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Development of a multiatmosphere UV-preionized carbon dioxide laser amplifier Stuart, Gregory C. 1986

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DEVELOPMEMT OF A MULTIATMOSPHERE UV-PREIONIZED CARBON DIOXIDE LASER AMPLIFIER By GREGORY C . STUART B . A . S c , U n i v e r s i t y of B r i t i s h C o l u m b i a , 1984 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n THE FACULTY OF GRADUATE STUDIES DEPARTMENT OF PHYSICS ( E n g i n e e r i n g P h y s i c s ) We accept t h i s t h e s i s as conforming to the r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA A p r i l 1986 © Gregory C . S t u a r t , 1986 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 his 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 PHYSICS The University of B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date 1 6 A P r i l 1986 >E-6 (.3/81) ) i i ABSTRACT A compact, high-pressure, UV-preionized TE C0 2 laser amplifier was designed and constructed. The amplifier uses a novel LC inversion c i r c u i t , with a single spark gap i n i t i a t i n g both the main discharge and the preionization. The e l e c t r i c a l and gain c h a r a c t e r i s t i c s were studied over a range of pressures and gas mixes. The voltage inversion was 96% e f f i c i e n t , and discharge currents reached 13.5 kA with risetimes of 28 ns. At ten atmospheres, with a 3% C0 2 mix, the peak small-signal gain of the 9P(18) l i n e was found to be 2.7 %/cm and the FWHM of the gain pulse was 500 ns. i i i TABLE OF CONTENTS ABSTRACT i i TABLE OF CONTENTS i i i LIST OF FIGURES v ACKNOWLEDGEMENTS v i i CHAPTER 1 INTRODUCTION 1.1 Objective and Motivation 1 1.2 High-pressure C0 2 Lasers 3 1.3 Thesis Organization 4 CHAPTER 2 THEORY 2.1 C0 2 Laser Theory 5 2.2 Pressure Broadening and Line Overlap 8 2.3 UV-Preionization 10 CHAPTER 3 AMPLIFIER DESIGN 3.1 Physical Description 11 3.2 E l e c t r i c a l Design 17 3.3 Inversion C i r c u i t r y 23 3.4 Charging Arrangement 25 CHAPTER 4 ELECTRICAL MEASUREMENTS 4.1 Voltage Measurements 26 4.2 Current Measurements 31 4.3 Preionizer Timing 34 4.4 Overall Amplifier Timing 37 i v CHAPTER 5 GAIN CHARACTERISTICS 5.1 S m a l l - S i g n a l G a i n 39 5.2 G a i n L i f e t i m e and R i s e t i m e 50 5.3 A m p l i f i e r as an O s c i l l a t o r 52 CHAPTER 6 DISCUSSION AND CONCLUSIONS 6.1 Summary of R e s u l t s 53 6.2 Recommendations 55 BIBLIOGRAPHY 57 V LIST OF FIGURES 2-1 Energy Level Diagram for C0 2 and N 2 6 2- 2 Lorentzian Lineshape 8 3- 1 End D e t a i l of Amplifier Main Body 12 3-2 Main Body Cross-section 13 3-3 Amplifier Cross-section 14 3-4 Top View of Amplifier 15 3-5 Main Discharge C i r c u i t 18 3-6 Preionization C i r c u i t 19 3-7 Equivalent Amplifier C i r c u i t 22 3-8 Inversion C i r c u i t Operation 24 3- 9 Pressurized Resistor Vessel 25 4- 1 Breakdown Voltage 27 4-2 Ringing Anode Voltage 28 4-3 Arcless Discharge Voltage 30 4-4 Arcing Voltage Trace 30 4-5 Derivative of Discharge Current 32 4-6 Discharge Current Pulse 32 4-7 Spark Gap and Preionizer Light 34 4-8 Main Body Light 35 4-9 Light Signal with Arc 35 4-10 Summary of Amplifier Timing 38 v i 5-1 S m a l l - S i g n a l G a i n Arrangement 40 5-2 T r i g g e r i n g Arrangement 42 5-3 G a i n P u l s e s 44 5-4 P r e s s u r e Dependence of G a i n 45 5-5 Gain as a F u n c t i o n of Energy D e n s i t y 47 5-6 S p a t i a l G a i n P r o f i l e 49 5-7 G a i n L i f e t i m e 51 5-8 Inverse G a i n R i s e t i m e 51 ACKNOWLEDGEMENTS I would l i k e to thank Dr. Jochen Meyer for the supervision and support provided throughout the course of t h i s work. I would also l i k e to thank Hubert Houtman for his experimental assistance and help with the c i r c u i t analysis. Thanks also go to Al Cheuck and Paul B u r r i l l for their technical assistance, and to Grant Mcintosh for many helpful discussions. 1 CHAPTER 1 INTRODUCTION 1.1 Objective and Motivation There has been substantial interest in high-pressure C0 2 lasers due to their usefulness for picosecond pulse production 1 and as broad-band tunable o s c i l l a t o r s . These applications are possible because the pronounced c o l l i s i o n a l broadening of individual r o t a t i o n a l - v i b r a t i o n a l l i n e s at multiatmospheric pressures leads to s i g n i f i c a n t l i n e overlap, resulting in a very large gain bandwidth. A high-pressure amplifier thus provides amplification of ultra-short C0 2 laser pulses while maintaining the temporal i n t e g r i t y of these pulses. This thesis describes the design, construction, and operation of a new amplifier which w i l l serve as the prototype for a chain of similar amplifiers. The chain w i l l be part of a high-power laser system, which i s presently under development. The U.B.C. Plasma Physics Group currently operates a large C0 2 l a s e r 2 . This laser, running at a wavelength of 10.6 /um and giving up to 10 J of energy in a 2 ns pulse, i s used to study a variety of laser-plasma interactions. It i s hoped to expand t h i s research to include the process of beatwave acceleration, where the beating of two laser waves in a magnetized plasma leads to the acceleration of electrons. The two incident laser beams are separated by a frequency equal to the plasma frequency, and another C0 2 laser system i s required to operate at 9.6 Mm. This system must provide output powers 2 comparable or superior to the present 5 GW system. By using multiatmospheric pressures, the new laser system w i l l generate pulses with durations of the order of 10 ps and with energies of up to 2 J. A ten atmosphere, Q-switched, mode-locked and cavity-dumped C 0 2 laser o s c i l l a t o r is now being developed. The 10 ps, 10 mJ pulses from this high-pressure o s c i l l a t o r w i l l then be amplified in the multiatmospheric amplifier chain. The use of picosecond rather than nanosecond pulses has the advantage of s i g n i f i c a n t l y reducing the overa l l size of the laser system. The new 2 0 0 GW ultra-short pulse system can be i n s t a l l e d on a single o p t i c a l table, while the present system i s room-sized. This compact high-pressure C 0 2 laser system w i l l also serve as a useful tool for infrared interferometry and other plasma diagnostics. 3 1.2 High-Pressure Carbon Dioxide Lasers The advantage of going to higher pressures was f i r s t proposed by Basov 3 et. a l . in 1971, and there has been research into a wide variety of methods of achieving multiatmospheric laser operation. E l e c t r o - i o n i z a t i o n lasers*' 5 have been successfully operated at pressures in excess of 50 atmospheres, but are not s u f f i c i e n t l y r e l i a b l e to be part of an amplifier chain. The thin f o i l , between the high-pressure C0 2 chamber and the electron-beam source, i s prone to f a i l u r e . O p t i c a l l y pumped lasers »' have also demonstrated multiatmospheric operation. Chang and Wood6 used a HBr laser to pump a 33 atmosphere C0 2 l a s e r . Multiatmospheric transverse-excitation (TE) C0 2 lasers are a natural progression from the atmospheric TEA lasers developed in the early 1970's. For these lasers the pumping i s achieved with a pulsed e l e c t r i c a l discharge. Among the multitude of ways used to obtain stable dischages are Laflamme double-discharges 7, pin showers 8, UV p r e i o n i z a t i o n 9 " ' 3 , and X-ray p r e i o n i z a t i o n 1 0 ' 1 5 . The r e l a t i v e s i m p l i c i t y and r e l i a b i l i t y of UV-preionized TE lasers make them suitable for both the production and amplification of ultra-short pulses. 