r g e , which was probably caused by i n s u f f i c i e n t p r e i o n i z a t i o n . Only the other three shots were analysed. The r e s u l t s are p l o t t e d in f i g u r e 4.5. F i g u r e 4.5: Graph of e l e c t r o n d e n s i t y vs time f o r t h r e e shots with a pure He gas (see f i g u r e 4.4). 39 aw 20nS ; 1 i A ^\ I AAAI . _ / i w d V j j Wv •V - I P L • :• • • • 1 ' ' ' ' t i —n L c F i g u r e 4.6: O s c i l l o g r a m s of the i n t e r f e r o m e t r y with a h a l f c o n c e n t r a t i o n (0.28% HC1 and 1.24% Xe i n 60 p s i a of He) gas mix. V c=35 kV. The analysed r e s u l t s are p l o t t e d i n f i g u r e 4.7. F i g u r e 4 . 7 : Graph of e l e c t r o n d e n s i t y vs time f o r three shots with a h a l f c o n c e n t r a t i o n gas mix (See f i g u r e 4 . 6 ) . F i g u r e 4.8: A, B, C: O s c i l l o g r a m s of the i n t e r f e r o m e t r y with a f u l l c o n c e n t r a t i o n (0.56% HCl and 2.48% Xe i n 60 p s i a of He) gas mix. V c=35 kV. The a n a l y s e d r e s u l t s are p l o t t e d i n f i g u r e 4.9. D: IR l i g h t output from the XeCl l a s e r . A germanium f l a t , which was opaque t o wavelengths s h o r t e r than 1.8 microns, was used to f i l t e r the output. A 10.6 micron i n t e r f e r e n c e f i l t e r was used to remove t h i s r a d i a t i o n . Note that the amplitude of t h i s s i g n a l i s f i f t y times that of the i n t e r f e r o m e t r y s i g n a l s . F i g u r e 4.9: Graph of e l e c t r o n d e n s i t y vs time f o r three shots with a f u l l c o n c e n t r a t i o n gas mix (See f i g u r e 4.8). F i g u r e 4.1u:Graph showing t h r e e shot averages of the e l e c t r o n d e n s i t y vs time with pure He, the h a l f c o n c e n t r a t i o n mix, and the f u l l c o n c e n t r a t i o n mix. See f i g u r e s 4.4 to 4.9. 44 CHAPTER 5: CONCLUSION S e c t i o n 5.1: D i s c u s s i o n of R e s u l t s By combining a l l of the measurements made, i t i s p o s s i b l e to c o n s t r u c t a f a i r l y complete p i c t u r e of the d i s c h a r g e process. F i g u r e 5.1 shows the r e l a t i v e t i m i n g of the v o l t a g e , c u r r e n t , e l e c t r o n d e n s i t y , and l a s e r output. When the l a s e r i s f i r e d , the v o l t a g e begins to i n v e r t , i n c r e a s i n g s t e a d i l y from zero u n t i l i t i s completely i n v e r t e d (to about 1.9xV ) or u n t i l a high enough p o t e n t i a l i s reached to achieve a breakdown a c r o s s the e l e c t r o d e gap. The breakdown p o t e n t i a l depends on the s i z e of the gap, and the contents and t o t a l pressure of the gas. If the breakdown p o t e n t i a l i s too high, the di s c h a r g e w i l l not occur, r e s u l t i n g i n a r i n g i n g s e r i e s LRC o s c i l l a t i o n , as shown i n f i g u r e 4.1 B. If the breakdown does occur, then the v o l t a g e begins to drop, and the c u r r e n t and the e l e c t r o n d e n s i t y i n the d i s c h a r g e both s t a r t to r i s e . The l a s e r output begins about halfway through the v o l t a g e drop, where the power i n the d i s c h a r g e , which i s the product of the v o l t a g e and the c u r r e n t , i s near i t s maximum. The v o l t a g e drops to zero i n about 35 ns, by which time both the c u r r e n t and the e l e c t r o n d e n s i t y have reached t h e i r peak v a l u e s . 45 0.0 100.0 200.0 300.0 100.0 500.0 TIME (n«J Figure 5 . 1 : Graph showing the r e l a t i v e timing of (from top to bottom) the voltage, current, electron density, and laser output traces. No v e r t i c a l scales. 