"Science, Faculty of"@en . "Physics and Astronomy, Department of"@en . "DSpace"@en . "UBCV"@en . "Ford, Joseph Earl"@en . "2010-05-13T22:22:30Z"@en . "1985"@en . "Master of Science - MSc"@en . "University of British Columbia"@en . "A discharge pumped LC inversion type XeCl excimer laser was constructed, and its discharge and output were examined. A maximum output energy of 167 mJ was achieved, with an efficiency of 0.56%, using 60 psi of a gas composed of 0.56% HCl, 2.48% Xe, 48.48% He, and 48.48% Ne. The 308 nm laser output pulse had a fwhm of 20 ns and a peak power of 8.6 MW. When charged to 35 kV, the voltage inversion reached a peak of ~45 kV and dropped to zero in ~35 ns. The fwhm of the discharge current was 46 ns, with a peak current of 15.3 kA. The electron density in the discharge was measured using an infrared Michelson interferometer, and found to have a fwhm of 30 ns and a peak value of 12\u00C2\u00B15xl0\u00C2\u00B9\u00E2\u0081\u00B4 cm\u00E2\u0081\u00BB\u00C2\u00B3."@en . "https://circle.library.ubc.ca/rest/handle/2429/24666?expand=metadata"@en . "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 \u00C2\u00A9 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\u00C2\u00B15xl0 1 f t cm\" 3. i i i TABLE OF CONTENTS ABSTRACT i i TABLE OF CONTENTS \u00E2\u0080\u00A2 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 \u00C2\u00A3 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 \u00E2\u0080\u0094 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\u00E2\u0080\u009E, and C 2 F 2 C 1 . Much higher output e n e r g i e s \u00E2\u0080\u0094 m o r e than one hundred m i l l i j o u l e s \u00E2\u0080\u0094 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 \u00C2\u00B12%. 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.\u00E2\u0080\u0094WV\\u00E2\u0080\u0094|l> -AVA\u00E2\u0080\u0094\u00E2\u0080\u00A2 +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 \ \u00E2\u0080\u0094 i 1 \u00E2\u0080\u0094I 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. "Thesis/Dissertation"@en . "10.14288/1.0085218"@en . "eng"@en . "Physics"@en . "Vancouver : University of British Columbia Library"@en . "University of British Columbia"@en . "For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use."@en . "Graduate"@en . "Investigations into the xenon chloride excimer laser"@en . "Text"@en . "http://hdl.handle.net/2429/24666"@en .