4 1.3 T h e s i s O r g a n i z a t i o n An understanding of high-pressure C0 2 laser amplifiers requires some knowledge of the relevant theory. Chapter 2 gives a summary of the general mechanics of C0 2 lasers, as well as the theory s p e c i f i c to high-pressure operation. For an amplifier of t h i s type the physical layout and the e l e c t r i c a l behaviour are intimately linked.* The d e t a i l s of the amplifier constuction and c i r c u i t r y are presented in Chapter 3. E l e c t r i c a l and gain measurements are described in Chapters 4 and 5 respectively. Chapter 4 also presents results of measurements of the preionization timing, and describes the ov e r a l l amplifier timing. Chapter 6 summarizes the important re s u l t s , gives suggestions for further study, and presents ideas for improving the performance of the amplifier. 5 CHAPTER 2 THEORY 2.1 CARBON D IOX IDE LASER THEORY In order to f a c i l i t a t e the discussion of discharge-pumped C0 2 laser amplifiers a brief review of laser theory and nomenclature i s appropriate. A p a r t i a l energy l e v e l diagram for C0 2 and N 2 i s presented in Figure 2-1. The v i b r a t i o n a l states for the C0 2 molecule are denoted by V \ V 2 ^ v 3 , where v. are the respective quantum numbers for the symmetric stretching, bending, and asymmetric stretching mode's, and / i s the angular momentum quantum number for the bending mode. The upper v i b r a t i o n a l l e v e l for C0 2 laser t r a n s i t i o n s , 00°1, thus represents the asymmetic stretching mode. Figure 2-1 also shows some of the rotational sublevels corresponding to the v i b r a t i o n a l states, these levels being characterized by the rotational quantum number J. Each of the possible laser t r a n s i t i o n s i s described as being either P(J) or R(J), where J represents the rotational quantum number of the lower laser l e v e l . P-branch tra n s i t i o n s have AJ = +1, and R-branch tr a n s i t i o n s are those with AJ = -1. The fact that the spacing of the rotational sublevels increases with increasing J means that the R-branch t r a n s i t i o n a l l i n e s are closer together in the frequency spectrum than are the P-branch l i n e s . Adjacent R-branch laser l i n e s are separated by 38 GHz, while adjacent P-branch l i n e s are separated by 54 GHz. 6 0 3 00°1 c OJ 0-1 radiative decays 00827 eV radiative) decay ca 0" 00°0 0-2913eV 0-2891 eV c o u X a* N ground state Figure 2-1 Energy L e v e l Diagram f o r C 0 2 and N 2 showing important r o t a t i o n a l - v i b r a t i o n a l l i n e s and l a s e r t r a n s i t i o n s . 7 [00°1—10°0] transitions correspond to wavelengths in the 10 Aim band, which i s centered about 10.4 /um, while [00°1-02°0] tr a n s i t i o n s are in the 9 nm band, centered about 9.4 nm. The pumping of the upper laser l e v e l for C0 2 i s improved by the addition of N 2. The f i r s t excited v i b r a t i o n a l state of the N 2 molecule i s very close in energy to the 00°1 l e v e l of C0 2. C o l l i s i o n s between these N 2 and C0 2 molecules, as well as direct electron impact, serve to pump the 00°1 l e v e l . The gas mix also normally contains helium, which improves the s t a b i l i t y of the e l e c t r i c a l discharge and, through c o l l i s i o n s , aids the depopulation of the long-lived 01 10 l e v e l of C0 2. As the operating pressure of a C0 2 amplifier increases, so do the c o l l i s i o n a l processes, leading to a decrease in both the risetime and l i f e t i m e of the gain. Typical high-pressure C0 2 laser mixes are composed of equal percentages of C0 2 and N 2, with He making up the balance of the mix. 8 2.2 Pressure Broadening and Line Overlap There are two forms of broadening of the lineshape function, g(v), which describes the spectral d i s t r i b u t i o n of a p a r t i c u l a r laser t r a n s i t i o n . Inhomogeneous, or Doppler, broadening i s due to the thermal motion of individual C0 2 molecules. Pressure broadening i s homogeneous, and i s due to c o l l i s i o n s which broaden the r o t a t i o n a l - v i b r a t i o n a l l e v e l s of the molecules. In thi s case, the lineshape i s Lorentzian, and is given by the following formula, g(v) = Lv / tr[ (v-vQ)2+Hv2] where v0 i s the frequency at line-center and i s the collision-broadened halfwidth at half maximum. Figure 2-2 gives the Lorentzian lineshape function for the 9P(18) laser t r a n s i t i o n at ten atmospheres, and shows locations of the adjacent laser l i n e s . i i i I I u 1 u _ 60 40 20 0 20 40 60 \v-v0\ (GHz) Figure 2-2 Lorentzian Lineshape due to Pressure Broadening. 9 C o l l i s i o n broadening of C0 2 laser t r a n s i t i o n s can be modelled by combining standard c o l l i s i o n a l theory with experimentally measured temperatures and c r o s s - s e c t i o n s 1 6 . For the 9 Mm band, [00°1-02°0] t r a n s i t i o n s , the halfwidths for the Lorentzians have been determined 1 7 to be, = 2.79 - 0.024[J±1] GHz/atm where the "+" i s for P-branch t r a n s i t i o n s and the i s for R-branch t r a n s i t i o n s . The 9R(16) l i n e has Lv = 2.43 GHz/atm while the 9P(18) l i n e has Lv = 2.33 GHz/atm. The gain c o e f f i c i e n t i s d i r e c t l y proportional to the lineshape function, and the gain at any p a r t i c u l a r frequency can be •obtained by summing the contributions from the overlapping lineshapes. At ten atmospheres, on the 9P(18) l i n e , one would expect the overlap to increase the peak gain c o e f f i c i e n t by a factor of about 1.7 times from the gain at atmospheric operation. Above twenty atmospheres, modifications to the Lorentzian lineshape must be accounted for, but below th i s pressure the Lorentzian theory i s usually s u f f i c i e n t l y accurate. The presence of helium in the gas mixture has been shown to cause a frequency s h i f t in the r o t a t i o n a l - v i b r a t i o n a l t r a n s i t i o n s of C 0 2 1 8 . This s h i f t , which i s of the order of 0.3 GHz/atm for a mix containing 90% helium, has to be taken into account when one requires a more detailed treatment of high-pressure laser theory. However, the important c h a r a c t e r i s t i c of multiatmospheric C0 2 lasers, namely the semi-continuous gain in the frequency spectrum, i s unaffected by these refinements. 