46 Unless breakdown occurs e x a c t l y at the peak of the v o l t a g e i n v e r s i o n , there w i l l be some energy l e f t i n the c a p a c i t o r s a f t e r breakdown. T h i s energy w i l l be spent in a e x p o n e n t i a l l y decaying r i n g i n g c i r c u i t which does not c o n t r i b u t e s i g n i f i c a n t l y to the d i s c h a r g e . The l a s e r l i g h t p u l s e , which has a f u l l width at h a l f maximum of about 20 ns and a t o t a l l ength of 30-40 ns, stops when the v o l t a g e has reached bottom, s i n c e there i s no more power in the d i s c h a r g e . The e l e c t r o n d e n s i t y curve and the c u r r e n t pulse t r a c e both l a s t approximately the same time. The c u r r e n t , l a s e r l i g h t , and v o l t a g e t r a c e s shown i n f i g u r e 5.1 are from a f u l l c o n c e n t r a t i o n (2.48% Xe and 0.56%HC1) gas mix using a h a l f He, h a l f Ne b u f f e r , while the e l e c t r o n d e n s i t y t r a c e i s from a gas mix using pure He as the b u f f e r , s i n c e i t was not p o s s i b l e to get an a n a l y z a b l e s i g n a l using the He/Ne gas mix, as mentioned i n s e c t i o n 4.4. An i n t e r e s t i n g r e s u l t of the e l e c t r o n d e n s i t y measurements made on v a r i o u s gas mixes was that the shape of the e l e c t r o n d e n s i t y curve changed c o n s i d e r a b l y when small changes were made in the c o n c e n t r a t i o n of the l a s i n g s p e c i e s . Looking at f i g u r e 4.10 i t i s c l e a r that the e l e c t r o n d e n s i t y i n c r e a s e s more r a p i d l y , reaches a higher peak, and descends more q u i c k l y , as the XeCl c o n c e n t r a t i o n i n c r e a s e s . The reason f o r t h i s i s probably that s i n c e the halogen, c h l o r i n e , i s a s t r o n g e l e c t r o n scavenger, i t i s i n i t i a l l y more d i f f i c u l t f o r the discharge to 47 begin, s i n c e the e l e c t r o n s which s t a r t the avalanche tend to be attach e d by the gas before they can gain enough v e l o c i t y to c r e a t e secondary e l e c t r o n s . When the discharge does begin, the vo l t a g e has r i s e n s l i g h t l y higher than i t would have with a lower XeCl c o n c e n t r a t i o n gas, so that the delayed d i s c h a r g e s t a r t s more a b r u p t l y and i s more intense than normal. When the vo l t a g e drops to near zero, the f r e e e l e c t r o n s are q u i c k l y 'eaten up' by the scavenger molecules i n the gas, so the long decaying t a i l seen i n measurements with pure helium i s tru n c a t e d , and the e l e c t r o n d e n s i t y r e t u r n s to zero i n a comparatively b r i e f time. S e c t i o n 5.2: Suggestions f o r Future Study As mentioned in S e c t i o n 3.3, the power output of the l a s e r can e a s i l y be in c r e a s e d by r a i s i n g the op e r a t i n g pressure (with the same percentage of the l a s i n g s p e c i e s ) . Since the l i m i t of 60 p s i a was q u i t e c o n s e r v a t i v e , i t should be p o s s i b l e to operate with gas p r e s s u r e s of at l e a s t 70-80 p s i a . A b e t t e r beam q u a l i t y and p o s s i b l y a higher e f f i c i e n c y can be achieved by using an unstable o p t i c a l resonator i n pl a c e of the plane-plane o p t i c s used f o r t h i s experiment. The necessary o p t i c a l components, with optimum r e f l e c t i v i t y i n the UV, have been a c q u i r e d and can be i n s t a l l e d at any time. Now