10 2.3 UV - P r e i o n i z a t i o n E f f i c i e n t operation of molecular-vibrational lasers, such as C0 2, requires low discharge f i e l d strengths and high current de n s i t i e s . Unfortunately these are conditions under which discharges are normally unstable. U l t r a v i o l e t photons do, however, have s u f f i c i e n t energy to ionize the constituent gas molecules and generate photo-electrons at the surfaces of the discharge electrodes. The presence of large numbers of electrons helps to reduce the production of streamers and arcs, leading to a more uniform glow discharge. In t h i s amplifier, preionization i s achieved through the use of two sliding-spark a r r a y s 1 9 , t h i s being a standard technique in high-pressure TE C0 2 lasers. 11 CHAPTER 3 AMPLIFIER DESIGN 3.1 Physical Description High-pressure amplifier design i s d i f f i c u l t since the design must simultaneously meet stringent mechanical and e l e c t r i c a l requirements. The problems of overcoming the large stresses and forces accompanying multiatmospheric pressures are i n t e n s i f i e d by the limited choice of materials due to e l e c t r i c a l considerations. The physical layout of the amplifier was c r i t i c a l because i t also governed the production, propagation, and timing of the various high voltages needed to create the discharge. The c h a r a c t e r i s t i c s of the preionization and main discharge c i r c u i t s were dependent on the locations of, and connections between, the various e l e c t r i c a l components. This section outlines the amplifier structure, the d e t a i l s of the amplifier c i r c u i t r y being discussed in following sections. The main body of the amplifier was constructed from a 40 cm long polyvinyl chloride (PVC) rod. The PVC, chosen for i t s insulating properties, was machined into a hollow tube of 5.7 cm I.D. and 7.62 cm O.D.. At ten atmospheres, the hoop stress was determined to be a, = PD./2t = 27.2 atm, well within hoop i the nominal y i e l d strength of PVC. The numerous holes, needed for the attachment of the the main electrodes, would have c e r t a i n l y reduced the y i e l d strength to below the nominal 12 value of 340 atm. These main electrodes were made of aluminum, and were contoured to give an e f f e c t i v e discharge volume of 1.5 cm x 1.5 cm x 35 cm. PVC flanges were cemented to the ends of the main body using high-pressure PVC cement. The Lucite end-flange arrangements were attached with 12 nylon bolts at each end, as shown in Figure 3 - 1 . The 5 mm thick potassium chloride (KC1) f l a t s were angled at 10° to avoid possible s e l f - l a s i n g due to r e f l e c t i o n s off their surfaces. Pressure seals were made with BUNA-N rubber O-rings. Figure 3 - 1 End De t a i l of Amplifier Main Body. 13 Figure 3-2 gives a cross-section of the main body of the amplifier. The main discharge electrodes were shaped to f i t on the inside wall of the PVC body, and each was attached to an aluminum capacitor plate with special brass f i t t i n g s . There were twelve of these connections on each side of the amplifier body. F l a t s t r i p s , 1.3 cm wide, were milled along the length of the PVC tube so as to provide surfaces for the O-ring seals. Figure 3-2 also shows the location of the two preionizer rods. Each rod was composed of s t a i n l e s s steel pieces f i t t e d on a length of 5 mm diameter glass tubing so as to give f i f t e e n spark gaps, spaced 2.54 cm apart. The preionizer spark gaps were 1 mm wide, with the two gaps at the ends of each rod being 20% wider to ensure adequate preionization at the ends of the discharge electrodes. preionizer rod scale:-2 cm Figure 3-2 Main Body Cross-Section showing electrode connection. 14 F i g u r e 3-3 Amplifier Cross-Section showing Capacitor Arrangement. •capacitor electrode PVC body preionizer 8 cm Doorknob capacitors, storing energy for the main discharge, were arranged about the main body in 8 banks of 11 capacitors each, as shown in the cross-section above. Figure 3-4 gives a top view of the amplifier, showing the spark gap and preionization arrangement. The preionization capacitors and the pressurized spark gap were located 24 cm above the center of the main body, connected to the upper and lower pairs of aluminum L-plates by lengths of 6 mm diameter copper rod. 16 The amplifier was supported on a 2.54 cm thick Lucite plate, which formed the base of Lucite box, with 5 mm sides and top. This box was covered with fine brass mesh to shield the external environment from discharge-generated e l e c t r i c a l noise. Gas connections to the amplifier and pressurized spark gap were made with 1/4 i n . Eastman Po l y f l o tubing and f i t t i n g s . The gas mix for the amplifier entered the main body through an adapted Pol y f l o f i t t i n g in one of the end-flange arrangements and exited at the other end. This ensured a good a x i a l flow for gas replacement between shots. The gas volume of the main amplifier body was 753 cm3. Gas flows were monitored using curved-tube flowmeters 2 0, with 5/32 i n . b a l l s inside 1/4 i n . tubing. Typical flow rates for the amplifier gas mix were 30 cm 3/s. Throughout the course of t h i s study the gas mix had an equal percentage of carbon dioxide and nitrogen, with the remainder being helium. S p e c i f i c a l l y , the amplifier mixes used were (C0 2:N 2:He) = (3:3:94), (6:6:88), (8:8:84), and (15:15:70). From th i s point onwards, the mixes w i l l be simply referred to as the 3%, 6%, 8%, and 15% C0 2 mixes. 17 3.2 E l e c t r i c a l Design The most common method of producing high voltages in large-aperture multiatmospheric TE C0 2 amplifiers i s through the use of Marx banks 8' 9' 1 2. The systems using Marx bank c i r c u i t r y , requiring multiple spark gaps and pulse-forming networks which need to be immersed in o i l , are bulky and complicated. For very small amplifiers, Blumlein c i r c u i t s have been successfully demonstrated 1 3' 2 1' 2 2, but implementation of this method in larger systems i s impractical. LC inversion c i r c u i t s can provide large aperture discharges with the benefits of o v e r a l l s i m p l i c i t y and compactness. The double-sided, four-fold LC inversion c i r c u i t with automatic preionization was proposed by Hubert Houtman for use in this amplifier, and d e t a i l s are presented in section 3.3. The energy for the main discharge was stored in the 88 Murata high-voltage ceramic capacitors. Each of these doorknob capacitors was rated at 2.7 nF, 40 kV, giving a t o t a l capacitance of 237.6 nF. Assuming a discharge area of 225 mm2, the input energy density, in units of Joules per l i t r e atmosphere, i s given by, E. = 1.504 V 2/ P in c where V i s the charging voltage in kV and P i s the absolute pressure in atmospheres. Figure 3-5 gives the c i r c u i t diagram for the main discharge, and shows the connections to the pressurized spark gap. 1 MS2 grounding r e s i s t o r s were used in order to hold the appropriate c i r c u i t points at a zero D.C. p o t e n t i a l . 18 F i g u r e 3-5 Main D i s c h a r g e C i r c u i t of the A m p l i f i e r . 19 T h e p r e i o n i z a t i o n c i r c u i t u s e d a f u r t h e r e i g h t o f t h e d o o r k n o b c a p a c i t o r s , f o u r f o r e a c h o f t h e p r e i o n i z e r r o d s . F i g u r e 3-6 g i v e s t h e p r e i o n i z a t i o n c i r c u i t r y , a n d s h o w s t h e t r i g g e r i n g a r r a n g e m e n t f o r t h e p r e s s u r i z e d s p a r k g a p . "H C = 2-7nF R = 1 Ma 0 preionizer trigger pulse 4-1 , step-up transformer F i g u r e 3-6 P r e i o n i z a t i o n C i r c u i t D i a g r a m . T r i g g e r i n g o f t h e p r e s s u r i z e d s p a r k g a p w a s a c c o m p l i s h e d b y a t r i g g e r p i n r u n n i n g u p t h e c e n t e r o f t h e s p a r k g a p c a t h o d e . T h i s t r i g g e r p i n w a s c o u p l e d , t h o u g h a 4 : 1 s t e p - u p 20 transformer, to the output of a krytron unit. This kryton unit, using an EG&G KN-22 krytron, supplied -8 kV trigger pulses with a j i t t e r of 5 ns. The spark gap was operated at pressures just in excess of those for which untriggered breakdown of the gap occurred, minimizing the j i t t e r . A transversely excited amplifier must have a s u f f i c i e n t l y fast main discharge c i r c u i t in order that the stored e l e c t r i c a l charge can flow between the electrodes in the form of a clean glow discharge. As time progresses, i t i s ch a r a c t e r i s t i c for the glow of a pulsed discharge to constr i c t and eventually develop into an arc. A fast c i r c u i t permits the transfer of most of the energy to the glow phase, reducing both the l i k e l i h o o d and the seriousness of arcing in the discharge. Since the risetime for the discharge current is proportional to the square root of the product of the c i r c u i t inductance and the c i r c u i t capacitance, there i s a need to compromise between energy and speed. Tr a d i t i o n a l Marx bank c i r c u i t s are inherently slow, with risetimes of the order of 100 ns or more. To get e f f i c i e n t input energy densities, the discharge volume must therefore be limited. The discharge c i r c u i t of t h i s amplifier was designed to be not only simpler than Marx bank methods, but also faster, allowing for an unusually large discharge volume. 21 The speed of the main discharge c i r c u i t can be estimated by considering the amplifier geometry. A current loop, going through the capacitor banks of one side of the amplifier and passing through the discharge, encloses an e f f e c t i v e area of A = 131 cm2. The inductance can be taken to be that of a current sheet, giving L = uoA/l where / i s the discharge length and uQ i s the free-space permeability. For / = 35 cm, the inductance i s 47 nH, the net capacitance for one side of the capacitor arrangement being 7.425 nF. If the discharge were to be perfectly impedance-matched, the current risetime would be that of a critically-damped c i r c u i t , T = /LC = 18.7 ns r This value represents the fastest possible estimate, the risetime being larger for imperfectly-matched discharge resistances. For the case of t o t a l underdamping, the risetime would be Tt/fc/LC = 29 ns. Figure 3-7 gives the equivalent discharge c i r c u i t for the amplifier i f i t i s assumed to be critically-damped. The capacitance i s twice that of one side, C„ = 14.85 nF, while the inductance i s half the value as E determined above, L„ = 23.5 nH. The resistance of the E discharge for impedance matching i s R E = 2/UE^E = 2 , 5 °* 22 Figure 3-7 E q u i v a l e n t A m p l i f i e r C i r c u i t f o r p r e d i c t e d i n d u c t a n c e a n d c r i t i c a l d a m p i n g . 14-85 n F 23 5 n H 25 n W i t h t h e p r e i o n i z e r r o d s i n r e l a t i v e l y c l o s e p r o x i m i t y t o t h e m a i n d i s c h a r g e e l e c t r o d e s , t h e p o s s i b i l i t y o f a r c i n g t o t h e p r e i o n i z e r s m u s t b e c o n s i d e r e d . H o w e v e r , t h e s y m m e t r y i n b o t h t h e m a i n a n d t h e p r e i o n i z e r c i r c u i t r y o f t h i s a m p l i f i e r d e s i g n e f f e c t i v e l y e l i m i n a t e d t h i s p r o b l e m . W i t h o u t t h i s s y m m e t r y , t h e r e w o u l d b e l i t t l e h o p e o f s u c c e s s f u l l y o p e r a t i n g t h e a m p l i f i e r w i t h a s i n g l e t r i g g e r e d s p a r k g a p , a s o n e w o u l d b e f o r c e d t o d e c o u p l e t h e m a i n a n d p r e i o n i z a t i o n c i r c u i t s . 23 3 . 3 Inversion C i r c u i t r y The main discharge c i r c u i t of the amplifier r e l i e s on the p r i n c i p l e of vector i n v e r s i o n 2 3 to generate transient high voltages. The d i r e c t i o n of the e l e c t r i c vector of a charged capacitor i s reversed when a spark gap, in p a r a l l e l with the capacitor, i s triggered. As shown in Figure 3 - 8 , charging the main discharge capacitor banks to voltages ±V^ gave an i n i t i a l vector configuration such that there was no voltage across the main electrodes. When the pressurized spark gap was f i r e d , the vectors of the capacitors connected in p a r a l l e l with the spark gap inverted, giving a t h e o r e t i c a l l y maximum potential of 4 V ^ between the electrodes. Lumped LC inversion c i r c u i t s are commonly used as "voltage doublers" for small-aperture l a s e r s 1 0 - 2 * . However, by s p l i t t i n g the capacitor bank for the main discharge, and putting half of the capacitors on each side of the laser, one gains a substantial increase in the speed of the discharge c i r c u i t . The concept of symmetrically s p l i t t i n g the capacitors for LC inversion lasers was developed by Hubert Houtman, and has been successfully applied to nitrogen, C02, and excimer 2 5 lasers at U.B.C.. The side-to-side symmetry of the amplifier increases the discharge c i r c u i t speed, by reducing the e f f e c t i v e inductance. The top-to-bottom symmetry gives the four - f o l d voltage m u l t i p l i c a t i o n , and further increases the speed of the discharge by reducing the e f f e c t i v e capacitance. 24 This LC inversion method produced the very high pulsed voltages needed to i n i t i a t e the main discharge from charging voltages which were low enough to be r e l a t i v e l y easy to work with. The highest voltages in the system were formed only at the locations where they were needed, namely at the two main electrodes and down the two preionization rod cables. There was no need for the insulating o i l required by Marx-bank systems, and the amplifier was thus very simple and compact. before inversion after inversion F i g u r e 3-8 Inversion C i r c u i t Operation. 25 3 . 