that the l a s e r and the in t e r f e r o m e t e r have been b u i l t , i t would be a s t r a i g h t f o r w a r d p r o j e c t to optimize the power and 48 make e l e c t r o n d e n s i t y measurements of a v a r i e t y of other excimer gas mixes, such as KrF and XeBr, e t c . . Since p l e n t y of f r i n g e s were observed, the i n t e r f e r o m e t e r c o u l d be m o d i f i e d i n t o a two pass, i n s t e a d of a four pass, system. T h i s would reduce the probe beam a b s o r p t i o n s i g n i f i c a n t l y and s t i l l leave enough f r i n g e s to an a l y s e . The power output of the C0 2 probe l a s e r c o u l d be i n c r e a s e d by a d j u s t i n g i t to operate on the 10.6 urn band. I t was a c c i d e n t a l l y s h i f t e d to the lower power 10.74 urn band before the i n t e r f e r o m e t e r measurements were made, and the change was not dete c t e d u n t i l the data had a l l been taken. The s t a t e of the a r t i n excimer l a s e r s has advanced r a p i d l y i n recent y e a r s , and i t would c e r t a i n l y be p o s s i b l e to de s i g n and c o n s t r u c t an improved l a s e r . For example, one that used an LC i n v e r s i o n f i r i n g c i r c u i t with the c a p a c i t o r s c o n t a i n e d w i t h i n a c y l i n d r i c a l high pressure gas chamber, with automatic p r e i o n i z a t i o n sent through the ground e l e c t r o d e . However, th e r e i s at t h i s time no need to c o n s i d e r b u i l d i n g a new excimer l a s e r , s i n c e the one c o n s t r u c t e d f o r t h i s experiment has a l r e a d y f u l f i l l e d i t s primary p u r p o s e — a l l o w i n g the e l e c t r o n d e n s i t y measurement to be made—and w i l l , i n the f u t u r e , serve as a u s e f u l p i e c e of the l a b o r a t o r y ' s equipment. 49 REFERENCES 1) F.G. Houtermans, Helv. Phys. A c t a . Vol.33, p933-937 (1960) 2) N.G. Basov, V.A. Danalychev, and Yu.M. Popov, Sov. J . Quant. E l e c . V ol.1, No.1, pl8-22 (1971) 3) H.A. Koehler, L . J . F e r d e r b e r , D.L. Redhead, and P.J. Ebert, Appl. Phys. L e t t . Vol.21, No.5, pl98-200 (1972) 4) J.E. Velazco and D.W. S e t s e r , J . Chem. Phys. Vol.62, No.5, p1990-1991 (1975) 5) J . J . Ewing and C.A. Brau, Phys. Rev. A Vol.21, No.1, p129-132, (1975) 6) C.A. Brau and J . J . Ewing, A p p l . Phys. L e t t . Vol.27, No.8, P435-437 (1975) 7) J . J . Ewing and C.A. Brau, A p p l . Phys. L e t t . Vol.27, No.6, p350-352 (1975) 8) S.K. S e a r l e s and G.A. Hart, Appl. Phys. L e t t . Vol.27, No.4, p243-245 (1975) 9) V.N. Ischenko, V.N. L i s i t s y n , and A.M. Razhev, Opt. Comm. Vol.21, No.1 p30-32 (1977) 10) R. Burnham, Opt. Comm. Vol.24, No.2, pi 61 -163 (1978) 11) R.C. Sze and P.B. S c o t t , A p p l . Phys. L e t t . Vol.33, No.5, P419-421 (1978) 12) R.C. Sze, J . Appl. Phys. Vol.50, No.7 p4596-4598 (1979) 13) J . Chen, S. Ju, and M. L i u , Appl. Phys. L e t t . Vol.37, No.10,p883-885 (1980) 14) W.H. Long, J r . , M.J. Plummer, and E.A. Stap p a e r t s , Appl. Phys. L e t t . Vol.43, No.8, p735-737 (1983) 15) K. Mi y a z a k i , Y. Toda, T. Hasama, and T. Sato, Rev. S c i . Instrum. Vol.56, No.2, p20V-204 (1985) 16) E. Arm a n d i l l o , F. Bonanni, and G. Grasso, Opt. Comm. Vol.24, No.1 p63-66 (1982) 17) R. Burnham, Appl. Phys. L e t t . Vol.29, No.1 p30-32 (1976) 18) H. Houtman and J Meyer, Rev. S c i . Instrum. Vol.54, No.12 p1629-1630 (1983) 19) P.E. Dyer and B.L. T a i t , J . Phys. E: S c i . Instrum. Vol.17, p637-638 (1984) 

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