4 Charging Arrangement The amplifier was charged using a dual power supply to provide both pos i t i v e and negative high voltages. This supply was constructed from two Hipotronics model 30B power packs, each rated at 30 kV, 5 mA D.C. Output voltages were contr o l l e d by Powerstat variable transformers and each voltage was monitored i n d i v i d u a l l y . The capacitors were charged though two 50 Mfl resistances. These charging resistances were p a r a l l e l - s e r i e s combinations of four 50 MO TRW Tiger r e s i s t o r s , each r e s i s t o r having a rating of 15 kV, 3 W. To.reduce problems due to corona, the charging r e s i s t o r s were enclosed in Lucite vessels and pressurized with dry a i r to about two atmospheres. This charging arrangement gave an RC time constant of six seconds, the amplifier requiring half a minute to reach 99% of f u l l charge. Lucite ]i 50 MO i — i — i 0 2 /.cm Figure 3-9 Pressurized Resistor Vessel. 26 CHAPTER 4 ELECTRICAL MEASUREMENTS 4.1 Voltage Measurements The voltages on the main discharge electrodes were determined with the aid of two high-voltage probes. These probes were voltage div i d e r s , using the 50 fl of terminated RG-58 coaxial signal cables in series with 7 kfl resistances connected to the electrodes' capacitor plates. The probes thus provided voltage attenuations of 141 times. The 7 kfl resistances were each composed of sixty-three 1 kfl carbon r e s i s t o r s , arranged in low inductance, p a r a l l e l - s e r i e s configurations. The risetimes of the probes were measured to be 30.6 ± 0.8 ns. The breakdown c h a r a c t e r i s t i c s of the amplifier were studied as a function of both the gas mixture and the amplifier pressure. The electrode voltage signals, attenuated by a further factor of 100, were put on the two channels of a Tektronix 466 storage oscilloscope, operating with a 20 MHz bandwidth. The positi v e and negative voltages could be displayed i n d i v i d u a l l y , or, by adding the channels, the t o t a l p otential across the electrodes could be obtained. The anode and cathode voltage traces were observed to mirror each other, as expected from the symmetry of both the amplifier and the measurement technique. Figure 4-1 gives the breakdown voltages, which show the expected linear r i s e in voltage with increasing pressure. 27 i r i r i r KOh 120h 100b > & 8 0 o > 15% %C0 2 = % N 2 balance is He % co 2 slope (kV/atm) — 15 1057 — 8 912 — 6 8-50 3 6-63 7 8 9 10 11 12 13 pressure (atm) Figure 4-1 Breakdown Voltages as a function of Gas Mix and Pressure. 28 By operating the amplifier with conditions for which no main gap breakdown occurred, the actual parameters of the LC inversion c i r c u i t were determined. Peak voltages across the electrodes reached 3.84 times the magnitude of the charging voltage, which represented 96% of the maximum theoreti c a l value. For a given gas mix, the charging voltage, V , c required for breakdown at a p a r t i c u l a r pressure could be conveniently found by r e f e r r i n g to the results of Figure 4-1. With a 6% C0 2 mix, at 7.8 atmospheres, and for V = ±14 c kV, there was no breakdown, re s u l t i n g in the ringing anode voltage of Figure 4-2. F i g u r e 4-2 Ringing Anode Voltage and measured parameters. 2 9 The s i t u a t i o n corresponded to an under-damped L R C c i r c u i t , with a ringing frequency, / = [ 4 T T L ] - V 4 L / C - R*= and an exponential-decay time constant, T = 2 L / R . From the values of Figure 4 - 2 , and with the known capacitance C = 2 9 . 7 nF, the inductance and resistance of the inversion c i r c u i t were calculated to be L = 3 9 4 nH and R = 0 . 2 1 1 n respectively. Voltage risetimes were 3 0 0 ns for the situations where the main discharge broke down at the peak of the r i s i n g voltage. The probe risetimes were taken into account by taking the true voltage risetime to be, where r and r are the measured and probe risetimes. This m p result i s exact for Gaussian pulses, but i t i s commonly used to obtain estimates of the actual risetimes. Figure 4 - 3 , where V^ = ± 2 0 kV, P = 4 . 4 atm, for a 6% mix, shows a voltage c h a r a c t e r i s t i c of an arcless glow discharge. The t o t a l f a l l - t i m e of the voltage pulse was about 9 0 ns. Arcing discharges gave voltage traces such as that of Figure 4-4, which represents the cathode potential for operating conditions of V = ± 2 4 kV, P = 7 . 8 atm, again with a 6% mix. For an arc, the discharge resistance e f f e c t i v e l y f a l l s to zero, which results in the ringing as shown on the voltage trace of Figure 4-4. 30 ~«>6 ' 6 0 0 ' 8 0 0 ' 1000 ' 1200 ' K O O t ime ( ns) Figure 4-4 Arcing Voltage Trace. 31 4.2 Current Measurements The current flowing through the main discharge was measured using a Rogowski c o i l which was placed along the outside of the PVC main body. The c o i l , 4.8 cm long, consisting of 6 turns of 2.7 mm diameter, was connected to RG-58 coaxial cable. Both the c o i l and the signal cable were in a length of Poly f l o tubing, which provided the needed e l e c t r i c a l i n s u l a t i o n . The voltage signals from t h i s Rogowski c o i l were proportional to the derivatives of the discharge currents. With a calculated c o i l inductance of 5.4 nH, and the 50 fi impedance of the coaxial cable, the. i n t r i n s i c L/R risetime of the arrangement was 108 ps. The measurement was thus scope-limited in thi s case. After being attenuated 100 times, the voltage signals were displayed on a Tektronix 466 scope with a 100 MHz bandwidth. Figure 4-5 shows the Rogowski c o i l signal for conditions of V = ±18 kV, P = 6 atm, with a 3% C0 2 mix. The la t e r parts c of the traces showed s i g n i f i c a n t shot-to-shot v a r i a t i o n , as indicated by the dotted l i n e . Integrating the signal gave the discharge current pulse of Figure 4-6, and the current risetime was seen to be 28 ns. The second hump in the current pulse corresponded to the development of an arc in the discharge. This arc formed some 100 ns after the star t of the discharge. time (ns) F i g u r e 4-6 D i s c h a r g e C u r r e n t P u l s e . 33 By integrating the current pulse again so as to get the t o t a l charge flowing through the discharge, Q, the constant of proportionality of the Rogowski c o i l , K, was obtained. With t h i s value, the true magnitude of the discharge current was determined. The signal voltage from the c o i l , S = K dl/dt so that the current and charge were, I = J S / K dt = A / K Q = /I dt = /A//c dt But the charge that flowed through the discharge can also be taken to be, Q = CF f c/l6 where C = 237.6 nF i s the t o t a l capacitance and V, i s the b breakdown voltage. This gave K = 7.81 ± 0 . 2 4 O 0 5 ) V/Cns 2, and hence the peak discharge current was 13.5 ± 1 kA. At the peak current, the discharge voltage was 28 kV, which gives a discharge resistance of R = 2.4 fi. For c r i t i c a l damping, the results of the analysis of page 21 predict a resistance R £ = 2.5 SJ. This analysis also gives a maximum current, I = K e" V C /L = 16.4 kA, m b E E which i s s l i g h t l y larger than the measured discharge current. 34 4 . 3 Preionizer Timing By looking at the v i s i b l e l i g h t signals produced by the pressurized spark gap, preionizer rods, and the main discharge, the r e l a t i v e timing of the UV-preionization was determined. The l i g h t was channeled into a screened room through an op t i c a l f i b r e , and converted to an e l e c t r i c a l signal with a Hewlett Packard 4220 pin diode. This signal was then displayed on a 20 MHz Tektronix 466 scope. Figure 4-7 shows the spark gap l i g h t , attenuated 10 times, leading the preionizer l i g h t by some 85 ns. In thi s case the amplifier was operated with conditions for which there was no main gap breakdown (15% mix, 8 atm, and V - ±20 kV). Figure 4 - 8 gives c the l i g h t signal, obtained by imaging, through the Lucite end-flange arrangement, the i n t e r i o r of the main body when there was a main discharge (15% mix, 3 atm, V = ±18 kV). In c t h i s case the l i g h t from a clean glow discharge was seen in addition to the preionizer l i g h t . 300 400 500 600 700 800 timt (ns) Figure 4-7 Spark Gap and Preionizer Light. 10r -i r i r -i 1 1 r preionizer light main discharge light I i I . L 400 800 time (ns) 1200 F i g u r e 4-8 Main Body L i g h t showing l i g h t from p r e i o n i z e r s and glow d i s c h a r g e . > £ cn S 2-5 o > c 5? w 0 1 1 • I ' i arc-—\ i 1 glow—, preionizer—i • I . I . I . 400 500 600 700 800 time (ns) F i g u r e 4-9 L i g h t S i g n a l w i t h A r c . 36 The 30 ns glow-light risetime corresponded c l o s e l y to the previously measured 28 ns risetime of the discharge current. The average time from the start of the preionization to the st a r t of the main discharge was seen to be 175 ns. Figure 4-9 shows a l i g h t signal where the glow discharge developed into an arc after approximately 90 ns. This glow-to-arc time was similar to the results from the discharge current pulses (Fig. 4 - 6 ) . The detector signal due to the arc l i g h t was more than forty times larger than that of the glow-discharge l i g h t . 37 4.4 Overall Amplifier Timing The results of the voltage, current, gain, and v i s i b l e l i g h t measurements provide a good description of the timing of the amplifier, which i s summarized in Figure 4-10. The pressurized spark gap was observed to f i r e some 350 ns after the i n i t i a l trigger signal reached the krytron unit. This delay was att r i b u t a b l e to the EG&G TR-130 transformer used, within the krytron unit, to trigger the krytron. The voltage across the main discharge electrodes starts to develop almost immediately upon the breakdown of the spark gap, while the l i g h t from the preionizers was observed to start 85 ns l a t e r . The exact point at which the main discharge broke down was dependent on the gas mix and operating pressure, but for the situation depicted t h i s occurred some 250 ns after the spark-gap breakdown. The preionization thus started 175 ns before the'start of the main discharge. The discharge current reached a peak with a 28 ns risetime, and had a FWHM of approximately 70 ns. The peak gain was t y p i c a l l y to seen occur 300 ns after the i n i t i a l s t a r t of the current, i . e . 900 ns after the i n i t i a l t r i g g e r i n g of the krytron unit. 38 1 1 T 1 r discharge current voltage _i i : i i i i i 300 500 700 900 time (ns) F i g u r e 4-10 Summary of A m p l i f i e r T i m i n g . 39 CHAPTER 5 GAIN MEASUREMENTS 5.1 Small-Signal Gain The gain of multiatmosphere TE C0 2 lasers has been extensively studied for a wide range of pressures, gas mixes, and excitation methods 9' 1 2' 1 3' 2 4. The purpose of t h i s thesis was not, however, to perform an in-depth investigation of the amplifier gain, but rather to develop an i n i t i a l understanding of the conditions under which gain could be obtained. A large number of factors determine the gain, such as the operating pressure, C0 2 concentration, charging voltages, and the p a r t i c u l a r r o t a t i o n a l - v i b r a t i o n a l l i n e on which the gain i s measured. Since the amplifier i s intended for use in the 9 /urn laser system, only the gain for selected l i n e s in t h i s band were measured in t h i s study. The small-signal gain of the amplifier was measured using the arrangement of Figure 5-1. A TEM 0 0, =2 W C0 2 probe beam of 5mm diameter was produced by a grating-tuned CW la s e r . Alignment of the CW laser was f a c i l i t a t e d by an i n t r a c a v i t y He-Ne beam, t h i s being introduced into the laser cavity by r e f l e c t i o n off the KC1 f l a t at one end of the C0 2 laser body. A second He-Ne beam was made coaxial with the ho r i z o n t a l l y - p o l a r i z e d output C0 2 beam from the blazed grating. This was done by using a Germanium etalon set at. the appropriate C0 2 Brewster angle. 40 F i g u r e 5-1 Small o p t i c a l layout -Signal Gain Arrangement showing for Single-Pass Measurements. 41 The probe beam was chopped, passed through the high-pressure amplifier, and focused, with a 20 cm focal length KC1 lens, down onto a Boston Electronics P005 HgCdTe infrared detector. This detector had a s e n s i t i v i t y of 5 mV/W. Both the detector and the oscilloscope were within an e l e c t r i c a l l y screened room. The chopper, consisting of a slotted brass disk spun at 1500 rpm by a small e l e c t r i c motor, provided a chopped probe beam with 8% duty cycle. The wavelength of the grating-tuned CW C0 2 laser was monitored with an Optical Engineering model 16A C0 2 spectrum analyzer. The CW laser had a stable output when running on the 9P(18) l i n e , which was the l i n e for which most of the gain measurements were made. The triggering and timing methods for the gain study are summarized in Figure 5-2. The result of t h i s arrangement was that the amplifier only f i r e d when a probe C0 2 pulse was within the discharge volume. F i r i n g of the amplifier was i n i t i a t e d by a squeeze bulb, with this signal being put in coincidence with an e l e c t r i c a l pulse t r a i n from the beam-chopper. The coincidence result was the input of a delay unit, which put out a 40 V pulse, 20 MS long, with a 75 ns risetime. This pulse triggered both the krytron unit, f i r i n g the amplifier spark gap, and the oscilloscope used to display the signal from the infrared detector. high vo l tage power supply -V, cw b e a m chopper a m p l i f i e r -o a - t r a n s f o r m e r spark 1 — g a p \JT32kV sc reened r o o m 6 squeeze bulb o sc i l l o scope tr igger input o--8kV 43 I n i t i a l l y , gain signals were displayed on a 100 MHz Tektronix 7904 oscilloscope, with -1.25 mV/div s e n s i t i v i t y provided by using two 7A12 amplifier units. However the gain signals were s u f f i c i e n t l y large that a Tektronix 466 storage scope, with a 2.5 mV/div display, was used for the majority of the measurements. Figure 5-3 shows two gain traces, both for a 3% C0 2 mix, with one at 10 atm and the other at 6.4 atm. The i n i t i a l dip in the transmitted probe beam can be attributed to inverse bremstrahlung absorption occurring in the d i s c h a r g e 1 3 . If the i n i t i a l i ntensity of the probe beam was I 0 , then the peak output intensity was, I = I 0 exp[ff/ ] where a was the small-signal gain and / was the length of the gain region. For the case of a single pass of the amplifier, with / = 35 cm, the small-signal gain, in units of %/cm, was given by, a = 2.86 l n [ l / I 0 l . The gain, as a function of pressure, was measured on the 9P(18) l i n e for 3%, 6%, 8%, and 15% C0 2 mixes, and the results are summarized in Figure 5-4. The plotted points represent the maximum gains seen at each pressure. A l l of these measurements were made with the probe beam passing down the center of the discharge volume. 44 1 « 1 1 1 r 2\-3% mix 9P(18) 6-4 atm o s 11 o cn J u J_ 400 800 1200 time (ns) 1600 i 1 1 1 1 " r i r 2h u a c 3 % mix 9P(18) 10 atm 1200 time (ns) F i g u r e 5-3 G a i n P u l s e s . • average uncertainty typical input energy density * 130 J / [ l atm] 1 1 6 7 8 pressure (atm) 10 46 The results showed the expected r e l a t i o n s h i p between the gain and the richness of the gas mix, in that the highest gains occurred with the 15% C0 2 mix. A l l the mixes, except the 15% one, showed an increasing gain with increasing pressure, at least i n i t i a l l y for the 6% and 8% mixes. The f a l l o f f in the gain for the 15% mix, and the l a t e r drops with the 6% and 8% mixes, were the result of arcing problems in the discharge. Only the 3% and 6% mixes were observed to have gain at the highest pressures, and the 3% mix was the only one for which gain was r e l i a b l y obtained at ten atmospheres. Though the pump energy density did vary somewhat for the data of Figure 5-4, i t can be taken to be 130 Joules per l i t r e atmosphere for the entire figure. For the 3% C0 2 mix, the small-signal gain was seen to ri s e with increasing pressure over the whole pressure range, consistent with pressure-broadened gain theory. The gain at 10 atm was 1.5 times that at 3 atm. However the gain was independent of the nominal input energy density at t h i s pressure. This was l i k e l y due to arcing, with any additional energy being channeled into the arc rather than increasing the energy density of the glow region of the discharge. The gain for a 3% C0 2 mix at ten atmospheres, on the 9R(16) l i n e , was observed to be 20% higher than the gain on the 9P(18) l i n e . There were no measurable differences between results on the 9P(18) and 9P(20) l i n e s . 47 Only at the lower operating pressures was the nominal input energy density t r u l y meaningful. Figure 5-5 shows the dependence of the gain on the energy density, with the gain c h a r a c t e r i s t i c a l l y r i s i n g and then l e v e l l i n g o f f , i f not f a l l i n g s l i g h t l y , as the input energy density reached and surpassed some c r i t i c a l value. A 1 — i 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 £ 3 < 2 > - ~ 27atm,8° /omix c a cn 2 4-7 a tm. 67o mix • ° — 2 7 atm,6%mix 1 J I 50 100 150 200 250 energy density ( J / [ I a tm]) F i g u r e 5-5 Gain as a Function of the nominal Input Energy Density. 48 As previously mentioned, the small-signal gain results were influenced by arcing problems, the seriousness of which increased as the operating pressure of the amplifier was raised. With the 15% and 8% gas mixes, at pressures above six atmospheres, severe arcing could be observed to be occurring on both sides of the discharge. For the 6% and 3% mixes, arcing at higher pressures was confined to only one side of the discharge. The center of the discharge was e s s e n t i a l l y free of arcs for a l l the mixes and at a l l amplifier pressures. The s p a t i a l d i s t r i b u t i o n of the gain on the central plane of the discharge was measured for a 3% C0 2 mix at 10 atm, and the results are given in Figure 5 - 6 . The horizontal position of each point represents the center of the probing C0 2 beam. The s p a t i a l gain p r o f i l e was e n t i r e l y consistent with the observed presence of an arc down the right-hand edge of the discharge. At t h i s pressure, the useful region of the discharge was seen to be confined to a 7 mm wide area at the center of the electrodes. The propensity for arcs to occur along one side of the discharge was due to the fact that the electrodes were not absolutely p a r a l l e l . There was an average of 2% misalignment, with the electrodes being 0.3 mm closer together on the "arcing" side. This misalignment was due to the holes in one side of the main body being at a s l i g h t angle to the holes on the other side. F i g u r e 5-6 S p a t i a l G a i n P r o f i l e . 50 5.2 Gain Lifetime and Risetime Both the gain l i f e t i m e and risetime were seen to have pressure dependences in good agreement with standard c o l l i s i o n a l theory. Figure 5-7 gives the f u l l width at half maximum of the gain plotted against the pressure, while Figure 5-8 gives the inverse gain risetime versus the pressure. The data, for both figures, was for a 6% C0 2 mix, and represents a combination of single-pass and double-pass measurements. These results are consistent with known theory, where the relaxation times of the laser l e v e l s scale inversely with the pressure. This scaling relationship • has been experimentally confirmed by other investigators, and Alcock 9 et. a l . have measured similar pressure dependence of gain l i f e t i m e and risetime. slope s -155 ns/atm 5 6 7 pressure (atm) 10 Figure 5-7 G a i n L i f e t i m e . 5 6 7 8 pressure (atm) F i g u r e 5-8 Inverse G a i n R i s e t i m e . 52 5.3 Amplifier as an O s c i l l a t o r In order to see what sort of energies could be obtained from the amplifier i t was set up as an o s c i l l a t o r . The stable resonating cavity consisted of a f u l l y - r e f l e c t i n g gold mirror of 5 m radius and a 85%-reflecting coated germanium f l a t , spaced 80 cm apart. With a 15% C0 2 mix, the amplifier was charged to V = ±20 kV at a pressure of 2.7 atmospheres. The output laser pulse was measured with a GenTec ED 200 pyroelectric detector, and found to be 1.45 J. This represented an e f f i c i e n c y of approximately 3%. The germanium f l a t was damaged after only a few shots, but there was no vi s u a l damage to the the 5 m mirror. 53 CHAPTER 6 DISCUSSION AND CONCLUSIONS 6.1 Summary of Results The unique e l e c t r i c a l c i r c u i t r y of the multiatmospheric amplifier has proven to be e f f e c t i v e , with measured results c l o s e l y matching the predicted values. The voltage inversion achieved e f f i c i e n c i e s of up to 96%, with the potential across the main discharge electrodes reaching 3.84 times the magnitude of the charging voltage in ringing s i t u a t i o n s . For discharges, where main-gap breakdown occurred, r e l i a b l e operation could be obtained with 90% inversions. The measured discharge current risetime of 28 ns was c e r t a i n l y within the predicted range, and suggested that the discharge was not impedance-matched in i t s early stages. However, the 2.4 Q discharge resistance at the peak of the current pulse matched the 2.5 fi value for c r i t i c a l damping. The resistance of the discharge, changing throughout the discharge period, approached the impedance-matched value at the peak of the discharge current. The preionization was seen to occur approximately 175 ns before the star t of the main discharge. This delay was fixed for a p a r t i c u l a r gas mix and amplifier pressure, with the triggering of the single pressurized spark gap starting both the preionization and the LC voltage inversion. The ef f e c t i v e delay between the preionization and the main discharge was 54 greatest when the breakdown of the main gap came near the peak of the voltage inversion. Arcing in the discharge, when i t did happen, was evident in the current, voltage, v i s i b l e l i g h t , and s p a t i a l gain measurements. In the current and v i s i b l e l i g h t results, any arcing was seen to occur about 100 ns after the start of the discharge. By thi s time, most of the energy had been deposited in the form of a clean glow discharge. Arcing was confined to one side of the discharge volume, as confirmed by the s p a t i a l gain p r o f i l e . The small-signal gain r e s u l t s , at least for the weak C0 2 mixes, were consistent with the theory of c o l l i s i o n a l broadening at high pressures. Both the gain l i f e t i m e s and inverse gain risetimes were seen to be l i n e a r l y dependent on pressure. Peak small-signal gains of 2.7 %/cm for a 3% C0 2 mix at 10 atm are similar to those reported by Corkum 2 6 for a Lumonics 880 TE module operating with the same mix. The gain for richer gas mixes at higher pressures was seriously affected by arcing in the discharge. 55 6.2 Recommendations The e l e c t r i c a l and timing c h a r a c t e r i s t i c s of the amplifier have been found to be e n t i r e l y suitable. The only problem to be overcome i s that of arcing in the main discharge. For the highest pressures t h i s arcing e f f e c t i v e l y l i m i t s the useful gain volume to a 7 mm wide region at the center of the discharge. If the amplifier i s to be used for multi-pass amplification the entire discharge volume must be u t i l i z e d . The arcing down both sides of the discharge for the richer gas mixes suggests that the electrodes are too sharply curved along their edges. The misalignment of the electrodes results in the single-sided arcing observed for the weak gas mixes at high pressures. For t h i s amplifier, and future units, the electrodes should be c a r e f u l l y aligned, and a more gradual curve given to their surfaces. The danger in reshaping the discharge electrodes l i e s in the p o s s i b i l i t y of merely moving the arcing from the edges of the discharge to the center, which i s even less desirable. The addition of trace amounts of low ionization seed gases to high-pressure UV-preionized lasers has been demonstrated to be an e f f e c t i v e technique for reducing the propensity for arcing. M i l l e r 1 3 et. a l . achieved a 50% increase in the maximum operating pressure of a multiatmospheric laser by adding tri-n-propylamine to a 10% C0 2 mix. In small enough concentrations, there is no measurable ef f e c t with respect to the gain. Even without any 56 changes to the amplifier electrodes, one could expect the use of tri-n-propylamine to allow arc-free operation of 3% and 6% C0 2 mixes at pressures of up to 10 atmospheres. Once i t i s known exactly which l i n e the output of the high-pressure mode-locked C0 2 o s c i l l a t o r w i l l be centered on, further studies of the small-signal gain should be carried out. With improvements to the discharge electrodes, and the addition of tri-n-propylamine, the amplifier can be expected to work r e l i a b l y at 10 atm with an 8% C0 2 mix. The timing of the amplifier gain must be c a r e f u l l y determined for these conditions so as to allow for correct synchronization between the amplifier and other components in the ultra-short pulse laser chain. The amplifier has been successfully charged to V = ±34 kV, at which point corona st a r t s to become a problem. By flushing the amplifier box with dry a i r , r e l i a b l e charging would be possible up to at least V = ±37 kV, corresponding to nominal input energy densities of 206 Joules per l i t r e atmosphere for amplifier pressures of ten atmospheres. Under these conditions, with an 8% C0 2 mix, one expects gains of 3.5 %/cm on the 9R(16) l i n e 9 , representing a 3.4 fold i n tensity increase per pass through the amplifier. 57 BIBLIOGRAPHY 1. Alcock, A. J . , and Corkum, P. B., P h i l . Trans. R. Soc. Lond. A., 298, 365 (1980) 2. Bernard, J . E., "The C0 2 Laser at U.B.C.", Plasma Physics Laboratory Report #104 (1985) 3. Basov, N. G., Belenov, E. M., Danilychev, V. A., et. a l . , Sov. Phys. JETP Lett., _1_4, 285 (1971) 4. Basov, N. G., Belenov, E. M., Danilychev, V. A., et. a l . , Sov. Phys. JETP, 32, 58 (1973) 5 . Harris, N. W. , O'Neill, F., and Whitney, W. T., Appl. Phys. Lett., 25, 148 (1974) 6. Chang, T. Y., and Wood, 0. R., Appl. Phys. Lett., 23, 370 (1973) 7. Blanchard, M., G i l b e r t , J . , Rheault, F., et. a l . , J . Appl. Phys., 45, 1311 (1974) 8. Hidson, D. J . , Makios, V., and Morrison, R. W., Phys. Lett., 40A, 413 (1972) 9 . Alcock, A. J . , Fedosejevs, R., and Walker, A. C , IEEE J . Quantum Ele c t . , QE-11, 767 (1975) 10. Carmen, T., and Dyer, P. E., J . Appl. Phys., 49, 3742 (1978) 11. Wan, C. Y., Werling, U., and Renk, K. F., J . Appl. Phys., 57, 990 (1985) 12. Taylor, R. S., Alcock, A. J., Sargeant, W. J., and Leopold, K. E., IEEE J . Quantum El e c t . , QE-15, 1131 (1979) 58 1 3 . M i l l e r , J . L., Ross, A. H. M., and George, E. v., Appl. Phys. Lett., 26, 523 (1975) 1 4 . Dyer, P. E., and Raouf, D. N., Opt. Commun., 53, 36 (1985) 1 5 . Jayaram, K., and Alcock, A. J., J . Appl. Phys., 58, 1719 (1985) 1 6 . Patel, B. S., and Swarup, P., J . Phys. D.: Appl. Phys., 6, 1670 (1973) 1 7 . Chang, N. C., and Tavis, M. T., IEEE J . Quantum Elect., QE-10, 372 (1974) 1 8 . Agalakov, Y. G. , Bulanin, M. 0., Bertsev,. V. V., et. a l . , Opt. Spectrosc. (USSR), 58, 298 (1985) 1 9 . Norris, B., and Smith, A. L. S., J . Phys. E: S c i . Instrum., j_0, 551 ( 1977) 20. Houtman, H., Rev. S c i . Instrum., 56, 2295 (1985) 2 1 . Brink, D. J . , Hasson, V., and Salamon, T. I., J . Phys. E: S c i . Instrum., J_0, 370 (1977) 22. Brink, D. J., and Hasson, V., J . Appl. Phys., 49, 2250 (1978) 2 3 . F i t c h , R. A., and Howell, V. T. S., Proc. I EE, 111, 849 (1964) 2 4 . Olbertz, A. H. M., and Witteman, W. J . , Opt. Commun., 30, 447 (1979) 2 5 . Ford, J . E., Meyer, J., and Houtman, H., Appl. Phys. Lett., (to be published) (1986) 2 6 . Corkum, P. B., IEEE J . Quantum El e c t . , QE-21, 216 (1985) 

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